Annotation of embedaddon/pcre/doc/pcre.txt, revision 1.1.1.1
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:
28: The current implementation of PCRE corresponds approximately with Perl
29: 5.12, including support for UTF-8 encoded strings and Unicode general
30: category properties. However, UTF-8 and Unicode support has to be
31: explicitly enabled; it is not the default. The Unicode tables corre-
32: spond to Unicode release 6.0.0.
33:
34: In addition to the Perl-compatible matching function, PCRE contains an
35: alternative function that matches the same compiled patterns in a dif-
36: ferent way. In certain circumstances, the alternative function has some
37: advantages. For a discussion of the two matching algorithms, see the
38: pcrematching page.
39:
40: PCRE is written in C and released as a C library. A number of people
41: have written wrappers and interfaces of various kinds. In particular,
42: Google Inc. have provided a comprehensive C++ wrapper. This is now
43: included as part of the PCRE distribution. The pcrecpp page has details
44: of this interface. Other people's contributions can be found in the
45: Contrib directory at the primary FTP site, which is:
46:
47: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
48:
49: Details of exactly which Perl regular expression features are and are
50: not supported by PCRE are given in separate documents. See the pcrepat-
51: tern and pcrecompat pages. There is a syntax summary in the pcresyntax
52: page.
53:
54: Some features of PCRE can be included, excluded, or changed when the
55: library is built. The pcre_config() function makes it possible for a
56: client to discover which features are available. The features them-
57: selves are described in the pcrebuild page. Documentation about build-
58: ing PCRE for various operating systems can be found in the README and
59: NON-UNIX-USE files in the source distribution.
60:
61: The library contains a number of undocumented internal functions and
62: data tables that are used by more than one of the exported external
63: functions, but which are not intended for use by external callers.
64: Their names all begin with "_pcre_", which hopefully will not provoke
65: any name clashes. In some environments, it is possible to control which
66: external symbols are exported when a shared library is built, and in
67: these cases the undocumented symbols are not exported.
68:
69:
70: USER DOCUMENTATION
71:
72: The user documentation for PCRE comprises a number of different sec-
73: tions. In the "man" format, each of these is a separate "man page". In
74: the HTML format, each is a separate page, linked from the index page.
75: In the plain text format, all the sections, except the pcredemo sec-
76: tion, are concatenated, for ease of searching. The sections are as fol-
77: lows:
78:
79: pcre this document
80: pcre-config show PCRE installation configuration information
81: pcreapi details of PCRE's native C API
82: pcrebuild options for building PCRE
83: pcrecallout details of the callout feature
84: pcrecompat discussion of Perl compatibility
85: pcrecpp details of the C++ wrapper
86: pcredemo a demonstration C program that uses PCRE
87: pcregrep description of the pcregrep command
88: pcrejit discussion of the just-in-time optimization support
89: pcrelimits details of size and other limits
90: pcrematching discussion of the two matching algorithms
91: pcrepartial details of the partial matching facility
92: pcrepattern syntax and semantics of supported
93: regular expressions
94: pcreperform discussion of performance issues
95: pcreposix the POSIX-compatible C API
96: pcreprecompile details of saving and re-using precompiled patterns
97: pcresample discussion of the pcredemo program
98: pcrestack discussion of stack usage
99: pcresyntax quick syntax reference
100: pcretest description of the pcretest testing command
101: pcreunicode discussion of Unicode and UTF-8 support
102:
103: In addition, in the "man" and HTML formats, there is a short page for
104: each C library function, listing its arguments and results.
105:
106:
107: AUTHOR
108:
109: Philip Hazel
110: University Computing Service
111: Cambridge CB2 3QH, England.
112:
113: Putting an actual email address here seems to have been a spam magnet,
114: so I've taken it away. If you want to email me, use my two initials,
115: followed by the two digits 10, at the domain cam.ac.uk.
116:
117:
118: REVISION
119:
120: Last updated: 24 August 2011
121: Copyright (c) 1997-2011 University of Cambridge.
122: ------------------------------------------------------------------------------
123:
124:
125: PCREBUILD(3) PCREBUILD(3)
126:
127:
128: NAME
129: PCRE - Perl-compatible regular expressions
130:
131:
132: PCRE BUILD-TIME OPTIONS
133:
134: This document describes the optional features of PCRE that can be
135: selected when the library is compiled. It assumes use of the configure
136: script, where the optional features are selected or deselected by pro-
137: viding options to configure before running the make command. However,
138: the same options can be selected in both Unix-like and non-Unix-like
139: environments using the GUI facility of cmake-gui if you are using CMake
140: instead of configure to build PCRE.
141:
142: There is a lot more information about building PCRE in non-Unix-like
143: environments in the file called NON_UNIX_USE, which is part of the PCRE
144: distribution. You should consult this file as well as the README file
145: if you are building in a non-Unix-like environment.
146:
147: The complete list of options for configure (which includes the standard
148: ones such as the selection of the installation directory) can be
149: obtained by running
150:
151: ./configure --help
152:
153: The following sections include descriptions of options whose names
154: begin with --enable or --disable. These settings specify changes to the
155: defaults for the configure command. Because of the way that configure
156: works, --enable and --disable always come in pairs, so the complemen-
157: tary option always exists as well, but as it specifies the default, it
158: is not described.
159:
160:
161: BUILDING SHARED AND STATIC LIBRARIES
162:
163: The PCRE building process uses libtool to build both shared and static
164: Unix libraries by default. You can suppress one of these by adding one
165: of
166:
167: --disable-shared
168: --disable-static
169:
170: to the configure command, as required.
171:
172:
173: C++ SUPPORT
174:
175: By default, the configure script will search for a C++ compiler and C++
176: header files. If it finds them, it automatically builds the C++ wrapper
177: library for PCRE. You can disable this by adding
178:
179: --disable-cpp
180:
181: to the configure command.
182:
183:
184: UTF-8 SUPPORT
185:
186: To build PCRE with support for UTF-8 Unicode character strings, add
187:
188: --enable-utf8
189:
190: to the configure command. Of itself, this does not make PCRE treat
191: strings as UTF-8. As well as compiling PCRE with this option, you also
192: have have to set the PCRE_UTF8 option when you call the pcre_compile()
193: or pcre_compile2() functions.
194:
195: If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE
196: expects its input to be either ASCII or UTF-8 (depending on the runtime
197: option). It is not possible to support both EBCDIC and UTF-8 codes in
198: the same version of the library. Consequently, --enable-utf8 and
199: --enable-ebcdic are mutually exclusive.
200:
201:
202: UNICODE CHARACTER PROPERTY SUPPORT
203:
204: UTF-8 support allows PCRE to process character values greater than 255
205: in the strings that it handles. On its own, however, it does not pro-
206: vide any facilities for accessing the properties of such characters. If
207: you want to be able to use the pattern escapes \P, \p, and \X, which
208: refer to Unicode character properties, you must add
209:
210: --enable-unicode-properties
211:
212: to the configure command. This implies UTF-8 support, even if you have
213: not explicitly requested it.
214:
215: Including Unicode property support adds around 30K of tables to the
216: PCRE library. Only the general category properties such as Lu and Nd
217: are supported. Details are given in the pcrepattern documentation.
218:
219:
220: JUST-IN-TIME COMPILER SUPPORT
221:
222: Just-in-time compiler support is included in the build by specifying
223:
224: --enable-jit
225:
226: This support is available only for certain hardware architectures. If
227: this option is set for an unsupported architecture, a compile time
228: error occurs. See the pcrejit documentation for a discussion of JIT
229: usage. When JIT support is enabled, pcregrep automatically makes use of
230: it, unless you add
231:
232: --disable-pcregrep-jit
233:
234: to the "configure" command.
235:
236:
237: CODE VALUE OF NEWLINE
238:
239: By default, PCRE interprets the linefeed (LF) character as indicating
240: the end of a line. This is the normal newline character on Unix-like
241: systems. You can compile PCRE to use carriage return (CR) instead, by
242: adding
243:
244: --enable-newline-is-cr
245:
246: to the configure command. There is also a --enable-newline-is-lf
247: option, which explicitly specifies linefeed as the newline character.
248:
249: Alternatively, you can specify that line endings are to be indicated by
250: the two character sequence CRLF. If you want this, add
251:
252: --enable-newline-is-crlf
253:
254: to the configure command. There is a fourth option, specified by
255:
256: --enable-newline-is-anycrlf
257:
258: which causes PCRE to recognize any of the three sequences CR, LF, or
259: CRLF as indicating a line ending. Finally, a fifth option, specified by
260:
261: --enable-newline-is-any
262:
263: causes PCRE to recognize any Unicode newline sequence.
264:
265: Whatever line ending convention is selected when PCRE is built can be
266: overridden when the library functions are called. At build time it is
267: conventional to use the standard for your operating system.
268:
269:
270: WHAT \R MATCHES
271:
272: By default, the sequence \R in a pattern matches any Unicode newline
273: sequence, whatever has been selected as the line ending sequence. If
274: you specify
275:
276: --enable-bsr-anycrlf
277:
278: the default is changed so that \R matches only CR, LF, or CRLF. What-
279: ever is selected when PCRE is built can be overridden when the library
280: functions are called.
281:
282:
283: POSIX MALLOC USAGE
284:
285: When PCRE is called through the POSIX interface (see the pcreposix doc-
286: umentation), additional working storage is required for holding the
287: pointers to capturing substrings, because PCRE requires three integers
288: per substring, whereas the POSIX interface provides only two. If the
289: number of expected substrings is small, the wrapper function uses space
290: on the stack, because this is faster than using malloc() for each call.
291: The default threshold above which the stack is no longer used is 10; it
292: can be changed by adding a setting such as
293:
294: --with-posix-malloc-threshold=20
295:
296: to the configure command.
297:
298:
299: HANDLING VERY LARGE PATTERNS
300:
301: Within a compiled pattern, offset values are used to point from one
302: part to another (for example, from an opening parenthesis to an alter-
303: nation metacharacter). By default, two-byte values are used for these
304: offsets, leading to a maximum size for a compiled pattern of around
305: 64K. This is sufficient to handle all but the most gigantic patterns.
306: Nevertheless, some people do want to process truyl enormous patterns,
307: so it is possible to compile PCRE to use three-byte or four-byte off-
308: sets by adding a setting such as
309:
310: --with-link-size=3
311:
312: to the configure command. The value given must be 2, 3, or 4. Using
313: longer offsets slows down the operation of PCRE because it has to load
314: additional bytes when handling them.
315:
316:
317: AVOIDING EXCESSIVE STACK USAGE
318:
319: When matching with the pcre_exec() function, PCRE implements backtrack-
320: ing by making recursive calls to an internal function called match().
321: In environments where the size of the stack is limited, this can se-
322: verely limit PCRE's operation. (The Unix environment does not usually
323: suffer from this problem, but it may sometimes be necessary to increase
324: the maximum stack size. There is a discussion in the pcrestack docu-
325: mentation.) An alternative approach to recursion that uses memory from
326: the heap to remember data, instead of using recursive function calls,
327: has been implemented to work round the problem of limited stack size.
328: If you want to build a version of PCRE that works this way, add
329:
330: --disable-stack-for-recursion
331:
332: to the configure command. With this configuration, PCRE will use the
333: pcre_stack_malloc and pcre_stack_free variables to call memory manage-
334: ment functions. By default these point to malloc() and free(), but you
335: can replace the pointers so that your own functions are used instead.
336:
337: Separate functions are provided rather than using pcre_malloc and
338: pcre_free because the usage is very predictable: the block sizes
339: requested are always the same, and the blocks are always freed in
340: reverse order. A calling program might be able to implement optimized
341: functions that perform better than malloc() and free(). PCRE runs
342: noticeably more slowly when built in this way. This option affects only
343: the pcre_exec() function; it is not relevant for pcre_dfa_exec().
344:
345:
346: LIMITING PCRE RESOURCE USAGE
347:
348: Internally, PCRE has a function called match(), which it calls repeat-
349: edly (sometimes recursively) when matching a pattern with the
350: pcre_exec() function. By controlling the maximum number of times this
351: function may be called during a single matching operation, a limit can
352: be placed on the resources used by a single call to pcre_exec(). The
353: limit can be changed at run time, as described in the pcreapi documen-
354: tation. The default is 10 million, but this can be changed by adding a
355: setting such as
356:
357: --with-match-limit=500000
358:
359: to the configure command. This setting has no effect on the
360: pcre_dfa_exec() matching function.
361:
362: In some environments it is desirable to limit the depth of recursive
363: calls of match() more strictly than the total number of calls, in order
364: to restrict the maximum amount of stack (or heap, if --disable-stack-
365: for-recursion is specified) that is used. A second limit controls this;
366: it defaults to the value that is set for --with-match-limit, which
367: imposes no additional constraints. However, you can set a lower limit
368: by adding, for example,
369:
370: --with-match-limit-recursion=10000
371:
372: to the configure command. This value can also be overridden at run
373: time.
374:
375:
376: CREATING CHARACTER TABLES AT BUILD TIME
377:
378: PCRE uses fixed tables for processing characters whose code values are
379: less than 256. By default, PCRE is built with a set of tables that are
380: distributed in the file pcre_chartables.c.dist. These tables are for
381: ASCII codes only. If you add
382:
383: --enable-rebuild-chartables
384:
385: to the configure command, the distributed tables are no longer used.
386: Instead, a program called dftables is compiled and run. This outputs
387: the source for new set of tables, created in the default locale of your
388: C runtime system. (This method of replacing the tables does not work if
389: you are cross compiling, because dftables is run on the local host. If
390: you need to create alternative tables when cross compiling, you will
391: have to do so "by hand".)
392:
393:
394: USING EBCDIC CODE
395:
396: PCRE assumes by default that it will run in an environment where the
397: character code is ASCII (or Unicode, which is a superset of ASCII).
398: This is the case for most computer operating systems. PCRE can, how-
399: ever, be compiled to run in an EBCDIC environment by adding
400:
401: --enable-ebcdic
402:
403: to the configure command. This setting implies --enable-rebuild-charta-
404: bles. You should only use it if you know that you are in an EBCDIC
405: environment (for example, an IBM mainframe operating system). The
406: --enable-ebcdic option is incompatible with --enable-utf8.
407:
408:
409: PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
410:
411: By default, pcregrep reads all files as plain text. You can build it so
412: that it recognizes files whose names end in .gz or .bz2, and reads them
413: with libz or libbz2, respectively, by adding one or both of
414:
415: --enable-pcregrep-libz
416: --enable-pcregrep-libbz2
417:
418: to the configure command. These options naturally require that the rel-
419: evant libraries are installed on your system. Configuration will fail
420: if they are not.
421:
422:
423: PCREGREP BUFFER SIZE
424:
425: pcregrep uses an internal buffer to hold a "window" on the file it is
426: scanning, in order to be able to output "before" and "after" lines when
427: it finds a match. The size of the buffer is controlled by a parameter
428: whose default value is 20K. The buffer itself is three times this size,
429: but because of the way it is used for holding "before" lines, the long-
430: est line that is guaranteed to be processable is the parameter size.
431: You can change the default parameter value by adding, for example,
432:
433: --with-pcregrep-bufsize=50K
434:
435: to the configure command. The caller of pcregrep can, however, override
436: this value by specifying a run-time option.
437:
438:
439: PCRETEST OPTION FOR LIBREADLINE SUPPORT
440:
441: If you add
442:
443: --enable-pcretest-libreadline
444:
445: to the configure command, pcretest is linked with the libreadline
446: library, and when its input is from a terminal, it reads it using the
447: readline() function. This provides line-editing and history facilities.
448: Note that libreadline is GPL-licensed, so if you distribute a binary of
449: pcretest linked in this way, there may be licensing issues.
450:
451: Setting this option causes the -lreadline option to be added to the
452: pcretest build. In many operating environments with a sytem-installed
453: libreadline this is sufficient. However, in some environments (e.g. if
454: an unmodified distribution version of readline is in use), some extra
455: configuration may be necessary. The INSTALL file for libreadline says
456: this:
457:
458: "Readline uses the termcap functions, but does not link with the
459: termcap or curses library itself, allowing applications which link
460: with readline the to choose an appropriate library."
461:
462: If your environment has not been set up so that an appropriate library
463: is automatically included, you may need to add something like
464:
465: LIBS="-ncurses"
466:
467: immediately before the configure command.
468:
469:
470: SEE ALSO
471:
472: pcreapi(3), pcre_config(3).
473:
474:
475: AUTHOR
476:
477: Philip Hazel
478: University Computing Service
479: Cambridge CB2 3QH, England.
480:
481:
482: REVISION
483:
484: Last updated: 06 September 2011
485: Copyright (c) 1997-2011 University of Cambridge.
486: ------------------------------------------------------------------------------
487:
488:
489: PCREMATCHING(3) PCREMATCHING(3)
490:
491:
492: NAME
493: PCRE - Perl-compatible regular expressions
494:
495:
496: PCRE MATCHING ALGORITHMS
497:
498: This document describes the two different algorithms that are available
499: in PCRE for matching a compiled regular expression against a given sub-
500: ject string. The "standard" algorithm is the one provided by the
501: pcre_exec() function. This works in the same was as Perl's matching
502: function, and provides a Perl-compatible matching operation.
503:
504: An alternative algorithm is provided by the pcre_dfa_exec() function;
505: this operates in a different way, and is not Perl-compatible. It has
506: advantages and disadvantages compared with the standard algorithm, and
507: these are described below.
508:
509: When there is only one possible way in which a given subject string can
510: match a pattern, the two algorithms give the same answer. A difference
511: arises, however, when there are multiple possibilities. For example, if
512: the pattern
513:
514: ^<.*>
515:
516: is matched against the string
517:
518: <something> <something else> <something further>
519:
520: there are three possible answers. The standard algorithm finds only one
521: of them, whereas the alternative algorithm finds all three.
522:
523:
524: REGULAR EXPRESSIONS AS TREES
525:
526: The set of strings that are matched by a regular expression can be rep-
527: resented as a tree structure. An unlimited repetition in the pattern
528: makes the tree of infinite size, but it is still a tree. Matching the
529: pattern to a given subject string (from a given starting point) can be
530: thought of as a search of the tree. There are two ways to search a
531: tree: depth-first and breadth-first, and these correspond to the two
532: matching algorithms provided by PCRE.
533:
534:
535: THE STANDARD MATCHING ALGORITHM
536:
537: In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
538: sions", the standard algorithm is an "NFA algorithm". It conducts a
539: depth-first search of the pattern tree. That is, it proceeds along a
540: single path through the tree, checking that the subject matches what is
541: required. When there is a mismatch, the algorithm tries any alterna-
542: tives at the current point, and if they all fail, it backs up to the
543: previous branch point in the tree, and tries the next alternative
544: branch at that level. This often involves backing up (moving to the
545: left) in the subject string as well. The order in which repetition
546: branches are tried is controlled by the greedy or ungreedy nature of
547: the quantifier.
548:
549: If a leaf node is reached, a matching string has been found, and at
550: that point the algorithm stops. Thus, if there is more than one possi-
551: ble match, this algorithm returns the first one that it finds. Whether
552: this is the shortest, the longest, or some intermediate length depends
553: on the way the greedy and ungreedy repetition quantifiers are specified
554: in the pattern.
555:
556: Because it ends up with a single path through the tree, it is rela-
557: tively straightforward for this algorithm to keep track of the sub-
558: strings that are matched by portions of the pattern in parentheses.
559: This provides support for capturing parentheses and back references.
560:
561:
562: THE ALTERNATIVE MATCHING ALGORITHM
563:
564: This algorithm conducts a breadth-first search of the tree. Starting
565: from the first matching point in the subject, it scans the subject
566: string from left to right, once, character by character, and as it does
567: this, it remembers all the paths through the tree that represent valid
568: matches. In Friedl's terminology, this is a kind of "DFA algorithm",
569: though it is not implemented as a traditional finite state machine (it
570: keeps multiple states active simultaneously).
571:
572: Although the general principle of this matching algorithm is that it
573: scans the subject string only once, without backtracking, there is one
574: exception: when a lookaround assertion is encountered, the characters
575: following or preceding the current point have to be independently
576: inspected.
577:
578: The scan continues until either the end of the subject is reached, or
579: there are no more unterminated paths. At this point, terminated paths
580: represent the different matching possibilities (if there are none, the
581: match has failed). Thus, if there is more than one possible match,
582: this algorithm finds all of them, and in particular, it finds the long-
583: est. The matches are returned in decreasing order of length. There is
584: an option to stop the algorithm after the first match (which is neces-
585: sarily the shortest) is found.
586:
587: Note that all the matches that are found start at the same point in the
588: subject. If the pattern
589:
590: cat(er(pillar)?)?
591:
592: is matched against the string "the caterpillar catchment", the result
593: will be the three strings "caterpillar", "cater", and "cat" that start
594: at the fifth character of the subject. The algorithm does not automati-
595: cally move on to find matches that start at later positions.
596:
597: There are a number of features of PCRE regular expressions that are not
598: supported by the alternative matching algorithm. They are as follows:
599:
600: 1. Because the algorithm finds all possible matches, the greedy or
601: ungreedy nature of repetition quantifiers is not relevant. Greedy and
602: ungreedy quantifiers are treated in exactly the same way. However, pos-
603: sessive quantifiers can make a difference when what follows could also
604: match what is quantified, for example in a pattern like this:
605:
606: ^a++\w!
607:
608: This pattern matches "aaab!" but not "aaa!", which would be matched by
609: a non-possessive quantifier. Similarly, if an atomic group is present,
610: it is matched as if it were a standalone pattern at the current point,
611: and the longest match is then "locked in" for the rest of the overall
612: pattern.
613:
614: 2. When dealing with multiple paths through the tree simultaneously, it
615: is not straightforward to keep track of captured substrings for the
616: different matching possibilities, and PCRE's implementation of this
617: algorithm does not attempt to do this. This means that no captured sub-
618: strings are available.
619:
620: 3. Because no substrings are captured, back references within the pat-
621: tern are not supported, and cause errors if encountered.
622:
623: 4. For the same reason, conditional expressions that use a backrefer-
624: ence as the condition or test for a specific group recursion are not
625: supported.
626:
627: 5. Because many paths through the tree may be active, the \K escape
628: sequence, which resets the start of the match when encountered (but may
629: be on some paths and not on others), is not supported. It causes an
630: error if encountered.
631:
632: 6. Callouts are supported, but the value of the capture_top field is
633: always 1, and the value of the capture_last field is always -1.
634:
635: 7. The \C escape sequence, which (in the standard algorithm) matches a
636: single byte, even in UTF-8 mode, is not supported in UTF-8 mode,
637: because the alternative algorithm moves through the subject string one
638: character at a time, for all active paths through the tree.
639:
640: 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
641: are not supported. (*FAIL) is supported, and behaves like a failing
642: negative assertion.
643:
644:
645: ADVANTAGES OF THE ALTERNATIVE ALGORITHM
646:
647: Using the alternative matching algorithm provides the following advan-
648: tages:
649:
650: 1. All possible matches (at a single point in the subject) are automat-
651: ically found, and in particular, the longest match is found. To find
652: more than one match using the standard algorithm, you have to do kludgy
653: things with callouts.
654:
655: 2. Because the alternative algorithm scans the subject string just
656: once, and never needs to backtrack, it is possible to pass very long
657: subject strings to the matching function in several pieces, checking
658: for partial matching each time. Although it is possible to do multi-
659: segment matching using the standard algorithm (pcre_exec()), by retain-
660: ing partially matched substrings, it is more complicated. The pcrepar-
661: tial documentation gives details of partial matching and discusses
662: multi-segment matching.
663:
664:
665: DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
666:
667: The alternative algorithm suffers from a number of disadvantages:
668:
669: 1. It is substantially slower than the standard algorithm. This is
670: partly because it has to search for all possible matches, but is also
671: because it is less susceptible to optimization.
672:
673: 2. Capturing parentheses and back references are not supported.
674:
675: 3. Although atomic groups are supported, their use does not provide the
676: performance advantage that it does for the standard algorithm.
677:
678:
679: AUTHOR
680:
681: Philip Hazel
682: University Computing Service
683: Cambridge CB2 3QH, England.
684:
685:
686: REVISION
687:
688: Last updated: 19 November 2011
689: Copyright (c) 1997-2010 University of Cambridge.
690: ------------------------------------------------------------------------------
691:
692:
693: PCREAPI(3) PCREAPI(3)
694:
695:
696: NAME
697: PCRE - Perl-compatible regular expressions
698:
699:
700: PCRE NATIVE API BASIC FUNCTIONS
701:
702: #include <pcre.h>
703:
704: pcre *pcre_compile(const char *pattern, int options,
705: const char **errptr, int *erroffset,
706: const unsigned char *tableptr);
707:
708: pcre *pcre_compile2(const char *pattern, int options,
709: int *errorcodeptr,
710: const char **errptr, int *erroffset,
711: const unsigned char *tableptr);
712:
713: pcre_extra *pcre_study(const pcre *code, int options,
714: const char **errptr);
715:
716: void pcre_free_study(pcre_extra *extra);
717:
718: int pcre_exec(const pcre *code, const pcre_extra *extra,
719: const char *subject, int length, int startoffset,
720: int options, int *ovector, int ovecsize);
721:
722:
723: PCRE NATIVE API AUXILIARY FUNCTIONS
724:
725: pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
726:
727: void pcre_jit_stack_free(pcre_jit_stack *stack);
728:
729: void pcre_assign_jit_stack(pcre_extra *extra,
730: pcre_jit_callback callback, void *data);
731:
732: int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
733: const char *subject, int length, int startoffset,
734: int options, int *ovector, int ovecsize,
735: int *workspace, int wscount);
736:
737: int pcre_copy_named_substring(const pcre *code,
738: const char *subject, int *ovector,
739: int stringcount, const char *stringname,
740: char *buffer, int buffersize);
741:
742: int pcre_copy_substring(const char *subject, int *ovector,
743: int stringcount, int stringnumber, char *buffer,
744: int buffersize);
745:
746: int pcre_get_named_substring(const pcre *code,
747: const char *subject, int *ovector,
748: int stringcount, const char *stringname,
749: const char **stringptr);
750:
751: int pcre_get_stringnumber(const pcre *code,
752: const char *name);
753:
754: int pcre_get_stringtable_entries(const pcre *code,
755: const char *name, char **first, char **last);
756:
757: int pcre_get_substring(const char *subject, int *ovector,
758: int stringcount, int stringnumber,
759: const char **stringptr);
760:
761: int pcre_get_substring_list(const char *subject,
762: int *ovector, int stringcount, const char ***listptr);
763:
764: void pcre_free_substring(const char *stringptr);
765:
766: void pcre_free_substring_list(const char **stringptr);
767:
768: const unsigned char *pcre_maketables(void);
769:
770: int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
771: int what, void *where);
772:
773: int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
774:
775: int pcre_refcount(pcre *code, int adjust);
776:
777: int pcre_config(int what, void *where);
778:
779: char *pcre_version(void);
780:
781:
782: PCRE NATIVE API INDIRECTED FUNCTIONS
783:
784: void *(*pcre_malloc)(size_t);
785:
786: void (*pcre_free)(void *);
787:
788: void *(*pcre_stack_malloc)(size_t);
789:
790: void (*pcre_stack_free)(void *);
791:
792: int (*pcre_callout)(pcre_callout_block *);
793:
794:
795: PCRE API OVERVIEW
796:
797: PCRE has its own native API, which is described in this document. There
798: are also some wrapper functions that correspond to the POSIX regular
799: expression API, but they do not give access to all the functionality.
800: They are described in the pcreposix documentation. Both of these APIs
801: define a set of C function calls. A C++ wrapper is also distributed
802: with PCRE. It is documented in the pcrecpp page.
803:
804: The native API C function prototypes are defined in the header file
805: pcre.h, and on Unix systems the library itself is called libpcre. It
806: can normally be accessed by adding -lpcre to the command for linking an
807: application that uses PCRE. The header file defines the macros
808: PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
809: bers for the library. Applications can use these to include support
810: for different releases of PCRE.
811:
812: In a Windows environment, if you want to statically link an application
813: program against a non-dll pcre.a file, you must define PCRE_STATIC
814: before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
815: loc() and pcre_free() exported functions will be declared
816: __declspec(dllimport), with unwanted results.
817:
818: The functions pcre_compile(), pcre_compile2(), pcre_study(), and
819: pcre_exec() are used for compiling and matching regular expressions in
820: a Perl-compatible manner. A sample program that demonstrates the sim-
821: plest way of using them is provided in the file called pcredemo.c in
822: the PCRE source distribution. A listing of this program is given in the
823: pcredemo documentation, and the pcresample documentation describes how
824: to compile and run it.
825:
826: Just-in-time compiler support is an optional feature of PCRE that can
827: be built in appropriate hardware environments. It greatly speeds up the
828: matching performance of many patterns. Simple programs can easily
829: request that it be used if available, by setting an option that is
830: ignored when it is not relevant. More complicated programs might need
831: to make use of the functions pcre_jit_stack_alloc(),
832: pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control
833: the JIT code's memory usage. These functions are discussed in the
834: pcrejit documentation.
835:
836: A second matching function, pcre_dfa_exec(), which is not Perl-compati-
837: ble, is also provided. This uses a different algorithm for the match-
838: ing. The alternative algorithm finds all possible matches (at a given
839: point in the subject), and scans the subject just once (unless there
840: are lookbehind assertions). However, this algorithm does not return
841: captured substrings. A description of the two matching algorithms and
842: their advantages and disadvantages is given in the pcrematching docu-
843: mentation.
844:
845: In addition to the main compiling and matching functions, there are
846: convenience functions for extracting captured substrings from a subject
847: string that is matched by pcre_exec(). They are:
848:
849: pcre_copy_substring()
850: pcre_copy_named_substring()
851: pcre_get_substring()
852: pcre_get_named_substring()
853: pcre_get_substring_list()
854: pcre_get_stringnumber()
855: pcre_get_stringtable_entries()
856:
857: pcre_free_substring() and pcre_free_substring_list() are also provided,
858: to free the memory used for extracted strings.
859:
860: The function pcre_maketables() is used to build a set of character
861: tables in the current locale for passing to pcre_compile(),
862: pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
863: provided for specialist use. Most commonly, no special tables are
864: passed, in which case internal tables that are generated when PCRE is
865: built are used.
866:
867: The function pcre_fullinfo() is used to find out information about a
868: compiled pattern; pcre_info() is an obsolete version that returns only
869: some of the available information, but is retained for backwards com-
870: patibility. The function pcre_version() returns a pointer to a string
871: containing the version of PCRE and its date of release.
872:
873: The function pcre_refcount() maintains a reference count in a data
874: block containing a compiled pattern. This is provided for the benefit
875: of object-oriented applications.
876:
877: The global variables pcre_malloc and pcre_free initially contain the
878: entry points of the standard malloc() and free() functions, respec-
879: tively. PCRE calls the memory management functions via these variables,
880: so a calling program can replace them if it wishes to intercept the
881: calls. This should be done before calling any PCRE functions.
882:
883: The global variables pcre_stack_malloc and pcre_stack_free are also
884: indirections to memory management functions. These special functions
885: are used only when PCRE is compiled to use the heap for remembering
886: data, instead of recursive function calls, when running the pcre_exec()
887: function. See the pcrebuild documentation for details of how to do
888: this. It is a non-standard way of building PCRE, for use in environ-
889: ments that have limited stacks. Because of the greater use of memory
890: management, it runs more slowly. Separate functions are provided so
891: that special-purpose external code can be used for this case. When
892: used, these functions are always called in a stack-like manner (last
893: obtained, first freed), and always for memory blocks of the same size.
