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1.1 misho 1: -----------------------------------------------------------------------------
2: This file contains a concatenation of the PCRE man pages, converted to plain
3: text format for ease of searching with a text editor, or for use on systems
4: that do not have a man page processor. The small individual files that give
5: synopses of each function in the library have not been included. Neither has
6: the pcredemo program. There are separate text files for the pcregrep and
7: pcretest commands.
8: -----------------------------------------------------------------------------
9:
10:
11: PCRE(3) PCRE(3)
12:
13:
14: NAME
15: PCRE - Perl-compatible regular expressions
16:
17:
18: INTRODUCTION
19:
20: The PCRE library is a set of functions that implement regular expres-
21: sion pattern matching using the same syntax and semantics as Perl, with
22: just a few differences. Some features that appeared in Python and PCRE
23: before they appeared in Perl are also available using the Python syn-
24: tax, there is some support for one or two .NET and Oniguruma syntax
25: items, and there is an option for requesting some minor changes that
26: give better JavaScript compatibility.
27:
1.1.1.2 misho 28: Starting with release 8.30, it is possible to compile two separate PCRE
29: libraries: the original, which supports 8-bit character strings
30: (including UTF-8 strings), and a second library that supports 16-bit
31: character strings (including UTF-16 strings). The build process allows
32: either one or both to be built. The majority of the work to make this
33: possible was done by Zoltan Herczeg.
34:
35: The two libraries contain identical sets of functions, except that the
36: names in the 16-bit library start with pcre16_ instead of pcre_. To
37: avoid over-complication and reduce the documentation maintenance load,
38: most of the documentation describes the 8-bit library, with the differ-
39: ences for the 16-bit library described separately in the pcre16 page.
40: References to functions or structures of the form pcre[16]_xxx should
41: be read as meaning "pcre_xxx when using the 8-bit library and
42: pcre16_xxx when using the 16-bit library".
43:
1.1 misho 44: The current implementation of PCRE corresponds approximately with Perl
1.1.1.2 misho 45: 5.12, including support for UTF-8/16 encoded strings and Unicode gen-
46: eral category properties. However, UTF-8/16 and Unicode support has to
47: be explicitly enabled; it is not the default. The Unicode tables corre-
1.1 misho 48: spond to Unicode release 6.0.0.
49:
50: In addition to the Perl-compatible matching function, PCRE contains an
51: alternative function that matches the same compiled patterns in a dif-
52: ferent way. In certain circumstances, the alternative function has some
53: advantages. For a discussion of the two matching algorithms, see the
54: pcrematching page.
55:
56: PCRE is written in C and released as a C library. A number of people
57: have written wrappers and interfaces of various kinds. In particular,
1.1.1.2 misho 58: Google Inc. have provided a comprehensive C++ wrapper for the 8-bit
59: library. This is now included as part of the PCRE distribution. The
60: pcrecpp page has details of this interface. Other people's contribu-
61: tions can be found in the Contrib directory at the primary FTP site,
62: which is:
1.1 misho 63:
64: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
65:
1.1.1.2 misho 66: Details of exactly which Perl regular expression features are and are
1.1 misho 67: not supported by PCRE are given in separate documents. See the pcrepat-
1.1.1.2 misho 68: tern and pcrecompat pages. There is a syntax summary in the pcresyntax
1.1 misho 69: page.
70:
1.1.1.2 misho 71: Some features of PCRE can be included, excluded, or changed when the
72: library is built. The pcre_config() function makes it possible for a
73: client to discover which features are available. The features them-
74: selves are described in the pcrebuild page. Documentation about build-
75: ing PCRE for various operating systems can be found in the README and
1.1 misho 76: NON-UNIX-USE files in the source distribution.
77:
1.1.1.2 misho 78: The libraries contains a number of undocumented internal functions and
79: data tables that are used by more than one of the exported external
80: functions, but which are not intended for use by external callers.
81: Their names all begin with "_pcre_" or "_pcre16_", which hopefully will
82: not provoke any name clashes. In some environments, it is possible to
83: control which external symbols are exported when a shared library is
84: built, and in these cases the undocumented symbols are not exported.
1.1 misho 85:
86:
87: USER DOCUMENTATION
88:
1.1.1.2 misho 89: The user documentation for PCRE comprises a number of different sec-
90: tions. In the "man" format, each of these is a separate "man page". In
91: the HTML format, each is a separate page, linked from the index page.
92: In the plain text format, all the sections, except the pcredemo sec-
1.1 misho 93: tion, are concatenated, for ease of searching. The sections are as fol-
94: lows:
95:
96: pcre this document
1.1.1.2 misho 97: pcre16 details of the 16-bit library
1.1 misho 98: pcre-config show PCRE installation configuration information
99: pcreapi details of PCRE's native C API
100: pcrebuild options for building PCRE
101: pcrecallout details of the callout feature
102: pcrecompat discussion of Perl compatibility
1.1.1.2 misho 103: pcrecpp details of the C++ wrapper for the 8-bit library
1.1 misho 104: pcredemo a demonstration C program that uses PCRE
1.1.1.2 misho 105: pcregrep description of the pcregrep command (8-bit only)
1.1 misho 106: pcrejit discussion of the just-in-time optimization support
107: pcrelimits details of size and other limits
108: pcrematching discussion of the two matching algorithms
109: pcrepartial details of the partial matching facility
110: pcrepattern syntax and semantics of supported
111: regular expressions
112: pcreperform discussion of performance issues
1.1.1.2 misho 113: pcreposix the POSIX-compatible C API for the 8-bit library
1.1 misho 114: pcreprecompile details of saving and re-using precompiled patterns
115: pcresample discussion of the pcredemo program
116: pcrestack discussion of stack usage
117: pcresyntax quick syntax reference
118: pcretest description of the pcretest testing command
1.1.1.2 misho 119: pcreunicode discussion of Unicode and UTF-8/16 support
1.1 misho 120:
1.1.1.2 misho 121: In addition, in the "man" and HTML formats, there is a short page for
122: each 8-bit C library function, listing its arguments and results.
1.1 misho 123:
124:
125: AUTHOR
126:
127: Philip Hazel
128: University Computing Service
129: Cambridge CB2 3QH, England.
130:
1.1.1.2 misho 131: Putting an actual email address here seems to have been a spam magnet,
132: so I've taken it away. If you want to email me, use my two initials,
1.1 misho 133: followed by the two digits 10, at the domain cam.ac.uk.
134:
135:
136: REVISION
137:
1.1.1.2 misho 138: Last updated: 10 January 2012
139: Copyright (c) 1997-2012 University of Cambridge.
140: ------------------------------------------------------------------------------
141:
142:
143: PCRE(3) PCRE(3)
144:
145:
146: NAME
147: PCRE - Perl-compatible regular expressions
148:
149: #include <pcre.h>
150:
151:
152: PCRE 16-BIT API BASIC FUNCTIONS
153:
154: pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options,
155: const char **errptr, int *erroffset,
156: const unsigned char *tableptr);
157:
158: pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options,
159: int *errorcodeptr,
160: const char **errptr, int *erroffset,
161: const unsigned char *tableptr);
162:
163: pcre16_extra *pcre16_study(const pcre16 *code, int options,
164: const char **errptr);
165:
166: void pcre16_free_study(pcre16_extra *extra);
167:
168: int pcre16_exec(const pcre16 *code, const pcre16_extra *extra,
169: PCRE_SPTR16 subject, int length, int startoffset,
170: int options, int *ovector, int ovecsize);
171:
172: int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra,
173: PCRE_SPTR16 subject, int length, int startoffset,
174: int options, int *ovector, int ovecsize,
175: int *workspace, int wscount);
176:
177:
178: PCRE 16-BIT API STRING EXTRACTION FUNCTIONS
179:
180: int pcre16_copy_named_substring(const pcre16 *code,
181: PCRE_SPTR16 subject, int *ovector,
182: int stringcount, PCRE_SPTR16 stringname,
183: PCRE_UCHAR16 *buffer, int buffersize);
184:
185: int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector,
186: int stringcount, int stringnumber, PCRE_UCHAR16 *buffer,
187: int buffersize);
188:
189: int pcre16_get_named_substring(const pcre16 *code,
190: PCRE_SPTR16 subject, int *ovector,
191: int stringcount, PCRE_SPTR16 stringname,
192: PCRE_SPTR16 *stringptr);
193:
194: int pcre16_get_stringnumber(const pcre16 *code,
195: PCRE_SPTR16 name);
196:
197: int pcre16_get_stringtable_entries(const pcre16 *code,
198: PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last);
199:
200: int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector,
201: int stringcount, int stringnumber,
202: PCRE_SPTR16 *stringptr);
203:
204: int pcre16_get_substring_list(PCRE_SPTR16 subject,
205: int *ovector, int stringcount, PCRE_SPTR16 **listptr);
206:
207: void pcre16_free_substring(PCRE_SPTR16 stringptr);
208:
209: void pcre16_free_substring_list(PCRE_SPTR16 *stringptr);
210:
211:
212: PCRE 16-BIT API AUXILIARY FUNCTIONS
213:
214: pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize);
215:
216: void pcre16_jit_stack_free(pcre16_jit_stack *stack);
217:
218: void pcre16_assign_jit_stack(pcre16_extra *extra,
219: pcre16_jit_callback callback, void *data);
220:
221: const unsigned char *pcre16_maketables(void);
222:
223: int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra,
224: int what, void *where);
225:
226: int pcre16_refcount(pcre16 *code, int adjust);
227:
228: int pcre16_config(int what, void *where);
229:
230: const char *pcre16_version(void);
231:
232: int pcre16_pattern_to_host_byte_order(pcre16 *code,
233: pcre16_extra *extra, const unsigned char *tables);
234:
235:
236: PCRE 16-BIT API INDIRECTED FUNCTIONS
237:
238: void *(*pcre16_malloc)(size_t);
239:
240: void (*pcre16_free)(void *);
241:
242: void *(*pcre16_stack_malloc)(size_t);
243:
244: void (*pcre16_stack_free)(void *);
245:
246: int (*pcre16_callout)(pcre16_callout_block *);
247:
248:
249: PCRE 16-BIT API 16-BIT-ONLY FUNCTION
250:
251: int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output,
252: PCRE_SPTR16 input, int length, int *byte_order,
253: int keep_boms);
254:
255:
256: THE PCRE 16-BIT LIBRARY
257:
258: Starting with release 8.30, it is possible to compile a PCRE library
259: that supports 16-bit character strings, including UTF-16 strings, as
260: well as or instead of the original 8-bit library. The majority of the
261: work to make this possible was done by Zoltan Herczeg. The two
262: libraries contain identical sets of functions, used in exactly the same
263: way. Only the names of the functions and the data types of their argu-
264: ments and results are different. To avoid over-complication and reduce
265: the documentation maintenance load, most of the PCRE documentation
266: describes the 8-bit library, with only occasional references to the
267: 16-bit library. This page describes what is different when you use the
268: 16-bit library.
269:
270: WARNING: A single application can be linked with both libraries, but
271: you must take care when processing any particular pattern to use func-
272: tions from just one library. For example, if you want to study a pat-
273: tern that was compiled with pcre16_compile(), you must do so with
274: pcre16_study(), not pcre_study(), and you must free the study data with
275: pcre16_free_study().
276:
277:
278: THE HEADER FILE
279:
280: There is only one header file, pcre.h. It contains prototypes for all
281: the functions in both libraries, as well as definitions of flags,
282: structures, error codes, etc.
283:
284:
285: THE LIBRARY NAME
286:
287: In Unix-like systems, the 16-bit library is called libpcre16, and can
288: normally be accesss by adding -lpcre16 to the command for linking an
289: application that uses PCRE.
290:
291:
292: STRING TYPES
293:
294: In the 8-bit library, strings are passed to PCRE library functions as
295: vectors of bytes with the C type "char *". In the 16-bit library,
296: strings are passed as vectors of unsigned 16-bit quantities. The macro
297: PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is
298: defined as "const PCRE_UCHAR16 *". In very many environments, "short
299: int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16
300: as "short int", but checks that it really is a 16-bit data type. If it
301: is not, the build fails with an error message telling the maintainer to
302: modify the definition appropriately.
303:
304:
305: STRUCTURE TYPES
306:
307: The types of the opaque structures that are used for compiled 16-bit
308: patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively.
309: The type of the user-accessible structure that is returned by
310: pcre16_study() is pcre16_extra, and the type of the structure that is
311: used for passing data to a callout function is pcre16_callout_block.
312: These structures contain the same fields, with the same names, as their
313: 8-bit counterparts. The only difference is that pointers to character
314: strings are 16-bit instead of 8-bit types.
315:
316:
317: 16-BIT FUNCTIONS
318:
319: For every function in the 8-bit library there is a corresponding func-
320: tion in the 16-bit library with a name that starts with pcre16_ instead
321: of pcre_. The prototypes are listed above. In addition, there is one
322: extra function, pcre16_utf16_to_host_byte_order(). This is a utility
323: function that converts a UTF-16 character string to host byte order if
324: necessary. The other 16-bit functions expect the strings they are
325: passed to be in host byte order.
326:
327: The input and output arguments of pcre16_utf16_to_host_byte_order() may
328: point to the same address, that is, conversion in place is supported.
329: The output buffer must be at least as long as the input.
330:
331: The length argument specifies the number of 16-bit data units in the
332: input string; a negative value specifies a zero-terminated string.
333:
334: If byte_order is NULL, it is assumed that the string starts off in host
335: byte order. This may be changed by byte-order marks (BOMs) anywhere in
336: the string (commonly as the first character).
337:
338: If byte_order is not NULL, a non-zero value of the integer to which it
339: points means that the input starts off in host byte order, otherwise
340: the opposite order is assumed. Again, BOMs in the string can change
341: this. The final byte order is passed back at the end of processing.
342:
343: If keep_boms is not zero, byte-order mark characters (0xfeff) are
344: copied into the output string. Otherwise they are discarded.
345:
346: The result of the function is the number of 16-bit units placed into
347: the output buffer, including the zero terminator if the string was
348: zero-terminated.
349:
350:
351: SUBJECT STRING OFFSETS
352:
353: The offsets within subject strings that are returned by the matching
354: functions are in 16-bit units rather than bytes.
355:
356:
357: NAMED SUBPATTERNS
358:
359: The name-to-number translation table that is maintained for named sub-
360: patterns uses 16-bit characters. The pcre16_get_stringtable_entries()
361: function returns the length of each entry in the table as the number of
362: 16-bit data units.
363:
364:
365: OPTION NAMES
366:
367: There are two new general option names, PCRE_UTF16 and
368: PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and
369: PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options
1.1.1.3 ! misho 370: define the same bits in the options word. There is a discussion about
! 371: the validity of UTF-16 strings in the pcreunicode page.
1.1.1.2 misho 372:
1.1.1.3 ! misho 373: For the pcre16_config() function there is an option PCRE_CONFIG_UTF16
! 374: that returns 1 if UTF-16 support is configured, otherwise 0. If this
! 375: option is given to pcre_config(), or if the PCRE_CONFIG_UTF8 option is
1.1.1.2 misho 376: given to pcre16_config(), the result is the PCRE_ERROR_BADOPTION error.
377:
378:
379: CHARACTER CODES
380:
1.1.1.3 ! misho 381: In 16-bit mode, when PCRE_UTF16 is not set, character values are
1.1.1.2 misho 382: treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
1.1.1.3 ! misho 383: that they can range from 0 to 0xffff instead of 0 to 0xff. Character
! 384: types for characters less than 0xff can therefore be influenced by the
! 385: locale in the same way as before. Characters greater than 0xff have
1.1.1.2 misho 386: only one case, and no "type" (such as letter or digit).
387:
1.1.1.3 ! misho 388: In UTF-16 mode, the character code is Unicode, in the range 0 to
! 389: 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff
! 390: because those are "surrogate" values that are used in pairs to encode
1.1.1.2 misho 391: values greater than 0xffff.
392:
1.1.1.3 ! misho 393: A UTF-16 string can indicate its endianness by special code knows as a
1.1.1.2 misho 394: byte-order mark (BOM). The PCRE functions do not handle this, expecting
1.1.1.3 ! misho 395: strings to be in host byte order. A utility function called
! 396: pcre16_utf16_to_host_byte_order() is provided to help with this (see
1.1.1.2 misho 397: above).
398:
399:
400: ERROR NAMES
401:
1.1.1.3 ! misho 402: The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre-
! 403: spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is
! 404: given when a compiled pattern is passed to a function that processes
! 405: patterns in the other mode, for example, if a pattern compiled with
1.1.1.2 misho 406: pcre_compile() is passed to pcre16_exec().
407:
1.1.1.3 ! misho 408: There are new error codes whose names begin with PCRE_UTF16_ERR for
! 409: invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for
! 410: UTF-8 strings that are described in the section entitled "Reason codes
! 411: for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors
1.1.1.2 misho 412: are:
413:
414: PCRE_UTF16_ERR1 Missing low surrogate at end of string
415: PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate
416: PCRE_UTF16_ERR3 Isolated low surrogate
417: PCRE_UTF16_ERR4 Invalid character 0xfffe
418:
419:
420: ERROR TEXTS
421:
1.1.1.3 ! misho 422: If there is an error while compiling a pattern, the error text that is
! 423: passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit
1.1.1.2 misho 424: character string, zero-terminated.
425:
426:
427: CALLOUTS
428:
1.1.1.3 ! misho 429: The subject and mark fields in the callout block that is passed to a
1.1.1.2 misho 430: callout function point to 16-bit vectors.
431:
432:
433: TESTING
434:
1.1.1.3 ! misho 435: The pcretest program continues to operate with 8-bit input and output
! 436: files, but it can be used for testing the 16-bit library. If it is run
1.1.1.2 misho 437: with the command line option -16, patterns and subject strings are con-
438: verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit
1.1.1.3 ! misho 439: library functions are used instead of the 8-bit ones. Returned 16-bit
1.1.1.2 misho 440: strings are converted to 8-bit for output. If the 8-bit library was not
441: compiled, pcretest defaults to 16-bit and the -16 option is ignored.
442:
1.1.1.3 ! misho 443: When PCRE is being built, the RunTest script that is called by "make
! 444: check" uses the pcretest -C option to discover which of the 8-bit and
1.1.1.2 misho 445: 16-bit libraries has been built, and runs the tests appropriately.
446:
447:
448: NOT SUPPORTED IN 16-BIT MODE
449:
450: Not all the features of the 8-bit library are available with the 16-bit
1.1.1.3 ! misho 451: library. The C++ and POSIX wrapper functions support only the 8-bit
1.1.1.2 misho 452: library, and the pcregrep program is at present 8-bit only.
453:
454:
455: AUTHOR
456:
457: Philip Hazel
458: University Computing Service
459: Cambridge CB2 3QH, England.
460:
461:
462: REVISION
463:
1.1.1.3 ! misho 464: Last updated: 14 April 2012
1.1.1.2 misho 465: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 466: ------------------------------------------------------------------------------
467:
468:
469: PCREBUILD(3) PCREBUILD(3)
470:
471:
472: NAME
473: PCRE - Perl-compatible regular expressions
474:
475:
476: PCRE BUILD-TIME OPTIONS
477:
478: This document describes the optional features of PCRE that can be
479: selected when the library is compiled. It assumes use of the configure
480: script, where the optional features are selected or deselected by pro-
481: viding options to configure before running the make command. However,
482: the same options can be selected in both Unix-like and non-Unix-like
483: environments using the GUI facility of cmake-gui if you are using CMake
484: instead of configure to build PCRE.
485:
486: There is a lot more information about building PCRE in non-Unix-like
487: environments in the file called NON_UNIX_USE, which is part of the PCRE
488: distribution. You should consult this file as well as the README file
489: if you are building in a non-Unix-like environment.
490:
491: The complete list of options for configure (which includes the standard
492: ones such as the selection of the installation directory) can be
493: obtained by running
494:
495: ./configure --help
496:
497: The following sections include descriptions of options whose names
498: begin with --enable or --disable. These settings specify changes to the
499: defaults for the configure command. Because of the way that configure
500: works, --enable and --disable always come in pairs, so the complemen-
501: tary option always exists as well, but as it specifies the default, it
502: is not described.
503:
504:
1.1.1.2 misho 505: BUILDING 8-BIT and 16-BIT LIBRARIES
506:
507: By default, a library called libpcre is built, containing functions
508: that take string arguments contained in vectors of bytes, either as
509: single-byte characters, or interpreted as UTF-8 strings. You can also
510: build a separate library, called libpcre16, in which strings are con-
511: tained in vectors of 16-bit data units and interpreted either as sin-
512: gle-unit characters or UTF-16 strings, by adding
513:
514: --enable-pcre16
515:
516: to the configure command. If you do not want the 8-bit library, add
517:
518: --disable-pcre8
519:
520: as well. At least one of the two libraries must be built. Note that the
521: C++ and POSIX wrappers are for the 8-bit library only, and that pcre-
522: grep is an 8-bit program. None of these are built if you select only
523: the 16-bit library.
524:
525:
1.1 misho 526: BUILDING SHARED AND STATIC LIBRARIES
527:
528: The PCRE building process uses libtool to build both shared and static
529: Unix libraries by default. You can suppress one of these by adding one
530: of
531:
532: --disable-shared
533: --disable-static
534:
535: to the configure command, as required.
536:
537:
538: C++ SUPPORT
539:
1.1.1.2 misho 540: By default, if the 8-bit library is being built, the configure script
541: will search for a C++ compiler and C++ header files. If it finds them,
542: it automatically builds the C++ wrapper library (which supports only
543: 8-bit strings). You can disable this by adding
1.1 misho 544:
545: --disable-cpp
546:
547: to the configure command.
548:
549:
1.1.1.2 misho 550: UTF-8 and UTF-16 SUPPORT
1.1 misho 551:
1.1.1.2 misho 552: To build PCRE with support for UTF Unicode character strings, add
1.1 misho 553:
1.1.1.2 misho 554: --enable-utf
1.1 misho 555:
1.1.1.2 misho 556: to the configure command. This setting applies to both libraries,
557: adding support for UTF-8 to the 8-bit library and support for UTF-16 to
558: the 16-bit library. There are no separate options for enabling UTF-8
559: and UTF-16 independently because that would allow ridiculous settings
560: such as requesting UTF-16 support while building only the 8-bit
561: library. It is not possible to build one library with UTF support and
562: the other without in the same configuration. (For backwards compatibil-
563: ity, --enable-utf8 is a synonym of --enable-utf.)
564:
565: Of itself, this setting does not make PCRE treat strings as UTF-8 or
566: UTF-16. As well as compiling PCRE with this option, you also have have
567: to set the PCRE_UTF8 or PCRE_UTF16 option when you call one of the pat-
568: tern compiling functions.
1.1 misho 569:
1.1.1.2 misho 570: If you set --enable-utf when compiling in an EBCDIC environment, PCRE
1.1.1.3 ! misho 571: expects its input to be either ASCII or UTF-8 (depending on the run-
! 572: time option). It is not possible to support both EBCDIC and UTF-8 codes
! 573: in the same version of the library. Consequently, --enable-utf and
1.1 misho 574: --enable-ebcdic are mutually exclusive.
575:
576:
577: UNICODE CHARACTER PROPERTY SUPPORT
578:
1.1.1.2 misho 579: UTF support allows the libraries to process character codepoints up to
580: 0x10ffff in the strings that they handle. On its own, however, it does
581: not provide any facilities for accessing the properties of such charac-
582: ters. If you want to be able to use the pattern escapes \P, \p, and \X,
583: which refer to Unicode character properties, you must add
1.1 misho 584:
585: --enable-unicode-properties
586:
1.1.1.2 misho 587: to the configure command. This implies UTF support, even if you have
1.1 misho 588: not explicitly requested it.
589:
590: Including Unicode property support adds around 30K of tables to the
591: PCRE library. Only the general category properties such as Lu and Nd
592: are supported. Details are given in the pcrepattern documentation.
593:
594:
595: JUST-IN-TIME COMPILER SUPPORT
596:
597: Just-in-time compiler support is included in the build by specifying
598:
599: --enable-jit
600:
601: This support is available only for certain hardware architectures. If
602: this option is set for an unsupported architecture, a compile time
603: error occurs. See the pcrejit documentation for a discussion of JIT
604: usage. When JIT support is enabled, pcregrep automatically makes use of
605: it, unless you add
606:
607: --disable-pcregrep-jit
608:
609: to the "configure" command.
610:
611:
612: CODE VALUE OF NEWLINE
613:
614: By default, PCRE interprets the linefeed (LF) character as indicating
615: the end of a line. This is the normal newline character on Unix-like
616: systems. You can compile PCRE to use carriage return (CR) instead, by
617: adding
618:
619: --enable-newline-is-cr
620:
621: to the configure command. There is also a --enable-newline-is-lf
622: option, which explicitly specifies linefeed as the newline character.
623:
624: Alternatively, you can specify that line endings are to be indicated by
625: the two character sequence CRLF. If you want this, add
626:
627: --enable-newline-is-crlf
628:
629: to the configure command. There is a fourth option, specified by
630:
631: --enable-newline-is-anycrlf
632:
633: which causes PCRE to recognize any of the three sequences CR, LF, or
634: CRLF as indicating a line ending. Finally, a fifth option, specified by
635:
636: --enable-newline-is-any
637:
638: causes PCRE to recognize any Unicode newline sequence.
639:
640: Whatever line ending convention is selected when PCRE is built can be
641: overridden when the library functions are called. At build time it is
642: conventional to use the standard for your operating system.
643:
644:
645: WHAT \R MATCHES
646:
647: By default, the sequence \R in a pattern matches any Unicode newline
648: sequence, whatever has been selected as the line ending sequence. If
649: you specify
650:
651: --enable-bsr-anycrlf
652:
653: the default is changed so that \R matches only CR, LF, or CRLF. What-
654: ever is selected when PCRE is built can be overridden when the library
655: functions are called.
656:
657:
658: POSIX MALLOC USAGE
659:
1.1.1.2 misho 660: When the 8-bit library is called through the POSIX interface (see the
661: pcreposix documentation), additional working storage is required for
662: holding the pointers to capturing substrings, because PCRE requires
663: three integers per substring, whereas the POSIX interface provides only
664: two. If the number of expected substrings is small, the wrapper func-
665: tion uses space on the stack, because this is faster than using mal-
666: loc() for each call. The default threshold above which the stack is no
667: longer used is 10; it can be changed by adding a setting such as
1.1 misho 668:
669: --with-posix-malloc-threshold=20
670:
671: to the configure command.
672:
673:
674: HANDLING VERY LARGE PATTERNS
675:
676: Within a compiled pattern, offset values are used to point from one
677: part to another (for example, from an opening parenthesis to an alter-
678: nation metacharacter). By default, two-byte values are used for these
679: offsets, leading to a maximum size for a compiled pattern of around
680: 64K. This is sufficient to handle all but the most gigantic patterns.
1.1.1.2 misho 681: Nevertheless, some people do want to process truly enormous patterns,
1.1 misho 682: so it is possible to compile PCRE to use three-byte or four-byte off-
683: sets by adding a setting such as
684:
685: --with-link-size=3
686:
1.1.1.2 misho 687: to the configure command. The value given must be 2, 3, or 4. For the
688: 16-bit library, a value of 3 is rounded up to 4. Using longer offsets
689: slows down the operation of PCRE because it has to load additional data
690: when handling them.
1.1 misho 691:
692:
693: AVOIDING EXCESSIVE STACK USAGE
694:
695: When matching with the pcre_exec() function, PCRE implements backtrack-
1.1.1.2 misho 696: ing by making recursive calls to an internal function called match().
697: In environments where the size of the stack is limited, this can se-
698: verely limit PCRE's operation. (The Unix environment does not usually
1.1 misho 699: suffer from this problem, but it may sometimes be necessary to increase
1.1.1.2 misho 700: the maximum stack size. There is a discussion in the pcrestack docu-
701: mentation.) An alternative approach to recursion that uses memory from
702: the heap to remember data, instead of using recursive function calls,
703: has been implemented to work round the problem of limited stack size.
1.1 misho 704: If you want to build a version of PCRE that works this way, add
705:
706: --disable-stack-for-recursion
707:
1.1.1.2 misho 708: to the configure command. With this configuration, PCRE will use the
709: pcre_stack_malloc and pcre_stack_free variables to call memory manage-
710: ment functions. By default these point to malloc() and free(), but you
1.1 misho 711: can replace the pointers so that your own functions are used instead.
712:
1.1.1.2 misho 713: Separate functions are provided rather than using pcre_malloc and
714: pcre_free because the usage is very predictable: the block sizes
715: requested are always the same, and the blocks are always freed in
716: reverse order. A calling program might be able to implement optimized
717: functions that perform better than malloc() and free(). PCRE runs
1.1 misho 718: noticeably more slowly when built in this way. This option affects only
719: the pcre_exec() function; it is not relevant for pcre_dfa_exec().
720:
721:
722: LIMITING PCRE RESOURCE USAGE
723:
1.1.1.2 misho 724: Internally, PCRE has a function called match(), which it calls repeat-
725: edly (sometimes recursively) when matching a pattern with the
726: pcre_exec() function. By controlling the maximum number of times this
727: function may be called during a single matching operation, a limit can
728: be placed on the resources used by a single call to pcre_exec(). The
729: limit can be changed at run time, as described in the pcreapi documen-
730: tation. The default is 10 million, but this can be changed by adding a
1.1 misho 731: setting such as
732:
733: --with-match-limit=500000
734:
1.1.1.2 misho 735: to the configure command. This setting has no effect on the
1.1 misho 736: pcre_dfa_exec() matching function.
737:
1.1.1.2 misho 738: In some environments it is desirable to limit the depth of recursive
1.1 misho 739: calls of match() more strictly than the total number of calls, in order
1.1.1.2 misho 740: to restrict the maximum amount of stack (or heap, if --disable-stack-
1.1 misho 741: for-recursion is specified) that is used. A second limit controls this;
1.1.1.2 misho 742: it defaults to the value that is set for --with-match-limit, which
743: imposes no additional constraints. However, you can set a lower limit
1.1 misho 744: by adding, for example,
745:
746: --with-match-limit-recursion=10000
747:
1.1.1.2 misho 748: to the configure command. This value can also be overridden at run
1.1 misho 749: time.
750:
751:
752: CREATING CHARACTER TABLES AT BUILD TIME
753:
1.1.1.2 misho 754: PCRE uses fixed tables for processing characters whose code values are
755: less than 256. By default, PCRE is built with a set of tables that are
756: distributed in the file pcre_chartables.c.dist. These tables are for
1.1 misho 757: ASCII codes only. If you add
758:
759: --enable-rebuild-chartables
760:
1.1.1.2 misho 761: to the configure command, the distributed tables are no longer used.
762: Instead, a program called dftables is compiled and run. This outputs
1.1 misho 763: the source for new set of tables, created in the default locale of your
1.1.1.3 ! misho 764: C run-time system. (This method of replacing the tables does not work
! 765: if you are cross compiling, because dftables is run on the local host.
! 766: If you need to create alternative tables when cross compiling, you will
1.1 misho 767: have to do so "by hand".)
768:
769:
770: USING EBCDIC CODE
771:
1.1.1.2 misho 772: PCRE assumes by default that it will run in an environment where the
773: character code is ASCII (or Unicode, which is a superset of ASCII).
774: This is the case for most computer operating systems. PCRE can, how-
1.1 misho 775: ever, be compiled to run in an EBCDIC environment by adding
776:
777: --enable-ebcdic
778:
779: to the configure command. This setting implies --enable-rebuild-charta-
1.1.1.2 misho 780: bles. You should only use it if you know that you are in an EBCDIC
781: environment (for example, an IBM mainframe operating system). The
782: --enable-ebcdic option is incompatible with --enable-utf.
1.1 misho 783:
784:
785: PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
786:
787: By default, pcregrep reads all files as plain text. You can build it so
788: that it recognizes files whose names end in .gz or .bz2, and reads them
789: with libz or libbz2, respectively, by adding one or both of
790:
791: --enable-pcregrep-libz
792: --enable-pcregrep-libbz2
793:
794: to the configure command. These options naturally require that the rel-
1.1.1.2 misho 795: evant libraries are installed on your system. Configuration will fail
1.1 misho 796: if they are not.
797:
798:
799: PCREGREP BUFFER SIZE
800:
1.1.1.2 misho 801: pcregrep uses an internal buffer to hold a "window" on the file it is
1.1 misho 802: scanning, in order to be able to output "before" and "after" lines when
1.1.1.2 misho 803: it finds a match. The size of the buffer is controlled by a parameter
1.1 misho 804: whose default value is 20K. The buffer itself is three times this size,
805: but because of the way it is used for holding "before" lines, the long-
1.1.1.2 misho 806: est line that is guaranteed to be processable is the parameter size.
1.1 misho 807: You can change the default parameter value by adding, for example,
808:
809: --with-pcregrep-bufsize=50K
810:
811: to the configure command. The caller of pcregrep can, however, override
812: this value by specifying a run-time option.
813:
814:
815: PCRETEST OPTION FOR LIBREADLINE SUPPORT
816:
817: If you add
818:
819: --enable-pcretest-libreadline
820:
1.1.1.2 misho 821: to the configure command, pcretest is linked with the libreadline
822: library, and when its input is from a terminal, it reads it using the
1.1 misho 823: readline() function. This provides line-editing and history facilities.
824: Note that libreadline is GPL-licensed, so if you distribute a binary of
825: pcretest linked in this way, there may be licensing issues.
826:
1.1.1.2 misho 827: Setting this option causes the -lreadline option to be added to the
828: pcretest build. In many operating environments with a sytem-installed
1.1 misho 829: libreadline this is sufficient. However, in some environments (e.g. if
1.1.1.2 misho 830: an unmodified distribution version of readline is in use), some extra
831: configuration may be necessary. The INSTALL file for libreadline says
1.1 misho 832: this:
833:
834: "Readline uses the termcap functions, but does not link with the
835: termcap or curses library itself, allowing applications which link
836: with readline the to choose an appropriate library."
837:
1.1.1.2 misho 838: If your environment has not been set up so that an appropriate library
1.1 misho 839: is automatically included, you may need to add something like
840:
841: LIBS="-ncurses"
842:
843: immediately before the configure command.
844:
845:
846: SEE ALSO
847:
1.1.1.2 misho 848: pcreapi(3), pcre16, pcre_config(3).
1.1 misho 849:
850:
851: AUTHOR
852:
853: Philip Hazel
854: University Computing Service
855: Cambridge CB2 3QH, England.
856:
857:
858: REVISION
859:
1.1.1.2 misho 860: Last updated: 07 January 2012
861: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 862: ------------------------------------------------------------------------------
863:
864:
865: PCREMATCHING(3) PCREMATCHING(3)
866:
867:
868: NAME
869: PCRE - Perl-compatible regular expressions
870:
871:
872: PCRE MATCHING ALGORITHMS
873:
874: This document describes the two different algorithms that are available
875: in PCRE for matching a compiled regular expression against a given sub-
876: ject string. The "standard" algorithm is the one provided by the
1.1.1.2 misho 877: pcre_exec() and pcre16_exec() functions. These work in the same was as
878: Perl's matching function, and provide a Perl-compatible matching opera-
879: tion. The just-in-time (JIT) optimization that is described in the
880: pcrejit documentation is compatible with these functions.
881:
882: An alternative algorithm is provided by the pcre_dfa_exec() and
883: pcre16_dfa_exec() functions; they operate in a different way, and are
884: not Perl-compatible. This alternative has advantages and disadvantages
885: compared with the standard algorithm, and these are described below.
1.1 misho 886:
887: When there is only one possible way in which a given subject string can
888: match a pattern, the two algorithms give the same answer. A difference
889: arises, however, when there are multiple possibilities. For example, if
890: the pattern
891:
892: ^<.*>
893:
894: is matched against the string
895:
896: <something> <something else> <something further>
897:
898: there are three possible answers. The standard algorithm finds only one
899: of them, whereas the alternative algorithm finds all three.
900:
901:
902: REGULAR EXPRESSIONS AS TREES
903:
904: The set of strings that are matched by a regular expression can be rep-
905: resented as a tree structure. An unlimited repetition in the pattern
906: makes the tree of infinite size, but it is still a tree. Matching the
907: pattern to a given subject string (from a given starting point) can be
908: thought of as a search of the tree. There are two ways to search a
909: tree: depth-first and breadth-first, and these correspond to the two
910: matching algorithms provided by PCRE.
911:
912:
913: THE STANDARD MATCHING ALGORITHM
914:
915: In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
916: sions", the standard algorithm is an "NFA algorithm". It conducts a
917: depth-first search of the pattern tree. That is, it proceeds along a
918: single path through the tree, checking that the subject matches what is
919: required. When there is a mismatch, the algorithm tries any alterna-
920: tives at the current point, and if they all fail, it backs up to the
921: previous branch point in the tree, and tries the next alternative
922: branch at that level. This often involves backing up (moving to the
923: left) in the subject string as well. The order in which repetition
924: branches are tried is controlled by the greedy or ungreedy nature of
925: the quantifier.
926:
927: If a leaf node is reached, a matching string has been found, and at
928: that point the algorithm stops. Thus, if there is more than one possi-
929: ble match, this algorithm returns the first one that it finds. Whether
930: this is the shortest, the longest, or some intermediate length depends
931: on the way the greedy and ungreedy repetition quantifiers are specified
932: in the pattern.
933:
934: Because it ends up with a single path through the tree, it is rela-
935: tively straightforward for this algorithm to keep track of the sub-
936: strings that are matched by portions of the pattern in parentheses.
937: This provides support for capturing parentheses and back references.
938:
939:
940: THE ALTERNATIVE MATCHING ALGORITHM
941:
942: This algorithm conducts a breadth-first search of the tree. Starting
943: from the first matching point in the subject, it scans the subject
944: string from left to right, once, character by character, and as it does
945: this, it remembers all the paths through the tree that represent valid
946: matches. In Friedl's terminology, this is a kind of "DFA algorithm",
947: though it is not implemented as a traditional finite state machine (it
948: keeps multiple states active simultaneously).
949:
950: Although the general principle of this matching algorithm is that it
951: scans the subject string only once, without backtracking, there is one
952: exception: when a lookaround assertion is encountered, the characters
953: following or preceding the current point have to be independently
954: inspected.
955:
956: The scan continues until either the end of the subject is reached, or
957: there are no more unterminated paths. At this point, terminated paths
958: represent the different matching possibilities (if there are none, the
959: match has failed). Thus, if there is more than one possible match,
960: this algorithm finds all of them, and in particular, it finds the long-
961: est. The matches are returned in decreasing order of length. There is
962: an option to stop the algorithm after the first match (which is neces-
963: sarily the shortest) is found.
964:
965: Note that all the matches that are found start at the same point in the
966: subject. If the pattern
967:
968: cat(er(pillar)?)?
969:
970: is matched against the string "the caterpillar catchment", the result
971: will be the three strings "caterpillar", "cater", and "cat" that start
972: at the fifth character of the subject. The algorithm does not automati-
973: cally move on to find matches that start at later positions.
974:
975: There are a number of features of PCRE regular expressions that are not
976: supported by the alternative matching algorithm. They are as follows:
977:
978: 1. Because the algorithm finds all possible matches, the greedy or
979: ungreedy nature of repetition quantifiers is not relevant. Greedy and
980: ungreedy quantifiers are treated in exactly the same way. However, pos-
981: sessive quantifiers can make a difference when what follows could also
982: match what is quantified, for example in a pattern like this:
983:
984: ^a++\w!
985:
986: This pattern matches "aaab!" but not "aaa!", which would be matched by
987: a non-possessive quantifier. Similarly, if an atomic group is present,
988: it is matched as if it were a standalone pattern at the current point,
989: and the longest match is then "locked in" for the rest of the overall
990: pattern.
991:
992: 2. When dealing with multiple paths through the tree simultaneously, it
993: is not straightforward to keep track of captured substrings for the
994: different matching possibilities, and PCRE's implementation of this
995: algorithm does not attempt to do this. This means that no captured sub-
996: strings are available.
997:
998: 3. Because no substrings are captured, back references within the pat-
999: tern are not supported, and cause errors if encountered.
1000:
1001: 4. For the same reason, conditional expressions that use a backrefer-
1002: ence as the condition or test for a specific group recursion are not
1003: supported.
1004:
1005: 5. Because many paths through the tree may be active, the \K escape
1006: sequence, which resets the start of the match when encountered (but may
1007: be on some paths and not on others), is not supported. It causes an
1008: error if encountered.
1009:
1010: 6. Callouts are supported, but the value of the capture_top field is
1011: always 1, and the value of the capture_last field is always -1.
1012:
1.1.1.2 misho 1013: 7. The \C escape sequence, which (in the standard algorithm) always
1014: matches a single data unit, even in UTF-8 or UTF-16 modes, is not sup-
1015: ported in these modes, because the alternative algorithm moves through
1016: the subject string one character (not data unit) at a time, for all
1017: active paths through the tree.
1.1 misho 1018:
1.1.1.2 misho 1019: 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
1020: are not supported. (*FAIL) is supported, and behaves like a failing
1.1 misho 1021: negative assertion.
1022:
1023:
1024: ADVANTAGES OF THE ALTERNATIVE ALGORITHM
1025:
1.1.1.2 misho 1026: Using the alternative matching algorithm provides the following advan-
1.1 misho 1027: tages:
1028:
1029: 1. All possible matches (at a single point in the subject) are automat-
1.1.1.2 misho 1030: ically found, and in particular, the longest match is found. To find
1.1 misho 1031: more than one match using the standard algorithm, you have to do kludgy
1032: things with callouts.
1033:
1.1.1.2 misho 1034: 2. Because the alternative algorithm scans the subject string just
1035: once, and never needs to backtrack (except for lookbehinds), it is pos-
1036: sible to pass very long subject strings to the matching function in
1037: several pieces, checking for partial matching each time. Although it is
1038: possible to do multi-segment matching using the standard algorithm by
1039: retaining partially matched substrings, it is more complicated. The
1040: pcrepartial documentation gives details of partial matching and dis-
1041: cusses multi-segment matching.
1.1 misho 1042:
1043:
1044: DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
1045:
1046: The alternative algorithm suffers from a number of disadvantages:
1047:
1.1.1.2 misho 1048: 1. It is substantially slower than the standard algorithm. This is
1049: partly because it has to search for all possible matches, but is also
1.1 misho 1050: because it is less susceptible to optimization.
1051:
1052: 2. Capturing parentheses and back references are not supported.
1053:
1054: 3. Although atomic groups are supported, their use does not provide the
1055: performance advantage that it does for the standard algorithm.
1056:
1057:
1058: AUTHOR
1059:
1060: Philip Hazel
1061: University Computing Service
1062: Cambridge CB2 3QH, England.
1063:
1064:
1065: REVISION
1066:
1.1.1.2 misho 1067: Last updated: 08 January 2012
1068: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 1069: ------------------------------------------------------------------------------
1070:
1071:
1072: PCREAPI(3) PCREAPI(3)
1073:
1074:
1075: NAME
1076: PCRE - Perl-compatible regular expressions
1077:
1.1.1.2 misho 1078: #include <pcre.h>
1.1 misho 1079:
1080:
1.1.1.2 misho 1081: PCRE NATIVE API BASIC FUNCTIONS
1.1 misho 1082:
1083: pcre *pcre_compile(const char *pattern, int options,
1084: const char **errptr, int *erroffset,
1085: const unsigned char *tableptr);
1086:
1087: pcre *pcre_compile2(const char *pattern, int options,
1088: int *errorcodeptr,
1089: const char **errptr, int *erroffset,
1090: const unsigned char *tableptr);
1091:
1092: pcre_extra *pcre_study(const pcre *code, int options,
1093: const char **errptr);
1094:
1095: void pcre_free_study(pcre_extra *extra);
1096:
1097: int pcre_exec(const pcre *code, const pcre_extra *extra,
1098: const char *subject, int length, int startoffset,
1099: int options, int *ovector, int ovecsize);
1100:
1101: int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
1102: const char *subject, int length, int startoffset,
1103: int options, int *ovector, int ovecsize,
1104: int *workspace, int wscount);
1105:
1.1.1.2 misho 1106:
1107: PCRE NATIVE API STRING EXTRACTION FUNCTIONS
1108:
1.1 misho 1109: int pcre_copy_named_substring(const pcre *code,
1110: const char *subject, int *ovector,
1111: int stringcount, const char *stringname,
1112: char *buffer, int buffersize);
1113:
1114: int pcre_copy_substring(const char *subject, int *ovector,
1115: int stringcount, int stringnumber, char *buffer,
1116: int buffersize);
1117:
1118: int pcre_get_named_substring(const pcre *code,
1119: const char *subject, int *ovector,
1120: int stringcount, const char *stringname,
1121: const char **stringptr);
1122:
1123: int pcre_get_stringnumber(const pcre *code,
1124: const char *name);
1125:
1126: int pcre_get_stringtable_entries(const pcre *code,
1127: const char *name, char **first, char **last);
1128:
1129: int pcre_get_substring(const char *subject, int *ovector,
1130: int stringcount, int stringnumber,
1131: const char **stringptr);
1132:
1133: int pcre_get_substring_list(const char *subject,
1134: int *ovector, int stringcount, const char ***listptr);
1135:
1136: void pcre_free_substring(const char *stringptr);
1137:
1138: void pcre_free_substring_list(const char **stringptr);
1139:
1.1.1.2 misho 1140:
1141: PCRE NATIVE API AUXILIARY FUNCTIONS
1142:
1143: pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
1144:
1145: void pcre_jit_stack_free(pcre_jit_stack *stack);
1146:
1147: void pcre_assign_jit_stack(pcre_extra *extra,
1148: pcre_jit_callback callback, void *data);
1149:
1.1 misho 1150: const unsigned char *pcre_maketables(void);
1151:
1152: int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1153: int what, void *where);
1154:
1155: int pcre_refcount(pcre *code, int adjust);
1156:
1157: int pcre_config(int what, void *where);
1158:
1.1.1.2 misho 1159: const char *pcre_version(void);
1160:
1161: int pcre_pattern_to_host_byte_order(pcre *code,
1162: pcre_extra *extra, const unsigned char *tables);
1.1 misho 1163:
1164:
1165: PCRE NATIVE API INDIRECTED FUNCTIONS
1166:
1167: void *(*pcre_malloc)(size_t);
1168:
1169: void (*pcre_free)(void *);
1170:
1171: void *(*pcre_stack_malloc)(size_t);
1172:
1173: void (*pcre_stack_free)(void *);
1174:
1175: int (*pcre_callout)(pcre_callout_block *);
1176:
1177:
1.1.1.2 misho 1178: PCRE 8-BIT AND 16-BIT LIBRARIES
1179:
1180: From release 8.30, PCRE can be compiled as a library for handling
1181: 16-bit character strings as well as, or instead of, the original
1182: library that handles 8-bit character strings. To avoid too much compli-
1183: cation, this document describes the 8-bit versions of the functions,
1184: with only occasional references to the 16-bit library.
1185:
1186: The 16-bit functions operate in the same way as their 8-bit counter-
1187: parts; they just use different data types for their arguments and
1188: results, and their names start with pcre16_ instead of pcre_. For every
1189: option that has UTF8 in its name (for example, PCRE_UTF8), there is a
1190: corresponding 16-bit name with UTF8 replaced by UTF16. This facility is
1191: in fact just cosmetic; the 16-bit option names define the same bit val-
1192: ues.
1193:
1194: References to bytes and UTF-8 in this document should be read as refer-
1195: ences to 16-bit data quantities and UTF-16 when using the 16-bit
1196: library, unless specified otherwise. More details of the specific dif-
1197: ferences for the 16-bit library are given in the pcre16 page.
1198:
1199:
1.1 misho 1200: PCRE API OVERVIEW
1201:
1202: PCRE has its own native API, which is described in this document. There
1.1.1.2 misho 1203: are also some wrapper functions (for the 8-bit library only) that cor-
1204: respond to the POSIX regular expression API, but they do not give
1205: access to all the functionality. They are described in the pcreposix
1206: documentation. Both of these APIs define a set of C function calls. A
1207: C++ wrapper (again for the 8-bit library only) is also distributed with
1208: PCRE. It is documented in the pcrecpp page.
1.1 misho 1209:
1210: The native API C function prototypes are defined in the header file
1.1.1.2 misho 1211: pcre.h, and on Unix-like systems the (8-bit) library itself is called
1212: libpcre. It can normally be accessed by adding -lpcre to the command
1213: for linking an application that uses PCRE. The header file defines the
1214: macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release
1215: numbers for the library. Applications can use these to include support
1.1 misho 1216: for different releases of PCRE.
1217:
1218: In a Windows environment, if you want to statically link an application
1219: program against a non-dll pcre.a file, you must define PCRE_STATIC
1220: before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
1221: loc() and pcre_free() exported functions will be declared
1222: __declspec(dllimport), with unwanted results.
1223:
1224: The functions pcre_compile(), pcre_compile2(), pcre_study(), and
1225: pcre_exec() are used for compiling and matching regular expressions in
1226: a Perl-compatible manner. A sample program that demonstrates the sim-
1227: plest way of using them is provided in the file called pcredemo.c in
1228: the PCRE source distribution. A listing of this program is given in the
1229: pcredemo documentation, and the pcresample documentation describes how
1230: to compile and run it.
1231:
1232: Just-in-time compiler support is an optional feature of PCRE that can
1233: be built in appropriate hardware environments. It greatly speeds up the
1234: matching performance of many patterns. Simple programs can easily
1235: request that it be used if available, by setting an option that is
1236: ignored when it is not relevant. More complicated programs might need
1237: to make use of the functions pcre_jit_stack_alloc(),
1238: pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control
1239: the JIT code's memory usage. These functions are discussed in the
1240: pcrejit documentation.
1241:
1242: A second matching function, pcre_dfa_exec(), which is not Perl-compati-
1243: ble, is also provided. This uses a different algorithm for the match-
1244: ing. The alternative algorithm finds all possible matches (at a given
1245: point in the subject), and scans the subject just once (unless there
1246: are lookbehind assertions). However, this algorithm does not return
1247: captured substrings. A description of the two matching algorithms and
1248: their advantages and disadvantages is given in the pcrematching docu-
1249: mentation.
1250:
1251: In addition to the main compiling and matching functions, there are
1252: convenience functions for extracting captured substrings from a subject
1253: string that is matched by pcre_exec(). They are:
1254:
1255: pcre_copy_substring()
1256: pcre_copy_named_substring()
1257: pcre_get_substring()
1258: pcre_get_named_substring()
1259: pcre_get_substring_list()
1260: pcre_get_stringnumber()
1261: pcre_get_stringtable_entries()
1262:
1263: pcre_free_substring() and pcre_free_substring_list() are also provided,
1264: to free the memory used for extracted strings.
1265:
1266: The function pcre_maketables() is used to build a set of character
1267: tables in the current locale for passing to pcre_compile(),
1268: pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
1269: provided for specialist use. Most commonly, no special tables are
1270: passed, in which case internal tables that are generated when PCRE is
1271: built are used.
1272:
1273: The function pcre_fullinfo() is used to find out information about a
1.1.1.2 misho 1274: compiled pattern. The function pcre_version() returns a pointer to a
1275: string containing the version of PCRE and its date of release.
1.1 misho 1276:
1277: The function pcre_refcount() maintains a reference count in a data
1278: block containing a compiled pattern. This is provided for the benefit
1279: of object-oriented applications.
1280:
1281: The global variables pcre_malloc and pcre_free initially contain the
1282: entry points of the standard malloc() and free() functions, respec-
1283: tively. PCRE calls the memory management functions via these variables,
1284: so a calling program can replace them if it wishes to intercept the
1285: calls. This should be done before calling any PCRE functions.