894: There is a discussion about PCRE's stack usage in the pcrestack docu-
895: mentation.
896:
897: The global variable pcre_callout initially contains NULL. It can be set
898: by the caller to a "callout" function, which PCRE will then call at
899: specified points during a matching operation. Details are given in the
900: pcrecallout documentation.
901:
902:
903: NEWLINES
904:
905: PCRE supports five different conventions for indicating line breaks in
906: strings: a single CR (carriage return) character, a single LF (line-
907: feed) character, the two-character sequence CRLF, any of the three pre-
908: ceding, or any Unicode newline sequence. The Unicode newline sequences
909: are the three just mentioned, plus the single characters VT (vertical
910: tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
911: separator, U+2028), and PS (paragraph separator, U+2029).
912:
913: Each of the first three conventions is used by at least one operating
914: system as its standard newline sequence. When PCRE is built, a default
915: can be specified. The default default is LF, which is the Unix stan-
916: dard. When PCRE is run, the default can be overridden, either when a
917: pattern is compiled, or when it is matched.
918:
919: At compile time, the newline convention can be specified by the options
920: argument of pcre_compile(), or it can be specified by special text at
921: the start of the pattern itself; this overrides any other settings. See
922: the pcrepattern page for details of the special character sequences.
923:
924: In the PCRE documentation the word "newline" is used to mean "the char-
925: acter or pair of characters that indicate a line break". The choice of
926: newline convention affects the handling of the dot, circumflex, and
927: dollar metacharacters, the handling of #-comments in /x mode, and, when
928: CRLF is a recognized line ending sequence, the match position advance-
929: ment for a non-anchored pattern. There is more detail about this in the
930: section on pcre_exec() options below.
931:
932: The choice of newline convention does not affect the interpretation of
933: the \n or \r escape sequences, nor does it affect what \R matches,
934: which is controlled in a similar way, but by separate options.
935:
936:
937: MULTITHREADING
938:
939: The PCRE functions can be used in multi-threading applications, with
940: the proviso that the memory management functions pointed to by
941: pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
942: callout function pointed to by pcre_callout, are shared by all threads.
943:
944: The compiled form of a regular expression is not altered during match-
945: ing, so the same compiled pattern can safely be used by several threads
946: at once.
947:
948: If the just-in-time optimization feature is being used, it needs sepa-
949: rate memory stack areas for each thread. See the pcrejit documentation
950: for more details.
951:
952:
953: SAVING PRECOMPILED PATTERNS FOR LATER USE
954:
955: The compiled form of a regular expression can be saved and re-used at a
956: later time, possibly by a different program, and even on a host other
957: than the one on which it was compiled. Details are given in the
958: pcreprecompile documentation. However, compiling a regular expression
959: with one version of PCRE for use with a different version is not guar-
960: anteed to work and may cause crashes.
961:
962:
963: CHECKING BUILD-TIME OPTIONS
964:
965: int pcre_config(int what, void *where);
966:
967: The function pcre_config() makes it possible for a PCRE client to dis-
968: cover which optional features have been compiled into the PCRE library.
969: The pcrebuild documentation has more details about these optional fea-
970: tures.
971:
972: The first argument for pcre_config() is an integer, specifying which
973: information is required; the second argument is a pointer to a variable
974: into which the information is placed. The following information is
975: available:
976:
977: PCRE_CONFIG_UTF8
978:
979: The output is an integer that is set to one if UTF-8 support is avail-
980: able; otherwise it is set to zero.
981:
982: PCRE_CONFIG_UNICODE_PROPERTIES
983:
984: The output is an integer that is set to one if support for Unicode
985: character properties is available; otherwise it is set to zero.
986:
987: PCRE_CONFIG_JIT
988:
989: The output is an integer that is set to one if support for just-in-time
990: compiling is available; otherwise it is set to zero.
991:
992: PCRE_CONFIG_NEWLINE
993:
994: The output is an integer whose value specifies the default character
995: sequence that is recognized as meaning "newline". The four values that
996: are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
997: and -1 for ANY. Though they are derived from ASCII, the same values
998: are returned in EBCDIC environments. The default should normally corre-
999: spond to the standard sequence for your operating system.
1000:
1001: PCRE_CONFIG_BSR
1002:
1003: The output is an integer whose value indicates what character sequences
1004: the \R escape sequence matches by default. A value of 0 means that \R
1005: matches any Unicode line ending sequence; a value of 1 means that \R
1006: matches only CR, LF, or CRLF. The default can be overridden when a pat-
1007: tern is compiled or matched.
1008:
1009: PCRE_CONFIG_LINK_SIZE
1010:
1011: The output is an integer that contains the number of bytes used for
1012: internal linkage in compiled regular expressions. The value is 2, 3, or
1013: 4. Larger values allow larger regular expressions to be compiled, at
1014: the expense of slower matching. The default value of 2 is sufficient
1015: for all but the most massive patterns, since it allows the compiled
1016: pattern to be up to 64K in size.
1017:
1018: PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1019:
1020: The output is an integer that contains the threshold above which the
1021: POSIX interface uses malloc() for output vectors. Further details are
1022: given in the pcreposix documentation.
1023:
1024: PCRE_CONFIG_MATCH_LIMIT
1025:
1026: The output is a long integer that gives the default limit for the num-
1027: ber of internal matching function calls in a pcre_exec() execution.
1028: Further details are given with pcre_exec() below.
1029:
1030: PCRE_CONFIG_MATCH_LIMIT_RECURSION
1031:
1032: The output is a long integer that gives the default limit for the depth
1033: of recursion when calling the internal matching function in a
1034: pcre_exec() execution. Further details are given with pcre_exec()
1035: below.
1036:
1037: PCRE_CONFIG_STACKRECURSE
1038:
1039: The output is an integer that is set to one if internal recursion when
1040: running pcre_exec() is implemented by recursive function calls that use
1041: the stack to remember their state. This is the usual way that PCRE is
1042: compiled. The output is zero if PCRE was compiled to use blocks of data
1043: on the heap instead of recursive function calls. In this case,
1044: pcre_stack_malloc and pcre_stack_free are called to manage memory
1045: blocks on the heap, thus avoiding the use of the stack.
1046:
1047:
1048: COMPILING A PATTERN
1049:
1050: pcre *pcre_compile(const char *pattern, int options,
1051: const char **errptr, int *erroffset,
1052: const unsigned char *tableptr);
1053:
1054: pcre *pcre_compile2(const char *pattern, int options,
1055: int *errorcodeptr,
1056: const char **errptr, int *erroffset,
1057: const unsigned char *tableptr);
1058:
1059: Either of the functions pcre_compile() or pcre_compile2() can be called
1060: to compile a pattern into an internal form. The only difference between
1061: the two interfaces is that pcre_compile2() has an additional argument,
1062: errorcodeptr, via which a numerical error code can be returned. To
1063: avoid too much repetition, we refer just to pcre_compile() below, but
1064: the information applies equally to pcre_compile2().
1065:
1066: The pattern is a C string terminated by a binary zero, and is passed in
1067: the pattern argument. A pointer to a single block of memory that is
1068: obtained via pcre_malloc is returned. This contains the compiled code
1069: and related data. The pcre type is defined for the returned block; this
1070: is a typedef for a structure whose contents are not externally defined.
1071: It is up to the caller to free the memory (via pcre_free) when it is no
1072: longer required.
1073:
1074: Although the compiled code of a PCRE regex is relocatable, that is, it
1075: does not depend on memory location, the complete pcre data block is not
1076: fully relocatable, because it may contain a copy of the tableptr argu-
1077: ment, which is an address (see below).
1078:
1079: The options argument contains various bit settings that affect the com-
1080: pilation. It should be zero if no options are required. The available
1081: options are described below. Some of them (in particular, those that
1082: are compatible with Perl, but some others as well) can also be set and
1083: unset from within the pattern (see the detailed description in the
1084: pcrepattern documentation). For those options that can be different in
1085: different parts of the pattern, the contents of the options argument
1086: specifies their settings at the start of compilation and execution. The
1087: PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
1088: PCRE_NO_START_OPT options can be set at the time of matching as well as
1089: at compile time.
1090:
1091: If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1092: if compilation of a pattern fails, pcre_compile() returns NULL, and
1093: sets the variable pointed to by errptr to point to a textual error mes-
1094: sage. This is a static string that is part of the library. You must not
1095: try to free it. Normally, the offset from the start of the pattern to
1096: the byte that was being processed when the error was discovered is
1097: placed in the variable pointed to by erroffset, which must not be NULL
1098: (if it is, an immediate error is given). However, for an invalid UTF-8
1099: string, the offset is that of the first byte of the failing character.
1100: Also, some errors are not detected until checks are carried out when
1101: the whole pattern has been scanned; in these cases the offset passed
1102: back is the length of the pattern.
1103:
1104: Note that the offset is in bytes, not characters, even in UTF-8 mode.
1105: It may sometimes point into the middle of a UTF-8 character.
1106:
1107: If pcre_compile2() is used instead of pcre_compile(), and the error-
1108: codeptr argument is not NULL, a non-zero error code number is returned
1109: via this argument in the event of an error. This is in addition to the
1110: textual error message. Error codes and messages are listed below.
1111:
1112: If the final argument, tableptr, is NULL, PCRE uses a default set of
1113: character tables that are built when PCRE is compiled, using the
1114: default C locale. Otherwise, tableptr must be an address that is the
1115: result of a call to pcre_maketables(). This value is stored with the
1116: compiled pattern, and used again by pcre_exec(), unless another table
1117: pointer is passed to it. For more discussion, see the section on locale
1118: support below.
1119:
1120: This code fragment shows a typical straightforward call to pcre_com-
1121: pile():
1122:
1123: pcre *re;
1124: const char *error;
1125: int erroffset;
1126: re = pcre_compile(
1127: "^A.*Z", /* the pattern */
1128: 0, /* default options */
1129: &error, /* for error message */
1130: &erroffset, /* for error offset */
1131: NULL); /* use default character tables */
1132:
1133: The following names for option bits are defined in the pcre.h header
1134: file:
1135:
1136: PCRE_ANCHORED
1137:
1138: If this bit is set, the pattern is forced to be "anchored", that is, it
1139: is constrained to match only at the first matching point in the string
1140: that is being searched (the "subject string"). This effect can also be
1141: achieved by appropriate constructs in the pattern itself, which is the
1142: only way to do it in Perl.
1143:
1144: PCRE_AUTO_CALLOUT
1145:
1146: If this bit is set, pcre_compile() automatically inserts callout items,
1147: all with number 255, before each pattern item. For discussion of the
1148: callout facility, see the pcrecallout documentation.
1149:
1150: PCRE_BSR_ANYCRLF
1151: PCRE_BSR_UNICODE
1152:
1153: These options (which are mutually exclusive) control what the \R escape
1154: sequence matches. The choice is either to match only CR, LF, or CRLF,
1155: or to match any Unicode newline sequence. The default is specified when
1156: PCRE is built. It can be overridden from within the pattern, or by set-
1157: ting an option when a compiled pattern is matched.
1158:
1159: PCRE_CASELESS
1160:
1161: If this bit is set, letters in the pattern match both upper and lower
1162: case letters. It is equivalent to Perl's /i option, and it can be
1163: changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1164: always understands the concept of case for characters whose values are
1165: less than 128, so caseless matching is always possible. For characters
1166: with higher values, the concept of case is supported if PCRE is com-
1167: piled with Unicode property support, but not otherwise. If you want to
1168: use caseless matching for characters 128 and above, you must ensure
1169: that PCRE is compiled with Unicode property support as well as with
1170: UTF-8 support.
1171:
1172: PCRE_DOLLAR_ENDONLY
1173:
1174: If this bit is set, a dollar metacharacter in the pattern matches only
1175: at the end of the subject string. Without this option, a dollar also
1176: matches immediately before a newline at the end of the string (but not
1177: before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1178: if PCRE_MULTILINE is set. There is no equivalent to this option in
1179: Perl, and no way to set it within a pattern.
1180:
1181: PCRE_DOTALL
1182:
1183: If this bit is set, a dot metacharacter in the pattern matches a char-
1184: acter of any value, including one that indicates a newline. However, it
1185: only ever matches one character, even if newlines are coded as CRLF.
1186: Without this option, a dot does not match when the current position is
1187: at a newline. This option is equivalent to Perl's /s option, and it can
1188: be changed within a pattern by a (?s) option setting. A negative class
1189: such as [^a] always matches newline characters, independent of the set-
1190: ting of this option.
1191:
1192: PCRE_DUPNAMES
1193:
1194: If this bit is set, names used to identify capturing subpatterns need
1195: not be unique. This can be helpful for certain types of pattern when it
1196: is known that only one instance of the named subpattern can ever be
1197: matched. There are more details of named subpatterns below; see also
1198: the pcrepattern documentation.
1199:
1200: PCRE_EXTENDED
1201:
1202: If this bit is set, whitespace data characters in the pattern are
1203: totally ignored except when escaped or inside a character class. White-
1204: space does not include the VT character (code 11). In addition, charac-
1205: ters between an unescaped # outside a character class and the next new-
1206: line, inclusive, are also ignored. This is equivalent to Perl's /x
1207: option, and it can be changed within a pattern by a (?x) option set-
1208: ting.
1209:
1210: Which characters are interpreted as newlines is controlled by the
1211: options passed to pcre_compile() or by a special sequence at the start
1212: of the pattern, as described in the section entitled "Newline conven-
1213: tions" in the pcrepattern documentation. Note that the end of this type
1214: of comment is a literal newline sequence in the pattern; escape
1215: sequences that happen to represent a newline do not count.
1216:
1217: This option makes it possible to include comments inside complicated
1218: patterns. Note, however, that this applies only to data characters.
1219: Whitespace characters may never appear within special character
1220: sequences in a pattern, for example within the sequence (?( that intro-
1221: duces a conditional subpattern.
1222:
1223: PCRE_EXTRA
1224:
1225: This option was invented in order to turn on additional functionality
1226: of PCRE that is incompatible with Perl, but it is currently of very
1227: little use. When set, any backslash in a pattern that is followed by a
1228: letter that has no special meaning causes an error, thus reserving
1229: these combinations for future expansion. By default, as in Perl, a
1230: backslash followed by a letter with no special meaning is treated as a
1231: literal. (Perl can, however, be persuaded to give an error for this, by
1232: running it with the -w option.) There are at present no other features
1233: controlled by this option. It can also be set by a (?X) option setting
1234: within a pattern.
1235:
1236: PCRE_FIRSTLINE
1237:
1238: If this option is set, an unanchored pattern is required to match
1239: before or at the first newline in the subject string, though the
1240: matched text may continue over the newline.
1241:
1242: PCRE_JAVASCRIPT_COMPAT
1243:
1244: If this option is set, PCRE's behaviour is changed in some ways so that
1245: it is compatible with JavaScript rather than Perl. The changes are as
1246: follows:
1247:
1248: (1) A lone closing square bracket in a pattern causes a compile-time
1249: error, because this is illegal in JavaScript (by default it is treated
1250: as a data character). Thus, the pattern AB]CD becomes illegal when this
1251: option is set.
1252:
1253: (2) At run time, a back reference to an unset subpattern group matches
1254: an empty string (by default this causes the current matching alterna-
1255: tive to fail). A pattern such as (\1)(a) succeeds when this option is
1256: set (assuming it can find an "a" in the subject), whereas it fails by
1257: default, for Perl compatibility.
1258:
1259: (3) \U matches an upper case "U" character; by default \U causes a com-
1260: pile time error (Perl uses \U to upper case subsequent characters).
1261:
1262: (4) \u matches a lower case "u" character unless it is followed by four
1263: hexadecimal digits, in which case the hexadecimal number defines the
1264: code point to match. By default, \u causes a compile time error (Perl
1265: uses it to upper case the following character).
1266:
1267: (5) \x matches a lower case "x" character unless it is followed by two
1268: hexadecimal digits, in which case the hexadecimal number defines the
1269: code point to match. By default, as in Perl, a hexadecimal number is
1270: always expected after \x, but it may have zero, one, or two digits (so,
1271: for example, \xz matches a binary zero character followed by z).
1272:
1273: PCRE_MULTILINE
1274:
1275: By default, PCRE treats the subject string as consisting of a single
1276: line of characters (even if it actually contains newlines). The "start
1277: of line" metacharacter (^) matches only at the start of the string,
1278: while the "end of line" metacharacter ($) matches only at the end of
1279: the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1280: is set). This is the same as Perl.
1281:
1282: When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1283: constructs match immediately following or immediately before internal
1284: newlines in the subject string, respectively, as well as at the very
1285: start and end. This is equivalent to Perl's /m option, and it can be
1286: changed within a pattern by a (?m) option setting. If there are no new-
1287: lines in a subject string, or no occurrences of ^ or $ in a pattern,
1288: setting PCRE_MULTILINE has no effect.
1289:
1290: PCRE_NEWLINE_CR
1291: PCRE_NEWLINE_LF
1292: PCRE_NEWLINE_CRLF
1293: PCRE_NEWLINE_ANYCRLF
1294: PCRE_NEWLINE_ANY
1295:
1296: These options override the default newline definition that was chosen
1297: when PCRE was built. Setting the first or the second specifies that a
1298: newline is indicated by a single character (CR or LF, respectively).
1299: Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1300: two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1301: that any of the three preceding sequences should be recognized. Setting
1302: PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1303: recognized. The Unicode newline sequences are the three just mentioned,
1304: plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1305: U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1306: (paragraph separator, U+2029). The last two are recognized only in
1307: UTF-8 mode.
1308:
1309: The newline setting in the options word uses three bits that are
1310: treated as a number, giving eight possibilities. Currently only six are
1311: used (default plus the five values above). This means that if you set
1312: more than one newline option, the combination may or may not be sensi-
1313: ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1314: PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1315: cause an error.
1316:
1317: The only time that a line break in a pattern is specially recognized
1318: when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace
1319: characters, and so are ignored in this mode. Also, an unescaped # out-
1320: side a character class indicates a comment that lasts until after the
1321: next line break sequence. In other circumstances, line break sequences
1322: in patterns are treated as literal data.
1323:
1324: The newline option that is set at compile time becomes the default that
1325: is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1326:
1327: PCRE_NO_AUTO_CAPTURE
1328:
1329: If this option is set, it disables the use of numbered capturing paren-
1330: theses in the pattern. Any opening parenthesis that is not followed by
1331: ? behaves as if it were followed by ?: but named parentheses can still
1332: be used for capturing (and they acquire numbers in the usual way).
1333: There is no equivalent of this option in Perl.
1334:
1335: NO_START_OPTIMIZE
1336:
1337: This is an option that acts at matching time; that is, it is really an
1338: option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1339: time, it is remembered with the compiled pattern and assumed at match-
1340: ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1341: below.
1342:
1343: PCRE_UCP
1344:
1345: This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1346: \w, and some of the POSIX character classes. By default, only ASCII
1347: characters are recognized, but if PCRE_UCP is set, Unicode properties
1348: are used instead to classify characters. More details are given in the
1349: section on generic character types in the pcrepattern page. If you set
1350: PCRE_UCP, matching one of the items it affects takes much longer. The
1351: option is available only if PCRE has been compiled with Unicode prop-
1352: erty support.
1353:
1354: PCRE_UNGREEDY
1355:
1356: This option inverts the "greediness" of the quantifiers so that they
1357: are not greedy by default, but become greedy if followed by "?". It is
1358: not compatible with Perl. It can also be set by a (?U) option setting
1359: within the pattern.
1360:
1361: PCRE_UTF8
1362:
1363: This option causes PCRE to regard both the pattern and the subject as
1364: strings of UTF-8 characters instead of single-byte character strings.
1365: However, it is available only when PCRE is built to include UTF-8 sup-
1366: port. If not, the use of this option provokes an error. Details of how
1367: this option changes the behaviour of PCRE are given in the pcreunicode
1368: page.
1369:
1370: PCRE_NO_UTF8_CHECK
1371:
1372: When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1373: automatically checked. There is a discussion about the validity of
1374: UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1375: bytes is found, pcre_compile() returns an error. If you already know
1376: that your pattern is valid, and you want to skip this check for perfor-
1377: mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1378: set, the effect of passing an invalid UTF-8 string as a pattern is
1379: undefined. It may cause your program to crash. Note that this option
1380: can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1381: UTF-8 validity checking of subject strings.
1382:
1383:
1384: COMPILATION ERROR CODES
1385:
1386: The following table lists the error codes than may be returned by
1387: pcre_compile2(), along with the error messages that may be returned by
1388: both compiling functions. As PCRE has developed, some error codes have
1389: fallen out of use. To avoid confusion, they have not been re-used.
1390:
1391: 0 no error
1392: 1 \ at end of pattern
1393: 2 \c at end of pattern
1394: 3 unrecognized character follows \
1395: 4 numbers out of order in {} quantifier
1396: 5 number too big in {} quantifier
1397: 6 missing terminating ] for character class
1398: 7 invalid escape sequence in character class
1399: 8 range out of order in character class
1400: 9 nothing to repeat
1401: 10 [this code is not in use]
1402: 11 internal error: unexpected repeat
1403: 12 unrecognized character after (? or (?-
1404: 13 POSIX named classes are supported only within a class
1405: 14 missing )
1406: 15 reference to non-existent subpattern
1407: 16 erroffset passed as NULL
1408: 17 unknown option bit(s) set
1409: 18 missing ) after comment
1410: 19 [this code is not in use]
1411: 20 regular expression is too large
1412: 21 failed to get memory
1413: 22 unmatched parentheses
1414: 23 internal error: code overflow
1415: 24 unrecognized character after (?<
1416: 25 lookbehind assertion is not fixed length
1417: 26 malformed number or name after (?(
1418: 27 conditional group contains more than two branches
1419: 28 assertion expected after (?(
1420: 29 (?R or (?[+-]digits must be followed by )
1421: 30 unknown POSIX class name
1422: 31 POSIX collating elements are not supported
1423: 32 this version of PCRE is not compiled with PCRE_UTF8 support
1424: 33 [this code is not in use]
1425: 34 character value in \x{...} sequence is too large
1426: 35 invalid condition (?(0)
1427: 36 \C not allowed in lookbehind assertion
1428: 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1429: 38 number after (?C is > 255
1430: 39 closing ) for (?C expected
1431: 40 recursive call could loop indefinitely
1432: 41 unrecognized character after (?P
1433: 42 syntax error in subpattern name (missing terminator)
1434: 43 two named subpatterns have the same name
1435: 44 invalid UTF-8 string
1436: 45 support for \P, \p, and \X has not been compiled
1437: 46 malformed \P or \p sequence
1438: 47 unknown property name after \P or \p
1439: 48 subpattern name is too long (maximum 32 characters)
1440: 49 too many named subpatterns (maximum 10000)
1441: 50 [this code is not in use]
1442: 51 octal value is greater than \377 (not in UTF-8 mode)
1443: 52 internal error: overran compiling workspace
1444: 53 internal error: previously-checked referenced subpattern
1445: not found
1446: 54 DEFINE group contains more than one branch
1447: 55 repeating a DEFINE group is not allowed
1448: 56 inconsistent NEWLINE options
1449: 57 \g is not followed by a braced, angle-bracketed, or quoted
1450: name/number or by a plain number
1451: 58 a numbered reference must not be zero
1452: 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1453: 60 (*VERB) not recognized
1454: 61 number is too big
1455: 62 subpattern name expected
1456: 63 digit expected after (?+
1457: 64 ] is an invalid data character in JavaScript compatibility mode
1458: 65 different names for subpatterns of the same number are
1459: not allowed
1460: 66 (*MARK) must have an argument
1461: 67 this version of PCRE is not compiled with PCRE_UCP support
1462: 68 \c must be followed by an ASCII character
1463: 69 \k is not followed by a braced, angle-bracketed, or quoted name
1464:
1465: The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1466: values may be used if the limits were changed when PCRE was built.
1467:
1468:
1469: STUDYING A PATTERN
1470:
1471: pcre_extra *pcre_study(const pcre *code, int options
1472: const char **errptr);
1473:
1474: If a compiled pattern is going to be used several times, it is worth
1475: spending more time analyzing it in order to speed up the time taken for
1476: matching. The function pcre_study() takes a pointer to a compiled pat-
1477: tern as its first argument. If studying the pattern produces additional
1478: information that will help speed up matching, pcre_study() returns a
1479: pointer to a pcre_extra block, in which the study_data field points to
1480: the results of the study.
1481:
1482: The returned value from pcre_study() can be passed directly to
1483: pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1484: tains other fields that can be set by the caller before the block is
1485: passed; these are described below in the section on matching a pattern.
1486:
1487: If studying the pattern does not produce any useful information,
1488: pcre_study() returns NULL. In that circumstance, if the calling program
1489: wants to pass any of the other fields to pcre_exec() or
1490: pcre_dfa_exec(), it must set up its own pcre_extra block.
1491:
1492: The second argument of pcre_study() contains option bits. There is only
1493: one option: PCRE_STUDY_JIT_COMPILE. If this is set, and the just-in-
1494: time compiler is available, the pattern is further compiled into
1495: machine code that executes much faster than the pcre_exec() matching
1496: function. If the just-in-time compiler is not available, this option is
1497: ignored. All other bits in the options argument must be zero.
1498:
1499: JIT compilation is a heavyweight optimization. It can take some time
1500: for patterns to be analyzed, and for one-off matches and simple pat-
1501: terns the benefit of faster execution might be offset by a much slower
1502: study time. Not all patterns can be optimized by the JIT compiler. For
1503: those that cannot be handled, matching automatically falls back to the
1504: pcre_exec() interpreter. For more details, see the pcrejit documenta-
1505: tion.
1506:
1507: The third argument for pcre_study() is a pointer for an error message.
1508: If studying succeeds (even if no data is returned), the variable it
1509: points to is set to NULL. Otherwise it is set to point to a textual
1510: error message. This is a static string that is part of the library. You
1511: must not try to free it. You should test the error pointer for NULL
1512: after calling pcre_study(), to be sure that it has run successfully.
1513:
1514: When you are finished with a pattern, you can free the memory used for
1515: the study data by calling pcre_free_study(). This function was added to
1516: the API for release 8.20. For earlier versions, the memory could be
1517: freed with pcre_free(), just like the pattern itself. This will still
1518: work in cases where PCRE_STUDY_JIT_COMPILE is not used, but it is
1519: advisable to change to the new function when convenient.
1520:
1521: This is a typical way in which pcre_study() is used (except that in a
1522: real application there should be tests for errors):
1523:
1524: int rc;
1525: pcre *re;
1526: pcre_extra *sd;
1527: re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
1528: sd = pcre_study(
1529: re, /* result of pcre_compile() */
1530: 0, /* no options */
1531: &error); /* set to NULL or points to a message */
1532: rc = pcre_exec( /* see below for details of pcre_exec() options */
1533: re, sd, "subject", 7, 0, 0, ovector, 30);
1534: ...
1535: pcre_free_study(sd);
1536: pcre_free(re);
1537:
1538: Studying a pattern does two things: first, a lower bound for the length
1539: of subject string that is needed to match the pattern is computed. This
1540: does not mean that there are any strings of that length that match, but
1541: it does guarantee that no shorter strings match. The value is used by
1542: pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1543: match strings that are shorter than the lower bound. You can find out
1544: the value in a calling program via the pcre_fullinfo() function.
1545:
1546: Studying a pattern is also useful for non-anchored patterns that do not
1547: have a single fixed starting character. A bitmap of possible starting
1548: bytes is created. This speeds up finding a position in the subject at
1549: which to start matching.
1550:
1551: These two optimizations apply to both pcre_exec() and pcre_dfa_exec().
1552: However, they are not used by pcre_exec() if pcre_study() is called
1553: with the PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling is
1554: successful. The optimizations can be disabled by setting the
1555: PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or
1556: pcre_dfa_exec(). You might want to do this if your pattern contains
1557: callouts or (*MARK) (which cannot be handled by the JIT compiler), and
1558: you want to make use of these facilities in cases where matching fails.
1559: See the discussion of PCRE_NO_START_OPTIMIZE below.
1560:
1561:
1562: LOCALE SUPPORT
1563:
1564: PCRE handles caseless matching, and determines whether characters are
1565: letters, digits, or whatever, by reference to a set of tables, indexed
1566: by character value. When running in UTF-8 mode, this applies only to
1567: characters with codes less than 128. By default, higher-valued codes
1568: never match escapes such as \w or \d, but they can be tested with \p if
1569: PCRE is built with Unicode character property support. Alternatively,
1570: the PCRE_UCP option can be set at compile time; this causes \w and
1571: friends to use Unicode property support instead of built-in tables. The
1572: use of locales with Unicode is discouraged. If you are handling charac-
1573: ters with codes greater than 128, you should either use UTF-8 and Uni-
1574: code, or use locales, but not try to mix the two.
1575:
1576: PCRE contains an internal set of tables that are used when the final
1577: argument of pcre_compile() is NULL. These are sufficient for many
1578: applications. Normally, the internal tables recognize only ASCII char-
1579: acters. However, when PCRE is built, it is possible to cause the inter-
1580: nal tables to be rebuilt in the default "C" locale of the local system,
1581: which may cause them to be different.
1582:
1583: The internal tables can always be overridden by tables supplied by the
1584: application that calls PCRE. These may be created in a different locale
1585: from the default. As more and more applications change to using Uni-
1586: code, the need for this locale support is expected to die away.
1587:
1588: External tables are built by calling the pcre_maketables() function,
1589: which has no arguments, in the relevant locale. The result can then be
1590: passed to pcre_compile() or pcre_exec() as often as necessary. For
1591: example, to build and use tables that are appropriate for the French
1592: locale (where accented characters with values greater than 128 are
1593: treated as letters), the following code could be used:
1594:
1595: setlocale(LC_CTYPE, "fr_FR");
1596: tables = pcre_maketables();
1597: re = pcre_compile(..., tables);
1598:
1599: The locale name "fr_FR" is used on Linux and other Unix-like systems;
1600: if you are using Windows, the name for the French locale is "french".
1601:
1602: When pcre_maketables() runs, the tables are built in memory that is
1603: obtained via pcre_malloc. It is the caller's responsibility to ensure
1604: that the memory containing the tables remains available for as long as
1605: it is needed.
1606:
1607: The pointer that is passed to pcre_compile() is saved with the compiled
1608: pattern, and the same tables are used via this pointer by pcre_study()
1609: and normally also by pcre_exec(). Thus, by default, for any single pat-
1610: tern, compilation, studying and matching all happen in the same locale,
1611: but different patterns can be compiled in different locales.
1612:
1613: It is possible to pass a table pointer or NULL (indicating the use of
1614: the internal tables) to pcre_exec(). Although not intended for this
1615: purpose, this facility could be used to match a pattern in a different
1616: locale from the one in which it was compiled. Passing table pointers at
1617: run time is discussed below in the section on matching a pattern.
1618:
1619:
1620: INFORMATION ABOUT A PATTERN
1621:
1622: int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1623: int what, void *where);
1624:
1625: The pcre_fullinfo() function returns information about a compiled pat-
1626: tern. It replaces the obsolete pcre_info() function, which is neverthe-
1627: less retained for backwards compability (and is documented below).
1628:
1629: The first argument for pcre_fullinfo() is a pointer to the compiled
1630: pattern. The second argument is the result of pcre_study(), or NULL if
1631: the pattern was not studied. The third argument specifies which piece
1632: of information is required, and the fourth argument is a pointer to a
1633: variable to receive the data. The yield of the function is zero for
1634: success, or one of the following negative numbers:
1635:
1636: PCRE_ERROR_NULL the argument code was NULL
1637: the argument where was NULL
1638: PCRE_ERROR_BADMAGIC the "magic number" was not found
1639: PCRE_ERROR_BADOPTION the value of what was invalid
1640:
1641: The "magic number" is placed at the start of each compiled pattern as
1642: an simple check against passing an arbitrary memory pointer. Here is a
1643: typical call of pcre_fullinfo(), to obtain the length of the compiled
1644: pattern:
1645:
1646: int rc;
1647: size_t length;
1648: rc = pcre_fullinfo(
1649: re, /* result of pcre_compile() */
1650: sd, /* result of pcre_study(), or NULL */
1651: PCRE_INFO_SIZE, /* what is required */
1652: &length); /* where to put the data */
1653:
1654: The possible values for the third argument are defined in pcre.h, and
1655: are as follows:
1656:
1657: PCRE_INFO_BACKREFMAX
1658:
1659: Return the number of the highest back reference in the pattern. The
1660: fourth argument should point to an int variable. Zero is returned if
1661: there are no back references.
1662:
1663: PCRE_INFO_CAPTURECOUNT
1664:
1665: Return the number of capturing subpatterns in the pattern. The fourth
1666: argument should point to an int variable.
1667:
1668: PCRE_INFO_DEFAULT_TABLES
1669:
1670: Return a pointer to the internal default character tables within PCRE.
1671: The fourth argument should point to an unsigned char * variable. This
1672: information call is provided for internal use by the pcre_study() func-
1673: tion. External callers can cause PCRE to use its internal tables by
1674: passing a NULL table pointer.
1675:
1676: PCRE_INFO_FIRSTBYTE
1677:
1678: Return information about the first byte of any matched string, for a
1679: non-anchored pattern. The fourth argument should point to an int vari-
1680: able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1681: is still recognized for backwards compatibility.)
1682:
1683: If there is a fixed first byte, for example, from a pattern such as
1684: (cat|cow|coyote), its value is returned. Otherwise, if either
1685:
1686: (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1687: branch starts with "^", or
1688:
1689: (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1690: set (if it were set, the pattern would be anchored),
1691:
1692: -1 is returned, indicating that the pattern matches only at the start
1693: of a subject string or after any newline within the string. Otherwise
1694: -2 is returned. For anchored patterns, -2 is returned.
1695:
1696: PCRE_INFO_FIRSTTABLE
1697:
1698: If the pattern was studied, and this resulted in the construction of a
1699: 256-bit table indicating a fixed set of bytes for the first byte in any
1700: matching string, a pointer to the table is returned. Otherwise NULL is
1701: returned. The fourth argument should point to an unsigned char * vari-
1702: able.
1703:
1704: PCRE_INFO_HASCRORLF
1705:
1706: Return 1 if the pattern contains any explicit matches for CR or LF
1707: characters, otherwise 0. The fourth argument should point to an int
1708: variable. An explicit match is either a literal CR or LF character, or
1709: \r or \n.
1710:
1711: PCRE_INFO_JCHANGED
1712:
1713: Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1714: otherwise 0. The fourth argument should point to an int variable. (?J)
1715: and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1716:
1717: PCRE_INFO_JIT
1718:
1719: Return 1 if the pattern was studied with the PCRE_STUDY_JIT_COMPILE
1720: option, and just-in-time compiling was successful. The fourth argument
1721: should point to an int variable. A return value of 0 means that JIT
1722: support is not available in this version of PCRE, or that the pattern
1723: was not studied with the PCRE_STUDY_JIT_COMPILE option, or that the JIT
1724: compiler could not handle this particular pattern. See the pcrejit doc-
1725: umentation for details of what can and cannot be handled.
1726:
1727: PCRE_INFO_JITSIZE
1728:
1729: If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
1730: option, return the size of the JIT compiled code, otherwise return
1731: zero. The fourth argument should point to a size_t variable.
1732:
1733: PCRE_INFO_LASTLITERAL
1734:
1735: Return the value of the rightmost literal byte that must exist in any
1736: matched string, other than at its start, if such a byte has been
1737: recorded. The fourth argument should point to an int variable. If there
1738: is no such byte, -1 is returned. For anchored patterns, a last literal
1739: byte is recorded only if it follows something of variable length. For
1740: example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1741: /^a\dz\d/ the returned value is -1.
1742:
1743: PCRE_INFO_MINLENGTH
1744:
1745: If the pattern was studied and a minimum length for matching subject
1746: strings was computed, its value is returned. Otherwise the returned
1747: value is -1. The value is a number of characters, not bytes (this may
1748: be relevant in UTF-8 mode). The fourth argument should point to an int
1749: variable. A non-negative value is a lower bound to the length of any
1750: matching string. There may not be any strings of that length that do
1751: actually match, but every string that does match is at least that long.
1752:
1753: PCRE_INFO_NAMECOUNT
1754: PCRE_INFO_NAMEENTRYSIZE
1755: PCRE_INFO_NAMETABLE
1756:
1757: PCRE supports the use of named as well as numbered capturing parenthe-
1758: ses. The names are just an additional way of identifying the parenthe-
1759: ses, which still acquire numbers. Several convenience functions such as
1760: pcre_get_named_substring() are provided for extracting captured sub-
1761: strings by name. It is also possible to extract the data directly, by
1762: first converting the name to a number in order to access the correct
1763: pointers in the output vector (described with pcre_exec() below). To do
1764: the conversion, you need to use the name-to-number map, which is
1765: described by these three values.
1766:
1767: The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1768: gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1769: of each entry; both of these return an int value. The entry size
1770: depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1771: a pointer to the first entry of the table (a pointer to char). The
1772: first two bytes of each entry are the number of the capturing parenthe-
1773: sis, most significant byte first. The rest of the entry is the corre-
1774: sponding name, zero terminated.
1775:
1776: The names are in alphabetical order. Duplicate names may appear if (?|
1777: is used to create multiple groups with the same number, as described in
1778: the section on duplicate subpattern numbers in the pcrepattern page.
1779: Duplicate names for subpatterns with different numbers are permitted
1780: only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
1781: appear in the table in the order in which they were found in the pat-
1782: tern. In the absence of (?| this is the order of increasing number;
1783: when (?| is used this is not necessarily the case because later subpat-
1784: terns may have lower numbers.
1785:
1786: As a simple example of the name/number table, consider the following
1787: pattern (assume PCRE_EXTENDED is set, so white space - including new-
1788: lines - is ignored):
1789:
1790: (?<date> (?<year>(\d\d)?\d\d) -
1791: (?<month>\d\d) - (?<day>\d\d) )
1792:
1793: There are four named subpatterns, so the table has four entries, and
1794: each entry in the table is eight bytes long. The table is as follows,
1795: with non-printing bytes shows in hexadecimal, and undefined bytes shown
1796: as ??:
1797:
1798: 00 01 d a t e 00 ??
1799: 00 05 d a y 00 ?? ??
1800: 00 04 m o n t h 00
1801: 00 02 y e a r 00 ??
1802:
1803: When writing code to extract data from named subpatterns using the
1804: name-to-number map, remember that the length of the entries is likely
1805: to be different for each compiled pattern.
1806:
1807: PCRE_INFO_OKPARTIAL
1808:
1809: Return 1 if the pattern can be used for partial matching with
1810: pcre_exec(), otherwise 0. The fourth argument should point to an int
1811: variable. From release 8.00, this always returns 1, because the
1812: restrictions that previously applied to partial matching have been
1813: lifted. The pcrepartial documentation gives details of partial match-
1814: ing.
1815:
1816: PCRE_INFO_OPTIONS
1817:
1818: Return a copy of the options with which the pattern was compiled. The
1819: fourth argument should point to an unsigned long int variable. These
1820: option bits are those specified in the call to pcre_compile(), modified
1821: by any top-level option settings at the start of the pattern itself. In
1822: other words, they are the options that will be in force when matching
1823: starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1824: the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1825: and PCRE_EXTENDED.
1826:
1827: A pattern is automatically anchored by PCRE if all of its top-level
1828: alternatives begin with one of the following:
1829:
1830: ^ unless PCRE_MULTILINE is set
1831: \A always
1832: \G always
1833: .* if PCRE_DOTALL is set and there are no back
1834: references to the subpattern in which .* appears
1835:
1836: For such patterns, the PCRE_ANCHORED bit is set in the options returned
1837: by pcre_fullinfo().
1838:
1839: PCRE_INFO_SIZE
1840:
1841: Return the size of the compiled pattern. The fourth argument should
1842: point to a size_t variable. This value does not include the size of the
1843: pcre structure that is returned by pcre_compile(). The value that is
1844: passed as the argument to pcre_malloc() when pcre_compile() is getting
1845: memory in which to place the compiled data is the value returned by
1846: this option plus the size of the pcre structure. Studying a compiled
1847: pattern, with or without JIT, does not alter the value returned by this
1848: option.
1849:
1850: PCRE_INFO_STUDYSIZE
1851:
1852: Return the size of the data block pointed to by the study_data field in
1853: a pcre_extra block. If pcre_extra is NULL, or there is no study data,
1854: zero is returned. The fourth argument should point to a size_t vari-
1855: able. The study_data field is set by pcre_study() to record informa-
1856: tion that will speed up matching (see the section entitled "Studying a
1857: pattern" above). The format of the study_data block is private, but its
1858: length is made available via this option so that it can be saved and
1859: restored (see the pcreprecompile documentation for details).
1860:
1861:
1862: OBSOLETE INFO FUNCTION
1863:
1864: int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1865:
1866: The pcre_info() function is now obsolete because its interface is too
1867: restrictive to return all the available data about a compiled pattern.
1868: New programs should use pcre_fullinfo() instead. The yield of
1869: pcre_info() is the number of capturing subpatterns, or one of the fol-
1870: lowing negative numbers:
1871:
1872: PCRE_ERROR_NULL the argument code was NULL
1873: PCRE_ERROR_BADMAGIC the "magic number" was not found
1874:
1875: If the optptr argument is not NULL, a copy of the options with which
1876: the pattern was compiled is placed in the integer it points to (see
1877: PCRE_INFO_OPTIONS above).
1878:
1879: If the pattern is not anchored and the firstcharptr argument is not
1880: NULL, it is used to pass back information about the first character of
1881: any matched string (see PCRE_INFO_FIRSTBYTE above).
1882:
1883:
1884: REFERENCE COUNTS
1885:
1886: int pcre_refcount(pcre *code, int adjust);
1887:
1888: The pcre_refcount() function is used to maintain a reference count in
1889: the data block that contains a compiled pattern. It is provided for the
1890: benefit of applications that operate in an object-oriented manner,
1891: where different parts of the application may be using the same compiled
1892: pattern, but you want to free the block when they are all done.
1893:
1894: When a pattern is compiled, the reference count field is initialized to
1895: zero. It is changed only by calling this function, whose action is to
1896: add the adjust value (which may be positive or negative) to it. The
1897: yield of the function is the new value. However, the value of the count
1898: is constrained to lie between 0 and 65535, inclusive. If the new value
1899: is outside these limits, it is forced to the appropriate limit value.
1900:
1901: Except when it is zero, the reference count is not correctly preserved
1902: if a pattern is compiled on one host and then transferred to a host
1903: whose byte-order is different. (This seems a highly unlikely scenario.)
1904:
1905:
1906: MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1907:
1908: int pcre_exec(const pcre *code, const pcre_extra *extra,
1909: const char *subject, int length, int startoffset,
1910: int options, int *ovector, int ovecsize);
1911:
1912: The function pcre_exec() is called to match a subject string against a
1913: compiled pattern, which is passed in the code argument. If the pattern
1914: was studied, the result of the study should be passed in the extra
1915: argument. You can call pcre_exec() with the same code and extra argu-
1916: ments as many times as you like, in order to match different subject
1917: strings with the same pattern.
1918:
1919: This function is the main matching facility of the library, and it
1920: operates in a Perl-like manner. For specialist use there is also an
1921: alternative matching function, which is described below in the section
1922: about the pcre_dfa_exec() function.
1923:
1924: In most applications, the pattern will have been compiled (and option-
1925: ally studied) in the same process that calls pcre_exec(). However, it
1926: is possible to save compiled patterns and study data, and then use them
1927: later in different processes, possibly even on different hosts. For a
1928: discussion about this, see the pcreprecompile documentation.
1929:
1930: Here is an example of a simple call to pcre_exec():
1931:
1932: int rc;
1933: int ovector[30];
1934: rc = pcre_exec(
1935: re, /* result of pcre_compile() */
1936: NULL, /* we didn't study the pattern */
1937: "some string", /* the subject string */
1938: 11, /* the length of the subject string */
1939: 0, /* start at offset 0 in the subject */
1940: 0, /* default options */
1941: ovector, /* vector of integers for substring information */
1942: 30); /* number of elements (NOT size in bytes) */
1943:
1944: Extra data for pcre_exec()
1945:
1946: If the extra argument is not NULL, it must point to a pcre_extra data
1947: block. The pcre_study() function returns such a block (when it doesn't
1948: return NULL), but you can also create one for yourself, and pass addi-
1949: tional information in it. The pcre_extra block contains the following
1950: fields (not necessarily in this order):
1951:
1952: unsigned long int flags;
1953: void *study_data;
1954: void *executable_jit;
1955: unsigned long int match_limit;
1956: unsigned long int match_limit_recursion;
1957: void *callout_data;
1958: const unsigned char *tables;
1959: unsigned char **mark;
1960:
1961: The flags field is a bitmap that specifies which of the other fields
1962: are set. The flag bits are:
1963:
1964: PCRE_EXTRA_STUDY_DATA
1965: PCRE_EXTRA_EXECUTABLE_JIT
1966: PCRE_EXTRA_MATCH_LIMIT
1967: PCRE_EXTRA_MATCH_LIMIT_RECURSION
1968: PCRE_EXTRA_CALLOUT_DATA
1969: PCRE_EXTRA_TABLES
1970: PCRE_EXTRA_MARK
1971:
1972: Other flag bits should be set to zero. The study_data field and some-
1973: times the executable_jit field are set in the pcre_extra block that is
1974: returned by pcre_study(), together with the appropriate flag bits. You
1975: should not set these yourself, but you may add to the block by setting
1976: the other fields and their corresponding flag bits.
1977:
1978: The match_limit field provides a means of preventing PCRE from using up
1979: a vast amount of resources when running patterns that are not going to
1980: match, but which have a very large number of possibilities in their
1981: search trees. The classic example is a pattern that uses nested unlim-
1982: ited repeats.
1983:
1984: Internally, pcre_exec() uses a function called match(), which it calls
1985: repeatedly (sometimes recursively). The limit set by match_limit is
1986: imposed on the number of times this function is called during a match,
1987: which has the effect of limiting the amount of backtracking that can
1988: take place. For patterns that are not anchored, the count restarts from
1989: zero for each position in the subject string.
1990:
1991: When pcre_exec() is called with a pattern that was successfully studied
1992: with the PCRE_STUDY_JIT_COMPILE option, the way that the matching is
1993: executed is entirely different. However, there is still the possibility
1994: of runaway matching that goes on for a very long time, and so the
1995: match_limit value is also used in this case (but in a different way) to
1996: limit how long the matching can continue.
1997:
1998: The default value for the limit can be set when PCRE is built; the
1999: default default is 10 million, which handles all but the most extreme
2000: cases. You can override the default by suppling pcre_exec() with a
2001: pcre_extra block in which match_limit is set, and
2002: PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2003: exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2004:
2005: The match_limit_recursion field is similar to match_limit, but instead
2006: of limiting the total number of times that match() is called, it limits
2007: the depth of recursion. The recursion depth is a smaller number than
2008: the total number of calls, because not all calls to match() are recur-
2009: sive. This limit is of use only if it is set smaller than match_limit.
2010:
2011: Limiting the recursion depth limits the amount of machine stack that
2012: can be used, or, when PCRE has been compiled to use memory on the heap
2013: instead of the stack, the amount of heap memory that can be used. This
2014: limit is not relevant, and is ignored, if the pattern was successfully
2015: studied with PCRE_STUDY_JIT_COMPILE.
2016:
2017: The default value for match_limit_recursion can be set when PCRE is
2018: built; the default default is the same value as the default for
2019: match_limit. You can override the default by suppling pcre_exec() with
2020: a pcre_extra block in which match_limit_recursion is set, and
2021: PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2022: limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2023:
2024: The callout_data field is used in conjunction with the "callout" fea-
2025: ture, and is described in the pcrecallout documentation.
2026:
2027: The tables field is used to pass a character tables pointer to
2028: pcre_exec(); this overrides the value that is stored with the compiled
2029: pattern. A non-NULL value is stored with the compiled pattern only if
2030: custom tables were supplied to pcre_compile() via its tableptr argu-
2031: ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2032: PCRE's internal tables to be used. This facility is helpful when re-
2033: using patterns that have been saved after compiling with an external
2034: set of tables, because the external tables might be at a different
2035: address when pcre_exec() is called. See the pcreprecompile documenta-
2036: tion for a discussion of saving compiled patterns for later use.
2037:
2038: If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2039: set to point to a char * variable. If the pattern contains any back-
2040: tracking control verbs such as (*MARK:NAME), and the execution ends up
2041: with a name to pass back, a pointer to the name string (zero termi-
2042: nated) is placed in the variable pointed to by the mark field. The
2043: names are within the compiled pattern; if you wish to retain such a
2044: name you must copy it before freeing the memory of a compiled pattern.
2045: If there is no name to pass back, the variable pointed to by the mark
2046: field set to NULL. For details of the backtracking control verbs, see
2047: the section entitled "Backtracking control" in the pcrepattern documen-
2048: tation.
2049:
2050: Option bits for pcre_exec()
2051:
2052: The unused bits of the options argument for pcre_exec() must be zero.
2053: The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2054: PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
2055: PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_SOFT, and
2056: PCRE_PARTIAL_HARD.
2057:
2058: If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
2059: option, the only supported options for JIT execution are
2060: PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
2061: PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
2062: supported. If an unsupported option is used, JIT execution is disabled
2063: and the normal interpretive code in pcre_exec() is run.
2064:
2065: PCRE_ANCHORED
2066:
2067: The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2068: matching position. If a pattern was compiled with PCRE_ANCHORED, or
2069: turned out to be anchored by virtue of its contents, it cannot be made
2070: unachored at matching time.
2071:
2072: PCRE_BSR_ANYCRLF
2073: PCRE_BSR_UNICODE
2074:
2075: These options (which are mutually exclusive) control what the \R escape
2076: sequence matches. The choice is either to match only CR, LF, or CRLF,
2077: or to match any Unicode newline sequence. These options override the
2078: choice that was made or defaulted when the pattern was compiled.
2079:
2080: PCRE_NEWLINE_CR
2081: PCRE_NEWLINE_LF
2082: PCRE_NEWLINE_CRLF
2083: PCRE_NEWLINE_ANYCRLF
2084: PCRE_NEWLINE_ANY
2085:
2086: These options override the newline definition that was chosen or
2087: defaulted when the pattern was compiled. For details, see the descrip-
2088: tion of pcre_compile() above. During matching, the newline choice
2089: affects the behaviour of the dot, circumflex, and dollar metacharac-
2090: ters. It may also alter the way the match position is advanced after a
2091: match failure for an unanchored pattern.
2092:
2093: When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
2094: set, and a match attempt for an unanchored pattern fails when the cur-
2095: rent position is at a CRLF sequence, and the pattern contains no
2096: explicit matches for CR or LF characters, the match position is
2097: advanced by two characters instead of one, in other words, to after the
2098: CRLF.
2099:
2100: The above rule is a compromise that makes the most common cases work as
2101: expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2102: option is not set), it does not match the string "\r\nA" because, after
2103: failing at the start, it skips both the CR and the LF before retrying.
2104: However, the pattern [\r\n]A does match that string, because it con-
2105: tains an explicit CR or LF reference, and so advances only by one char-
2106: acter after the first failure.
2107:
2108: An explicit match for CR of LF is either a literal appearance of one of
2109: those characters, or one of the \r or \n escape sequences. Implicit
2110: matches such as [^X] do not count, nor does \s (which includes CR and
2111: LF in the characters that it matches).
2112:
2113: Notwithstanding the above, anomalous effects may still occur when CRLF
2114: is a valid newline sequence and explicit \r or \n escapes appear in the
2115: pattern.
2116:
2117: PCRE_NOTBOL
2118:
2119: This option specifies that first character of the subject string is not
2120: the beginning of a line, so the circumflex metacharacter should not
2121: match before it. Setting this without PCRE_MULTILINE (at compile time)
2122: causes circumflex never to match. This option affects only the behav-
2123: iour of the circumflex metacharacter. It does not affect \A.
2124:
2125: PCRE_NOTEOL
2126:
2127: This option specifies that the end of the subject string is not the end
2128: of a line, so the dollar metacharacter should not match it nor (except
2129: in multiline mode) a newline immediately before it. Setting this with-
2130: out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2131: option affects only the behaviour of the dollar metacharacter. It does
2132: not affect \Z or \z.
2133:
2134: PCRE_NOTEMPTY
2135:
2136: An empty string is not considered to be a valid match if this option is
2137: set. If there are alternatives in the pattern, they are tried. If all
2138: the alternatives match the empty string, the entire match fails. For
2139: example, if the pattern
2140:
2141: a?b?
2142:
2143: is applied to a string not beginning with "a" or "b", it matches an
2144: empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2145: match is not valid, so PCRE searches further into the string for occur-
2146: rences of "a" or "b".
2147:
2148: PCRE_NOTEMPTY_ATSTART
2149:
2150: This is like PCRE_NOTEMPTY, except that an empty string match that is
2151: not at the start of the subject is permitted. If the pattern is
2152: anchored, such a match can occur only if the pattern contains \K.
2153:
2154: Perl has no direct equivalent of PCRE_NOTEMPTY or
2155: PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2156: match of the empty string within its split() function, and when using
2157: the /g modifier. It is possible to emulate Perl's behaviour after
2158: matching a null string by first trying the match again at the same off-
2159: set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2160: fails, by advancing the starting offset (see below) and trying an ordi-
2161: nary match again. There is some code that demonstrates how to do this
2162: in the pcredemo sample program. In the most general case, you have to
2163: check to see if the newline convention recognizes CRLF as a newline,
2164: and if so, and the current character is CR followed by LF, advance the
2165: starting offset by two characters instead of one.
2166:
2167: PCRE_NO_START_OPTIMIZE
2168:
2169: There are a number of optimizations that pcre_exec() uses at the start
2170: of a match, in order to speed up the process. For example, if it is
2171: known that an unanchored match must start with a specific character, it
2172: searches the subject for that character, and fails immediately if it
2173: cannot find it, without actually running the main matching function.
2174: This means that a special item such as (*COMMIT) at the start of a pat-
2175: tern is not considered until after a suitable starting point for the
2176: match has been found. When callouts or (*MARK) items are in use, these
2177: "start-up" optimizations can cause them to be skipped if the pattern is
2178: never actually used. The start-up optimizations are in effect a pre-
2179: scan of the subject that takes place before the pattern is run.
2180:
2181: The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2182: possibly causing performance to suffer, but ensuring that in cases
2183: where the result is "no match", the callouts do occur, and that items
2184: such as (*COMMIT) and (*MARK) are considered at every possible starting
2185: position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
2186: compile time, it cannot be unset at matching time.
2187:
2188: Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2189: operation. Consider the pattern
2190:
2191: (*COMMIT)ABC
2192:
2193: When this is compiled, PCRE records the fact that a match must start
2194: with the character "A". Suppose the subject string is "DEFABC". The
2195: start-up optimization scans along the subject, finds "A" and runs the
2196: first match attempt from there. The (*COMMIT) item means that the pat-
2197: tern must match the current starting position, which in this case, it
2198: does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2199: set, the initial scan along the subject string does not happen. The
2200: first match attempt is run starting from "D" and when this fails,
2201: (*COMMIT) prevents any further matches being tried, so the overall
2202: result is "no match". If the pattern is studied, more start-up opti-
2203: mizations may be used. For example, a minimum length for the subject
2204: may be recorded. Consider the pattern
2205:
2206: (*MARK:A)(X|Y)
2207:
2208: The minimum length for a match is one character. If the subject is
2209: "ABC", there will be attempts to match "ABC", "BC", "C", and then
2210: finally an empty string. If the pattern is studied, the final attempt
2211: does not take place, because PCRE knows that the subject is too short,
2212: and so the (*MARK) is never encountered. In this case, studying the
2213: pattern does not affect the overall match result, which is still "no
2214: match", but it does affect the auxiliary information that is returned.
2215:
2216: PCRE_NO_UTF8_CHECK
2217:
2218: When PCRE_UTF8 is set at compile time, the validity of the subject as a
2219: UTF-8 string is automatically checked when pcre_exec() is subsequently
2220: called. The value of startoffset is also checked to ensure that it
2221: points to the start of a UTF-8 character. There is a discussion about
2222: the validity of UTF-8 strings in the section on UTF-8 support in the
2223: main pcre page. If an invalid UTF-8 sequence of bytes is found,
2224: pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PAR-
2225: TIAL_HARD is set and the problem is a truncated UTF-8 character at the
2226: end of the subject, PCRE_ERROR_SHORTUTF8. In both cases, information
2227: about the precise nature of the error may also be returned (see the
2228: descriptions of these errors in the section entitled Error return val-
2229: ues from pcre_exec() below). If startoffset contains a value that does
2230: not point to the start of a UTF-8 character (or to the end of the sub-
2231: ject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2232:
2233: If you already know that your subject is valid, and you want to skip
2234: these checks for performance reasons, you can set the
2235: PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2236: do this for the second and subsequent calls to pcre_exec() if you are
2237: making repeated calls to find all the matches in a single subject
2238: string. However, you should be sure that the value of startoffset
2239: points to the start of a UTF-8 character (or the end of the subject).
2240: When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8
2241: string as a subject or an invalid value of startoffset is undefined.
2242: Your program may crash.
2243:
2244: PCRE_PARTIAL_HARD
2245: PCRE_PARTIAL_SOFT
2246:
2247: These options turn on the partial matching feature. For backwards com-
2248: patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2249: match occurs if the end of the subject string is reached successfully,
2250: but there are not enough subject characters to complete the match. If
2251: this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
2252: matching continues by testing any remaining alternatives. Only if no
2253: complete match can be found is PCRE_ERROR_PARTIAL returned instead of
2254: PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
2255: caller is prepared to handle a partial match, but only if no complete
2256: match can be found.
2257:
2258: If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
2259: case, if a partial match is found, pcre_exec() immediately returns
2260: PCRE_ERROR_PARTIAL, without considering any other alternatives. In
2261: other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
2262: ered to be more important that an alternative complete match.
2263:
2264: In both cases, the portion of the string that was inspected when the
2265: partial match was found is set as the first matching string. There is a
2266: more detailed discussion of partial and multi-segment matching, with
2267: examples, in the pcrepartial documentation.
2268:
2269: The string to be matched by pcre_exec()
2270:
2271: The subject string is passed to pcre_exec() as a pointer in subject, a
2272: length (in bytes) in length, and a starting byte offset in startoffset.
2273: If this is negative or greater than the length of the subject,
2274: pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
2275: zero, the search for a match starts at the beginning of the subject,
2276: and this is by far the most common case. In UTF-8 mode, the byte offset
2277: must point to the start of a UTF-8 character (or the end of the sub-
2278: ject). Unlike the pattern string, the subject may contain binary zero
2279: bytes.
2280:
2281: A non-zero starting offset is useful when searching for another match
2282: in the same subject by calling pcre_exec() again after a previous suc-
2283: cess. Setting startoffset differs from just passing over a shortened
2284: string and setting PCRE_NOTBOL in the case of a pattern that begins
2285: with any kind of lookbehind. For example, consider the pattern
2286:
2287: \Biss\B
2288:
2289: which finds occurrences of "iss" in the middle of words. (\B matches
2290: only if the current position in the subject is not a word boundary.)
2291: When applied to the string "Mississipi" the first call to pcre_exec()
2292: finds the first occurrence. If pcre_exec() is called again with just
2293: the remainder of the subject, namely "issipi", it does not match,
2294: because \B is always false at the start of the subject, which is deemed
2295: to be a word boundary. However, if pcre_exec() is passed the entire
2296: string again, but with startoffset set to 4, it finds the second occur-
2297: rence of "iss" because it is able to look behind the starting point to
2298: discover that it is preceded by a letter.
2299:
2300: Finding all the matches in a subject is tricky when the pattern can
2301: match an empty string. It is possible to emulate Perl's /g behaviour by
2302: first trying the match again at the same offset, with the
2303: PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
2304: fails, advancing the starting offset and trying an ordinary match
2305: again. There is some code that demonstrates how to do this in the pcre-
2306: demo sample program. In the most general case, you have to check to see
2307: if the newline convention recognizes CRLF as a newline, and if so, and
2308: the current character is CR followed by LF, advance the starting offset
2309: by two characters instead of one.
2310:
2311: If a non-zero starting offset is passed when the pattern is anchored,
2312: one attempt to match at the given offset is made. This can only succeed
2313: if the pattern does not require the match to be at the start of the
2314: subject.
2315:
2316: How pcre_exec() returns captured substrings
2317:
2318: In general, a pattern matches a certain portion of the subject, and in
2319: addition, further substrings from the subject may be picked out by
2320: parts of the pattern. Following the usage in Jeffrey Friedl's book,
2321: this is called "capturing" in what follows, and the phrase "capturing
2322: subpattern" is used for a fragment of a pattern that picks out a sub-
2323: string. PCRE supports several other kinds of parenthesized subpattern
2324: that do not cause substrings to be captured.
2325:
2326: Captured substrings are returned to the caller via a vector of integers
2327: whose address is passed in ovector. The number of elements in the vec-
2328: tor is passed in ovecsize, which must be a non-negative number. Note:
2329: this argument is NOT the size of ovector in bytes.
2330:
2331: The first two-thirds of the vector is used to pass back captured sub-
2332: strings, each substring using a pair of integers. The remaining third
2333: of the vector is used as workspace by pcre_exec() while matching cap-
2334: turing subpatterns, and is not available for passing back information.
2335: The number passed in ovecsize should always be a multiple of three. If
2336: it is not, it is rounded down.
2337:
2338: When a match is successful, information about captured substrings is
2339: returned in pairs of integers, starting at the beginning of ovector,
2340: and continuing up to two-thirds of its length at the most. The first
2341: element of each pair is set to the byte offset of the first character
2342: in a substring, and the second is set to the byte offset of the first
2343: character after the end of a substring. Note: these values are always
2344: byte offsets, even in UTF-8 mode. They are not character counts.
2345:
2346: The first pair of integers, ovector[0] and ovector[1], identify the
2347: portion of the subject string matched by the entire pattern. The next
2348: pair is used for the first capturing subpattern, and so on. The value
2349: returned by pcre_exec() is one more than the highest numbered pair that
2350: has been set. For example, if two substrings have been captured, the
2351: returned value is 3. If there are no capturing subpatterns, the return
2352: value from a successful match is 1, indicating that just the first pair
2353: of offsets has been set.
2354:
2355: If a capturing subpattern is matched repeatedly, it is the last portion
2356: of the string that it matched that is returned.