1286:
1287: The global variables pcre_stack_malloc and pcre_stack_free are also
1288: indirections to memory management functions. These special functions
1289: are used only when PCRE is compiled to use the heap for remembering
1290: data, instead of recursive function calls, when running the pcre_exec()
1291: function. See the pcrebuild documentation for details of how to do
1292: this. It is a non-standard way of building PCRE, for use in environ-
1293: ments that have limited stacks. Because of the greater use of memory
1294: management, it runs more slowly. Separate functions are provided so
1295: that special-purpose external code can be used for this case. When
1296: used, these functions are always called in a stack-like manner (last
1297: obtained, first freed), and always for memory blocks of the same size.
1298: There is a discussion about PCRE's stack usage in the pcrestack docu-
1299: mentation.
1300:
1301: The global variable pcre_callout initially contains NULL. It can be set
1302: by the caller to a "callout" function, which PCRE will then call at
1303: specified points during a matching operation. Details are given in the
1304: pcrecallout documentation.
1305:
1306:
1307: NEWLINES
1308:
1309: PCRE supports five different conventions for indicating line breaks in
1310: strings: a single CR (carriage return) character, a single LF (line-
1311: feed) character, the two-character sequence CRLF, any of the three pre-
1312: ceding, or any Unicode newline sequence. The Unicode newline sequences
1313: are the three just mentioned, plus the single characters VT (vertical
1.1.1.3 ! misho 1314: tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
1.1 misho 1315: separator, U+2028), and PS (paragraph separator, U+2029).
1316:
1317: Each of the first three conventions is used by at least one operating
1318: system as its standard newline sequence. When PCRE is built, a default
1319: can be specified. The default default is LF, which is the Unix stan-
1320: dard. When PCRE is run, the default can be overridden, either when a
1321: pattern is compiled, or when it is matched.
1322:
1323: At compile time, the newline convention can be specified by the options
1324: argument of pcre_compile(), or it can be specified by special text at
1325: the start of the pattern itself; this overrides any other settings. See
1326: the pcrepattern page for details of the special character sequences.
1327:
1328: In the PCRE documentation the word "newline" is used to mean "the char-
1329: acter or pair of characters that indicate a line break". The choice of
1330: newline convention affects the handling of the dot, circumflex, and
1331: dollar metacharacters, the handling of #-comments in /x mode, and, when
1332: CRLF is a recognized line ending sequence, the match position advance-
1333: ment for a non-anchored pattern. There is more detail about this in the
1334: section on pcre_exec() options below.
1335:
1336: The choice of newline convention does not affect the interpretation of
1337: the \n or \r escape sequences, nor does it affect what \R matches,
1338: which is controlled in a similar way, but by separate options.
1339:
1340:
1341: MULTITHREADING
1342:
1343: The PCRE functions can be used in multi-threading applications, with
1344: the proviso that the memory management functions pointed to by
1345: pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1346: callout function pointed to by pcre_callout, are shared by all threads.
1347:
1348: The compiled form of a regular expression is not altered during match-
1349: ing, so the same compiled pattern can safely be used by several threads
1350: at once.
1351:
1352: If the just-in-time optimization feature is being used, it needs sepa-
1353: rate memory stack areas for each thread. See the pcrejit documentation
1354: for more details.
1355:
1356:
1357: SAVING PRECOMPILED PATTERNS FOR LATER USE
1358:
1359: The compiled form of a regular expression can be saved and re-used at a
1360: later time, possibly by a different program, and even on a host other
1361: than the one on which it was compiled. Details are given in the
1.1.1.2 misho 1362: pcreprecompile documentation, which includes a description of the
1363: pcre_pattern_to_host_byte_order() function. However, compiling a regu-
1364: lar expression with one version of PCRE for use with a different ver-
1365: sion is not guaranteed to work and may cause crashes.
1.1 misho 1366:
1367:
1368: CHECKING BUILD-TIME OPTIONS
1369:
1370: int pcre_config(int what, void *where);
1371:
1.1.1.2 misho 1372: The function pcre_config() makes it possible for a PCRE client to dis-
1.1 misho 1373: cover which optional features have been compiled into the PCRE library.
1.1.1.2 misho 1374: The pcrebuild documentation has more details about these optional fea-
1.1 misho 1375: tures.
1376:
1.1.1.2 misho 1377: The first argument for pcre_config() is an integer, specifying which
1.1 misho 1378: information is required; the second argument is a pointer to a variable
1.1.1.2 misho 1379: into which the information is placed. The returned value is zero on
1380: success, or the negative error code PCRE_ERROR_BADOPTION if the value
1381: in the first argument is not recognized. The following information is
1.1 misho 1382: available:
1383:
1384: PCRE_CONFIG_UTF8
1385:
1.1.1.2 misho 1386: The output is an integer that is set to one if UTF-8 support is avail-
1387: able; otherwise it is set to zero. If this option is given to the
1388: 16-bit version of this function, pcre16_config(), the result is
1389: PCRE_ERROR_BADOPTION.
1390:
1391: PCRE_CONFIG_UTF16
1392:
1393: The output is an integer that is set to one if UTF-16 support is avail-
1394: able; otherwise it is set to zero. This value should normally be given
1395: to the 16-bit version of this function, pcre16_config(). If it is given
1396: to the 8-bit version of this function, the result is PCRE_ERROR_BADOP-
1397: TION.
1.1 misho 1398:
1399: PCRE_CONFIG_UNICODE_PROPERTIES
1400:
1.1.1.2 misho 1401: The output is an integer that is set to one if support for Unicode
1.1 misho 1402: character properties is available; otherwise it is set to zero.
1403:
1404: PCRE_CONFIG_JIT
1405:
1406: The output is an integer that is set to one if support for just-in-time
1407: compiling is available; otherwise it is set to zero.
1408:
1.1.1.2 misho 1409: PCRE_CONFIG_JITTARGET
1410:
1411: The output is a pointer to a zero-terminated "const char *" string. If
1412: JIT support is available, the string contains the name of the architec-
1413: ture for which the JIT compiler is configured, for example "x86 32bit
1414: (little endian + unaligned)". If JIT support is not available, the
1415: result is NULL.
1416:
1.1 misho 1417: PCRE_CONFIG_NEWLINE
1418:
1.1.1.2 misho 1419: The output is an integer whose value specifies the default character
1420: sequence that is recognized as meaning "newline". The four values that
1.1 misho 1421: are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1.1.1.2 misho 1422: and -1 for ANY. Though they are derived from ASCII, the same values
1.1 misho 1423: are returned in EBCDIC environments. The default should normally corre-
1424: spond to the standard sequence for your operating system.
1425:
1426: PCRE_CONFIG_BSR
1427:
1428: The output is an integer whose value indicates what character sequences
1.1.1.2 misho 1429: the \R escape sequence matches by default. A value of 0 means that \R
1430: matches any Unicode line ending sequence; a value of 1 means that \R
1.1 misho 1431: matches only CR, LF, or CRLF. The default can be overridden when a pat-
1432: tern is compiled or matched.
1433:
1434: PCRE_CONFIG_LINK_SIZE
1435:
1.1.1.2 misho 1436: The output is an integer that contains the number of bytes used for
1437: internal linkage in compiled regular expressions. For the 8-bit
1438: library, the value can be 2, 3, or 4. For the 16-bit library, the value
1439: is either 2 or 4 and is still a number of bytes. The default value of 2
1440: is sufficient for all but the most massive patterns, since it allows
1441: the compiled pattern to be up to 64K in size. Larger values allow
1442: larger regular expressions to be compiled, at the expense of slower
1443: matching.
1.1 misho 1444:
1445: PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1446:
1.1.1.2 misho 1447: The output is an integer that contains the threshold above which the
1448: POSIX interface uses malloc() for output vectors. Further details are
1.1 misho 1449: given in the pcreposix documentation.
1450:
1451: PCRE_CONFIG_MATCH_LIMIT
1452:
1.1.1.2 misho 1453: The output is a long integer that gives the default limit for the num-
1454: ber of internal matching function calls in a pcre_exec() execution.
1.1 misho 1455: Further details are given with pcre_exec() below.
1456:
1457: PCRE_CONFIG_MATCH_LIMIT_RECURSION
1458:
1459: The output is a long integer that gives the default limit for the depth
1.1.1.2 misho 1460: of recursion when calling the internal matching function in a
1461: pcre_exec() execution. Further details are given with pcre_exec()
1.1 misho 1462: below.
1463:
1464: PCRE_CONFIG_STACKRECURSE
1465:
1.1.1.2 misho 1466: The output is an integer that is set to one if internal recursion when
1.1 misho 1467: running pcre_exec() is implemented by recursive function calls that use
1.1.1.2 misho 1468: the stack to remember their state. This is the usual way that PCRE is
1.1 misho 1469: compiled. The output is zero if PCRE was compiled to use blocks of data
1.1.1.2 misho 1470: on the heap instead of recursive function calls. In this case,
1471: pcre_stack_malloc and pcre_stack_free are called to manage memory
1.1 misho 1472: blocks on the heap, thus avoiding the use of the stack.
1473:
1474:
1475: COMPILING A PATTERN
1476:
1477: pcre *pcre_compile(const char *pattern, int options,
1478: const char **errptr, int *erroffset,
1479: const unsigned char *tableptr);
1480:
1481: pcre *pcre_compile2(const char *pattern, int options,
1482: int *errorcodeptr,
1483: const char **errptr, int *erroffset,
1484: const unsigned char *tableptr);
1485:
1486: Either of the functions pcre_compile() or pcre_compile2() can be called
1487: to compile a pattern into an internal form. The only difference between
1.1.1.2 misho 1488: the two interfaces is that pcre_compile2() has an additional argument,
1489: errorcodeptr, via which a numerical error code can be returned. To
1490: avoid too much repetition, we refer just to pcre_compile() below, but
1.1 misho 1491: the information applies equally to pcre_compile2().
1492:
1493: The pattern is a C string terminated by a binary zero, and is passed in
1.1.1.2 misho 1494: the pattern argument. A pointer to a single block of memory that is
1495: obtained via pcre_malloc is returned. This contains the compiled code
1.1 misho 1496: and related data. The pcre type is defined for the returned block; this
1497: is a typedef for a structure whose contents are not externally defined.
1498: It is up to the caller to free the memory (via pcre_free) when it is no
1499: longer required.
1500:
1.1.1.2 misho 1501: Although the compiled code of a PCRE regex is relocatable, that is, it
1.1 misho 1502: does not depend on memory location, the complete pcre data block is not
1.1.1.2 misho 1503: fully relocatable, because it may contain a copy of the tableptr argu-
1.1 misho 1504: ment, which is an address (see below).
1505:
1506: The options argument contains various bit settings that affect the com-
1.1.1.2 misho 1507: pilation. It should be zero if no options are required. The available
1508: options are described below. Some of them (in particular, those that
1509: are compatible with Perl, but some others as well) can also be set and
1510: unset from within the pattern (see the detailed description in the
1511: pcrepattern documentation). For those options that can be different in
1512: different parts of the pattern, the contents of the options argument
1.1 misho 1513: specifies their settings at the start of compilation and execution. The
1.1.1.2 misho 1514: PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
1.1.1.3 ! misho 1515: PCRE_NO_START_OPTIMIZE options can be set at the time of matching as
! 1516: well as at compile time.
1.1 misho 1517:
1518: If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1.1.1.2 misho 1519: if compilation of a pattern fails, pcre_compile() returns NULL, and
1.1 misho 1520: sets the variable pointed to by errptr to point to a textual error mes-
1521: sage. This is a static string that is part of the library. You must not
1.1.1.2 misho 1522: try to free it. Normally, the offset from the start of the pattern to
1523: the byte that was being processed when the error was discovered is
1524: placed in the variable pointed to by erroffset, which must not be NULL
1525: (if it is, an immediate error is given). However, for an invalid UTF-8
1526: string, the offset is that of the first byte of the failing character.
1.1 misho 1527:
1.1.1.2 misho 1528: Some errors are not detected until the whole pattern has been scanned;
1529: in these cases, the offset passed back is the length of the pattern.
1.1 misho 1530: Note that the offset is in bytes, not characters, even in UTF-8 mode.
1531: It may sometimes point into the middle of a UTF-8 character.
1532:
1533: If pcre_compile2() is used instead of pcre_compile(), and the error-
1534: codeptr argument is not NULL, a non-zero error code number is returned
1535: via this argument in the event of an error. This is in addition to the
1536: textual error message. Error codes and messages are listed below.
1537:
1538: If the final argument, tableptr, is NULL, PCRE uses a default set of
1539: character tables that are built when PCRE is compiled, using the
1540: default C locale. Otherwise, tableptr must be an address that is the
1541: result of a call to pcre_maketables(). This value is stored with the
1542: compiled pattern, and used again by pcre_exec(), unless another table
1543: pointer is passed to it. For more discussion, see the section on locale
1544: support below.
1545:
1546: This code fragment shows a typical straightforward call to pcre_com-
1547: pile():
1548:
1549: pcre *re;
1550: const char *error;
1551: int erroffset;
1552: re = pcre_compile(
1553: "^A.*Z", /* the pattern */
1554: 0, /* default options */
1555: &error, /* for error message */
1556: &erroffset, /* for error offset */
1557: NULL); /* use default character tables */
1558:
1559: The following names for option bits are defined in the pcre.h header
1560: file:
1561:
1562: PCRE_ANCHORED
1563:
1564: If this bit is set, the pattern is forced to be "anchored", that is, it
1565: is constrained to match only at the first matching point in the string
1566: that is being searched (the "subject string"). This effect can also be
1567: achieved by appropriate constructs in the pattern itself, which is the
1568: only way to do it in Perl.
1569:
1570: PCRE_AUTO_CALLOUT
1571:
1572: If this bit is set, pcre_compile() automatically inserts callout items,
1573: all with number 255, before each pattern item. For discussion of the
1574: callout facility, see the pcrecallout documentation.
1575:
1576: PCRE_BSR_ANYCRLF
1577: PCRE_BSR_UNICODE
1578:
1579: These options (which are mutually exclusive) control what the \R escape
1580: sequence matches. The choice is either to match only CR, LF, or CRLF,
1581: or to match any Unicode newline sequence. The default is specified when
1582: PCRE is built. It can be overridden from within the pattern, or by set-
1583: ting an option when a compiled pattern is matched.
1584:
1585: PCRE_CASELESS
1586:
1587: If this bit is set, letters in the pattern match both upper and lower
1588: case letters. It is equivalent to Perl's /i option, and it can be
1589: changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1590: always understands the concept of case for characters whose values are
1591: less than 128, so caseless matching is always possible. For characters
1592: with higher values, the concept of case is supported if PCRE is com-
1593: piled with Unicode property support, but not otherwise. If you want to
1594: use caseless matching for characters 128 and above, you must ensure
1595: that PCRE is compiled with Unicode property support as well as with
1596: UTF-8 support.
1597:
1598: PCRE_DOLLAR_ENDONLY
1599:
1600: If this bit is set, a dollar metacharacter in the pattern matches only
1601: at the end of the subject string. Without this option, a dollar also
1602: matches immediately before a newline at the end of the string (but not
1603: before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1604: if PCRE_MULTILINE is set. There is no equivalent to this option in
1605: Perl, and no way to set it within a pattern.
1606:
1607: PCRE_DOTALL
1608:
1609: If this bit is set, a dot metacharacter in the pattern matches a char-
1610: acter of any value, including one that indicates a newline. However, it
1611: only ever matches one character, even if newlines are coded as CRLF.
1612: Without this option, a dot does not match when the current position is
1613: at a newline. This option is equivalent to Perl's /s option, and it can
1614: be changed within a pattern by a (?s) option setting. A negative class
1615: such as [^a] always matches newline characters, independent of the set-
1616: ting of this option.
1617:
1618: PCRE_DUPNAMES
1619:
1620: If this bit is set, names used to identify capturing subpatterns need
1621: not be unique. This can be helpful for certain types of pattern when it
1622: is known that only one instance of the named subpattern can ever be
1623: matched. There are more details of named subpatterns below; see also
1624: the pcrepattern documentation.
1625:
1626: PCRE_EXTENDED
1627:
1.1.1.3 ! misho 1628: If this bit is set, white space data characters in the pattern are
! 1629: totally ignored except when escaped or inside a character class. White
1.1 misho 1630: space does not include the VT character (code 11). In addition, charac-
1631: ters between an unescaped # outside a character class and the next new-
1632: line, inclusive, are also ignored. This is equivalent to Perl's /x
1633: option, and it can be changed within a pattern by a (?x) option set-
1634: ting.
1635:
1636: Which characters are interpreted as newlines is controlled by the
1637: options passed to pcre_compile() or by a special sequence at the start
1638: of the pattern, as described in the section entitled "Newline conven-
1639: tions" in the pcrepattern documentation. Note that the end of this type
1640: of comment is a literal newline sequence in the pattern; escape
1641: sequences that happen to represent a newline do not count.
1642:
1643: This option makes it possible to include comments inside complicated
1644: patterns. Note, however, that this applies only to data characters.
1.1.1.3 ! misho 1645: White space characters may never appear within special character
1.1 misho 1646: sequences in a pattern, for example within the sequence (?( that intro-
1647: duces a conditional subpattern.
1648:
1649: PCRE_EXTRA
1650:
1651: This option was invented in order to turn on additional functionality
1652: of PCRE that is incompatible with Perl, but it is currently of very
1653: little use. When set, any backslash in a pattern that is followed by a
1654: letter that has no special meaning causes an error, thus reserving
1655: these combinations for future expansion. By default, as in Perl, a
1656: backslash followed by a letter with no special meaning is treated as a
1657: literal. (Perl can, however, be persuaded to give an error for this, by
1658: running it with the -w option.) There are at present no other features
1659: controlled by this option. It can also be set by a (?X) option setting
1660: within a pattern.
1661:
1662: PCRE_FIRSTLINE
1663:
1664: If this option is set, an unanchored pattern is required to match
1665: before or at the first newline in the subject string, though the
1666: matched text may continue over the newline.
1667:
1668: PCRE_JAVASCRIPT_COMPAT
1669:
1670: If this option is set, PCRE's behaviour is changed in some ways so that
1671: it is compatible with JavaScript rather than Perl. The changes are as
1672: follows:
1673:
1674: (1) A lone closing square bracket in a pattern causes a compile-time
1675: error, because this is illegal in JavaScript (by default it is treated
1676: as a data character). Thus, the pattern AB]CD becomes illegal when this
1677: option is set.
1678:
1679: (2) At run time, a back reference to an unset subpattern group matches
1680: an empty string (by default this causes the current matching alterna-
1681: tive to fail). A pattern such as (\1)(a) succeeds when this option is
1682: set (assuming it can find an "a" in the subject), whereas it fails by
1683: default, for Perl compatibility.
1684:
1685: (3) \U matches an upper case "U" character; by default \U causes a com-
1686: pile time error (Perl uses \U to upper case subsequent characters).
1687:
1688: (4) \u matches a lower case "u" character unless it is followed by four
1689: hexadecimal digits, in which case the hexadecimal number defines the
1690: code point to match. By default, \u causes a compile time error (Perl
1691: uses it to upper case the following character).
1692:
1693: (5) \x matches a lower case "x" character unless it is followed by two
1694: hexadecimal digits, in which case the hexadecimal number defines the
1695: code point to match. By default, as in Perl, a hexadecimal number is
1696: always expected after \x, but it may have zero, one, or two digits (so,
1697: for example, \xz matches a binary zero character followed by z).
1698:
1699: PCRE_MULTILINE
1700:
1701: By default, PCRE treats the subject string as consisting of a single
1702: line of characters (even if it actually contains newlines). The "start
1703: of line" metacharacter (^) matches only at the start of the string,
1704: while the "end of line" metacharacter ($) matches only at the end of
1705: the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1706: is set). This is the same as Perl.
1707:
1708: When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1709: constructs match immediately following or immediately before internal
1710: newlines in the subject string, respectively, as well as at the very
1711: start and end. This is equivalent to Perl's /m option, and it can be
1712: changed within a pattern by a (?m) option setting. If there are no new-
1713: lines in a subject string, or no occurrences of ^ or $ in a pattern,
1714: setting PCRE_MULTILINE has no effect.
1715:
1716: PCRE_NEWLINE_CR
1717: PCRE_NEWLINE_LF
1718: PCRE_NEWLINE_CRLF
1719: PCRE_NEWLINE_ANYCRLF
1720: PCRE_NEWLINE_ANY
1721:
1722: These options override the default newline definition that was chosen
1723: when PCRE was built. Setting the first or the second specifies that a
1724: newline is indicated by a single character (CR or LF, respectively).
1725: Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1726: two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1727: that any of the three preceding sequences should be recognized. Setting
1728: PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1729: recognized. The Unicode newline sequences are the three just mentioned,
1.1.1.3 ! misho 1730: plus the single characters VT (vertical tab, U+000B), FF (form feed,
1.1 misho 1731: U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1.1.1.2 misho 1732: (paragraph separator, U+2029). For the 8-bit library, the last two are
1733: recognized only in UTF-8 mode.
1.1 misho 1734:
1735: The newline setting in the options word uses three bits that are
1736: treated as a number, giving eight possibilities. Currently only six are
1737: used (default plus the five values above). This means that if you set
1738: more than one newline option, the combination may or may not be sensi-
1739: ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1740: PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1741: cause an error.
1742:
1743: The only time that a line break in a pattern is specially recognized
1.1.1.3 ! misho 1744: when compiling is when PCRE_EXTENDED is set. CR and LF are white space
1.1 misho 1745: characters, and so are ignored in this mode. Also, an unescaped # out-
1746: side a character class indicates a comment that lasts until after the
1747: next line break sequence. In other circumstances, line break sequences
1748: in patterns are treated as literal data.
1749:
1750: The newline option that is set at compile time becomes the default that
1751: is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1752:
1753: PCRE_NO_AUTO_CAPTURE
1754:
1755: If this option is set, it disables the use of numbered capturing paren-
1756: theses in the pattern. Any opening parenthesis that is not followed by
1757: ? behaves as if it were followed by ?: but named parentheses can still
1758: be used for capturing (and they acquire numbers in the usual way).
1759: There is no equivalent of this option in Perl.
1760:
1761: NO_START_OPTIMIZE
1762:
1763: This is an option that acts at matching time; that is, it is really an
1764: option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1765: time, it is remembered with the compiled pattern and assumed at match-
1766: ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1767: below.
1768:
1769: PCRE_UCP
1770:
1771: This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1772: \w, and some of the POSIX character classes. By default, only ASCII
1773: characters are recognized, but if PCRE_UCP is set, Unicode properties
1774: are used instead to classify characters. More details are given in the
1775: section on generic character types in the pcrepattern page. If you set
1776: PCRE_UCP, matching one of the items it affects takes much longer. The
1777: option is available only if PCRE has been compiled with Unicode prop-
1778: erty support.
1779:
1780: PCRE_UNGREEDY
1781:
1782: This option inverts the "greediness" of the quantifiers so that they
1783: are not greedy by default, but become greedy if followed by "?". It is
1784: not compatible with Perl. It can also be set by a (?U) option setting
1785: within the pattern.
1786:
1787: PCRE_UTF8
1788:
1789: This option causes PCRE to regard both the pattern and the subject as
1.1.1.2 misho 1790: strings of UTF-8 characters instead of single-byte strings. However, it
1791: is available only when PCRE is built to include UTF support. If not,
1792: the use of this option provokes an error. Details of how this option
1793: changes the behaviour of PCRE are given in the pcreunicode page.
1.1 misho 1794:
1795: PCRE_NO_UTF8_CHECK
1796:
1797: When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1.1.1.2 misho 1798: automatically checked. There is a discussion about the validity of
1799: UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is
1800: found, pcre_compile() returns an error. If you already know that your
1801: pattern is valid, and you want to skip this check for performance rea-
1802: sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the
1803: effect of passing an invalid UTF-8 string as a pattern is undefined. It
1804: may cause your program to crash. Note that this option can also be
1805: passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity
1806: checking of subject strings.
1.1 misho 1807:
1808:
1809: COMPILATION ERROR CODES
1810:
1.1.1.2 misho 1811: The following table lists the error codes than may be returned by
1812: pcre_compile2(), along with the error messages that may be returned by
1813: both compiling functions. Note that error messages are always 8-bit
1814: ASCII strings, even in 16-bit mode. As PCRE has developed, some error
1815: codes have fallen out of use. To avoid confusion, they have not been
1816: re-used.
1.1 misho 1817:
1818: 0 no error
1819: 1 \ at end of pattern
1820: 2 \c at end of pattern
1821: 3 unrecognized character follows \
1822: 4 numbers out of order in {} quantifier
1823: 5 number too big in {} quantifier
1824: 6 missing terminating ] for character class
1825: 7 invalid escape sequence in character class
1826: 8 range out of order in character class
1827: 9 nothing to repeat
1828: 10 [this code is not in use]
1829: 11 internal error: unexpected repeat
1830: 12 unrecognized character after (? or (?-
1831: 13 POSIX named classes are supported only within a class
1832: 14 missing )
1833: 15 reference to non-existent subpattern
1834: 16 erroffset passed as NULL
1835: 17 unknown option bit(s) set
1836: 18 missing ) after comment
1837: 19 [this code is not in use]
1838: 20 regular expression is too large
1839: 21 failed to get memory
1840: 22 unmatched parentheses
1841: 23 internal error: code overflow
1842: 24 unrecognized character after (?<
1843: 25 lookbehind assertion is not fixed length
1844: 26 malformed number or name after (?(
1845: 27 conditional group contains more than two branches
1846: 28 assertion expected after (?(
1847: 29 (?R or (?[+-]digits must be followed by )
1848: 30 unknown POSIX class name
1849: 31 POSIX collating elements are not supported
1.1.1.2 misho 1850: 32 this version of PCRE is compiled without UTF support
1.1 misho 1851: 33 [this code is not in use]
1852: 34 character value in \x{...} sequence is too large
1853: 35 invalid condition (?(0)
1854: 36 \C not allowed in lookbehind assertion
1855: 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1856: 38 number after (?C is > 255
1857: 39 closing ) for (?C expected
1858: 40 recursive call could loop indefinitely
1859: 41 unrecognized character after (?P
1860: 42 syntax error in subpattern name (missing terminator)
1861: 43 two named subpatterns have the same name
1.1.1.2 misho 1862: 44 invalid UTF-8 string (specifically UTF-8)
1.1 misho 1863: 45 support for \P, \p, and \X has not been compiled
1864: 46 malformed \P or \p sequence
1865: 47 unknown property name after \P or \p
1866: 48 subpattern name is too long (maximum 32 characters)
1867: 49 too many named subpatterns (maximum 10000)
1868: 50 [this code is not in use]
1.1.1.2 misho 1869: 51 octal value is greater than \377 in 8-bit non-UTF-8 mode
1.1 misho 1870: 52 internal error: overran compiling workspace
1871: 53 internal error: previously-checked referenced subpattern
1872: not found
1873: 54 DEFINE group contains more than one branch
1874: 55 repeating a DEFINE group is not allowed
1875: 56 inconsistent NEWLINE options
1876: 57 \g is not followed by a braced, angle-bracketed, or quoted
1877: name/number or by a plain number
1878: 58 a numbered reference must not be zero
1879: 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1880: 60 (*VERB) not recognized
1881: 61 number is too big
1882: 62 subpattern name expected
1883: 63 digit expected after (?+
1884: 64 ] is an invalid data character in JavaScript compatibility mode
1885: 65 different names for subpatterns of the same number are
1886: not allowed
1887: 66 (*MARK) must have an argument
1.1.1.2 misho 1888: 67 this version of PCRE is not compiled with Unicode property
1889: support
1.1 misho 1890: 68 \c must be followed by an ASCII character
1891: 69 \k is not followed by a braced, angle-bracketed, or quoted name
1.1.1.2 misho 1892: 70 internal error: unknown opcode in find_fixedlength()
1893: 71 \N is not supported in a class
1894: 72 too many forward references
1895: 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
1896: 74 invalid UTF-16 string (specifically UTF-16)
1.1.1.3 ! misho 1897: 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)
! 1898: 76 character value in \u.... sequence is too large
1.1 misho 1899:
1.1.1.2 misho 1900: The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1.1 misho 1901: values may be used if the limits were changed when PCRE was built.
1902:
1903:
1904: STUDYING A PATTERN
1905:
1906: pcre_extra *pcre_study(const pcre *code, int options
1907: const char **errptr);
1908:
1.1.1.2 misho 1909: If a compiled pattern is going to be used several times, it is worth
1.1 misho 1910: spending more time analyzing it in order to speed up the time taken for
1.1.1.2 misho 1911: matching. The function pcre_study() takes a pointer to a compiled pat-
1.1 misho 1912: tern as its first argument. If studying the pattern produces additional
1.1.1.2 misho 1913: information that will help speed up matching, pcre_study() returns a
1914: pointer to a pcre_extra block, in which the study_data field points to
1.1 misho 1915: the results of the study.
1916:
1917: The returned value from pcre_study() can be passed directly to
1.1.1.2 misho 1918: pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1919: tains other fields that can be set by the caller before the block is
1.1 misho 1920: passed; these are described below in the section on matching a pattern.
1921:
1.1.1.2 misho 1922: If studying the pattern does not produce any useful information,
1.1 misho 1923: pcre_study() returns NULL. In that circumstance, if the calling program
1.1.1.2 misho 1924: wants to pass any of the other fields to pcre_exec() or
1.1 misho 1925: pcre_dfa_exec(), it must set up its own pcre_extra block.
1926:
1.1.1.3 ! misho 1927: The second argument of pcre_study() contains option bits. There are
! 1928: three options:
! 1929:
! 1930: PCRE_STUDY_JIT_COMPILE
! 1931: PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
! 1932: PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
! 1933:
! 1934: If any of these are set, and the just-in-time compiler is available,
! 1935: the pattern is further compiled into machine code that executes much
! 1936: faster than the pcre_exec() interpretive matching function. If the
! 1937: just-in-time compiler is not available, these options are ignored. All
! 1938: other bits in the options argument must be zero.
1.1 misho 1939:
1.1.1.2 misho 1940: JIT compilation is a heavyweight optimization. It can take some time
1941: for patterns to be analyzed, and for one-off matches and simple pat-
1942: terns the benefit of faster execution might be offset by a much slower
1.1 misho 1943: study time. Not all patterns can be optimized by the JIT compiler. For
1.1.1.2 misho 1944: those that cannot be handled, matching automatically falls back to the
1945: pcre_exec() interpreter. For more details, see the pcrejit documenta-
1.1 misho 1946: tion.
1947:
1.1.1.2 misho 1948: The third argument for pcre_study() is a pointer for an error message.
1949: If studying succeeds (even if no data is returned), the variable it
1950: points to is set to NULL. Otherwise it is set to point to a textual
1.1 misho 1951: error message. This is a static string that is part of the library. You
1.1.1.2 misho 1952: must not try to free it. You should test the error pointer for NULL
1.1 misho 1953: after calling pcre_study(), to be sure that it has run successfully.
1954:
1.1.1.2 misho 1955: When you are finished with a pattern, you can free the memory used for
1.1 misho 1956: the study data by calling pcre_free_study(). This function was added to
1.1.1.2 misho 1957: the API for release 8.20. For earlier versions, the memory could be
1958: freed with pcre_free(), just like the pattern itself. This will still
1.1.1.3 ! misho 1959: work in cases where JIT optimization is not used, but it is advisable
! 1960: to change to the new function when convenient.
1.1 misho 1961:
1.1.1.2 misho 1962: This is a typical way in which pcre_study() is used (except that in a
1.1 misho 1963: real application there should be tests for errors):
1964:
1965: int rc;
1966: pcre *re;
1967: pcre_extra *sd;
1968: re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
1969: sd = pcre_study(
1970: re, /* result of pcre_compile() */
1971: 0, /* no options */
1972: &error); /* set to NULL or points to a message */
1973: rc = pcre_exec( /* see below for details of pcre_exec() options */
1974: re, sd, "subject", 7, 0, 0, ovector, 30);
1975: ...
1976: pcre_free_study(sd);
1977: pcre_free(re);
1978:
1979: Studying a pattern does two things: first, a lower bound for the length
1980: of subject string that is needed to match the pattern is computed. This
1981: does not mean that there are any strings of that length that match, but
1.1.1.2 misho 1982: it does guarantee that no shorter strings match. The value is used by
1983: pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1984: match strings that are shorter than the lower bound. You can find out
1.1 misho 1985: the value in a calling program via the pcre_fullinfo() function.
1986:
1987: Studying a pattern is also useful for non-anchored patterns that do not
1.1.1.2 misho 1988: have a single fixed starting character. A bitmap of possible starting
1989: bytes is created. This speeds up finding a position in the subject at
1990: which to start matching. (In 16-bit mode, the bitmap is used for 16-bit
1991: values less than 256.)
1.1 misho 1992:
1.1.1.3 ! misho 1993: These two optimizations apply to both pcre_exec() and pcre_dfa_exec(),
! 1994: and the information is also used by the JIT compiler. The optimiza-
! 1995: tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option when
! 1996: calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execu-
! 1997: tion is also disabled. You might want to do this if your pattern con-
! 1998: tains callouts or (*MARK) and you want to make use of these facilities
! 1999: in cases where matching fails. See the discussion of
! 2000: PCRE_NO_START_OPTIMIZE below.
1.1 misho 2001:
2002:
2003: LOCALE SUPPORT
2004:
1.1.1.3 ! misho 2005: PCRE handles caseless matching, and determines whether characters are
! 2006: letters, digits, or whatever, by reference to a set of tables, indexed
! 2007: by character value. When running in UTF-8 mode, this applies only to
! 2008: characters with codes less than 128. By default, higher-valued codes
1.1 misho 2009: never match escapes such as \w or \d, but they can be tested with \p if
1.1.1.3 ! misho 2010: PCRE is built with Unicode character property support. Alternatively,
! 2011: the PCRE_UCP option can be set at compile time; this causes \w and
1.1 misho 2012: friends to use Unicode property support instead of built-in tables. The
2013: use of locales with Unicode is discouraged. If you are handling charac-
1.1.1.3 ! misho 2014: ters with codes greater than 128, you should either use UTF-8 and Uni-
1.1 misho 2015: code, or use locales, but not try to mix the two.
2016:
1.1.1.3 ! misho 2017: PCRE contains an internal set of tables that are used when the final
! 2018: argument of pcre_compile() is NULL. These are sufficient for many
1.1 misho 2019: applications. Normally, the internal tables recognize only ASCII char-
2020: acters. However, when PCRE is built, it is possible to cause the inter-
2021: nal tables to be rebuilt in the default "C" locale of the local system,
2022: which may cause them to be different.
2023:
1.1.1.3 ! misho 2024: The internal tables can always be overridden by tables supplied by the
1.1 misho 2025: application that calls PCRE. These may be created in a different locale
1.1.1.3 ! misho 2026: from the default. As more and more applications change to using Uni-
1.1 misho 2027: code, the need for this locale support is expected to die away.
2028:
1.1.1.3 ! misho 2029: External tables are built by calling the pcre_maketables() function,
! 2030: which has no arguments, in the relevant locale. The result can then be
! 2031: passed to pcre_compile() or pcre_exec() as often as necessary. For
! 2032: example, to build and use tables that are appropriate for the French
! 2033: locale (where accented characters with values greater than 128 are
1.1 misho 2034: treated as letters), the following code could be used:
2035:
2036: setlocale(LC_CTYPE, "fr_FR");
2037: tables = pcre_maketables();
2038: re = pcre_compile(..., tables);
2039:
1.1.1.3 ! misho 2040: The locale name "fr_FR" is used on Linux and other Unix-like systems;
1.1 misho 2041: if you are using Windows, the name for the French locale is "french".
2042:
1.1.1.3 ! misho 2043: When pcre_maketables() runs, the tables are built in memory that is
! 2044: obtained via pcre_malloc. It is the caller's responsibility to ensure
! 2045: that the memory containing the tables remains available for as long as
1.1 misho 2046: it is needed.
2047:
2048: The pointer that is passed to pcre_compile() is saved with the compiled
1.1.1.3 ! misho 2049: pattern, and the same tables are used via this pointer by pcre_study()
1.1 misho 2050: and normally also by pcre_exec(). Thus, by default, for any single pat-
2051: tern, compilation, studying and matching all happen in the same locale,
2052: but different patterns can be compiled in different locales.
2053:
1.1.1.3 ! misho 2054: It is possible to pass a table pointer or NULL (indicating the use of
! 2055: the internal tables) to pcre_exec(). Although not intended for this
! 2056: purpose, this facility could be used to match a pattern in a different
1.1 misho 2057: locale from the one in which it was compiled. Passing table pointers at
2058: run time is discussed below in the section on matching a pattern.
2059:
2060:
2061: INFORMATION ABOUT A PATTERN
2062:
2063: int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
2064: int what, void *where);
2065:
1.1.1.3 ! misho 2066: The pcre_fullinfo() function returns information about a compiled pat-
! 2067: tern. It replaces the pcre_info() function, which was removed from the
1.1.1.2 misho 2068: library at version 8.30, after more than 10 years of obsolescence.
1.1 misho 2069:
1.1.1.3 ! misho 2070: The first argument for pcre_fullinfo() is a pointer to the compiled
! 2071: pattern. The second argument is the result of pcre_study(), or NULL if
! 2072: the pattern was not studied. The third argument specifies which piece
! 2073: of information is required, and the fourth argument is a pointer to a
! 2074: variable to receive the data. The yield of the function is zero for
1.1 misho 2075: success, or one of the following negative numbers:
2076:
1.1.1.2 misho 2077: PCRE_ERROR_NULL the argument code was NULL
2078: the argument where was NULL
2079: PCRE_ERROR_BADMAGIC the "magic number" was not found
2080: PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
2081: endianness
2082: PCRE_ERROR_BADOPTION the value of what was invalid
1.1 misho 2083:
1.1.1.3 ! misho 2084: The "magic number" is placed at the start of each compiled pattern as
! 2085: an simple check against passing an arbitrary memory pointer. The endi-
1.1.1.2 misho 2086: anness error can occur if a compiled pattern is saved and reloaded on a
1.1.1.3 ! misho 2087: different host. Here is a typical call of pcre_fullinfo(), to obtain
1.1.1.2 misho 2088: the length of the compiled pattern:
1.1 misho 2089:
2090: int rc;
2091: size_t length;
2092: rc = pcre_fullinfo(
2093: re, /* result of pcre_compile() */
2094: sd, /* result of pcre_study(), or NULL */
2095: PCRE_INFO_SIZE, /* what is required */
2096: &length); /* where to put the data */
2097:
1.1.1.3 ! misho 2098: The possible values for the third argument are defined in pcre.h, and
1.1 misho 2099: are as follows:
2100:
2101: PCRE_INFO_BACKREFMAX
2102:
1.1.1.3 ! misho 2103: Return the number of the highest back reference in the pattern. The
! 2104: fourth argument should point to an int variable. Zero is returned if
1.1 misho 2105: there are no back references.
2106:
2107: PCRE_INFO_CAPTURECOUNT
2108:
1.1.1.3 ! misho 2109: Return the number of capturing subpatterns in the pattern. The fourth
1.1 misho 2110: argument should point to an int variable.
2111:
2112: PCRE_INFO_DEFAULT_TABLES
2113:
1.1.1.3 ! misho 2114: Return a pointer to the internal default character tables within PCRE.
! 2115: The fourth argument should point to an unsigned char * variable. This
1.1 misho 2116: information call is provided for internal use by the pcre_study() func-
1.1.1.3 ! misho 2117: tion. External callers can cause PCRE to use its internal tables by
1.1 misho 2118: passing a NULL table pointer.
2119:
2120: PCRE_INFO_FIRSTBYTE
2121:
1.1.1.2 misho 2122: Return information about the first data unit of any matched string, for
1.1.1.3 ! misho 2123: a non-anchored pattern. (The name of this option refers to the 8-bit
! 2124: library, where data units are bytes.) The fourth argument should point
1.1.1.2 misho 2125: to an int variable.
2126:
1.1.1.3 ! misho 2127: If there is a fixed first value, for example, the letter "c" from a
! 2128: pattern such as (cat|cow|coyote), its value is returned. In the 8-bit
! 2129: library, the value is always less than 256; in the 16-bit library the
1.1.1.2 misho 2130: value can be up to 0xffff.
1.1 misho 2131:
1.1.1.2 misho 2132: If there is no fixed first value, and if either
1.1 misho 2133:
1.1.1.3 ! misho 2134: (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1.1 misho 2135: branch starts with "^", or
2136:
2137: (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
2138: set (if it were set, the pattern would be anchored),
2139:
1.1.1.3 ! misho 2140: -1 is returned, indicating that the pattern matches only at the start
! 2141: of a subject string or after any newline within the string. Otherwise
1.1 misho 2142: -2 is returned. For anchored patterns, -2 is returned.
2143:
2144: PCRE_INFO_FIRSTTABLE
2145:
1.1.1.3 ! misho 2146: If the pattern was studied, and this resulted in the construction of a
! 2147: 256-bit table indicating a fixed set of values for the first data unit
! 2148: in any matching string, a pointer to the table is returned. Otherwise
! 2149: NULL is returned. The fourth argument should point to an unsigned char
1.1.1.2 misho 2150: * variable.
1.1 misho 2151:
2152: PCRE_INFO_HASCRORLF
2153:
1.1.1.3 ! misho 2154: Return 1 if the pattern contains any explicit matches for CR or LF
! 2155: characters, otherwise 0. The fourth argument should point to an int
! 2156: variable. An explicit match is either a literal CR or LF character, or
1.1 misho 2157: \r or \n.
2158:
2159: PCRE_INFO_JCHANGED
2160:
1.1.1.3 ! misho 2161: Return 1 if the (?J) or (?-J) option setting is used in the pattern,
! 2162: otherwise 0. The fourth argument should point to an int variable. (?J)
1.1 misho 2163: and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
2164:
2165: PCRE_INFO_JIT
2166:
1.1.1.3 ! misho 2167: Return 1 if the pattern was studied with one of the JIT options, and
! 2168: just-in-time compiling was successful. The fourth argument should point
! 2169: to an int variable. A return value of 0 means that JIT support is not
! 2170: available in this version of PCRE, or that the pattern was not studied
! 2171: with a JIT option, or that the JIT compiler could not handle this par-
! 2172: ticular pattern. See the pcrejit documentation for details of what can
! 2173: and cannot be handled.
1.1 misho 2174:
2175: PCRE_INFO_JITSIZE
2176:
1.1.1.3 ! misho 2177: If the pattern was successfully studied with a JIT option, return the
! 2178: size of the JIT compiled code, otherwise return zero. The fourth argu-
! 2179: ment should point to a size_t variable.
1.1 misho 2180:
2181: PCRE_INFO_LASTLITERAL
2182:
1.1.1.3 ! misho 2183: Return the value of the rightmost literal data unit that must exist in
! 2184: any matched string, other than at its start, if such a value has been
1.1 misho 2185: recorded. The fourth argument should point to an int variable. If there
1.1.1.2 misho 2186: is no such value, -1 is returned. For anchored patterns, a last literal
1.1.1.3 ! misho 2187: value is recorded only if it follows something of variable length. For
1.1 misho 2188: example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
2189: /^a\dz\d/ the returned value is -1.
2190:
1.1.1.3 ! misho 2191: PCRE_INFO_MAXLOOKBEHIND
! 2192:
! 2193: Return the number of characters (NB not bytes) in the longest lookbe-
! 2194: hind assertion in the pattern. Note that the simple assertions \b and
! 2195: \B require a one-character lookbehind. This information is useful when
! 2196: doing multi-segment matching using the partial matching facilities.
! 2197:
1.1 misho 2198: PCRE_INFO_MINLENGTH
2199:
1.1.1.2 misho 2200: If the pattern was studied and a minimum length for matching subject
2201: strings was computed, its value is returned. Otherwise the returned
2202: value is -1. The value is a number of characters, which in UTF-8 mode
2203: may be different from the number of bytes. The fourth argument should
2204: point to an int variable. A non-negative value is a lower bound to the
2205: length of any matching string. There may not be any strings of that
2206: length that do actually match, but every string that does match is at
2207: least that long.
1.1 misho 2208:
2209: PCRE_INFO_NAMECOUNT
2210: PCRE_INFO_NAMEENTRYSIZE
2211: PCRE_INFO_NAMETABLE
2212:
2213: PCRE supports the use of named as well as numbered capturing parenthe-
2214: ses. The names are just an additional way of identifying the parenthe-
2215: ses, which still acquire numbers. Several convenience functions such as
2216: pcre_get_named_substring() are provided for extracting captured sub-
2217: strings by name. It is also possible to extract the data directly, by
2218: first converting the name to a number in order to access the correct
2219: pointers in the output vector (described with pcre_exec() below). To do
2220: the conversion, you need to use the name-to-number map, which is
2221: described by these three values.
2222:
2223: The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
2224: gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
2225: of each entry; both of these return an int value. The entry size
2226: depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1.1.1.2 misho 2227: a pointer to the first entry of the table. This is a pointer to char in
2228: the 8-bit library, where the first two bytes of each entry are the num-
2229: ber of the capturing parenthesis, most significant byte first. In the
2230: 16-bit library, the pointer points to 16-bit data units, the first of
2231: which contains the parenthesis number. The rest of the entry is the
2232: corresponding name, zero terminated.
1.1 misho 2233:
2234: The names are in alphabetical order. Duplicate names may appear if (?|
2235: is used to create multiple groups with the same number, as described in
2236: the section on duplicate subpattern numbers in the pcrepattern page.
2237: Duplicate names for subpatterns with different numbers are permitted
2238: only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
2239: appear in the table in the order in which they were found in the pat-
2240: tern. In the absence of (?| this is the order of increasing number;
2241: when (?| is used this is not necessarily the case because later subpat-
2242: terns may have lower numbers.
2243:
2244: As a simple example of the name/number table, consider the following
1.1.1.2 misho 2245: pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is
2246: set, so white space - including newlines - is ignored):
1.1 misho 2247:
2248: (?<date> (?<year>(\d\d)?\d\d) -
2249: (?<month>\d\d) - (?<day>\d\d) )
2250:
2251: There are four named subpatterns, so the table has four entries, and
2252: each entry in the table is eight bytes long. The table is as follows,
2253: with non-printing bytes shows in hexadecimal, and undefined bytes shown
2254: as ??:
2255:
2256: 00 01 d a t e 00 ??
2257: 00 05 d a y 00 ?? ??
2258: 00 04 m o n t h 00
2259: 00 02 y e a r 00 ??
2260:
2261: When writing code to extract data from named subpatterns using the
2262: name-to-number map, remember that the length of the entries is likely
2263: to be different for each compiled pattern.
2264:
2265: PCRE_INFO_OKPARTIAL
2266:
2267: Return 1 if the pattern can be used for partial matching with
2268: pcre_exec(), otherwise 0. The fourth argument should point to an int
2269: variable. From release 8.00, this always returns 1, because the
2270: restrictions that previously applied to partial matching have been
2271: lifted. The pcrepartial documentation gives details of partial match-
2272: ing.
2273:
2274: PCRE_INFO_OPTIONS
2275:
2276: Return a copy of the options with which the pattern was compiled. The
2277: fourth argument should point to an unsigned long int variable. These
2278: option bits are those specified in the call to pcre_compile(), modified
2279: by any top-level option settings at the start of the pattern itself. In
2280: other words, they are the options that will be in force when matching
2281: starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
2282: the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
2283: and PCRE_EXTENDED.
2284:
2285: A pattern is automatically anchored by PCRE if all of its top-level
2286: alternatives begin with one of the following:
2287:
2288: ^ unless PCRE_MULTILINE is set
2289: \A always
2290: \G always
2291: .* if PCRE_DOTALL is set and there are no back
2292: references to the subpattern in which .* appears
2293:
2294: For such patterns, the PCRE_ANCHORED bit is set in the options returned
2295: by pcre_fullinfo().
2296:
2297: PCRE_INFO_SIZE
2298:
1.1.1.2 misho 2299: Return the size of the compiled pattern in bytes (for both libraries).
2300: The fourth argument should point to a size_t variable. This value does
2301: not include the size of the pcre structure that is returned by
2302: pcre_compile(). The value that is passed as the argument to pcre_mal-
2303: loc() when pcre_compile() is getting memory in which to place the com-
2304: piled data is the value returned by this option plus the size of the
2305: pcre structure. Studying a compiled pattern, with or without JIT, does
2306: not alter the value returned by this option.
1.1 misho 2307:
2308: PCRE_INFO_STUDYSIZE
2309:
1.1.1.2 misho 2310: Return the size in bytes of the data block pointed to by the study_data
2311: field in a pcre_extra block. If pcre_extra is NULL, or there is no
2312: study data, zero is returned. The fourth argument should point to a
2313: size_t variable. The study_data field is set by pcre_study() to record
2314: information that will speed up matching (see the section entitled
2315: "Studying a pattern" above). The format of the study_data block is pri-
2316: vate, but its length is made available via this option so that it can
2317: be saved and restored (see the pcreprecompile documentation for
2318: details).
1.1 misho 2319:
2320:
2321: REFERENCE COUNTS
2322:
2323: int pcre_refcount(pcre *code, int adjust);
2324:
1.1.1.2 misho 2325: The pcre_refcount() function is used to maintain a reference count in
1.1 misho 2326: the data block that contains a compiled pattern. It is provided for the
1.1.1.2 misho 2327: benefit of applications that operate in an object-oriented manner,
1.1 misho 2328: where different parts of the application may be using the same compiled
2329: pattern, but you want to free the block when they are all done.
2330:
2331: When a pattern is compiled, the reference count field is initialized to
1.1.1.2 misho 2332: zero. It is changed only by calling this function, whose action is to
2333: add the adjust value (which may be positive or negative) to it. The
1.1 misho 2334: yield of the function is the new value. However, the value of the count
1.1.1.2 misho 2335: is constrained to lie between 0 and 65535, inclusive. If the new value
1.1 misho 2336: is outside these limits, it is forced to the appropriate limit value.
2337:
1.1.1.2 misho 2338: Except when it is zero, the reference count is not correctly preserved
2339: if a pattern is compiled on one host and then transferred to a host
1.1 misho 2340: whose byte-order is different. (This seems a highly unlikely scenario.)
2341:
2342:
2343: MATCHING A PATTERN: THE TRADITIONAL FUNCTION
2344:
2345: int pcre_exec(const pcre *code, const pcre_extra *extra,
2346: const char *subject, int length, int startoffset,
2347: int options, int *ovector, int ovecsize);
2348:
1.1.1.2 misho 2349: The function pcre_exec() is called to match a subject string against a
2350: compiled pattern, which is passed in the code argument. If the pattern
2351: was studied, the result of the study should be passed in the extra
2352: argument. You can call pcre_exec() with the same code and extra argu-
2353: ments as many times as you like, in order to match different subject
1.1 misho 2354: strings with the same pattern.
2355:
1.1.1.2 misho 2356: This function is the main matching facility of the library, and it
2357: operates in a Perl-like manner. For specialist use there is also an
2358: alternative matching function, which is described below in the section
1.1 misho 2359: about the pcre_dfa_exec() function.
2360:
1.1.1.2 misho 2361: In most applications, the pattern will have been compiled (and option-
2362: ally studied) in the same process that calls pcre_exec(). However, it
1.1 misho 2363: is possible to save compiled patterns and study data, and then use them
1.1.1.2 misho 2364: later in different processes, possibly even on different hosts. For a
1.1 misho 2365: discussion about this, see the pcreprecompile documentation.
2366:
2367: Here is an example of a simple call to pcre_exec():
2368:
2369: int rc;
2370: int ovector[30];
2371: rc = pcre_exec(
2372: re, /* result of pcre_compile() */
2373: NULL, /* we didn't study the pattern */
2374: "some string", /* the subject string */
2375: 11, /* the length of the subject string */
2376: 0, /* start at offset 0 in the subject */
2377: 0, /* default options */
2378: ovector, /* vector of integers for substring information */
2379: 30); /* number of elements (NOT size in bytes) */
2380:
2381: Extra data for pcre_exec()
2382:
1.1.1.2 misho 2383: If the extra argument is not NULL, it must point to a pcre_extra data
2384: block. The pcre_study() function returns such a block (when it doesn't
2385: return NULL), but you can also create one for yourself, and pass addi-
2386: tional information in it. The pcre_extra block contains the following
1.1 misho 2387: fields (not necessarily in this order):
2388:
2389: unsigned long int flags;
2390: void *study_data;
2391: void *executable_jit;
2392: unsigned long int match_limit;
2393: unsigned long int match_limit_recursion;
2394: void *callout_data;
2395: const unsigned char *tables;
2396: unsigned char **mark;
2397:
1.1.1.2 misho 2398: In the 16-bit version of this structure, the mark field has type
2399: "PCRE_UCHAR16 **".
2400:
1.1.1.3 ! misho 2401: The flags field is used to specify which of the other fields are set.
! 2402: The flag bits are:
1.1 misho 2403:
1.1.1.3 ! misho 2404: PCRE_EXTRA_CALLOUT_DATA
1.1 misho 2405: PCRE_EXTRA_EXECUTABLE_JIT
1.1.1.3 ! misho 2406: PCRE_EXTRA_MARK
1.1 misho 2407: PCRE_EXTRA_MATCH_LIMIT
2408: PCRE_EXTRA_MATCH_LIMIT_RECURSION
1.1.1.3 ! misho 2409: PCRE_EXTRA_STUDY_DATA
1.1 misho 2410: PCRE_EXTRA_TABLES
2411:
2412: Other flag bits should be set to zero. The study_data field and some-
2413: times the executable_jit field are set in the pcre_extra block that is
2414: returned by pcre_study(), together with the appropriate flag bits. You
2415: should not set these yourself, but you may add to the block by setting
1.1.1.3 ! misho 2416: other fields and their corresponding flag bits.
1.1 misho 2417:
2418: The match_limit field provides a means of preventing PCRE from using up
2419: a vast amount of resources when running patterns that are not going to
2420: match, but which have a very large number of possibilities in their
2421: search trees. The classic example is a pattern that uses nested unlim-
2422: ited repeats.
2423:
2424: Internally, pcre_exec() uses a function called match(), which it calls
2425: repeatedly (sometimes recursively). The limit set by match_limit is
2426: imposed on the number of times this function is called during a match,
2427: which has the effect of limiting the amount of backtracking that can
2428: take place. For patterns that are not anchored, the count restarts from
2429: zero for each position in the subject string.
2430:
2431: When pcre_exec() is called with a pattern that was successfully studied
1.1.1.3 ! misho 2432: with a JIT option, the way that the matching is executed is entirely
! 2433: different. However, there is still the possibility of runaway matching
! 2434: that goes on for a very long time, and so the match_limit value is also
! 2435: used in this case (but in a different way) to limit how long the match-
! 2436: ing can continue.
1.1 misho 2437:
2438: The default value for the limit can be set when PCRE is built; the
2439: default default is 10 million, which handles all but the most extreme
2440: cases. You can override the default by suppling pcre_exec() with a
2441: pcre_extra block in which match_limit is set, and
2442: PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2443: exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2444:
2445: The match_limit_recursion field is similar to match_limit, but instead
2446: of limiting the total number of times that match() is called, it limits
2447: the depth of recursion. The recursion depth is a smaller number than
2448: the total number of calls, because not all calls to match() are recur-
2449: sive. This limit is of use only if it is set smaller than match_limit.
2450:
2451: Limiting the recursion depth limits the amount of machine stack that
2452: can be used, or, when PCRE has been compiled to use memory on the heap
2453: instead of the stack, the amount of heap memory that can be used. This
1.1.1.3 ! misho 2454: limit is not relevant, and is ignored, when matching is done using JIT
! 2455: compiled code.
1.1 misho 2456:
2457: The default value for match_limit_recursion can be set when PCRE is
2458: built; the default default is the same value as the default for
2459: match_limit. You can override the default by suppling pcre_exec() with
2460: a pcre_extra block in which match_limit_recursion is set, and
2461: PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2462: limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2463:
2464: The callout_data field is used in conjunction with the "callout" fea-
2465: ture, and is described in the pcrecallout documentation.
2466:
2467: The tables field is used to pass a character tables pointer to
2468: pcre_exec(); this overrides the value that is stored with the compiled
2469: pattern. A non-NULL value is stored with the compiled pattern only if
2470: custom tables were supplied to pcre_compile() via its tableptr argu-
2471: ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2472: PCRE's internal tables to be used. This facility is helpful when re-
2473: using patterns that have been saved after compiling with an external
2474: set of tables, because the external tables might be at a different
2475: address when pcre_exec() is called. See the pcreprecompile documenta-
2476: tion for a discussion of saving compiled patterns for later use.
2477:
2478: If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
1.1.1.2 misho 2479: set to point to a suitable variable. If the pattern contains any back-
1.1 misho 2480: tracking control verbs such as (*MARK:NAME), and the execution ends up
2481: with a name to pass back, a pointer to the name string (zero termi-
2482: nated) is placed in the variable pointed to by the mark field. The
2483: names are within the compiled pattern; if you wish to retain such a
2484: name you must copy it before freeing the memory of a compiled pattern.
2485: If there is no name to pass back, the variable pointed to by the mark
1.1.1.2 misho 2486: field is set to NULL. For details of the backtracking control verbs,
2487: see the section entitled "Backtracking control" in the pcrepattern doc-
2488: umentation.
1.1 misho 2489:
2490: Option bits for pcre_exec()
2491:
2492: The unused bits of the options argument for pcre_exec() must be zero.
2493: The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2494: PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
1.1.1.3 ! misho 2495: PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and
! 2496: PCRE_PARTIAL_SOFT.
1.1 misho 2497:
1.1.1.3 ! misho 2498: If the pattern was successfully studied with one of the just-in-time
! 2499: (JIT) compile options, the only supported options for JIT execution are
! 2500: PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
! 2501: PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an
! 2502: unsupported option is used, JIT execution is disabled and the normal
! 2503: interpretive code in pcre_exec() is run.
1.1 misho 2504:
2505: PCRE_ANCHORED
2506:
2507: The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2508: matching position. If a pattern was compiled with PCRE_ANCHORED, or
2509: turned out to be anchored by virtue of its contents, it cannot be made
2510: unachored at matching time.
2511:
2512: PCRE_BSR_ANYCRLF
2513: PCRE_BSR_UNICODE
2514:
2515: These options (which are mutually exclusive) control what the \R escape
2516: sequence matches. The choice is either to match only CR, LF, or CRLF,
2517: or to match any Unicode newline sequence. These options override the
2518: choice that was made or defaulted when the pattern was compiled.
2519:
2520: PCRE_NEWLINE_CR
2521: PCRE_NEWLINE_LF
2522: PCRE_NEWLINE_CRLF
2523: PCRE_NEWLINE_ANYCRLF
2524: PCRE_NEWLINE_ANY
2525:
2526: These options override the newline definition that was chosen or
2527: defaulted when the pattern was compiled. For details, see the descrip-
2528: tion of pcre_compile() above. During matching, the newline choice
2529: affects the behaviour of the dot, circumflex, and dollar metacharac-
2530: ters. It may also alter the way the match position is advanced after a
2531: match failure for an unanchored pattern.
2532:
2533: When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
2534: set, and a match attempt for an unanchored pattern fails when the cur-
2535: rent position is at a CRLF sequence, and the pattern contains no
2536: explicit matches for CR or LF characters, the match position is
2537: advanced by two characters instead of one, in other words, to after the
2538: CRLF.
2539:
2540: The above rule is a compromise that makes the most common cases work as
2541: expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2542: option is not set), it does not match the string "\r\nA" because, after
2543: failing at the start, it skips both the CR and the LF before retrying.
2544: However, the pattern [\r\n]A does match that string, because it con-
2545: tains an explicit CR or LF reference, and so advances only by one char-
2546: acter after the first failure.
2547:
2548: An explicit match for CR of LF is either a literal appearance of one of
2549: those characters, or one of the \r or \n escape sequences. Implicit
2550: matches such as [^X] do not count, nor does \s (which includes CR and
2551: LF in the characters that it matches).
2552:
2553: Notwithstanding the above, anomalous effects may still occur when CRLF
2554: is a valid newline sequence and explicit \r or \n escapes appear in the
2555: pattern.
2556:
2557: PCRE_NOTBOL
2558:
2559: This option specifies that first character of the subject string is not
2560: the beginning of a line, so the circumflex metacharacter should not
2561: match before it. Setting this without PCRE_MULTILINE (at compile time)
2562: causes circumflex never to match. This option affects only the behav-
2563: iour of the circumflex metacharacter. It does not affect \A.
2564:
2565: PCRE_NOTEOL
2566:
2567: This option specifies that the end of the subject string is not the end
2568: of a line, so the dollar metacharacter should not match it nor (except
2569: in multiline mode) a newline immediately before it. Setting this with-
2570: out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2571: option affects only the behaviour of the dollar metacharacter. It does
2572: not affect \Z or \z.
2573:
2574: PCRE_NOTEMPTY
2575:
2576: An empty string is not considered to be a valid match if this option is
2577: set. If there are alternatives in the pattern, they are tried. If all
2578: the alternatives match the empty string, the entire match fails. For
2579: example, if the pattern
2580:
2581: a?b?
2582:
2583: is applied to a string not beginning with "a" or "b", it matches an
2584: empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2585: match is not valid, so PCRE searches further into the string for occur-
2586: rences of "a" or "b".
2587:
2588: PCRE_NOTEMPTY_ATSTART
2589:
2590: This is like PCRE_NOTEMPTY, except that an empty string match that is
2591: not at the start of the subject is permitted. If the pattern is
2592: anchored, such a match can occur only if the pattern contains \K.
2593:
2594: Perl has no direct equivalent of PCRE_NOTEMPTY or
2595: PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2596: match of the empty string within its split() function, and when using
2597: the /g modifier. It is possible to emulate Perl's behaviour after
2598: matching a null string by first trying the match again at the same off-
2599: set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2600: fails, by advancing the starting offset (see below) and trying an ordi-
2601: nary match again. There is some code that demonstrates how to do this
2602: in the pcredemo sample program. In the most general case, you have to
2603: check to see if the newline convention recognizes CRLF as a newline,
2604: and if so, and the current character is CR followed by LF, advance the
2605: starting offset by two characters instead of one.
2606:
2607: PCRE_NO_START_OPTIMIZE
2608:
2609: There are a number of optimizations that pcre_exec() uses at the start
2610: of a match, in order to speed up the process. For example, if it is
2611: known that an unanchored match must start with a specific character, it
2612: searches the subject for that character, and fails immediately if it
2613: cannot find it, without actually running the main matching function.
2614: This means that a special item such as (*COMMIT) at the start of a pat-
2615: tern is not considered until after a suitable starting point for the
2616: match has been found. When callouts or (*MARK) items are in use, these
2617: "start-up" optimizations can cause them to be skipped if the pattern is
2618: never actually used. The start-up optimizations are in effect a pre-
2619: scan of the subject that takes place before the pattern is run.
2620:
2621: The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2622: possibly causing performance to suffer, but ensuring that in cases
2623: where the result is "no match", the callouts do occur, and that items
2624: such as (*COMMIT) and (*MARK) are considered at every possible starting
2625: position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
1.1.1.3 ! misho 2626: compile time, it cannot be unset at matching time. The use of
! 2627: PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching
! 2628: is always done using interpretively.
1.1 misho 2629:
2630: Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2631: operation. Consider the pattern
2632:
2633: (*COMMIT)ABC
2634:
2635: When this is compiled, PCRE records the fact that a match must start
2636: with the character "A". Suppose the subject string is "DEFABC". The
2637: start-up optimization scans along the subject, finds "A" and runs the
2638: first match attempt from there. The (*COMMIT) item means that the pat-
2639: tern must match the current starting position, which in this case, it
2640: does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2641: set, the initial scan along the subject string does not happen. The
2642: first match attempt is run starting from "D" and when this fails,
2643: (*COMMIT) prevents any further matches being tried, so the overall
2644: result is "no match". If the pattern is studied, more start-up opti-
2645: mizations may be used. For example, a minimum length for the subject
2646: may be recorded. Consider the pattern
2647:
2648: (*MARK:A)(X|Y)
2649:
2650: The minimum length for a match is one character. If the subject is
2651: "ABC", there will be attempts to match "ABC", "BC", "C", and then
2652: finally an empty string. If the pattern is studied, the final attempt
2653: does not take place, because PCRE knows that the subject is too short,
2654: and so the (*MARK) is never encountered. In this case, studying the
2655: pattern does not affect the overall match result, which is still "no
2656: match", but it does affect the auxiliary information that is returned.
2657:
2658: PCRE_NO_UTF8_CHECK
2659:
2660: When PCRE_UTF8 is set at compile time, the validity of the subject as a
2661: UTF-8 string is automatically checked when pcre_exec() is subsequently
1.1.1.3 ! misho 2662: called. The entire string is checked before any other processing takes
! 2663: place. The value of startoffset is also checked to ensure that it
! 2664: points to the start of a UTF-8 character. There is a discussion about
! 2665: the validity of UTF-8 strings in the pcreunicode page. If an invalid
! 2666: sequence of bytes is found, pcre_exec() returns the error
1.1.1.2 misho 2667: PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
2668: truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
1.1.1.3 ! misho 2669: both cases, information about the precise nature of the error may also
! 2670: be returned (see the descriptions of these errors in the section enti-
! 2671: tled Error return values from pcre_exec() below). If startoffset con-
1.1.1.2 misho 2672: tains a value that does not point to the start of a UTF-8 character (or
2673: to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2674:
1.1.1.3 ! misho 2675: If you already know that your subject is valid, and you want to skip
! 2676: these checks for performance reasons, you can set the
! 2677: PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
! 2678: do this for the second and subsequent calls to pcre_exec() if you are
! 2679: making repeated calls to find all the matches in a single subject
! 2680: string. However, you should be sure that the value of startoffset
! 2681: points to the start of a character (or the end of the subject). When
1.1.1.2 misho 2682: PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a
1.1.1.3 ! misho 2683: subject or an invalid value of startoffset is undefined. Your program
1.1.1.2 misho 2684: may crash.
1.1 misho 2685:
2686: PCRE_PARTIAL_HARD
2687: PCRE_PARTIAL_SOFT
2688:
1.1.1.3 ! misho 2689: These options turn on the partial matching feature. For backwards com-
! 2690: patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
! 2691: match occurs if the end of the subject string is reached successfully,
! 2692: but there are not enough subject characters to complete the match. If
1.1 misho 2693: this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
1.1.1.3 ! misho 2694: matching continues by testing any remaining alternatives. Only if no
! 2695: complete match can be found is PCRE_ERROR_PARTIAL returned instead of
! 2696: PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
! 2697: caller is prepared to handle a partial match, but only if no complete
1.1 misho 2698: match can be found.
2699:
1.1.1.3 ! misho 2700: If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
! 2701: case, if a partial match is found, pcre_exec() immediately returns
! 2702: PCRE_ERROR_PARTIAL, without considering any other alternatives. In
! 2703: other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
1.1 misho 2704: ered to be more important that an alternative complete match.
2705:
1.1.1.3 ! misho 2706: In both cases, the portion of the string that was inspected when the
1.1 misho 2707: partial match was found is set as the first matching string. There is a
1.1.1.3 ! misho 2708: more detailed discussion of partial and multi-segment matching, with
1.1 misho 2709: examples, in the pcrepartial documentation.
2710:
2711: The string to be matched by pcre_exec()
2712:
1.1.1.3 ! misho 2713: The subject string is passed to pcre_exec() as a pointer in subject, a
! 2714: length in bytes in length, and a starting byte offset in startoffset.
! 2715: If this is negative or greater than the length of the subject,
! 2716: pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
! 2717: zero, the search for a match starts at the beginning of the subject,
1.1 misho 2718: and this is by far the most common case. In UTF-8 mode, the byte offset
1.1.1.3 ! misho 2719: must point to the start of a UTF-8 character (or the end of the sub-
! 2720: ject). Unlike the pattern string, the subject may contain binary zero
1.1 misho 2721: bytes.
2722:
1.1.1.3 ! misho 2723: A non-zero starting offset is useful when searching for another match
! 2724: in the same subject by calling pcre_exec() again after a previous suc-
! 2725: cess. Setting startoffset differs from just passing over a shortened
! 2726: string and setting PCRE_NOTBOL in the case of a pattern that begins
1.1 misho 2727: with any kind of lookbehind. For example, consider the pattern
2728:
2729: \Biss\B
2730:
1.1.1.3 ! misho 2731: which finds occurrences of "iss" in the middle of words. (\B matches
! 2732: only if the current position in the subject is not a word boundary.)
! 2733: When applied to the string "Mississipi" the first call to pcre_exec()
! 2734: finds the first occurrence. If pcre_exec() is called again with just
! 2735: the remainder of the subject, namely "issipi", it does not match,
1.1 misho 2736: because \B is always false at the start of the subject, which is deemed
1.1.1.3 ! misho 2737: to be a word boundary. However, if pcre_exec() is passed the entire
1.1 misho 2738: string again, but with startoffset set to 4, it finds the second occur-
1.1.1.3 ! misho 2739: rence of "iss" because it is able to look behind the starting point to
1.1 misho 2740: discover that it is preceded by a letter.
2741:
1.1.1.3 ! misho 2742: Finding all the matches in a subject is tricky when the pattern can
1.1 misho 2743: match an empty string. It is possible to emulate Perl's /g behaviour by
1.1.1.3 ! misho 2744: first trying the match again at the same offset, with the
! 2745: PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
! 2746: fails, advancing the starting offset and trying an ordinary match
1.1 misho 2747: again. There is some code that demonstrates how to do this in the pcre-
2748: demo sample program. In the most general case, you have to check to see
1.1.1.3 ! misho 2749: if the newline convention recognizes CRLF as a newline, and if so, and
1.1 misho 2750: the current character is CR followed by LF, advance the starting offset
2751: by two characters instead of one.
2752:
1.1.1.3 ! misho 2753: If a non-zero starting offset is passed when the pattern is anchored,
1.1 misho 2754: one attempt to match at the given offset is made. This can only succeed
1.1.1.3 ! misho 2755: if the pattern does not require the match to be at the start of the
1.1 misho 2756: subject.
2757:
2758: How pcre_exec() returns captured substrings
2759:
1.1.1.3 ! misho 2760: In general, a pattern matches a certain portion of the subject, and in
! 2761: addition, further substrings from the subject may be picked out by
! 2762: parts of the pattern. Following the usage in Jeffrey Friedl's book,
! 2763: this is called "capturing" in what follows, and the phrase "capturing
! 2764: subpattern" is used for a fragment of a pattern that picks out a sub-
! 2765: string. PCRE supports several other kinds of parenthesized subpattern
1.1 misho 2766: that do not cause substrings to be captured.
2767:
2768: Captured substrings are returned to the caller via a vector of integers
1.1.1.3 ! misho 2769: whose address is passed in ovector. The number of elements in the vec-
! 2770: tor is passed in ovecsize, which must be a non-negative number. Note:
1.1 misho 2771: this argument is NOT the size of ovector in bytes.
2772:
1.1.1.3 ! misho 2773: The first two-thirds of the vector is used to pass back captured sub-
! 2774: strings, each substring using a pair of integers. The remaining third
! 2775: of the vector is used as workspace by pcre_exec() while matching cap-
! 2776: turing subpatterns, and is not available for passing back information.
! 2777: The number passed in ovecsize should always be a multiple of three. If
1.1 misho 2778: it is not, it is rounded down.
2779:
1.1.1.3 ! misho 2780: When a match is successful, information about captured substrings is
! 2781: returned in pairs of integers, starting at the beginning of ovector,
! 2782: and continuing up to two-thirds of its length at the most. The first
! 2783: element of each pair is set to the byte offset of the first character
! 2784: in a substring, and the second is set to the byte offset of the first
! 2785: character after the end of a substring. Note: these values are always
1.1 misho 2786: byte offsets, even in UTF-8 mode. They are not character counts.
2787:
1.1.1.3 ! misho 2788: The first pair of integers, ovector[0] and ovector[1], identify the
! 2789: portion of the subject string matched by the entire pattern. The next
! 2790: pair is used for the first capturing subpattern, and so on. The value
1.1 misho 2791: returned by pcre_exec() is one more than the highest numbered pair that
1.1.1.3 ! misho 2792: has been set. For example, if two substrings have been captured, the
! 2793: returned value is 3. If there are no capturing subpatterns, the return
1.1 misho 2794: value from a successful match is 1, indicating that just the first pair
2795: of offsets has been set.
2796:
2797: If a capturing subpattern is matched repeatedly, it is the last portion
2798: of the string that it matched that is returned.
2799:
1.1.1.3 ! misho 2800: If the vector is too small to hold all the captured substring offsets,
1.1 misho 2801: it is used as far as possible (up to two-thirds of its length), and the
1.1.1.3 ! misho 2802: function returns a value of zero. If neither the actual string matched
! 2803: nor any captured substrings are of interest, pcre_exec() may be called
! 2804: with ovector passed as NULL and ovecsize as zero. However, if the pat-
! 2805: tern contains back references and the ovector is not big enough to
! 2806: remember the related substrings, PCRE has to get additional memory for
! 2807: use during matching. Thus it is usually advisable to supply an ovector
1.1 misho 2808: of reasonable size.
2809:
1.1.1.3 ! misho 2810: There are some cases where zero is returned (indicating vector over-
! 2811: flow) when in fact the vector is exactly the right size for the final
1.1 misho 2812: match. For example, consider the pattern
2813:
2814: (a)(?:(b)c|bd)
2815:
1.1.1.3 ! misho 2816: If a vector of 6 elements (allowing for only 1 captured substring) is
1.1 misho 2817: given with subject string "abd", pcre_exec() will try to set the second
2818: captured string, thereby recording a vector overflow, before failing to
1.1.1.3 ! misho 2819: match "c" and backing up to try the second alternative. The zero
! 2820: return, however, does correctly indicate that the maximum number of
1.1 misho 2821: slots (namely 2) have been filled. In similar cases where there is tem-
1.1.1.3 ! misho 2822: porary overflow, but the final number of used slots is actually less
1.1 misho 2823: than the maximum, a non-zero value is returned.
2824:
2825: The pcre_fullinfo() function can be used to find out how many capturing
1.1.1.3 ! misho 2826: subpatterns there are in a compiled pattern. The smallest size for
! 2827: ovector that will allow for n captured substrings, in addition to the
1.1 misho 2828: offsets of the substring matched by the whole pattern, is (n+1)*3.
2829:
1.1.1.3 ! misho 2830: It is possible for capturing subpattern number n+1 to match some part
1.1 misho 2831: of the subject when subpattern n has not been used at all. For example,
1.1.1.3 ! misho 2832: if the string "abc" is matched against the pattern (a|(z))(bc) the
1.1 misho 2833: return from the function is 4, and subpatterns 1 and 3 are matched, but
1.1.1.3 ! misho 2834: 2 is not. When this happens, both values in the offset pairs corre-
1.1 misho 2835: sponding to unused subpatterns are set to -1.
2836:
1.1.1.3 ! misho 2837: Offset values that correspond to unused subpatterns at the end of the
! 2838: expression are also set to -1. For example, if the string "abc" is
! 2839: matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
! 2840: matched. The return from the function is 2, because the highest used
! 2841: capturing subpattern number is 1, and the offsets for for the second
! 2842: and third capturing subpatterns (assuming the vector is large enough,
1.1 misho 2843: of course) are set to -1.
2844:
1.1.1.3 ! misho 2845: Note: Elements in the first two-thirds of ovector that do not corre-
! 2846: spond to capturing parentheses in the pattern are never changed. That
! 2847: is, if a pattern contains n capturing parentheses, no more than ovec-
! 2848: tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
1.1 misho 2849: the first two-thirds) retain whatever values they previously had.
2850:
1.1.1.3 ! misho 2851: Some convenience functions are provided for extracting the captured
1.1 misho 2852: substrings as separate strings. These are described below.
2853:
2854: Error return values from pcre_exec()
2855:
1.1.1.3 ! misho 2856: If pcre_exec() fails, it returns a negative number. The following are
1.1 misho 2857: defined in the header file:
2858:
2859: PCRE_ERROR_NOMATCH (-1)
2860:
2861: The subject string did not match the pattern.
2862:
2863: PCRE_ERROR_NULL (-2)
2864:
1.1.1.3 ! misho 2865: Either code or subject was passed as NULL, or ovector was NULL and
1.1 misho 2866: ovecsize was not zero.
2867:
2868: PCRE_ERROR_BADOPTION (-3)
2869:
2870: An unrecognized bit was set in the options argument.
2871:
2872: PCRE_ERROR_BADMAGIC (-4)
2873:
1.1.1.3 ! misho 2874: PCRE stores a 4-byte "magic number" at the start of the compiled code,
1.1 misho 2875: to catch the case when it is passed a junk pointer and to detect when a
2876: pattern that was compiled in an environment of one endianness is run in
1.1.1.3 ! misho 2877: an environment with the other endianness. This is the error that PCRE
1.1 misho 2878: gives when the magic number is not present.
2879:
2880: PCRE_ERROR_UNKNOWN_OPCODE (-5)
2881:
2882: While running the pattern match, an unknown item was encountered in the
1.1.1.3 ! misho 2883: compiled pattern. This error could be caused by a bug in PCRE or by
1.1 misho 2884: overwriting of the compiled pattern.
2885:
2886: PCRE_ERROR_NOMEMORY (-6)
2887:
1.1.1.3 ! misho 2888: If a pattern contains back references, but the ovector that is passed
1.1 misho 2889: to pcre_exec() is not big enough to remember the referenced substrings,
1.1.1.3 ! misho 2890: PCRE gets a block of memory at the start of matching to use for this
! 2891: purpose. If the call via pcre_malloc() fails, this error is given. The
1.1 misho 2892: memory is automatically freed at the end of matching.
2893:
1.1.1.3 ! misho 2894: This error is also given if pcre_stack_malloc() fails in pcre_exec().
! 2895: This can happen only when PCRE has been compiled with --disable-stack-
1.1 misho 2896: for-recursion.
2897:
2898: PCRE_ERROR_NOSUBSTRING (-7)
2899:
1.1.1.3 ! misho 2900: This error is used by the pcre_copy_substring(), pcre_get_substring(),
1.1 misho 2901: and pcre_get_substring_list() functions (see below). It is never
2902: returned by pcre_exec().
2903:
2904: PCRE_ERROR_MATCHLIMIT (-8)
2905:
1.1.1.3 ! misho 2906: The backtracking limit, as specified by the match_limit field in a
! 2907: pcre_extra structure (or defaulted) was reached. See the description
1.1 misho 2908: above.
2909:
2910: PCRE_ERROR_CALLOUT (-9)
2911:
2912: This error is never generated by pcre_exec() itself. It is provided for
1.1.1.3 ! misho 2913: use by callout functions that want to yield a distinctive error code.
1.1 misho 2914: See the pcrecallout documentation for details.
2915:
2916: PCRE_ERROR_BADUTF8 (-10)
2917:
1.1.1.3 ! misho 2918: A string that contains an invalid UTF-8 byte sequence was passed as a
! 2919: subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
! 2920: the output vector (ovecsize) is at least 2, the byte offset to the
! 2921: start of the the invalid UTF-8 character is placed in the first ele-
! 2922: ment, and a reason code is placed in the second element. The reason
1.1 misho 2923: codes are listed in the following section. For backward compatibility,
1.1.1.3 ! misho 2924: if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
! 2925: acter at the end of the subject (reason codes 1 to 5),
1.1 misho 2926: PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2927:
2928: PCRE_ERROR_BADUTF8_OFFSET (-11)
2929:
1.1.1.3 ! misho 2930: The UTF-8 byte sequence that was passed as a subject was checked and
! 2931: found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
! 2932: value of startoffset did not point to the beginning of a UTF-8 charac-
1.1 misho 2933: ter or the end of the subject.
2934:
2935: PCRE_ERROR_PARTIAL (-12)
2936:
1.1.1.3 ! misho 2937: The subject string did not match, but it did match partially. See the
1.1 misho 2938: pcrepartial documentation for details of partial matching.
2939:
2940: PCRE_ERROR_BADPARTIAL (-13)
2941:
1.1.1.3 ! misho 2942: This code is no longer in use. It was formerly returned when the
! 2943: PCRE_PARTIAL option was used with a compiled pattern containing items
! 2944: that were not supported for partial matching. From release 8.00
1.1 misho 2945: onwards, there are no restrictions on partial matching.
2946:
2947: PCRE_ERROR_INTERNAL (-14)
2948:
1.1.1.3 ! misho 2949: An unexpected internal error has occurred. This error could be caused
1.1 misho 2950: by a bug in PCRE or by overwriting of the compiled pattern.
2951:
2952: PCRE_ERROR_BADCOUNT (-15)
2953:
2954: This error is given if the value of the ovecsize argument is negative.
2955:
2956: PCRE_ERROR_RECURSIONLIMIT (-21)
2957:
2958: The internal recursion limit, as specified by the match_limit_recursion
1.1.1.3 ! misho 2959: field in a pcre_extra structure (or defaulted) was reached. See the
1.1 misho 2960: description above.
2961:
2962: PCRE_ERROR_BADNEWLINE (-23)
2963:
2964: An invalid combination of PCRE_NEWLINE_xxx options was given.
2965:
2966: PCRE_ERROR_BADOFFSET (-24)
2967:
2968: The value of startoffset was negative or greater than the length of the
2969: subject, that is, the value in length.
2970:
2971: PCRE_ERROR_SHORTUTF8 (-25)
2972:
1.1.1.3 ! misho 2973: This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
! 2974: string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
! 2975: option is set. Information about the failure is returned as for
! 2976: PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
! 2977: this special error code for PCRE_PARTIAL_HARD precedes the implementa-
! 2978: tion of returned information; it is retained for backwards compatibil-
1.1 misho 2979: ity.
2980:
2981: PCRE_ERROR_RECURSELOOP (-26)
2982:
2983: This error is returned when pcre_exec() detects a recursion loop within
1.1.1.3 ! misho 2984: the pattern. Specifically, it means that either the whole pattern or a
! 2985: subpattern has been called recursively for the second time at the same
1.1 misho 2986: position in the subject string. Some simple patterns that might do this
1.1.1.3 ! misho 2987: are detected and faulted at compile time, but more complicated cases,
1.1 misho 2988: in particular mutual recursions between two different subpatterns, can-
2989: not be detected until run time.
2990:
2991: PCRE_ERROR_JIT_STACKLIMIT (-27)
2992:
1.1.1.3 ! misho 2993: This error is returned when a pattern that was successfully studied
! 2994: using a JIT compile option is being matched, but the memory available
! 2995: for the just-in-time processing stack is not large enough. See the
! 2996: pcrejit documentation for more details.
1.1 misho 2997:
1.1.1.3 ! misho 2998: PCRE_ERROR_BADMODE (-28)
1.1.1.2 misho 2999:
3000: This error is given if a pattern that was compiled by the 8-bit library
3001: is passed to a 16-bit library function, or vice versa.
3002:
1.1.1.3 ! misho 3003: PCRE_ERROR_BADENDIANNESS (-29)
1.1.1.2 misho 3004:
1.1.1.3 ! misho 3005: This error is given if a pattern that was compiled and saved is
! 3006: reloaded on a host with different endianness. The utility function
1.1.1.2 misho 3007: pcre_pattern_to_host_byte_order() can be used to convert such a pattern
3008: so that it runs on the new host.
3009:
1.1.1.3 ! misho 3010: Error numbers -16 to -20, -22, and -30 are not used by pcre_exec().
1.1 misho 3011:
3012: Reason codes for invalid UTF-8 strings
3013:
1.1.1.3 ! misho 3014: This section applies only to the 8-bit library. The corresponding
1.1.1.2 misho 3015: information for the 16-bit library is given in the pcre16 page.
3016:
1.1 misho 3017: When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
1.1.1.3 ! misho 3018: UTF8, and the size of the output vector (ovecsize) is at least 2, the
! 3019: offset of the start of the invalid UTF-8 character is placed in the
1.1 misho 3020: first output vector element (ovector[0]) and a reason code is placed in
1.1.1.3 ! misho 3021: the second element (ovector[1]). The reason codes are given names in
1.1 misho 3022: the pcre.h header file:
3023:
3024: PCRE_UTF8_ERR1
3025: PCRE_UTF8_ERR2
3026: PCRE_UTF8_ERR3
3027: PCRE_UTF8_ERR4
3028: PCRE_UTF8_ERR5
3029:
1.1.1.3 ! misho 3030: The string ends with a truncated UTF-8 character; the code specifies
! 3031: how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
! 3032: characters to be no longer than 4 bytes, the encoding scheme (origi-
! 3033: nally defined by RFC 2279) allows for up to 6 bytes, and this is
1.1 misho 3034: checked first; hence the possibility of 4 or 5 missing bytes.
3035:
3036: PCRE_UTF8_ERR6
3037: PCRE_UTF8_ERR7
3038: PCRE_UTF8_ERR8
3039: PCRE_UTF8_ERR9
3040: PCRE_UTF8_ERR10
3041:
3042: The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
1.1.1.3 ! misho 3043: the character do not have the binary value 0b10 (that is, either the
1.1 misho 3044: most significant bit is 0, or the next bit is 1).
3045:
3046: PCRE_UTF8_ERR11
3047: PCRE_UTF8_ERR12
3048:
1.1.1.3 ! misho 3049: A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
1.1 misho 3050: long; these code points are excluded by RFC 3629.
3051:
3052: PCRE_UTF8_ERR13
3053:
1.1.1.3 ! misho 3054: A 4-byte character has a value greater than 0x10fff; these code points
1.1 misho 3055: are excluded by RFC 3629.
3056:
3057: PCRE_UTF8_ERR14
3058:
1.1.1.3 ! misho 3059: A 3-byte character has a value in the range 0xd800 to 0xdfff; this
! 3060: range of code points are reserved by RFC 3629 for use with UTF-16, and
1.1 misho 3061: so are excluded from UTF-8.
3062:
3063: PCRE_UTF8_ERR15
3064: PCRE_UTF8_ERR16
3065: PCRE_UTF8_ERR17
3066: PCRE_UTF8_ERR18
3067: PCRE_UTF8_ERR19
3068:
1.1.1.3 ! misho 3069: A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
! 3070: for a value that can be represented by fewer bytes, which is invalid.
! 3071: For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
1.1 misho 3072: rect coding uses just one byte.
3073:
3074: PCRE_UTF8_ERR20
3075:
3076: The two most significant bits of the first byte of a character have the
1.1.1.3 ! misho 3077: binary value 0b10 (that is, the most significant bit is 1 and the sec-
! 3078: ond is 0). Such a byte can only validly occur as the second or subse-
1.1 misho 3079: quent byte of a multi-byte character.
3080:
3081: PCRE_UTF8_ERR21
3082:
1.1.1.3 ! misho 3083: The first byte of a character has the value 0xfe or 0xff. These values
1.1 misho 3084: can never occur in a valid UTF-8 string.
3085:
3086:
3087: EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
3088:
3089: int pcre_copy_substring(const char *subject, int *ovector,
3090: int stringcount, int stringnumber, char *buffer,
3091: int buffersize);
3092:
3093: int pcre_get_substring(const char *subject, int *ovector,
3094: int stringcount, int stringnumber,
3095: const char **stringptr);
3096:
3097: int pcre_get_substring_list(const char *subject,
3098: int *ovector, int stringcount, const char ***listptr);
3099:
1.1.1.3 ! misho 3100: Captured substrings can be accessed directly by using the offsets
! 3101: returned by pcre_exec() in ovector. For convenience, the functions
1.1 misho 3102: pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
1.1.1.3 ! misho 3103: string_list() are provided for extracting captured substrings as new,
! 3104: separate, zero-terminated strings. These functions identify substrings
! 3105: by number. The next section describes functions for extracting named
1.1 misho 3106: substrings.
3107:
1.1.1.3 ! misho 3108: A substring that contains a binary zero is correctly extracted and has
! 3109: a further zero added on the end, but the result is not, of course, a C
! 3110: string. However, you can process such a string by referring to the
! 3111: length that is returned by pcre_copy_substring() and pcre_get_sub-
1.1 misho 3112: string(). Unfortunately, the interface to pcre_get_substring_list() is
1.1.1.3 ! misho 3113: not adequate for handling strings containing binary zeros, because the
1.1 misho 3114: end of the final string is not independently indicated.
3115:
1.1.1.3 ! misho 3116: The first three arguments are the same for all three of these func-
! 3117: tions: subject is the subject string that has just been successfully
1.1 misho 3118: matched, ovector is a pointer to the vector of integer offsets that was
3119: passed to pcre_exec(), and stringcount is the number of substrings that
1.1.1.3 ! misho 3120: were captured by the match, including the substring that matched the
1.1 misho 3121: entire regular expression. This is the value returned by pcre_exec() if
1.1.1.3 ! misho 3122: it is greater than zero. If pcre_exec() returned zero, indicating that
! 3123: it ran out of space in ovector, the value passed as stringcount should
1.1 misho 3124: be the number of elements in the vector divided by three.
3125:
1.1.1.3 ! misho 3126: The functions pcre_copy_substring() and pcre_get_substring() extract a
! 3127: single substring, whose number is given as stringnumber. A value of
! 3128: zero extracts the substring that matched the entire pattern, whereas
! 3129: higher values extract the captured substrings. For pcre_copy_sub-
! 3130: string(), the string is placed in buffer, whose length is given by
! 3131: buffersize, while for pcre_get_substring() a new block of memory is
! 3132: obtained via pcre_malloc, and its address is returned via stringptr.
! 3133: The yield of the function is the length of the string, not including
1.1 misho 3134: the terminating zero, or one of these error codes:
3135:
3136: PCRE_ERROR_NOMEMORY (-6)
3137:
1.1.1.3 ! misho 3138: The buffer was too small for pcre_copy_substring(), or the attempt to
1.1 misho 3139: get memory failed for pcre_get_substring().
3140:
3141: PCRE_ERROR_NOSUBSTRING (-7)
3142:
3143: There is no substring whose number is stringnumber.
3144:
1.1.1.3 ! misho 3145: The pcre_get_substring_list() function extracts all available sub-
! 3146: strings and builds a list of pointers to them. All this is done in a
1.1 misho 3147: single block of memory that is obtained via pcre_malloc. The address of
1.1.1.3 ! misho 3148: the memory block is returned via listptr, which is also the start of
! 3149: the list of string pointers. The end of the list is marked by a NULL
! 3150: pointer. The yield of the function is zero if all went well, or the
1.1 misho 3151: error code
3152:
3153: PCRE_ERROR_NOMEMORY (-6)
3154:
3155: if the attempt to get the memory block failed.
3156:
1.1.1.3 ! misho 3157: When any of these functions encounter a substring that is unset, which
! 3158: can happen when capturing subpattern number n+1 matches some part of
! 3159: the subject, but subpattern n has not been used at all, they return an
1.1 misho 3160: empty string. This can be distinguished from a genuine zero-length sub-
1.1.1.3 ! misho 3161: string by inspecting the appropriate offset in ovector, which is nega-
1.1 misho 3162: tive for unset substrings.
3163:
1.1.1.3 ! misho 3164: The two convenience functions pcre_free_substring() and pcre_free_sub-
! 3165: string_list() can be used to free the memory returned by a previous
1.1 misho 3166: call of pcre_get_substring() or pcre_get_substring_list(), respec-
1.1.1.3 ! misho 3167: tively. They do nothing more than call the function pointed to by
! 3168: pcre_free, which of course could be called directly from a C program.
! 3169: However, PCRE is used in some situations where it is linked via a spe-
! 3170: cial interface to another programming language that cannot use
! 3171: pcre_free directly; it is for these cases that the functions are pro-
1.1 misho 3172: vided.
3173:
3174:
3175: EXTRACTING CAPTURED SUBSTRINGS BY NAME
3176:
3177: int pcre_get_stringnumber(const pcre *code,
3178: const char *name);
3179:
3180: int pcre_copy_named_substring(const pcre *code,
3181: const char *subject, int *ovector,
3182: int stringcount, const char *stringname,
3183: char *buffer, int buffersize);
3184:
3185: int pcre_get_named_substring(const pcre *code,
3186: const char *subject, int *ovector,
3187: int stringcount, const char *stringname,
3188: const char **stringptr);
3189:
1.1.1.3 ! misho 3190: To extract a substring by name, you first have to find associated num-
1.1 misho 3191: ber. For example, for this pattern
3192:
3193: (a+)b(?<xxx>\d+)...
3194:
3195: the number of the subpattern called "xxx" is 2. If the name is known to
3196: be unique (PCRE_DUPNAMES was not set), you can find the number from the
3197: name by calling pcre_get_stringnumber(). The first argument is the com-
3198: piled pattern, and the second is the name. The yield of the function is
1.1.1.3 ! misho 3199: the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
1.1 misho 3200: subpattern of that name.
3201:
3202: Given the number, you can extract the substring directly, or use one of
3203: the functions described in the previous section. For convenience, there
3204: are also two functions that do the whole job.
3205:
1.1.1.3 ! misho 3206: Most of the arguments of pcre_copy_named_substring() and
! 3207: pcre_get_named_substring() are the same as those for the similarly
! 3208: named functions that extract by number. As these are described in the
! 3209: previous section, they are not re-described here. There are just two
1.1 misho 3210: differences:
3211:
1.1.1.3 ! misho 3212: First, instead of a substring number, a substring name is given. Sec-
1.1 misho 3213: ond, there is an extra argument, given at the start, which is a pointer
1.1.1.3 ! misho 3214: to the compiled pattern. This is needed in order to gain access to the
1.1 misho 3215: name-to-number translation table.
3216:
1.1.1.3 ! misho 3217: These functions call pcre_get_stringnumber(), and if it succeeds, they
! 3218: then call pcre_copy_substring() or pcre_get_substring(), as appropri-
! 3219: ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
1.1 misho 3220: behaviour may not be what you want (see the next section).
3221:
3222: Warning: If the pattern uses the (?| feature to set up multiple subpat-
1.1.1.3 ! misho 3223: terns with the same number, as described in the section on duplicate
! 3224: subpattern numbers in the pcrepattern page, you cannot use names to
! 3225: distinguish the different subpatterns, because names are not included
! 3226: in the compiled code. The matching process uses only numbers. For this
! 3227: reason, the use of different names for subpatterns of the same number
1.1 misho 3228: causes an error at compile time.
3229:
3230:
3231: DUPLICATE SUBPATTERN NAMES
3232:
3233: int pcre_get_stringtable_entries(const pcre *code,
3234: const char *name, char **first, char **last);
3235:
1.1.1.3 ! misho 3236: When a pattern is compiled with the PCRE_DUPNAMES option, names for
! 3237: subpatterns are not required to be unique. (Duplicate names are always
! 3238: allowed for subpatterns with the same number, created by using the (?|
! 3239: feature. Indeed, if such subpatterns are named, they are required to
1.1 misho 3240: use the same names.)
3241:
3242: Normally, patterns with duplicate names are such that in any one match,
1.1.1.3 ! misho 3243: only one of the named subpatterns participates. An example is shown in
1.1 misho 3244: the pcrepattern documentation.
3245:
1.1.1.3 ! misho 3246: When duplicates are present, pcre_copy_named_substring() and
! 3247: pcre_get_named_substring() return the first substring corresponding to
! 3248: the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
! 3249: (-7) is returned; no data is returned. The pcre_get_stringnumber()
! 3250: function returns one of the numbers that are associated with the name,
1.1 misho 3251: but it is not defined which it is.
3252:
1.1.1.3 ! misho 3253: If you want to get full details of all captured substrings for a given
! 3254: name, you must use the pcre_get_stringtable_entries() function. The
1.1 misho 3255: first argument is the compiled pattern, and the second is the name. The
1.1.1.3 ! misho 3256: third and fourth are pointers to variables which are updated by the
1.1 misho 3257: function. After it has run, they point to the first and last entries in
1.1.1.3 ! misho 3258: the name-to-number table for the given name. The function itself
! 3259: returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
! 3260: there are none. The format of the table is described above in the sec-
! 3261: tion entitled Information about a pattern above. Given all the rele-
! 3262: vant entries for the name, you can extract each of their numbers, and
1.1 misho 3263: hence the captured data, if any.
3264:
3265:
3266: FINDING ALL POSSIBLE MATCHES
3267:
1.1.1.3 ! misho 3268: The traditional matching function uses a similar algorithm to Perl,
1.1 misho 3269: which stops when it finds the first match, starting at a given point in
1.1.1.3 ! misho 3270: the subject. If you want to find all possible matches, or the longest
! 3271: possible match, consider using the alternative matching function (see
! 3272: below) instead. If you cannot use the alternative function, but still
! 3273: need to find all possible matches, you can kludge it up by making use
1.1 misho 3274: of the callout facility, which is described in the pcrecallout documen-
3275: tation.
3276:
3277: What you have to do is to insert a callout right at the end of the pat-
1.1.1.3 ! misho 3278: tern. When your callout function is called, extract and save the cur-
! 3279: rent matched substring. Then return 1, which forces pcre_exec() to
! 3280: backtrack and try other alternatives. Ultimately, when it runs out of
1.1 misho 3281: matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
3282:
3283:
1.1.1.2 misho 3284: OBTAINING AN ESTIMATE OF STACK USAGE
3285:
1.1.1.3 ! misho 3286: Matching certain patterns using pcre_exec() can use a lot of process
! 3287: stack, which in certain environments can be rather limited in size.
! 3288: Some users find it helpful to have an estimate of the amount of stack
! 3289: that is used by pcre_exec(), to help them set recursion limits, as
! 3290: described in the pcrestack documentation. The estimate that is output
1.1.1.2 misho 3291: by pcretest when called with the -m and -C options is obtained by call-
1.1.1.3 ! misho 3292: ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its
1.1.1.2 misho 3293: first five arguments.
3294:
1.1.1.3 ! misho 3295: Normally, if its first argument is NULL, pcre_exec() immediately
! 3296: returns the negative error code PCRE_ERROR_NULL, but with this special
! 3297: combination of arguments, it returns instead a negative number whose
! 3298: absolute value is the approximate stack frame size in bytes. (A nega-
! 3299: tive number is used so that it is clear that no match has happened.)
! 3300: The value is approximate because in some cases, recursive calls to
1.1.1.2 misho 3301: pcre_exec() occur when there are one or two additional variables on the
3302: stack.
3303:
1.1.1.3 ! misho 3304: If PCRE has been compiled to use the heap instead of the stack for
! 3305: recursion, the value returned is the size of each block that is
1.1.1.2 misho 3306: obtained from the heap.
3307:
3308:
1.1 misho 3309: MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
3310:
3311: int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
3312: const char *subject, int length, int startoffset,
3313: int options, int *ovector, int ovecsize,
3314: int *workspace, int wscount);
3315:
1.1.1.3 ! misho 3316: The function pcre_dfa_exec() is called to match a subject string
! 3317: against a compiled pattern, using a matching algorithm that scans the
! 3318: subject string just once, and does not backtrack. This has different
! 3319: characteristics to the normal algorithm, and is not compatible with
! 3320: Perl. Some of the features of PCRE patterns are not supported. Never-
! 3321: theless, there are times when this kind of matching can be useful. For
! 3322: a discussion of the two matching algorithms, and a list of features
! 3323: that pcre_dfa_exec() does not support, see the pcrematching documenta-
1.1 misho 3324: tion.
3325:
1.1.1.3 ! misho 3326: The arguments for the pcre_dfa_exec() function are the same as for
1.1 misho 3327: pcre_exec(), plus two extras. The ovector argument is used in a differ-
1.1.1.3 ! misho 3328: ent way, and this is described below. The other common arguments are
! 3329: used in the same way as for pcre_exec(), so their description is not
1.1 misho 3330: repeated here.
3331:
1.1.1.3 ! misho 3332: The two additional arguments provide workspace for the function. The
! 3333: workspace vector should contain at least 20 elements. It is used for
1.1 misho 3334: keeping track of multiple paths through the pattern tree. More
1.1.1.3 ! misho 3335: workspace will be needed for patterns and subjects where there are a
1.1 misho 3336: lot of potential matches.
3337:
3338: Here is an example of a simple call to pcre_dfa_exec():
3339:
3340: int rc;
3341: int ovector[10];
3342: int wspace[20];
3343: rc = pcre_dfa_exec(
3344: re, /* result of pcre_compile() */
3345: NULL, /* we didn't study the pattern */
3346: "some string", /* the subject string */
3347: 11, /* the length of the subject string */
3348: 0, /* start at offset 0 in the subject */
3349: 0, /* default options */
3350: ovector, /* vector of integers for substring information */
3351: 10, /* number of elements (NOT size in bytes) */
3352: wspace, /* working space vector */
3353: 20); /* number of elements (NOT size in bytes) */
3354:
3355: Option bits for pcre_dfa_exec()
3356:
1.1.1.3 ! misho 3357: The unused bits of the options argument for pcre_dfa_exec() must be
! 3358: zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
1.1 misho 3359: LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
1.1.1.3 ! misho 3360: PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF,
! 3361: PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
! 3362: TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
! 3363: four of these are exactly the same as for pcre_exec(), so their
1.1 misho 3364: description is not repeated here.
3365:
3366: PCRE_PARTIAL_HARD
3367: PCRE_PARTIAL_SOFT
3368:
1.1.1.3 ! misho 3369: These have the same general effect as they do for pcre_exec(), but the
! 3370: details are slightly different. When PCRE_PARTIAL_HARD is set for
! 3371: pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
! 3372: ject is reached and there is still at least one matching possibility
1.1 misho 3373: that requires additional characters. This happens even if some complete
3374: matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
3375: code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
1.1.1.3 ! misho 3376: of the subject is reached, there have been no complete matches, but
! 3377: there is still at least one matching possibility. The portion of the
! 3378: string that was inspected when the longest partial match was found is
! 3379: set as the first matching string in both cases. There is a more
! 3380: detailed discussion of partial and multi-segment matching, with exam-
1.1 misho 3381: ples, in the pcrepartial documentation.
3382:
3383: PCRE_DFA_SHORTEST
3384:
1.1.1.3 ! misho 3385: Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
1.1 misho 3386: stop as soon as it has found one match. Because of the way the alterna-
1.1.1.3 ! misho 3387: tive algorithm works, this is necessarily the shortest possible match
1.1 misho 3388: at the first possible matching point in the subject string.
3389:
3390: PCRE_DFA_RESTART
3391:
3392: When pcre_dfa_exec() returns a partial match, it is possible to call it
1.1.1.3 ! misho 3393: again, with additional subject characters, and have it continue with
! 3394: the same match. The PCRE_DFA_RESTART option requests this action; when
! 3395: it is set, the workspace and wscount options must reference the same
! 3396: vector as before because data about the match so far is left in them
1.1 misho 3397: after a partial match. There is more discussion of this facility in the
3398: pcrepartial documentation.
3399:
3400: Successful returns from pcre_dfa_exec()
3401:
1.1.1.3 ! misho 3402: When pcre_dfa_exec() succeeds, it may have matched more than one sub-
1.1 misho 3403: string in the subject. Note, however, that all the matches from one run
1.1.1.3 ! misho 3404: of the function start at the same point in the subject. The shorter
! 3405: matches are all initial substrings of the longer matches. For example,
1.1 misho 3406: if the pattern
3407:
3408: <.*>
3409:
3410: is matched against the string
3411:
3412: This is <something> <something else> <something further> no more
3413:
3414: the three matched strings are
3415:
3416: <something>
3417: <something> <something else>
3418: <something> <something else> <something further>
3419:
1.1.1.3 ! misho 3420: On success, the yield of the function is a number greater than zero,
! 3421: which is the number of matched substrings. The substrings themselves
! 3422: are returned in ovector. Each string uses two elements; the first is
! 3423: the offset to the start, and the second is the offset to the end. In
! 3424: fact, all the strings have the same start offset. (Space could have
! 3425: been saved by giving this only once, but it was decided to retain some
! 3426: compatibility with the way pcre_exec() returns data, even though the
1.1 misho 3427: meaning of the strings is different.)