2357:
2358: If the vector is too small to hold all the captured substring offsets,
2359: it is used as far as possible (up to two-thirds of its length), and the
2360: function returns a value of zero. If neither the actual string matched
2361: not any captured substrings are of interest, pcre_exec() may be called
2362: with ovector passed as NULL and ovecsize as zero. However, if the pat-
2363: tern contains back references and the ovector is not big enough to
2364: remember the related substrings, PCRE has to get additional memory for
2365: use during matching. Thus it is usually advisable to supply an ovector
2366: of reasonable size.
2367:
2368: There are some cases where zero is returned (indicating vector over-
2369: flow) when in fact the vector is exactly the right size for the final
2370: match. For example, consider the pattern
2371:
2372: (a)(?:(b)c|bd)
2373:
2374: If a vector of 6 elements (allowing for only 1 captured substring) is
2375: given with subject string "abd", pcre_exec() will try to set the second
2376: captured string, thereby recording a vector overflow, before failing to
2377: match "c" and backing up to try the second alternative. The zero
2378: return, however, does correctly indicate that the maximum number of
2379: slots (namely 2) have been filled. In similar cases where there is tem-
2380: porary overflow, but the final number of used slots is actually less
2381: than the maximum, a non-zero value is returned.
2382:
2383: The pcre_fullinfo() function can be used to find out how many capturing
2384: subpatterns there are in a compiled pattern. The smallest size for
2385: ovector that will allow for n captured substrings, in addition to the
2386: offsets of the substring matched by the whole pattern, is (n+1)*3.
2387:
2388: It is possible for capturing subpattern number n+1 to match some part
2389: of the subject when subpattern n has not been used at all. For example,
2390: if the string "abc" is matched against the pattern (a|(z))(bc) the
2391: return from the function is 4, and subpatterns 1 and 3 are matched, but
2392: 2 is not. When this happens, both values in the offset pairs corre-
2393: sponding to unused subpatterns are set to -1.
2394:
2395: Offset values that correspond to unused subpatterns at the end of the
2396: expression are also set to -1. For example, if the string "abc" is
2397: matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2398: matched. The return from the function is 2, because the highest used
2399: capturing subpattern number is 1, and the offsets for for the second
2400: and third capturing subpatterns (assuming the vector is large enough,
2401: of course) are set to -1.
2402:
2403: Note: Elements in the first two-thirds of ovector that do not corre-
2404: spond to capturing parentheses in the pattern are never changed. That
2405: is, if a pattern contains n capturing parentheses, no more than ovec-
2406: tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
2407: the first two-thirds) retain whatever values they previously had.
2408:
2409: Some convenience functions are provided for extracting the captured
2410: substrings as separate strings. These are described below.
2411:
2412: Error return values from pcre_exec()
2413:
2414: If pcre_exec() fails, it returns a negative number. The following are
2415: defined in the header file:
2416:
2417: PCRE_ERROR_NOMATCH (-1)
2418:
2419: The subject string did not match the pattern.
2420:
2421: PCRE_ERROR_NULL (-2)
2422:
2423: Either code or subject was passed as NULL, or ovector was NULL and
2424: ovecsize was not zero.
2425:
2426: PCRE_ERROR_BADOPTION (-3)
2427:
2428: An unrecognized bit was set in the options argument.
2429:
2430: PCRE_ERROR_BADMAGIC (-4)
2431:
2432: PCRE stores a 4-byte "magic number" at the start of the compiled code,
2433: to catch the case when it is passed a junk pointer and to detect when a
2434: pattern that was compiled in an environment of one endianness is run in
2435: an environment with the other endianness. This is the error that PCRE
2436: gives when the magic number is not present.
2437:
2438: PCRE_ERROR_UNKNOWN_OPCODE (-5)
2439:
2440: While running the pattern match, an unknown item was encountered in the
2441: compiled pattern. This error could be caused by a bug in PCRE or by
2442: overwriting of the compiled pattern.
2443:
2444: PCRE_ERROR_NOMEMORY (-6)
2445:
2446: If a pattern contains back references, but the ovector that is passed
2447: to pcre_exec() is not big enough to remember the referenced substrings,
2448: PCRE gets a block of memory at the start of matching to use for this
2449: purpose. If the call via pcre_malloc() fails, this error is given. The
2450: memory is automatically freed at the end of matching.
2451:
2452: This error is also given if pcre_stack_malloc() fails in pcre_exec().
2453: This can happen only when PCRE has been compiled with --disable-stack-
2454: for-recursion.
2455:
2456: PCRE_ERROR_NOSUBSTRING (-7)
2457:
2458: This error is used by the pcre_copy_substring(), pcre_get_substring(),
2459: and pcre_get_substring_list() functions (see below). It is never
2460: returned by pcre_exec().
2461:
2462: PCRE_ERROR_MATCHLIMIT (-8)
2463:
2464: The backtracking limit, as specified by the match_limit field in a
2465: pcre_extra structure (or defaulted) was reached. See the description
2466: above.
2467:
2468: PCRE_ERROR_CALLOUT (-9)
2469:
2470: This error is never generated by pcre_exec() itself. It is provided for
2471: use by callout functions that want to yield a distinctive error code.
2472: See the pcrecallout documentation for details.
2473:
2474: PCRE_ERROR_BADUTF8 (-10)
2475:
2476: A string that contains an invalid UTF-8 byte sequence was passed as a
2477: subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
2478: the output vector (ovecsize) is at least 2, the byte offset to the
2479: start of the the invalid UTF-8 character is placed in the first ele-
2480: ment, and a reason code is placed in the second element. The reason
2481: codes are listed in the following section. For backward compatibility,
2482: if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
2483: acter at the end of the subject (reason codes 1 to 5),
2484: PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2485:
2486: PCRE_ERROR_BADUTF8_OFFSET (-11)
2487:
2488: The UTF-8 byte sequence that was passed as a subject was checked and
2489: found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
2490: value of startoffset did not point to the beginning of a UTF-8 charac-
2491: ter or the end of the subject.
2492:
2493: PCRE_ERROR_PARTIAL (-12)
2494:
2495: The subject string did not match, but it did match partially. See the
2496: pcrepartial documentation for details of partial matching.
2497:
2498: PCRE_ERROR_BADPARTIAL (-13)
2499:
2500: This code is no longer in use. It was formerly returned when the
2501: PCRE_PARTIAL option was used with a compiled pattern containing items
2502: that were not supported for partial matching. From release 8.00
2503: onwards, there are no restrictions on partial matching.
2504:
2505: PCRE_ERROR_INTERNAL (-14)
2506:
2507: An unexpected internal error has occurred. This error could be caused
2508: by a bug in PCRE or by overwriting of the compiled pattern.
2509:
2510: PCRE_ERROR_BADCOUNT (-15)
2511:
2512: This error is given if the value of the ovecsize argument is negative.
2513:
2514: PCRE_ERROR_RECURSIONLIMIT (-21)
2515:
2516: The internal recursion limit, as specified by the match_limit_recursion
2517: field in a pcre_extra structure (or defaulted) was reached. See the
2518: description above.
2519:
2520: PCRE_ERROR_BADNEWLINE (-23)
2521:
2522: An invalid combination of PCRE_NEWLINE_xxx options was given.
2523:
2524: PCRE_ERROR_BADOFFSET (-24)
2525:
2526: The value of startoffset was negative or greater than the length of the
2527: subject, that is, the value in length.
2528:
2529: PCRE_ERROR_SHORTUTF8 (-25)
2530:
2531: This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
2532: string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
2533: option is set. Information about the failure is returned as for
2534: PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
2535: this special error code for PCRE_PARTIAL_HARD precedes the implementa-
2536: tion of returned information; it is retained for backwards compatibil-
2537: ity.
2538:
2539: PCRE_ERROR_RECURSELOOP (-26)
2540:
2541: This error is returned when pcre_exec() detects a recursion loop within
2542: the pattern. Specifically, it means that either the whole pattern or a
2543: subpattern has been called recursively for the second time at the same
2544: position in the subject string. Some simple patterns that might do this
2545: are detected and faulted at compile time, but more complicated cases,
2546: in particular mutual recursions between two different subpatterns, can-
2547: not be detected until run time.
2548:
2549: PCRE_ERROR_JIT_STACKLIMIT (-27)
2550:
2551: This error is returned when a pattern that was successfully studied
2552: using the PCRE_STUDY_JIT_COMPILE option is being matched, but the mem-
2553: ory available for the just-in-time processing stack is not large
2554: enough. See the pcrejit documentation for more details.
2555:
2556: Error numbers -16 to -20 and -22 are not used by pcre_exec().
2557:
2558: Reason codes for invalid UTF-8 strings
2559:
2560: When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
2561: UTF8, and the size of the output vector (ovecsize) is at least 2, the
2562: offset of the start of the invalid UTF-8 character is placed in the
2563: first output vector element (ovector[0]) and a reason code is placed in
2564: the second element (ovector[1]). The reason codes are given names in
2565: the pcre.h header file:
2566:
2567: PCRE_UTF8_ERR1
2568: PCRE_UTF8_ERR2
2569: PCRE_UTF8_ERR3
2570: PCRE_UTF8_ERR4
2571: PCRE_UTF8_ERR5
2572:
2573: The string ends with a truncated UTF-8 character; the code specifies
2574: how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
2575: characters to be no longer than 4 bytes, the encoding scheme (origi-
2576: nally defined by RFC 2279) allows for up to 6 bytes, and this is
2577: checked first; hence the possibility of 4 or 5 missing bytes.
2578:
2579: PCRE_UTF8_ERR6
2580: PCRE_UTF8_ERR7
2581: PCRE_UTF8_ERR8
2582: PCRE_UTF8_ERR9
2583: PCRE_UTF8_ERR10
2584:
2585: The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
2586: the character do not have the binary value 0b10 (that is, either the
2587: most significant bit is 0, or the next bit is 1).
2588:
2589: PCRE_UTF8_ERR11
2590: PCRE_UTF8_ERR12
2591:
2592: A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
2593: long; these code points are excluded by RFC 3629.
2594:
2595: PCRE_UTF8_ERR13
2596:
2597: A 4-byte character has a value greater than 0x10fff; these code points
2598: are excluded by RFC 3629.
2599:
2600: PCRE_UTF8_ERR14
2601:
2602: A 3-byte character has a value in the range 0xd800 to 0xdfff; this
2603: range of code points are reserved by RFC 3629 for use with UTF-16, and
2604: so are excluded from UTF-8.
2605:
2606: PCRE_UTF8_ERR15
2607: PCRE_UTF8_ERR16
2608: PCRE_UTF8_ERR17
2609: PCRE_UTF8_ERR18
2610: PCRE_UTF8_ERR19
2611:
2612: A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
2613: for a value that can be represented by fewer bytes, which is invalid.
2614: For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
2615: rect coding uses just one byte.
2616:
2617: PCRE_UTF8_ERR20
2618:
2619: The two most significant bits of the first byte of a character have the
2620: binary value 0b10 (that is, the most significant bit is 1 and the sec-
2621: ond is 0). Such a byte can only validly occur as the second or subse-
2622: quent byte of a multi-byte character.
2623:
2624: PCRE_UTF8_ERR21
2625:
2626: The first byte of a character has the value 0xfe or 0xff. These values
2627: can never occur in a valid UTF-8 string.
2628:
2629:
2630: EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2631:
2632: int pcre_copy_substring(const char *subject, int *ovector,
2633: int stringcount, int stringnumber, char *buffer,
2634: int buffersize);
2635:
2636: int pcre_get_substring(const char *subject, int *ovector,
2637: int stringcount, int stringnumber,
2638: const char **stringptr);
2639:
2640: int pcre_get_substring_list(const char *subject,
2641: int *ovector, int stringcount, const char ***listptr);
2642:
2643: Captured substrings can be accessed directly by using the offsets
2644: returned by pcre_exec() in ovector. For convenience, the functions
2645: pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2646: string_list() are provided for extracting captured substrings as new,
2647: separate, zero-terminated strings. These functions identify substrings
2648: by number. The next section describes functions for extracting named
2649: substrings.
2650:
2651: A substring that contains a binary zero is correctly extracted and has
2652: a further zero added on the end, but the result is not, of course, a C
2653: string. However, you can process such a string by referring to the
2654: length that is returned by pcre_copy_substring() and pcre_get_sub-
2655: string(). Unfortunately, the interface to pcre_get_substring_list() is
2656: not adequate for handling strings containing binary zeros, because the
2657: end of the final string is not independently indicated.
2658:
2659: The first three arguments are the same for all three of these func-
2660: tions: subject is the subject string that has just been successfully
2661: matched, ovector is a pointer to the vector of integer offsets that was
2662: passed to pcre_exec(), and stringcount is the number of substrings that
2663: were captured by the match, including the substring that matched the
2664: entire regular expression. This is the value returned by pcre_exec() if
2665: it is greater than zero. If pcre_exec() returned zero, indicating that
2666: it ran out of space in ovector, the value passed as stringcount should
2667: be the number of elements in the vector divided by three.
2668:
2669: The functions pcre_copy_substring() and pcre_get_substring() extract a
2670: single substring, whose number is given as stringnumber. A value of
2671: zero extracts the substring that matched the entire pattern, whereas
2672: higher values extract the captured substrings. For pcre_copy_sub-
2673: string(), the string is placed in buffer, whose length is given by
2674: buffersize, while for pcre_get_substring() a new block of memory is
2675: obtained via pcre_malloc, and its address is returned via stringptr.
2676: The yield of the function is the length of the string, not including
2677: the terminating zero, or one of these error codes:
2678:
2679: PCRE_ERROR_NOMEMORY (-6)
2680:
2681: The buffer was too small for pcre_copy_substring(), or the attempt to
2682: get memory failed for pcre_get_substring().
2683:
2684: PCRE_ERROR_NOSUBSTRING (-7)
2685:
2686: There is no substring whose number is stringnumber.
2687:
2688: The pcre_get_substring_list() function extracts all available sub-
2689: strings and builds a list of pointers to them. All this is done in a
2690: single block of memory that is obtained via pcre_malloc. The address of
2691: the memory block is returned via listptr, which is also the start of
2692: the list of string pointers. The end of the list is marked by a NULL
2693: pointer. The yield of the function is zero if all went well, or the
2694: error code
2695:
2696: PCRE_ERROR_NOMEMORY (-6)
2697:
2698: if the attempt to get the memory block failed.
2699:
2700: When any of these functions encounter a substring that is unset, which
2701: can happen when capturing subpattern number n+1 matches some part of
2702: the subject, but subpattern n has not been used at all, they return an
2703: empty string. This can be distinguished from a genuine zero-length sub-
2704: string by inspecting the appropriate offset in ovector, which is nega-
2705: tive for unset substrings.
2706:
2707: The two convenience functions pcre_free_substring() and pcre_free_sub-
2708: string_list() can be used to free the memory returned by a previous
2709: call of pcre_get_substring() or pcre_get_substring_list(), respec-
2710: tively. They do nothing more than call the function pointed to by
2711: pcre_free, which of course could be called directly from a C program.
2712: However, PCRE is used in some situations where it is linked via a spe-
2713: cial interface to another programming language that cannot use
2714: pcre_free directly; it is for these cases that the functions are pro-
2715: vided.
2716:
2717:
2718: EXTRACTING CAPTURED SUBSTRINGS BY NAME
2719:
2720: int pcre_get_stringnumber(const pcre *code,
2721: const char *name);
2722:
2723: int pcre_copy_named_substring(const pcre *code,
2724: const char *subject, int *ovector,
2725: int stringcount, const char *stringname,
2726: char *buffer, int buffersize);
2727:
2728: int pcre_get_named_substring(const pcre *code,
2729: const char *subject, int *ovector,
2730: int stringcount, const char *stringname,
2731: const char **stringptr);
2732:
2733: To extract a substring by name, you first have to find associated num-
2734: ber. For example, for this pattern
2735:
2736: (a+)b(?<xxx>\d+)...
2737:
2738: the number of the subpattern called "xxx" is 2. If the name is known to
2739: be unique (PCRE_DUPNAMES was not set), you can find the number from the
2740: name by calling pcre_get_stringnumber(). The first argument is the com-
2741: piled pattern, and the second is the name. The yield of the function is
2742: the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2743: subpattern of that name.
2744:
2745: Given the number, you can extract the substring directly, or use one of
2746: the functions described in the previous section. For convenience, there
2747: are also two functions that do the whole job.
2748:
2749: Most of the arguments of pcre_copy_named_substring() and
2750: pcre_get_named_substring() are the same as those for the similarly
2751: named functions that extract by number. As these are described in the
2752: previous section, they are not re-described here. There are just two
2753: differences:
2754:
2755: First, instead of a substring number, a substring name is given. Sec-
2756: ond, there is an extra argument, given at the start, which is a pointer
2757: to the compiled pattern. This is needed in order to gain access to the
2758: name-to-number translation table.
2759:
2760: These functions call pcre_get_stringnumber(), and if it succeeds, they
2761: then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2762: ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2763: behaviour may not be what you want (see the next section).
2764:
2765: Warning: If the pattern uses the (?| feature to set up multiple subpat-
2766: terns with the same number, as described in the section on duplicate
2767: subpattern numbers in the pcrepattern page, you cannot use names to
2768: distinguish the different subpatterns, because names are not included
2769: in the compiled code. The matching process uses only numbers. For this
2770: reason, the use of different names for subpatterns of the same number
2771: causes an error at compile time.
2772:
2773:
2774: DUPLICATE SUBPATTERN NAMES
2775:
2776: int pcre_get_stringtable_entries(const pcre *code,
2777: const char *name, char **first, char **last);
2778:
2779: When a pattern is compiled with the PCRE_DUPNAMES option, names for
2780: subpatterns are not required to be unique. (Duplicate names are always
2781: allowed for subpatterns with the same number, created by using the (?|
2782: feature. Indeed, if such subpatterns are named, they are required to
2783: use the same names.)
2784:
2785: Normally, patterns with duplicate names are such that in any one match,
2786: only one of the named subpatterns participates. An example is shown in
2787: the pcrepattern documentation.
2788:
2789: When duplicates are present, pcre_copy_named_substring() and
2790: pcre_get_named_substring() return the first substring corresponding to
2791: the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2792: (-7) is returned; no data is returned. The pcre_get_stringnumber()
2793: function returns one of the numbers that are associated with the name,
2794: but it is not defined which it is.
2795:
2796: If you want to get full details of all captured substrings for a given
2797: name, you must use the pcre_get_stringtable_entries() function. The
2798: first argument is the compiled pattern, and the second is the name. The
2799: third and fourth are pointers to variables which are updated by the
2800: function. After it has run, they point to the first and last entries in
2801: the name-to-number table for the given name. The function itself
2802: returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2803: there are none. The format of the table is described above in the sec-
2804: tion entitled Information about a pattern above. Given all the rele-
2805: vant entries for the name, you can extract each of their numbers, and
2806: hence the captured data, if any.
2807:
2808:
2809: FINDING ALL POSSIBLE MATCHES
2810:
2811: The traditional matching function uses a similar algorithm to Perl,
2812: which stops when it finds the first match, starting at a given point in
2813: the subject. If you want to find all possible matches, or the longest
2814: possible match, consider using the alternative matching function (see
2815: below) instead. If you cannot use the alternative function, but still
2816: need to find all possible matches, you can kludge it up by making use
2817: of the callout facility, which is described in the pcrecallout documen-
2818: tation.
2819:
2820: What you have to do is to insert a callout right at the end of the pat-
2821: tern. When your callout function is called, extract and save the cur-
2822: rent matched substring. Then return 1, which forces pcre_exec() to
2823: backtrack and try other alternatives. Ultimately, when it runs out of
2824: matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2825:
2826:
2827: MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2828:
2829: int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2830: const char *subject, int length, int startoffset,
2831: int options, int *ovector, int ovecsize,
2832: int *workspace, int wscount);
2833:
2834: The function pcre_dfa_exec() is called to match a subject string
2835: against a compiled pattern, using a matching algorithm that scans the
2836: subject string just once, and does not backtrack. This has different
2837: characteristics to the normal algorithm, and is not compatible with
2838: Perl. Some of the features of PCRE patterns are not supported. Never-
2839: theless, there are times when this kind of matching can be useful. For
2840: a discussion of the two matching algorithms, and a list of features
2841: that pcre_dfa_exec() does not support, see the pcrematching documenta-
2842: tion.
2843:
2844: The arguments for the pcre_dfa_exec() function are the same as for
2845: pcre_exec(), plus two extras. The ovector argument is used in a differ-
2846: ent way, and this is described below. The other common arguments are
2847: used in the same way as for pcre_exec(), so their description is not
2848: repeated here.
2849:
2850: The two additional arguments provide workspace for the function. The
2851: workspace vector should contain at least 20 elements. It is used for
2852: keeping track of multiple paths through the pattern tree. More
2853: workspace will be needed for patterns and subjects where there are a
2854: lot of potential matches.
2855:
2856: Here is an example of a simple call to pcre_dfa_exec():
2857:
2858: int rc;
2859: int ovector[10];
2860: int wspace[20];
2861: rc = pcre_dfa_exec(
2862: re, /* result of pcre_compile() */
2863: NULL, /* we didn't study the pattern */
2864: "some string", /* the subject string */
2865: 11, /* the length of the subject string */
2866: 0, /* start at offset 0 in the subject */
2867: 0, /* default options */
2868: ovector, /* vector of integers for substring information */
2869: 10, /* number of elements (NOT size in bytes) */
2870: wspace, /* working space vector */
2871: 20); /* number of elements (NOT size in bytes) */
2872:
2873: Option bits for pcre_dfa_exec()
2874:
2875: The unused bits of the options argument for pcre_dfa_exec() must be
2876: zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2877: LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
2878: PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF,
2879: PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
2880: TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2881: four of these are exactly the same as for pcre_exec(), so their
2882: description is not repeated here.
2883:
2884: PCRE_PARTIAL_HARD
2885: PCRE_PARTIAL_SOFT
2886:
2887: These have the same general effect as they do for pcre_exec(), but the
2888: details are slightly different. When PCRE_PARTIAL_HARD is set for
2889: pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
2890: ject is reached and there is still at least one matching possibility
2891: that requires additional characters. This happens even if some complete
2892: matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
2893: code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
2894: of the subject is reached, there have been no complete matches, but
2895: there is still at least one matching possibility. The portion of the
2896: string that was inspected when the longest partial match was found is
2897: set as the first matching string in both cases. There is a more
2898: detailed discussion of partial and multi-segment matching, with exam-
2899: ples, in the pcrepartial documentation.
2900:
2901: PCRE_DFA_SHORTEST
2902:
2903: Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2904: stop as soon as it has found one match. Because of the way the alterna-
2905: tive algorithm works, this is necessarily the shortest possible match
2906: at the first possible matching point in the subject string.
2907:
2908: PCRE_DFA_RESTART
2909:
2910: When pcre_dfa_exec() returns a partial match, it is possible to call it
2911: again, with additional subject characters, and have it continue with
2912: the same match. The PCRE_DFA_RESTART option requests this action; when
2913: it is set, the workspace and wscount options must reference the same
2914: vector as before because data about the match so far is left in them
2915: after a partial match. There is more discussion of this facility in the
2916: pcrepartial documentation.
2917:
2918: Successful returns from pcre_dfa_exec()
2919:
2920: When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2921: string in the subject. Note, however, that all the matches from one run
2922: of the function start at the same point in the subject. The shorter
2923: matches are all initial substrings of the longer matches. For example,
2924: if the pattern
2925:
2926: <.*>
2927:
2928: is matched against the string
2929:
2930: This is <something> <something else> <something further> no more
2931:
2932: the three matched strings are
2933:
2934: <something>
2935: <something> <something else>
2936: <something> <something else> <something further>
2937:
2938: On success, the yield of the function is a number greater than zero,
2939: which is the number of matched substrings. The substrings themselves
2940: are returned in ovector. Each string uses two elements; the first is
2941: the offset to the start, and the second is the offset to the end. In
2942: fact, all the strings have the same start offset. (Space could have
2943: been saved by giving this only once, but it was decided to retain some
2944: compatibility with the way pcre_exec() returns data, even though the
2945: meaning of the strings is different.)
2946:
2947: The strings are returned in reverse order of length; that is, the long-
2948: est matching string is given first. If there were too many matches to
2949: fit into ovector, the yield of the function is zero, and the vector is
2950: filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
2951: can use the entire ovector for returning matched strings.
2952:
2953: Error returns from pcre_dfa_exec()
2954:
2955: The pcre_dfa_exec() function returns a negative number when it fails.
2956: Many of the errors are the same as for pcre_exec(), and these are
2957: described above. There are in addition the following errors that are
2958: specific to pcre_dfa_exec():
2959:
2960: PCRE_ERROR_DFA_UITEM (-16)
2961:
2962: This return is given if pcre_dfa_exec() encounters an item in the pat-
2963: tern that it does not support, for instance, the use of \C or a back
2964: reference.
2965:
2966: PCRE_ERROR_DFA_UCOND (-17)
2967:
2968: This return is given if pcre_dfa_exec() encounters a condition item
2969: that uses a back reference for the condition, or a test for recursion
2970: in a specific group. These are not supported.
2971:
2972: PCRE_ERROR_DFA_UMLIMIT (-18)
2973:
2974: This return is given if pcre_dfa_exec() is called with an extra block
2975: that contains a setting of the match_limit or match_limit_recursion
2976: fields. This is not supported (these fields are meaningless for DFA
2977: matching).
2978:
2979: PCRE_ERROR_DFA_WSSIZE (-19)
2980:
2981: This return is given if pcre_dfa_exec() runs out of space in the
2982: workspace vector.
2983:
2984: PCRE_ERROR_DFA_RECURSE (-20)
2985:
2986: When a recursive subpattern is processed, the matching function calls
2987: itself recursively, using private vectors for ovector and workspace.
2988: This error is given if the output vector is not large enough. This
2989: should be extremely rare, as a vector of size 1000 is used.
2990:
2991:
2992: SEE ALSO
2993:
2994: pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2995: tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2996:
2997:
2998: AUTHOR
2999:
3000: Philip Hazel
3001: University Computing Service
3002: Cambridge CB2 3QH, England.
3003:
3004:
3005: REVISION
3006:
3007: Last updated: 02 December 2011
3008: Copyright (c) 1997-2011 University of Cambridge.
3009: ------------------------------------------------------------------------------
3010:
3011:
3012: PCRECALLOUT(3) PCRECALLOUT(3)
3013:
3014:
3015: NAME
3016: PCRE - Perl-compatible regular expressions
3017:
3018:
3019: PCRE CALLOUTS
3020:
3021: int (*pcre_callout)(pcre_callout_block *);
3022:
3023: PCRE provides a feature called "callout", which is a means of temporar-
3024: ily passing control to the caller of PCRE in the middle of pattern
3025: matching. The caller of PCRE provides an external function by putting
3026: its entry point in the global variable pcre_callout. By default, this
3027: variable contains NULL, which disables all calling out.
3028:
3029: Within a regular expression, (?C) indicates the points at which the
3030: external function is to be called. Different callout points can be
3031: identified by putting a number less than 256 after the letter C. The
3032: default value is zero. For example, this pattern has two callout
3033: points:
3034:
3035: (?C1)abc(?C2)def
3036:
3037: If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or
3038: pcre_compile2() is called, PCRE automatically inserts callouts, all
3039: with number 255, before each item in the pattern. For example, if
3040: PCRE_AUTO_CALLOUT is used with the pattern
3041:
3042: A(\d{2}|--)
3043:
3044: it is processed as if it were
3045:
3046: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3047:
3048: Notice that there is a callout before and after each parenthesis and
3049: alternation bar. Automatic callouts can be used for tracking the
3050: progress of pattern matching. The pcretest command has an option that
3051: sets automatic callouts; when it is used, the output indicates how the
3052: pattern is matched. This is useful information when you are trying to
3053: optimize the performance of a particular pattern.
3054:
3055: The use of callouts in a pattern makes it ineligible for optimization
3056: by the just-in-time compiler. Studying such a pattern with the
3057: PCRE_STUDY_JIT_COMPILE option always fails.
3058:
3059:
3060: MISSING CALLOUTS
3061:
3062: You should be aware that, because of optimizations in the way PCRE
3063: matches patterns by default, callouts sometimes do not happen. For
3064: example, if the pattern is
3065:
3066: ab(?C4)cd
3067:
3068: PCRE knows that any matching string must contain the letter "d". If the
3069: subject string is "abyz", the lack of "d" means that matching doesn't
3070: ever start, and the callout is never reached. However, with "abyd",
3071: though the result is still no match, the callout is obeyed.
3072:
3073: If the pattern is studied, PCRE knows the minimum length of a matching
3074: string, and will immediately give a "no match" return without actually
3075: running a match if the subject is not long enough, or, for unanchored
3076: patterns, if it has been scanned far enough.
3077:
3078: You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3079: MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(), or by
3080: starting the pattern with (*NO_START_OPT). This slows down the matching
3081: process, but does ensure that callouts such as the example above are
3082: obeyed.
3083:
3084:
3085: THE CALLOUT INTERFACE
3086:
3087: During matching, when PCRE reaches a callout point, the external func-
3088: tion defined by pcre_callout is called (if it is set). This applies to
3089: both the pcre_exec() and the pcre_dfa_exec() matching functions. The
3090: only argument to the callout function is a pointer to a pcre_callout
3091: block. This structure contains the following fields:
3092:
3093: int version;
3094: int callout_number;
3095: int *offset_vector;
3096: const char *subject;
3097: int subject_length;
3098: int start_match;
3099: int current_position;
3100: int capture_top;
3101: int capture_last;
3102: void *callout_data;
3103: int pattern_position;
3104: int next_item_length;
3105: const unsigned char *mark;
3106:
3107: The version field is an integer containing the version number of the
3108: block format. The initial version was 0; the current version is 2. The
3109: version number will change again in future if additional fields are
3110: added, but the intention is never to remove any of the existing fields.
3111:
3112: The callout_number field contains the number of the callout, as com-
3113: piled into the pattern (that is, the number after ?C for manual call-
3114: outs, and 255 for automatically generated callouts).
3115:
3116: The offset_vector field is a pointer to the vector of offsets that was
3117: passed by the caller to pcre_exec() or pcre_dfa_exec(). When
3118: pcre_exec() is used, the contents can be inspected in order to extract
3119: substrings that have been matched so far, in the same way as for
3120: extracting substrings after a match has completed. For pcre_dfa_exec()
3121: this field is not useful.
3122:
3123: The subject and subject_length fields contain copies of the values that
3124: were passed to pcre_exec().
3125:
3126: The start_match field normally contains the offset within the subject
3127: at which the current match attempt started. However, if the escape
3128: sequence \K has been encountered, this value is changed to reflect the
3129: modified starting point. If the pattern is not anchored, the callout
3130: function may be called several times from the same point in the pattern
3131: for different starting points in the subject.
3132:
3133: The current_position field contains the offset within the subject of
3134: the current match pointer.
3135:
3136: When the pcre_exec() function is used, the capture_top field contains
3137: one more than the number of the highest numbered captured substring so
3138: far. If no substrings have been captured, the value of capture_top is
3139: one. This is always the case when pcre_dfa_exec() is used, because it
3140: does not support captured substrings.
3141:
3142: The capture_last field contains the number of the most recently cap-
3143: tured substring. If no substrings have been captured, its value is -1.
3144: This is always the case when pcre_dfa_exec() is used.
3145:
3146: The callout_data field contains a value that is passed to pcre_exec()
3147: or pcre_dfa_exec() specifically so that it can be passed back in call-
3148: outs. It is passed in the pcre_callout field of the pcre_extra data
3149: structure. If no such data was passed, the value of callout_data in a
3150: pcre_callout block is NULL. There is a description of the pcre_extra
3151: structure in the pcreapi documentation.