3428:
3429: The strings are returned in reverse order of length; that is, the long-
1.1.1.3 ! misho 3430: est matching string is given first. If there were too many matches to
! 3431: fit into ovector, the yield of the function is zero, and the vector is
! 3432: filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
1.1 misho 3433: can use the entire ovector for returning matched strings.
3434:
3435: Error returns from pcre_dfa_exec()
3436:
1.1.1.3 ! misho 3437: The pcre_dfa_exec() function returns a negative number when it fails.
! 3438: Many of the errors are the same as for pcre_exec(), and these are
! 3439: described above. There are in addition the following errors that are
1.1 misho 3440: specific to pcre_dfa_exec():
3441:
3442: PCRE_ERROR_DFA_UITEM (-16)
3443:
1.1.1.3 ! misho 3444: This return is given if pcre_dfa_exec() encounters an item in the pat-
! 3445: tern that it does not support, for instance, the use of \C or a back
1.1 misho 3446: reference.
3447:
3448: PCRE_ERROR_DFA_UCOND (-17)
3449:
1.1.1.3 ! misho 3450: This return is given if pcre_dfa_exec() encounters a condition item
! 3451: that uses a back reference for the condition, or a test for recursion
1.1 misho 3452: in a specific group. These are not supported.
3453:
3454: PCRE_ERROR_DFA_UMLIMIT (-18)
3455:
1.1.1.3 ! misho 3456: This return is given if pcre_dfa_exec() is called with an extra block
! 3457: that contains a setting of the match_limit or match_limit_recursion
! 3458: fields. This is not supported (these fields are meaningless for DFA
1.1 misho 3459: matching).
3460:
3461: PCRE_ERROR_DFA_WSSIZE (-19)
3462:
1.1.1.3 ! misho 3463: This return is given if pcre_dfa_exec() runs out of space in the
1.1 misho 3464: workspace vector.
3465:
3466: PCRE_ERROR_DFA_RECURSE (-20)
3467:
1.1.1.3 ! misho 3468: When a recursive subpattern is processed, the matching function calls
! 3469: itself recursively, using private vectors for ovector and workspace.
! 3470: This error is given if the output vector is not large enough. This
1.1 misho 3471: should be extremely rare, as a vector of size 1000 is used.
3472:
1.1.1.3 ! misho 3473: PCRE_ERROR_DFA_BADRESTART (-30)
! 3474:
! 3475: When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some
! 3476: plausibility checks are made on the contents of the workspace, which
! 3477: should contain data about the previous partial match. If any of these
! 3478: checks fail, this error is given.
! 3479:
1.1 misho 3480:
3481: SEE ALSO
3482:
1.1.1.2 misho 3483: pcre16(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematch-
3484: ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3),
3485: pcrestack(3).
1.1 misho 3486:
3487:
3488: AUTHOR
3489:
3490: Philip Hazel
3491: University Computing Service
3492: Cambridge CB2 3QH, England.
3493:
3494:
3495: REVISION
3496:
1.1.1.3 ! misho 3497: Last updated: 17 June 2012
1.1.1.2 misho 3498: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 3499: ------------------------------------------------------------------------------
3500:
3501:
3502: PCRECALLOUT(3) PCRECALLOUT(3)
3503:
3504:
3505: NAME
3506: PCRE - Perl-compatible regular expressions
3507:
3508:
3509: PCRE CALLOUTS
3510:
3511: int (*pcre_callout)(pcre_callout_block *);
3512:
1.1.1.2 misho 3513: int (*pcre16_callout)(pcre16_callout_block *);
3514:
1.1 misho 3515: PCRE provides a feature called "callout", which is a means of temporar-
3516: ily passing control to the caller of PCRE in the middle of pattern
3517: matching. The caller of PCRE provides an external function by putting
1.1.1.2 misho 3518: its entry point in the global variable pcre_callout (pcre16_callout for
3519: the 16-bit library). By default, this variable contains NULL, which
3520: disables all calling out.
1.1 misho 3521:
1.1.1.2 misho 3522: Within a regular expression, (?C) indicates the points at which the
3523: external function is to be called. Different callout points can be
3524: identified by putting a number less than 256 after the letter C. The
3525: default value is zero. For example, this pattern has two callout
1.1 misho 3526: points:
3527:
3528: (?C1)abc(?C2)def
3529:
1.1.1.2 misho 3530: If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled,
3531: PCRE automatically inserts callouts, all with number 255, before each
3532: item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
3533: pattern
1.1 misho 3534:
3535: A(\d{2}|--)
3536:
3537: it is processed as if it were
3538:
3539: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3540:
1.1.1.2 misho 3541: Notice that there is a callout before and after each parenthesis and
3542: alternation bar. Automatic callouts can be used for tracking the
3543: progress of pattern matching. The pcretest command has an option that
3544: sets automatic callouts; when it is used, the output indicates how the
3545: pattern is matched. This is useful information when you are trying to
1.1 misho 3546: optimize the performance of a particular pattern.
3547:
1.1.1.2 misho 3548: The use of callouts in a pattern makes it ineligible for optimization
1.1 misho 3549: by the just-in-time compiler. Studying such a pattern with the
3550: PCRE_STUDY_JIT_COMPILE option always fails.
3551:
3552:
3553: MISSING CALLOUTS
3554:
1.1.1.2 misho 3555: You should be aware that, because of optimizations in the way PCRE
3556: matches patterns by default, callouts sometimes do not happen. For
1.1 misho 3557: example, if the pattern is
3558:
3559: ab(?C4)cd
3560:
3561: PCRE knows that any matching string must contain the letter "d". If the
1.1.1.2 misho 3562: subject string is "abyz", the lack of "d" means that matching doesn't
3563: ever start, and the callout is never reached. However, with "abyd",
1.1 misho 3564: though the result is still no match, the callout is obeyed.
3565:
1.1.1.2 misho 3566: If the pattern is studied, PCRE knows the minimum length of a matching
3567: string, and will immediately give a "no match" return without actually
3568: running a match if the subject is not long enough, or, for unanchored
1.1 misho 3569: patterns, if it has been scanned far enough.
3570:
1.1.1.2 misho 3571: You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3572: MIZE option to the matching function, or by starting the pattern with
3573: (*NO_START_OPT). This slows down the matching process, but does ensure
3574: that callouts such as the example above are obeyed.
1.1 misho 3575:
3576:
3577: THE CALLOUT INTERFACE
3578:
3579: During matching, when PCRE reaches a callout point, the external func-
1.1.1.2 misho 3580: tion defined by pcre_callout or pcre16_callout is called (if it is
3581: set). This applies to both normal and DFA matching. The only argument
3582: to the callout function is a pointer to a pcre_callout or pcre16_call-
3583: out block. These structures contains the following fields:
3584:
3585: int version;
3586: int callout_number;
3587: int *offset_vector;
3588: const char *subject; (8-bit version)
3589: PCRE_SPTR16 subject; (16-bit version)
3590: int subject_length;
3591: int start_match;
3592: int current_position;
3593: int capture_top;
3594: int capture_last;
3595: void *callout_data;
3596: int pattern_position;
3597: int next_item_length;
3598: const unsigned char *mark; (8-bit version)
3599: const PCRE_UCHAR16 *mark; (16-bit version)
1.1 misho 3600:
3601: The version field is an integer containing the version number of the
3602: block format. The initial version was 0; the current version is 2. The
3603: version number will change again in future if additional fields are
3604: added, but the intention is never to remove any of the existing fields.
3605:
3606: The callout_number field contains the number of the callout, as com-
3607: piled into the pattern (that is, the number after ?C for manual call-
3608: outs, and 255 for automatically generated callouts).
3609:
3610: The offset_vector field is a pointer to the vector of offsets that was
1.1.1.2 misho 3611: passed by the caller to the matching function. When pcre_exec() or
3612: pcre16_exec() is used, the contents can be inspected, in order to
3613: extract substrings that have been matched so far, in the same way as
3614: for extracting substrings after a match has completed. For the DFA
3615: matching functions, this field is not useful.
1.1 misho 3616:
3617: The subject and subject_length fields contain copies of the values that
1.1.1.2 misho 3618: were passed to the matching function.
1.1 misho 3619:
3620: The start_match field normally contains the offset within the subject
3621: at which the current match attempt started. However, if the escape
3622: sequence \K has been encountered, this value is changed to reflect the
3623: modified starting point. If the pattern is not anchored, the callout
3624: function may be called several times from the same point in the pattern
3625: for different starting points in the subject.
3626:
3627: The current_position field contains the offset within the subject of
3628: the current match pointer.
3629:
1.1.1.2 misho 3630: When the pcre_exec() or pcre16_exec() is used, the capture_top field
3631: contains one more than the number of the highest numbered captured sub-
3632: string so far. If no substrings have been captured, the value of cap-
3633: ture_top is one. This is always the case when the DFA functions are
3634: used, because they do not support captured substrings.
1.1 misho 3635:
3636: The capture_last field contains the number of the most recently cap-
3637: tured substring. If no substrings have been captured, its value is -1.
1.1.1.2 misho 3638: This is always the case for the DFA matching functions.
1.1 misho 3639:
1.1.1.2 misho 3640: The callout_data field contains a value that is passed to a matching
3641: function specifically so that it can be passed back in callouts. It is
3642: passed in the callout_data field of a pcre_extra or pcre16_extra data
1.1 misho 3643: structure. If no such data was passed, the value of callout_data in a
1.1.1.2 misho 3644: callout block is NULL. There is a description of the pcre_extra struc-
3645: ture in the pcreapi documentation.
1.1 misho 3646:
1.1.1.2 misho 3647: The pattern_position field is present from version 1 of the callout
3648: structure. It contains the offset to the next item to be matched in the
3649: pattern string.
3650:
3651: The next_item_length field is present from version 1 of the callout
3652: structure. It contains the length of the next item to be matched in the
3653: pattern string. When the callout immediately precedes an alternation
3654: bar, a closing parenthesis, or the end of the pattern, the length is
3655: zero. When the callout precedes an opening parenthesis, the length is
3656: that of the entire subpattern.
1.1 misho 3657:
3658: The pattern_position and next_item_length fields are intended to help
3659: in distinguishing between different automatic callouts, which all have
3660: the same callout number. However, they are set for all callouts.
3661:
1.1.1.2 misho 3662: The mark field is present from version 2 of the callout structure. In
3663: callouts from pcre_exec() or pcre16_exec() it contains a pointer to the
3664: zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
3665: (*THEN) item in the match, or NULL if no such items have been passed.
3666: Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
3667: previous (*MARK). In callouts from the DFA matching functions this
3668: field always contains NULL.
1.1 misho 3669:
3670:
3671: RETURN VALUES
3672:
3673: The external callout function returns an integer to PCRE. If the value
3674: is zero, matching proceeds as normal. If the value is greater than
3675: zero, matching fails at the current point, but the testing of other
3676: matching possibilities goes ahead, just as if a lookahead assertion had
1.1.1.2 misho 3677: failed. If the value is less than zero, the match is abandoned, the
3678: matching function returns the negative value.
1.1 misho 3679:
3680: Negative values should normally be chosen from the set of
3681: PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3682: dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3683: reserved for use by callout functions; it will never be used by PCRE
3684: itself.
3685:
3686:
3687: AUTHOR
3688:
3689: Philip Hazel
3690: University Computing Service
3691: Cambridge CB2 3QH, England.
3692:
3693:
3694: REVISION
3695:
1.1.1.2 misho 3696: Last updated: 08 Janurary 2012
3697: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 3698: ------------------------------------------------------------------------------
3699:
3700:
3701: PCRECOMPAT(3) PCRECOMPAT(3)
3702:
3703:
3704: NAME
3705: PCRE - Perl-compatible regular expressions
3706:
3707:
3708: DIFFERENCES BETWEEN PCRE AND PERL
3709:
3710: This document describes the differences in the ways that PCRE and Perl
3711: handle regular expressions. The differences described here are with
3712: respect to Perl versions 5.10 and above.
3713:
1.1.1.2 misho 3714: 1. PCRE has only a subset of Perl's Unicode support. Details of what it
3715: does have are given in the pcreunicode page.
1.1 misho 3716:
3717: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3718: they do not mean what you might think. For example, (?!a){3} does not
3719: assert that the next three characters are not "a". It just asserts that
3720: the next character is not "a" three times (in principle: PCRE optimizes
3721: this to run the assertion just once). Perl allows repeat quantifiers on
3722: other assertions such as \b, but these do not seem to have any use.
3723:
3724: 3. Capturing subpatterns that occur inside negative lookahead asser-
3725: tions are counted, but their entries in the offsets vector are never
3726: set. Perl sets its numerical variables from any such patterns that are
3727: matched before the assertion fails to match something (thereby succeed-
3728: ing), but only if the negative lookahead assertion contains just one
3729: branch.
3730:
3731: 4. Though binary zero characters are supported in the subject string,
3732: they are not allowed in a pattern string because it is passed as a nor-
3733: mal C string, terminated by zero. The escape sequence \0 can be used in
3734: the pattern to represent a binary zero.
3735:
3736: 5. The following Perl escape sequences are not supported: \l, \u, \L,
3737: \U, and \N when followed by a character name or Unicode value. (\N on
3738: its own, matching a non-newline character, is supported.) In fact these
3739: are implemented by Perl's general string-handling and are not part of
3740: its pattern matching engine. If any of these are encountered by PCRE,
3741: an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
3742: PAT option is set, \U and \u are interpreted as JavaScript interprets
3743: them.
3744:
3745: 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3746: is built with Unicode character property support. The properties that
3747: can be tested with \p and \P are limited to the general category prop-
3748: erties such as Lu and Nd, script names such as Greek or Han, and the
3749: derived properties Any and L&. PCRE does support the Cs (surrogate)
3750: property, which Perl does not; the Perl documentation says "Because
3751: Perl hides the need for the user to understand the internal representa-
3752: tion of Unicode characters, there is no need to implement the somewhat
3753: messy concept of surrogates."
3754:
3755: 7. PCRE implements a simpler version of \X than Perl, which changed to
3756: make \X match what Unicode calls an "extended grapheme cluster". This
3757: is more complicated than an extended Unicode sequence, which is what
3758: PCRE matches.
3759:
3760: 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3761: ters in between are treated as literals. This is slightly different
3762: from Perl in that $ and @ are also handled as literals inside the
3763: quotes. In Perl, they cause variable interpolation (but of course PCRE
3764: does not have variables). Note the following examples:
3765:
3766: Pattern PCRE matches Perl matches
3767:
3768: \Qabc$xyz\E abc$xyz abc followed by the
3769: contents of $xyz
3770: \Qabc\$xyz\E abc\$xyz abc\$xyz
3771: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3772:
3773: The \Q...\E sequence is recognized both inside and outside character
3774: classes.
3775:
3776: 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3777: constructions. However, there is support for recursive patterns. This
3778: is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3779: "callout" feature allows an external function to be called during pat-
3780: tern matching. See the pcrecallout documentation for details.
3781:
3782: 10. Subpatterns that are called as subroutines (whether or not recur-
3783: sively) are always treated as atomic groups in PCRE. This is like
3784: Python, but unlike Perl. Captured values that are set outside a sub-
3785: routine call can be reference from inside in PCRE, but not in Perl.
3786: There is a discussion that explains these differences in more detail in
3787: the section on recursion differences from Perl in the pcrepattern page.
3788:
1.1.1.3 ! misho 3789: 11. If any of the backtracking control verbs are used in an assertion
! 3790: or in a subpattern that is called as a subroutine (whether or not
! 3791: recursively), their effect is confined to that subpattern; it does not
! 3792: extend to the surrounding pattern. This is not always the case in Perl.
! 3793: In particular, if (*THEN) is present in a group that is called as a
! 3794: subroutine, its action is limited to that group, even if the group does
! 3795: not contain any | characters. There is one exception to this: the name
! 3796: from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a success-
! 3797: ful positive assertion is passed back when a match succeeds (compare
! 3798: capturing parentheses in assertions). Note that such subpatterns are
! 3799: processed as anchored at the point where they are tested.
1.1 misho 3800:
3801: 12. There are some differences that are concerned with the settings of
3802: captured strings when part of a pattern is repeated. For example,
3803: matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3804: unset, but in PCRE it is set to "b".
3805:
3806: 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3807: pattern names is not as general as Perl's. This is a consequence of the
3808: fact the PCRE works internally just with numbers, using an external ta-
3809: ble to translate between numbers and names. In particular, a pattern
3810: such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3811: the same number but different names, is not supported, and causes an
3812: error at compile time. If it were allowed, it would not be possible to
3813: distinguish which parentheses matched, because both names map to cap-
3814: turing subpattern number 1. To avoid this confusing situation, an error
3815: is given at compile time.
3816:
3817: 14. Perl recognizes comments in some places that PCRE does not, for
3818: example, between the ( and ? at the start of a subpattern. If the /x
1.1.1.3 ! misho 3819: modifier is set, Perl allows white space between ( and ? but PCRE never
1.1 misho 3820: does, even if the PCRE_EXTENDED option is set.
3821:
3822: 15. PCRE provides some extensions to the Perl regular expression facil-
3823: ities. Perl 5.10 includes new features that are not in earlier ver-
3824: sions of Perl, some of which (such as named parentheses) have been in
3825: PCRE for some time. This list is with respect to Perl 5.10:
3826:
3827: (a) Although lookbehind assertions in PCRE must match fixed length
3828: strings, each alternative branch of a lookbehind assertion can match a
3829: different length of string. Perl requires them all to have the same
3830: length.
3831:
3832: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3833: meta-character matches only at the very end of the string.
3834:
3835: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3836: cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3837: ignored. (Perl can be made to issue a warning.)
3838:
3839: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3840: fiers is inverted, that is, by default they are not greedy, but if fol-
3841: lowed by a question mark they are.
3842:
3843: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3844: tried only at the first matching position in the subject string.
3845:
3846: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
3847: and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3848: lents.
3849:
3850: (g) The \R escape sequence can be restricted to match only CR, LF, or
3851: CRLF by the PCRE_BSR_ANYCRLF option.
3852:
3853: (h) The callout facility is PCRE-specific.
3854:
3855: (i) The partial matching facility is PCRE-specific.
3856:
3857: (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3858: even on different hosts that have the other endianness. However, this
3859: does not apply to optimized data created by the just-in-time compiler.
3860:
1.1.1.2 misho 3861: (k) The alternative matching functions (pcre_dfa_exec() and
3862: pcre16_dfa_exec()) match in a different way and are not Perl-compati-
3863: ble.
1.1 misho 3864:
1.1.1.2 misho 3865: (l) PCRE recognizes some special sequences such as (*CR) at the start
1.1 misho 3866: of a pattern that set overall options that cannot be changed within the
3867: pattern.
3868:
3869:
3870: AUTHOR
3871:
3872: Philip Hazel
3873: University Computing Service
3874: Cambridge CB2 3QH, England.
3875:
3876:
3877: REVISION
3878:
1.1.1.3 ! misho 3879: Last updated: 01 June 2012
1.1.1.2 misho 3880: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 3881: ------------------------------------------------------------------------------
3882:
3883:
3884: PCREPATTERN(3) PCREPATTERN(3)
3885:
3886:
3887: NAME
3888: PCRE - Perl-compatible regular expressions
3889:
3890:
3891: PCRE REGULAR EXPRESSION DETAILS
3892:
3893: The syntax and semantics of the regular expressions that are supported
3894: by PCRE are described in detail below. There is a quick-reference syn-
3895: tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3896: semantics as closely as it can. PCRE also supports some alternative
3897: regular expression syntax (which does not conflict with the Perl syn-
3898: tax) in order to provide some compatibility with regular expressions in
3899: Python, .NET, and Oniguruma.
3900:
3901: Perl's regular expressions are described in its own documentation, and
3902: regular expressions in general are covered in a number of books, some
3903: of which have copious examples. Jeffrey Friedl's "Mastering Regular
3904: Expressions", published by O'Reilly, covers regular expressions in
3905: great detail. This description of PCRE's regular expressions is
3906: intended as reference material.
3907:
3908: The original operation of PCRE was on strings of one-byte characters.
1.1.1.2 misho 3909: However, there is now also support for UTF-8 strings in the original
3910: library, and a second library that supports 16-bit and UTF-16 character
3911: strings. To use these features, PCRE must be built to include appropri-
3912: ate support. When using UTF strings you must either call the compiling
3913: function with the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
3914: start with one of these special sequences:
1.1 misho 3915:
3916: (*UTF8)
1.1.1.2 misho 3917: (*UTF16)
1.1 misho 3918:
1.1.1.2 misho 3919: Starting a pattern with such a sequence is equivalent to setting the
3920: relevant option. This feature is not Perl-compatible. How setting a UTF
3921: mode affects pattern matching is mentioned in several places below.
3922: There is also a summary of features in the pcreunicode page.
1.1 misho 3923:
1.1.1.2 misho 3924: Another special sequence that may appear at the start of a pattern or
3925: in combination with (*UTF8) or (*UTF16) is:
1.1 misho 3926:
3927: (*UCP)
3928:
1.1.1.2 misho 3929: This has the same effect as setting the PCRE_UCP option: it causes
3930: sequences such as \d and \w to use Unicode properties to determine
1.1 misho 3931: character types, instead of recognizing only characters with codes less
3932: than 128 via a lookup table.
3933:
1.1.1.2 misho 3934: If a pattern starts with (*NO_START_OPT), it has the same effect as
1.1 misho 3935: setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3936: time. There are also some more of these special sequences that are con-
3937: cerned with the handling of newlines; they are described below.
3938:
1.1.1.2 misho 3939: The remainder of this document discusses the patterns that are sup-
3940: ported by PCRE when one its main matching functions, pcre_exec()
3941: (8-bit) or pcre16_exec() (16-bit), is used. PCRE also has alternative
3942: matching functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
3943: using a different algorithm that is not Perl-compatible. Some of the
3944: features discussed below are not available when DFA matching is used.
3945: The advantages and disadvantages of the alternative functions, and how
3946: they differ from the normal functions, are discussed in the pcrematch-
3947: ing page.
1.1 misho 3948:
3949:
3950: NEWLINE CONVENTIONS
3951:
3952: PCRE supports five different conventions for indicating line breaks in
3953: strings: a single CR (carriage return) character, a single LF (line-
3954: feed) character, the two-character sequence CRLF, any of the three pre-
3955: ceding, or any Unicode newline sequence. The pcreapi page has further
3956: discussion about newlines, and shows how to set the newline convention
3957: in the options arguments for the compiling and matching functions.
3958:
3959: It is also possible to specify a newline convention by starting a pat-
3960: tern string with one of the following five sequences:
3961:
3962: (*CR) carriage return
3963: (*LF) linefeed
3964: (*CRLF) carriage return, followed by linefeed
3965: (*ANYCRLF) any of the three above
3966: (*ANY) all Unicode newline sequences
3967:
1.1.1.2 misho 3968: These override the default and the options given to the compiling func-
3969: tion. For example, on a Unix system where LF is the default newline
3970: sequence, the pattern
1.1 misho 3971:
3972: (*CR)a.b
3973:
3974: changes the convention to CR. That pattern matches "a\nb" because LF is
3975: no longer a newline. Note that these special settings, which are not
3976: Perl-compatible, are recognized only at the very start of a pattern,
3977: and that they must be in upper case. If more than one of them is
3978: present, the last one is used.
3979:
3980: The newline convention affects the interpretation of the dot metachar-
3981: acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3982: ever, it does not affect what the \R escape sequence matches. By
3983: default, this is any Unicode newline sequence, for Perl compatibility.
3984: However, this can be changed; see the description of \R in the section
3985: entitled "Newline sequences" below. A change of \R setting can be com-
3986: bined with a change of newline convention.
3987:
3988:
3989: CHARACTERS AND METACHARACTERS
3990:
3991: A regular expression is a pattern that is matched against a subject
3992: string from left to right. Most characters stand for themselves in a
3993: pattern, and match the corresponding characters in the subject. As a
3994: trivial example, the pattern
3995:
3996: The quick brown fox
3997:
3998: matches a portion of a subject string that is identical to itself. When
3999: caseless matching is specified (the PCRE_CASELESS option), letters are
1.1.1.2 misho 4000: matched independently of case. In a UTF mode, PCRE always understands
1.1 misho 4001: the concept of case for characters whose values are less than 128, so
4002: caseless matching is always possible. For characters with higher val-
4003: ues, the concept of case is supported if PCRE is compiled with Unicode
4004: property support, but not otherwise. If you want to use caseless
4005: matching for characters 128 and above, you must ensure that PCRE is
1.1.1.2 misho 4006: compiled with Unicode property support as well as with UTF support.
1.1 misho 4007:
4008: The power of regular expressions comes from the ability to include
4009: alternatives and repetitions in the pattern. These are encoded in the
4010: pattern by the use of metacharacters, which do not stand for themselves
4011: but instead are interpreted in some special way.
4012:
4013: There are two different sets of metacharacters: those that are recog-
4014: nized anywhere in the pattern except within square brackets, and those
4015: that are recognized within square brackets. Outside square brackets,
4016: the metacharacters are as follows:
4017:
4018: \ general escape character with several uses
4019: ^ assert start of string (or line, in multiline mode)
4020: $ assert end of string (or line, in multiline mode)
4021: . match any character except newline (by default)
4022: [ start character class definition
4023: | start of alternative branch
4024: ( start subpattern
4025: ) end subpattern
4026: ? extends the meaning of (
4027: also 0 or 1 quantifier
4028: also quantifier minimizer
4029: * 0 or more quantifier
4030: + 1 or more quantifier
4031: also "possessive quantifier"
4032: { start min/max quantifier
4033:
4034: Part of a pattern that is in square brackets is called a "character
4035: class". In a character class the only metacharacters are:
4036:
4037: \ general escape character
4038: ^ negate the class, but only if the first character
4039: - indicates character range
4040: [ POSIX character class (only if followed by POSIX
4041: syntax)
4042: ] terminates the character class
4043:
4044: The following sections describe the use of each of the metacharacters.
4045:
4046:
4047: BACKSLASH
4048:
4049: The backslash character has several uses. Firstly, if it is followed by
4050: a character that is not a number or a letter, it takes away any special
4051: meaning that character may have. This use of backslash as an escape
4052: character applies both inside and outside character classes.
4053:
4054: For example, if you want to match a * character, you write \* in the
4055: pattern. This escaping action applies whether or not the following
4056: character would otherwise be interpreted as a metacharacter, so it is
4057: always safe to precede a non-alphanumeric with backslash to specify
4058: that it stands for itself. In particular, if you want to match a back-
4059: slash, you write \\.
4060:
1.1.1.2 misho 4061: In a UTF mode, only ASCII numbers and letters have any special meaning
1.1 misho 4062: after a backslash. All other characters (in particular, those whose
4063: codepoints are greater than 127) are treated as literals.
4064:
1.1.1.3 ! misho 4065: If a pattern is compiled with the PCRE_EXTENDED option, white space in
1.1 misho 4066: the pattern (other than in a character class) and characters between a
4067: # outside a character class and the next newline are ignored. An escap-
1.1.1.3 ! misho 4068: ing backslash can be used to include a white space or # character as
1.1 misho 4069: part of the pattern.
4070:
4071: If you want to remove the special meaning from a sequence of charac-
4072: ters, you can do so by putting them between \Q and \E. This is differ-
4073: ent from Perl in that $ and @ are handled as literals in \Q...\E
4074: sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
4075: tion. Note the following examples:
4076:
4077: Pattern PCRE matches Perl matches
4078:
4079: \Qabc$xyz\E abc$xyz abc followed by the
4080: contents of $xyz
4081: \Qabc\$xyz\E abc\$xyz abc\$xyz
4082: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
4083:
4084: The \Q...\E sequence is recognized both inside and outside character
4085: classes. An isolated \E that is not preceded by \Q is ignored. If \Q
4086: is not followed by \E later in the pattern, the literal interpretation
4087: continues to the end of the pattern (that is, \E is assumed at the
4088: end). If the isolated \Q is inside a character class, this causes an
4089: error, because the character class is not terminated.
4090:
4091: Non-printing characters
4092:
4093: A second use of backslash provides a way of encoding non-printing char-
4094: acters in patterns in a visible manner. There is no restriction on the
4095: appearance of non-printing characters, apart from the binary zero that
4096: terminates a pattern, but when a pattern is being prepared by text
4097: editing, it is often easier to use one of the following escape
4098: sequences than the binary character it represents:
4099:
4100: \a alarm, that is, the BEL character (hex 07)
4101: \cx "control-x", where x is any ASCII character
4102: \e escape (hex 1B)
1.1.1.3 ! misho 4103: \f form feed (hex 0C)
1.1 misho 4104: \n linefeed (hex 0A)
4105: \r carriage return (hex 0D)
4106: \t tab (hex 09)
4107: \ddd character with octal code ddd, or back reference
4108: \xhh character with hex code hh
4109: \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
4110: \uhhhh character with hex code hhhh (JavaScript mode only)
4111:
4112: The precise effect of \cx is as follows: if x is a lower case letter,
4113: it is converted to upper case. Then bit 6 of the character (hex 40) is
4114: inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
4115: is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
4116: has a value greater than 127, a compile-time error occurs. This locks
1.1.1.2 misho 4117: out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
4118: mode, all byte values are valid. A lower case letter is converted to
4119: upper case, and then the 0xc0 bits are flipped.)
1.1 misho 4120:
4121: By default, after \x, from zero to two hexadecimal digits are read
4122: (letters can be in upper or lower case). Any number of hexadecimal dig-
1.1.1.2 misho 4123: its may appear between \x{ and }, but the character code is constrained
4124: as follows:
4125:
4126: 8-bit non-UTF mode less than 0x100
4127: 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
4128: 16-bit non-UTF mode less than 0x10000
4129: 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
1.1 misho 4130:
1.1.1.2 misho 4131: Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
4132: called "surrogate" codepoints).
4133:
4134: If characters other than hexadecimal digits appear between \x{ and },
1.1 misho 4135: or if there is no terminating }, this form of escape is not recognized.
1.1.1.2 misho 4136: Instead, the initial \x will be interpreted as a basic hexadecimal
4137: escape, with no following digits, giving a character whose value is
1.1 misho 4138: zero.
4139:
1.1.1.2 misho 4140: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
4141: is as just described only when it is followed by two hexadecimal dig-
4142: its. Otherwise, it matches a literal "x" character. In JavaScript
1.1 misho 4143: mode, support for code points greater than 256 is provided by \u, which
1.1.1.2 misho 4144: must be followed by four hexadecimal digits; otherwise it matches a
1.1.1.3 ! misho 4145: literal "u" character. Character codes specified by \u in JavaScript
! 4146: mode are constrained in the same was as those specified by \x in non-
! 4147: JavaScript mode.
1.1 misho 4148:
4149: Characters whose value is less than 256 can be defined by either of the
1.1.1.2 misho 4150: two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
1.1 misho 4151: ence in the way they are handled. For example, \xdc is exactly the same
4152: as \x{dc} (or \u00dc in JavaScript mode).
4153:
1.1.1.2 misho 4154: After \0 up to two further octal digits are read. If there are fewer
4155: than two digits, just those that are present are used. Thus the
1.1 misho 4156: sequence \0\x\07 specifies two binary zeros followed by a BEL character
1.1.1.2 misho 4157: (code value 7). Make sure you supply two digits after the initial zero
1.1 misho 4158: if the pattern character that follows is itself an octal digit.
4159:
4160: The handling of a backslash followed by a digit other than 0 is compli-
4161: cated. Outside a character class, PCRE reads it and any following dig-
1.1.1.2 misho 4162: its as a decimal number. If the number is less than 10, or if there
1.1 misho 4163: have been at least that many previous capturing left parentheses in the
1.1.1.2 misho 4164: expression, the entire sequence is taken as a back reference. A
4165: description of how this works is given later, following the discussion
1.1 misho 4166: of parenthesized subpatterns.
4167:
1.1.1.2 misho 4168: Inside a character class, or if the decimal number is greater than 9
4169: and there have not been that many capturing subpatterns, PCRE re-reads
1.1 misho 4170: up to three octal digits following the backslash, and uses them to gen-
1.1.1.2 misho 4171: erate a data character. Any subsequent digits stand for themselves. The
4172: value of the character is constrained in the same way as characters
4173: specified in hexadecimal. For example:
1.1 misho 4174:
4175: \040 is another way of writing a space
4176: \40 is the same, provided there are fewer than 40
4177: previous capturing subpatterns
4178: \7 is always a back reference
4179: \11 might be a back reference, or another way of
4180: writing a tab
4181: \011 is always a tab
4182: \0113 is a tab followed by the character "3"
4183: \113 might be a back reference, otherwise the
4184: character with octal code 113
4185: \377 might be a back reference, otherwise
1.1.1.2 misho 4186: the value 255 (decimal)
1.1 misho 4187: \81 is either a back reference, or a binary zero
4188: followed by the two characters "8" and "1"
4189:
4190: Note that octal values of 100 or greater must not be introduced by a
4191: leading zero, because no more than three octal digits are ever read.
4192:
4193: All the sequences that define a single character value can be used both
4194: inside and outside character classes. In addition, inside a character
4195: class, \b is interpreted as the backspace character (hex 08).
4196:
4197: \N is not allowed in a character class. \B, \R, and \X are not special
4198: inside a character class. Like other unrecognized escape sequences,
4199: they are treated as the literal characters "B", "R", and "X" by
4200: default, but cause an error if the PCRE_EXTRA option is set. Outside a
4201: character class, these sequences have different meanings.
4202:
4203: Unsupported escape sequences
4204:
4205: In Perl, the sequences \l, \L, \u, and \U are recognized by its string
4206: handler and used to modify the case of following characters. By
4207: default, PCRE does not support these escape sequences. However, if the
4208: PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
4209: \u can be used to define a character by code point, as described in the
4210: previous section.
4211:
4212: Absolute and relative back references
4213:
4214: The sequence \g followed by an unsigned or a negative number, option-
4215: ally enclosed in braces, is an absolute or relative back reference. A
4216: named back reference can be coded as \g{name}. Back references are dis-
4217: cussed later, following the discussion of parenthesized subpatterns.
4218:
4219: Absolute and relative subroutine calls
4220:
4221: For compatibility with Oniguruma, the non-Perl syntax \g followed by a
4222: name or a number enclosed either in angle brackets or single quotes, is
4223: an alternative syntax for referencing a subpattern as a "subroutine".
4224: Details are discussed later. Note that \g{...} (Perl syntax) and
4225: \g<...> (Oniguruma syntax) are not synonymous. The former is a back
4226: reference; the latter is a subroutine call.
4227:
4228: Generic character types
4229:
4230: Another use of backslash is for specifying generic character types:
4231:
4232: \d any decimal digit
4233: \D any character that is not a decimal digit
1.1.1.3 ! misho 4234: \h any horizontal white space character
! 4235: \H any character that is not a horizontal white space character
! 4236: \s any white space character
! 4237: \S any character that is not a white space character
! 4238: \v any vertical white space character
! 4239: \V any character that is not a vertical white space character
1.1 misho 4240: \w any "word" character
4241: \W any "non-word" character
4242:
4243: There is also the single sequence \N, which matches a non-newline char-
4244: acter. This is the same as the "." metacharacter when PCRE_DOTALL is
4245: not set. Perl also uses \N to match characters by name; PCRE does not
4246: support this.
4247:
4248: Each pair of lower and upper case escape sequences partitions the com-
4249: plete set of characters into two disjoint sets. Any given character
4250: matches one, and only one, of each pair. The sequences can appear both
4251: inside and outside character classes. They each match one character of
4252: the appropriate type. If the current matching point is at the end of
4253: the subject string, all of them fail, because there is no character to
4254: match.
4255:
4256: For compatibility with Perl, \s does not match the VT character (code
4257: 11). This makes it different from the the POSIX "space" class. The \s
4258: characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
4259: "use locale;" is included in a Perl script, \s may match the VT charac-
4260: ter. In PCRE, it never does.
4261:
4262: A "word" character is an underscore or any character that is a letter
4263: or digit. By default, the definition of letters and digits is con-
4264: trolled by PCRE's low-valued character tables, and may vary if locale-
4265: specific matching is taking place (see "Locale support" in the pcreapi
4266: page). For example, in a French locale such as "fr_FR" in Unix-like
4267: systems, or "french" in Windows, some character codes greater than 128
4268: are used for accented letters, and these are then matched by \w. The
4269: use of locales with Unicode is discouraged.
4270:
1.1.1.2 misho 4271: By default, in a UTF mode, characters with values greater than 128
1.1 misho 4272: never match \d, \s, or \w, and always match \D, \S, and \W. These
1.1.1.2 misho 4273: sequences retain their original meanings from before UTF support was
1.1 misho 4274: available, mainly for efficiency reasons. However, if PCRE is compiled
4275: with Unicode property support, and the PCRE_UCP option is set, the be-
4276: haviour is changed so that Unicode properties are used to determine
4277: character types, as follows:
4278:
4279: \d any character that \p{Nd} matches (decimal digit)
4280: \s any character that \p{Z} matches, plus HT, LF, FF, CR
4281: \w any character that \p{L} or \p{N} matches, plus underscore
4282:
4283: The upper case escapes match the inverse sets of characters. Note that
4284: \d matches only decimal digits, whereas \w matches any Unicode digit,
4285: as well as any Unicode letter, and underscore. Note also that PCRE_UCP
4286: affects \b, and \B because they are defined in terms of \w and \W.
4287: Matching these sequences is noticeably slower when PCRE_UCP is set.
4288:
4289: The sequences \h, \H, \v, and \V are features that were added to Perl
4290: at release 5.10. In contrast to the other sequences, which match only
4291: ASCII characters by default, these always match certain high-valued
1.1.1.2 misho 4292: codepoints, whether or not PCRE_UCP is set. The horizontal space char-
4293: acters are:
1.1 misho 4294:
4295: U+0009 Horizontal tab
4296: U+0020 Space
4297: U+00A0 Non-break space
4298: U+1680 Ogham space mark
4299: U+180E Mongolian vowel separator
4300: U+2000 En quad
4301: U+2001 Em quad
4302: U+2002 En space
4303: U+2003 Em space
4304: U+2004 Three-per-em space
4305: U+2005 Four-per-em space
4306: U+2006 Six-per-em space
4307: U+2007 Figure space
4308: U+2008 Punctuation space
4309: U+2009 Thin space
4310: U+200A Hair space
4311: U+202F Narrow no-break space
4312: U+205F Medium mathematical space
4313: U+3000 Ideographic space
4314:
4315: The vertical space characters are:
4316:
4317: U+000A Linefeed
4318: U+000B Vertical tab
1.1.1.3 ! misho 4319: U+000C Form feed
1.1 misho 4320: U+000D Carriage return
4321: U+0085 Next line
4322: U+2028 Line separator
4323: U+2029 Paragraph separator
4324:
1.1.1.2 misho 4325: In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
4326: 256 are relevant.
4327:
1.1 misho 4328: Newline sequences
4329:
1.1.1.2 misho 4330: Outside a character class, by default, the escape sequence \R matches
4331: any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
4332: to the following:
1.1 misho 4333:
4334: (?>\r\n|\n|\x0b|\f|\r|\x85)
4335:
1.1.1.2 misho 4336: This is an example of an "atomic group", details of which are given
1.1 misho 4337: below. This particular group matches either the two-character sequence
1.1.1.2 misho 4338: CR followed by LF, or one of the single characters LF (linefeed,
1.1.1.3 ! misho 4339: U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car-
! 4340: riage return, U+000D), or NEL (next line, U+0085). The two-character
! 4341: sequence is treated as a single unit that cannot be split.
1.1 misho 4342:
1.1.1.2 misho 4343: In other modes, two additional characters whose codepoints are greater
1.1 misho 4344: than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
1.1.1.2 misho 4345: rator, U+2029). Unicode character property support is not needed for
1.1 misho 4346: these characters to be recognized.
4347:
4348: It is possible to restrict \R to match only CR, LF, or CRLF (instead of
1.1.1.2 misho 4349: the complete set of Unicode line endings) by setting the option
1.1 misho 4350: PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
4351: (BSR is an abbrevation for "backslash R".) This can be made the default
1.1.1.2 misho 4352: when PCRE is built; if this is the case, the other behaviour can be
4353: requested via the PCRE_BSR_UNICODE option. It is also possible to
4354: specify these settings by starting a pattern string with one of the
1.1 misho 4355: following sequences:
4356:
4357: (*BSR_ANYCRLF) CR, LF, or CRLF only
4358: (*BSR_UNICODE) any Unicode newline sequence
4359:
1.1.1.2 misho 4360: These override the default and the options given to the compiling func-
4361: tion, but they can themselves be overridden by options given to a
4362: matching function. Note that these special settings, which are not
4363: Perl-compatible, are recognized only at the very start of a pattern,
4364: and that they must be in upper case. If more than one of them is
4365: present, the last one is used. They can be combined with a change of
1.1 misho 4366: newline convention; for example, a pattern can start with:
4367:
4368: (*ANY)(*BSR_ANYCRLF)
4369:
1.1.1.2 misho 4370: They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
4371: sequences. Inside a character class, \R is treated as an unrecognized
4372: escape sequence, and so matches the letter "R" by default, but causes
4373: an error if PCRE_EXTRA is set.
1.1 misho 4374:
4375: Unicode character properties
4376:
4377: When PCRE is built with Unicode character property support, three addi-
1.1.1.2 misho 4378: tional escape sequences that match characters with specific properties
4379: are available. When in 8-bit non-UTF-8 mode, these sequences are of
4380: course limited to testing characters whose codepoints are less than
4381: 256, but they do work in this mode. The extra escape sequences are:
1.1 misho 4382:
4383: \p{xx} a character with the xx property
4384: \P{xx} a character without the xx property
4385: \X an extended Unicode sequence
4386:
1.1.1.2 misho 4387: The property names represented by xx above are limited to the Unicode
1.1 misho 4388: script names, the general category properties, "Any", which matches any
1.1.1.2 misho 4389: character (including newline), and some special PCRE properties
4390: (described in the next section). Other Perl properties such as "InMu-
4391: sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
1.1 misho 4392: does not match any characters, so always causes a match failure.
4393:
4394: Sets of Unicode characters are defined as belonging to certain scripts.
1.1.1.2 misho 4395: A character from one of these sets can be matched using a script name.
1.1 misho 4396: For example:
4397:
4398: \p{Greek}
4399: \P{Han}
4400:
1.1.1.2 misho 4401: Those that are not part of an identified script are lumped together as
1.1 misho 4402: "Common". The current list of scripts is:
4403:
1.1.1.3 ! misho 4404: Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
! 4405: Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
! 4406: Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
! 4407: Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic,
! 4408: Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
! 4409: gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip-
! 4410: tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
! 4411: Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
! 4412: Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive,
! 4413: Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko,
! 4414: Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic,
! 4415: Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari-
! 4416: tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese,
! 4417: Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
! 4418: Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
! 4419: Yi.
1.1 misho 4420:
4421: Each character has exactly one Unicode general category property, spec-
1.1.1.2 misho 4422: ified by a two-letter abbreviation. For compatibility with Perl, nega-
4423: tion can be specified by including a circumflex between the opening
4424: brace and the property name. For example, \p{^Lu} is the same as
1.1 misho 4425: \P{Lu}.
4426:
4427: If only one letter is specified with \p or \P, it includes all the gen-
1.1.1.2 misho 4428: eral category properties that start with that letter. In this case, in
4429: the absence of negation, the curly brackets in the escape sequence are
1.1 misho 4430: optional; these two examples have the same effect:
4431:
4432: \p{L}
4433: \pL
4434:
4435: The following general category property codes are supported:
4436:
4437: C Other
4438: Cc Control
4439: Cf Format
4440: Cn Unassigned
4441: Co Private use
4442: Cs Surrogate
4443:
4444: L Letter
4445: Ll Lower case letter
4446: Lm Modifier letter
4447: Lo Other letter
4448: Lt Title case letter
4449: Lu Upper case letter
4450:
4451: M Mark
4452: Mc Spacing mark
4453: Me Enclosing mark
4454: Mn Non-spacing mark
4455:
4456: N Number
4457: Nd Decimal number
4458: Nl Letter number
4459: No Other number
4460:
4461: P Punctuation
4462: Pc Connector punctuation
4463: Pd Dash punctuation
4464: Pe Close punctuation
4465: Pf Final punctuation
4466: Pi Initial punctuation
4467: Po Other punctuation
4468: Ps Open punctuation
4469:
4470: S Symbol
4471: Sc Currency symbol
4472: Sk Modifier symbol
4473: Sm Mathematical symbol
4474: So Other symbol
4475:
4476: Z Separator
4477: Zl Line separator
4478: Zp Paragraph separator
4479: Zs Space separator
4480:
1.1.1.2 misho 4481: The special property L& is also supported: it matches a character that
4482: has the Lu, Ll, or Lt property, in other words, a letter that is not
1.1 misho 4483: classified as a modifier or "other".
4484:
1.1.1.2 misho 4485: The Cs (Surrogate) property applies only to characters in the range
4486: U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
4487: so cannot be tested by PCRE, unless UTF validity checking has been
4488: turned off (see the discussion of PCRE_NO_UTF8_CHECK and
4489: PCRE_NO_UTF16_CHECK in the pcreapi page). Perl does not support the Cs
4490: property.
1.1 misho 4491:
4492: The long synonyms for property names that Perl supports (such as
4493: \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
4494: any of these properties with "Is".
4495:
4496: No character that is in the Unicode table has the Cn (unassigned) prop-
4497: erty. Instead, this property is assumed for any code point that is not
4498: in the Unicode table.
4499:
4500: Specifying caseless matching does not affect these escape sequences.
4501: For example, \p{Lu} always matches only upper case letters.
4502:
4503: The \X escape matches any number of Unicode characters that form an
4504: extended Unicode sequence. \X is equivalent to
4505:
4506: (?>\PM\pM*)
4507:
4508: That is, it matches a character without the "mark" property, followed
4509: by zero or more characters with the "mark" property, and treats the
4510: sequence as an atomic group (see below). Characters with the "mark"
4511: property are typically accents that affect the preceding character.
1.1.1.2 misho 4512: None of them have codepoints less than 256, so in 8-bit non-UTF-8 mode
4513: \X matches any one character.
1.1 misho 4514:
4515: Note that recent versions of Perl have changed \X to match what Unicode
4516: calls an "extended grapheme cluster", which has a more complicated def-
4517: inition.
4518:
4519: Matching characters by Unicode property is not fast, because PCRE has
4520: to search a structure that contains data for over fifteen thousand
4521: characters. That is why the traditional escape sequences such as \d and
4522: \w do not use Unicode properties in PCRE by default, though you can
1.1.1.2 misho 4523: make them do so by setting the PCRE_UCP option or by starting the pat-
4524: tern with (*UCP).
1.1 misho 4525:
4526: PCRE's additional properties
4527:
4528: As well as the standard Unicode properties described in the previous
4529: section, PCRE supports four more that make it possible to convert tra-
4530: ditional escape sequences such as \w and \s and POSIX character classes
4531: to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
4532: erties internally when PCRE_UCP is set. They are:
4533:
4534: Xan Any alphanumeric character
4535: Xps Any POSIX space character
4536: Xsp Any Perl space character
4537: Xwd Any Perl "word" character
4538:
4539: Xan matches characters that have either the L (letter) or the N (num-
4540: ber) property. Xps matches the characters tab, linefeed, vertical tab,
1.1.1.3 ! misho 4541: form feed, or carriage return, and any other character that has the Z
1.1 misho 4542: (separator) property. Xsp is the same as Xps, except that vertical tab
4543: is excluded. Xwd matches the same characters as Xan, plus underscore.
4544:
4545: Resetting the match start
4546:
4547: The escape sequence \K causes any previously matched characters not to
4548: be included in the final matched sequence. For example, the pattern:
4549:
4550: foo\Kbar
4551:
4552: matches "foobar", but reports that it has matched "bar". This feature
4553: is similar to a lookbehind assertion (described below). However, in
4554: this case, the part of the subject before the real match does not have
4555: to be of fixed length, as lookbehind assertions do. The use of \K does
4556: not interfere with the setting of captured substrings. For example,
4557: when the pattern
4558:
4559: (foo)\Kbar
4560:
4561: matches "foobar", the first substring is still set to "foo".
4562:
4563: Perl documents that the use of \K within assertions is "not well
4564: defined". In PCRE, \K is acted upon when it occurs inside positive
4565: assertions, but is ignored in negative assertions.
4566:
4567: Simple assertions
4568:
4569: The final use of backslash is for certain simple assertions. An asser-
4570: tion specifies a condition that has to be met at a particular point in
4571: a match, without consuming any characters from the subject string. The
4572: use of subpatterns for more complicated assertions is described below.
4573: The backslashed assertions are:
4574:
4575: \b matches at a word boundary
4576: \B matches when not at a word boundary
4577: \A matches at the start of the subject
4578: \Z matches at the end of the subject
4579: also matches before a newline at the end of the subject
4580: \z matches only at the end of the subject
4581: \G matches at the first matching position in the subject
4582:
4583: Inside a character class, \b has a different meaning; it matches the
4584: backspace character. If any other of these assertions appears in a
4585: character class, by default it matches the corresponding literal char-
4586: acter (for example, \B matches the letter B). However, if the
4587: PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4588: ated instead.
4589:
4590: A word boundary is a position in the subject string where the current
4591: character and the previous character do not both match \w or \W (i.e.
4592: one matches \w and the other matches \W), or the start or end of the
1.1.1.2 misho 4593: string if the first or last character matches \w, respectively. In a
4594: UTF mode, the meanings of \w and \W can be changed by setting the
1.1 misho 4595: PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4596: PCRE nor Perl has a separate "start of word" or "end of word" metase-
4597: quence. However, whatever follows \b normally determines which it is.
4598: For example, the fragment \ba matches "a" at the start of a word.
4599:
4600: The \A, \Z, and \z assertions differ from the traditional circumflex
4601: and dollar (described in the next section) in that they only ever match
4602: at the very start and end of the subject string, whatever options are
4603: set. Thus, they are independent of multiline mode. These three asser-
4604: tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4605: affect only the behaviour of the circumflex and dollar metacharacters.
4606: However, if the startoffset argument of pcre_exec() is non-zero, indi-
4607: cating that matching is to start at a point other than the beginning of
4608: the subject, \A can never match. The difference between \Z and \z is
4609: that \Z matches before a newline at the end of the string as well as at
4610: the very end, whereas \z matches only at the end.
4611:
4612: The \G assertion is true only when the current matching position is at
4613: the start point of the match, as specified by the startoffset argument
4614: of pcre_exec(). It differs from \A when the value of startoffset is
4615: non-zero. By calling pcre_exec() multiple times with appropriate argu-
4616: ments, you can mimic Perl's /g option, and it is in this kind of imple-
4617: mentation where \G can be useful.
4618:
4619: Note, however, that PCRE's interpretation of \G, as the start of the
4620: current match, is subtly different from Perl's, which defines it as the
4621: end of the previous match. In Perl, these can be different when the
4622: previously matched string was empty. Because PCRE does just one match
4623: at a time, it cannot reproduce this behaviour.
4624:
4625: If all the alternatives of a pattern begin with \G, the expression is
4626: anchored to the starting match position, and the "anchored" flag is set
4627: in the compiled regular expression.
4628:
4629:
4630: CIRCUMFLEX AND DOLLAR
4631:
4632: Outside a character class, in the default matching mode, the circumflex
4633: character is an assertion that is true only if the current matching
4634: point is at the start of the subject string. If the startoffset argu-
4635: ment of pcre_exec() is non-zero, circumflex can never match if the
4636: PCRE_MULTILINE option is unset. Inside a character class, circumflex
4637: has an entirely different meaning (see below).
4638:
4639: Circumflex need not be the first character of the pattern if a number
4640: of alternatives are involved, but it should be the first thing in each
4641: alternative in which it appears if the pattern is ever to match that
4642: branch. If all possible alternatives start with a circumflex, that is,
4643: if the pattern is constrained to match only at the start of the sub-
4644: ject, it is said to be an "anchored" pattern. (There are also other
4645: constructs that can cause a pattern to be anchored.)
4646:
4647: A dollar character is an assertion that is true only if the current
4648: matching point is at the end of the subject string, or immediately
4649: before a newline at the end of the string (by default). Dollar need not
4650: be the last character of the pattern if a number of alternatives are
4651: involved, but it should be the last item in any branch in which it
4652: appears. Dollar has no special meaning in a character class.
4653:
4654: The meaning of dollar can be changed so that it matches only at the
4655: very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4656: compile time. This does not affect the \Z assertion.
4657:
4658: The meanings of the circumflex and dollar characters are changed if the
4659: PCRE_MULTILINE option is set. When this is the case, a circumflex
4660: matches immediately after internal newlines as well as at the start of
4661: the subject string. It does not match after a newline that ends the
4662: string. A dollar matches before any newlines in the string, as well as
4663: at the very end, when PCRE_MULTILINE is set. When newline is specified
4664: as the two-character sequence CRLF, isolated CR and LF characters do
4665: not indicate newlines.
4666:
4667: For example, the pattern /^abc$/ matches the subject string "def\nabc"
4668: (where \n represents a newline) in multiline mode, but not otherwise.
4669: Consequently, patterns that are anchored in single line mode because
4670: all branches start with ^ are not anchored in multiline mode, and a
4671: match for circumflex is possible when the startoffset argument of
4672: pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4673: PCRE_MULTILINE is set.
4674:
4675: Note that the sequences \A, \Z, and \z can be used to match the start
4676: and end of the subject in both modes, and if all branches of a pattern
4677: start with \A it is always anchored, whether or not PCRE_MULTILINE is
4678: set.
4679:
4680:
4681: FULL STOP (PERIOD, DOT) AND \N
4682:
4683: Outside a character class, a dot in the pattern matches any one charac-
4684: ter in the subject string except (by default) a character that signi-
1.1.1.2 misho 4685: fies the end of a line.
1.1 misho 4686:
1.1.1.2 misho 4687: When a line ending is defined as a single character, dot never matches
4688: that character; when the two-character sequence CRLF is used, dot does
4689: not match CR if it is immediately followed by LF, but otherwise it
4690: matches all characters (including isolated CRs and LFs). When any Uni-
4691: code line endings are being recognized, dot does not match CR or LF or
1.1 misho 4692: any of the other line ending characters.
4693:
1.1.1.2 misho 4694: The behaviour of dot with regard to newlines can be changed. If the
4695: PCRE_DOTALL option is set, a dot matches any one character, without
1.1 misho 4696: exception. If the two-character sequence CRLF is present in the subject
4697: string, it takes two dots to match it.
4698:
1.1.1.2 misho 4699: The handling of dot is entirely independent of the handling of circum-
4700: flex and dollar, the only relationship being that they both involve
1.1 misho 4701: newlines. Dot has no special meaning in a character class.
4702:
1.1.1.2 misho 4703: The escape sequence \N behaves like a dot, except that it is not
4704: affected by the PCRE_DOTALL option. In other words, it matches any
4705: character except one that signifies the end of a line. Perl also uses
1.1 misho 4706: \N to match characters by name; PCRE does not support this.
4707:
4708:
1.1.1.2 misho 4709: MATCHING A SINGLE DATA UNIT
1.1 misho 4710:
1.1.1.2 misho 4711: Outside a character class, the escape sequence \C matches any one data
4712: unit, whether or not a UTF mode is set. In the 8-bit library, one data
4713: unit is one byte; in the 16-bit library it is a 16-bit unit. Unlike a
4714: dot, \C always matches line-ending characters. The feature is provided
4715: in Perl in order to match individual bytes in UTF-8 mode, but it is
4716: unclear how it can usefully be used. Because \C breaks up characters
4717: into individual data units, matching one unit with \C in a UTF mode
4718: means that the rest of the string may start with a malformed UTF char-
4719: acter. This has undefined results, because PCRE assumes that it is
4720: dealing with valid UTF strings (and by default it checks this at the
1.1.1.3 ! misho 4721: start of processing unless the PCRE_NO_UTF8_CHECK or
! 4722: PCRE_NO_UTF16_CHECK option is used).
1.1 misho 4723:
1.1.1.3 ! misho 4724: PCRE does not allow \C to appear in lookbehind assertions (described
! 4725: below) in a UTF mode, because this would make it impossible to calcu-
1.1 misho 4726: late the length of the lookbehind.
4727:
1.1.1.2 misho 4728: In general, the \C escape sequence is best avoided. However, one way of
1.1.1.3 ! misho 4729: using it that avoids the problem of malformed UTF characters is to use
! 4730: a lookahead to check the length of the next character, as in this pat-
! 4731: tern, which could be used with a UTF-8 string (ignore white space and
1.1.1.2 misho 4732: line breaks):
1.1 misho 4733:
4734: (?| (?=[\x00-\x7f])(\C) |
4735: (?=[\x80-\x{7ff}])(\C)(\C) |
4736: (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
4737: (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
4738:
1.1.1.3 ! misho 4739: A group that starts with (?| resets the capturing parentheses numbers
! 4740: in each alternative (see "Duplicate Subpattern Numbers" below). The
! 4741: assertions at the start of each branch check the next UTF-8 character
! 4742: for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
! 4743: character's individual bytes are then captured by the appropriate num-
1.1 misho 4744: ber of groups.
4745:
4746:
4747: SQUARE BRACKETS AND CHARACTER CLASSES
4748:
4749: An opening square bracket introduces a character class, terminated by a
4750: closing square bracket. A closing square bracket on its own is not spe-
4751: cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4752: a lone closing square bracket causes a compile-time error. If a closing
1.1.1.3 ! misho 4753: square bracket is required as a member of the class, it should be the
! 4754: first data character in the class (after an initial circumflex, if
1.1 misho 4755: present) or escaped with a backslash.
4756:
1.1.1.3 ! misho 4757: A character class matches a single character in the subject. In a UTF
! 4758: mode, the character may be more than one data unit long. A matched
1.1.1.2 misho 4759: character must be in the set of characters defined by the class, unless
1.1.1.3 ! misho 4760: the first character in the class definition is a circumflex, in which
1.1.1.2 misho 4761: case the subject character must not be in the set defined by the class.
1.1.1.3 ! misho 4762: If a circumflex is actually required as a member of the class, ensure
1.1.1.2 misho 4763: it is not the first character, or escape it with a backslash.
1.1 misho 4764:
1.1.1.3 ! misho 4765: For example, the character class [aeiou] matches any lower case vowel,
! 4766: while [^aeiou] matches any character that is not a lower case vowel.
1.1 misho 4767: Note that a circumflex is just a convenient notation for specifying the
1.1.1.3 ! misho 4768: characters that are in the class by enumerating those that are not. A
! 4769: class that starts with a circumflex is not an assertion; it still con-
! 4770: sumes a character from the subject string, and therefore it fails if
1.1 misho 4771: the current pointer is at the end of the string.
4772:
1.1.1.3 ! misho 4773: In UTF-8 (UTF-16) mode, characters with values greater than 255
! 4774: (0xffff) can be included in a class as a literal string of data units,
1.1.1.2 misho 4775: or by using the \x{ escaping mechanism.
4776:
1.1.1.3 ! misho 4777: When caseless matching is set, any letters in a class represent both
! 4778: their upper case and lower case versions, so for example, a caseless
! 4779: [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
! 4780: match "A", whereas a caseful version would. In a UTF mode, PCRE always
! 4781: understands the concept of case for characters whose values are less
! 4782: than 128, so caseless matching is always possible. For characters with
! 4783: higher values, the concept of case is supported if PCRE is compiled
! 4784: with Unicode property support, but not otherwise. If you want to use
! 4785: caseless matching in a UTF mode for characters 128 and above, you must
! 4786: ensure that PCRE is compiled with Unicode property support as well as
1.1.1.2 misho 4787: with UTF support.
4788:
1.1.1.3 ! misho 4789: Characters that might indicate line breaks are never treated in any
! 4790: special way when matching character classes, whatever line-ending
! 4791: sequence is in use, and whatever setting of the PCRE_DOTALL and
1.1 misho 4792: PCRE_MULTILINE options is used. A class such as [^a] always matches one
4793: of these characters.
4794:
1.1.1.3 ! misho 4795: The minus (hyphen) character can be used to specify a range of charac-
! 4796: ters in a character class. For example, [d-m] matches any letter
! 4797: between d and m, inclusive. If a minus character is required in a
! 4798: class, it must be escaped with a backslash or appear in a position
! 4799: where it cannot be interpreted as indicating a range, typically as the
1.1 misho 4800: first or last character in the class.
4801:
4802: It is not possible to have the literal character "]" as the end charac-
1.1.1.3 ! misho 4803: ter of a range. A pattern such as [W-]46] is interpreted as a class of
! 4804: two characters ("W" and "-") followed by a literal string "46]", so it
! 4805: would match "W46]" or "-46]". However, if the "]" is escaped with a
! 4806: backslash it is interpreted as the end of range, so [W-\]46] is inter-
! 4807: preted as a class containing a range followed by two other characters.
! 4808: The octal or hexadecimal representation of "]" can also be used to end
1.1 misho 4809: a range.
4810:
1.1.1.3 ! misho 4811: Ranges operate in the collating sequence of character values. They can
! 4812: also be used for characters specified numerically, for example
! 4813: [\000-\037]. Ranges can include any characters that are valid for the
1.1.1.2 misho 4814: current mode.
1.1 misho 4815:
4816: If a range that includes letters is used when caseless matching is set,
4817: it matches the letters in either case. For example, [W-c] is equivalent
1.1.1.3 ! misho 4818: to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
! 4819: character tables for a French locale are in use, [\xc8-\xcb] matches
! 4820: accented E characters in both cases. In UTF modes, PCRE supports the
! 4821: concept of case for characters with values greater than 128 only when
1.1 misho 4822: it is compiled with Unicode property support.
4823:
1.1.1.3 ! misho 4824: The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
1.1 misho 4825: \w, and \W may appear in a character class, and add the characters that
1.1.1.3 ! misho 4826: they match to the class. For example, [\dABCDEF] matches any hexadeci-
! 4827: mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
! 4828: \d, \s, \w and their upper case partners, just as it does when they
! 4829: appear outside a character class, as described in the section entitled
1.1 misho 4830: "Generic character types" above. The escape sequence \b has a different
1.1.1.3 ! misho 4831: meaning inside a character class; it matches the backspace character.
! 4832: The sequences \B, \N, \R, and \X are not special inside a character
! 4833: class. Like any other unrecognized escape sequences, they are treated
! 4834: as the literal characters "B", "N", "R", and "X" by default, but cause
1.1 misho 4835: an error if the PCRE_EXTRA option is set.
4836:
1.1.1.3 ! misho 4837: A circumflex can conveniently be used with the upper case character
! 4838: types to specify a more restricted set of characters than the matching
! 4839: lower case type. For example, the class [^\W_] matches any letter or
1.1 misho 4840: digit, but not underscore, whereas [\w] includes underscore. A positive
4841: character class should be read as "something OR something OR ..." and a
4842: negative class as "NOT something AND NOT something AND NOT ...".
4843:
1.1.1.3 ! misho 4844: The only metacharacters that are recognized in character classes are
! 4845: backslash, hyphen (only where it can be interpreted as specifying a
! 4846: range), circumflex (only at the start), opening square bracket (only
! 4847: when it can be interpreted as introducing a POSIX class name - see the
! 4848: next section), and the terminating closing square bracket. However,
1.1 misho 4849: escaping other non-alphanumeric characters does no harm.
4850:
4851:
4852: POSIX CHARACTER CLASSES
4853:
4854: Perl supports the POSIX notation for character classes. This uses names
1.1.1.3 ! misho 4855: enclosed by [: and :] within the enclosing square brackets. PCRE also
1.1 misho 4856: supports this notation. For example,
4857:
4858: [01[:alpha:]%]
4859:
4860: matches "0", "1", any alphabetic character, or "%". The supported class
4861: names are:
4862:
4863: alnum letters and digits
4864: alpha letters
4865: ascii character codes 0 - 127
4866: blank space or tab only
4867: cntrl control characters
4868: digit decimal digits (same as \d)
4869: graph printing characters, excluding space
4870: lower lower case letters
4871: print printing characters, including space
4872: punct printing characters, excluding letters and digits and space
4873: space white space (not quite the same as \s)
4874: upper upper case letters
4875: word "word" characters (same as \w)
4876: xdigit hexadecimal digits
4877:
1.1.1.3 ! misho 4878: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
! 4879: and space (32). Notice that this list includes the VT character (code
1.1 misho 4880: 11). This makes "space" different to \s, which does not include VT (for
4881: Perl compatibility).
4882:
1.1.1.3 ! misho 4883: The name "word" is a Perl extension, and "blank" is a GNU extension
! 4884: from Perl 5.8. Another Perl extension is negation, which is indicated
1.1 misho 4885: by a ^ character after the colon. For example,
4886:
4887: [12[:^digit:]]
4888:
1.1.1.3 ! misho 4889: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
1.1 misho 4890: POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4891: these are not supported, and an error is given if they are encountered.
4892:
1.1.1.3 ! misho 4893: By default, in UTF modes, characters with values greater than 128 do
! 4894: not match any of the POSIX character classes. However, if the PCRE_UCP
! 4895: option is passed to pcre_compile(), some of the classes are changed so
1.1 misho 4896: that Unicode character properties are used. This is achieved by replac-
4897: ing the POSIX classes by other sequences, as follows:
4898:
4899: [:alnum:] becomes \p{Xan}
4900: [:alpha:] becomes \p{L}
4901: [:blank:] becomes \h
4902: [:digit:] becomes \p{Nd}
4903: [:lower:] becomes \p{Ll}
4904: [:space:] becomes \p{Xps}
4905: [:upper:] becomes \p{Lu}
4906: [:word:] becomes \p{Xwd}
4907:
1.1.1.3 ! misho 4908: Negated versions, such as [:^alpha:] use \P instead of \p. The other
1.1 misho 4909: POSIX classes are unchanged, and match only characters with code points
4910: less than 128.
4911:
4912:
4913: VERTICAL BAR
4914:
1.1.1.3 ! misho 4915: Vertical bar characters are used to separate alternative patterns. For
1.1 misho 4916: example, the pattern
4917:
4918: gilbert|sullivan
4919:
1.1.1.3 ! misho 4920: matches either "gilbert" or "sullivan". Any number of alternatives may
! 4921: appear, and an empty alternative is permitted (matching the empty
1.1 misho 4922: string). The matching process tries each alternative in turn, from left
1.1.1.3 ! misho 4923: to right, and the first one that succeeds is used. If the alternatives
! 4924: are within a subpattern (defined below), "succeeds" means matching the
1.1 misho 4925: rest of the main pattern as well as the alternative in the subpattern.
4926:
4927:
4928: INTERNAL OPTION SETTING
4929:
1.1.1.3 ! misho 4930: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
! 4931: PCRE_EXTENDED options (which are Perl-compatible) can be changed from
! 4932: within the pattern by a sequence of Perl option letters enclosed
1.1 misho 4933: between "(?" and ")". The option letters are
4934:
4935: i for PCRE_CASELESS
4936: m for PCRE_MULTILINE
4937: s for PCRE_DOTALL
4938: x for PCRE_EXTENDED
4939:
4940: For example, (?im) sets caseless, multiline matching. It is also possi-
4941: ble to unset these options by preceding the letter with a hyphen, and a
1.1.1.3 ! misho 4942: combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
! 4943: LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
! 4944: is also permitted. If a letter appears both before and after the
1.1 misho 4945: hyphen, the option is unset.
4946:
1.1.1.3 ! misho 4947: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
! 4948: can be changed in the same way as the Perl-compatible options by using
1.1 misho 4949: the characters J, U and X respectively.
4950:
1.1.1.3 ! misho 4951: When one of these option changes occurs at top level (that is, not
! 4952: inside subpattern parentheses), the change applies to the remainder of
1.1 misho 4953: the pattern that follows. If the change is placed right at the start of
4954: a pattern, PCRE extracts it into the global options (and it will there-
4955: fore show up in data extracted by the pcre_fullinfo() function).
4956:
1.1.1.3 ! misho 4957: An option change within a subpattern (see below for a description of
! 4958: subpatterns) affects only that part of the subpattern that follows it,
1.1 misho 4959: so
4960:
4961: (a(?i)b)c
4962:
4963: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
1.1.1.3 ! misho 4964: used). By this means, options can be made to have different settings
! 4965: in different parts of the pattern. Any changes made in one alternative
! 4966: do carry on into subsequent branches within the same subpattern. For
1.1 misho 4967: example,
4968:
4969: (a(?i)b|c)
4970:
1.1.1.3 ! misho 4971: matches "ab", "aB", "c", and "C", even though when matching "C" the
! 4972: first branch is abandoned before the option setting. This is because
! 4973: the effects of option settings happen at compile time. There would be
1.1 misho 4974: some very weird behaviour otherwise.
4975:
1.1.1.3 ! misho 4976: Note: There are other PCRE-specific options that can be set by the
! 4977: application when the compiling or matching functions are called. In
! 4978: some cases the pattern can contain special leading sequences such as
! 4979: (*CRLF) to override what the application has set or what has been
! 4980: defaulted. Details are given in the section entitled "Newline
! 4981: sequences" above. There are also the (*UTF8), (*UTF16), and (*UCP)
! 4982: leading sequences that can be used to set UTF and Unicode property
! 4983: modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, and
1.1.1.2 misho 4984: the PCRE_UCP options, respectively.
1.1 misho 4985:
4986:
4987: SUBPATTERNS
4988:
4989: Subpatterns are delimited by parentheses (round brackets), which can be
4990: nested. Turning part of a pattern into a subpattern does two things:
4991:
4992: 1. It localizes a set of alternatives. For example, the pattern
4993:
4994: cat(aract|erpillar|)
4995:
1.1.1.3 ! misho 4996: matches "cataract", "caterpillar", or "cat". Without the parentheses,
1.1 misho 4997: it would match "cataract", "erpillar" or an empty string.
4998:
1.1.1.3 ! misho 4999: 2. It sets up the subpattern as a capturing subpattern. This means
! 5000: that, when the whole pattern matches, that portion of the subject
1.1 misho 5001: string that matched the subpattern is passed back to the caller via the
1.1.1.3 ! misho 5002: ovector argument of the matching function. (This applies only to the
! 5003: traditional matching functions; the DFA matching functions do not sup-
1.1.1.2 misho 5004: port capturing.)
5005:
5006: Opening parentheses are counted from left to right (starting from 1) to
1.1.1.3 ! misho 5007: obtain numbers for the capturing subpatterns. For example, if the
1.1.1.2 misho 5008: string "the red king" is matched against the pattern
1.1 misho 5009:
5010: the ((red|white) (king|queen))
5011:
5012: the captured substrings are "red king", "red", and "king", and are num-
5013: bered 1, 2, and 3, respectively.
5014:
1.1.1.3 ! misho 5015: The fact that plain parentheses fulfil two functions is not always
! 5016: helpful. There are often times when a grouping subpattern is required
! 5017: without a capturing requirement. If an opening parenthesis is followed
! 5018: by a question mark and a colon, the subpattern does not do any captur-
! 5019: ing, and is not counted when computing the number of any subsequent
! 5020: capturing subpatterns. For example, if the string "the white queen" is
1.1 misho 5021: matched against the pattern
5022:
5023: the ((?:red|white) (king|queen))
5024:
5025: the captured substrings are "white queen" and "queen", and are numbered
5026: 1 and 2. The maximum number of capturing subpatterns is 65535.
5027:
1.1.1.3 ! misho 5028: As a convenient shorthand, if any option settings are required at the
! 5029: start of a non-capturing subpattern, the option letters may appear
1.1 misho 5030: between the "?" and the ":". Thus the two patterns
5031:
5032: (?i:saturday|sunday)
5033: (?:(?i)saturday|sunday)
5034:
5035: match exactly the same set of strings. Because alternative branches are
1.1.1.3 ! misho 5036: tried from left to right, and options are not reset until the end of
! 5037: the subpattern is reached, an option setting in one branch does affect
! 5038: subsequent branches, so the above patterns match "SUNDAY" as well as
1.1 misho 5039: "Saturday".
5040:
5041:
5042: DUPLICATE SUBPATTERN NUMBERS
5043:
5044: Perl 5.10 introduced a feature whereby each alternative in a subpattern
1.1.1.3 ! misho 5045: uses the same numbers for its capturing parentheses. Such a subpattern
! 5046: starts with (?| and is itself a non-capturing subpattern. For example,
1.1 misho 5047: consider this pattern:
5048:
5049: (?|(Sat)ur|(Sun))day
5050:
1.1.1.3 ! misho 5051: Because the two alternatives are inside a (?| group, both sets of cap-
! 5052: turing parentheses are numbered one. Thus, when the pattern matches,
! 5053: you can look at captured substring number one, whichever alternative
! 5054: matched. This construct is useful when you want to capture part, but
1.1 misho 5055: not all, of one of a number of alternatives. Inside a (?| group, paren-
1.1.1.3 ! misho 5056: theses are numbered as usual, but the number is reset at the start of
! 5057: each branch. The numbers of any capturing parentheses that follow the
! 5058: subpattern start after the highest number used in any branch. The fol-
1.1 misho 5059: lowing example is taken from the Perl documentation. The numbers under-
5060: neath show in which buffer the captured content will be stored.
5061:
5062: # before ---------------branch-reset----------- after
5063: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
5064: # 1 2 2 3 2 3 4
5065:
1.1.1.3 ! misho 5066: A back reference to a numbered subpattern uses the most recent value
! 5067: that is set for that number by any subpattern. The following pattern
1.1 misho 5068: matches "abcabc" or "defdef":
5069:
5070: /(?|(abc)|(def))\1/
5071:
1.1.1.3 ! misho 5072: In contrast, a subroutine call to a numbered subpattern always refers
! 5073: to the first one in the pattern with the given number. The following
1.1 misho 5074: pattern matches "abcabc" or "defabc":
5075:
5076: /(?|(abc)|(def))(?1)/
5077:
1.1.1.3 ! misho 5078: If a condition test for a subpattern's having matched refers to a non-
! 5079: unique number, the test is true if any of the subpatterns of that num-
1.1 misho 5080: ber have matched.
5081:
1.1.1.3 ! misho 5082: An alternative approach to using this "branch reset" feature is to use
1.1 misho 5083: duplicate named subpatterns, as described in the next section.
5084:
5085:
5086: NAMED SUBPATTERNS
5087:
1.1.1.3 ! misho 5088: Identifying capturing parentheses by number is simple, but it can be
! 5089: very hard to keep track of the numbers in complicated regular expres-
! 5090: sions. Furthermore, if an expression is modified, the numbers may
! 5091: change. To help with this difficulty, PCRE supports the naming of sub-
1.1 misho 5092: patterns. This feature was not added to Perl until release 5.10. Python
1.1.1.3 ! misho 5093: had the feature earlier, and PCRE introduced it at release 4.0, using
! 5094: the Python syntax. PCRE now supports both the Perl and the Python syn-
! 5095: tax. Perl allows identically numbered subpatterns to have different
1.1 misho 5096: names, but PCRE does not.
5097:
1.1.1.3 ! misho 5098: In PCRE, a subpattern can be named in one of three ways: (?<name>...)
! 5099: or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
! 5100: to capturing parentheses from other parts of the pattern, such as back
! 5101: references, recursion, and conditions, can be made by name as well as
1.1 misho 5102: by number.
5103:
1.1.1.3 ! misho 5104: Names consist of up to 32 alphanumeric characters and underscores.
! 5105: Named capturing parentheses are still allocated numbers as well as
! 5106: names, exactly as if the names were not present. The PCRE API provides
1.1 misho 5107: function calls for extracting the name-to-number translation table from
5108: a compiled pattern. There is also a convenience function for extracting
5109: a captured substring by name.
5110:
1.1.1.3 ! misho 5111: By default, a name must be unique within a pattern, but it is possible
1.1 misho 5112: to relax this constraint by setting the PCRE_DUPNAMES option at compile
1.1.1.3 ! misho 5113: time. (Duplicate names are also always permitted for subpatterns with
! 5114: the same number, set up as described in the previous section.) Dupli-
! 5115: cate names can be useful for patterns where only one instance of the
! 5116: named parentheses can match. Suppose you want to match the name of a
! 5117: weekday, either as a 3-letter abbreviation or as the full name, and in
1.1 misho 5118: both cases you want to extract the abbreviation. This pattern (ignoring
5119: the line breaks) does the job:
5120:
5121: (?<DN>Mon|Fri|Sun)(?:day)?|
5122: (?<DN>Tue)(?:sday)?|
5123: (?<DN>Wed)(?:nesday)?|
5124: (?<DN>Thu)(?:rsday)?|
5125: (?<DN>Sat)(?:urday)?
5126:
1.1.1.3 ! misho 5127: There are five capturing substrings, but only one is ever set after a
1.1 misho 5128: match. (An alternative way of solving this problem is to use a "branch
5129: reset" subpattern, as described in the previous section.)
5130:
1.1.1.3 ! misho 5131: The convenience function for extracting the data by name returns the
! 5132: substring for the first (and in this example, the only) subpattern of
! 5133: that name that matched. This saves searching to find which numbered
1.1 misho 5134: subpattern it was.
5135:
1.1.1.3 ! misho 5136: If you make a back reference to a non-unique named subpattern from
! 5137: elsewhere in the pattern, the one that corresponds to the first occur-
1.1 misho 5138: rence of the name is used. In the absence of duplicate numbers (see the
1.1.1.3 ! misho 5139: previous section) this is the one with the lowest number. If you use a
! 5140: named reference in a condition test (see the section about conditions
! 5141: below), either to check whether a subpattern has matched, or to check
! 5142: for recursion, all subpatterns with the same name are tested. If the
! 5143: condition is true for any one of them, the overall condition is true.
1.1 misho 5144: This is the same behaviour as testing by number. For further details of
5145: the interfaces for handling named subpatterns, see the pcreapi documen-
5146: tation.
5147:
5148: Warning: You cannot use different names to distinguish between two sub-
1.1.1.3 ! misho 5149: patterns with the same number because PCRE uses only the numbers when
1.1 misho 5150: matching. For this reason, an error is given at compile time if differ-
1.1.1.3 ! misho 5151: ent names are given to subpatterns with the same number. However, you
! 5152: can give the same name to subpatterns with the same number, even when
1.1 misho 5153: PCRE_DUPNAMES is not set.
5154:
5155:
5156: REPETITION
5157:
1.1.1.3 ! misho 5158: Repetition is specified by quantifiers, which can follow any of the
1.1 misho 5159: following items:
5160:
5161: a literal data character
5162: the dot metacharacter
5163: the \C escape sequence
1.1.1.2 misho 5164: the \X escape sequence
1.1 misho 5165: the \R escape sequence
5166: an escape such as \d or \pL that matches a single character
5167: a character class
5168: a back reference (see next section)
5169: a parenthesized subpattern (including assertions)
5170: a subroutine call to a subpattern (recursive or otherwise)
5171:
1.1.1.3 ! misho 5172: The general repetition quantifier specifies a minimum and maximum num-
! 5173: ber of permitted matches, by giving the two numbers in curly brackets
! 5174: (braces), separated by a comma. The numbers must be less than 65536,
1.1 misho 5175: and the first must be less than or equal to the second. For example:
5176:
5177: z{2,4}
5178:
1.1.1.3 ! misho 5179: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
! 5180: special character. If the second number is omitted, but the comma is
! 5181: present, there is no upper limit; if the second number and the comma
! 5182: are both omitted, the quantifier specifies an exact number of required
1.1 misho 5183: matches. Thus
5184:
5185: [aeiou]{3,}
5186:
5187: matches at least 3 successive vowels, but may match many more, while
5188:
5189: \d{8}
5190:
1.1.1.3 ! misho 5191: matches exactly 8 digits. An opening curly bracket that appears in a
! 5192: position where a quantifier is not allowed, or one that does not match
! 5193: the syntax of a quantifier, is taken as a literal character. For exam-
1.1 misho 5194: ple, {,6} is not a quantifier, but a literal string of four characters.
5195:
1.1.1.2 misho 5196: In UTF modes, quantifiers apply to characters rather than to individual
1.1.1.3 ! misho 5197: data units. Thus, for example, \x{100}{2} matches two characters, each
1.1.1.2 misho 5198: of which is represented by a two-byte sequence in a UTF-8 string. Simi-
1.1.1.3 ! misho 5199: larly, \X{3} matches three Unicode extended sequences, each of which
1.1.1.2 misho 5200: may be several data units long (and they may be of different lengths).
1.1 misho 5201:
5202: The quantifier {0} is permitted, causing the expression to behave as if
5203: the previous item and the quantifier were not present. This may be use-
1.1.1.3 ! misho 5204: ful for subpatterns that are referenced as subroutines from elsewhere
1.1 misho 5205: in the pattern (but see also the section entitled "Defining subpatterns
1.1.1.3 ! misho 5206: for use by reference only" below). Items other than subpatterns that
1.1 misho 5207: have a {0} quantifier are omitted from the compiled pattern.
5208:
1.1.1.3 ! misho 5209: For convenience, the three most common quantifiers have single-charac-
1.1 misho 5210: ter abbreviations:
5211:
5212: * is equivalent to {0,}
5213: + is equivalent to {1,}
5214: ? is equivalent to {0,1}
5215:
1.1.1.3 ! misho 5216: It is possible to construct infinite loops by following a subpattern
1.1 misho 5217: that can match no characters with a quantifier that has no upper limit,
5218: for example:
5219:
5220: (a?)*
5221:
5222: Earlier versions of Perl and PCRE used to give an error at compile time
1.1.1.3 ! misho 5223: for such patterns. However, because there are cases where this can be
! 5224: useful, such patterns are now accepted, but if any repetition of the
! 5225: subpattern does in fact match no characters, the loop is forcibly bro-
1.1 misho 5226: ken.
5227:
1.1.1.3 ! misho 5228: By default, the quantifiers are "greedy", that is, they match as much
! 5229: as possible (up to the maximum number of permitted times), without
! 5230: causing the rest of the pattern to fail. The classic example of where
1.1 misho 5231: this gives problems is in trying to match comments in C programs. These
1.1.1.3 ! misho 5232: appear between /* and */ and within the comment, individual * and /
! 5233: characters may appear. An attempt to match C comments by applying the
1.1 misho 5234: pattern
5235:
5236: /\*.*\*/
5237:
5238: to the string
5239:
5240: /* first comment */ not comment /* second comment */
5241:
1.1.1.3 ! misho 5242: fails, because it matches the entire string owing to the greediness of
1.1 misho 5243: the .* item.
5244:
1.1.1.3 ! misho 5245: However, if a quantifier is followed by a question mark, it ceases to
1.1 misho 5246: be greedy, and instead matches the minimum number of times possible, so
5247: the pattern
5248:
5249: /\*.*?\*/
5250:
1.1.1.3 ! misho 5251: does the right thing with the C comments. The meaning of the various
! 5252: quantifiers is not otherwise changed, just the preferred number of
! 5253: matches. Do not confuse this use of question mark with its use as a
! 5254: quantifier in its own right. Because it has two uses, it can sometimes
1.1 misho 5255: appear doubled, as in
5256:
5257: \d??\d
5258:
5259: which matches one digit by preference, but can match two if that is the
5260: only way the rest of the pattern matches.
5261:
1.1.1.3 ! misho 5262: If the PCRE_UNGREEDY option is set (an option that is not available in
! 5263: Perl), the quantifiers are not greedy by default, but individual ones
! 5264: can be made greedy by following them with a question mark. In other
1.1 misho 5265: words, it inverts the default behaviour.
5266:
1.1.1.3 ! misho 5267: When a parenthesized subpattern is quantified with a minimum repeat
! 5268: count that is greater than 1 or with a limited maximum, more memory is
! 5269: required for the compiled pattern, in proportion to the size of the
1.1 misho 5270: minimum or maximum.
5271:
5272: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
1.1.1.3 ! misho 5273: alent to Perl's /s) is set, thus allowing the dot to match newlines,
! 5274: the pattern is implicitly anchored, because whatever follows will be
! 5275: tried against every character position in the subject string, so there
! 5276: is no point in retrying the overall match at any position after the
! 5277: first. PCRE normally treats such a pattern as though it were preceded
1.1 misho 5278: by \A.
5279:
1.1.1.3 ! misho 5280: In cases where it is known that the subject string contains no new-
! 5281: lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
1.1 misho 5282: mization, or alternatively using ^ to indicate anchoring explicitly.
5283:
1.1.1.3 ! misho 5284: However, there is one situation where the optimization cannot be used.
1.1 misho 5285: When .* is inside capturing parentheses that are the subject of a back
5286: reference elsewhere in the pattern, a match at the start may fail where
5287: a later one succeeds. Consider, for example:
5288:
5289: (.*)abc\1
5290:
1.1.1.3 ! misho 5291: If the subject is "xyz123abc123" the match point is the fourth charac-
1.1 misho 5292: ter. For this reason, such a pattern is not implicitly anchored.
5293:
5294: When a capturing subpattern is repeated, the value captured is the sub-
5295: string that matched the final iteration. For example, after
5296:
5297: (tweedle[dume]{3}\s*)+
5298:
5299: has matched "tweedledum tweedledee" the value of the captured substring
1.1.1.3 ! misho 5300: is "tweedledee". However, if there are nested capturing subpatterns,
! 5301: the corresponding captured values may have been set in previous itera-
1.1 misho 5302: tions. For example, after
5303:
5304: /(a|(b))+/
5305:
5306: matches "aba" the value of the second captured substring is "b".
5307:
5308:
5309: ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
5310:
1.1.1.3 ! misho 5311: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
! 5312: repetition, failure of what follows normally causes the repeated item
! 5313: to be re-evaluated to see if a different number of repeats allows the
! 5314: rest of the pattern to match. Sometimes it is useful to prevent this,
! 5315: either to change the nature of the match, or to cause it fail earlier
! 5316: than it otherwise might, when the author of the pattern knows there is
1.1 misho 5317: no point in carrying on.
5318:
1.1.1.3 ! misho 5319: Consider, for example, the pattern \d+foo when applied to the subject
1.1 misho 5320: line
5321:
5322: 123456bar
5323:
5324: After matching all 6 digits and then failing to match "foo", the normal
1.1.1.3 ! misho 5325: action of the matcher is to try again with only 5 digits matching the
! 5326: \d+ item, and then with 4, and so on, before ultimately failing.
! 5327: "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
! 5328: the means for specifying that once a subpattern has matched, it is not
1.1 misho 5329: to be re-evaluated in this way.
5330:
1.1.1.3 ! misho 5331: If we use atomic grouping for the previous example, the matcher gives
! 5332: up immediately on failing to match "foo" the first time. The notation
1.1 misho 5333: is a kind of special parenthesis, starting with (?> as in this example:
5334:
5335: (?>\d+)foo
5336:
1.1.1.3 ! misho 5337: This kind of parenthesis "locks up" the part of the pattern it con-
! 5338: tains once it has matched, and a failure further into the pattern is
! 5339: prevented from backtracking into it. Backtracking past it to previous
1.1 misho 5340: items, however, works as normal.
5341:
1.1.1.3 ! misho 5342: An alternative description is that a subpattern of this type matches
! 5343: the string of characters that an identical standalone pattern would
1.1 misho 5344: match, if anchored at the current point in the subject string.
5345:
5346: Atomic grouping subpatterns are not capturing subpatterns. Simple cases
5347: such as the above example can be thought of as a maximizing repeat that
1.1.1.3 ! misho 5348: must swallow everything it can. So, while both \d+ and \d+? are pre-
! 5349: pared to adjust the number of digits they match in order to make the
1.1 misho 5350: rest of the pattern match, (?>\d+) can only match an entire sequence of
5351: digits.
5352:
1.1.1.3 ! misho 5353: Atomic groups in general can of course contain arbitrarily complicated
! 5354: subpatterns, and can be nested. However, when the subpattern for an
1.1 misho 5355: atomic group is just a single repeated item, as in the example above, a
1.1.1.3 ! misho 5356: simpler notation, called a "possessive quantifier" can be used. This
! 5357: consists of an additional + character following a quantifier. Using
1.1 misho 5358: this notation, the previous example can be rewritten as
5359:
5360: \d++foo
5361:
5362: Note that a possessive quantifier can be used with an entire group, for
5363: example:
5364:
5365: (abc|xyz){2,3}+
5366:
1.1.1.3 ! misho 5367: Possessive quantifiers are always greedy; the setting of the
1.1 misho 5368: PCRE_UNGREEDY option is ignored. They are a convenient notation for the
1.1.1.3 ! misho 5369: simpler forms of atomic group. However, there is no difference in the
! 5370: meaning of a possessive quantifier and the equivalent atomic group,
! 5371: though there may be a performance difference; possessive quantifiers
1.1 misho 5372: should be slightly faster.
5373:
1.1.1.3 ! misho 5374: The possessive quantifier syntax is an extension to the Perl 5.8 syn-
! 5375: tax. Jeffrey Friedl originated the idea (and the name) in the first
1.1 misho 5376: edition of his book. Mike McCloskey liked it, so implemented it when he
1.1.1.3 ! misho 5377: built Sun's Java package, and PCRE copied it from there. It ultimately
1.1 misho 5378: found its way into Perl at release 5.10.
5379:
5380: PCRE has an optimization that automatically "possessifies" certain sim-
1.1.1.3 ! misho 5381: ple pattern constructs. For example, the sequence A+B is treated as
! 5382: A++B because there is no point in backtracking into a sequence of A's
1.1 misho 5383: when B must follow.
5384:
1.1.1.3 ! misho 5385: When a pattern contains an unlimited repeat inside a subpattern that
! 5386: can itself be repeated an unlimited number of times, the use of an
! 5387: atomic group is the only way to avoid some failing matches taking a
1.1 misho 5388: very long time indeed. The pattern
5389:
5390: (\D+|<\d+>)*[!?]
5391:
1.1.1.3 ! misho 5392: matches an unlimited number of substrings that either consist of non-
! 5393: digits, or digits enclosed in <>, followed by either ! or ?. When it
1.1 misho 5394: matches, it runs quickly. However, if it is applied to
5395:
5396: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5397:
1.1.1.3 ! misho 5398: it takes a long time before reporting failure. This is because the
! 5399: string can be divided between the internal \D+ repeat and the external
! 5400: * repeat in a large number of ways, and all have to be tried. (The
! 5401: example uses [!?] rather than a single character at the end, because
! 5402: both PCRE and Perl have an optimization that allows for fast failure
! 5403: when a single character is used. They remember the last single charac-
! 5404: ter that is required for a match, and fail early if it is not present
! 5405: in the string.) If the pattern is changed so that it uses an atomic
1.1 misho 5406: group, like this:
5407:
5408: ((?>\D+)|<\d+>)*[!?]
5409:
5410: sequences of non-digits cannot be broken, and failure happens quickly.
5411:
5412:
5413: BACK REFERENCES
5414:
5415: Outside a character class, a backslash followed by a digit greater than
5416: 0 (and possibly further digits) is a back reference to a capturing sub-
1.1.1.3 ! misho 5417: pattern earlier (that is, to its left) in the pattern, provided there
1.1 misho 5418: have been that many previous capturing left parentheses.
5419:
5420: However, if the decimal number following the backslash is less than 10,
1.1.1.3 ! misho 5421: it is always taken as a back reference, and causes an error only if
! 5422: there are not that many capturing left parentheses in the entire pat-
! 5423: tern. In other words, the parentheses that are referenced need not be
! 5424: to the left of the reference for numbers less than 10. A "forward back
! 5425: reference" of this type can make sense when a repetition is involved
! 5426: and the subpattern to the right has participated in an earlier itera-
1.1 misho 5427: tion.
5428:
1.1.1.3 ! misho 5429: It is not possible to have a numerical "forward back reference" to a
! 5430: subpattern whose number is 10 or more using this syntax because a
! 5431: sequence such as \50 is interpreted as a character defined in octal.
1.1 misho 5432: See the subsection entitled "Non-printing characters" above for further
1.1.1.3 ! misho 5433: details of the handling of digits following a backslash. There is no
! 5434: such problem when named parentheses are used. A back reference to any
1.1 misho 5435: subpattern is possible using named parentheses (see below).
5436:
1.1.1.3 ! misho 5437: Another way of avoiding the ambiguity inherent in the use of digits
! 5438: following a backslash is to use the \g escape sequence. This escape
1.1 misho 5439: must be followed by an unsigned number or a negative number, optionally
5440: enclosed in braces. These examples are all identical:
5441:
5442: (ring), \1
5443: (ring), \g1
5444: (ring), \g{1}
5445:
1.1.1.3 ! misho 5446: An unsigned number specifies an absolute reference without the ambigu-
1.1 misho 5447: ity that is present in the older syntax. It is also useful when literal
5448: digits follow the reference. A negative number is a relative reference.
5449: Consider this example:
5450:
5451: (abc(def)ghi)\g{-1}
5452:
5453: The sequence \g{-1} is a reference to the most recently started captur-
5454: ing subpattern before \g, that is, is it equivalent to \2 in this exam-
1.1.1.3 ! misho 5455: ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
! 5456: references can be helpful in long patterns, and also in patterns that
! 5457: are created by joining together fragments that contain references
1.1 misho 5458: within themselves.
5459:
1.1.1.3 ! misho 5460: A back reference matches whatever actually matched the capturing sub-
! 5461: pattern in the current subject string, rather than anything matching
1.1 misho 5462: the subpattern itself (see "Subpatterns as subroutines" below for a way
5463: of doing that). So the pattern
5464:
5465: (sens|respons)e and \1ibility
5466:
1.1.1.3 ! misho 5467: matches "sense and sensibility" and "response and responsibility", but
! 5468: not "sense and responsibility". If caseful matching is in force at the
! 5469: time of the back reference, the case of letters is relevant. For exam-
1.1 misho 5470: ple,
5471:
5472: ((?i)rah)\s+\1
5473:
1.1.1.3 ! misho 5474: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
1.1 misho 5475: original capturing subpattern is matched caselessly.
5476:
1.1.1.3 ! misho 5477: There are several different ways of writing back references to named
! 5478: subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
! 5479: \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
1.1 misho 5480: unified back reference syntax, in which \g can be used for both numeric
1.1.1.3 ! misho 5481: and named references, is also supported. We could rewrite the above
1.1 misho 5482: example in any of the following ways:
5483:
5484: (?<p1>(?i)rah)\s+\k<p1>
5485: (?'p1'(?i)rah)\s+\k{p1}
5486: (?P<p1>(?i)rah)\s+(?P=p1)
5487: (?<p1>(?i)rah)\s+\g{p1}
5488:
1.1.1.3 ! misho 5489: A subpattern that is referenced by name may appear in the pattern
1.1 misho 5490: before or after the reference.
5491:
1.1.1.3 ! misho 5492: There may be more than one back reference to the same subpattern. If a
! 5493: subpattern has not actually been used in a particular match, any back
1.1 misho 5494: references to it always fail by default. For example, the pattern
5495:
5496: (a|(bc))\2
5497:
1.1.1.3 ! misho 5498: always fails if it starts to match "a" rather than "bc". However, if
1.1 misho 5499: the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
5500: ence to an unset value matches an empty string.
5501:
1.1.1.3 ! misho 5502: Because there may be many capturing parentheses in a pattern, all dig-
! 5503: its following a backslash are taken as part of a potential back refer-
! 5504: ence number. If the pattern continues with a digit character, some
! 5505: delimiter must be used to terminate the back reference. If the
! 5506: PCRE_EXTENDED option is set, this can be white space. Otherwise, the
! 5507: \g{ syntax or an empty comment (see "Comments" below) can be used.
1.1 misho 5508:
5509: Recursive back references
5510:
1.1.1.3 ! misho 5511: A back reference that occurs inside the parentheses to which it refers
! 5512: fails when the subpattern is first used, so, for example, (a\1) never
! 5513: matches. However, such references can be useful inside repeated sub-
1.1 misho 5514: patterns. For example, the pattern
5515:
5516: (a|b\1)+
5517:
5518: matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
1.1.1.3 ! misho 5519: ation of the subpattern, the back reference matches the character
! 5520: string corresponding to the previous iteration. In order for this to
! 5521: work, the pattern must be such that the first iteration does not need
! 5522: to match the back reference. This can be done using alternation, as in
1.1 misho 5523: the example above, or by a quantifier with a minimum of zero.
5524:
1.1.1.3 ! misho 5525: Back references of this type cause the group that they reference to be
! 5526: treated as an atomic group. Once the whole group has been matched, a
! 5527: subsequent matching failure cannot cause backtracking into the middle
1.1 misho 5528: of the group.
5529:
5530:
5531: ASSERTIONS
5532:
1.1.1.3 ! misho 5533: An assertion is a test on the characters following or preceding the
! 5534: current matching point that does not actually consume any characters.
! 5535: The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
1.1 misho 5536: described above.
5537:
1.1.1.3 ! misho 5538: More complicated assertions are coded as subpatterns. There are two
! 5539: kinds: those that look ahead of the current position in the subject
! 5540: string, and those that look behind it. An assertion subpattern is
! 5541: matched in the normal way, except that it does not cause the current
1.1 misho 5542: matching position to be changed.
5543:
1.1.1.3 ! misho 5544: Assertion subpatterns are not capturing subpatterns. If such an asser-
! 5545: tion contains capturing subpatterns within it, these are counted for
! 5546: the purposes of numbering the capturing subpatterns in the whole pat-
! 5547: tern. However, substring capturing is carried out only for positive
1.1 misho 5548: assertions, because it does not make sense for negative assertions.
5549:
1.1.1.3 ! misho 5550: For compatibility with Perl, assertion subpatterns may be repeated;
! 5551: though it makes no sense to assert the same thing several times, the
! 5552: side effect of capturing parentheses may occasionally be useful. In
1.1 misho 5553: practice, there only three cases:
5554:
1.1.1.3 ! misho 5555: (1) If the quantifier is {0}, the assertion is never obeyed during
! 5556: matching. However, it may contain internal capturing parenthesized
1.1 misho 5557: groups that are called from elsewhere via the subroutine mechanism.
5558:
1.1.1.3 ! misho 5559: (2) If quantifier is {0,n} where n is greater than zero, it is treated
! 5560: as if it were {0,1}. At run time, the rest of the pattern match is
1.1 misho 5561: tried with and without the assertion, the order depending on the greed-
5562: iness of the quantifier.
5563:
1.1.1.3 ! misho 5564: (3) If the minimum repetition is greater than zero, the quantifier is
! 5565: ignored. The assertion is obeyed just once when encountered during
1.1 misho 5566: matching.
5567:
5568: Lookahead assertions
5569:
5570: Lookahead assertions start with (?= for positive assertions and (?! for
5571: negative assertions. For example,
5572:
5573: \w+(?=;)
5574:
1.1.1.3 ! misho 5575: matches a word followed by a semicolon, but does not include the semi-
1.1 misho 5576: colon in the match, and
5577:
5578: foo(?!bar)
5579:
1.1.1.3 ! misho 5580: matches any occurrence of "foo" that is not followed by "bar". Note
1.1 misho 5581: that the apparently similar pattern
5582:
5583: (?!foo)bar
5584:
1.1.1.3 ! misho 5585: does not find an occurrence of "bar" that is preceded by something
! 5586: other than "foo"; it finds any occurrence of "bar" whatsoever, because
1.1 misho 5587: the assertion (?!foo) is always true when the next three characters are
5588: "bar". A lookbehind assertion is needed to achieve the other effect.
5589:
5590: If you want to force a matching failure at some point in a pattern, the
1.1.1.3 ! misho 5591: most convenient way to do it is with (?!) because an empty string
! 5592: always matches, so an assertion that requires there not to be an empty
1.1 misho 5593: string must always fail. The backtracking control verb (*FAIL) or (*F)
5594: is a synonym for (?!).
5595:
5596: Lookbehind assertions
5597:
1.1.1.3 ! misho 5598: Lookbehind assertions start with (?<= for positive assertions and (?<!
1.1 misho 5599: for negative assertions. For example,
5600:
5601: (?<!foo)bar
5602:
1.1.1.3 ! misho 5603: does find an occurrence of "bar" that is not preceded by "foo". The
! 5604: contents of a lookbehind assertion are restricted such that all the
1.1 misho 5605: strings it matches must have a fixed length. However, if there are sev-
1.1.1.3 ! misho 5606: eral top-level alternatives, they do not all have to have the same
1.1 misho 5607: fixed length. Thus
5608:
5609: (?<=bullock|donkey)
5610:
5611: is permitted, but
5612:
5613: (?<!dogs?|cats?)
5614:
1.1.1.3 ! misho 5615: causes an error at compile time. Branches that match different length
! 5616: strings are permitted only at the top level of a lookbehind assertion.
1.1 misho 5617: This is an extension compared with Perl, which requires all branches to
5618: match the same length of string. An assertion such as
5619:
5620: (?<=ab(c|de))
5621:
1.1.1.3 ! misho 5622: is not permitted, because its single top-level branch can match two
1.1 misho 5623: different lengths, but it is acceptable to PCRE if rewritten to use two
5624: top-level branches:
5625:
5626: (?<=abc|abde)
5627:
1.1.1.3 ! misho 5628: In some cases, the escape sequence \K (see above) can be used instead
1.1 misho 5629: of a lookbehind assertion to get round the fixed-length restriction.
5630:
1.1.1.3 ! misho 5631: The implementation of lookbehind assertions is, for each alternative,
! 5632: to temporarily move the current position back by the fixed length and
1.1 misho 5633: then try to match. If there are insufficient characters before the cur-
5634: rent position, the assertion fails.
5635:
1.1.1.3 ! misho 5636: In a UTF mode, PCRE does not allow the \C escape (which matches a sin-
! 5637: gle data unit even in a UTF mode) to appear in lookbehind assertions,
! 5638: because it makes it impossible to calculate the length of the lookbe-
! 5639: hind. The \X and \R escapes, which can match different numbers of data
1.1.1.2 misho 5640: units, are also not permitted.
1.1 misho 5641:
1.1.1.3 ! misho 5642: "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
! 5643: lookbehinds, as long as the subpattern matches a fixed-length string.
1.1 misho 5644: Recursion, however, is not supported.
5645:
1.1.1.3 ! misho 5646: Possessive quantifiers can be used in conjunction with lookbehind
1.1 misho 5647: assertions to specify efficient matching of fixed-length strings at the
5648: end of subject strings. Consider a simple pattern such as
5649:
5650: abcd$
5651:
1.1.1.3 ! misho 5652: when applied to a long string that does not match. Because matching
1.1 misho 5653: proceeds from left to right, PCRE will look for each "a" in the subject
1.1.1.3 ! misho 5654: and then see if what follows matches the rest of the pattern. If the
1.1 misho 5655: pattern is specified as
5656:
5657: ^.*abcd$
5658:
1.1.1.3 ! misho 5659: the initial .* matches the entire string at first, but when this fails
1.1 misho 5660: (because there is no following "a"), it backtracks to match all but the
1.1.1.3 ! misho 5661: last character, then all but the last two characters, and so on. Once
! 5662: again the search for "a" covers the entire string, from right to left,
1.1 misho 5663: so we are no better off. However, if the pattern is written as
5664:
5665: ^.*+(?<=abcd)
5666:
1.1.1.3 ! misho 5667: there can be no backtracking for the .*+ item; it can match only the
! 5668: entire string. The subsequent lookbehind assertion does a single test
! 5669: on the last four characters. If it fails, the match fails immediately.
! 5670: For long strings, this approach makes a significant difference to the
1.1 misho 5671: processing time.