3152:
3153: The pattern_position field is present from version 1 of the pcre_call-
3154: out structure. It contains the offset to the next item to be matched in
3155: the pattern string.
3156:
3157: The next_item_length field is present from version 1 of the pcre_call-
3158: out structure. It contains the length of the next item to be matched in
3159: the pattern string. When the callout immediately precedes an alterna-
3160: tion bar, a closing parenthesis, or the end of the pattern, the length
3161: is zero. When the callout precedes an opening parenthesis, the length
3162: is that of the entire subpattern.
3163:
3164: The pattern_position and next_item_length fields are intended to help
3165: in distinguishing between different automatic callouts, which all have
3166: the same callout number. However, they are set for all callouts.
3167:
3168: The mark field is present from version 2 of the pcre_callout structure.
3169: In callouts from pcre_exec() it contains a pointer to the zero-termi-
3170: nated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
3171: item in the match, or NULL if no such items have been passed. Instances
3172: of (*PRUNE) or (*THEN) without a name do not obliterate a previous
3173: (*MARK). In callouts from pcre_dfa_exec() this field always contains
3174: NULL.
3175:
3176:
3177: RETURN VALUES
3178:
3179: The external callout function returns an integer to PCRE. If the value
3180: is zero, matching proceeds as normal. If the value is greater than
3181: zero, matching fails at the current point, but the testing of other
3182: matching possibilities goes ahead, just as if a lookahead assertion had
3183: failed. If the value is less than zero, the match is abandoned, and
3184: pcre_exec() or pcre_dfa_exec() returns the negative value.
3185:
3186: Negative values should normally be chosen from the set of
3187: PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3188: dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3189: reserved for use by callout functions; it will never be used by PCRE
3190: itself.
3191:
3192:
3193: AUTHOR
3194:
3195: Philip Hazel
3196: University Computing Service
3197: Cambridge CB2 3QH, England.
3198:
3199:
3200: REVISION
3201:
3202: Last updated: 30 November 2011
3203: Copyright (c) 1997-2011 University of Cambridge.
3204: ------------------------------------------------------------------------------
3205:
3206:
3207: PCRECOMPAT(3) PCRECOMPAT(3)
3208:
3209:
3210: NAME
3211: PCRE - Perl-compatible regular expressions
3212:
3213:
3214: DIFFERENCES BETWEEN PCRE AND PERL
3215:
3216: This document describes the differences in the ways that PCRE and Perl
3217: handle regular expressions. The differences described here are with
3218: respect to Perl versions 5.10 and above.
3219:
3220: 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
3221: of what it does have are given in the pcreunicode page.
3222:
3223: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3224: they do not mean what you might think. For example, (?!a){3} does not
3225: assert that the next three characters are not "a". It just asserts that
3226: the next character is not "a" three times (in principle: PCRE optimizes
3227: this to run the assertion just once). Perl allows repeat quantifiers on
3228: other assertions such as \b, but these do not seem to have any use.
3229:
3230: 3. Capturing subpatterns that occur inside negative lookahead asser-
3231: tions are counted, but their entries in the offsets vector are never
3232: set. Perl sets its numerical variables from any such patterns that are
3233: matched before the assertion fails to match something (thereby succeed-
3234: ing), but only if the negative lookahead assertion contains just one
3235: branch.
3236:
3237: 4. Though binary zero characters are supported in the subject string,
3238: they are not allowed in a pattern string because it is passed as a nor-
3239: mal C string, terminated by zero. The escape sequence \0 can be used in
3240: the pattern to represent a binary zero.
3241:
3242: 5. The following Perl escape sequences are not supported: \l, \u, \L,
3243: \U, and \N when followed by a character name or Unicode value. (\N on
3244: its own, matching a non-newline character, is supported.) In fact these
3245: are implemented by Perl's general string-handling and are not part of
3246: its pattern matching engine. If any of these are encountered by PCRE,
3247: an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
3248: PAT option is set, \U and \u are interpreted as JavaScript interprets
3249: them.
3250:
3251: 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3252: is built with Unicode character property support. The properties that
3253: can be tested with \p and \P are limited to the general category prop-
3254: erties such as Lu and Nd, script names such as Greek or Han, and the
3255: derived properties Any and L&. PCRE does support the Cs (surrogate)
3256: property, which Perl does not; the Perl documentation says "Because
3257: Perl hides the need for the user to understand the internal representa-
3258: tion of Unicode characters, there is no need to implement the somewhat
3259: messy concept of surrogates."
3260:
3261: 7. PCRE implements a simpler version of \X than Perl, which changed to
3262: make \X match what Unicode calls an "extended grapheme cluster". This
3263: is more complicated than an extended Unicode sequence, which is what
3264: PCRE matches.
3265:
3266: 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3267: ters in between are treated as literals. This is slightly different
3268: from Perl in that $ and @ are also handled as literals inside the
3269: quotes. In Perl, they cause variable interpolation (but of course PCRE
3270: does not have variables). Note the following examples:
3271:
3272: Pattern PCRE matches Perl matches
3273:
3274: \Qabc$xyz\E abc$xyz abc followed by the
3275: contents of $xyz
3276: \Qabc\$xyz\E abc\$xyz abc\$xyz
3277: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3278:
3279: The \Q...\E sequence is recognized both inside and outside character
3280: classes.
3281:
3282: 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3283: constructions. However, there is support for recursive patterns. This
3284: is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3285: "callout" feature allows an external function to be called during pat-
3286: tern matching. See the pcrecallout documentation for details.
3287:
3288: 10. Subpatterns that are called as subroutines (whether or not recur-
3289: sively) are always treated as atomic groups in PCRE. This is like
3290: Python, but unlike Perl. Captured values that are set outside a sub-
3291: routine call can be reference from inside in PCRE, but not in Perl.
3292: There is a discussion that explains these differences in more detail in
3293: the section on recursion differences from Perl in the pcrepattern page.
3294:
3295: 11. If (*THEN) is present in a group that is called as a subroutine,
3296: its action is limited to that group, even if the group does not contain
3297: any | characters.
3298:
3299: 12. There are some differences that are concerned with the settings of
3300: captured strings when part of a pattern is repeated. For example,
3301: matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3302: unset, but in PCRE it is set to "b".
3303:
3304: 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3305: pattern names is not as general as Perl's. This is a consequence of the
3306: fact the PCRE works internally just with numbers, using an external ta-
3307: ble to translate between numbers and names. In particular, a pattern
3308: such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3309: the same number but different names, is not supported, and causes an
3310: error at compile time. If it were allowed, it would not be possible to
3311: distinguish which parentheses matched, because both names map to cap-
3312: turing subpattern number 1. To avoid this confusing situation, an error
3313: is given at compile time.
3314:
3315: 14. Perl recognizes comments in some places that PCRE does not, for
3316: example, between the ( and ? at the start of a subpattern. If the /x
3317: modifier is set, Perl allows whitespace between ( and ? but PCRE never
3318: does, even if the PCRE_EXTENDED option is set.
3319:
3320: 15. PCRE provides some extensions to the Perl regular expression facil-
3321: ities. Perl 5.10 includes new features that are not in earlier ver-
3322: sions of Perl, some of which (such as named parentheses) have been in
3323: PCRE for some time. This list is with respect to Perl 5.10:
3324:
3325: (a) Although lookbehind assertions in PCRE must match fixed length
3326: strings, each alternative branch of a lookbehind assertion can match a
3327: different length of string. Perl requires them all to have the same
3328: length.
3329:
3330: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3331: meta-character matches only at the very end of the string.
3332:
3333: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3334: cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3335: ignored. (Perl can be made to issue a warning.)
3336:
3337: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3338: fiers is inverted, that is, by default they are not greedy, but if fol-
3339: lowed by a question mark they are.
3340:
3341: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3342: tried only at the first matching position in the subject string.
3343:
3344: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
3345: and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3346: lents.
3347:
3348: (g) The \R escape sequence can be restricted to match only CR, LF, or
3349: CRLF by the PCRE_BSR_ANYCRLF option.
3350:
3351: (h) The callout facility is PCRE-specific.
3352:
3353: (i) The partial matching facility is PCRE-specific.
3354:
3355: (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3356: even on different hosts that have the other endianness. However, this
3357: does not apply to optimized data created by the just-in-time compiler.
3358:
3359: (k) The alternative matching function (pcre_dfa_exec()) matches in a
3360: different way and is not Perl-compatible.
3361:
3362: (l) PCRE recognizes some special sequences such as (*CR) at the start
3363: of a pattern that set overall options that cannot be changed within the
3364: pattern.
3365:
3366:
3367: AUTHOR
3368:
3369: Philip Hazel
3370: University Computing Service
3371: Cambridge CB2 3QH, England.
3372:
3373:
3374: REVISION
3375:
3376: Last updated: 14 November 2011
3377: Copyright (c) 1997-2011 University of Cambridge.
3378: ------------------------------------------------------------------------------
3379:
3380:
3381: PCREPATTERN(3) PCREPATTERN(3)
3382:
3383:
3384: NAME
3385: PCRE - Perl-compatible regular expressions
3386:
3387:
3388: PCRE REGULAR EXPRESSION DETAILS
3389:
3390: The syntax and semantics of the regular expressions that are supported
3391: by PCRE are described in detail below. There is a quick-reference syn-
3392: tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3393: semantics as closely as it can. PCRE also supports some alternative
3394: regular expression syntax (which does not conflict with the Perl syn-
3395: tax) in order to provide some compatibility with regular expressions in
3396: Python, .NET, and Oniguruma.
3397:
3398: Perl's regular expressions are described in its own documentation, and
3399: regular expressions in general are covered in a number of books, some
3400: of which have copious examples. Jeffrey Friedl's "Mastering Regular
3401: Expressions", published by O'Reilly, covers regular expressions in
3402: great detail. This description of PCRE's regular expressions is
3403: intended as reference material.
3404:
3405: The original operation of PCRE was on strings of one-byte characters.
3406: However, there is now also support for UTF-8 character strings. To use
3407: this, PCRE must be built to include UTF-8 support, and you must call
3408: pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is
3409: also a special sequence that can be given at the start of a pattern:
3410:
3411: (*UTF8)
3412:
3413: Starting a pattern with this sequence is equivalent to setting the
3414: PCRE_UTF8 option. This feature is not Perl-compatible. How setting
3415: UTF-8 mode affects pattern matching is mentioned in several places
3416: below. There is also a summary of UTF-8 features in the pcreunicode
3417: page.
3418:
3419: Another special sequence that may appear at the start of a pattern or
3420: in combination with (*UTF8) is:
3421:
3422: (*UCP)
3423:
3424: This has the same effect as setting the PCRE_UCP option: it causes
3425: sequences such as \d and \w to use Unicode properties to determine
3426: character types, instead of recognizing only characters with codes less
3427: than 128 via a lookup table.
3428:
3429: If a pattern starts with (*NO_START_OPT), it has the same effect as
3430: setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3431: time. There are also some more of these special sequences that are con-
3432: cerned with the handling of newlines; they are described below.
3433:
3434: The remainder of this document discusses the patterns that are sup-
3435: ported by PCRE when its main matching function, pcre_exec(), is used.
3436: From release 6.0, PCRE offers a second matching function,
3437: pcre_dfa_exec(), which matches using a different algorithm that is not
3438: Perl-compatible. Some of the features discussed below are not available
3439: when pcre_dfa_exec() is used. The advantages and disadvantages of the
3440: alternative function, and how it differs from the normal function, are
3441: discussed in the pcrematching page.
3442:
3443:
3444: NEWLINE CONVENTIONS
3445:
3446: PCRE supports five different conventions for indicating line breaks in
3447: strings: a single CR (carriage return) character, a single LF (line-
3448: feed) character, the two-character sequence CRLF, any of the three pre-
3449: ceding, or any Unicode newline sequence. The pcreapi page has further
3450: discussion about newlines, and shows how to set the newline convention
3451: in the options arguments for the compiling and matching functions.
3452:
3453: It is also possible to specify a newline convention by starting a pat-
3454: tern string with one of the following five sequences:
3455:
3456: (*CR) carriage return
3457: (*LF) linefeed
3458: (*CRLF) carriage return, followed by linefeed
3459: (*ANYCRLF) any of the three above
3460: (*ANY) all Unicode newline sequences
3461:
3462: These override the default and the options given to pcre_compile() or
3463: pcre_compile2(). For example, on a Unix system where LF is the default
3464: newline sequence, the pattern
3465:
3466: (*CR)a.b
3467:
3468: changes the convention to CR. That pattern matches "a\nb" because LF is
3469: no longer a newline. Note that these special settings, which are not
3470: Perl-compatible, are recognized only at the very start of a pattern,
3471: and that they must be in upper case. If more than one of them is
3472: present, the last one is used.
3473:
3474: The newline convention affects the interpretation of the dot metachar-
3475: acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3476: ever, it does not affect what the \R escape sequence matches. By
3477: default, this is any Unicode newline sequence, for Perl compatibility.
3478: However, this can be changed; see the description of \R in the section
3479: entitled "Newline sequences" below. A change of \R setting can be com-
3480: bined with a change of newline convention.
3481:
3482:
3483: CHARACTERS AND METACHARACTERS
3484:
3485: A regular expression is a pattern that is matched against a subject
3486: string from left to right. Most characters stand for themselves in a
3487: pattern, and match the corresponding characters in the subject. As a
3488: trivial example, the pattern
3489:
3490: The quick brown fox
3491:
3492: matches a portion of a subject string that is identical to itself. When
3493: caseless matching is specified (the PCRE_CASELESS option), letters are
3494: matched independently of case. In UTF-8 mode, PCRE always understands
3495: the concept of case for characters whose values are less than 128, so
3496: caseless matching is always possible. For characters with higher val-
3497: ues, the concept of case is supported if PCRE is compiled with Unicode
3498: property support, but not otherwise. If you want to use caseless
3499: matching for characters 128 and above, you must ensure that PCRE is
3500: compiled with Unicode property support as well as with UTF-8 support.
3501:
3502: The power of regular expressions comes from the ability to include
3503: alternatives and repetitions in the pattern. These are encoded in the
3504: pattern by the use of metacharacters, which do not stand for themselves
3505: but instead are interpreted in some special way.
3506:
3507: There are two different sets of metacharacters: those that are recog-
3508: nized anywhere in the pattern except within square brackets, and those
3509: that are recognized within square brackets. Outside square brackets,
3510: the metacharacters are as follows:
3511:
3512: \ general escape character with several uses
3513: ^ assert start of string (or line, in multiline mode)
3514: $ assert end of string (or line, in multiline mode)
3515: . match any character except newline (by default)
3516: [ start character class definition
3517: | start of alternative branch
3518: ( start subpattern
3519: ) end subpattern
3520: ? extends the meaning of (
3521: also 0 or 1 quantifier
3522: also quantifier minimizer
3523: * 0 or more quantifier
3524: + 1 or more quantifier
3525: also "possessive quantifier"
3526: { start min/max quantifier
3527:
3528: Part of a pattern that is in square brackets is called a "character
3529: class". In a character class the only metacharacters are:
3530:
3531: \ general escape character
3532: ^ negate the class, but only if the first character
3533: - indicates character range
3534: [ POSIX character class (only if followed by POSIX
3535: syntax)
3536: ] terminates the character class
3537:
3538: The following sections describe the use of each of the metacharacters.
3539:
3540:
3541: BACKSLASH
3542:
3543: The backslash character has several uses. Firstly, if it is followed by
3544: a character that is not a number or a letter, it takes away any special
3545: meaning that character may have. This use of backslash as an escape
3546: character applies both inside and outside character classes.
3547:
3548: For example, if you want to match a * character, you write \* in the
3549: pattern. This escaping action applies whether or not the following
3550: character would otherwise be interpreted as a metacharacter, so it is
3551: always safe to precede a non-alphanumeric with backslash to specify
3552: that it stands for itself. In particular, if you want to match a back-
3553: slash, you write \\.
3554:
3555: In UTF-8 mode, only ASCII numbers and letters have any special meaning
3556: after a backslash. All other characters (in particular, those whose
3557: codepoints are greater than 127) are treated as literals.
3558:
3559: If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3560: the pattern (other than in a character class) and characters between a
3561: # outside a character class and the next newline are ignored. An escap-
3562: ing backslash can be used to include a whitespace or # character as
3563: part of the pattern.
3564:
3565: If you want to remove the special meaning from a sequence of charac-
3566: ters, you can do so by putting them between \Q and \E. This is differ-
3567: ent from Perl in that $ and @ are handled as literals in \Q...\E
3568: sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3569: tion. Note the following examples:
3570:
3571: Pattern PCRE matches Perl matches
3572:
3573: \Qabc$xyz\E abc$xyz abc followed by the
3574: contents of $xyz
3575: \Qabc\$xyz\E abc\$xyz abc\$xyz
3576: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3577:
3578: The \Q...\E sequence is recognized both inside and outside character
3579: classes. An isolated \E that is not preceded by \Q is ignored. If \Q
3580: is not followed by \E later in the pattern, the literal interpretation
3581: continues to the end of the pattern (that is, \E is assumed at the
3582: end). If the isolated \Q is inside a character class, this causes an
3583: error, because the character class is not terminated.
3584:
3585: Non-printing characters
3586:
3587: A second use of backslash provides a way of encoding non-printing char-
3588: acters in patterns in a visible manner. There is no restriction on the
3589: appearance of non-printing characters, apart from the binary zero that
3590: terminates a pattern, but when a pattern is being prepared by text
3591: editing, it is often easier to use one of the following escape
3592: sequences than the binary character it represents:
3593:
3594: \a alarm, that is, the BEL character (hex 07)
3595: \cx "control-x", where x is any ASCII character
3596: \e escape (hex 1B)
3597: \f formfeed (hex 0C)
3598: \n linefeed (hex 0A)
3599: \r carriage return (hex 0D)
3600: \t tab (hex 09)
3601: \ddd character with octal code ddd, or back reference
3602: \xhh character with hex code hh
3603: \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
3604: \uhhhh character with hex code hhhh (JavaScript mode only)
3605:
3606: The precise effect of \cx is as follows: if x is a lower case letter,
3607: it is converted to upper case. Then bit 6 of the character (hex 40) is
3608: inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
3609: is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
3610: has a value greater than 127, a compile-time error occurs. This locks
3611: out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE
3612: is compiled in EBCDIC mode, all byte values are valid. A lower case
3613: letter is converted to upper case, and then the 0xc0 bits are flipped.)
3614:
3615: By default, after \x, from zero to two hexadecimal digits are read
3616: (letters can be in upper or lower case). Any number of hexadecimal dig-
3617: its may appear between \x{ and }, but the value of the character code
3618: must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8
3619: mode. That is, the maximum value in hexadecimal is 7FFFFFFF. Note that
3620: this is bigger than the largest Unicode code point, which is 10FFFF.
3621:
3622: If characters other than hexadecimal digits appear between \x{ and },
3623: or if there is no terminating }, this form of escape is not recognized.
3624: Instead, the initial \x will be interpreted as a basic hexadecimal
3625: escape, with no following digits, giving a character whose value is
3626: zero.
3627:
3628: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
3629: is as just described only when it is followed by two hexadecimal dig-
3630: its. Otherwise, it matches a literal "x" character. In JavaScript
3631: mode, support for code points greater than 256 is provided by \u, which
3632: must be followed by four hexadecimal digits; otherwise it matches a
3633: literal "u" character.
3634:
3635: Characters whose value is less than 256 can be defined by either of the
3636: two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
3637: ence in the way they are handled. For example, \xdc is exactly the same
3638: as \x{dc} (or \u00dc in JavaScript mode).
3639:
3640: After \0 up to two further octal digits are read. If there are fewer
3641: than two digits, just those that are present are used. Thus the
3642: sequence \0\x\07 specifies two binary zeros followed by a BEL character
3643: (code value 7). Make sure you supply two digits after the initial zero
3644: if the pattern character that follows is itself an octal digit.
3645:
3646: The handling of a backslash followed by a digit other than 0 is compli-
3647: cated. Outside a character class, PCRE reads it and any following dig-
3648: its as a decimal number. If the number is less than 10, or if there
3649: have been at least that many previous capturing left parentheses in the
3650: expression, the entire sequence is taken as a back reference. A
3651: description of how this works is given later, following the discussion
3652: of parenthesized subpatterns.
3653:
3654: Inside a character class, or if the decimal number is greater than 9
3655: and there have not been that many capturing subpatterns, PCRE re-reads
3656: up to three octal digits following the backslash, and uses them to gen-
3657: erate a data character. Any subsequent digits stand for themselves. In
3658: non-UTF-8 mode, the value of a character specified in octal must be
3659: less than \400. In UTF-8 mode, values up to \777 are permitted. For
3660: example:
3661:
3662: \040 is another way of writing a space
3663: \40 is the same, provided there are fewer than 40
3664: previous capturing subpatterns
3665: \7 is always a back reference
3666: \11 might be a back reference, or another way of
3667: writing a tab
3668: \011 is always a tab
3669: \0113 is a tab followed by the character "3"
3670: \113 might be a back reference, otherwise the
3671: character with octal code 113
3672: \377 might be a back reference, otherwise
3673: the byte consisting entirely of 1 bits
3674: \81 is either a back reference, or a binary zero
3675: followed by the two characters "8" and "1"
3676:
3677: Note that octal values of 100 or greater must not be introduced by a
3678: leading zero, because no more than three octal digits are ever read.
3679:
3680: All the sequences that define a single character value can be used both
3681: inside and outside character classes. In addition, inside a character
3682: class, \b is interpreted as the backspace character (hex 08).
3683:
3684: \N is not allowed in a character class. \B, \R, and \X are not special
3685: inside a character class. Like other unrecognized escape sequences,
3686: they are treated as the literal characters "B", "R", and "X" by
3687: default, but cause an error if the PCRE_EXTRA option is set. Outside a
3688: character class, these sequences have different meanings.
3689:
3690: Unsupported escape sequences
3691:
3692: In Perl, the sequences \l, \L, \u, and \U are recognized by its string
3693: handler and used to modify the case of following characters. By
3694: default, PCRE does not support these escape sequences. However, if the
3695: PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
3696: \u can be used to define a character by code point, as described in the
3697: previous section.
3698:
3699: Absolute and relative back references
3700:
3701: The sequence \g followed by an unsigned or a negative number, option-
3702: ally enclosed in braces, is an absolute or relative back reference. A
3703: named back reference can be coded as \g{name}. Back references are dis-
3704: cussed later, following the discussion of parenthesized subpatterns.
3705:
3706: Absolute and relative subroutine calls
3707:
3708: For compatibility with Oniguruma, the non-Perl syntax \g followed by a
3709: name or a number enclosed either in angle brackets or single quotes, is
3710: an alternative syntax for referencing a subpattern as a "subroutine".
3711: Details are discussed later. Note that \g{...} (Perl syntax) and
3712: \g<...> (Oniguruma syntax) are not synonymous. The former is a back
3713: reference; the latter is a subroutine call.
3714:
3715: Generic character types
3716:
3717: Another use of backslash is for specifying generic character types:
3718:
3719: \d any decimal digit
3720: \D any character that is not a decimal digit
3721: \h any horizontal whitespace character
3722: \H any character that is not a horizontal whitespace character
3723: \s any whitespace character
3724: \S any character that is not a whitespace character
3725: \v any vertical whitespace character
3726: \V any character that is not a vertical whitespace character
3727: \w any "word" character
3728: \W any "non-word" character
3729:
3730: There is also the single sequence \N, which matches a non-newline char-
3731: acter. This is the same as the "." metacharacter when PCRE_DOTALL is
3732: not set. Perl also uses \N to match characters by name; PCRE does not
3733: support this.
3734:
3735: Each pair of lower and upper case escape sequences partitions the com-
3736: plete set of characters into two disjoint sets. Any given character
3737: matches one, and only one, of each pair. The sequences can appear both
3738: inside and outside character classes. They each match one character of
3739: the appropriate type. If the current matching point is at the end of
3740: the subject string, all of them fail, because there is no character to
3741: match.
3742:
3743: For compatibility with Perl, \s does not match the VT character (code
3744: 11). This makes it different from the the POSIX "space" class. The \s
3745: characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3746: "use locale;" is included in a Perl script, \s may match the VT charac-
3747: ter. In PCRE, it never does.
3748:
3749: A "word" character is an underscore or any character that is a letter
3750: or digit. By default, the definition of letters and digits is con-
3751: trolled by PCRE's low-valued character tables, and may vary if locale-
3752: specific matching is taking place (see "Locale support" in the pcreapi
3753: page). For example, in a French locale such as "fr_FR" in Unix-like
3754: systems, or "french" in Windows, some character codes greater than 128
3755: are used for accented letters, and these are then matched by \w. The
3756: use of locales with Unicode is discouraged.
3757:
3758: By default, in UTF-8 mode, characters with values greater than 128
3759: never match \d, \s, or \w, and always match \D, \S, and \W. These
3760: sequences retain their original meanings from before UTF-8 support was
3761: available, mainly for efficiency reasons. However, if PCRE is compiled
3762: with Unicode property support, and the PCRE_UCP option is set, the be-
3763: haviour is changed so that Unicode properties are used to determine
3764: character types, as follows:
3765:
3766: \d any character that \p{Nd} matches (decimal digit)
3767: \s any character that \p{Z} matches, plus HT, LF, FF, CR
3768: \w any character that \p{L} or \p{N} matches, plus underscore
3769:
3770: The upper case escapes match the inverse sets of characters. Note that
3771: \d matches only decimal digits, whereas \w matches any Unicode digit,
3772: as well as any Unicode letter, and underscore. Note also that PCRE_UCP
3773: affects \b, and \B because they are defined in terms of \w and \W.
3774: Matching these sequences is noticeably slower when PCRE_UCP is set.
3775:
3776: The sequences \h, \H, \v, and \V are features that were added to Perl
3777: at release 5.10. In contrast to the other sequences, which match only
3778: ASCII characters by default, these always match certain high-valued
3779: codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The horizon-
3780: tal space characters are:
3781:
3782: U+0009 Horizontal tab
3783: U+0020 Space
3784: U+00A0 Non-break space
3785: U+1680 Ogham space mark
3786: U+180E Mongolian vowel separator
3787: U+2000 En quad
3788: U+2001 Em quad
3789: U+2002 En space
3790: U+2003 Em space
3791: U+2004 Three-per-em space
3792: U+2005 Four-per-em space
3793: U+2006 Six-per-em space
3794: U+2007 Figure space
3795: U+2008 Punctuation space
3796: U+2009 Thin space
3797: U+200A Hair space
3798: U+202F Narrow no-break space
3799: U+205F Medium mathematical space
3800: U+3000 Ideographic space
3801:
3802: The vertical space characters are:
3803:
3804: U+000A Linefeed
3805: U+000B Vertical tab
3806: U+000C Formfeed
3807: U+000D Carriage return
3808: U+0085 Next line
3809: U+2028 Line separator
3810: U+2029 Paragraph separator
3811:
3812: Newline sequences
3813:
3814: Outside a character class, by default, the escape sequence \R matches
3815: any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
3816: following:
3817:
3818: (?>\r\n|\n|\x0b|\f|\r|\x85)
3819:
3820: This is an example of an "atomic group", details of which are given
3821: below. This particular group matches either the two-character sequence
3822: CR followed by LF, or one of the single characters LF (linefeed,
3823: U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3824: return, U+000D), or NEL (next line, U+0085). The two-character sequence
3825: is treated as a single unit that cannot be split.
3826:
3827: In UTF-8 mode, two additional characters whose codepoints are greater
3828: than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3829: rator, U+2029). Unicode character property support is not needed for
3830: these characters to be recognized.
3831:
3832: It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3833: the complete set of Unicode line endings) by setting the option
3834: PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3835: (BSR is an abbrevation for "backslash R".) This can be made the default
3836: when PCRE is built; if this is the case, the other behaviour can be
3837: requested via the PCRE_BSR_UNICODE option. It is also possible to
3838: specify these settings by starting a pattern string with one of the
3839: following sequences:
3840:
3841: (*BSR_ANYCRLF) CR, LF, or CRLF only
3842: (*BSR_UNICODE) any Unicode newline sequence
3843:
3844: These override the default and the options given to pcre_compile() or
3845: pcre_compile2(), but they can be overridden by options given to
3846: pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
3847: are not Perl-compatible, are recognized only at the very start of a
3848: pattern, and that they must be in upper case. If more than one of them
3849: is present, the last one is used. They can be combined with a change of
3850: newline convention; for example, a pattern can start with:
3851:
3852: (*ANY)(*BSR_ANYCRLF)
3853:
3854: They can also be combined with the (*UTF8) or (*UCP) special sequences.
3855: Inside a character class, \R is treated as an unrecognized escape
3856: sequence, and so matches the letter "R" by default, but causes an error
3857: if PCRE_EXTRA is set.
3858:
3859: Unicode character properties
3860:
3861: When PCRE is built with Unicode character property support, three addi-
3862: tional escape sequences that match characters with specific properties
3863: are available. When not in UTF-8 mode, these sequences are of course
3864: limited to testing characters whose codepoints are less than 256, but
3865: they do work in this mode. The extra escape sequences are:
3866:
3867: \p{xx} a character with the xx property
3868: \P{xx} a character without the xx property
3869: \X an extended Unicode sequence
3870:
3871: The property names represented by xx above are limited to the Unicode
3872: script names, the general category properties, "Any", which matches any
3873: character (including newline), and some special PCRE properties
3874: (described in the next section). Other Perl properties such as "InMu-
3875: sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
3876: does not match any characters, so always causes a match failure.
3877:
3878: Sets of Unicode characters are defined as belonging to certain scripts.
3879: A character from one of these sets can be matched using a script name.
3880: For example:
3881:
3882: \p{Greek}
3883: \P{Han}
3884:
3885: Those that are not part of an identified script are lumped together as
3886: "Common". The current list of scripts is:
3887:
3888: Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
3889: Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
3890: Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
3891: tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
3892: Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
3893: rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
3894: Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
3895: Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
3896: Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
3897: Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
3898: Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
3899: Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
3900: Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
3901: Ugaritic, Vai, Yi.
3902:
3903: Each character has exactly one Unicode general category property, spec-
3904: ified by a two-letter abbreviation. For compatibility with Perl, nega-
3905: tion can be specified by including a circumflex between the opening
3906: brace and the property name. For example, \p{^Lu} is the same as
3907: \P{Lu}.
3908:
3909: If only one letter is specified with \p or \P, it includes all the gen-
3910: eral category properties that start with that letter. In this case, in
3911: the absence of negation, the curly brackets in the escape sequence are
3912: optional; these two examples have the same effect:
3913:
3914: \p{L}
3915: \pL
3916:
3917: The following general category property codes are supported:
3918:
3919: C Other
3920: Cc Control
3921: Cf Format
3922: Cn Unassigned
3923: Co Private use
3924: Cs Surrogate
3925:
3926: L Letter
3927: Ll Lower case letter
3928: Lm Modifier letter
3929: Lo Other letter
3930: Lt Title case letter
3931: Lu Upper case letter
3932:
3933: M Mark
3934: Mc Spacing mark
3935: Me Enclosing mark
3936: Mn Non-spacing mark
3937:
3938: N Number
3939: Nd Decimal number
3940: Nl Letter number
3941: No Other number
3942:
3943: P Punctuation
3944: Pc Connector punctuation
3945: Pd Dash punctuation
3946: Pe Close punctuation
3947: Pf Final punctuation
3948: Pi Initial punctuation
3949: Po Other punctuation
3950: Ps Open punctuation
3951:
3952: S Symbol
3953: Sc Currency symbol
3954: Sk Modifier symbol
3955: Sm Mathematical symbol
3956: So Other symbol
3957:
3958: Z Separator
3959: Zl Line separator
3960: Zp Paragraph separator
3961: Zs Space separator
3962:
3963: The special property L& is also supported: it matches a character that
3964: has the Lu, Ll, or Lt property, in other words, a letter that is not
3965: classified as a modifier or "other".
3966:
3967: The Cs (Surrogate) property applies only to characters in the range
3968: U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3969: RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3970: ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3971: the pcreapi page). Perl does not support the Cs property.
3972:
3973: The long synonyms for property names that Perl supports (such as
3974: \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3975: any of these properties with "Is".
3976:
3977: No character that is in the Unicode table has the Cn (unassigned) prop-
3978: erty. Instead, this property is assumed for any code point that is not
3979: in the Unicode table.
3980:
3981: Specifying caseless matching does not affect these escape sequences.
3982: For example, \p{Lu} always matches only upper case letters.
3983:
3984: The \X escape matches any number of Unicode characters that form an
3985: extended Unicode sequence. \X is equivalent to
3986:
3987: (?>\PM\pM*)
3988:
3989: That is, it matches a character without the "mark" property, followed
3990: by zero or more characters with the "mark" property, and treats the
3991: sequence as an atomic group (see below). Characters with the "mark"
3992: property are typically accents that affect the preceding character.
3993: None of them have codepoints less than 256, so in non-UTF-8 mode \X
3994: matches any one character.
3995:
3996: Note that recent versions of Perl have changed \X to match what Unicode
3997: calls an "extended grapheme cluster", which has a more complicated def-
3998: inition.
3999:
4000: Matching characters by Unicode property is not fast, because PCRE has
4001: to search a structure that contains data for over fifteen thousand
4002: characters. That is why the traditional escape sequences such as \d and
4003: \w do not use Unicode properties in PCRE by default, though you can
4004: make them do so by setting the PCRE_UCP option for pcre_compile() or by
4005: starting the pattern with (*UCP).
4006:
4007: PCRE's additional properties
4008:
4009: As well as the standard Unicode properties described in the previous
4010: section, PCRE supports four more that make it possible to convert tra-
4011: ditional escape sequences such as \w and \s and POSIX character classes
4012: to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
4013: erties internally when PCRE_UCP is set. They are:
4014:
4015: Xan Any alphanumeric character
4016: Xps Any POSIX space character
4017: Xsp Any Perl space character
4018: Xwd Any Perl "word" character
4019:
4020: Xan matches characters that have either the L (letter) or the N (num-
4021: ber) property. Xps matches the characters tab, linefeed, vertical tab,
4022: formfeed, or carriage return, and any other character that has the Z
4023: (separator) property. Xsp is the same as Xps, except that vertical tab
4024: is excluded. Xwd matches the same characters as Xan, plus underscore.
4025:
4026: Resetting the match start
4027:
4028: The escape sequence \K causes any previously matched characters not to
4029: be included in the final matched sequence. For example, the pattern:
4030:
4031: foo\Kbar
4032:
4033: matches "foobar", but reports that it has matched "bar". This feature
4034: is similar to a lookbehind assertion (described below). However, in
4035: this case, the part of the subject before the real match does not have
4036: to be of fixed length, as lookbehind assertions do. The use of \K does
4037: not interfere with the setting of captured substrings. For example,
4038: when the pattern
4039:
4040: (foo)\Kbar
4041:
4042: matches "foobar", the first substring is still set to "foo".
4043:
4044: Perl documents that the use of \K within assertions is "not well
4045: defined". In PCRE, \K is acted upon when it occurs inside positive
4046: assertions, but is ignored in negative assertions.
4047:
4048: Simple assertions
4049:
4050: The final use of backslash is for certain simple assertions. An asser-
4051: tion specifies a condition that has to be met at a particular point in
4052: a match, without consuming any characters from the subject string. The
4053: use of subpatterns for more complicated assertions is described below.
4054: The backslashed assertions are:
4055:
4056: \b matches at a word boundary
4057: \B matches when not at a word boundary
4058: \A matches at the start of the subject
4059: \Z matches at the end of the subject
4060: also matches before a newline at the end of the subject
4061: \z matches only at the end of the subject
4062: \G matches at the first matching position in the subject
4063:
4064: Inside a character class, \b has a different meaning; it matches the
4065: backspace character. If any other of these assertions appears in a
4066: character class, by default it matches the corresponding literal char-
4067: acter (for example, \B matches the letter B). However, if the
4068: PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4069: ated instead.
4070:
4071: A word boundary is a position in the subject string where the current
4072: character and the previous character do not both match \w or \W (i.e.
4073: one matches \w and the other matches \W), or the start or end of the
4074: string if the first or last character matches \w, respectively. In
4075: UTF-8 mode, the meanings of \w and \W can be changed by setting the
4076: PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4077: PCRE nor Perl has a separate "start of word" or "end of word" metase-
4078: quence. However, whatever follows \b normally determines which it is.
4079: For example, the fragment \ba matches "a" at the start of a word.
4080:
4081: The \A, \Z, and \z assertions differ from the traditional circumflex
4082: and dollar (described in the next section) in that they only ever match
4083: at the very start and end of the subject string, whatever options are
4084: set. Thus, they are independent of multiline mode. These three asser-
4085: tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4086: affect only the behaviour of the circumflex and dollar metacharacters.
4087: However, if the startoffset argument of pcre_exec() is non-zero, indi-
4088: cating that matching is to start at a point other than the beginning of
4089: the subject, \A can never match. The difference between \Z and \z is
4090: that \Z matches before a newline at the end of the string as well as at
4091: the very end, whereas \z matches only at the end.
4092:
4093: The \G assertion is true only when the current matching position is at
4094: the start point of the match, as specified by the startoffset argument
4095: of pcre_exec(). It differs from \A when the value of startoffset is
4096: non-zero. By calling pcre_exec() multiple times with appropriate argu-
4097: ments, you can mimic Perl's /g option, and it is in this kind of imple-
4098: mentation where \G can be useful.
4099:
4100: Note, however, that PCRE's interpretation of \G, as the start of the
4101: current match, is subtly different from Perl's, which defines it as the
4102: end of the previous match. In Perl, these can be different when the
4103: previously matched string was empty. Because PCRE does just one match
4104: at a time, it cannot reproduce this behaviour.
4105:
4106: If all the alternatives of a pattern begin with \G, the expression is
4107: anchored to the starting match position, and the "anchored" flag is set
4108: in the compiled regular expression.
4109:
4110:
4111: CIRCUMFLEX AND DOLLAR
4112:
4113: Outside a character class, in the default matching mode, the circumflex
4114: character is an assertion that is true only if the current matching
4115: point is at the start of the subject string. If the startoffset argu-
4116: ment of pcre_exec() is non-zero, circumflex can never match if the
4117: PCRE_MULTILINE option is unset. Inside a character class, circumflex
4118: has an entirely different meaning (see below).
4119:
4120: Circumflex need not be the first character of the pattern if a number
4121: of alternatives are involved, but it should be the first thing in each
4122: alternative in which it appears if the pattern is ever to match that
4123: branch. If all possible alternatives start with a circumflex, that is,
4124: if the pattern is constrained to match only at the start of the sub-
4125: ject, it is said to be an "anchored" pattern. (There are also other
4126: constructs that can cause a pattern to be anchored.)
4127:
4128: A dollar character is an assertion that is true only if the current
4129: matching point is at the end of the subject string, or immediately
4130: before a newline at the end of the string (by default). Dollar need not
4131: be the last character of the pattern if a number of alternatives are
4132: involved, but it should be the last item in any branch in which it
4133: appears. Dollar has no special meaning in a character class.
4134:
4135: The meaning of dollar can be changed so that it matches only at the
4136: very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4137: compile time. This does not affect the \Z assertion.
4138:
4139: The meanings of the circumflex and dollar characters are changed if the
4140: PCRE_MULTILINE option is set. When this is the case, a circumflex
4141: matches immediately after internal newlines as well as at the start of
4142: the subject string. It does not match after a newline that ends the
4143: string. A dollar matches before any newlines in the string, as well as
4144: at the very end, when PCRE_MULTILINE is set. When newline is specified
4145: as the two-character sequence CRLF, isolated CR and LF characters do
4146: not indicate newlines.
4147:
4148: For example, the pattern /^abc$/ matches the subject string "def\nabc"
4149: (where \n represents a newline) in multiline mode, but not otherwise.
4150: Consequently, patterns that are anchored in single line mode because
4151: all branches start with ^ are not anchored in multiline mode, and a
4152: match for circumflex is possible when the startoffset argument of
4153: pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4154: PCRE_MULTILINE is set.
4155:
4156: Note that the sequences \A, \Z, and \z can be used to match the start
4157: and end of the subject in both modes, and if all branches of a pattern
4158: start with \A it is always anchored, whether or not PCRE_MULTILINE is
4159: set.
4160:
4161:
4162: FULL STOP (PERIOD, DOT) AND \N
4163:
4164: Outside a character class, a dot in the pattern matches any one charac-
4165: ter in the subject string except (by default) a character that signi-
4166: fies the end of a line. In UTF-8 mode, the matched character may be
4167: more than one byte long.
4168:
4169: When a line ending is defined as a single character, dot never matches
4170: that character; when the two-character sequence CRLF is used, dot does
4171: not match CR if it is immediately followed by LF, but otherwise it
4172: matches all characters (including isolated CRs and LFs). When any Uni-
4173: code line endings are being recognized, dot does not match CR or LF or
4174: any of the other line ending characters.
4175:
4176: The behaviour of dot with regard to newlines can be changed. If the
4177: PCRE_DOTALL option is set, a dot matches any one character, without
4178: exception. If the two-character sequence CRLF is present in the subject
4179: string, it takes two dots to match it.
4180:
4181: The handling of dot is entirely independent of the handling of circum-
4182: flex and dollar, the only relationship being that they both involve
4183: newlines. Dot has no special meaning in a character class.
4184:
4185: The escape sequence \N behaves like a dot, except that it is not
4186: affected by the PCRE_DOTALL option. In other words, it matches any
4187: character except one that signifies the end of a line. Perl also uses
4188: \N to match characters by name; PCRE does not support this.
4189:
4190:
4191: MATCHING A SINGLE BYTE
4192:
4193: Outside a character class, the escape sequence \C matches any one byte,
4194: both in and out of UTF-8 mode. Unlike a dot, it always matches line-
4195: ending characters. The feature is provided in Perl in order to match
4196: individual bytes in UTF-8 mode, but it is unclear how it can usefully
4197: be used. Because \C breaks up characters into individual bytes, match-
4198: ing one byte with \C in UTF-8 mode means that the rest of the string
4199: may start with a malformed UTF-8 character. This has undefined results,
4200: because PCRE assumes that it is dealing with valid UTF-8 strings (and
4201: by default it checks this at the start of processing unless the
4202: PCRE_NO_UTF8_CHECK option is used).
4203:
4204: PCRE does not allow \C to appear in lookbehind assertions (described
4205: below) in UTF-8 mode, because this would make it impossible to calcu-
4206: late the length of the lookbehind.
4207:
4208: In general, the \C escape sequence is best avoided in UTF-8 mode. How-
4209: ever, one way of using it that avoids the problem of malformed UTF-8
4210: characters is to use a lookahead to check the length of the next char-
4211: acter, as in this pattern (ignore white space and line breaks):
4212:
4213: (?| (?=[\x00-\x7f])(\C) |
4214: (?=[\x80-\x{7ff}])(\C)(\C) |
4215: (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
4216: (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
4217:
4218: A group that starts with (?| resets the capturing parentheses numbers
4219: in each alternative (see "Duplicate Subpattern Numbers" below). The
4220: assertions at the start of each branch check the next UTF-8 character
4221: for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
4222: character's individual bytes are then captured by the appropriate num-
4223: ber of groups.
4224:
4225:
4226: SQUARE BRACKETS AND CHARACTER CLASSES
4227:
4228: An opening square bracket introduces a character class, terminated by a
4229: closing square bracket. A closing square bracket on its own is not spe-
4230: cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4231: a lone closing square bracket causes a compile-time error. If a closing
4232: square bracket is required as a member of the class, it should be the
4233: first data character in the class (after an initial circumflex, if
4234: present) or escaped with a backslash.
4235:
4236: A character class matches a single character in the subject. In UTF-8
4237: mode, the character may be more than one byte long. A matched character
4238: must be in the set of characters defined by the class, unless the first
4239: character in the class definition is a circumflex, in which case the
4240: subject character must not be in the set defined by the class. If a
4241: circumflex is actually required as a member of the class, ensure it is
4242: not the first character, or escape it with a backslash.
4243:
4244: For example, the character class [aeiou] matches any lower case vowel,
4245: while [^aeiou] matches any character that is not a lower case vowel.
4246: Note that a circumflex is just a convenient notation for specifying the
4247: characters that are in the class by enumerating those that are not. A
4248: class that starts with a circumflex is not an assertion; it still con-
4249: sumes a character from the subject string, and therefore it fails if
4250: the current pointer is at the end of the string.
4251:
4252: In UTF-8 mode, characters with values greater than 255 can be included
4253: in a class as a literal string of bytes, or by using the \x{ escaping
4254: mechanism.
4255:
4256: When caseless matching is set, any letters in a class represent both
4257: their upper case and lower case versions, so for example, a caseless
4258: [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
4259: match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
4260: understands the concept of case for characters whose values are less
4261: than 128, so caseless matching is always possible. For characters with
4262: higher values, the concept of case is supported if PCRE is compiled
4263: with Unicode property support, but not otherwise. If you want to use
4264: caseless matching in UTF8-mode for characters 128 and above, you must
4265: ensure that PCRE is compiled with Unicode property support as well as
4266: with UTF-8 support.
4267:
4268: Characters that might indicate line breaks are never treated in any
4269: special way when matching character classes, whatever line-ending
4270: sequence is in use, and whatever setting of the PCRE_DOTALL and
4271: PCRE_MULTILINE options is used. A class such as [^a] always matches one
4272: of these characters.
4273:
4274: The minus (hyphen) character can be used to specify a range of charac-
4275: ters in a character class. For example, [d-m] matches any letter
4276: between d and m, inclusive. If a minus character is required in a
4277: class, it must be escaped with a backslash or appear in a position
4278: where it cannot be interpreted as indicating a range, typically as the
4279: first or last character in the class.
4280:
4281: It is not possible to have the literal character "]" as the end charac-
4282: ter of a range. A pattern such as [W-]46] is interpreted as a class of
4283: two characters ("W" and "-") followed by a literal string "46]", so it
4284: would match "W46]" or "-46]". However, if the "]" is escaped with a
4285: backslash it is interpreted as the end of range, so [W-\]46] is inter-
4286: preted as a class containing a range followed by two other characters.
4287: The octal or hexadecimal representation of "]" can also be used to end
4288: a range.
4289:
4290: Ranges operate in the collating sequence of character values. They can
4291: also be used for characters specified numerically, for example
4292: [\000-\037]. In UTF-8 mode, ranges can include characters whose values
4293: are greater than 255, for example [\x{100}-\x{2ff}].
4294:
4295: If a range that includes letters is used when caseless matching is set,
4296: it matches the letters in either case. For example, [W-c] is equivalent
4297: to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
4298: character tables for a French locale are in use, [\xc8-\xcb] matches
4299: accented E characters in both cases. In UTF-8 mode, PCRE supports the
4300: concept of case for characters with values greater than 128 only when
4301: it is compiled with Unicode property support.
4302:
4303: The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
4304: \w, and \W may appear in a character class, and add the characters that
4305: they match to the class. For example, [\dABCDEF] matches any hexadeci-
4306: mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of
4307: \d, \s, \w and their upper case partners, just as it does when they
4308: appear outside a character class, as described in the section entitled
4309: "Generic character types" above. The escape sequence \b has a different
4310: meaning inside a character class; it matches the backspace character.
4311: The sequences \B, \N, \R, and \X are not special inside a character
4312: class. Like any other unrecognized escape sequences, they are treated
4313: as the literal characters "B", "N", "R", and "X" by default, but cause
4314: an error if the PCRE_EXTRA option is set.
4315:
4316: A circumflex can conveniently be used with the upper case character
4317: types to specify a more restricted set of characters than the matching
4318: lower case type. For example, the class [^\W_] matches any letter or
4319: digit, but not underscore, whereas [\w] includes underscore. A positive
4320: character class should be read as "something OR something OR ..." and a
4321: negative class as "NOT something AND NOT something AND NOT ...".
4322:
4323: The only metacharacters that are recognized in character classes are
4324: backslash, hyphen (only where it can be interpreted as specifying a
4325: range), circumflex (only at the start), opening square bracket (only
4326: when it can be interpreted as introducing a POSIX class name - see the
4327: next section), and the terminating closing square bracket. However,
4328: escaping other non-alphanumeric characters does no harm.
4329:
4330:
4331: POSIX CHARACTER CLASSES
4332:
4333: Perl supports the POSIX notation for character classes. This uses names
4334: enclosed by [: and :] within the enclosing square brackets. PCRE also
4335: supports this notation. For example,
4336:
4337: [01[:alpha:]%]
4338:
4339: matches "0", "1", any alphabetic character, or "%". The supported class
4340: names are:
4341:
4342: alnum letters and digits
4343: alpha letters
4344: ascii character codes 0 - 127
4345: blank space or tab only
4346: cntrl control characters
4347: digit decimal digits (same as \d)
4348: graph printing characters, excluding space
4349: lower lower case letters
4350: print printing characters, including space
4351: punct printing characters, excluding letters and digits and space
4352: space white space (not quite the same as \s)
4353: upper upper case letters
4354: word "word" characters (same as \w)
4355: xdigit hexadecimal digits
4356:
4357: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4358: and space (32). Notice that this list includes the VT character (code
4359: 11). This makes "space" different to \s, which does not include VT (for
4360: Perl compatibility).
4361:
4362: The name "word" is a Perl extension, and "blank" is a GNU extension
4363: from Perl 5.8. Another Perl extension is negation, which is indicated
4364: by a ^ character after the colon. For example,
4365:
4366: [12[:^digit:]]
4367:
4368: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4369: POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4370: these are not supported, and an error is given if they are encountered.
4371:
4372: By default, in UTF-8 mode, characters with values greater than 128 do
4373: not match any of the POSIX character classes. However, if the PCRE_UCP
4374: option is passed to pcre_compile(), some of the classes are changed so
4375: that Unicode character properties are used. This is achieved by replac-
4376: ing the POSIX classes by other sequences, as follows:
4377:
4378: [:alnum:] becomes \p{Xan}
4379: [:alpha:] becomes \p{L}
4380: [:blank:] becomes \h
4381: [:digit:] becomes \p{Nd}
4382: [:lower:] becomes \p{Ll}
4383: [:space:] becomes \p{Xps}
4384: [:upper:] becomes \p{Lu}
4385: [:word:] becomes \p{Xwd}
4386:
4387: Negated versions, such as [:^alpha:] use \P instead of \p. The other
4388: POSIX classes are unchanged, and match only characters with code points
4389: less than 128.
4390:
4391:
4392: VERTICAL BAR
4393:
4394: Vertical bar characters are used to separate alternative patterns. For
4395: example, the pattern
4396:
4397: gilbert|sullivan
4398:
4399: matches either "gilbert" or "sullivan". Any number of alternatives may
4400: appear, and an empty alternative is permitted (matching the empty
4401: string). The matching process tries each alternative in turn, from left
4402: to right, and the first one that succeeds is used. If the alternatives
4403: are within a subpattern (defined below), "succeeds" means matching the
4404: rest of the main pattern as well as the alternative in the subpattern.
4405:
4406:
4407: INTERNAL OPTION SETTING
4408:
4409: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4410: PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4411: within the pattern by a sequence of Perl option letters enclosed
4412: between "(?" and ")". The option letters are
4413:
4414: i for PCRE_CASELESS
4415: m for PCRE_MULTILINE
4416: s for PCRE_DOTALL
4417: x for PCRE_EXTENDED
4418:
4419: For example, (?im) sets caseless, multiline matching. It is also possi-
4420: ble to unset these options by preceding the letter with a hyphen, and a
4421: combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4422: LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
4423: is also permitted. If a letter appears both before and after the
4424: hyphen, the option is unset.
4425:
4426: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
4427: can be changed in the same way as the Perl-compatible options by using
4428: the characters J, U and X respectively.
4429:
4430: When one of these option changes occurs at top level (that is, not
4431: inside subpattern parentheses), the change applies to the remainder of
4432: the pattern that follows. If the change is placed right at the start of
4433: a pattern, PCRE extracts it into the global options (and it will there-
4434: fore show up in data extracted by the pcre_fullinfo() function).
4435:
4436: An option change within a subpattern (see below for a description of
4437: subpatterns) affects only that part of the subpattern that follows it,
4438: so
4439:
4440: (a(?i)b)c
4441:
4442: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4443: used). By this means, options can be made to have different settings
4444: in different parts of the pattern. Any changes made in one alternative
4445: do carry on into subsequent branches within the same subpattern. For
4446: example,
4447:
4448: (a(?i)b|c)
4449:
4450: matches "ab", "aB", "c", and "C", even though when matching "C" the
4451: first branch is abandoned before the option setting. This is because
4452: the effects of option settings happen at compile time. There would be
4453: some very weird behaviour otherwise.
4454:
4455: Note: There are other PCRE-specific options that can be set by the
4456: application when the compile or match functions are called. In some
4457: cases the pattern can contain special leading sequences such as (*CRLF)
4458: to override what the application has set or what has been defaulted.
4459: Details are given in the section entitled "Newline sequences" above.
4460: There are also the (*UTF8) and (*UCP) leading sequences that can be
4461: used to set UTF-8 and Unicode property modes; they are equivalent to
4462: setting the PCRE_UTF8 and the PCRE_UCP options, respectively.
4463:
4464:
4465: SUBPATTERNS
4466:
4467: Subpatterns are delimited by parentheses (round brackets), which can be
4468: nested. Turning part of a pattern into a subpattern does two things:
4469:
4470: 1. It localizes a set of alternatives. For example, the pattern
4471:
4472: cat(aract|erpillar|)
4473:
4474: matches "cataract", "caterpillar", or "cat". Without the parentheses,
4475: it would match "cataract", "erpillar" or an empty string.
4476:
4477: 2. It sets up the subpattern as a capturing subpattern. This means
4478: that, when the whole pattern matches, that portion of the subject
4479: string that matched the subpattern is passed back to the caller via the
4480: ovector argument of pcre_exec(). Opening parentheses are counted from
4481: left to right (starting from 1) to obtain numbers for the capturing
4482: subpatterns. For example, if the string "the red king" is matched
4483: against the pattern
4484:
4485: the ((red|white) (king|queen))
4486:
4487: the captured substrings are "red king", "red", and "king", and are num-
4488: bered 1, 2, and 3, respectively.
4489:
4490: The fact that plain parentheses fulfil two functions is not always
4491: helpful. There are often times when a grouping subpattern is required
4492: without a capturing requirement. If an opening parenthesis is followed
4493: by a question mark and a colon, the subpattern does not do any captur-
4494: ing, and is not counted when computing the number of any subsequent
4495: capturing subpatterns. For example, if the string "the white queen" is
4496: matched against the pattern
4497:
4498: the ((?:red|white) (king|queen))
4499:
4500: the captured substrings are "white queen" and "queen", and are numbered
4501: 1 and 2. The maximum number of capturing subpatterns is 65535.
4502:
4503: As a convenient shorthand, if any option settings are required at the
4504: start of a non-capturing subpattern, the option letters may appear
4505: between the "?" and the ":". Thus the two patterns
4506:
4507: (?i:saturday|sunday)
4508: (?:(?i)saturday|sunday)
4509:
4510: match exactly the same set of strings. Because alternative branches are
4511: tried from left to right, and options are not reset until the end of
4512: the subpattern is reached, an option setting in one branch does affect
4513: subsequent branches, so the above patterns match "SUNDAY" as well as
4514: "Saturday".
4515:
4516:
4517: DUPLICATE SUBPATTERN NUMBERS
4518:
4519: Perl 5.10 introduced a feature whereby each alternative in a subpattern
4520: uses the same numbers for its capturing parentheses. Such a subpattern
4521: starts with (?| and is itself a non-capturing subpattern. For example,
4522: consider this pattern:
4523:
4524: (?|(Sat)ur|(Sun))day
4525:
4526: Because the two alternatives are inside a (?| group, both sets of cap-
4527: turing parentheses are numbered one. Thus, when the pattern matches,
4528: you can look at captured substring number one, whichever alternative
4529: matched. This construct is useful when you want to capture part, but
4530: not all, of one of a number of alternatives. Inside a (?| group, paren-
4531: theses are numbered as usual, but the number is reset at the start of
4532: each branch. The numbers of any capturing parentheses that follow the
4533: subpattern start after the highest number used in any branch. The fol-
4534: lowing example is taken from the Perl documentation. The numbers under-
4535: neath show in which buffer the captured content will be stored.
4536:
4537: # before ---------------branch-reset----------- after
4538: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
4539: # 1 2 2 3 2 3 4
4540:
4541: A back reference to a numbered subpattern uses the most recent value
4542: that is set for that number by any subpattern. The following pattern
4543: matches "abcabc" or "defdef":
4544:
4545: /(?|(abc)|(def))\1/
4546:
4547: In contrast, a subroutine call to a numbered subpattern always refers
4548: to the first one in the pattern with the given number. The following
4549: pattern matches "abcabc" or "defabc":
4550:
4551: /(?|(abc)|(def))(?1)/
4552:
4553: If a condition test for a subpattern's having matched refers to a non-
4554: unique number, the test is true if any of the subpatterns of that num-
4555: ber have matched.
4556:
4557: An alternative approach to using this "branch reset" feature is to use
4558: duplicate named subpatterns, as described in the next section.
4559:
4560:
4561: NAMED SUBPATTERNS
4562:
4563: Identifying capturing parentheses by number is simple, but it can be
4564: very hard to keep track of the numbers in complicated regular expres-
4565: sions. Furthermore, if an expression is modified, the numbers may
4566: change. To help with this difficulty, PCRE supports the naming of sub-
4567: patterns. This feature was not added to Perl until release 5.10. Python
4568: had the feature earlier, and PCRE introduced it at release 4.0, using
4569: the Python syntax. PCRE now supports both the Perl and the Python syn-
4570: tax. Perl allows identically numbered subpatterns to have different
4571: names, but PCRE does not.
4572:
4573: In PCRE, a subpattern can be named in one of three ways: (?<name>...)
4574: or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
4575: to capturing parentheses from other parts of the pattern, such as back
4576: references, recursion, and conditions, can be made by name as well as
4577: by number.
4578:
4579: Names consist of up to 32 alphanumeric characters and underscores.
4580: Named capturing parentheses are still allocated numbers as well as
4581: names, exactly as if the names were not present. The PCRE API provides
4582: function calls for extracting the name-to-number translation table from
4583: a compiled pattern. There is also a convenience function for extracting
4584: a captured substring by name.
4585:
4586: By default, a name must be unique within a pattern, but it is possible
4587: to relax this constraint by setting the PCRE_DUPNAMES option at compile
4588: time. (Duplicate names are also always permitted for subpatterns with
4589: the same number, set up as described in the previous section.) Dupli-
4590: cate names can be useful for patterns where only one instance of the
4591: named parentheses can match. Suppose you want to match the name of a
4592: weekday, either as a 3-letter abbreviation or as the full name, and in
4593: both cases you want to extract the abbreviation. This pattern (ignoring
4594: the line breaks) does the job:
4595:
4596: (?<DN>Mon|Fri|Sun)(?:day)?|
4597: (?<DN>Tue)(?:sday)?|
4598: (?<DN>Wed)(?:nesday)?|
4599: (?<DN>Thu)(?:rsday)?|
4600: (?<DN>Sat)(?:urday)?
4601:
4602: There are five capturing substrings, but only one is ever set after a
4603: match. (An alternative way of solving this problem is to use a "branch
4604: reset" subpattern, as described in the previous section.)
4605:
4606: The convenience function for extracting the data by name returns the
4607: substring for the first (and in this example, the only) subpattern of
4608: that name that matched. This saves searching to find which numbered
4609: subpattern it was.
4610:
4611: If you make a back reference to a non-unique named subpattern from
4612: elsewhere in the pattern, the one that corresponds to the first occur-
4613: rence of the name is used. In the absence of duplicate numbers (see the
4614: previous section) this is the one with the lowest number. If you use a
4615: named reference in a condition test (see the section about conditions
4616: below), either to check whether a subpattern has matched, or to check
4617: for recursion, all subpatterns with the same name are tested. If the
4618: condition is true for any one of them, the overall condition is true.
4619: This is the same behaviour as testing by number. For further details of
4620: the interfaces for handling named subpatterns, see the pcreapi documen-
4621: tation.
4622:
4623: Warning: You cannot use different names to distinguish between two sub-
4624: patterns with the same number because PCRE uses only the numbers when
4625: matching. For this reason, an error is given at compile time if differ-
4626: ent names are given to subpatterns with the same number. However, you
4627: can give the same name to subpatterns with the same number, even when
4628: PCRE_DUPNAMES is not set.
4629:
4630:
4631: REPETITION
4632:
4633: Repetition is specified by quantifiers, which can follow any of the
4634: following items:
4635:
4636: a literal data character
4637: the dot metacharacter
4638: the \C escape sequence
4639: the \X escape sequence (in UTF-8 mode with Unicode properties)
4640: the \R escape sequence
4641: an escape such as \d or \pL that matches a single character
4642: a character class
4643: a back reference (see next section)
4644: a parenthesized subpattern (including assertions)
4645: a subroutine call to a subpattern (recursive or otherwise)
4646:
4647: The general repetition quantifier specifies a minimum and maximum num-
4648: ber of permitted matches, by giving the two numbers in curly brackets
4649: (braces), separated by a comma. The numbers must be less than 65536,
4650: and the first must be less than or equal to the second. For example:
4651:
4652: z{2,4}
4653:
4654: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
4655: special character. If the second number is omitted, but the comma is
4656: present, there is no upper limit; if the second number and the comma
4657: are both omitted, the quantifier specifies an exact number of required
4658: matches. Thus
4659:
4660: [aeiou]{3,}
4661:
4662: matches at least 3 successive vowels, but may match many more, while
4663:
4664: \d{8}
4665:
4666: matches exactly 8 digits. An opening curly bracket that appears in a
4667: position where a quantifier is not allowed, or one that does not match
4668: the syntax of a quantifier, is taken as a literal character. For exam-
4669: ple, {,6} is not a quantifier, but a literal string of four characters.
4670:
4671: In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
4672: individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4673: acters, each of which is represented by a two-byte sequence. Similarly,
4674: when Unicode property support is available, \X{3} matches three Unicode
4675: extended sequences, each of which may be several bytes long (and they
4676: may be of different lengths).
4677:
4678: The quantifier {0} is permitted, causing the expression to behave as if
4679: the previous item and the quantifier were not present. This may be use-
4680: ful for subpatterns that are referenced as subroutines from elsewhere
4681: in the pattern (but see also the section entitled "Defining subpatterns
4682: for use by reference only" below). Items other than subpatterns that
4683: have a {0} quantifier are omitted from the compiled pattern.