5672:
5673: Using multiple assertions
5674:
5675: Several assertions (of any sort) may occur in succession. For example,
5676:
5677: (?<=\d{3})(?<!999)foo
5678:
1.1.1.3 ! misho 5679: matches "foo" preceded by three digits that are not "999". Notice that
! 5680: each of the assertions is applied independently at the same point in
! 5681: the subject string. First there is a check that the previous three
! 5682: characters are all digits, and then there is a check that the same
1.1 misho 5683: three characters are not "999". This pattern does not match "foo" pre-
1.1.1.3 ! misho 5684: ceded by six characters, the first of which are digits and the last
! 5685: three of which are not "999". For example, it doesn't match "123abc-
1.1 misho 5686: foo". A pattern to do that is
5687:
5688: (?<=\d{3}...)(?<!999)foo
5689:
1.1.1.3 ! misho 5690: This time the first assertion looks at the preceding six characters,
1.1 misho 5691: checking that the first three are digits, and then the second assertion
5692: checks that the preceding three characters are not "999".
5693:
5694: Assertions can be nested in any combination. For example,
5695:
5696: (?<=(?<!foo)bar)baz
5697:
1.1.1.3 ! misho 5698: matches an occurrence of "baz" that is preceded by "bar" which in turn
1.1 misho 5699: is not preceded by "foo", while
5700:
5701: (?<=\d{3}(?!999)...)foo
5702:
1.1.1.3 ! misho 5703: is another pattern that matches "foo" preceded by three digits and any
1.1 misho 5704: three characters that are not "999".
5705:
5706:
5707: CONDITIONAL SUBPATTERNS
5708:
1.1.1.3 ! misho 5709: It is possible to cause the matching process to obey a subpattern con-
! 5710: ditionally or to choose between two alternative subpatterns, depending
! 5711: on the result of an assertion, or whether a specific capturing subpat-
! 5712: tern has already been matched. The two possible forms of conditional
1.1 misho 5713: subpattern are:
5714:
5715: (?(condition)yes-pattern)
5716: (?(condition)yes-pattern|no-pattern)
5717:
1.1.1.3 ! misho 5718: If the condition is satisfied, the yes-pattern is used; otherwise the
! 5719: no-pattern (if present) is used. If there are more than two alterna-
! 5720: tives in the subpattern, a compile-time error occurs. Each of the two
1.1 misho 5721: alternatives may itself contain nested subpatterns of any form, includ-
5722: ing conditional subpatterns; the restriction to two alternatives
5723: applies only at the level of the condition. This pattern fragment is an
5724: example where the alternatives are complex:
5725:
5726: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5727:
5728:
1.1.1.3 ! misho 5729: There are four kinds of condition: references to subpatterns, refer-
1.1 misho 5730: ences to recursion, a pseudo-condition called DEFINE, and assertions.
5731:
5732: Checking for a used subpattern by number
5733:
1.1.1.3 ! misho 5734: If the text between the parentheses consists of a sequence of digits,
1.1 misho 5735: the condition is true if a capturing subpattern of that number has pre-
1.1.1.3 ! misho 5736: viously matched. If there is more than one capturing subpattern with
! 5737: the same number (see the earlier section about duplicate subpattern
! 5738: numbers), the condition is true if any of them have matched. An alter-
! 5739: native notation is to precede the digits with a plus or minus sign. In
! 5740: this case, the subpattern number is relative rather than absolute. The
! 5741: most recently opened parentheses can be referenced by (?(-1), the next
! 5742: most recent by (?(-2), and so on. Inside loops it can also make sense
1.1 misho 5743: to refer to subsequent groups. The next parentheses to be opened can be
1.1.1.3 ! misho 5744: referenced as (?(+1), and so on. (The value zero in any of these forms
1.1 misho 5745: is not used; it provokes a compile-time error.)
5746:
1.1.1.3 ! misho 5747: Consider the following pattern, which contains non-significant white
1.1 misho 5748: space to make it more readable (assume the PCRE_EXTENDED option) and to
5749: divide it into three parts for ease of discussion:
5750:
5751: ( $ )? [^()]+ (?(1) $ )
5752:
1.1.1.3 ! misho 5753: The first part matches an optional opening parenthesis, and if that
1.1 misho 5754: character is present, sets it as the first captured substring. The sec-
1.1.1.3 ! misho 5755: ond part matches one or more characters that are not parentheses. The
! 5756: third part is a conditional subpattern that tests whether or not the
! 5757: first set of parentheses matched. If they did, that is, if subject
! 5758: started with an opening parenthesis, the condition is true, and so the
! 5759: yes-pattern is executed and a closing parenthesis is required. Other-
! 5760: wise, since no-pattern is not present, the subpattern matches nothing.
! 5761: In other words, this pattern matches a sequence of non-parentheses,
1.1 misho 5762: optionally enclosed in parentheses.
5763:
1.1.1.3 ! misho 5764: If you were embedding this pattern in a larger one, you could use a
1.1 misho 5765: relative reference:
5766:
5767: ...other stuff... ( $ )? [^()]+ (?(-1) $ ) ...
5768:
1.1.1.3 ! misho 5769: This makes the fragment independent of the parentheses in the larger
1.1 misho 5770: pattern.
5771:
5772: Checking for a used subpattern by name
5773:
1.1.1.3 ! misho 5774: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
! 5775: used subpattern by name. For compatibility with earlier versions of
! 5776: PCRE, which had this facility before Perl, the syntax (?(name)...) is
! 5777: also recognized. However, there is a possible ambiguity with this syn-
! 5778: tax, because subpattern names may consist entirely of digits. PCRE
! 5779: looks first for a named subpattern; if it cannot find one and the name
! 5780: consists entirely of digits, PCRE looks for a subpattern of that num-
! 5781: ber, which must be greater than zero. Using subpattern names that con-
1.1 misho 5782: sist entirely of digits is not recommended.
5783:
5784: Rewriting the above example to use a named subpattern gives this:
5785:
5786: (?<OPEN> $ )? [^()]+ (?(<OPEN>) $ )
5787:
1.1.1.3 ! misho 5788: If the name used in a condition of this kind is a duplicate, the test
! 5789: is applied to all subpatterns of the same name, and is true if any one
1.1 misho 5790: of them has matched.
5791:
5792: Checking for pattern recursion
5793:
5794: If the condition is the string (R), and there is no subpattern with the
1.1.1.3 ! misho 5795: name R, the condition is true if a recursive call to the whole pattern
1.1 misho 5796: or any subpattern has been made. If digits or a name preceded by amper-
5797: sand follow the letter R, for example:
5798:
5799: (?(R3)...) or (?(R&name)...)
5800:
5801: the condition is true if the most recent recursion is into a subpattern
5802: whose number or name is given. This condition does not check the entire
1.1.1.3 ! misho 5803: recursion stack. If the name used in a condition of this kind is a
1.1 misho 5804: duplicate, the test is applied to all subpatterns of the same name, and
5805: is true if any one of them is the most recent recursion.
5806:
1.1.1.3 ! misho 5807: At "top level", all these recursion test conditions are false. The
1.1 misho 5808: syntax for recursive patterns is described below.
5809:
5810: Defining subpatterns for use by reference only
5811:
1.1.1.3 ! misho 5812: If the condition is the string (DEFINE), and there is no subpattern
! 5813: with the name DEFINE, the condition is always false. In this case,
! 5814: there may be only one alternative in the subpattern. It is always
! 5815: skipped if control reaches this point in the pattern; the idea of
! 5816: DEFINE is that it can be used to define subroutines that can be refer-
! 5817: enced from elsewhere. (The use of subroutines is described below.) For
! 5818: example, a pattern to match an IPv4 address such as "192.168.23.245"
! 5819: could be written like this (ignore white space and line breaks):
1.1 misho 5820:
5821: (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5822: \b (?&byte) (\.(?&byte)){3} \b
5823:
1.1.1.3 ! misho 5824: The first part of the pattern is a DEFINE group inside which a another
! 5825: group named "byte" is defined. This matches an individual component of
! 5826: an IPv4 address (a number less than 256). When matching takes place,
! 5827: this part of the pattern is skipped because DEFINE acts like a false
! 5828: condition. The rest of the pattern uses references to the named group
! 5829: to match the four dot-separated components of an IPv4 address, insist-
1.1 misho 5830: ing on a word boundary at each end.
5831:
5832: Assertion conditions
5833:
1.1.1.3 ! misho 5834: If the condition is not in any of the above formats, it must be an
! 5835: assertion. This may be a positive or negative lookahead or lookbehind
! 5836: assertion. Consider this pattern, again containing non-significant
1.1 misho 5837: white space, and with the two alternatives on the second line:
5838:
5839: (?(?=[^a-z]*[a-z])
5840: \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5841:
1.1.1.3 ! misho 5842: The condition is a positive lookahead assertion that matches an
! 5843: optional sequence of non-letters followed by a letter. In other words,
! 5844: it tests for the presence of at least one letter in the subject. If a
! 5845: letter is found, the subject is matched against the first alternative;
! 5846: otherwise it is matched against the second. This pattern matches
! 5847: strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
1.1 misho 5848: letters and dd are digits.
5849:
5850:
5851: COMMENTS
5852:
5853: There are two ways of including comments in patterns that are processed
5854: by PCRE. In both cases, the start of the comment must not be in a char-
5855: acter class, nor in the middle of any other sequence of related charac-
1.1.1.3 ! misho 5856: ters such as (?: or a subpattern name or number. The characters that
1.1 misho 5857: make up a comment play no part in the pattern matching.
5858:
1.1.1.3 ! misho 5859: The sequence (?# marks the start of a comment that continues up to the
! 5860: next closing parenthesis. Nested parentheses are not permitted. If the
1.1 misho 5861: PCRE_EXTENDED option is set, an unescaped # character also introduces a
1.1.1.3 ! misho 5862: comment, which in this case continues to immediately after the next
! 5863: newline character or character sequence in the pattern. Which charac-
1.1 misho 5864: ters are interpreted as newlines is controlled by the options passed to
1.1.1.3 ! misho 5865: a compiling function or by a special sequence at the start of the pat-
1.1.1.2 misho 5866: tern, as described in the section entitled "Newline conventions" above.
5867: Note that the end of this type of comment is a literal newline sequence
1.1.1.3 ! misho 5868: in the pattern; escape sequences that happen to represent a newline do
! 5869: not count. For example, consider this pattern when PCRE_EXTENDED is
1.1.1.2 misho 5870: set, and the default newline convention is in force:
1.1 misho 5871:
5872: abc #comment \n still comment
5873:
1.1.1.3 ! misho 5874: On encountering the # character, pcre_compile() skips along, looking
! 5875: for a newline in the pattern. The sequence \n is still literal at this
! 5876: stage, so it does not terminate the comment. Only an actual character
1.1 misho 5877: with the code value 0x0a (the default newline) does so.
5878:
5879:
5880: RECURSIVE PATTERNS
5881:
1.1.1.3 ! misho 5882: Consider the problem of matching a string in parentheses, allowing for
! 5883: unlimited nested parentheses. Without the use of recursion, the best
! 5884: that can be done is to use a pattern that matches up to some fixed
! 5885: depth of nesting. It is not possible to handle an arbitrary nesting
1.1 misho 5886: depth.
5887:
5888: For some time, Perl has provided a facility that allows regular expres-
1.1.1.3 ! misho 5889: sions to recurse (amongst other things). It does this by interpolating
! 5890: Perl code in the expression at run time, and the code can refer to the
1.1 misho 5891: expression itself. A Perl pattern using code interpolation to solve the
5892: parentheses problem can be created like this:
5893:
5894: $re = qr{$ (?: (?>[^()]+) | (?p{$re}) )* $}x;
5895:
5896: The (?p{...}) item interpolates Perl code at run time, and in this case
5897: refers recursively to the pattern in which it appears.
5898:
5899: Obviously, PCRE cannot support the interpolation of Perl code. Instead,
1.1.1.3 ! misho 5900: it supports special syntax for recursion of the entire pattern, and
! 5901: also for individual subpattern recursion. After its introduction in
! 5902: PCRE and Python, this kind of recursion was subsequently introduced
1.1 misho 5903: into Perl at release 5.10.
5904:
1.1.1.3 ! misho 5905: A special item that consists of (? followed by a number greater than
! 5906: zero and a closing parenthesis is a recursive subroutine call of the
! 5907: subpattern of the given number, provided that it occurs inside that
! 5908: subpattern. (If not, it is a non-recursive subroutine call, which is
! 5909: described in the next section.) The special item (?R) or (?0) is a
1.1 misho 5910: recursive call of the entire regular expression.
5911:
1.1.1.3 ! misho 5912: This PCRE pattern solves the nested parentheses problem (assume the
1.1 misho 5913: PCRE_EXTENDED option is set so that white space is ignored):
5914:
5915: $ ( [^()]++ | (?R) )* $
5916:
1.1.1.3 ! misho 5917: First it matches an opening parenthesis. Then it matches any number of
! 5918: substrings which can either be a sequence of non-parentheses, or a
! 5919: recursive match of the pattern itself (that is, a correctly parenthe-
1.1 misho 5920: sized substring). Finally there is a closing parenthesis. Note the use
5921: of a possessive quantifier to avoid backtracking into sequences of non-
5922: parentheses.
5923:
1.1.1.3 ! misho 5924: If this were part of a larger pattern, you would not want to recurse
1.1 misho 5925: the entire pattern, so instead you could use this:
5926:
5927: ( $ ( [^()]++ | (?1) )* $ )
5928:
1.1.1.3 ! misho 5929: We have put the pattern into parentheses, and caused the recursion to
1.1 misho 5930: refer to them instead of the whole pattern.
5931:
1.1.1.3 ! misho 5932: In a larger pattern, keeping track of parenthesis numbers can be
! 5933: tricky. This is made easier by the use of relative references. Instead
1.1 misho 5934: of (?1) in the pattern above you can write (?-2) to refer to the second
1.1.1.3 ! misho 5935: most recently opened parentheses preceding the recursion. In other
! 5936: words, a negative number counts capturing parentheses leftwards from
1.1 misho 5937: the point at which it is encountered.
5938:
1.1.1.3 ! misho 5939: It is also possible to refer to subsequently opened parentheses, by
! 5940: writing references such as (?+2). However, these cannot be recursive
! 5941: because the reference is not inside the parentheses that are refer-
! 5942: enced. They are always non-recursive subroutine calls, as described in
1.1 misho 5943: the next section.
5944:
1.1.1.3 ! misho 5945: An alternative approach is to use named parentheses instead. The Perl
! 5946: syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
1.1 misho 5947: supported. We could rewrite the above example as follows:
5948:
5949: (?<pn> $ ( [^()]++ | (?&pn) )* $ )
5950:
1.1.1.3 ! misho 5951: If there is more than one subpattern with the same name, the earliest
1.1 misho 5952: one is used.
5953:
1.1.1.3 ! misho 5954: This particular example pattern that we have been looking at contains
1.1 misho 5955: nested unlimited repeats, and so the use of a possessive quantifier for
5956: matching strings of non-parentheses is important when applying the pat-
1.1.1.3 ! misho 5957: tern to strings that do not match. For example, when this pattern is
1.1 misho 5958: applied to
5959:
5960: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5961:
1.1.1.3 ! misho 5962: it yields "no match" quickly. However, if a possessive quantifier is
! 5963: not used, the match runs for a very long time indeed because there are
! 5964: so many different ways the + and * repeats can carve up the subject,
1.1 misho 5965: and all have to be tested before failure can be reported.
5966:
1.1.1.3 ! misho 5967: At the end of a match, the values of capturing parentheses are those
! 5968: from the outermost level. If you want to obtain intermediate values, a
! 5969: callout function can be used (see below and the pcrecallout documenta-
1.1 misho 5970: tion). If the pattern above is matched against
5971:
5972: (ab(cd)ef)
5973:
1.1.1.3 ! misho 5974: the value for the inner capturing parentheses (numbered 2) is "ef",
! 5975: which is the last value taken on at the top level. If a capturing sub-
! 5976: pattern is not matched at the top level, its final captured value is
! 5977: unset, even if it was (temporarily) set at a deeper level during the
1.1 misho 5978: matching process.
5979:
1.1.1.3 ! misho 5980: If there are more than 15 capturing parentheses in a pattern, PCRE has
! 5981: to obtain extra memory to store data during a recursion, which it does
1.1 misho 5982: by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5983: can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5984:
1.1.1.3 ! misho 5985: Do not confuse the (?R) item with the condition (R), which tests for
! 5986: recursion. Consider this pattern, which matches text in angle brack-
! 5987: ets, allowing for arbitrary nesting. Only digits are allowed in nested
! 5988: brackets (that is, when recursing), whereas any characters are permit-
1.1 misho 5989: ted at the outer level.
5990:
5991: < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5992:
1.1.1.3 ! misho 5993: In this pattern, (?(R) is the start of a conditional subpattern, with
! 5994: two different alternatives for the recursive and non-recursive cases.
1.1 misho 5995: The (?R) item is the actual recursive call.
5996:
5997: Differences in recursion processing between PCRE and Perl
5998:
1.1.1.3 ! misho 5999: Recursion processing in PCRE differs from Perl in two important ways.
! 6000: In PCRE (like Python, but unlike Perl), a recursive subpattern call is
1.1 misho 6001: always treated as an atomic group. That is, once it has matched some of
6002: the subject string, it is never re-entered, even if it contains untried
1.1.1.3 ! misho 6003: alternatives and there is a subsequent matching failure. This can be
! 6004: illustrated by the following pattern, which purports to match a palin-
! 6005: dromic string that contains an odd number of characters (for example,
1.1 misho 6006: "a", "aba", "abcba", "abcdcba"):
6007:
6008: ^(.|(.)(?1)\2)$
6009:
6010: The idea is that it either matches a single character, or two identical
1.1.1.3 ! misho 6011: characters surrounding a sub-palindrome. In Perl, this pattern works;
! 6012: in PCRE it does not if the pattern is longer than three characters.
1.1 misho 6013: Consider the subject string "abcba":
6014:
1.1.1.3 ! misho 6015: At the top level, the first character is matched, but as it is not at
1.1 misho 6016: the end of the string, the first alternative fails; the second alterna-
6017: tive is taken and the recursion kicks in. The recursive call to subpat-
1.1.1.3 ! misho 6018: tern 1 successfully matches the next character ("b"). (Note that the
1.1 misho 6019: beginning and end of line tests are not part of the recursion).
6020:
1.1.1.3 ! misho 6021: Back at the top level, the next character ("c") is compared with what
! 6022: subpattern 2 matched, which was "a". This fails. Because the recursion
! 6023: is treated as an atomic group, there are now no backtracking points,
! 6024: and so the entire match fails. (Perl is able, at this point, to re-
! 6025: enter the recursion and try the second alternative.) However, if the
1.1 misho 6026: pattern is written with the alternatives in the other order, things are
6027: different:
6028:
6029: ^((.)(?1)\2|.)$
6030:
1.1.1.3 ! misho 6031: This time, the recursing alternative is tried first, and continues to
! 6032: recurse until it runs out of characters, at which point the recursion
! 6033: fails. But this time we do have another alternative to try at the
! 6034: higher level. That is the big difference: in the previous case the
1.1 misho 6035: remaining alternative is at a deeper recursion level, which PCRE cannot
6036: use.
6037:
1.1.1.3 ! misho 6038: To change the pattern so that it matches all palindromic strings, not
! 6039: just those with an odd number of characters, it is tempting to change
1.1 misho 6040: the pattern to this:
6041:
6042: ^((.)(?1)\2|.?)$
6043:
1.1.1.3 ! misho 6044: Again, this works in Perl, but not in PCRE, and for the same reason.
! 6045: When a deeper recursion has matched a single character, it cannot be
! 6046: entered again in order to match an empty string. The solution is to
! 6047: separate the two cases, and write out the odd and even cases as alter-
1.1 misho 6048: natives at the higher level:
6049:
6050: ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
6051:
1.1.1.3 ! misho 6052: If you want to match typical palindromic phrases, the pattern has to
1.1 misho 6053: ignore all non-word characters, which can be done like this:
6054:
6055: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
6056:
6057: If run with the PCRE_CASELESS option, this pattern matches phrases such
6058: as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
1.1.1.3 ! misho 6059: Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
! 6060: ing into sequences of non-word characters. Without this, PCRE takes a
! 6061: great deal longer (ten times or more) to match typical phrases, and
1.1 misho 6062: Perl takes so long that you think it has gone into a loop.
6063:
1.1.1.3 ! misho 6064: WARNING: The palindrome-matching patterns above work only if the sub-
! 6065: ject string does not start with a palindrome that is shorter than the
! 6066: entire string. For example, although "abcba" is correctly matched, if
! 6067: the subject is "ababa", PCRE finds the palindrome "aba" at the start,
! 6068: then fails at top level because the end of the string does not follow.
! 6069: Once again, it cannot jump back into the recursion to try other alter-
1.1 misho 6070: natives, so the entire match fails.
6071:
1.1.1.3 ! misho 6072: The second way in which PCRE and Perl differ in their recursion pro-
! 6073: cessing is in the handling of captured values. In Perl, when a subpat-
! 6074: tern is called recursively or as a subpattern (see the next section),
! 6075: it has no access to any values that were captured outside the recur-
! 6076: sion, whereas in PCRE these values can be referenced. Consider this
1.1 misho 6077: pattern:
6078:
6079: ^(.)(\1|a(?2))
6080:
1.1.1.3 ! misho 6081: In PCRE, this pattern matches "bab". The first capturing parentheses
! 6082: match "b", then in the second group, when the back reference \1 fails
! 6083: to match "b", the second alternative matches "a" and then recurses. In
! 6084: the recursion, \1 does now match "b" and so the whole match succeeds.
! 6085: In Perl, the pattern fails to match because inside the recursive call
1.1 misho 6086: \1 cannot access the externally set value.
6087:
6088:
6089: SUBPATTERNS AS SUBROUTINES
6090:
1.1.1.3 ! misho 6091: If the syntax for a recursive subpattern call (either by number or by
! 6092: name) is used outside the parentheses to which it refers, it operates
! 6093: like a subroutine in a programming language. The called subpattern may
! 6094: be defined before or after the reference. A numbered reference can be
1.1 misho 6095: absolute or relative, as in these examples:
6096:
6097: (...(absolute)...)...(?2)...
6098: (...(relative)...)...(?-1)...
6099: (...(?+1)...(relative)...
6100:
6101: An earlier example pointed out that the pattern
6102:
6103: (sens|respons)e and \1ibility
6104:
1.1.1.3 ! misho 6105: matches "sense and sensibility" and "response and responsibility", but
1.1 misho 6106: not "sense and responsibility". If instead the pattern
6107:
6108: (sens|respons)e and (?1)ibility
6109:
1.1.1.3 ! misho 6110: is used, it does match "sense and responsibility" as well as the other
! 6111: two strings. Another example is given in the discussion of DEFINE
1.1 misho 6112: above.
6113:
1.1.1.3 ! misho 6114: All subroutine calls, whether recursive or not, are always treated as
! 6115: atomic groups. That is, once a subroutine has matched some of the sub-
1.1 misho 6116: ject string, it is never re-entered, even if it contains untried alter-
1.1.1.3 ! misho 6117: natives and there is a subsequent matching failure. Any capturing
! 6118: parentheses that are set during the subroutine call revert to their
1.1 misho 6119: previous values afterwards.
6120:
1.1.1.3 ! misho 6121: Processing options such as case-independence are fixed when a subpat-
! 6122: tern is defined, so if it is used as a subroutine, such options cannot
1.1 misho 6123: be changed for different calls. For example, consider this pattern:
6124:
6125: (abc)(?i:(?-1))
6126:
1.1.1.3 ! misho 6127: It matches "abcabc". It does not match "abcABC" because the change of
1.1 misho 6128: processing option does not affect the called subpattern.
6129:
6130:
6131: ONIGURUMA SUBROUTINE SYNTAX
6132:
1.1.1.3 ! misho 6133: For compatibility with Oniguruma, the non-Perl syntax \g followed by a
1.1 misho 6134: name or a number enclosed either in angle brackets or single quotes, is
1.1.1.3 ! misho 6135: an alternative syntax for referencing a subpattern as a subroutine,
! 6136: possibly recursively. Here are two of the examples used above, rewrit-
1.1 misho 6137: ten using this syntax:
6138:
6139: (?<pn> $ ( (?>[^()]+) | \g<pn> )* $ )
6140: (sens|respons)e and \g'1'ibility
6141:
1.1.1.3 ! misho 6142: PCRE supports an extension to Oniguruma: if a number is preceded by a
1.1 misho 6143: plus or a minus sign it is taken as a relative reference. For example:
6144:
6145: (abc)(?i:\g<-1>)
6146:
1.1.1.3 ! misho 6147: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
! 6148: synonymous. The former is a back reference; the latter is a subroutine
1.1 misho 6149: call.
6150:
6151:
6152: CALLOUTS
6153:
6154: Perl has a feature whereby using the sequence (?{...}) causes arbitrary
1.1.1.3 ! misho 6155: Perl code to be obeyed in the middle of matching a regular expression.
1.1 misho 6156: This makes it possible, amongst other things, to extract different sub-
6157: strings that match the same pair of parentheses when there is a repeti-
6158: tion.
6159:
6160: PCRE provides a similar feature, but of course it cannot obey arbitrary
6161: Perl code. The feature is called "callout". The caller of PCRE provides
1.1.1.3 ! misho 6162: an external function by putting its entry point in the global variable
! 6163: pcre_callout (8-bit library) or pcre16_callout (16-bit library). By
1.1.1.2 misho 6164: default, this variable contains NULL, which disables all calling out.
1.1 misho 6165:
1.1.1.3 ! misho 6166: Within a regular expression, (?C) indicates the points at which the
! 6167: external function is to be called. If you want to identify different
! 6168: callout points, you can put a number less than 256 after the letter C.
! 6169: The default value is zero. For example, this pattern has two callout
1.1 misho 6170: points:
6171:
6172: (?C1)abc(?C2)def
6173:
1.1.1.3 ! misho 6174: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call-
! 6175: outs are automatically installed before each item in the pattern. They
1.1.1.2 misho 6176: are all numbered 255.
6177:
1.1.1.3 ! misho 6178: During matching, when PCRE reaches a callout point, the external func-
! 6179: tion is called. It is provided with the number of the callout, the
! 6180: position in the pattern, and, optionally, one item of data originally
! 6181: supplied by the caller of the matching function. The callout function
! 6182: may cause matching to proceed, to backtrack, or to fail altogether. A
! 6183: complete description of the interface to the callout function is given
1.1.1.2 misho 6184: in the pcrecallout documentation.
1.1 misho 6185:
6186:
6187: BACKTRACKING CONTROL
6188:
1.1.1.3 ! misho 6189: Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
1.1 misho 6190: which are described in the Perl documentation as "experimental and sub-
1.1.1.3 ! misho 6191: ject to change or removal in a future version of Perl". It goes on to
! 6192: say: "Their usage in production code should be noted to avoid problems
1.1 misho 6193: during upgrades." The same remarks apply to the PCRE features described
6194: in this section.
6195:
1.1.1.3 ! misho 6196: Since these verbs are specifically related to backtracking, most of
! 6197: them can be used only when the pattern is to be matched using one of
1.1.1.2 misho 6198: the traditional matching functions, which use a backtracking algorithm.
1.1.1.3 ! misho 6199: With the exception of (*FAIL), which behaves like a failing negative
! 6200: assertion, they cause an error if encountered by a DFA matching func-
1.1.1.2 misho 6201: tion.
1.1 misho 6202:
1.1.1.3 ! misho 6203: If any of these verbs are used in an assertion or in a subpattern that
1.1 misho 6204: is called as a subroutine (whether or not recursively), their effect is
6205: confined to that subpattern; it does not extend to the surrounding pat-
6206: tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
1.1.1.3 ! misho 6207: that is encountered in a successful positive assertion is passed back
! 6208: when a match succeeds (compare capturing parentheses in assertions).
1.1 misho 6209: Note that such subpatterns are processed as anchored at the point where
1.1.1.3 ! misho 6210: they are tested. Note also that Perl's treatment of subroutines and
! 6211: assertions is different in some cases.
1.1 misho 6212:
1.1.1.3 ! misho 6213: The new verbs make use of what was previously invalid syntax: an open-
1.1 misho 6214: ing parenthesis followed by an asterisk. They are generally of the form
1.1.1.3 ! misho 6215: (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
! 6216: haviour, depending on whether or not an argument is present. A name is
1.1 misho 6217: any sequence of characters that does not include a closing parenthesis.
1.1.1.3 ! misho 6218: The maximum length of name is 255 in the 8-bit library and 65535 in the
! 6219: 16-bit library. If the name is empty, that is, if the closing parenthe-
! 6220: sis immediately follows the colon, the effect is as if the colon were
! 6221: not there. Any number of these verbs may occur in a pattern.
! 6222:
! 6223: Optimizations that affect backtracking verbs
1.1 misho 6224:
1.1.1.2 misho 6225: PCRE contains some optimizations that are used to speed up matching by
1.1 misho 6226: running some checks at the start of each match attempt. For example, it
1.1.1.2 misho 6227: may know the minimum length of matching subject, or that a particular
6228: character must be present. When one of these optimizations suppresses
6229: the running of a match, any included backtracking verbs will not, of
1.1 misho 6230: course, be processed. You can suppress the start-of-match optimizations
1.1.1.2 misho 6231: by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
1.1 misho 6232: pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
1.1.1.3 ! misho 6233: There is more discussion of this option in the section entitled "Option
! 6234: bits for pcre_exec()" in the pcreapi documentation.
1.1 misho 6235:
1.1.1.2 misho 6236: Experiments with Perl suggest that it too has similar optimizations,
1.1 misho 6237: sometimes leading to anomalous results.
6238:
6239: Verbs that act immediately
6240:
1.1.1.2 misho 6241: The following verbs act as soon as they are encountered. They may not
1.1 misho 6242: be followed by a name.
6243:
6244: (*ACCEPT)
6245:
1.1.1.2 misho 6246: This verb causes the match to end successfully, skipping the remainder
6247: of the pattern. However, when it is inside a subpattern that is called
6248: as a subroutine, only that subpattern is ended successfully. Matching
6249: then continues at the outer level. If (*ACCEPT) is inside capturing
1.1 misho 6250: parentheses, the data so far is captured. For example:
6251:
6252: A((?:A|B(*ACCEPT)|C)D)
6253:
1.1.1.2 misho 6254: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
1.1 misho 6255: tured by the outer parentheses.
6256:
6257: (*FAIL) or (*F)
6258:
1.1.1.2 misho 6259: This verb causes a matching failure, forcing backtracking to occur. It
6260: is equivalent to (?!) but easier to read. The Perl documentation notes
6261: that it is probably useful only when combined with (?{}) or (??{}).
6262: Those are, of course, Perl features that are not present in PCRE. The
6263: nearest equivalent is the callout feature, as for example in this pat-
1.1 misho 6264: tern:
6265:
6266: a+(?C)(*FAIL)
6267:
1.1.1.2 misho 6268: A match with the string "aaaa" always fails, but the callout is taken
1.1 misho 6269: before each backtrack happens (in this example, 10 times).
6270:
6271: Recording which path was taken
6272:
1.1.1.2 misho 6273: There is one verb whose main purpose is to track how a match was
6274: arrived at, though it also has a secondary use in conjunction with
1.1 misho 6275: advancing the match starting point (see (*SKIP) below).
6276:
6277: (*MARK:NAME) or (*:NAME)
6278:
1.1.1.2 misho 6279: A name is always required with this verb. There may be as many
6280: instances of (*MARK) as you like in a pattern, and their names do not
1.1 misho 6281: have to be unique.
6282:
1.1.1.2 misho 6283: When a match succeeds, the name of the last-encountered (*MARK) on the
6284: matching path is passed back to the caller as described in the section
6285: entitled "Extra data for pcre_exec()" in the pcreapi documentation.
6286: Here is an example of pcretest output, where the /K modifier requests
6287: the retrieval and outputting of (*MARK) data:
1.1 misho 6288:
6289: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
6290: data> XY
6291: 0: XY
6292: MK: A
6293: XZ
6294: 0: XZ
6295: MK: B
6296:
6297: The (*MARK) name is tagged with "MK:" in this output, and in this exam-
1.1.1.2 misho 6298: ple it indicates which of the two alternatives matched. This is a more
6299: efficient way of obtaining this information than putting each alterna-
1.1 misho 6300: tive in its own capturing parentheses.
6301:
6302: If (*MARK) is encountered in a positive assertion, its name is recorded
6303: and passed back if it is the last-encountered. This does not happen for
6304: negative assertions.
6305:
1.1.1.2 misho 6306: After a partial match or a failed match, the name of the last encoun-
1.1 misho 6307: tered (*MARK) in the entire match process is returned. For example:
6308:
6309: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
6310: data> XP
6311: No match, mark = B
6312:
1.1.1.2 misho 6313: Note that in this unanchored example the mark is retained from the
1.1.1.3 ! misho 6314: match attempt that started at the letter "X" in the subject. Subsequent
! 6315: match attempts starting at "P" and then with an empty string do not get
! 6316: as far as the (*MARK) item, but nevertheless do not reset it.
! 6317:
! 6318: If you are interested in (*MARK) values after failed matches, you
! 6319: should probably set the PCRE_NO_START_OPTIMIZE option (see above) to
! 6320: ensure that the match is always attempted.
1.1 misho 6321:
6322: Verbs that act after backtracking
6323:
6324: The following verbs do nothing when they are encountered. Matching con-
1.1.1.2 misho 6325: tinues with what follows, but if there is no subsequent match, causing
6326: a backtrack to the verb, a failure is forced. That is, backtracking
6327: cannot pass to the left of the verb. However, when one of these verbs
6328: appears inside an atomic group, its effect is confined to that group,
6329: because once the group has been matched, there is never any backtrack-
6330: ing into it. In this situation, backtracking can "jump back" to the
6331: left of the entire atomic group. (Remember also, as stated above, that
1.1 misho 6332: this localization also applies in subroutine calls and assertions.)
6333:
1.1.1.2 misho 6334: These verbs differ in exactly what kind of failure occurs when back-
1.1 misho 6335: tracking reaches them.
6336:
6337: (*COMMIT)
6338:
1.1.1.2 misho 6339: This verb, which may not be followed by a name, causes the whole match
1.1 misho 6340: to fail outright if the rest of the pattern does not match. Even if the
6341: pattern is unanchored, no further attempts to find a match by advancing
6342: the starting point take place. Once (*COMMIT) has been passed,
1.1.1.2 misho 6343: pcre_exec() is committed to finding a match at the current starting
1.1 misho 6344: point, or not at all. For example:
6345:
6346: a+(*COMMIT)b
6347:
1.1.1.2 misho 6348: This matches "xxaab" but not "aacaab". It can be thought of as a kind
1.1 misho 6349: of dynamic anchor, or "I've started, so I must finish." The name of the
1.1.1.2 misho 6350: most recently passed (*MARK) in the path is passed back when (*COMMIT)
1.1 misho 6351: forces a match failure.
6352:
1.1.1.2 misho 6353: Note that (*COMMIT) at the start of a pattern is not the same as an
6354: anchor, unless PCRE's start-of-match optimizations are turned off, as
1.1 misho 6355: shown in this pcretest example:
6356:
6357: re> /(*COMMIT)abc/
6358: data> xyzabc
6359: 0: abc
6360: xyzabc\Y
6361: No match
6362:
1.1.1.2 misho 6363: PCRE knows that any match must start with "a", so the optimization
6364: skips along the subject to "a" before running the first match attempt,
6365: which succeeds. When the optimization is disabled by the \Y escape in
1.1 misho 6366: the second subject, the match starts at "x" and so the (*COMMIT) causes
6367: it to fail without trying any other starting points.
6368:
6369: (*PRUNE) or (*PRUNE:NAME)
6370:
1.1.1.2 misho 6371: This verb causes the match to fail at the current starting position in
6372: the subject if the rest of the pattern does not match. If the pattern
6373: is unanchored, the normal "bumpalong" advance to the next starting
6374: character then happens. Backtracking can occur as usual to the left of
6375: (*PRUNE), before it is reached, or when matching to the right of
6376: (*PRUNE), but if there is no match to the right, backtracking cannot
6377: cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
6378: native to an atomic group or possessive quantifier, but there are some
1.1 misho 6379: uses of (*PRUNE) that cannot be expressed in any other way. The behav-
1.1.1.2 misho 6380: iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
1.1 misho 6381: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
6382:
6383: (*SKIP)
6384:
1.1.1.2 misho 6385: This verb, when given without a name, is like (*PRUNE), except that if
6386: the pattern is unanchored, the "bumpalong" advance is not to the next
1.1 misho 6387: character, but to the position in the subject where (*SKIP) was encoun-
1.1.1.2 misho 6388: tered. (*SKIP) signifies that whatever text was matched leading up to
1.1 misho 6389: it cannot be part of a successful match. Consider:
6390:
6391: a+(*SKIP)b
6392:
1.1.1.2 misho 6393: If the subject is "aaaac...", after the first match attempt fails
6394: (starting at the first character in the string), the starting point
1.1 misho 6395: skips on to start the next attempt at "c". Note that a possessive quan-
1.1.1.2 misho 6396: tifer does not have the same effect as this example; although it would
6397: suppress backtracking during the first match attempt, the second
6398: attempt would start at the second character instead of skipping on to
1.1 misho 6399: "c".
6400:
6401: (*SKIP:NAME)
6402:
1.1.1.2 misho 6403: When (*SKIP) has an associated name, its behaviour is modified. If the
1.1 misho 6404: following pattern fails to match, the previous path through the pattern
1.1.1.2 misho 6405: is searched for the most recent (*MARK) that has the same name. If one
6406: is found, the "bumpalong" advance is to the subject position that cor-
6407: responds to that (*MARK) instead of to where (*SKIP) was encountered.
1.1 misho 6408: If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
6409:
6410: (*THEN) or (*THEN:NAME)
6411:
1.1.1.2 misho 6412: This verb causes a skip to the next innermost alternative if the rest
6413: of the pattern does not match. That is, it cancels pending backtrack-
6414: ing, but only within the current alternative. Its name comes from the
1.1 misho 6415: observation that it can be used for a pattern-based if-then-else block:
6416:
6417: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
6418:
1.1.1.2 misho 6419: If the COND1 pattern matches, FOO is tried (and possibly further items
6420: after the end of the group if FOO succeeds); on failure, the matcher
6421: skips to the second alternative and tries COND2, without backtracking
6422: into COND1. The behaviour of (*THEN:NAME) is exactly the same as
6423: (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts
1.1 misho 6424: like (*PRUNE).
6425:
1.1.1.2 misho 6426: Note that a subpattern that does not contain a | character is just a
6427: part of the enclosing alternative; it is not a nested alternation with
6428: only one alternative. The effect of (*THEN) extends beyond such a sub-
6429: pattern to the enclosing alternative. Consider this pattern, where A,
1.1 misho 6430: B, etc. are complex pattern fragments that do not contain any | charac-
6431: ters at this level:
6432:
6433: A (B(*THEN)C) | D
6434:
1.1.1.2 misho 6435: If A and B are matched, but there is a failure in C, matching does not
1.1 misho 6436: backtrack into A; instead it moves to the next alternative, that is, D.
1.1.1.2 misho 6437: However, if the subpattern containing (*THEN) is given an alternative,
1.1 misho 6438: it behaves differently:
6439:
6440: A (B(*THEN)C | (*FAIL)) | D
6441:
1.1.1.2 misho 6442: The effect of (*THEN) is now confined to the inner subpattern. After a
1.1 misho 6443: failure in C, matching moves to (*FAIL), which causes the whole subpat-
1.1.1.2 misho 6444: tern to fail because there are no more alternatives to try. In this
1.1 misho 6445: case, matching does now backtrack into A.
6446:
6447: Note also that a conditional subpattern is not considered as having two
1.1.1.2 misho 6448: alternatives, because only one is ever used. In other words, the |
1.1 misho 6449: character in a conditional subpattern has a different meaning. Ignoring
6450: white space, consider:
6451:
6452: ^.*? (?(?=a) a | b(*THEN)c )
6453:
1.1.1.2 misho 6454: If the subject is "ba", this pattern does not match. Because .*? is
6455: ungreedy, it initially matches zero characters. The condition (?=a)
6456: then fails, the character "b" is matched, but "c" is not. At this
6457: point, matching does not backtrack to .*? as might perhaps be expected
6458: from the presence of the | character. The conditional subpattern is
1.1 misho 6459: part of the single alternative that comprises the whole pattern, and so
1.1.1.2 misho 6460: the match fails. (If there was a backtrack into .*?, allowing it to
1.1 misho 6461: match "b", the match would succeed.)
6462:
1.1.1.2 misho 6463: The verbs just described provide four different "strengths" of control
1.1 misho 6464: when subsequent matching fails. (*THEN) is the weakest, carrying on the
1.1.1.2 misho 6465: match at the next alternative. (*PRUNE) comes next, failing the match
6466: at the current starting position, but allowing an advance to the next
6467: character (for an unanchored pattern). (*SKIP) is similar, except that
1.1 misho 6468: the advance may be more than one character. (*COMMIT) is the strongest,
6469: causing the entire match to fail.
6470:
6471: If more than one such verb is present in a pattern, the "strongest" one
6472: wins. For example, consider this pattern, where A, B, etc. are complex
6473: pattern fragments:
6474:
6475: (A(*COMMIT)B(*THEN)C|D)
6476:
1.1.1.2 misho 6477: Once A has matched, PCRE is committed to this match, at the current
6478: starting position. If subsequently B matches, but C does not, the nor-
1.1 misho 6479: mal (*THEN) action of trying the next alternative (that is, D) does not
6480: happen because (*COMMIT) overrides.
6481:
6482:
6483: SEE ALSO
6484:
1.1.1.2 misho 6485: pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
6486: pcre16(3).
1.1 misho 6487:
6488:
6489: AUTHOR
6490:
6491: Philip Hazel
6492: University Computing Service
6493: Cambridge CB2 3QH, England.
6494:
6495:
6496: REVISION
6497:
1.1.1.3 ! misho 6498: Last updated: 17 June 2012
1.1.1.2 misho 6499: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 6500: ------------------------------------------------------------------------------
6501:
6502:
6503: PCRESYNTAX(3) PCRESYNTAX(3)
6504:
6505:
6506: NAME
6507: PCRE - Perl-compatible regular expressions
6508:
6509:
6510: PCRE REGULAR EXPRESSION SYNTAX SUMMARY
6511:
6512: The full syntax and semantics of the regular expressions that are sup-
6513: ported by PCRE are described in the pcrepattern documentation. This
1.1.1.2 misho 6514: document contains a quick-reference summary of the syntax.
1.1 misho 6515:
6516:
6517: QUOTING
6518:
6519: \x where x is non-alphanumeric is a literal x
6520: \Q...\E treat enclosed characters as literal
6521:
6522:
6523: CHARACTERS
6524:
6525: \a alarm, that is, the BEL character (hex 07)
6526: \cx "control-x", where x is any ASCII character
6527: \e escape (hex 1B)
1.1.1.3 ! misho 6528: \f form feed (hex 0C)
1.1 misho 6529: \n newline (hex 0A)
6530: \r carriage return (hex 0D)
6531: \t tab (hex 09)
6532: \ddd character with octal code ddd, or backreference
6533: \xhh character with hex code hh
6534: \x{hhh..} character with hex code hhh..
6535:
6536:
6537: CHARACTER TYPES
6538:
6539: . any character except newline;
6540: in dotall mode, any character whatsoever
1.1.1.2 misho 6541: \C one data unit, even in UTF mode (best avoided)
1.1 misho 6542: \d a decimal digit
6543: \D a character that is not a decimal digit
1.1.1.3 ! misho 6544: \h a horizontal white space character
! 6545: \H a character that is not a horizontal white space character
1.1 misho 6546: \N a character that is not a newline
6547: \p{xx} a character with the xx property
6548: \P{xx} a character without the xx property
6549: \R a newline sequence
1.1.1.3 ! misho 6550: \s a white space character
! 6551: \S a character that is not a white space character
! 6552: \v a vertical white space character
! 6553: \V a character that is not a vertical white space character
1.1 misho 6554: \w a "word" character
6555: \W a "non-word" character
6556: \X an extended Unicode sequence
6557:
6558: In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII
1.1.1.2 misho 6559: characters, even in a UTF mode. However, this can be changed by setting
1.1 misho 6560: the PCRE_UCP option.
6561:
6562:
6563: GENERAL CATEGORY PROPERTIES FOR \p and \P
6564:
6565: C Other
6566: Cc Control
6567: Cf Format
6568: Cn Unassigned
6569: Co Private use
6570: Cs Surrogate
6571:
6572: L Letter
6573: Ll Lower case letter
6574: Lm Modifier letter
6575: Lo Other letter
6576: Lt Title case letter
6577: Lu Upper case letter
6578: L& Ll, Lu, or Lt
6579:
6580: M Mark
6581: Mc Spacing mark
6582: Me Enclosing mark
6583: Mn Non-spacing mark
6584:
6585: N Number
6586: Nd Decimal number
6587: Nl Letter number
6588: No Other number
6589:
6590: P Punctuation
6591: Pc Connector punctuation
6592: Pd Dash punctuation
6593: Pe Close punctuation
6594: Pf Final punctuation
6595: Pi Initial punctuation
6596: Po Other punctuation
6597: Ps Open punctuation
6598:
6599: S Symbol
6600: Sc Currency symbol
6601: Sk Modifier symbol
6602: Sm Mathematical symbol
6603: So Other symbol
6604:
6605: Z Separator
6606: Zl Line separator
6607: Zp Paragraph separator
6608: Zs Space separator
6609:
6610:
6611: PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P
6612:
6613: Xan Alphanumeric: union of properties L and N
6614: Xps POSIX space: property Z or tab, NL, VT, FF, CR
6615: Xsp Perl space: property Z or tab, NL, FF, CR
6616: Xwd Perl word: property Xan or underscore
6617:
6618:
6619: SCRIPT NAMES FOR \p AND \P
6620:
1.1.1.3 ! misho 6621: Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
! 6622: Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
! 6623: Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
! 6624: Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic,
! 6625: Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
! 6626: gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip-
! 6627: tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
! 6628: Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
! 6629: Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive,
! 6630: Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko,
! 6631: Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic,
! 6632: Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari-
! 6633: tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese,
! 6634: Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
! 6635: Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
! 6636: Yi.
1.1 misho 6637:
6638:
6639: CHARACTER CLASSES
6640:
6641: [...] positive character class
6642: [^...] negative character class
6643: [x-y] range (can be used for hex characters)
6644: [[:xxx:]] positive POSIX named set
6645: [[:^xxx:]] negative POSIX named set
6646:
6647: alnum alphanumeric
6648: alpha alphabetic
6649: ascii 0-127
6650: blank space or tab
6651: cntrl control character
6652: digit decimal digit
6653: graph printing, excluding space
6654: lower lower case letter
6655: print printing, including space
6656: punct printing, excluding alphanumeric
1.1.1.3 ! misho 6657: space white space
1.1 misho 6658: upper upper case letter
6659: word same as \w
6660: xdigit hexadecimal digit
6661:
6662: In PCRE, POSIX character set names recognize only ASCII characters by
6663: default, but some of them use Unicode properties if PCRE_UCP is set.
6664: You can use \Q...\E inside a character class.
6665:
6666:
6667: QUANTIFIERS
6668:
6669: ? 0 or 1, greedy
6670: ?+ 0 or 1, possessive
6671: ?? 0 or 1, lazy
6672: * 0 or more, greedy
6673: *+ 0 or more, possessive
6674: *? 0 or more, lazy
6675: + 1 or more, greedy
6676: ++ 1 or more, possessive
6677: +? 1 or more, lazy
6678: {n} exactly n
6679: {n,m} at least n, no more than m, greedy
6680: {n,m}+ at least n, no more than m, possessive
6681: {n,m}? at least n, no more than m, lazy
6682: {n,} n or more, greedy
6683: {n,}+ n or more, possessive
6684: {n,}? n or more, lazy
6685:
6686:
6687: ANCHORS AND SIMPLE ASSERTIONS
6688:
6689: \b word boundary
6690: \B not a word boundary
6691: ^ start of subject
6692: also after internal newline in multiline mode
6693: \A start of subject
6694: $ end of subject
6695: also before newline at end of subject
6696: also before internal newline in multiline mode
6697: \Z end of subject
6698: also before newline at end of subject
6699: \z end of subject
6700: \G first matching position in subject
6701:
6702:
6703: MATCH POINT RESET
6704:
6705: \K reset start of match
6706:
6707:
6708: ALTERNATION
6709:
6710: expr|expr|expr...
6711:
6712:
6713: CAPTURING
6714:
6715: (...) capturing group
6716: (?<name>...) named capturing group (Perl)
6717: (?'name'...) named capturing group (Perl)
6718: (?P<name>...) named capturing group (Python)
6719: (?:...) non-capturing group
6720: (?|...) non-capturing group; reset group numbers for
6721: capturing groups in each alternative
6722:
6723:
6724: ATOMIC GROUPS
6725:
6726: (?>...) atomic, non-capturing group
6727:
6728:
6729: COMMENT
6730:
6731: (?#....) comment (not nestable)
6732:
6733:
6734: OPTION SETTING
6735:
6736: (?i) caseless
6737: (?J) allow duplicate names
6738: (?m) multiline
6739: (?s) single line (dotall)
6740: (?U) default ungreedy (lazy)
6741: (?x) extended (ignore white space)
6742: (?-...) unset option(s)
6743:
6744: The following are recognized only at the start of a pattern or after
6745: one of the newline-setting options with similar syntax:
6746:
6747: (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
1.1.1.2 misho 6748: (*UTF8) set UTF-8 mode: 8-bit library (PCRE_UTF8)
6749: (*UTF16) set UTF-16 mode: 16-bit library (PCRE_UTF16)
1.1 misho 6750: (*UCP) set PCRE_UCP (use Unicode properties for \d etc)
6751:
6752:
6753: LOOKAHEAD AND LOOKBEHIND ASSERTIONS
6754:
6755: (?=...) positive look ahead
6756: (?!...) negative look ahead
6757: (?<=...) positive look behind
6758: (?<!...) negative look behind
6759:
6760: Each top-level branch of a look behind must be of a fixed length.
6761:
6762:
6763: BACKREFERENCES
6764:
6765: \n reference by number (can be ambiguous)
6766: \gn reference by number
6767: \g{n} reference by number
6768: \g{-n} relative reference by number
6769: \k<name> reference by name (Perl)
6770: \k'name' reference by name (Perl)
6771: \g{name} reference by name (Perl)
6772: \k{name} reference by name (.NET)
6773: (?P=name) reference by name (Python)
6774:
6775:
6776: SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
6777:
6778: (?R) recurse whole pattern
6779: (?n) call subpattern by absolute number
6780: (?+n) call subpattern by relative number
6781: (?-n) call subpattern by relative number
6782: (?&name) call subpattern by name (Perl)
6783: (?P>name) call subpattern by name (Python)
6784: \g<name> call subpattern by name (Oniguruma)
6785: \g'name' call subpattern by name (Oniguruma)
6786: \g<n> call subpattern by absolute number (Oniguruma)
6787: \g'n' call subpattern by absolute number (Oniguruma)
6788: \g<+n> call subpattern by relative number (PCRE extension)
6789: \g'+n' call subpattern by relative number (PCRE extension)
6790: \g<-n> call subpattern by relative number (PCRE extension)
6791: \g'-n' call subpattern by relative number (PCRE extension)
6792:
6793:
6794: CONDITIONAL PATTERNS
6795:
6796: (?(condition)yes-pattern)
6797: (?(condition)yes-pattern|no-pattern)
6798:
6799: (?(n)... absolute reference condition
6800: (?(+n)... relative reference condition
6801: (?(-n)... relative reference condition
6802: (?(<name>)... named reference condition (Perl)
6803: (?('name')... named reference condition (Perl)
6804: (?(name)... named reference condition (PCRE)
6805: (?(R)... overall recursion condition
6806: (?(Rn)... specific group recursion condition
6807: (?(R&name)... specific recursion condition
6808: (?(DEFINE)... define subpattern for reference
6809: (?(assert)... assertion condition
6810:
6811:
6812: BACKTRACKING CONTROL
6813:
6814: The following act immediately they are reached:
6815:
6816: (*ACCEPT) force successful match
6817: (*FAIL) force backtrack; synonym (*F)
1.1.1.2 misho 6818: (*MARK:NAME) set name to be passed back; synonym (*:NAME)
1.1 misho 6819:
6820: The following act only when a subsequent match failure causes a back-
6821: track to reach them. They all force a match failure, but they differ in
6822: what happens afterwards. Those that advance the start-of-match point do
6823: so only if the pattern is not anchored.