4684:
4685: For convenience, the three most common quantifiers have single-charac-
4686: ter abbreviations:
4687:
4688: * is equivalent to {0,}
4689: + is equivalent to {1,}
4690: ? is equivalent to {0,1}
4691:
4692: It is possible to construct infinite loops by following a subpattern
4693: that can match no characters with a quantifier that has no upper limit,
4694: for example:
4695:
4696: (a?)*
4697:
4698: Earlier versions of Perl and PCRE used to give an error at compile time
4699: for such patterns. However, because there are cases where this can be
4700: useful, such patterns are now accepted, but if any repetition of the
4701: subpattern does in fact match no characters, the loop is forcibly bro-
4702: ken.
4703:
4704: By default, the quantifiers are "greedy", that is, they match as much
4705: as possible (up to the maximum number of permitted times), without
4706: causing the rest of the pattern to fail. The classic example of where
4707: this gives problems is in trying to match comments in C programs. These
4708: appear between /* and */ and within the comment, individual * and /
4709: characters may appear. An attempt to match C comments by applying the
4710: pattern
4711:
4712: /\*.*\*/
4713:
4714: to the string
4715:
4716: /* first comment */ not comment /* second comment */
4717:
4718: fails, because it matches the entire string owing to the greediness of
4719: the .* item.
4720:
4721: However, if a quantifier is followed by a question mark, it ceases to
4722: be greedy, and instead matches the minimum number of times possible, so
4723: the pattern
4724:
4725: /\*.*?\*/
4726:
4727: does the right thing with the C comments. The meaning of the various
4728: quantifiers is not otherwise changed, just the preferred number of
4729: matches. Do not confuse this use of question mark with its use as a
4730: quantifier in its own right. Because it has two uses, it can sometimes
4731: appear doubled, as in
4732:
4733: \d??\d
4734:
4735: which matches one digit by preference, but can match two if that is the
4736: only way the rest of the pattern matches.
4737:
4738: If the PCRE_UNGREEDY option is set (an option that is not available in
4739: Perl), the quantifiers are not greedy by default, but individual ones
4740: can be made greedy by following them with a question mark. In other
4741: words, it inverts the default behaviour.
4742:
4743: When a parenthesized subpattern is quantified with a minimum repeat
4744: count that is greater than 1 or with a limited maximum, more memory is
4745: required for the compiled pattern, in proportion to the size of the
4746: minimum or maximum.
4747:
4748: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4749: alent to Perl's /s) is set, thus allowing the dot to match newlines,
4750: the pattern is implicitly anchored, because whatever follows will be
4751: tried against every character position in the subject string, so there
4752: is no point in retrying the overall match at any position after the
4753: first. PCRE normally treats such a pattern as though it were preceded
4754: by \A.
4755:
4756: In cases where it is known that the subject string contains no new-
4757: lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4758: mization, or alternatively using ^ to indicate anchoring explicitly.
4759:
4760: However, there is one situation where the optimization cannot be used.
4761: When .* is inside capturing parentheses that are the subject of a back
4762: reference elsewhere in the pattern, a match at the start may fail where
4763: a later one succeeds. Consider, for example:
4764:
4765: (.*)abc\1
4766:
4767: If the subject is "xyz123abc123" the match point is the fourth charac-
4768: ter. For this reason, such a pattern is not implicitly anchored.
4769:
4770: When a capturing subpattern is repeated, the value captured is the sub-
4771: string that matched the final iteration. For example, after
4772:
4773: (tweedle[dume]{3}\s*)+
4774:
4775: has matched "tweedledum tweedledee" the value of the captured substring
4776: is "tweedledee". However, if there are nested capturing subpatterns,
4777: the corresponding captured values may have been set in previous itera-
4778: tions. For example, after
4779:
4780: /(a|(b))+/
4781:
4782: matches "aba" the value of the second captured substring is "b".
4783:
4784:
4785: ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4786:
4787: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4788: repetition, failure of what follows normally causes the repeated item
4789: to be re-evaluated to see if a different number of repeats allows the
4790: rest of the pattern to match. Sometimes it is useful to prevent this,
4791: either to change the nature of the match, or to cause it fail earlier
4792: than it otherwise might, when the author of the pattern knows there is
4793: no point in carrying on.
4794:
4795: Consider, for example, the pattern \d+foo when applied to the subject
4796: line
4797:
4798: 123456bar
4799:
4800: After matching all 6 digits and then failing to match "foo", the normal
4801: action of the matcher is to try again with only 5 digits matching the
4802: \d+ item, and then with 4, and so on, before ultimately failing.
4803: "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4804: the means for specifying that once a subpattern has matched, it is not
4805: to be re-evaluated in this way.
4806:
4807: If we use atomic grouping for the previous example, the matcher gives
4808: up immediately on failing to match "foo" the first time. The notation
4809: is a kind of special parenthesis, starting with (?> as in this example:
4810:
4811: (?>\d+)foo
4812:
4813: This kind of parenthesis "locks up" the part of the pattern it con-
4814: tains once it has matched, and a failure further into the pattern is
4815: prevented from backtracking into it. Backtracking past it to previous
4816: items, however, works as normal.
4817:
4818: An alternative description is that a subpattern of this type matches
4819: the string of characters that an identical standalone pattern would
4820: match, if anchored at the current point in the subject string.
4821:
4822: Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4823: such as the above example can be thought of as a maximizing repeat that
4824: must swallow everything it can. So, while both \d+ and \d+? are pre-
4825: pared to adjust the number of digits they match in order to make the
4826: rest of the pattern match, (?>\d+) can only match an entire sequence of
4827: digits.
4828:
4829: Atomic groups in general can of course contain arbitrarily complicated
4830: subpatterns, and can be nested. However, when the subpattern for an
4831: atomic group is just a single repeated item, as in the example above, a
4832: simpler notation, called a "possessive quantifier" can be used. This
4833: consists of an additional + character following a quantifier. Using
4834: this notation, the previous example can be rewritten as
4835:
4836: \d++foo
4837:
4838: Note that a possessive quantifier can be used with an entire group, for
4839: example:
4840:
4841: (abc|xyz){2,3}+
4842:
4843: Possessive quantifiers are always greedy; the setting of the
4844: PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4845: simpler forms of atomic group. However, there is no difference in the
4846: meaning of a possessive quantifier and the equivalent atomic group,
4847: though there may be a performance difference; possessive quantifiers
4848: should be slightly faster.
4849:
4850: The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4851: tax. Jeffrey Friedl originated the idea (and the name) in the first
4852: edition of his book. Mike McCloskey liked it, so implemented it when he
4853: built Sun's Java package, and PCRE copied it from there. It ultimately
4854: found its way into Perl at release 5.10.
4855:
4856: PCRE has an optimization that automatically "possessifies" certain sim-
4857: ple pattern constructs. For example, the sequence A+B is treated as
4858: A++B because there is no point in backtracking into a sequence of A's
4859: when B must follow.
4860:
4861: When a pattern contains an unlimited repeat inside a subpattern that
4862: can itself be repeated an unlimited number of times, the use of an
4863: atomic group is the only way to avoid some failing matches taking a
4864: very long time indeed. The pattern
4865:
4866: (\D+|<\d+>)*[!?]
4867:
4868: matches an unlimited number of substrings that either consist of non-
4869: digits, or digits enclosed in <>, followed by either ! or ?. When it
4870: matches, it runs quickly. However, if it is applied to
4871:
4872: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4873:
4874: it takes a long time before reporting failure. This is because the
4875: string can be divided between the internal \D+ repeat and the external
4876: * repeat in a large number of ways, and all have to be tried. (The
4877: example uses [!?] rather than a single character at the end, because
4878: both PCRE and Perl have an optimization that allows for fast failure
4879: when a single character is used. They remember the last single charac-
4880: ter that is required for a match, and fail early if it is not present
4881: in the string.) If the pattern is changed so that it uses an atomic
4882: group, like this:
4883:
4884: ((?>\D+)|<\d+>)*[!?]
4885:
4886: sequences of non-digits cannot be broken, and failure happens quickly.
4887:
4888:
4889: BACK REFERENCES
4890:
4891: Outside a character class, a backslash followed by a digit greater than
4892: 0 (and possibly further digits) is a back reference to a capturing sub-
4893: pattern earlier (that is, to its left) in the pattern, provided there
4894: have been that many previous capturing left parentheses.
4895:
4896: However, if the decimal number following the backslash is less than 10,
4897: it is always taken as a back reference, and causes an error only if
4898: there are not that many capturing left parentheses in the entire pat-
4899: tern. In other words, the parentheses that are referenced need not be
4900: to the left of the reference for numbers less than 10. A "forward back
4901: reference" of this type can make sense when a repetition is involved
4902: and the subpattern to the right has participated in an earlier itera-
4903: tion.
4904:
4905: It is not possible to have a numerical "forward back reference" to a
4906: subpattern whose number is 10 or more using this syntax because a
4907: sequence such as \50 is interpreted as a character defined in octal.
4908: See the subsection entitled "Non-printing characters" above for further
4909: details of the handling of digits following a backslash. There is no
4910: such problem when named parentheses are used. A back reference to any
4911: subpattern is possible using named parentheses (see below).
4912:
4913: Another way of avoiding the ambiguity inherent in the use of digits
4914: following a backslash is to use the \g escape sequence. This escape
4915: must be followed by an unsigned number or a negative number, optionally
4916: enclosed in braces. These examples are all identical:
4917:
4918: (ring), \1
4919: (ring), \g1
4920: (ring), \g{1}
4921:
4922: An unsigned number specifies an absolute reference without the ambigu-
4923: ity that is present in the older syntax. It is also useful when literal
4924: digits follow the reference. A negative number is a relative reference.
4925: Consider this example:
4926:
4927: (abc(def)ghi)\g{-1}
4928:
4929: The sequence \g{-1} is a reference to the most recently started captur-
4930: ing subpattern before \g, that is, is it equivalent to \2 in this exam-
4931: ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
4932: references can be helpful in long patterns, and also in patterns that
4933: are created by joining together fragments that contain references
4934: within themselves.
4935:
4936: A back reference matches whatever actually matched the capturing sub-
4937: pattern in the current subject string, rather than anything matching
4938: the subpattern itself (see "Subpatterns as subroutines" below for a way
4939: of doing that). So the pattern
4940:
4941: (sens|respons)e and \1ibility
4942:
4943: matches "sense and sensibility" and "response and responsibility", but
4944: not "sense and responsibility". If caseful matching is in force at the
4945: time of the back reference, the case of letters is relevant. For exam-
4946: ple,
4947:
4948: ((?i)rah)\s+\1
4949:
4950: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4951: original capturing subpattern is matched caselessly.
4952:
4953: There are several different ways of writing back references to named
4954: subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4955: \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4956: unified back reference syntax, in which \g can be used for both numeric
4957: and named references, is also supported. We could rewrite the above
4958: example in any of the following ways:
4959:
4960: (?<p1>(?i)rah)\s+\k<p1>
4961: (?'p1'(?i)rah)\s+\k{p1}
4962: (?P<p1>(?i)rah)\s+(?P=p1)
4963: (?<p1>(?i)rah)\s+\g{p1}
4964:
4965: A subpattern that is referenced by name may appear in the pattern
4966: before or after the reference.
4967:
4968: There may be more than one back reference to the same subpattern. If a
4969: subpattern has not actually been used in a particular match, any back
4970: references to it always fail by default. For example, the pattern
4971:
4972: (a|(bc))\2
4973:
4974: always fails if it starts to match "a" rather than "bc". However, if
4975: the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
4976: ence to an unset value matches an empty string.
4977:
4978: Because there may be many capturing parentheses in a pattern, all dig-
4979: its following a backslash are taken as part of a potential back refer-
4980: ence number. If the pattern continues with a digit character, some
4981: delimiter must be used to terminate the back reference. If the
4982: PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
4983: syntax or an empty comment (see "Comments" below) can be used.
4984:
4985: Recursive back references
4986:
4987: A back reference that occurs inside the parentheses to which it refers
4988: fails when the subpattern is first used, so, for example, (a\1) never
4989: matches. However, such references can be useful inside repeated sub-
4990: patterns. For example, the pattern
4991:
4992: (a|b\1)+
4993:
4994: matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4995: ation of the subpattern, the back reference matches the character
4996: string corresponding to the previous iteration. In order for this to
4997: work, the pattern must be such that the first iteration does not need
4998: to match the back reference. This can be done using alternation, as in
4999: the example above, or by a quantifier with a minimum of zero.
5000:
5001: Back references of this type cause the group that they reference to be
5002: treated as an atomic group. Once the whole group has been matched, a
5003: subsequent matching failure cannot cause backtracking into the middle
5004: of the group.
5005:
5006:
5007: ASSERTIONS
5008:
5009: An assertion is a test on the characters following or preceding the
5010: current matching point that does not actually consume any characters.
5011: The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
5012: described above.
5013:
5014: More complicated assertions are coded as subpatterns. There are two
5015: kinds: those that look ahead of the current position in the subject
5016: string, and those that look behind it. An assertion subpattern is
5017: matched in the normal way, except that it does not cause the current
5018: matching position to be changed.
5019:
5020: Assertion subpatterns are not capturing subpatterns. If such an asser-
5021: tion contains capturing subpatterns within it, these are counted for
5022: the purposes of numbering the capturing subpatterns in the whole pat-
5023: tern. However, substring capturing is carried out only for positive
5024: assertions, because it does not make sense for negative assertions.
5025:
5026: For compatibility with Perl, assertion subpatterns may be repeated;
5027: though it makes no sense to assert the same thing several times, the
5028: side effect of capturing parentheses may occasionally be useful. In
5029: practice, there only three cases:
5030:
5031: (1) If the quantifier is {0}, the assertion is never obeyed during
5032: matching. However, it may contain internal capturing parenthesized
5033: groups that are called from elsewhere via the subroutine mechanism.
5034:
5035: (2) If quantifier is {0,n} where n is greater than zero, it is treated
5036: as if it were {0,1}. At run time, the rest of the pattern match is
5037: tried with and without the assertion, the order depending on the greed-
5038: iness of the quantifier.
5039:
5040: (3) If the minimum repetition is greater than zero, the quantifier is
5041: ignored. The assertion is obeyed just once when encountered during
5042: matching.
5043:
5044: Lookahead assertions
5045:
5046: Lookahead assertions start with (?= for positive assertions and (?! for
5047: negative assertions. For example,
5048:
5049: \w+(?=;)
5050:
5051: matches a word followed by a semicolon, but does not include the semi-
5052: colon in the match, and
5053:
5054: foo(?!bar)
5055:
5056: matches any occurrence of "foo" that is not followed by "bar". Note
5057: that the apparently similar pattern
5058:
5059: (?!foo)bar
5060:
5061: does not find an occurrence of "bar" that is preceded by something
5062: other than "foo"; it finds any occurrence of "bar" whatsoever, because
5063: the assertion (?!foo) is always true when the next three characters are
5064: "bar". A lookbehind assertion is needed to achieve the other effect.
5065:
5066: If you want to force a matching failure at some point in a pattern, the
5067: most convenient way to do it is with (?!) because an empty string
5068: always matches, so an assertion that requires there not to be an empty
5069: string must always fail. The backtracking control verb (*FAIL) or (*F)
5070: is a synonym for (?!).
5071:
5072: Lookbehind assertions
5073:
5074: Lookbehind assertions start with (?<= for positive assertions and (?<!
5075: for negative assertions. For example,
5076:
5077: (?<!foo)bar
5078:
5079: does find an occurrence of "bar" that is not preceded by "foo". The
5080: contents of a lookbehind assertion are restricted such that all the
5081: strings it matches must have a fixed length. However, if there are sev-
5082: eral top-level alternatives, they do not all have to have the same
5083: fixed length. Thus
5084:
5085: (?<=bullock|donkey)
5086:
5087: is permitted, but
5088:
5089: (?<!dogs?|cats?)
5090:
5091: causes an error at compile time. Branches that match different length
5092: strings are permitted only at the top level of a lookbehind assertion.
5093: This is an extension compared with Perl, which requires all branches to
5094: match the same length of string. An assertion such as
5095:
5096: (?<=ab(c|de))
5097:
5098: is not permitted, because its single top-level branch can match two
5099: different lengths, but it is acceptable to PCRE if rewritten to use two
5100: top-level branches:
5101:
5102: (?<=abc|abde)
5103:
5104: In some cases, the escape sequence \K (see above) can be used instead
5105: of a lookbehind assertion to get round the fixed-length restriction.
5106:
5107: The implementation of lookbehind assertions is, for each alternative,
5108: to temporarily move the current position back by the fixed length and
5109: then try to match. If there are insufficient characters before the cur-
5110: rent position, the assertion fails.
5111:
5112: In UTF-8 mode, PCRE does not allow the \C escape (which matches a sin-
5113: gle byte, even in UTF-8 mode) to appear in lookbehind assertions,
5114: because it makes it impossible to calculate the length of the lookbe-
5115: hind. The \X and \R escapes, which can match different numbers of
5116: bytes, are also not permitted.
5117:
5118: "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
5119: lookbehinds, as long as the subpattern matches a fixed-length string.
5120: Recursion, however, is not supported.
5121:
5122: Possessive quantifiers can be used in conjunction with lookbehind
5123: assertions to specify efficient matching of fixed-length strings at the
5124: end of subject strings. Consider a simple pattern such as
5125:
5126: abcd$
5127:
5128: when applied to a long string that does not match. Because matching
5129: proceeds from left to right, PCRE will look for each "a" in the subject
5130: and then see if what follows matches the rest of the pattern. If the
5131: pattern is specified as
5132:
5133: ^.*abcd$
5134:
5135: the initial .* matches the entire string at first, but when this fails
5136: (because there is no following "a"), it backtracks to match all but the
5137: last character, then all but the last two characters, and so on. Once
5138: again the search for "a" covers the entire string, from right to left,
5139: so we are no better off. However, if the pattern is written as
5140:
5141: ^.*+(?<=abcd)
5142:
5143: there can be no backtracking for the .*+ item; it can match only the
5144: entire string. The subsequent lookbehind assertion does a single test
5145: on the last four characters. If it fails, the match fails immediately.
5146: For long strings, this approach makes a significant difference to the
5147: processing time.
5148:
5149: Using multiple assertions
5150:
5151: Several assertions (of any sort) may occur in succession. For example,
5152:
5153: (?<=\d{3})(?<!999)foo
5154:
5155: matches "foo" preceded by three digits that are not "999". Notice that
5156: each of the assertions is applied independently at the same point in
5157: the subject string. First there is a check that the previous three
5158: characters are all digits, and then there is a check that the same
5159: three characters are not "999". This pattern does not match "foo" pre-
5160: ceded by six characters, the first of which are digits and the last
5161: three of which are not "999". For example, it doesn't match "123abc-
5162: foo". A pattern to do that is
5163:
5164: (?<=\d{3}...)(?<!999)foo
5165:
5166: This time the first assertion looks at the preceding six characters,
5167: checking that the first three are digits, and then the second assertion
5168: checks that the preceding three characters are not "999".
5169:
5170: Assertions can be nested in any combination. For example,
5171:
5172: (?<=(?<!foo)bar)baz
5173:
5174: matches an occurrence of "baz" that is preceded by "bar" which in turn
5175: is not preceded by "foo", while
5176:
5177: (?<=\d{3}(?!999)...)foo
5178:
5179: is another pattern that matches "foo" preceded by three digits and any
5180: three characters that are not "999".
5181:
5182:
5183: CONDITIONAL SUBPATTERNS
5184:
5185: It is possible to cause the matching process to obey a subpattern con-
5186: ditionally or to choose between two alternative subpatterns, depending
5187: on the result of an assertion, or whether a specific capturing subpat-
5188: tern has already been matched. The two possible forms of conditional
5189: subpattern are:
5190:
5191: (?(condition)yes-pattern)
5192: (?(condition)yes-pattern|no-pattern)
5193:
5194: If the condition is satisfied, the yes-pattern is used; otherwise the
5195: no-pattern (if present) is used. If there are more than two alterna-
5196: tives in the subpattern, a compile-time error occurs. Each of the two
5197: alternatives may itself contain nested subpatterns of any form, includ-
5198: ing conditional subpatterns; the restriction to two alternatives
5199: applies only at the level of the condition. This pattern fragment is an
5200: example where the alternatives are complex:
5201:
5202: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5203:
5204:
5205: There are four kinds of condition: references to subpatterns, refer-
5206: ences to recursion, a pseudo-condition called DEFINE, and assertions.
5207:
5208: Checking for a used subpattern by number
5209:
5210: If the text between the parentheses consists of a sequence of digits,
5211: the condition is true if a capturing subpattern of that number has pre-
5212: viously matched. If there is more than one capturing subpattern with
5213: the same number (see the earlier section about duplicate subpattern
5214: numbers), the condition is true if any of them have matched. An alter-
5215: native notation is to precede the digits with a plus or minus sign. In
5216: this case, the subpattern number is relative rather than absolute. The
5217: most recently opened parentheses can be referenced by (?(-1), the next
5218: most recent by (?(-2), and so on. Inside loops it can also make sense
5219: to refer to subsequent groups. The next parentheses to be opened can be
5220: referenced as (?(+1), and so on. (The value zero in any of these forms
5221: is not used; it provokes a compile-time error.)
5222:
5223: Consider the following pattern, which contains non-significant white
5224: space to make it more readable (assume the PCRE_EXTENDED option) and to
5225: divide it into three parts for ease of discussion:
5226:
5227: ( \( )? [^()]+ (?(1) \) )
5228:
5229: The first part matches an optional opening parenthesis, and if that
5230: character is present, sets it as the first captured substring. The sec-
5231: ond part matches one or more characters that are not parentheses. The
5232: third part is a conditional subpattern that tests whether or not the
5233: first set of parentheses matched. If they did, that is, if subject
5234: started with an opening parenthesis, the condition is true, and so the
5235: yes-pattern is executed and a closing parenthesis is required. Other-
5236: wise, since no-pattern is not present, the subpattern matches nothing.
5237: In other words, this pattern matches a sequence of non-parentheses,
5238: optionally enclosed in parentheses.
5239:
5240: If you were embedding this pattern in a larger one, you could use a
5241: relative reference:
5242:
5243: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
5244:
5245: This makes the fragment independent of the parentheses in the larger
5246: pattern.
5247:
5248: Checking for a used subpattern by name
5249:
5250: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
5251: used subpattern by name. For compatibility with earlier versions of
5252: PCRE, which had this facility before Perl, the syntax (?(name)...) is
5253: also recognized. However, there is a possible ambiguity with this syn-
5254: tax, because subpattern names may consist entirely of digits. PCRE
5255: looks first for a named subpattern; if it cannot find one and the name
5256: consists entirely of digits, PCRE looks for a subpattern of that num-
5257: ber, which must be greater than zero. Using subpattern names that con-
5258: sist entirely of digits is not recommended.
5259:
5260: Rewriting the above example to use a named subpattern gives this:
5261:
5262: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
5263:
5264: If the name used in a condition of this kind is a duplicate, the test
5265: is applied to all subpatterns of the same name, and is true if any one
5266: of them has matched.
5267:
5268: Checking for pattern recursion
5269:
5270: If the condition is the string (R), and there is no subpattern with the
5271: name R, the condition is true if a recursive call to the whole pattern
5272: or any subpattern has been made. If digits or a name preceded by amper-
5273: sand follow the letter R, for example:
5274:
5275: (?(R3)...) or (?(R&name)...)
5276:
5277: the condition is true if the most recent recursion is into a subpattern
5278: whose number or name is given. This condition does not check the entire
5279: recursion stack. If the name used in a condition of this kind is a
5280: duplicate, the test is applied to all subpatterns of the same name, and
5281: is true if any one of them is the most recent recursion.
5282:
5283: At "top level", all these recursion test conditions are false. The
5284: syntax for recursive patterns is described below.
5285:
5286: Defining subpatterns for use by reference only
5287:
5288: If the condition is the string (DEFINE), and there is no subpattern
5289: with the name DEFINE, the condition is always false. In this case,
5290: there may be only one alternative in the subpattern. It is always
5291: skipped if control reaches this point in the pattern; the idea of
5292: DEFINE is that it can be used to define subroutines that can be refer-
5293: enced from elsewhere. (The use of subroutines is described below.) For
5294: example, a pattern to match an IPv4 address such as "192.168.23.245"
5295: could be written like this (ignore whitespace and line breaks):
5296:
5297: (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5298: \b (?&byte) (\.(?&byte)){3} \b
5299:
5300: The first part of the pattern is a DEFINE group inside which a another
5301: group named "byte" is defined. This matches an individual component of
5302: an IPv4 address (a number less than 256). When matching takes place,
5303: this part of the pattern is skipped because DEFINE acts like a false
5304: condition. The rest of the pattern uses references to the named group
5305: to match the four dot-separated components of an IPv4 address, insist-
5306: ing on a word boundary at each end.
5307:
5308: Assertion conditions
5309:
5310: If the condition is not in any of the above formats, it must be an
5311: assertion. This may be a positive or negative lookahead or lookbehind
5312: assertion. Consider this pattern, again containing non-significant
5313: white space, and with the two alternatives on the second line:
5314:
5315: (?(?=[^a-z]*[a-z])
5316: \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5317:
5318: The condition is a positive lookahead assertion that matches an
5319: optional sequence of non-letters followed by a letter. In other words,
5320: it tests for the presence of at least one letter in the subject. If a
5321: letter is found, the subject is matched against the first alternative;
5322: otherwise it is matched against the second. This pattern matches
5323: strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
5324: letters and dd are digits.
5325:
5326:
5327: COMMENTS
5328:
5329: There are two ways of including comments in patterns that are processed
5330: by PCRE. In both cases, the start of the comment must not be in a char-
5331: acter class, nor in the middle of any other sequence of related charac-
5332: ters such as (?: or a subpattern name or number. The characters that
5333: make up a comment play no part in the pattern matching.
5334:
5335: The sequence (?# marks the start of a comment that continues up to the
5336: next closing parenthesis. Nested parentheses are not permitted. If the
5337: PCRE_EXTENDED option is set, an unescaped # character also introduces a
5338: comment, which in this case continues to immediately after the next
5339: newline character or character sequence in the pattern. Which charac-
5340: ters are interpreted as newlines is controlled by the options passed to
5341: pcre_compile() or by a special sequence at the start of the pattern, as
5342: described in the section entitled "Newline conventions" above. Note
5343: that the end of this type of comment is a literal newline sequence in
5344: the pattern; escape sequences that happen to represent a newline do not
5345: count. For example, consider this pattern when PCRE_EXTENDED is set,
5346: and the default newline convention is in force:
5347:
5348: abc #comment \n still comment
5349:
5350: On encountering the # character, pcre_compile() skips along, looking
5351: for a newline in the pattern. The sequence \n is still literal at this
5352: stage, so it does not terminate the comment. Only an actual character
5353: with the code value 0x0a (the default newline) does so.
5354:
5355:
5356: RECURSIVE PATTERNS
5357:
5358: Consider the problem of matching a string in parentheses, allowing for
5359: unlimited nested parentheses. Without the use of recursion, the best
5360: that can be done is to use a pattern that matches up to some fixed
5361: depth of nesting. It is not possible to handle an arbitrary nesting
5362: depth.
5363:
5364: For some time, Perl has provided a facility that allows regular expres-
5365: sions to recurse (amongst other things). It does this by interpolating
5366: Perl code in the expression at run time, and the code can refer to the
5367: expression itself. A Perl pattern using code interpolation to solve the
5368: parentheses problem can be created like this:
5369:
5370: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
5371:
5372: The (?p{...}) item interpolates Perl code at run time, and in this case
5373: refers recursively to the pattern in which it appears.
5374:
5375: Obviously, PCRE cannot support the interpolation of Perl code. Instead,
5376: it supports special syntax for recursion of the entire pattern, and
5377: also for individual subpattern recursion. After its introduction in
5378: PCRE and Python, this kind of recursion was subsequently introduced
5379: into Perl at release 5.10.
5380:
5381: A special item that consists of (? followed by a number greater than
5382: zero and a closing parenthesis is a recursive subroutine call of the
5383: subpattern of the given number, provided that it occurs inside that
5384: subpattern. (If not, it is a non-recursive subroutine call, which is
5385: described in the next section.) The special item (?R) or (?0) is a
5386: recursive call of the entire regular expression.
5387:
5388: This PCRE pattern solves the nested parentheses problem (assume the
5389: PCRE_EXTENDED option is set so that white space is ignored):
5390:
5391: \( ( [^()]++ | (?R) )* \)
5392:
5393: First it matches an opening parenthesis. Then it matches any number of
5394: substrings which can either be a sequence of non-parentheses, or a
5395: recursive match of the pattern itself (that is, a correctly parenthe-
5396: sized substring). Finally there is a closing parenthesis. Note the use
5397: of a possessive quantifier to avoid backtracking into sequences of non-
5398: parentheses.
5399:
5400: If this were part of a larger pattern, you would not want to recurse
5401: the entire pattern, so instead you could use this:
5402:
5403: ( \( ( [^()]++ | (?1) )* \) )
5404:
5405: We have put the pattern into parentheses, and caused the recursion to
5406: refer to them instead of the whole pattern.
5407:
5408: In a larger pattern, keeping track of parenthesis numbers can be
5409: tricky. This is made easier by the use of relative references. Instead
5410: of (?1) in the pattern above you can write (?-2) to refer to the second
5411: most recently opened parentheses preceding the recursion. In other
5412: words, a negative number counts capturing parentheses leftwards from
5413: the point at which it is encountered.
5414:
5415: It is also possible to refer to subsequently opened parentheses, by
5416: writing references such as (?+2). However, these cannot be recursive
5417: because the reference is not inside the parentheses that are refer-
5418: enced. They are always non-recursive subroutine calls, as described in
5419: the next section.
5420:
5421: An alternative approach is to use named parentheses instead. The Perl
5422: syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5423: supported. We could rewrite the above example as follows:
5424:
5425: (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5426:
5427: If there is more than one subpattern with the same name, the earliest
5428: one is used.
5429:
5430: This particular example pattern that we have been looking at contains
5431: nested unlimited repeats, and so the use of a possessive quantifier for
5432: matching strings of non-parentheses is important when applying the pat-
5433: tern to strings that do not match. For example, when this pattern is
5434: applied to
5435:
5436: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5437:
5438: it yields "no match" quickly. However, if a possessive quantifier is
5439: not used, the match runs for a very long time indeed because there are
5440: so many different ways the + and * repeats can carve up the subject,
5441: and all have to be tested before failure can be reported.
5442:
5443: At the end of a match, the values of capturing parentheses are those
5444: from the outermost level. If you want to obtain intermediate values, a
5445: callout function can be used (see below and the pcrecallout documenta-
5446: tion). If the pattern above is matched against
5447:
5448: (ab(cd)ef)
5449:
5450: the value for the inner capturing parentheses (numbered 2) is "ef",
5451: which is the last value taken on at the top level. If a capturing sub-
5452: pattern is not matched at the top level, its final captured value is
5453: unset, even if it was (temporarily) set at a deeper level during the
5454: matching process.
5455:
5456: If there are more than 15 capturing parentheses in a pattern, PCRE has
5457: to obtain extra memory to store data during a recursion, which it does
5458: by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5459: can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5460:
5461: Do not confuse the (?R) item with the condition (R), which tests for
5462: recursion. Consider this pattern, which matches text in angle brack-
5463: ets, allowing for arbitrary nesting. Only digits are allowed in nested
5464: brackets (that is, when recursing), whereas any characters are permit-
5465: ted at the outer level.