6824:
6825: (*COMMIT) overall failure, no advance of starting point
6826: (*PRUNE) advance to next starting character
1.1.1.2 misho 6827: (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE)
6828: (*SKIP) advance to current matching position
6829: (*SKIP:NAME) advance to position corresponding to an earlier
6830: (*MARK:NAME); if not found, the (*SKIP) is ignored
1.1 misho 6831: (*THEN) local failure, backtrack to next alternation
1.1.1.2 misho 6832: (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN)
1.1 misho 6833:
6834:
6835: NEWLINE CONVENTIONS
6836:
6837: These are recognized only at the very start of the pattern or after a
1.1.1.2 misho 6838: (*BSR_...), (*UTF8), (*UTF16) or (*UCP) option.
1.1 misho 6839:
6840: (*CR) carriage return only
6841: (*LF) linefeed only
6842: (*CRLF) carriage return followed by linefeed
6843: (*ANYCRLF) all three of the above
6844: (*ANY) any Unicode newline sequence
6845:
6846:
6847: WHAT \R MATCHES
6848:
6849: These are recognized only at the very start of the pattern or after a
1.1.1.2 misho 6850: (*...) option that sets the newline convention or a UTF or UCP mode.
1.1 misho 6851:
6852: (*BSR_ANYCRLF) CR, LF, or CRLF
6853: (*BSR_UNICODE) any Unicode newline sequence
6854:
6855:
6856: CALLOUTS
6857:
6858: (?C) callout
6859: (?Cn) callout with data n
6860:
6861:
6862: SEE ALSO
6863:
6864: pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
6865:
6866:
6867: AUTHOR
6868:
6869: Philip Hazel
6870: University Computing Service
6871: Cambridge CB2 3QH, England.
6872:
6873:
6874: REVISION
6875:
1.1.1.2 misho 6876: Last updated: 10 January 2012
6877: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 6878: ------------------------------------------------------------------------------
6879:
6880:
6881: PCREUNICODE(3) PCREUNICODE(3)
6882:
6883:
6884: NAME
6885: PCRE - Perl-compatible regular expressions
6886:
6887:
1.1.1.2 misho 6888: UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT
6889:
6890: From Release 8.30, in addition to its previous UTF-8 support, PCRE also
6891: supports UTF-16 by means of a separate 16-bit library. This can be
6892: built as well as, or instead of, the 8-bit library.
6893:
6894:
6895: UTF-8 SUPPORT
1.1 misho 6896:
1.1.1.2 misho 6897: In order process UTF-8 strings, you must build PCRE's 8-bit library
6898: with UTF support, and, in addition, you must call pcre_compile() with
6899: the PCRE_UTF8 option flag, or the pattern must start with the sequence
6900: (*UTF8). When either of these is the case, both the pattern and any
6901: subject strings that are matched against it are treated as UTF-8
6902: strings instead of strings of 1-byte characters.
1.1 misho 6903:
1.1.1.2 misho 6904:
6905: UTF-16 SUPPORT
6906:
6907: In order process UTF-16 strings, you must build PCRE's 16-bit library
6908: with UTF support, and, in addition, you must call pcre16_compile() with
6909: the PCRE_UTF16 option flag, or the pattern must start with the sequence
6910: (*UTF16). When either of these is the case, both the pattern and any
6911: subject strings that are matched against it are treated as UTF-16
6912: strings instead of strings of 16-bit characters.
6913:
6914:
6915: UTF SUPPORT OVERHEAD
6916:
6917: If you compile PCRE with UTF support, but do not use it at run time,
1.1 misho 6918: the library will be a bit bigger, but the additional run time overhead
1.1.1.2 misho 6919: is limited to testing the PCRE_UTF8/16 flag occasionally, so should not
6920: be very big.
6921:
6922:
6923: UNICODE PROPERTY SUPPORT
1.1 misho 6924:
6925: If PCRE is built with Unicode character property support (which implies
1.1.1.2 misho 6926: UTF support), the escape sequences \p{..}, \P{..}, and \X can be used.
6927: The available properties that can be tested are limited to the general
6928: category properties such as Lu for an upper case letter or Nd for a
6929: decimal number, the Unicode script names such as Arabic or Han, and the
6930: derived properties Any and L&. A full list is given in the pcrepattern
6931: documentation. Only the short names for properties are supported. For
6932: example, \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not
6933: supported. Furthermore, in Perl, many properties may optionally be
6934: prefixed by "Is", for compatibility with Perl 5.6. PCRE does not sup-
6935: port this.
1.1 misho 6936:
6937: Validity of UTF-8 strings
6938:
1.1.1.2 misho 6939: When you set the PCRE_UTF8 flag, the byte strings passed as patterns
6940: and subjects are (by default) checked for validity on entry to the rel-
1.1.1.3 ! misho 6941: evant functions. The entire string is checked before any other process-
! 6942: ing takes place. From release 7.3 of PCRE, the check is according the
1.1.1.2 misho 6943: rules of RFC 3629, which are themselves derived from the Unicode speci-
1.1.1.3 ! misho 6944: fication. Earlier releases of PCRE followed the rules of RFC 2279,
! 6945: which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The
! 6946: current check allows only values in the range U+0 to U+10FFFF, exclud-
1.1.1.2 misho 6947: ing U+D800 to U+DFFF.
6948:
1.1.1.3 ! misho 6949: The excluded code points are the "Surrogate Area" of Unicode. They are
! 6950: reserved for use by UTF-16, where they are used in pairs to encode
! 6951: codepoints with values greater than 0xFFFF. The code points that are
1.1.1.2 misho 6952: encoded by UTF-16 pairs are available independently in the UTF-8 encod-
1.1.1.3 ! misho 6953: ing. (In other words, the whole surrogate thing is a fudge for UTF-16
1.1.1.2 misho 6954: which unfortunately messes up UTF-8.)
1.1 misho 6955:
6956: If an invalid UTF-8 string is passed to PCRE, an error return is given.
1.1.1.3 ! misho 6957: At compile time, the only additional information is the offset to the
! 6958: first byte of the failing character. The run-time functions pcre_exec()
! 6959: and pcre_dfa_exec() also pass back this information, as well as a more
! 6960: detailed reason code if the caller has provided memory in which to do
1.1 misho 6961: this.
6962:
1.1.1.3 ! misho 6963: In some situations, you may already know that your strings are valid,
! 6964: and therefore want to skip these checks in order to improve perfor-
! 6965: mance, for example in the case of a long subject string that is being
! 6966: scanned repeatedly with different patterns. If you set the
! 6967: PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes
! 6968: that the pattern or subject it is given (respectively) contains only
! 6969: valid UTF-8 codes. In this case, it does not diagnose an invalid UTF-8
! 6970: string.
1.1 misho 6971:
1.1.1.3 ! misho 6972: If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
! 6973: what happens depends on why the string is invalid. If the string con-
1.1 misho 6974: forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
1.1.1.3 ! misho 6975: string of characters in the range 0 to 0x7FFFFFFF by pcre_dfa_exec()
! 6976: and the interpreted version of pcre_exec(). In other words, apart from
! 6977: the initial validity test, these functions (when in UTF-8 mode) handle
! 6978: strings according to the more liberal rules of RFC 2279. However, the
1.1 misho 6979: just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629.
1.1.1.3 ! misho 6980: If you are using JIT optimization, or if the string does not even con-
1.1 misho 6981: form to RFC 2279, the result is undefined. Your program may crash.
6982:
1.1.1.3 ! misho 6983: If you want to process strings of values in the full range 0 to
! 6984: 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
1.1 misho 6985: set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
1.1.1.3 ! misho 6986: this situation, you will have to apply your own validity check, and
1.1 misho 6987: avoid the use of JIT optimization.
6988:
1.1.1.2 misho 6989: Validity of UTF-16 strings
1.1 misho 6990:
1.1.1.2 misho 6991: When you set the PCRE_UTF16 flag, the strings of 16-bit data units that
6992: are passed as patterns and subjects are (by default) checked for valid-
1.1.1.3 ! misho 6993: ity on entry to the relevant functions. Values other than those in the
1.1.1.2 misho 6994: surrogate range U+D800 to U+DFFF are independent code points. Values in
6995: the surrogate range must be used in pairs in the correct manner.
6996:
1.1.1.3 ! misho 6997: If an invalid UTF-16 string is passed to PCRE, an error return is
! 6998: given. At compile time, the only additional information is the offset
! 6999: to the first data unit of the failing character. The run-time functions
1.1.1.2 misho 7000: pcre16_exec() and pcre16_dfa_exec() also pass back this information, as
1.1.1.3 ! misho 7001: well as a more detailed reason code if the caller has provided memory
1.1.1.2 misho 7002: in which to do this.
7003:
1.1.1.3 ! misho 7004: In some situations, you may already know that your strings are valid,
! 7005: and therefore want to skip these checks in order to improve perfor-
! 7006: mance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at
1.1.1.2 misho 7007: run time, PCRE assumes that the pattern or subject it is given (respec-
7008: tively) contains only valid UTF-16 sequences. In this case, it does not
7009: diagnose an invalid UTF-16 string.
7010:
7011: General comments about UTF modes
7012:
1.1.1.3 ! misho 7013: 1. Codepoints less than 256 can be specified by either braced or
! 7014: unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
1.1.1.2 misho 7015: Larger values have to use braced sequences.
7016:
1.1.1.3 ! misho 7017: 2. Octal numbers up to \777 are recognized, and in UTF-8 mode, they
1.1.1.2 misho 7018: match two-byte characters for values greater than \177.
7019:
7020: 3. Repeat quantifiers apply to complete UTF characters, not to individ-
7021: ual data units, for example: \x{100}{3}.
7022:
1.1.1.3 ! misho 7023: 4. The dot metacharacter matches one UTF character instead of a single
1.1.1.2 misho 7024: data unit.
7025:
1.1.1.3 ! misho 7026: 5. The escape sequence \C can be used to match a single byte in UTF-8
1.1.1.2 misho 7027: mode, or a single 16-bit data unit in UTF-16 mode, but its use can lead
7028: to some strange effects because it breaks up multi-unit characters (see
1.1.1.3 ! misho 7029: the description of \C in the pcrepattern documentation). The use of \C
! 7030: is not supported in the alternative matching function
! 7031: pcre[16]_dfa_exec(), nor is it supported in UTF mode by the JIT opti-
1.1.1.2 misho 7032: mization of pcre[16]_exec(). If JIT optimization is requested for a UTF
7033: pattern that contains \C, it will not succeed, and so the matching will
7034: be carried out by the normal interpretive function.
1.1 misho 7035:
1.1.1.3 ! misho 7036: 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
1.1 misho 7037: test characters of any code value, but, by default, the characters that
1.1.1.3 ! misho 7038: PCRE recognizes as digits, spaces, or word characters remain the same
! 7039: set as in non-UTF mode, all with values less than 256. This remains
! 7040: true even when PCRE is built to include Unicode property support,
1.1.1.2 misho 7041: because to do otherwise would slow down PCRE in many common cases. Note
1.1.1.3 ! misho 7042: in particular that this applies to \b and \B, because they are defined
1.1.1.2 misho 7043: in terms of \w and \W. If you really want to test for a wider sense of,
1.1.1.3 ! misho 7044: say, "digit", you can use explicit Unicode property tests such as
1.1.1.2 misho 7045: \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the
1.1.1.3 ! misho 7046: character escapes work is changed so that Unicode properties are used
1.1.1.2 misho 7047: to determine which characters match. There are more details in the sec-
7048: tion on generic character types in the pcrepattern documentation.
1.1 misho 7049:
1.1.1.3 ! misho 7050: 7. Similarly, characters that match the POSIX named character classes
1.1 misho 7051: are all low-valued characters, unless the PCRE_UCP option is set.
7052:
1.1.1.3 ! misho 7053: 8. However, the horizontal and vertical white space matching escapes
! 7054: (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
1.1 misho 7055: whether or not PCRE_UCP is set.
7056:
1.1.1.3 ! misho 7057: 9. Case-insensitive matching applies only to characters whose values
! 7058: are less than 128, unless PCRE is built with Unicode property support.
! 7059: Even when Unicode property support is available, PCRE still uses its
! 7060: own character tables when checking the case of low-valued characters,
! 7061: so as not to degrade performance. The Unicode property information is
1.1 misho 7062: used only for characters with higher values. Furthermore, PCRE supports
1.1.1.3 ! misho 7063: case-insensitive matching only when there is a one-to-one mapping
! 7064: between a letter's cases. There are a small number of many-to-one map-
1.1 misho 7065: pings in Unicode; these are not supported by PCRE.
7066:
7067:
7068: AUTHOR
7069:
7070: Philip Hazel
7071: University Computing Service
7072: Cambridge CB2 3QH, England.
7073:
7074:
7075: REVISION
7076:
1.1.1.3 ! misho 7077: Last updated: 14 April 2012
1.1.1.2 misho 7078: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 7079: ------------------------------------------------------------------------------
7080:
7081:
7082: PCREJIT(3) PCREJIT(3)
7083:
7084:
7085: NAME
7086: PCRE - Perl-compatible regular expressions
7087:
7088:
7089: PCRE JUST-IN-TIME COMPILER SUPPORT
7090:
7091: Just-in-time compiling is a heavyweight optimization that can greatly
7092: speed up pattern matching. However, it comes at the cost of extra pro-
7093: cessing before the match is performed. Therefore, it is of most benefit
7094: when the same pattern is going to be matched many times. This does not
1.1.1.2 misho 7095: necessarily mean many calls of a matching function; if the pattern is
7096: not anchored, matching attempts may take place many times at various
7097: positions in the subject, even for a single call. Therefore, if the
1.1 misho 7098: subject string is very long, it may still pay to use JIT for one-off
7099: matches.
7100:
1.1.1.2 misho 7101: JIT support applies only to the traditional Perl-compatible matching
7102: function. It does not apply when the DFA matching function is being
7103: used. The code for this support was written by Zoltan Herczeg.
7104:
7105:
7106: 8-BIT and 16-BIT SUPPORT
7107:
7108: JIT support is available for both the 8-bit and 16-bit PCRE libraries.
7109: To keep this documentation simple, only the 8-bit interface is
7110: described in what follows. If you are using the 16-bit library, substi-
7111: tute the 16-bit functions and 16-bit structures (for example,
7112: pcre16_jit_stack instead of pcre_jit_stack).
1.1 misho 7113:
7114:
7115: AVAILABILITY OF JIT SUPPORT
7116:
7117: JIT support is an optional feature of PCRE. The "configure" option
7118: --enable-jit (or equivalent CMake option) must be set when PCRE is
7119: built if you want to use JIT. The support is limited to the following
7120: hardware platforms:
7121:
7122: ARM v5, v7, and Thumb2
7123: Intel x86 32-bit and 64-bit
7124: MIPS 32-bit
1.1.1.2 misho 7125: Power PC 32-bit and 64-bit
1.1 misho 7126:
1.1.1.3 ! misho 7127: If --enable-jit is set on an unsupported platform, compilation fails.
1.1 misho 7128:
7129: A program that is linked with PCRE 8.20 or later can tell if JIT sup-
7130: port is available by calling pcre_config() with the PCRE_CONFIG_JIT
7131: option. The result is 1 when JIT is available, and 0 otherwise. How-
7132: ever, a simple program does not need to check this in order to use JIT.
1.1.1.3 ! misho 7133: The API is implemented in a way that falls back to the interpretive
1.1 misho 7134: code if JIT is not available.
7135:
7136: If your program may sometimes be linked with versions of PCRE that are
7137: older than 8.20, but you want to use JIT when it is available, you can
7138: test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT
7139: macro such as PCRE_CONFIG_JIT, for compile-time control of your code.
7140:
7141:
7142: SIMPLE USE OF JIT
7143:
7144: You have to do two things to make use of the JIT support in the sim-
7145: plest way:
7146:
7147: (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
7148: each compiled pattern, and pass the resulting pcre_extra block to
7149: pcre_exec().
7150:
7151: (2) Use pcre_free_study() to free the pcre_extra block when it is
1.1.1.3 ! misho 7152: no longer needed, instead of just freeing it yourself. This
1.1 misho 7153: ensures that any JIT data is also freed.
7154:
7155: For a program that may be linked with pre-8.20 versions of PCRE, you
7156: can insert
7157:
7158: #ifndef PCRE_STUDY_JIT_COMPILE
7159: #define PCRE_STUDY_JIT_COMPILE 0
7160: #endif
7161:
7162: so that no option is passed to pcre_study(), and then use something
7163: like this to free the study data:
7164:
7165: #ifdef PCRE_CONFIG_JIT
7166: pcre_free_study(study_ptr);
7167: #else
7168: pcre_free(study_ptr);
7169: #endif
7170:
1.1.1.3 ! misho 7171: PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for
! 7172: complete matches. If you want to run partial matches using the
! 7173: PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you
! 7174: should set one or both of the following options in addition to, or
! 7175: instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():
! 7176:
! 7177: PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
! 7178: PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
! 7179:
! 7180: The JIT compiler generates different optimized code for each of the
! 7181: three modes (normal, soft partial, hard partial). When pcre_exec() is
! 7182: called, the appropriate code is run if it is available. Otherwise, the
! 7183: pattern is matched using interpretive code.
! 7184:
! 7185: In some circumstances you may need to call additional functions. These
! 7186: are described in the section entitled "Controlling the JIT stack"
1.1 misho 7187: below.
7188:
1.1.1.3 ! misho 7189: If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are
! 7190: ignored, and no JIT data is created. Otherwise, the compiled pattern is
! 7191: passed to the JIT compiler, which turns it into machine code that exe-
! 7192: cutes much faster than the normal interpretive code. When pcre_exec()
! 7193: is passed a pcre_extra block containing a pointer to JIT code of the
! 7194: appropriate mode (normal or hard/soft partial), it obeys that code
! 7195: instead of running the interpreter. The result is identical, but the
! 7196: compiled JIT code runs much faster.
1.1 misho 7197:
7198: There are some pcre_exec() options that are not supported for JIT exe-
7199: cution. There are also some pattern items that JIT cannot handle.
7200: Details are given below. In both cases, execution automatically falls
1.1.1.3 ! misho 7201: back to the interpretive code. If you want to know whether JIT was
! 7202: actually used for a particular match, you should arrange for a JIT
! 7203: callback function to be set up as described in the section entitled
! 7204: "Controlling the JIT stack" below, even if you do not need to supply a
! 7205: non-default JIT stack. Such a callback function is called whenever JIT
! 7206: code is about to be obeyed. If the execution options are not right for
! 7207: JIT execution, the callback function is not obeyed.
1.1 misho 7208:
7209: If the JIT compiler finds an unsupported item, no JIT data is gener-
7210: ated. You can find out if JIT execution is available after studying a
7211: pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A
7212: result of 1 means that JIT compilation was successful. A result of 0
7213: means that JIT support is not available, or the pattern was not studied
1.1.1.3 ! misho 7214: with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to
! 7215: handle the pattern.
1.1 misho 7216:
7217: Once a pattern has been studied, with or without JIT, it can be used as
7218: many times as you like for matching different subject strings.
7219:
7220:
7221: UNSUPPORTED OPTIONS AND PATTERN ITEMS
7222:
7223: The only pcre_exec() options that are supported for JIT execution are
1.1.1.3 ! misho 7224: PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NOTBOL, PCRE_NOTEOL,
! 7225: PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PAR-
! 7226: TIAL_SOFT.
1.1 misho 7227:
7228: The unsupported pattern items are:
7229:
7230: \C match a single byte; not supported in UTF-8 mode
7231: (?Cn) callouts
1.1.1.3 ! misho 7232: (*PRUNE) )
! 7233: (*SKIP) ) backtracking control verbs
1.1 misho 7234: (*THEN) )
7235:
7236: Support for some of these may be added in future.
7237:
7238:
7239: RETURN VALUES FROM JIT EXECUTION
7240:
7241: When a pattern is matched using JIT execution, the return values are
7242: the same as those given by the interpretive pcre_exec() code, with the
7243: addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means
7244: that the memory used for the JIT stack was insufficient. See "Control-
7245: ling the JIT stack" below for a discussion of JIT stack usage. For com-
7246: patibility with the interpretive pcre_exec() code, no more than two-
7247: thirds of the ovector argument is used for passing back captured sub-
7248: strings.
7249:
7250: The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if
7251: searching a very large pattern tree goes on for too long, as it is in
7252: the same circumstance when JIT is not used, but the details of exactly
7253: what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error
7254: code is never returned by JIT execution.
7255:
7256:
7257: SAVING AND RESTORING COMPILED PATTERNS
7258:
7259: The code that is generated by the JIT compiler is architecture-spe-
7260: cific, and is also position dependent. For those reasons it cannot be
7261: saved (in a file or database) and restored later like the bytecode and
7262: other data of a compiled pattern. Saving and restoring compiled pat-
7263: terns is not something many people do. More detail about this facility
7264: is given in the pcreprecompile documentation. It should be possible to
7265: run pcre_study() on a saved and restored pattern, and thereby recreate
7266: the JIT data, but because JIT compilation uses significant resources,
7267: it is probably not worth doing this; you might as well recompile the
7268: original pattern.
7269:
7270:
7271: CONTROLLING THE JIT STACK
7272:
7273: When the compiled JIT code runs, it needs a block of memory to use as a
7274: stack. By default, it uses 32K on the machine stack. However, some
7275: large or complicated patterns need more than this. The error
7276: PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
7277: Three functions are provided for managing blocks of memory for use as
7278: JIT stacks. There is further discussion about the use of JIT stacks in
7279: the section entitled "JIT stack FAQ" below.
7280:
7281: The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments
7282: are a starting size and a maximum size, and it returns a pointer to an
7283: opaque structure of type pcre_jit_stack, or NULL if there is an error.
7284: The pcre_jit_stack_free() function can be used to free a stack that is
7285: no longer needed. (For the technically minded: the address space is
7286: allocated by mmap or VirtualAlloc.)
7287:
7288: JIT uses far less memory for recursion than the interpretive code, and
7289: a maximum stack size of 512K to 1M should be more than enough for any
7290: pattern.
7291:
7292: The pcre_assign_jit_stack() function specifies which stack JIT code
7293: should use. Its arguments are as follows:
7294:
7295: pcre_extra *extra
7296: pcre_jit_callback callback
7297: void *data
7298:
7299: The extra argument must be the result of studying a pattern with
1.1.1.3 ! misho 7300: PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the
1.1 misho 7301: other two options:
7302:
7303: (1) If callback is NULL and data is NULL, an internal 32K block
7304: on the machine stack is used.
7305:
7306: (2) If callback is NULL and data is not NULL, data must be
7307: a valid JIT stack, the result of calling pcre_jit_stack_alloc().
7308:
1.1.1.3 ! misho 7309: (3) If callback is not NULL, it must point to a function that is
! 7310: called with data as an argument at the start of matching, in
! 7311: order to set up a JIT stack. If the return from the callback
! 7312: function is NULL, the internal 32K stack is used; otherwise the
! 7313: return value must be a valid JIT stack, the result of calling
! 7314: pcre_jit_stack_alloc().
! 7315:
! 7316: A callback function is obeyed whenever JIT code is about to be run; it
! 7317: is not obeyed when pcre_exec() is called with options that are incom-
! 7318: patible for JIT execution. A callback function can therefore be used to
! 7319: determine whether a match operation was executed by JIT or by the
! 7320: interpreter.
! 7321:
! 7322: You may safely use the same JIT stack for more than one pattern (either
! 7323: by assigning directly or by callback), as long as the patterns are all
! 7324: matched sequentially in the same thread. In a multithread application,
! 7325: if you do not specify a JIT stack, or if you assign or pass back NULL
! 7326: from a callback, that is thread-safe, because each thread has its own
! 7327: machine stack. However, if you assign or pass back a non-NULL JIT
! 7328: stack, this must be a different stack for each thread so that the
! 7329: application is thread-safe.
! 7330:
! 7331: Strictly speaking, even more is allowed. You can assign the same non-
! 7332: NULL stack to any number of patterns as long as they are not used for
! 7333: matching by multiple threads at the same time. For example, you can
! 7334: assign the same stack to all compiled patterns, and use a global mutex
! 7335: in the callback to wait until the stack is available for use. However,
! 7336: this is an inefficient solution, and not recommended.
1.1 misho 7337:
1.1.1.3 ! misho 7338: This is a suggestion for how a multithreaded program that needs to set
! 7339: up non-default JIT stacks might operate:
1.1 misho 7340:
7341: During thread initalization
7342: thread_local_var = pcre_jit_stack_alloc(...)
7343:
7344: During thread exit
7345: pcre_jit_stack_free(thread_local_var)
7346:
7347: Use a one-line callback function
7348: return thread_local_var
7349:
1.1.1.3 ! misho 7350: All the functions described in this section do nothing if JIT is not
! 7351: available, and pcre_assign_jit_stack() does nothing unless the extra
! 7352: argument is non-NULL and points to a pcre_extra block that is the
! 7353: result of a successful study with PCRE_STUDY_JIT_COMPILE etc.
1.1 misho 7354:
7355:
7356: JIT STACK FAQ
7357:
7358: (1) Why do we need JIT stacks?
7359:
1.1.1.3 ! misho 7360: PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack
! 7361: where the local data of the current node is pushed before checking its
1.1 misho 7362: child nodes. Allocating real machine stack on some platforms is diffi-
7363: cult. For example, the stack chain needs to be updated every time if we
1.1.1.3 ! misho 7364: extend the stack on PowerPC. Although it is possible, its updating
1.1 misho 7365: time overhead decreases performance. So we do the recursion in memory.
7366:
7367: (2) Why don't we simply allocate blocks of memory with malloc()?
7368:
1.1.1.3 ! misho 7369: Modern operating systems have a nice feature: they can reserve an
1.1 misho 7370: address space instead of allocating memory. We can safely allocate mem-
1.1.1.3 ! misho 7371: ory pages inside this address space, so the stack could grow without
1.1 misho 7372: moving memory data (this is important because of pointers). Thus we can
1.1.1.3 ! misho 7373: allocate 1M address space, and use only a single memory page (usually
! 7374: 4K) if that is enough. However, we can still grow up to 1M anytime if
1.1 misho 7375: needed.
7376:
7377: (3) Who "owns" a JIT stack?
7378:
7379: The owner of the stack is the user program, not the JIT studied pattern
1.1.1.3 ! misho 7380: or anything else. The user program must ensure that if a stack is used
! 7381: by pcre_exec(), (that is, it is assigned to the pattern currently run-
1.1 misho 7382: ning), that stack must not be used by any other threads (to avoid over-
7383: writing the same memory area). The best practice for multithreaded pro-
1.1.1.3 ! misho 7384: grams is to allocate a stack for each thread, and return this stack
1.1 misho 7385: through the JIT callback function.
7386:
7387: (4) When should a JIT stack be freed?
7388:
7389: You can free a JIT stack at any time, as long as it will not be used by
1.1.1.3 ! misho 7390: pcre_exec() again. When you assign the stack to a pattern, only a
! 7391: pointer is set. There is no reference counting or any other magic. You
! 7392: can free the patterns and stacks in any order, anytime. Just do not
! 7393: call pcre_exec() with a pattern pointing to an already freed stack, as
! 7394: that will cause SEGFAULT. (Also, do not free a stack currently used by
! 7395: pcre_exec() in another thread). You can also replace the stack for a
! 7396: pattern at any time. You can even free the previous stack before
1.1 misho 7397: assigning a replacement.
7398:
1.1.1.3 ! misho 7399: (5) Should I allocate/free a stack every time before/after calling
1.1 misho 7400: pcre_exec()?
7401:
1.1.1.3 ! misho 7402: No, because this is too costly in terms of resources. However, you
! 7403: could implement some clever idea which release the stack if it is not
1.1 misho 7404: used in let's say two minutes. The JIT callback can help to achive this
7405: without keeping a list of the currently JIT studied patterns.
7406:
1.1.1.3 ! misho 7407: (6) OK, the stack is for long term memory allocation. But what happens
! 7408: if a pattern causes stack overflow with a stack of 1M? Is that 1M kept
1.1 misho 7409: until the stack is freed?
7410:
1.1.1.3 ! misho 7411: Especially on embedded sytems, it might be a good idea to release mem-
! 7412: ory sometimes without freeing the stack. There is no API for this at
! 7413: the moment. Probably a function call which returns with the currently
! 7414: allocated memory for any stack and another which allows releasing mem-
1.1 misho 7415: ory (shrinking the stack) would be a good idea if someone needs this.
7416:
7417: (7) This is too much of a headache. Isn't there any better solution for
7418: JIT stack handling?
7419:
1.1.1.3 ! misho 7420: No, thanks to Windows. If POSIX threads were used everywhere, we could
1.1 misho 7421: throw out this complicated API.
7422:
7423:
7424: EXAMPLE CODE
7425:
1.1.1.3 ! misho 7426: This is a single-threaded example that specifies a JIT stack without
1.1 misho 7427: using a callback.
7428:
7429: int rc;
7430: int ovector[30];
7431: pcre *re;
7432: pcre_extra *extra;
7433: pcre_jit_stack *jit_stack;
7434:
7435: re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
7436: /* Check for errors */
7437: extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
7438: jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
7439: /* Check for error (NULL) */
7440: pcre_assign_jit_stack(extra, NULL, jit_stack);
7441: rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
7442: /* Check results */
7443: pcre_free(re);
7444: pcre_free_study(extra);
7445: pcre_jit_stack_free(jit_stack);
7446:
7447:
7448: SEE ALSO
7449:
7450: pcreapi(3)
7451:
7452:
7453: AUTHOR
7454:
7455: Philip Hazel (FAQ by Zoltan Herczeg)
7456: University Computing Service
7457: Cambridge CB2 3QH, England.
7458:
7459:
7460: REVISION
7461:
1.1.1.3 ! misho 7462: Last updated: 04 May 2012
1.1.1.2 misho 7463: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 7464: ------------------------------------------------------------------------------
7465:
7466:
7467: PCREPARTIAL(3) PCREPARTIAL(3)
7468:
7469:
7470: NAME
7471: PCRE - Perl-compatible regular expressions
7472:
7473:
7474: PARTIAL MATCHING IN PCRE
7475:
1.1.1.2 misho 7476: In normal use of PCRE, if the subject string that is passed to a match-
7477: ing function matches as far as it goes, but is too short to match the
7478: entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances
7479: where it might be helpful to distinguish this case from other cases in
7480: which there is no match.
1.1 misho 7481:
7482: Consider, for example, an application where a human is required to type
7483: in data for a field with specific formatting requirements. An example
7484: might be a date in the form ddmmmyy, defined by this pattern:
7485:
7486: ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
7487:
7488: If the application sees the user's keystrokes one by one, and can check
7489: that what has been typed so far is potentially valid, it is able to
7490: raise an error as soon as a mistake is made, by beeping and not
7491: reflecting the character that has been typed, for example. This immedi-
7492: ate feedback is likely to be a better user interface than a check that
7493: is delayed until the entire string has been entered. Partial matching
7494: can also be useful when the subject string is very long and is not all
7495: available at once.
7496:
7497: PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
1.1.1.2 misho 7498: PCRE_PARTIAL_HARD options, which can be set when calling any of the
7499: matching functions. For backwards compatibility, PCRE_PARTIAL is a syn-
7500: onym for PCRE_PARTIAL_SOFT. The essential difference between the two
7501: options is whether or not a partial match is preferred to an alterna-
7502: tive complete match, though the details differ between the two types of
7503: matching function. If both options are set, PCRE_PARTIAL_HARD takes
7504: precedence.
7505:
1.1.1.3 ! misho 7506: If you want to use partial matching with just-in-time optimized code,
! 7507: you must call pcre_study() or pcre16_study() with one or both of these
! 7508: options:
! 7509:
! 7510: PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
! 7511: PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
! 7512:
! 7513: PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-
! 7514: partial matches on the same pattern. If the appropriate JIT study mode
! 7515: has not been set for a match, the interpretive matching code is used.
! 7516:
! 7517: Setting a partial matching option disables two of PCRE's standard opti-
! 7518: mizations. PCRE remembers the last literal data unit in a pattern, and
! 7519: abandons matching immediately if it is not present in the subject
1.1.1.2 misho 7520: string. This optimization cannot be used for a subject string that
7521: might match only partially. If the pattern was studied, PCRE knows the
7522: minimum length of a matching string, and does not bother to run the
7523: matching function on shorter strings. This optimization is also dis-
1.1 misho 7524: abled for partial matching.
7525:
7526:
1.1.1.2 misho 7527: PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()
1.1 misho 7528:
1.1.1.2 misho 7529: A partial match occurs during a call to pcre_exec() or pcre16_exec()
7530: when the end of the subject string is reached successfully, but match-
7531: ing cannot continue because more characters are needed. However, at
7532: least one character in the subject must have been inspected. This char-
7533: acter need not form part of the final matched string; lookbehind asser-
7534: tions and the \K escape sequence provide ways of inspecting characters
7535: before the start of a matched substring. The requirement for inspecting
7536: at least one character exists because an empty string can always be
7537: matched; without such a restriction there would always be a partial
7538: match of an empty string at the end of the subject.
7539:
7540: If there are at least two slots in the offsets vector when a partial
7541: match is returned, the first slot is set to the offset of the earliest
7542: character that was inspected. For convenience, the second offset points
7543: to the end of the subject so that a substring can easily be identified.
1.1 misho 7544:
7545: For the majority of patterns, the first offset identifies the start of
7546: the partially matched string. However, for patterns that contain look-
7547: behind assertions, or \K, or begin with \b or \B, earlier characters
7548: have been inspected while carrying out the match. For example:
7549:
7550: /(?<=abc)123/
7551:
7552: This pattern matches "123", but only if it is preceded by "abc". If the
7553: subject string is "xyzabc12", the offsets after a partial match are for
7554: the substring "abc12", because all these characters are needed if
7555: another match is tried with extra characters added to the subject.
7556:
7557: What happens when a partial match is identified depends on which of the
7558: two partial matching options are set.
7559:
1.1.1.2 misho 7560: PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()
1.1 misho 7561:
1.1.1.2 misho 7562: If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre16_exec() identi-
7563: fies a partial match, the partial match is remembered, but matching
7564: continues as normal, and other alternatives in the pattern are tried.
7565: If no complete match can be found, PCRE_ERROR_PARTIAL is returned
7566: instead of PCRE_ERROR_NOMATCH.
1.1 misho 7567:
7568: This option is "soft" because it prefers a complete match over a par-
7569: tial match. All the various matching items in a pattern behave as if
7570: the subject string is potentially complete. For example, \z, \Z, and $
7571: match at the end of the subject, as normal, and for \b and \B the end
7572: of the subject is treated as a non-alphanumeric.
7573:
7574: If there is more than one partial match, the first one that was found
7575: provides the data that is returned. Consider this pattern:
7576:
7577: /123\w+X|dogY/
7578:
7579: If this is matched against the subject string "abc123dog", both alter-
7580: natives fail to match, but the end of the subject is reached during
7581: matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3
7582: and 9, identifying "123dog" as the first partial match that was found.
7583: (In this example, there are two partial matches, because "dog" on its
7584: own partially matches the second alternative.)
7585:
1.1.1.2 misho 7586: PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()
1.1 misho 7587:
1.1.1.2 misho 7588: If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre16_exec(),
7589: PCRE_ERROR_PARTIAL is returned as soon as a partial match is found,
7590: without continuing to search for possible complete matches. This option
7591: is "hard" because it prefers an earlier partial match over a later com-
7592: plete match. For this reason, the assumption is made that the end of
7593: the supplied subject string may not be the true end of the available
7594: data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the
7595: subject, the result is PCRE_ERROR_PARTIAL, provided that at least one
7596: character in the subject has been inspected.
7597:
7598: Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject
7599: strings are checked for validity. Normally, an invalid sequence causes
7600: the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
7601: special case of a truncated character at the end of the subject,
7602: PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
7603: PCRE_PARTIAL_HARD is set.
1.1 misho 7604:
7605: Comparing hard and soft partial matching
7606:
7607: The difference between the two partial matching options can be illus-
7608: trated by a pattern such as:
7609:
7610: /dog(sbody)?/
7611:
7612: This matches either "dog" or "dogsbody", greedily (that is, it prefers
7613: the longer string if possible). If it is matched against the string
7614: "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog".
7615: However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL.
7616: On the other hand, if the pattern is made ungreedy the result is dif-
7617: ferent:
7618:
7619: /dog(sbody)??/
7620:
1.1.1.2 misho 7621: In this case the result is always a complete match because that is
7622: found first, and matching never continues after finding a complete
7623: match. It might be easier to follow this explanation by thinking of the
7624: two patterns like this:
1.1 misho 7625:
7626: /dog(sbody)?/ is the same as /dogsbody|dog/
7627: /dog(sbody)??/ is the same as /dog|dogsbody/
7628:
1.1.1.2 misho 7629: The second pattern will never match "dogsbody", because it will always
7630: find the shorter match first.
1.1 misho 7631:
7632:
1.1.1.2 misho 7633: PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()
1.1 misho 7634:
1.1.1.2 misho 7635: The DFA functions move along the subject string character by character,
7636: without backtracking, searching for all possible matches simultane-
7637: ously. If the end of the subject is reached before the end of the pat-
7638: tern, there is the possibility of a partial match, again provided that
7639: at least one character has been inspected.
1.1 misho 7640:
7641: When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if
7642: there have been no complete matches. Otherwise, the complete matches
7643: are returned. However, if PCRE_PARTIAL_HARD is set, a partial match
7644: takes precedence over any complete matches. The portion of the string
7645: that was inspected when the longest partial match was found is set as
7646: the first matching string, provided there are at least two slots in the
7647: offsets vector.
7648:
1.1.1.2 misho 7649: Because the DFA functions always search for all possible matches, and
7650: there is no difference between greedy and ungreedy repetition, their
7651: behaviour is different from the standard functions when PCRE_PAR-
7652: TIAL_HARD is set. Consider the string "dog" matched against the
7653: ungreedy pattern shown above:
1.1 misho 7654:
7655: /dog(sbody)??/
7656:
1.1.1.2 misho 7657: Whereas the standard functions stop as soon as they find the complete
7658: match for "dog", the DFA functions also find the partial match for
7659: "dogsbody", and so return that when PCRE_PARTIAL_HARD is set.
1.1 misho 7660:
7661:
7662: PARTIAL MATCHING AND WORD BOUNDARIES
7663:
7664: If a pattern ends with one of sequences \b or \B, which test for word
7665: boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-
7666: intuitive results. Consider this pattern:
7667:
7668: /\bcat\b/
7669:
7670: This matches "cat", provided there is a word boundary at either end. If
7671: the subject string is "the cat", the comparison of the final "t" with a
7672: following character cannot take place, so a partial match is found.
1.1.1.2 misho 7673: However, normal matching carries on, and \b matches at the end of the
7674: subject when the last character is a letter, so a complete match is
7675: found. The result, therefore, is not PCRE_ERROR_PARTIAL. Using
7676: PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
7677: then the partial match takes precedence.
1.1 misho 7678:
7679:
7680: FORMERLY RESTRICTED PATTERNS
7681:
7682: For releases of PCRE prior to 8.00, because of the way certain internal
1.1.1.2 misho 7683: optimizations were implemented in the pcre_exec() function, the
7684: PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be
7685: used with all patterns. From release 8.00 onwards, the restrictions no
7686: longer apply, and partial matching with can be requested for any pat-
7687: tern.
1.1 misho 7688:
7689: Items that were formerly restricted were repeated single characters and
1.1.1.2 misho 7690: repeated metasequences. If PCRE_PARTIAL was set for a pattern that did
7691: not conform to the restrictions, pcre_exec() returned the error code
7692: PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
7693: PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled
1.1 misho 7694: pattern can be used for partial matching now always returns 1.
7695:
7696:
7697: EXAMPLE OF PARTIAL MATCHING USING PCRETEST
7698:
1.1.1.2 misho 7699: If the escape sequence \P is present in a pcretest data line, the
7700: PCRE_PARTIAL_SOFT option is used for the match. Here is a run of
1.1 misho 7701: pcretest that uses the date example quoted above:
7702:
7703: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
7704: data> 25jun04\P
7705: 0: 25jun04
7706: 1: jun
7707: data> 25dec3\P
7708: Partial match: 23dec3
7709: data> 3ju\P
7710: Partial match: 3ju
7711: data> 3juj\P
7712: No match
7713: data> j\P
7714: No match
7715:
1.1.1.2 misho 7716: The first data string is matched completely, so pcretest shows the
7717: matched substrings. The remaining four strings do not match the com-
1.1 misho 7718: plete pattern, but the first two are partial matches. Similar output is
1.1.1.2 misho 7719: obtained if DFA matching is used.
1.1 misho 7720:
1.1.1.2 misho 7721: If the escape sequence \P is present more than once in a pcretest data
1.1 misho 7722: line, the PCRE_PARTIAL_HARD option is set for the match.
7723:
7724:
1.1.1.2 misho 7725: MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()
1.1 misho 7726:
1.1.1.2 misho 7727: When a partial match has been found using a DFA matching function, it
7728: is possible to continue the match by providing additional subject data
7729: and calling the function again with the same compiled regular expres-
7730: sion, this time setting the PCRE_DFA_RESTART option. You must pass the
1.1 misho 7731: same working space as before, because this is where details of the pre-
1.1.1.2 misho 7732: vious partial match are stored. Here is an example using pcretest,
7733: using the \R escape sequence to set the PCRE_DFA_RESTART option (\D
7734: specifies the use of the DFA matching function):
1.1 misho 7735:
7736: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
7737: data> 23ja\P\D
7738: Partial match: 23ja
7739: data> n05\R\D
7740: 0: n05
7741:
1.1.1.2 misho 7742: The first call has "23ja" as the subject, and requests partial match-
7743: ing; the second call has "n05" as the subject for the continued
7744: (restarted) match. Notice that when the match is complete, only the
7745: last part is shown; PCRE does not retain the previously partially-
7746: matched string. It is up to the calling program to do that if it needs
1.1 misho 7747: to.
7748:
1.1.1.2 misho 7749: You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
7750: PCRE_DFA_RESTART to continue partial matching over multiple segments.
7751: This facility can be used to pass very long subject strings to the DFA
7752: matching functions.
7753:
7754:
7755: MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()
7756:
7757: From release 8.00, the standard matching functions can also be used to
7758: do multi-segment matching. Unlike the DFA functions, it is not possible
7759: to restart the previous match with a new segment of data. Instead, new
7760: data must be added to the previous subject string, and the entire match
7761: re-run, starting from the point where the partial match occurred. Ear-
7762: lier data can be discarded.
7763:
7764: It is best to use PCRE_PARTIAL_HARD in this situation, because it does
7765: not treat the end of a segment as the end of the subject when matching
7766: \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches
7767: dates:
1.1 misho 7768:
7769: re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
7770: data> The date is 23ja\P\P
7771: Partial match: 23ja
7772:
1.1.1.2 misho 7773: At this stage, an application could discard the text preceding "23ja",
7774: add on text from the next segment, and call the matching function
1.1.1.3 ! misho 7775: again. Unlike the DFA matching functions, the entire matching string
1.1.1.2 misho 7776: must always be available, and the complete matching process occurs for
7777: each call, so more memory and more processing time is needed.
7778:
7779: Note: If the pattern contains lookbehind assertions, or \K, or starts
7780: with \b or \B, the string that is returned for a partial match includes
7781: characters that precede the partially matched string itself, because
7782: these must be retained when adding on more characters for a subsequent
1.1.1.3 ! misho 7783: matching attempt. However, in some cases you may need to retain even
! 7784: earlier characters, as discussed in the next section.
1.1 misho 7785:
7786:
7787: ISSUES WITH MULTI-SEGMENT MATCHING
7788:
7789: Certain types of pattern may give problems with multi-segment matching,
7790: whichever matching function is used.
7791:
7792: 1. If the pattern contains a test for the beginning of a line, you need
1.1.1.3 ! misho 7793: to pass the PCRE_NOTBOL option when the subject string for any call
! 7794: does start at the beginning of a line. There is also a PCRE_NOTEOL
1.1 misho 7795: option, but in practice when doing multi-segment matching you should be
7796: using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.
7797:
1.1.1.3 ! misho 7798: 2. Lookbehind assertions that have already been obeyed are catered for
! 7799: in the offsets that are returned for a partial match. However a lookbe-
! 7800: hind assertion later in the pattern could require even earlier charac-
! 7801: ters to be inspected. You can handle this case by using the
! 7802: PCRE_INFO_MAXLOOKBEHIND option of the pcre_fullinfo() or
! 7803: pcre16_fullinfo() functions to obtain the length of the largest lookbe-
! 7804: hind in the pattern. This length is given in characters, not bytes. If
! 7805: you always retain at least that many characters before the partially
! 7806: matched string, all should be well. (Of course, near the start of the
! 7807: subject, fewer characters may be present; in that case all characters
! 7808: should be retained.)
! 7809:
! 7810: 3. Because a partial match must always contain at least one character,
! 7811: what might be considered a partial match of an empty string actually
! 7812: gives a "no match" result. For example:
! 7813:
! 7814: re> /c(?<=abc)x/
! 7815: data> ab\P
! 7816: No match
! 7817:
! 7818: If the next segment begins "cx", a match should be found, but this will
! 7819: only happen if characters from the previous segment are retained. For
! 7820: this reason, a "no match" result should be interpreted as "partial
! 7821: match of an empty string" when the pattern contains lookbehinds.
1.1 misho 7822:
1.1.1.3 ! misho 7823: 4. Matching a subject string that is split into multiple segments may
1.1.1.2 misho 7824: not always produce exactly the same result as matching over one single
7825: long string, especially when PCRE_PARTIAL_SOFT is used. The section
7826: "Partial Matching and Word Boundaries" above describes an issue that
7827: arises if the pattern ends with \b or \B. Another kind of difference
7828: may occur when there are multiple matching possibilities, because (for
7829: PCRE_PARTIAL_SOFT) a partial match result is given only when there are
1.1 misho 7830: no completed matches. This means that as soon as the shortest match has
1.1.1.2 misho 7831: been found, continuation to a new subject segment is no longer possi-
1.1 misho 7832: ble. Consider again this pcretest example:
7833:
7834: re> /dog(sbody)?/
7835: data> dogsb\P
7836: 0: dog
7837: data> do\P\D
7838: Partial match: do
7839: data> gsb\R\P\D
7840: 0: g
7841: data> dogsbody\D
7842: 0: dogsbody
7843: 1: dog
7844:
1.1.1.2 misho 7845: The first data line passes the string "dogsb" to a standard matching
7846: function, setting the PCRE_PARTIAL_SOFT option. Although the string is
7847: a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL,
7848: because the shorter string "dog" is a complete match. Similarly, when
7849: the subject is presented to a DFA matching function in several parts
7850: ("do" and "gsb" being the first two) the match stops when "dog" has
7851: been found, and it is not possible to continue. On the other hand, if
7852: "dogsbody" is presented as a single string, a DFA matching function
7853: finds both matches.
1.1 misho 7854:
7855: Because of these problems, it is best to use PCRE_PARTIAL_HARD when
7856: matching multi-segment data. The example above then behaves differ-
7857: ently:
7858:
7859: re> /dog(sbody)?/
7860: data> dogsb\P\P
7861: Partial match: dogsb
7862: data> do\P\D
7863: Partial match: do
7864: data> gsb\R\P\P\D
7865: Partial match: gsb
7866:
1.1.1.3 ! misho 7867: 5. Patterns that contain alternatives at the top level which do not all
1.1 misho 7868: start with the same pattern item may not work as expected when
1.1.1.2 misho 7869: PCRE_DFA_RESTART is used. For example, consider this pattern:
1.1 misho 7870:
7871: 1234|3789
7872:
1.1.1.2 misho 7873: If the first part of the subject is "ABC123", a partial match of the
7874: first alternative is found at offset 3. There is no partial match for
1.1 misho 7875: the second alternative, because such a match does not start at the same
1.1.1.2 misho 7876: point in the subject string. Attempting to continue with the string
7877: "7890" does not yield a match because only those alternatives that
7878: match at one point in the subject are remembered. The problem arises
7879: because the start of the second alternative matches within the first
7880: alternative. There is no problem with anchored patterns or patterns
1.1 misho 7881: such as:
7882:
7883: 1234|ABCD
7884:
1.1.1.2 misho 7885: where no string can be a partial match for both alternatives. This is
7886: not a problem if a standard matching function is used, because the
7887: entire match has to be rerun each time:
1.1 misho 7888:
7889: re> /1234|3789/
7890: data> ABC123\P\P
7891: Partial match: 123
7892: data> 1237890
7893: 0: 3789
7894:
7895: Of course, instead of using PCRE_DFA_RESTART, the same technique of re-
1.1.1.2 misho 7896: running the entire match can also be used with the DFA matching func-
7897: tions. Another possibility is to work with two buffers. If a partial
7898: match at offset n in the first buffer is followed by "no match" when
7899: PCRE_DFA_RESTART is used on the second buffer, you can then try a new
7900: match starting at offset n+1 in the first buffer.
1.1 misho 7901:
7902:
7903: AUTHOR
7904:
7905: Philip Hazel
7906: University Computing Service
7907: Cambridge CB2 3QH, England.
7908:
7909:
7910: REVISION
7911:
1.1.1.3 ! misho 7912: Last updated: 24 February 2012
1.1.1.2 misho 7913: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 7914: ------------------------------------------------------------------------------
7915:
7916:
7917: PCREPRECOMPILE(3) PCREPRECOMPILE(3)
7918:
7919:
7920: NAME
7921: PCRE - Perl-compatible regular expressions
7922:
7923:
7924: SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
7925:
7926: If you are running an application that uses a large number of regular
7927: expression patterns, it may be useful to store them in a precompiled
7928: form instead of having to compile them every time the application is
7929: run. If you are not using any private character tables (see the
7930: pcre_maketables() documentation), this is relatively straightforward.
7931: If you are using private tables, it is a little bit more complicated.
1.1.1.2 misho 7932: However, if you are using the just-in-time optimization feature, it is
7933: not possible to save and reload the JIT data.
1.1 misho 7934:
7935: If you save compiled patterns to a file, you can copy them to a differ-
1.1.1.2 misho 7936: ent host and run them there. If the two hosts have different endianness
7937: (byte order), you should run the pcre[16]_pattern_to_host_byte_order()
7938: function on the new host before trying to match the pattern. The match-
7939: ing functions return PCRE_ERROR_BADENDIANNESS if they detect a pattern
7940: with the wrong endianness.
7941:
7942: Compiling regular expressions with one version of PCRE for use with a
7943: different version is not guaranteed to work and may cause crashes, and
7944: saving and restoring a compiled pattern loses any JIT optimization
7945: data.
1.1 misho 7946:
7947:
7948: SAVING A COMPILED PATTERN
7949:
1.1.1.2 misho 7950: The value returned by pcre[16]_compile() points to a single block of
7951: memory that holds the compiled pattern and associated data. You can
7952: find the length of this block in bytes by calling pcre[16]_fullinfo()
7953: with an argument of PCRE_INFO_SIZE. You can then save the data in any
7954: appropriate manner. Here is sample code for the 8-bit library that com-
7955: piles a pattern and writes it to a file. It assumes that the variable
7956: fd refers to a file that is open for output:
1.1 misho 7957:
7958: int erroroffset, rc, size;
7959: char *error;
7960: pcre *re;
7961:
7962: re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
7963: if (re == NULL) { ... handle errors ... }
7964: rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
7965: if (rc < 0) { ... handle errors ... }
7966: rc = fwrite(re, 1, size, fd);
7967: if (rc != size) { ... handle errors ... }
7968:
1.1.1.2 misho 7969: In this example, the bytes that comprise the compiled pattern are
7970: copied exactly. Note that this is binary data that may contain any of
7971: the 256 possible byte values. On systems that make a distinction
1.1 misho 7972: between binary and non-binary data, be sure that the file is opened for
7973: binary output.
7974:
1.1.1.2 misho 7975: If you want to write more than one pattern to a file, you will have to
7976: devise a way of separating them. For binary data, preceding each pat-
7977: tern with its length is probably the most straightforward approach.
7978: Another possibility is to write out the data in hexadecimal instead of
1.1 misho 7979: binary, one pattern to a line.
7980:
1.1.1.2 misho 7981: Saving compiled patterns in a file is only one possible way of storing
7982: them for later use. They could equally well be saved in a database, or
7983: in the memory of some daemon process that passes them via sockets to
1.1 misho 7984: the processes that want them.
7985:
7986: If the pattern has been studied, it is also possible to save the normal
7987: study data in a similar way to the compiled pattern itself. However, if
7988: the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre-
1.1.1.2 misho 7989: ated cannot be saved because it is too dependent on the current envi-
7990: ronment. When studying generates additional information,
7991: pcre[16]_study() returns a pointer to a pcre[16]_extra data block. Its
7992: format is defined in the section on matching a pattern in the pcreapi
7993: documentation. The study_data field points to the binary study data,
7994: and this is what you must save (not the pcre[16]_extra block itself).
7995: The length of the study data can be obtained by calling
7996: pcre[16]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remember
7997: to check that pcre[16]_study() did return a non-NULL value before try-
7998: ing to save the study data.
1.1 misho 7999:
8000:
8001: RE-USING A PRECOMPILED PATTERN
8002:
8003: Re-using a precompiled pattern is straightforward. Having reloaded it
1.1.1.2 misho 8004: into main memory, called pcre[16]_pattern_to_host_byte_order() if nec-
8005: essary, you pass its pointer to pcre[16]_exec() or pcre[16]_dfa_exec()
8006: in the usual way.
8007:
8008: However, if you passed a pointer to custom character tables when the
8009: pattern was compiled (the tableptr argument of pcre[16]_compile()), you
8010: must now pass a similar pointer to pcre[16]_exec() or
8011: pcre[16]_dfa_exec(), because the value saved with the compiled pattern
8012: will obviously be nonsense. A field in a pcre[16]_extra() block is used
8013: to pass this data, as described in the section on matching a pattern in
8014: the pcreapi documentation.
8015:
8016: If you did not provide custom character tables when the pattern was
8017: compiled, the pointer in the compiled pattern is NULL, which causes the
8018: matching functions to use PCRE's internal tables. Thus, you do not need
8019: to take any special action at run time in this case.
8020:
8021: If you saved study data with the compiled pattern, you need to create
8022: your own pcre[16]_extra data block and set the study_data field to
8023: point to the reloaded study data. You must also set the
8024: PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study
8025: data is present. Then pass the pcre[16]_extra block to the matching
8026: function in the usual way. If the pattern was studied for just-in-time
8027: optimization, that data cannot be saved, and so is lost by a
8028: save/restore cycle.
1.1 misho 8029:
8030:
8031: COMPATIBILITY WITH DIFFERENT PCRE RELEASES
8032:
8033: In general, it is safest to recompile all saved patterns when you
8034: update to a new PCRE release, though not all updates actually require
8035: this.
8036:
8037:
8038: AUTHOR
8039:
8040: Philip Hazel
8041: University Computing Service
8042: Cambridge CB2 3QH, England.
8043:
8044:
8045: REVISION
8046:
1.1.1.2 misho 8047: Last updated: 10 January 2012
8048: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 8049: ------------------------------------------------------------------------------
8050:
8051:
8052: PCREPERFORM(3) PCREPERFORM(3)
8053:
8054:
8055: NAME
8056: PCRE - Perl-compatible regular expressions
8057:
8058:
8059: PCRE PERFORMANCE
8060:
8061: Two aspects of performance are discussed below: memory usage and pro-
8062: cessing time. The way you express your pattern as a regular expression
8063: can affect both of them.
8064:
8065:
8066: COMPILED PATTERN MEMORY USAGE
8067:
1.1.1.2 misho 8068: Patterns are compiled by PCRE into a reasonably efficient interpretive
8069: code, so that most simple patterns do not use much memory. However,
8070: there is one case where the memory usage of a compiled pattern can be
8071: unexpectedly large. If a parenthesized subpattern has a quantifier with
8072: a minimum greater than 1 and/or a limited maximum, the whole subpattern
8073: is repeated in the compiled code. For example, the pattern
1.1 misho 8074:
8075: (abc|def){2,4}
8076:
8077: is compiled as if it were
8078:
8079: (abc|def)(abc|def)((abc|def)(abc|def)?)?
8080:
8081: (Technical aside: It is done this way so that backtrack points within
8082: each of the repetitions can be independently maintained.)
8083:
8084: For regular expressions whose quantifiers use only small numbers, this
8085: is not usually a problem. However, if the numbers are large, and par-
8086: ticularly if such repetitions are nested, the memory usage can become
8087: an embarrassment. For example, the very simple pattern
8088:
8089: ((ab){1,1000}c){1,3}
8090:
1.1.1.2 misho 8091: uses 51K bytes when compiled using the 8-bit library. When PCRE is com-
8092: piled with its default internal pointer size of two bytes, the size
8093: limit on a compiled pattern is 64K data units, and this is reached with
8094: the above pattern if the outer repetition is increased from 3 to 4.
8095: PCRE can be compiled to use larger internal pointers and thus handle
8096: larger compiled patterns, but it is better to try to rewrite your pat-
8097: tern to use less memory if you can.
1.1 misho 8098:
1.1.1.2 misho 8099: One way of reducing the memory usage for such patterns is to make use
1.1 misho 8100: of PCRE's "subroutine" facility. Re-writing the above pattern as
8101:
8102: ((ab)(?2){0,999}c)(?1){0,2}
8103:
8104: reduces the memory requirements to 18K, and indeed it remains under 20K
1.1.1.2 misho 8105: even with the outer repetition increased to 100. However, this pattern
8106: is not exactly equivalent, because the "subroutine" calls are treated
8107: as atomic groups into which there can be no backtracking if there is a
8108: subsequent matching failure. Therefore, PCRE cannot do this kind of
8109: rewriting automatically. Furthermore, there is a noticeable loss of
8110: speed when executing the modified pattern. Nevertheless, if the atomic
8111: grouping is not a problem and the loss of speed is acceptable, this
8112: kind of rewriting will allow you to process patterns that PCRE cannot
1.1 misho 8113: otherwise handle.
8114:
8115:
8116: STACK USAGE AT RUN TIME
8117:
1.1.1.2 misho 8118: When pcre_exec() or pcre16_exec() is used for matching, certain kinds
8119: of pattern can cause it to use large amounts of the process stack. In
8120: some environments the default process stack is quite small, and if it
8121: runs out the result is often SIGSEGV. This issue is probably the most
8122: frequently raised problem with PCRE. Rewriting your pattern can often
8123: help. The pcrestack documentation discusses this issue in detail.
1.1 misho 8124:
8125:
8126: PROCESSING TIME
8127:
1.1.1.2 misho 8128: Certain items in regular expression patterns are processed more effi-
1.1 misho 8129: ciently than others. It is more efficient to use a character class like
1.1.1.2 misho 8130: [aeiou] than a set of single-character alternatives such as
8131: (a|e|i|o|u). In general, the simplest construction that provides the
1.1 misho 8132: required behaviour is usually the most efficient. Jeffrey Friedl's book
1.1.1.2 misho 8133: contains a lot of useful general discussion about optimizing regular
8134: expressions for efficient performance. This document contains a few
1.1 misho 8135: observations about PCRE.
8136:
1.1.1.2 misho 8137: Using Unicode character properties (the \p, \P, and \X escapes) is
8138: slow, because PCRE has to scan a structure that contains data for over
8139: fifteen thousand characters whenever it needs a character's property.
8140: If you can find an alternative pattern that does not use character
1.1 misho 8141: properties, it will probably be faster.
8142:
1.1.1.2 misho 8143: By default, the escape sequences \b, \d, \s, and \w, and the POSIX
8144: character classes such as [:alpha:] do not use Unicode properties,
1.1 misho 8145: partly for backwards compatibility, and partly for performance reasons.
1.1.1.2 misho 8146: However, you can set PCRE_UCP if you want Unicode character properties
8147: to be used. This can double the matching time for items such as \d,
8148: when matched with a traditional matching function; the performance loss
8149: is less with a DFA matching function, and in both cases there is not
8150: much difference for \b.
1.1 misho 8151:
8152: When a pattern begins with .* not in parentheses, or in parentheses
8153: that are not the subject of a backreference, and the PCRE_DOTALL option
8154: is set, the pattern is implicitly anchored by PCRE, since it can match
8155: only at the start of a subject string. However, if PCRE_DOTALL is not
8156: set, PCRE cannot make this optimization, because the . metacharacter
8157: does not then match a newline, and if the subject string contains new-
8158: lines, the pattern may match from the character immediately following
8159: one of them instead of from the very start. For example, the pattern
8160:
8161: .*second
8162:
8163: matches the subject "first\nand second" (where \n stands for a newline
8164: character), with the match starting at the seventh character. In order
8165: to do this, PCRE has to retry the match starting after every newline in
8166: the subject.
8167:
8168: If you are using such a pattern with subject strings that do not con-
8169: tain newlines, the best performance is obtained by setting PCRE_DOTALL,
8170: or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
8171: ing. That saves PCRE from having to scan along the subject looking for
8172: a newline to restart at.
8173:
8174: Beware of patterns that contain nested indefinite repeats. These can
8175: take a long time to run when applied to a string that does not match.
8176: Consider the pattern fragment
8177:
8178: ^(a+)*
8179:
8180: This can match "aaaa" in 16 different ways, and this number increases
8181: very rapidly as the string gets longer. (The * repeat can match 0, 1,
8182: 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
8183: repeats can match different numbers of times.) When the remainder of
8184: the pattern is such that the entire match is going to fail, PCRE has in
8185: principle to try every possible variation, and this can take an
8186: extremely long time, even for relatively short strings.
8187:
8188: An optimization catches some of the more simple cases such as
8189:
8190: (a+)*b
8191:
8192: where a literal character follows. Before embarking on the standard
8193: matching procedure, PCRE checks that there is a "b" later in the sub-
8194: ject string, and if there is not, it fails the match immediately. How-
8195: ever, when there is no following literal this optimization cannot be
8196: used. You can see the difference by comparing the behaviour of
8197:
8198: (a+)*\d
8199:
8200: with the pattern above. The former gives a failure almost instantly
8201: when applied to a whole line of "a" characters, whereas the latter
8202: takes an appreciable time with strings longer than about 20 characters.
8203:
8204: In many cases, the solution to this kind of performance issue is to use
8205: an atomic group or a possessive quantifier.
8206:
8207:
8208: AUTHOR
8209:
8210: Philip Hazel
8211: University Computing Service
8212: Cambridge CB2 3QH, England.
8213:
8214:
8215: REVISION
8216:
1.1.1.2 misho 8217: Last updated: 09 January 2012
8218: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 8219: ------------------------------------------------------------------------------
8220:
8221:
8222: PCREPOSIX(3) PCREPOSIX(3)
8223:
8224:
8225: NAME
8226: PCRE - Perl-compatible regular expressions.
8227:
8228:
8229: SYNOPSIS OF POSIX API
8230:
8231: #include <pcreposix.h>
8232:
8233: int regcomp(regex_t *preg, const char *pattern,
8234: int cflags);
8235:
8236: int regexec(regex_t *preg, const char *string,
8237: size_t nmatch, regmatch_t pmatch[], int eflags);
8238:
8239: size_t regerror(int errcode, const regex_t *preg,
8240: char *errbuf, size_t errbuf_size);
8241:
8242: void regfree(regex_t *preg);
8243:
8244:
8245: DESCRIPTION
8246:
1.1.1.2 misho 8247: This set of functions provides a POSIX-style API for the PCRE regular
8248: expression 8-bit library. See the pcreapi documentation for a descrip-
8249: tion of PCRE's native API, which contains much additional functional-
8250: ity. There is no POSIX-style wrapper for PCRE's 16-bit library.
1.1 misho 8251:
8252: The functions described here are just wrapper functions that ultimately
8253: call the PCRE native API. Their prototypes are defined in the
1.1.1.2 misho 8254: pcreposix.h header file, and on Unix systems the library itself is
8255: called pcreposix.a, so can be accessed by adding -lpcreposix to the
8256: command for linking an application that uses them. Because the POSIX
1.1 misho 8257: functions call the native ones, it is also necessary to add -lpcre.
8258:
1.1.1.2 misho 8259: I have implemented only those POSIX option bits that can be reasonably
8260: mapped to PCRE native options. In addition, the option REG_EXTENDED is
8261: defined with the value zero. This has no effect, but since programs
8262: that are written to the POSIX interface often use it, this makes it
8263: easier to slot in PCRE as a replacement library. Other POSIX options
1.1 misho 8264: are not even defined.
8265:
1.1.1.2 misho 8266: There are also some other options that are not defined by POSIX. These
1.1 misho 8267: have been added at the request of users who want to make use of certain
8268: PCRE-specific features via the POSIX calling interface.
8269:
1.1.1.2 misho 8270: When PCRE is called via these functions, it is only the API that is
8271: POSIX-like in style. The syntax and semantics of the regular expres-
8272: sions themselves are still those of Perl, subject to the setting of
8273: various PCRE options, as described below. "POSIX-like in style" means
8274: that the API approximates to the POSIX definition; it is not fully
8275: POSIX-compatible, and in multi-byte encoding domains it is probably
1.1 misho 8276: even less compatible.
8277:
1.1.1.2 misho 8278: The header for these functions is supplied as pcreposix.h to avoid any
8279: potential clash with other POSIX libraries. It can, of course, be
1.1 misho 8280: renamed or aliased as regex.h, which is the "correct" name. It provides
1.1.1.2 misho 8281: two structure types, regex_t for compiled internal forms, and reg-
8282: match_t for returning captured substrings. It also defines some con-
8283: stants whose names start with "REG_"; these are used for setting
1.1 misho 8284: options and identifying error codes.
8285:
8286:
8287: COMPILING A PATTERN
8288:
1.1.1.2 misho 8289: The function regcomp() is called to compile a pattern into an internal
8290: form. The pattern is a C string terminated by a binary zero, and is
8291: passed in the argument pattern. The preg argument is a pointer to a
8292: regex_t structure that is used as a base for storing information about
1.1 misho 8293: the compiled regular expression.
8294:
8295: The argument cflags is either zero, or contains one or more of the bits
8296: defined by the following macros:
8297:
8298: REG_DOTALL
8299:
8300: The PCRE_DOTALL option is set when the regular expression is passed for
8301: compilation to the native function. Note that REG_DOTALL is not part of
8302: the POSIX standard.
8303:
8304: REG_ICASE
8305:
1.1.1.2 misho 8306: The PCRE_CASELESS option is set when the regular expression is passed
1.1 misho 8307: for compilation to the native function.
8308:
8309: REG_NEWLINE
8310:
1.1.1.2 misho 8311: The PCRE_MULTILINE option is set when the regular expression is passed
8312: for compilation to the native function. Note that this does not mimic
8313: the defined POSIX behaviour for REG_NEWLINE (see the following sec-
1.1 misho 8314: tion).
8315:
8316: REG_NOSUB
8317:
1.1.1.2 misho 8318: The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
1.1 misho 8319: passed for compilation to the native function. In addition, when a pat-
1.1.1.2 misho 8320: tern that is compiled with this flag is passed to regexec() for match-
8321: ing, the nmatch and pmatch arguments are ignored, and no captured
1.1 misho 8322: strings are returned.
8323:
8324: REG_UCP
8325:
1.1.1.2 misho 8326: The PCRE_UCP option is set when the regular expression is passed for
8327: compilation to the native function. This causes PCRE to use Unicode
8328: properties when matchine \d, \w, etc., instead of just recognizing
1.1 misho 8329: ASCII values. Note that REG_UTF8 is not part of the POSIX standard.
8330:
8331: REG_UNGREEDY
8332:
1.1.1.2 misho 8333: The PCRE_UNGREEDY option is set when the regular expression is passed
8334: for compilation to the native function. Note that REG_UNGREEDY is not
1.1 misho 8335: part of the POSIX standard.
8336:
8337: REG_UTF8
8338:
1.1.1.2 misho 8339: The PCRE_UTF8 option is set when the regular expression is passed for
8340: compilation to the native function. This causes the pattern itself and
8341: all data strings used for matching it to be treated as UTF-8 strings.
1.1 misho 8342: Note that REG_UTF8 is not part of the POSIX standard.
8343:
1.1.1.2 misho 8344: In the absence of these flags, no options are passed to the native
8345: function. This means the the regex is compiled with PCRE default
8346: semantics. In particular, the way it handles newline characters in the
8347: subject string is the Perl way, not the POSIX way. Note that setting
8348: PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
8349: It does not affect the way newlines are matched by . (they are not) or
1.1 misho 8350: by a negative class such as [^a] (they are).
8351:
1.1.1.2 misho 8352: The yield of regcomp() is zero on success, and non-zero otherwise. The
1.1 misho 8353: preg structure is filled in on success, and one member of the structure
1.1.1.2 misho 8354: is public: re_nsub contains the number of capturing subpatterns in the
1.1 misho 8355: regular expression. Various error codes are defined in the header file.
8356:
1.1.1.2 misho 8357: NOTE: If the yield of regcomp() is non-zero, you must not attempt to
1.1 misho 8358: use the contents of the preg structure. If, for example, you pass it to
8359: regexec(), the result is undefined and your program is likely to crash.
8360:
8361:
8362: MATCHING NEWLINE CHARACTERS
8363:
8364: This area is not simple, because POSIX and Perl take different views of
1.1.1.2 misho 8365: things. It is not possible to get PCRE to obey POSIX semantics, but
8366: then PCRE was never intended to be a POSIX engine. The following table
8367: lists the different possibilities for matching newline characters in
1.1 misho 8368: PCRE:
8369:
8370: Default Change with
8371:
8372: . matches newline no PCRE_DOTALL
8373: newline matches [^a] yes not changeable
8374: $ matches \n at end yes PCRE_DOLLARENDONLY
8375: $ matches \n in middle no PCRE_MULTILINE
8376: ^ matches \n in middle no PCRE_MULTILINE
8377:
8378: This is the equivalent table for POSIX:
8379:
8380: Default Change with
8381:
8382: . matches newline yes REG_NEWLINE
8383: newline matches [^a] yes REG_NEWLINE
8384: $ matches \n at end no REG_NEWLINE
8385: $ matches \n in middle no REG_NEWLINE
8386: ^ matches \n in middle no REG_NEWLINE
8387:
8388: PCRE's behaviour is the same as Perl's, except that there is no equiva-
1.1.1.2 misho 8389: lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
1.1 misho 8390: no way to stop newline from matching [^a].
8391:
1.1.1.2 misho 8392: The default POSIX newline handling can be obtained by setting
8393: PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
1.1 misho 8394: behave exactly as for the REG_NEWLINE action.
8395:
8396:
8397: MATCHING A PATTERN
8398:
1.1.1.2 misho 8399: The function regexec() is called to match a compiled pattern preg
8400: against a given string, which is by default terminated by a zero byte
8401: (but see REG_STARTEND below), subject to the options in eflags. These
1.1 misho 8402: can be:
8403:
8404: REG_NOTBOL
8405:
8406: The PCRE_NOTBOL option is set when calling the underlying PCRE matching
8407: function.
8408:
8409: REG_NOTEMPTY
8410:
8411: The PCRE_NOTEMPTY option is set when calling the underlying PCRE match-
8412: ing function. Note that REG_NOTEMPTY is not part of the POSIX standard.
8413: However, setting this option can give more POSIX-like behaviour in some
8414: situations.
8415:
8416: REG_NOTEOL
8417:
8418: The PCRE_NOTEOL option is set when calling the underlying PCRE matching
8419: function.
8420:
8421: REG_STARTEND
8422:
1.1.1.2 misho 8423: The string is considered to start at string + pmatch[0].rm_so and to
8424: have a terminating NUL located at string + pmatch[0].rm_eo (there need
8425: not actually be a NUL at that location), regardless of the value of
8426: nmatch. This is a BSD extension, compatible with but not specified by
8427: IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
1.1 misho 8428: software intended to be portable to other systems. Note that a non-zero
8429: rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
8430: of the string, not how it is matched.
8431:
1.1.1.2 misho 8432: If the pattern was compiled with the REG_NOSUB flag, no data about any
8433: matched strings is returned. The nmatch and pmatch arguments of
1.1 misho 8434: regexec() are ignored.
8435:
8436: If the value of nmatch is zero, or if the value pmatch is NULL, no data
8437: about any matched strings is returned.
8438:
8439: Otherwise,the portion of the string that was matched, and also any cap-
8440: tured substrings, are returned via the pmatch argument, which points to
1.1.1.2 misho 8441: an array of nmatch structures of type regmatch_t, containing the mem-
8442: bers rm_so and rm_eo. These contain the offset to the first character
8443: of each substring and the offset to the first character after the end
8444: of each substring, respectively. The 0th element of the vector relates
8445: to the entire portion of string that was matched; subsequent elements
8446: relate to the capturing subpatterns of the regular expression. Unused
1.1 misho 8447: entries in the array have both structure members set to -1.
8448:
1.1.1.2 misho 8449: A successful match yields a zero return; various error codes are
8450: defined in the header file, of which REG_NOMATCH is the "expected"
1.1 misho 8451: failure code.
8452:
8453:
8454: ERROR MESSAGES
8455:
8456: The regerror() function maps a non-zero errorcode from either regcomp()
1.1.1.2 misho 8457: or regexec() to a printable message. If preg is not NULL, the error
1.1 misho 8458: should have arisen from the use of that structure. A message terminated
1.1.1.2 misho 8459: by a binary zero is placed in errbuf. The length of the message,
8460: including the zero, is limited to errbuf_size. The yield of the func-
1.1 misho 8461: tion is the size of buffer needed to hold the whole message.
8462:
8463:
8464: MEMORY USAGE
8465:
1.1.1.2 misho 8466: Compiling a regular expression causes memory to be allocated and asso-
8467: ciated with the preg structure. The function regfree() frees all such
8468: memory, after which preg may no longer be used as a compiled expres-
1.1 misho 8469: sion.
8470:
8471:
8472: AUTHOR
8473:
8474: Philip Hazel
8475: University Computing Service
8476: Cambridge CB2 3QH, England.
8477:
8478:
8479: REVISION
8480:
1.1.1.2 misho 8481: Last updated: 09 January 2012
8482: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 8483: ------------------------------------------------------------------------------
8484:
8485:
8486: PCRECPP(3) PCRECPP(3)
8487:
8488:
8489: NAME
8490: PCRE - Perl-compatible regular expressions.
8491:
8492:
8493: SYNOPSIS OF C++ WRAPPER
8494:
8495: #include <pcrecpp.h>
8496:
8497:
8498: DESCRIPTION
8499:
8500: The C++ wrapper for PCRE was provided by Google Inc. Some additional
8501: functionality was added by Giuseppe Maxia. This brief man page was con-
8502: structed from the notes in the pcrecpp.h file, which should be con-
1.1.1.2 misho 8503: sulted for further details. Note that the C++ wrapper supports only the
8504: original 8-bit PCRE library. There is no 16-bit support at present.
1.1 misho 8505:
8506:
8507: MATCHING INTERFACE
8508:
1.1.1.2 misho 8509: The "FullMatch" operation checks that supplied text matches a supplied
8510: pattern exactly. If pointer arguments are supplied, it copies matched
1.1 misho 8511: sub-strings that match sub-patterns into them.
8512:
8513: Example: successful match
8514: pcrecpp::RE re("h.*o");
8515: re.FullMatch("hello");
8516:
8517: Example: unsuccessful match (requires full match):
8518: pcrecpp::RE re("e");
8519: !re.FullMatch("hello");
8520:
8521: Example: creating a temporary RE object:
8522: pcrecpp::RE("h.*o").FullMatch("hello");
8523:
1.1.1.2 misho 8524: You can pass in a "const char*" or a "string" for "text". The examples
8525: below tend to use a const char*. You can, as in the different examples
8526: above, store the RE object explicitly in a variable or use a temporary
8527: RE object. The examples below use one mode or the other arbitrarily.
1.1 misho 8528: Either could correctly be used for any of these examples.
8529:
8530: You must supply extra pointer arguments to extract matched subpieces.
8531:
8532: Example: extracts "ruby" into "s" and 1234 into "i"
8533: int i;
8534: string s;
8535: pcrecpp::RE re("(\\w+):(\\d+)");
8536: re.FullMatch("ruby:1234", &s, &i);
8537:
8538: Example: does not try to extract any extra sub-patterns
8539: re.FullMatch("ruby:1234", &s);
8540:
8541: Example: does not try to extract into NULL
8542: re.FullMatch("ruby:1234", NULL, &i);
8543:
8544: Example: integer overflow causes failure
8545: !re.FullMatch("ruby:1234567891234", NULL, &i);
8546:
8547: Example: fails because there aren't enough sub-patterns:
8548: !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
8549:
8550: Example: fails because string cannot be stored in integer
8551: !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
8552:
1.1.1.2 misho 8553: The provided pointer arguments can be pointers to any scalar numeric
1.1 misho 8554: type, or one of:
8555:
8556: string (matched piece is copied to string)
8557: StringPiece (StringPiece is mutated to point to matched piece)
8558: T (where "bool T::ParseFrom(const char*, int)" exists)
8559: NULL (the corresponding matched sub-pattern is not copied)
8560:
1.1.1.2 misho 8561: The function returns true iff all of the following conditions are sat-
1.1 misho 8562: isfied:
8563:
8564: a. "text" matches "pattern" exactly;
8565:
8566: b. The number of matched sub-patterns is >= number of supplied
8567: pointers;
8568:
8569: c. The "i"th argument has a suitable type for holding the
8570: string captured as the "i"th sub-pattern. If you pass in
8571: void * NULL for the "i"th argument, or a non-void * NULL
8572: of the correct type, or pass fewer arguments than the
8573: number of sub-patterns, "i"th captured sub-pattern is
8574: ignored.
8575:
1.1.1.2 misho 8576: CAVEAT: An optional sub-pattern that does not exist in the matched
8577: string is assigned the empty string. Therefore, the following will
1.1 misho 8578: return false (because the empty string is not a valid number):
8579:
8580: int number;
8581: pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
8582:
1.1.1.2 misho 8583: The matching interface supports at most 16 arguments per call. If you
8584: need more, consider using the more general interface
1.1 misho 8585: pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
8586:
1.1.1.2 misho 8587: NOTE: Do not use no_arg, which is used internally to mark the end of a
8588: list of optional arguments, as a placeholder for missing arguments, as
1.1 misho 8589: this can lead to segfaults.
8590:
8591:
8592: QUOTING METACHARACTERS
8593:
1.1.1.2 misho 8594: You can use the "QuoteMeta" operation to insert backslashes before all
8595: potentially meaningful characters in a string. The returned string,
1.1 misho 8596: used as a regular expression, will exactly match the original string.
8597:
8598: Example:
8599: string quoted = RE::QuoteMeta(unquoted);
8600:
1.1.1.2 misho 8601: Note that it's legal to escape a character even if it has no special
8602: meaning in a regular expression -- so this function does that. (This
8603: also makes it identical to the perl function of the same name; see
8604: "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
1.1 misho 8605: "1\.5\-2\.0\?".
8606:
8607:
8608: PARTIAL MATCHES
8609:
1.1.1.2 misho 8610: You can use the "PartialMatch" operation when you want the pattern to
1.1 misho 8611: match any substring of the text.
8612:
8613: Example: simple search for a string:
8614: pcrecpp::RE("ell").PartialMatch("hello");
8615:
8616: Example: find first number in a string:
8617: int number;
8618: pcrecpp::RE re("(\\d+)");
8619: re.PartialMatch("x*100 + 20", &number);
8620: assert(number == 100);
8621:
8622:
8623: UTF-8 AND THE MATCHING INTERFACE
8624:
1.1.1.2 misho 8625: By default, pattern and text are plain text, one byte per character.
8626: The UTF8 flag, passed to the constructor, causes both pattern and
1.1 misho 8627: string to be treated as UTF-8 text, still a byte stream but potentially
1.1.1.2 misho 8628: multiple bytes per character. In practice, the text is likelier to be
8629: UTF-8 than the pattern, but the match returned may depend on the UTF8
8630: flag, so always use it when matching UTF8 text. For example, "." will
8631: match one byte normally but with UTF8 set may match up to three bytes
1.1 misho 8632: of a multi-byte character.
8633:
8634: Example:
8635: pcrecpp::RE_Options options;
8636: options.set_utf8();
8637: pcrecpp::RE re(utf8_pattern, options);
8638: re.FullMatch(utf8_string);
8639:
8640: Example: using the convenience function UTF8():
8641: pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
8642: re.FullMatch(utf8_string);
8643:
8644: NOTE: The UTF8 flag is ignored if pcre was not configured with the
8645: --enable-utf8 flag.
8646:
8647:
8648: PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
8649:
1.1.1.2 misho 8650: PCRE defines some modifiers to change the behavior of the regular
8651: expression engine. The C++ wrapper defines an auxiliary class,
8652: RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
1.1 misho 8653: rently, the following modifiers are supported:
8654:
8655: modifier description Perl corresponding
8656:
8657: PCRE_CASELESS case insensitive match /i
8658: PCRE_MULTILINE multiple lines match /m
8659: PCRE_DOTALL dot matches newlines /s
8660: PCRE_DOLLAR_ENDONLY $ matches only at end N/A
8661: PCRE_EXTRA strict escape parsing N/A
1.1.1.3 ! misho 8662: PCRE_EXTENDED ignore white spaces /x
1.1 misho 8663: PCRE_UTF8 handles UTF8 chars built-in
8664: PCRE_UNGREEDY reverses * and *? N/A
8665: PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
8666:
1.1.1.2 misho 8667: (*) Both Perl and PCRE allow non capturing parentheses by means of the
8668: "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
1.1 misho 8669: ture, while (ab|cd) does.
8670:
1.1.1.2 misho 8671: For a full account on how each modifier works, please check the PCRE
1.1 misho 8672: API reference page.
8673:
1.1.1.2 misho 8674: For each modifier, there are two member functions whose name is made
8675: out of the modifier in lowercase, without the "PCRE_" prefix. For
1.1 misho 8676: instance, PCRE_CASELESS is handled by
8677:
8678: bool caseless()
8679:
8680: which returns true if the modifier is set, and
8681:
8682: RE_Options & set_caseless(bool)
8683:
8684: which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
1.1.1.2 misho 8685: be accessed through the set_match_limit() and match_limit() member
8686: functions. Setting match_limit to a non-zero value will limit the exe-
8687: cution of pcre to keep it from doing bad things like blowing the stack
8688: or taking an eternity to return a result. A value of 5000 is good
8689: enough to stop stack blowup in a 2MB thread stack. Setting match_limit
8690: to zero disables match limiting. Alternatively, you can call
8691: match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
8692: limit how much PCRE recurses. match_limit() limits the number of
1.1 misho 8693: matches PCRE does; match_limit_recursion() limits the depth of internal
8694: recursion, and therefore the amount of stack that is used.
8695:
1.1.1.2 misho 8696: Normally, to pass one or more modifiers to a RE class, you declare a
1.1 misho 8697: RE_Options object, set the appropriate options, and pass this object to
8698: a RE constructor. Example:
8699:
8700: RE_Options opt;
8701: opt.set_caseless(true);
8702: if (RE("HELLO", opt).PartialMatch("hello world")) ...
8703:
8704: RE_options has two constructors. The default constructor takes no argu-
1.1.1.2 misho 8705: ments and creates a set of flags that are off by default. The optional
8706: parameter option_flags is to facilitate transfer of legacy code from C
1.1 misho 8707: programs. This lets you do
8708:
8709: RE(pattern,
8710: RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
8711:
8712: However, new code is better off doing
8713:
8714: RE(pattern,
8715: RE_Options().set_caseless(true).set_multiline(true))
8716: .PartialMatch(str);
8717:
8718: If you are going to pass one of the most used modifiers, there are some
8719: convenience functions that return a RE_Options class with the appropri-
1.1.1.2 misho 8720: ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
1.1 misho 8721: and EXTENDED().
8722:
1.1.1.2 misho 8723: If you need to set several options at once, and you don't want to go
8724: through the pains of declaring a RE_Options object and setting several
8725: options, there is a parallel method that give you such ability on the
8726: fly. You can concatenate several set_xxxxx() member functions, since
8727: each of them returns a reference to its class object. For example, to
8728: pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
1.1 misho 8729: statement, you may write:
8730:
8731: RE(" ^ xyz \\s+ .* blah$",
8732: RE_Options()
8733: .set_caseless(true)
8734: .set_extended(true)
8735: .set_multiline(true)).PartialMatch(sometext);
8736:
8737:
8738: SCANNING TEXT INCREMENTALLY
8739:
1.1.1.2 misho 8740: The "Consume" operation may be useful if you want to repeatedly match
1.1 misho 8741: regular expressions at the front of a string and skip over them as they
1.1.1.2 misho 8742: match. This requires use of the "StringPiece" type, which represents a
8743: sub-range of a real string. Like RE, StringPiece is defined in the
1.1 misho 8744: pcrecpp namespace.
8745:
8746: Example: read lines of the form "var = value" from a string.
8747: string contents = ...; // Fill string somehow
8748: pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
8749:
8750: string var;
8751: int value;
8752: pcrecpp::RE re("(\\w+) = (\\d+)\n");
8753: while (re.Consume(&input, &var, &value)) {
8754: ...;
8755: }
8756:
1.1.1.2 misho 8757: Each successful call to "Consume" will set "var/value", and also
1.1 misho 8758: advance "input" so it points past the matched text.
8759:
1.1.1.2 misho 8760: The "FindAndConsume" operation is similar to "Consume" but does not
8761: anchor your match at the beginning of the string. For example, you
1.1 misho 8762: could extract all words from a string by repeatedly calling
8763:
8764: pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)
8765:
8766:
8767: PARSING HEX/OCTAL/C-RADIX NUMBERS
8768:
8769: By default, if you pass a pointer to a numeric value, the corresponding
1.1.1.2 misho 8770: text is interpreted as a base-10 number. You can instead wrap the
1.1 misho 8771: pointer with a call to one of the operators Hex(), Octal(), or CRadix()
1.1.1.2 misho 8772: to interpret the text in another base. The CRadix operator interprets
8773: C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to
1.1 misho 8774: base-10.
8775:
8776: Example:
8777: int a, b, c, d;
8778: pcrecpp::RE re("(.*) (.*) (.*) (.*)");
8779: re.FullMatch("100 40 0100 0x40",
8780: pcrecpp::Octal(&a), pcrecpp::Hex(&b),
8781: pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));
8782:
8783: will leave 64 in a, b, c, and d.
8784:
8785:
8786: REPLACING PARTS OF STRINGS
8787:
1.1.1.2 misho 8788: You can replace the first match of "pattern" in "str" with "rewrite".
8789: Within "rewrite", backslash-escaped digits (\1 to \9) can be used to
8790: insert text matching corresponding parenthesized group from the pat-
1.1 misho 8791: tern. \0 in "rewrite" refers to the entire matching text. For example:
8792:
8793: string s = "yabba dabba doo";
8794: pcrecpp::RE("b+").Replace("d", &s);
8795:
1.1.1.2 misho 8796: will leave "s" containing "yada dabba doo". The result is true if the
1.1 misho 8797: pattern matches and a replacement occurs, false otherwise.
8798:
1.1.1.2 misho 8799: GlobalReplace is like Replace except that it replaces all occurrences
8800: of the pattern in the string with the rewrite. Replacements are not
1.1 misho 8801: subject to re-matching. For example:
8802:
8803: string s = "yabba dabba doo";
8804: pcrecpp::RE("b+").GlobalReplace("d", &s);
8805:
1.1.1.2 misho 8806: will leave "s" containing "yada dada doo". It returns the number of
1.1 misho 8807: replacements made.
8808:
1.1.1.2 misho 8809: Extract is like Replace, except that if the pattern matches, "rewrite"
8810: is copied into "out" (an additional argument) with substitutions. The
8811: non-matching portions of "text" are ignored. Returns true iff a match
1.1 misho 8812: occurred and the extraction happened successfully; if no match occurs,
8813: the string is left unaffected.
8814:
8815:
8816: AUTHOR
8817:
8818: The C++ wrapper was contributed by Google Inc.
8819: Copyright (c) 2007 Google Inc.
8820:
8821:
8822: REVISION
8823:
1.1.1.2 misho 8824: Last updated: 08 January 2012
1.1 misho 8825: ------------------------------------------------------------------------------
8826:
8827:
8828: PCRESAMPLE(3) PCRESAMPLE(3)
8829:
8830:
8831: NAME
8832: PCRE - Perl-compatible regular expressions
8833:
8834:
8835: PCRE SAMPLE PROGRAM
8836:
8837: A simple, complete demonstration program, to get you started with using
8838: PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A
8839: listing of this program is given in the pcredemo documentation. If you
8840: do not have a copy of the PCRE distribution, you can save this listing
8841: to re-create pcredemo.c.
8842:
1.1.1.2 misho 8843: The demonstration program, which uses the original PCRE 8-bit library,
8844: compiles the regular expression that is its first argument, and matches
8845: it against the subject string in its second argument. No PCRE options
8846: are set, and default character tables are used. If matching succeeds,
8847: the program outputs the portion of the subject that matched, together
8848: with the contents of any captured substrings.
1.1 misho 8849:
8850: If the -g option is given on the command line, the program then goes on
8851: to check for further matches of the same regular expression in the same
1.1.1.2 misho 8852: subject string. The logic is a little bit tricky because of the possi-
8853: bility of matching an empty string. Comments in the code explain what
1.1 misho 8854: is going on.
8855:
1.1.1.2 misho 8856: If PCRE is installed in the standard include and library directories
1.1 misho 8857: for your operating system, you should be able to compile the demonstra-
8858: tion program using this command:
8859:
8860: gcc -o pcredemo pcredemo.c -lpcre
8861:
1.1.1.2 misho 8862: If PCRE is installed elsewhere, you may need to add additional options
8863: to the command line. For example, on a Unix-like system that has PCRE
8864: installed in /usr/local, you can compile the demonstration program
1.1 misho 8865: using a command like this:
8866:
8867: gcc -o pcredemo -I/usr/local/include pcredemo.c \
8868: -L/usr/local/lib -lpcre
8869:
1.1.1.2 misho 8870: In a Windows environment, if you want to statically link the program
1.1 misho 8871: against a non-dll pcre.a file, you must uncomment the line that defines
1.1.1.2 misho 8872: PCRE_STATIC before including pcre.h, because otherwise the pcre_mal-
1.1 misho 8873: loc() and pcre_free() exported functions will be declared
8874: __declspec(dllimport), with unwanted results.
8875:
1.1.1.2 misho 8876: Once you have compiled and linked the demonstration program, you can
1.1 misho 8877: run simple tests like this:
8878:
8879: ./pcredemo 'cat|dog' 'the cat sat on the mat'
8880: ./pcredemo -g 'cat|dog' 'the dog sat on the cat'
8881:
1.1.1.2 misho 8882: Note that there is a much more comprehensive test program, called
8883: pcretest, which supports many more facilities for testing regular
8884: expressions and both PCRE libraries. The pcredemo program is provided
8885: as a simple coding example.
1.1 misho 8886:
1.1.1.2 misho 8887: If you try to run pcredemo when PCRE is not installed in the standard
8888: library directory, you may get an error like this on some operating
1.1 misho 8889: systems (e.g. Solaris):
8890:
1.1.1.2 misho 8891: ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
1.1 misho 8892: directory
8893:
1.1.1.2 misho 8894: This is caused by the way shared library support works on those sys-
1.1 misho 8895: tems. You need to add
8896:
8897: -R/usr/local/lib
8898:
8899: (for example) to the compile command to get round this problem.
8900:
8901:
8902: AUTHOR
8903:
8904: Philip Hazel
8905: University Computing Service
8906: Cambridge CB2 3QH, England.
8907:
8908:
8909: REVISION
8910:
1.1.1.2 misho 8911: Last updated: 10 January 2012
8912: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 8913: ------------------------------------------------------------------------------
8914: PCRELIMITS(3) PCRELIMITS(3)
8915:
8916:
8917: NAME
8918: PCRE - Perl-compatible regular expressions
8919:
8920:
8921: SIZE AND OTHER LIMITATIONS
8922:
8923: There are some size limitations in PCRE but it is hoped that they will
8924: never in practice be relevant.
8925:
1.1.1.2 misho 8926: The maximum length of a compiled pattern is approximately 64K data
8927: units (bytes for the 8-bit library, 16-bit units for the 16-bit
8928: library) if PCRE is compiled with the default internal linkage size of
8929: 2 bytes. If you want to process regular expressions that are truly
8930: enormous, you can compile PCRE with an internal linkage size of 3 or 4
8931: (when building the 16-bit library, 3 is rounded up to 4). See the
8932: README file in the source distribution and the pcrebuild documentation
8933: for details. In these cases the limit is substantially larger. How-
8934: ever, the speed of execution is slower.
1.1 misho 8935:
8936: All values in repeating quantifiers must be less than 65536.
8937:
8938: There is no limit to the number of parenthesized subpatterns, but there
8939: can be no more than 65535 capturing subpatterns.
8940:
8941: There is a limit to the number of forward references to subsequent sub-
8942: patterns of around 200,000. Repeated forward references with fixed
8943: upper limits, for example, (?2){0,100} when subpattern number 2 is to
8944: the right, are included in the count. There is no limit to the number
8945: of backward references.
8946:
8947: The maximum length of name for a named subpattern is 32 characters, and
8948: the maximum number of named subpatterns is 10000.
8949:
1.1.1.3 ! misho 8950: The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or
! 8951: (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit
! 8952: library.
! 8953:
1.1 misho 8954: The maximum length of a subject string is the largest positive number
8955: that an integer variable can hold. However, when using the traditional
8956: matching function, PCRE uses recursion to handle subpatterns and indef-
8957: inite repetition. This means that the available stack space may limit
8958: the size of a subject string that can be processed by certain patterns.
8959: For a discussion of stack issues, see the pcrestack documentation.
8960:
8961:
8962: AUTHOR
8963:
8964: Philip Hazel
8965: University Computing Service
8966: Cambridge CB2 3QH, England.
8967:
8968:
8969: REVISION
8970:
1.1.1.3 ! misho 8971: Last updated: 04 May 2012
1.1.1.2 misho 8972: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 8973: ------------------------------------------------------------------------------
8974:
8975:
8976: PCRESTACK(3) PCRESTACK(3)
8977:
8978:
8979: NAME
8980: PCRE - Perl-compatible regular expressions
8981:
8982:
8983: PCRE DISCUSSION OF STACK USAGE
8984:
1.1.1.2 misho 8985: When you call pcre[16]_exec(), it makes use of an internal function
8986: called match(). This calls itself recursively at branch points in the
8987: pattern, in order to remember the state of the match so that it can
8988: back up and try a different alternative if the first one fails. As
8989: matching proceeds deeper and deeper into the tree of possibilities, the
8990: recursion depth increases. The match() function is also called in other
8991: circumstances, for example, whenever a parenthesized sub-pattern is
8992: entered, and in certain cases of repetition.
1.1 misho 8993:
8994: Not all calls of match() increase the recursion depth; for an item such
8995: as a* it may be called several times at the same level, after matching
8996: different numbers of a's. Furthermore, in a number of cases where the
8997: result of the recursive call would immediately be passed back as the
8998: result of the current call (a "tail recursion"), the function is just
8999: restarted instead.
9000:
1.1.1.2 misho 9001: The above comments apply when pcre[16]_exec() is run in its normal
9002: interpretive manner. If the pattern was studied with the
9003: PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was success-
9004: ful, and the options passed to pcre[16]_exec() were not incompatible,
9005: the matching process uses the JIT-compiled code instead of the match()
9006: function. In this case, the memory requirements are handled entirely
9007: differently. See the pcrejit documentation for details.
1.1 misho 9008:
1.1.1.2 misho 9009: The pcre[16]_dfa_exec() function operates in an entirely different way,
9010: and uses recursion only when there is a regular expression recursion or
1.1 misho 9011: subroutine call in the pattern. This includes the processing of asser-
9012: tion and "once-only" subpatterns, which are handled like subroutine
9013: calls. Normally, these are never very deep, and the limit on the com-
1.1.1.2 misho 9014: plexity of pcre[16]_dfa_exec() is controlled by the amount of workspace
9015: it is given. However, it is possible to write patterns with runaway
9016: infinite recursions; such patterns will cause pcre[16]_dfa_exec() to
9017: run out of stack. At present, there is no protection against this.
1.1 misho 9018:
1.1.1.2 misho 9019: The comments that follow do NOT apply to pcre[16]_dfa_exec(); they are
9020: relevant only for pcre[16]_exec() without the JIT optimization.
1.1 misho 9021:
1.1.1.2 misho 9022: Reducing pcre[16]_exec()'s stack usage
1.1 misho 9023:
9024: Each time that match() is actually called recursively, it uses memory
9025: from the process stack. For certain kinds of pattern and data, very
9026: large amounts of stack may be needed, despite the recognition of "tail
9027: recursion". You can often reduce the amount of recursion, and there-
9028: fore the amount of stack used, by modifying the pattern that is being
9029: matched. Consider, for example, this pattern:
9030:
9031: ([^<]|<(?!inet))+
9032:
9033: It matches from wherever it starts until it encounters "<inet" or the
9034: end of the data, and is the kind of pattern that might be used when
9035: processing an XML file. Each iteration of the outer parentheses matches
9036: either one character that is not "<" or a "<" that is not followed by
9037: "inet". However, each time a parenthesis is processed, a recursion
9038: occurs, so this formulation uses a stack frame for each matched charac-
9039: ter. For a long string, a lot of stack is required. Consider now this
9040: rewritten pattern, which matches exactly the same strings:
9041:
9042: ([^<]++|<(?!inet))+
9043:
9044: This uses very much less stack, because runs of characters that do not
9045: contain "<" are "swallowed" in one item inside the parentheses. Recur-
9046: sion happens only when a "<" character that is not followed by "inet"
9047: is encountered (and we assume this is relatively rare). A possessive
9048: quantifier is used to stop any backtracking into the runs of non-"<"
9049: characters, but that is not related to stack usage.
9050:
9051: This example shows that one way of avoiding stack problems when match-
9052: ing long subject strings is to write repeated parenthesized subpatterns
9053: to match more than one character whenever possible.
9054:
1.1.1.2 misho 9055: Compiling PCRE to use heap instead of stack for pcre[16]_exec()
1.1 misho 9056:
9057: In environments where stack memory is constrained, you might want to
9058: compile PCRE to use heap memory instead of stack for remembering back-
1.1.1.2 misho 9059: up points when pcre[16]_exec() is running. This makes it run a lot more
1.1 misho 9060: slowly, however. Details of how to do this are given in the pcrebuild
9061: documentation. When built in this way, instead of using the stack, PCRE
9062: obtains and frees memory by calling the functions that are pointed to
1.1.1.2 misho 9063: by the pcre[16]_stack_malloc and pcre[16]_stack_free variables. By
9064: default, these point to malloc() and free(), but you can replace the
9065: pointers to cause PCRE to use your own functions. Since the block sizes
9066: are always the same, and are always freed in reverse order, it may be
9067: possible to implement customized memory handlers that are more effi-
9068: cient than the standard functions.
1.1 misho 9069:
1.1.1.2 misho 9070: Limiting pcre[16]_exec()'s stack usage
1.1 misho 9071:
9072: You can set limits on the number of times that match() is called, both
1.1.1.2 misho 9073: in total and recursively. If a limit is exceeded, pcre[16]_exec()
9074: returns an error code. Setting suitable limits should prevent it from
9075: running out of stack. The default values of the limits are very large,
9076: and unlikely ever to operate. They can be changed when PCRE is built,
9077: and they can also be set when pcre[16]_exec() is called. For details of
9078: these interfaces, see the pcrebuild documentation and the section on
9079: extra data for pcre[16]_exec() in the pcreapi documentation.
1.1 misho 9080:
9081: As a very rough rule of thumb, you should reckon on about 500 bytes per
9082: recursion. Thus, if you want to limit your stack usage to 8Mb, you
9083: should set the limit at 16000 recursions. A 64Mb stack, on the other
9084: hand, can support around 128000 recursions.
9085:
9086: In Unix-like environments, the pcretest test program has a command line
9087: option (-S) that can be used to increase the size of its stack. As long
9088: as the stack is large enough, another option (-M) can be used to find
9089: the smallest limits that allow a particular pattern to match a given
1.1.1.2 misho 9090: subject string. This is done by calling pcre[16]_exec() repeatedly with
1.1 misho 9091: different limits.
9092:
1.1.1.2 misho 9093: Obtaining an estimate of stack usage
9094:
9095: The actual amount of stack used per recursion can vary quite a lot,
9096: depending on the compiler that was used to build PCRE and the optimiza-
9097: tion or debugging options that were set for it. The rule of thumb value
9098: of 500 bytes mentioned above may be larger or smaller than what is
9099: actually needed. A better approximation can be obtained by running this
9100: command:
9101:
9102: pcretest -m -C
9103:
9104: The -C option causes pcretest to output information about the options
9105: with which PCRE was compiled. When -m is also given (before -C), infor-
9106: mation about stack use is given in a line like this:
9107:
9108: Match recursion uses stack: approximate frame size = 640 bytes
9109:
9110: The value is approximate because some recursions need a bit more (up to
9111: perhaps 16 more bytes).
9112:
9113: If the above command is given when PCRE is compiled to use the heap
9114: instead of the stack for recursion, the value that is output is the
9115: size of each block that is obtained from the heap.
9116:
1.1 misho 9117: Changing stack size in Unix-like systems
9118:
9119: In Unix-like environments, there is not often a problem with the stack
9120: unless very long strings are involved, though the default limit on
9121: stack size varies from system to system. Values from 8Mb to 64Mb are
9122: common. You can find your default limit by running the command:
9123:
9124: ulimit -s
9125:
9126: Unfortunately, the effect of running out of stack is often SIGSEGV,
9127: though sometimes a more explicit error message is given. You can nor-
9128: mally increase the limit on stack size by code such as this:
9129:
9130: struct rlimit rlim;
9131: getrlimit(RLIMIT_STACK, &rlim);
9132: rlim.rlim_cur = 100*1024*1024;
9133: setrlimit(RLIMIT_STACK, &rlim);
9134:
9135: This reads the current limits (soft and hard) using getrlimit(), then
9136: attempts to increase the soft limit to 100Mb using setrlimit(). You
1.1.1.2 misho 9137: must do this before calling pcre[16]_exec().
1.1 misho 9138:
9139: Changing stack size in Mac OS X
9140:
9141: Using setrlimit(), as described above, should also work on Mac OS X. It
9142: is also possible to set a stack size when linking a program. There is a
9143: discussion about stack sizes in Mac OS X at this web site:
9144: http://developer.apple.com/qa/qa2005/qa1419.html.
9145:
9146:
9147: AUTHOR
9148:
9149: Philip Hazel
9150: University Computing Service
9151: Cambridge CB2 3QH, England.
9152:
9153:
9154: REVISION
9155:
1.1.1.2 misho 9156: Last updated: 21 January 2012
9157: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 9158: ------------------------------------------------------------------------------
9159:
9160:

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