5466:
5467: < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5468:
5469: In this pattern, (?(R) is the start of a conditional subpattern, with
5470: two different alternatives for the recursive and non-recursive cases.
5471: The (?R) item is the actual recursive call.
5472:
5473: Differences in recursion processing between PCRE and Perl
5474:
5475: Recursion processing in PCRE differs from Perl in two important ways.
5476: In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5477: always treated as an atomic group. That is, once it has matched some of
5478: the subject string, it is never re-entered, even if it contains untried
5479: alternatives and there is a subsequent matching failure. This can be
5480: illustrated by the following pattern, which purports to match a palin-
5481: dromic string that contains an odd number of characters (for example,
5482: "a", "aba", "abcba", "abcdcba"):
5483:
5484: ^(.|(.)(?1)\2)$
5485:
5486: The idea is that it either matches a single character, or two identical
5487: characters surrounding a sub-palindrome. In Perl, this pattern works;
5488: in PCRE it does not if the pattern is longer than three characters.
5489: Consider the subject string "abcba":
5490:
5491: At the top level, the first character is matched, but as it is not at
5492: the end of the string, the first alternative fails; the second alterna-
5493: tive is taken and the recursion kicks in. The recursive call to subpat-
5494: tern 1 successfully matches the next character ("b"). (Note that the
5495: beginning and end of line tests are not part of the recursion).
5496:
5497: Back at the top level, the next character ("c") is compared with what
5498: subpattern 2 matched, which was "a". This fails. Because the recursion
5499: is treated as an atomic group, there are now no backtracking points,
5500: and so the entire match fails. (Perl is able, at this point, to re-
5501: enter the recursion and try the second alternative.) However, if the
5502: pattern is written with the alternatives in the other order, things are
5503: different:
5504:
5505: ^((.)(?1)\2|.)$
5506:
5507: This time, the recursing alternative is tried first, and continues to
5508: recurse until it runs out of characters, at which point the recursion
5509: fails. But this time we do have another alternative to try at the
5510: higher level. That is the big difference: in the previous case the
5511: remaining alternative is at a deeper recursion level, which PCRE cannot
5512: use.
5513:
5514: To change the pattern so that it matches all palindromic strings, not
5515: just those with an odd number of characters, it is tempting to change
5516: the pattern to this:
5517:
5518: ^((.)(?1)\2|.?)$
5519:
5520: Again, this works in Perl, but not in PCRE, and for the same reason.
5521: When a deeper recursion has matched a single character, it cannot be
5522: entered again in order to match an empty string. The solution is to
5523: separate the two cases, and write out the odd and even cases as alter-
5524: natives at the higher level:
5525:
5526: ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
5527:
5528: If you want to match typical palindromic phrases, the pattern has to
5529: ignore all non-word characters, which can be done like this:
5530:
5531: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
5532:
5533: If run with the PCRE_CASELESS option, this pattern matches phrases such
5534: as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
5535: Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
5536: ing into sequences of non-word characters. Without this, PCRE takes a
5537: great deal longer (ten times or more) to match typical phrases, and
5538: Perl takes so long that you think it has gone into a loop.
5539:
5540: WARNING: The palindrome-matching patterns above work only if the sub-
5541: ject string does not start with a palindrome that is shorter than the
5542: entire string. For example, although "abcba" is correctly matched, if
5543: the subject is "ababa", PCRE finds the palindrome "aba" at the start,
5544: then fails at top level because the end of the string does not follow.
5545: Once again, it cannot jump back into the recursion to try other alter-
5546: natives, so the entire match fails.
5547:
5548: The second way in which PCRE and Perl differ in their recursion pro-
5549: cessing is in the handling of captured values. In Perl, when a subpat-
5550: tern is called recursively or as a subpattern (see the next section),
5551: it has no access to any values that were captured outside the recur-
5552: sion, whereas in PCRE these values can be referenced. Consider this
5553: pattern:
5554:
5555: ^(.)(\1|a(?2))
5556:
5557: In PCRE, this pattern matches "bab". The first capturing parentheses
5558: match "b", then in the second group, when the back reference \1 fails
5559: to match "b", the second alternative matches "a" and then recurses. In
5560: the recursion, \1 does now match "b" and so the whole match succeeds.
5561: In Perl, the pattern fails to match because inside the recursive call
5562: \1 cannot access the externally set value.
5563:
5564:
5565: SUBPATTERNS AS SUBROUTINES
5566:
5567: If the syntax for a recursive subpattern call (either by number or by
5568: name) is used outside the parentheses to which it refers, it operates
5569: like a subroutine in a programming language. The called subpattern may
5570: be defined before or after the reference. A numbered reference can be
5571: absolute or relative, as in these examples:
5572:
5573: (...(absolute)...)...(?2)...
5574: (...(relative)...)...(?-1)...
5575: (...(?+1)...(relative)...
5576:
5577: An earlier example pointed out that the pattern
5578:
5579: (sens|respons)e and \1ibility
5580:
5581: matches "sense and sensibility" and "response and responsibility", but
5582: not "sense and responsibility". If instead the pattern
5583:
5584: (sens|respons)e and (?1)ibility
5585:
5586: is used, it does match "sense and responsibility" as well as the other
5587: two strings. Another example is given in the discussion of DEFINE
5588: above.
5589:
5590: All subroutine calls, whether recursive or not, are always treated as
5591: atomic groups. That is, once a subroutine has matched some of the sub-
5592: ject string, it is never re-entered, even if it contains untried alter-
5593: natives and there is a subsequent matching failure. Any capturing
5594: parentheses that are set during the subroutine call revert to their
5595: previous values afterwards.
5596:
5597: Processing options such as case-independence are fixed when a subpat-
5598: tern is defined, so if it is used as a subroutine, such options cannot
5599: be changed for different calls. For example, consider this pattern:
5600:
5601: (abc)(?i:(?-1))
5602:
5603: It matches "abcabc". It does not match "abcABC" because the change of
5604: processing option does not affect the called subpattern.
5605:
5606:
5607: ONIGURUMA SUBROUTINE SYNTAX
5608:
5609: For compatibility with Oniguruma, the non-Perl syntax \g followed by a
5610: name or a number enclosed either in angle brackets or single quotes, is
5611: an alternative syntax for referencing a subpattern as a subroutine,
5612: possibly recursively. Here are two of the examples used above, rewrit-
5613: ten using this syntax:
5614:
5615: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
5616: (sens|respons)e and \g'1'ibility
5617:
5618: PCRE supports an extension to Oniguruma: if a number is preceded by a
5619: plus or a minus sign it is taken as a relative reference. For example:
5620:
5621: (abc)(?i:\g<-1>)
5622:
5623: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
5624: synonymous. The former is a back reference; the latter is a subroutine
5625: call.
5626:
5627:
5628: CALLOUTS
5629:
5630: Perl has a feature whereby using the sequence (?{...}) causes arbitrary
5631: Perl code to be obeyed in the middle of matching a regular expression.
5632: This makes it possible, amongst other things, to extract different sub-
5633: strings that match the same pair of parentheses when there is a repeti-
5634: tion.
5635:
5636: PCRE provides a similar feature, but of course it cannot obey arbitrary
5637: Perl code. The feature is called "callout". The caller of PCRE provides
5638: an external function by putting its entry point in the global variable
5639: pcre_callout. By default, this variable contains NULL, which disables
5640: all calling out.
5641:
5642: Within a regular expression, (?C) indicates the points at which the
5643: external function is to be called. If you want to identify different
5644: callout points, you can put a number less than 256 after the letter C.
5645: The default value is zero. For example, this pattern has two callout
5646: points:
5647:
5648: (?C1)abc(?C2)def
5649:
5650: If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
5651: automatically installed before each item in the pattern. They are all
5652: numbered 255.
5653:
5654: During matching, when PCRE reaches a callout point (and pcre_callout is
5655: set), the external function is called. It is provided with the number
5656: of the callout, the position in the pattern, and, optionally, one item
5657: of data originally supplied by the caller of pcre_exec(). The callout
5658: function may cause matching to proceed, to backtrack, or to fail alto-
5659: gether. A complete description of the interface to the callout function
5660: is given in the pcrecallout documentation.
5661:
5662:
5663: BACKTRACKING CONTROL
5664:
5665: Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
5666: which are described in the Perl documentation as "experimental and sub-
5667: ject to change or removal in a future version of Perl". It goes on to
5668: say: "Their usage in production code should be noted to avoid problems
5669: during upgrades." The same remarks apply to the PCRE features described
5670: in this section.
5671:
5672: Since these verbs are specifically related to backtracking, most of
5673: them can be used only when the pattern is to be matched using
5674: pcre_exec(), which uses a backtracking algorithm. With the exception of
5675: (*FAIL), which behaves like a failing negative assertion, they cause an
5676: error if encountered by pcre_dfa_exec().
5677:
5678: If any of these verbs are used in an assertion or in a subpattern that
5679: is called as a subroutine (whether or not recursively), their effect is
5680: confined to that subpattern; it does not extend to the surrounding pat-
5681: tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
5682: that is encountered in a successful positive assertion is passed back
5683: when a match succeeds (compare capturing parentheses in assertions).
5684: Note that such subpatterns are processed as anchored at the point where
5685: they are tested. Note also that Perl's treatment of subroutines is dif-
5686: ferent in some cases.
5687:
5688: The new verbs make use of what was previously invalid syntax: an open-
5689: ing parenthesis followed by an asterisk. They are generally of the form
5690: (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
5691: haviour, depending on whether or not an argument is present. A name is
5692: any sequence of characters that does not include a closing parenthesis.
5693: If the name is empty, that is, if the closing parenthesis immediately
5694: follows the colon, the effect is as if the colon were not there. Any
5695: number of these verbs may occur in a pattern.
5696:
5697: PCRE contains some optimizations that are used to speed up matching by
5698: running some checks at the start of each match attempt. For example, it
5699: may know the minimum length of matching subject, or that a particular
5700: character must be present. When one of these optimizations suppresses
5701: the running of a match, any included backtracking verbs will not, of
5702: course, be processed. You can suppress the start-of-match optimizations
5703: by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
5704: pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
5705:
5706: Experiments with Perl suggest that it too has similar optimizations,
5707: sometimes leading to anomalous results.
5708:
5709: Verbs that act immediately
5710:
5711: The following verbs act as soon as they are encountered. They may not
5712: be followed by a name.
5713:
5714: (*ACCEPT)
5715:
5716: This verb causes the match to end successfully, skipping the remainder
5717: of the pattern. However, when it is inside a subpattern that is called
5718: as a subroutine, only that subpattern is ended successfully. Matching
5719: then continues at the outer level. If (*ACCEPT) is inside capturing
5720: parentheses, the data so far is captured. For example:
5721:
5722: A((?:A|B(*ACCEPT)|C)D)
5723:
5724: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
5725: tured by the outer parentheses.
5726:
5727: (*FAIL) or (*F)
5728:
5729: This verb causes a matching failure, forcing backtracking to occur. It
5730: is equivalent to (?!) but easier to read. The Perl documentation notes
5731: that it is probably useful only when combined with (?{}) or (??{}).
5732: Those are, of course, Perl features that are not present in PCRE. The
5733: nearest equivalent is the callout feature, as for example in this pat-
5734: tern:
5735:
5736: a+(?C)(*FAIL)
5737:
5738: A match with the string "aaaa" always fails, but the callout is taken
5739: before each backtrack happens (in this example, 10 times).
5740:
5741: Recording which path was taken
5742:
5743: There is one verb whose main purpose is to track how a match was
5744: arrived at, though it also has a secondary use in conjunction with
5745: advancing the match starting point (see (*SKIP) below).
5746:
5747: (*MARK:NAME) or (*:NAME)
5748:
5749: A name is always required with this verb. There may be as many
5750: instances of (*MARK) as you like in a pattern, and their names do not
5751: have to be unique.
5752:
5753: When a match succeeds, the name of the last-encountered (*MARK) on the
5754: matching path is passed back to the caller via the pcre_extra data
5755: structure, as described in the section on pcre_extra in the pcreapi
5756: documentation. Here is an example of pcretest output, where the /K mod-
5757: ifier requests the retrieval and outputting of (*MARK) data:
5758:
5759: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
5760: data> XY
5761: 0: XY
5762: MK: A
5763: XZ
5764: 0: XZ
5765: MK: B
5766:
5767: The (*MARK) name is tagged with "MK:" in this output, and in this exam-
5768: ple it indicates which of the two alternatives matched. This is a more
5769: efficient way of obtaining this information than putting each alterna-
5770: tive in its own capturing parentheses.
5771:
5772: If (*MARK) is encountered in a positive assertion, its name is recorded
5773: and passed back if it is the last-encountered. This does not happen for
5774: negative assertions.
5775:
5776: After a partial match or a failed match, the name of the last encoun-
5777: tered (*MARK) in the entire match process is returned. For example:
5778:
5779: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
5780: data> XP
5781: No match, mark = B
5782:
5783: Note that in this unanchored example the mark is retained from the
5784: match attempt that started at the letter "X". Subsequent match attempts
5785: starting at "P" and then with an empty string do not get as far as the
5786: (*MARK) item, but nevertheless do not reset it.
5787:
5788: Verbs that act after backtracking
5789:
5790: The following verbs do nothing when they are encountered. Matching con-
5791: tinues with what follows, but if there is no subsequent match, causing
5792: a backtrack to the verb, a failure is forced. That is, backtracking
5793: cannot pass to the left of the verb. However, when one of these verbs
5794: appears inside an atomic group, its effect is confined to that group,
5795: because once the group has been matched, there is never any backtrack-
5796: ing into it. In this situation, backtracking can "jump back" to the
5797: left of the entire atomic group. (Remember also, as stated above, that
5798: this localization also applies in subroutine calls and assertions.)
5799:
5800: These verbs differ in exactly what kind of failure occurs when back-
5801: tracking reaches them.
5802:
5803: (*COMMIT)
5804:
5805: This verb, which may not be followed by a name, causes the whole match
5806: to fail outright if the rest of the pattern does not match. Even if the
5807: pattern is unanchored, no further attempts to find a match by advancing
5808: the starting point take place. Once (*COMMIT) has been passed,
5809: pcre_exec() is committed to finding a match at the current starting
5810: point, or not at all. For example:
5811:
5812: a+(*COMMIT)b
5813:
5814: This matches "xxaab" but not "aacaab". It can be thought of as a kind
5815: of dynamic anchor, or "I've started, so I must finish." The name of the
5816: most recently passed (*MARK) in the path is passed back when (*COMMIT)
5817: forces a match failure.
5818:
5819: Note that (*COMMIT) at the start of a pattern is not the same as an
5820: anchor, unless PCRE's start-of-match optimizations are turned off, as
5821: shown in this pcretest example:
5822:
5823: re> /(*COMMIT)abc/
5824: data> xyzabc
5825: 0: abc
5826: xyzabc\Y
5827: No match
5828:
5829: PCRE knows that any match must start with "a", so the optimization
5830: skips along the subject to "a" before running the first match attempt,
5831: which succeeds. When the optimization is disabled by the \Y escape in
5832: the second subject, the match starts at "x" and so the (*COMMIT) causes
5833: it to fail without trying any other starting points.
5834:
5835: (*PRUNE) or (*PRUNE:NAME)
5836:
5837: This verb causes the match to fail at the current starting position in
5838: the subject if the rest of the pattern does not match. If the pattern
5839: is unanchored, the normal "bumpalong" advance to the next starting
5840: character then happens. Backtracking can occur as usual to the left of
5841: (*PRUNE), before it is reached, or when matching to the right of
5842: (*PRUNE), but if there is no match to the right, backtracking cannot
5843: cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
5844: native to an atomic group or possessive quantifier, but there are some
5845: uses of (*PRUNE) that cannot be expressed in any other way. The behav-
5846: iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
5847: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
5848:
5849: (*SKIP)
5850:
5851: This verb, when given without a name, is like (*PRUNE), except that if
5852: the pattern is unanchored, the "bumpalong" advance is not to the next
5853: character, but to the position in the subject where (*SKIP) was encoun-
5854: tered. (*SKIP) signifies that whatever text was matched leading up to
5855: it cannot be part of a successful match. Consider:
5856:
5857: a+(*SKIP)b
5858:
5859: If the subject is "aaaac...", after the first match attempt fails
5860: (starting at the first character in the string), the starting point
5861: skips on to start the next attempt at "c". Note that a possessive quan-
5862: tifer does not have the same effect as this example; although it would
5863: suppress backtracking during the first match attempt, the second
5864: attempt would start at the second character instead of skipping on to
5865: "c".
5866:
5867: (*SKIP:NAME)
5868:
5869: When (*SKIP) has an associated name, its behaviour is modified. If the
5870: following pattern fails to match, the previous path through the pattern
5871: is searched for the most recent (*MARK) that has the same name. If one
5872: is found, the "bumpalong" advance is to the subject position that cor-
5873: responds to that (*MARK) instead of to where (*SKIP) was encountered.
5874: If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
5875:
5876: (*THEN) or (*THEN:NAME)
5877:
5878: This verb causes a skip to the next innermost alternative if the rest
5879: of the pattern does not match. That is, it cancels pending backtrack-
5880: ing, but only within the current alternative. Its name comes from the
5881: observation that it can be used for a pattern-based if-then-else block:
5882:
5883: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
5884:
5885: If the COND1 pattern matches, FOO is tried (and possibly further items
5886: after the end of the group if FOO succeeds); on failure, the matcher
5887: skips to the second alternative and tries COND2, without backtracking
5888: into COND1. The behaviour of (*THEN:NAME) is exactly the same as
5889: (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts
5890: like (*PRUNE).
5891:
5892: Note that a subpattern that does not contain a | character is just a
5893: part of the enclosing alternative; it is not a nested alternation with
5894: only one alternative. The effect of (*THEN) extends beyond such a sub-
5895: pattern to the enclosing alternative. Consider this pattern, where A,
5896: B, etc. are complex pattern fragments that do not contain any | charac-
5897: ters at this level:
5898:
5899: A (B(*THEN)C) | D
5900:
5901: If A and B are matched, but there is a failure in C, matching does not
5902: backtrack into A; instead it moves to the next alternative, that is, D.
5903: However, if the subpattern containing (*THEN) is given an alternative,
5904: it behaves differently:
5905:
5906: A (B(*THEN)C | (*FAIL)) | D
5907:
5908: The effect of (*THEN) is now confined to the inner subpattern. After a
5909: failure in C, matching moves to (*FAIL), which causes the whole subpat-
5910: tern to fail because there are no more alternatives to try. In this
5911: case, matching does now backtrack into A.
5912:
5913: Note also that a conditional subpattern is not considered as having two
5914: alternatives, because only one is ever used. In other words, the |
5915: character in a conditional subpattern has a different meaning. Ignoring
5916: white space, consider:
5917:
5918: ^.*? (?(?=a) a | b(*THEN)c )
5919:
5920: If the subject is "ba", this pattern does not match. Because .*? is
5921: ungreedy, it initially matches zero characters. The condition (?=a)
5922: then fails, the character "b" is matched, but "c" is not. At this
5923: point, matching does not backtrack to .*? as might perhaps be expected
5924: from the presence of the | character. The conditional subpattern is
5925: part of the single alternative that comprises the whole pattern, and so
5926: the match fails. (If there was a backtrack into .*?, allowing it to
5927: match "b", the match would succeed.)
5928:
5929: The verbs just described provide four different "strengths" of control
5930: when subsequent matching fails. (*THEN) is the weakest, carrying on the
5931: match at the next alternative. (*PRUNE) comes next, failing the match
5932: at the current starting position, but allowing an advance to the next
5933: character (for an unanchored pattern). (*SKIP) is similar, except that
5934: the advance may be more than one character. (*COMMIT) is the strongest,
5935: causing the entire match to fail.
5936:
5937: If more than one such verb is present in a pattern, the "strongest" one
5938: wins. For example, consider this pattern, where A, B, etc. are complex
5939: pattern fragments:
5940:
5941: (A(*COMMIT)B(*THEN)C|D)
5942:
5943: Once A has matched, PCRE is committed to this match, at the current
5944: starting position. If subsequently B matches, but C does not, the nor-
5945: mal (*THEN) action of trying the next alternative (that is, D) does not
5946: happen because (*COMMIT) overrides.
5947:
5948:
5949: SEE ALSO
5950:
5951: pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).
5952:
5953:
5954: AUTHOR
5955:
5956: Philip Hazel
5957: University Computing Service
5958: Cambridge CB2 3QH, England.
5959:
5960:
5961: REVISION
5962:
5963: Last updated: 29 November 2011
5964: Copyright (c) 1997-2011 University of Cambridge.
5965: ------------------------------------------------------------------------------
5966:
5967:
5968: PCRESYNTAX(3) PCRESYNTAX(3)
5969:
5970:
5971: NAME
5972: PCRE - Perl-compatible regular expressions
5973:
5974:
5975: PCRE REGULAR EXPRESSION SYNTAX SUMMARY
5976:
5977: The full syntax and semantics of the regular expressions that are sup-
5978: ported by PCRE are described in the pcrepattern documentation. This
5979: document contains just a quick-reference summary of the syntax.
5980:
5981:
5982: QUOTING
5983:
5984: \x where x is non-alphanumeric is a literal x
5985: \Q...\E treat enclosed characters as literal
5986:
5987:
5988: CHARACTERS
5989:
5990: \a alarm, that is, the BEL character (hex 07)
5991: \cx "control-x", where x is any ASCII character
5992: \e escape (hex 1B)
5993: \f formfeed (hex 0C)
5994: \n newline (hex 0A)
5995: \r carriage return (hex 0D)
5996: \t tab (hex 09)
5997: \ddd character with octal code ddd, or backreference
5998: \xhh character with hex code hh
5999: \x{hhh..} character with hex code hhh..
6000:
6001:
6002: CHARACTER TYPES
6003:
6004: . any character except newline;
6005: in dotall mode, any character whatsoever
6006: \C one byte, even in UTF-8 mode (best avoided)
6007: \d a decimal digit
6008: \D a character that is not a decimal digit
6009: \h a horizontal whitespace character
6010: \H a character that is not a horizontal whitespace character
6011: \N a character that is not a newline
6012: \p{xx} a character with the xx property
6013: \P{xx} a character without the xx property
6014: \R a newline sequence
6015: \s a whitespace character
6016: \S a character that is not a whitespace character
6017: \v a vertical whitespace character
6018: \V a character that is not a vertical whitespace character
6019: \w a "word" character
6020: \W a "non-word" character
6021: \X an extended Unicode sequence
6022:
6023: In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII
6024: characters, even in UTF-8 mode. However, this can be changed by setting
6025: the PCRE_UCP option.
6026:
6027:
6028: GENERAL CATEGORY PROPERTIES FOR \p and \P
6029:
6030: C Other
6031: Cc Control
6032: Cf Format
6033: Cn Unassigned
6034: Co Private use
6035: Cs Surrogate
6036:
6037: L Letter
6038: Ll Lower case letter
6039: Lm Modifier letter
6040: Lo Other letter
6041: Lt Title case letter
6042: Lu Upper case letter
6043: L& Ll, Lu, or Lt
6044:
6045: M Mark
6046: Mc Spacing mark
6047: Me Enclosing mark
6048: Mn Non-spacing mark
6049:
6050: N Number
6051: Nd Decimal number
6052: Nl Letter number
6053: No Other number
6054:
6055: P Punctuation
6056: Pc Connector punctuation
6057: Pd Dash punctuation
6058: Pe Close punctuation
6059: Pf Final punctuation
6060: Pi Initial punctuation
6061: Po Other punctuation
6062: Ps Open punctuation
6063:
6064: S Symbol
6065: Sc Currency symbol
6066: Sk Modifier symbol
6067: Sm Mathematical symbol
6068: So Other symbol
6069:
6070: Z Separator
6071: Zl Line separator
6072: Zp Paragraph separator
6073: Zs Space separator
6074:
6075:
6076: PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P
6077:
6078: Xan Alphanumeric: union of properties L and N
6079: Xps POSIX space: property Z or tab, NL, VT, FF, CR
6080: Xsp Perl space: property Z or tab, NL, FF, CR
6081: Xwd Perl word: property Xan or underscore
6082:
6083:
6084: SCRIPT NAMES FOR \p AND \P
6085:
6086: Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
6087: Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
6088: Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
6089: tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
6090: Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
6091: rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
6092: Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
6093: Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
6094: Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
6095: Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
6096: Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
6097: Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
6098: Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
6099: Ugaritic, Vai, Yi.
6100:
6101:
6102: CHARACTER CLASSES
6103:
6104: [...] positive character class
6105: [^...] negative character class
6106: [x-y] range (can be used for hex characters)
6107: [[:xxx:]] positive POSIX named set
6108: [[:^xxx:]] negative POSIX named set
6109:
6110: alnum alphanumeric
6111: alpha alphabetic
6112: ascii 0-127
6113: blank space or tab
6114: cntrl control character
6115: digit decimal digit
6116: graph printing, excluding space
6117: lower lower case letter
6118: print printing, including space
6119: punct printing, excluding alphanumeric
6120: space whitespace
6121: upper upper case letter
6122: word same as \w
6123: xdigit hexadecimal digit
6124:
6125: In PCRE, POSIX character set names recognize only ASCII characters by
6126: default, but some of them use Unicode properties if PCRE_UCP is set.
6127: You can use \Q...\E inside a character class.
6128:
6129:
6130: QUANTIFIERS
6131:
6132: ? 0 or 1, greedy
6133: ?+ 0 or 1, possessive
6134: ?? 0 or 1, lazy
6135: * 0 or more, greedy
6136: *+ 0 or more, possessive
6137: *? 0 or more, lazy
6138: + 1 or more, greedy
6139: ++ 1 or more, possessive
6140: +? 1 or more, lazy
6141: {n} exactly n
6142: {n,m} at least n, no more than m, greedy
6143: {n,m}+ at least n, no more than m, possessive
6144: {n,m}? at least n, no more than m, lazy
6145: {n,} n or more, greedy
6146: {n,}+ n or more, possessive
6147: {n,}? n or more, lazy
6148:
6149:
6150: ANCHORS AND SIMPLE ASSERTIONS
6151:
6152: \b word boundary
6153: \B not a word boundary
6154: ^ start of subject
6155: also after internal newline in multiline mode
6156: \A start of subject
6157: $ end of subject
6158: also before newline at end of subject
6159: also before internal newline in multiline mode
6160: \Z end of subject
6161: also before newline at end of subject
6162: \z end of subject
6163: \G first matching position in subject
6164:
6165:
6166: MATCH POINT RESET
6167:
6168: \K reset start of match
6169:
6170:
6171: ALTERNATION
6172:
6173: expr|expr|expr...
6174:
6175:
6176: CAPTURING
6177:
6178: (...) capturing group
6179: (?<name>...) named capturing group (Perl)
6180: (?'name'...) named capturing group (Perl)
6181: (?P<name>...) named capturing group (Python)
6182: (?:...) non-capturing group
6183: (?|...) non-capturing group; reset group numbers for
6184: capturing groups in each alternative
6185:
6186:
6187: ATOMIC GROUPS
6188:
6189: (?>...) atomic, non-capturing group
6190:
6191:
6192: COMMENT
6193:
6194: (?#....) comment (not nestable)
6195:
6196:
6197: OPTION SETTING
6198:
6199: (?i) caseless
6200: (?J) allow duplicate names
6201: (?m) multiline
6202: (?s) single line (dotall)
6203: (?U) default ungreedy (lazy)
6204: (?x) extended (ignore white space)
6205: (?-...) unset option(s)
6206:
6207: The following are recognized only at the start of a pattern or after
6208: one of the newline-setting options with similar syntax:
6209:
6210: (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
6211: (*UTF8) set UTF-8 mode (PCRE_UTF8)
6212: (*UCP) set PCRE_UCP (use Unicode properties for \d etc)
6213:
6214:
6215: LOOKAHEAD AND LOOKBEHIND ASSERTIONS
6216:
6217: (?=...) positive look ahead
6218: (?!...) negative look ahead
6219: (?<=...) positive look behind
6220: (?<!...) negative look behind
6221:
6222: Each top-level branch of a look behind must be of a fixed length.
6223:
6224:
6225: BACKREFERENCES
6226:
6227: \n reference by number (can be ambiguous)
6228: \gn reference by number
6229: \g{n} reference by number
6230: \g{-n} relative reference by number
6231: \k<name> reference by name (Perl)
6232: \k'name' reference by name (Perl)
6233: \g{name} reference by name (Perl)
6234: \k{name} reference by name (.NET)
6235: (?P=name) reference by name (Python)
6236:
6237:
6238: SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
6239:
6240: (?R) recurse whole pattern
6241: (?n) call subpattern by absolute number
6242: (?+n) call subpattern by relative number
6243: (?-n) call subpattern by relative number
6244: (?&name) call subpattern by name (Perl)
6245: (?P>name) call subpattern by name (Python)
6246: \g<name> call subpattern by name (Oniguruma)
6247: \g'name' call subpattern by name (Oniguruma)
6248: \g<n> call subpattern by absolute number (Oniguruma)
6249: \g'n' call subpattern by absolute number (Oniguruma)
6250: \g<+n> call subpattern by relative number (PCRE extension)
6251: \g'+n' call subpattern by relative number (PCRE extension)
6252: \g<-n> call subpattern by relative number (PCRE extension)
6253: \g'-n' call subpattern by relative number (PCRE extension)
6254:
6255:
6256: CONDITIONAL PATTERNS
6257:
6258: (?(condition)yes-pattern)
6259: (?(condition)yes-pattern|no-pattern)
6260:
6261: (?(n)... absolute reference condition
6262: (?(+n)... relative reference condition
6263: (?(-n)... relative reference condition
6264: (?(<name>)... named reference condition (Perl)
6265: (?('name')... named reference condition (Perl)
6266: (?(name)... named reference condition (PCRE)
6267: (?(R)... overall recursion condition
6268: (?(Rn)... specific group recursion condition
6269: (?(R&name)... specific recursion condition
6270: (?(DEFINE)... define subpattern for reference
6271: (?(assert)... assertion condition
6272:
6273:
6274: BACKTRACKING CONTROL
6275:
6276: The following act immediately they are reached:
6277:
6278: (*ACCEPT) force successful match
6279: (*FAIL) force backtrack; synonym (*F)
6280:
6281: The following act only when a subsequent match failure causes a back-
6282: track to reach them. They all force a match failure, but they differ in
6283: what happens afterwards. Those that advance the start-of-match point do
6284: so only if the pattern is not anchored.
6285:
6286: (*COMMIT) overall failure, no advance of starting point
6287: (*PRUNE) advance to next starting character
6288: (*SKIP) advance start to current matching position
6289: (*THEN) local failure, backtrack to next alternation
6290:
6291:
6292: NEWLINE CONVENTIONS
6293:
6294: These are recognized only at the very start of the pattern or after a
6295: (*BSR_...) or (*UTF8) or (*UCP) option.
6296:
6297: (*CR) carriage return only
6298: (*LF) linefeed only
6299: (*CRLF) carriage return followed by linefeed
6300: (*ANYCRLF) all three of the above
6301: (*ANY) any Unicode newline sequence
6302:
6303:
6304: WHAT \R MATCHES
6305:
6306: These are recognized only at the very start of the pattern or after a
6307: (*...) option that sets the newline convention or UTF-8 or UCP mode.
6308:
6309: (*BSR_ANYCRLF) CR, LF, or CRLF
6310: (*BSR_UNICODE) any Unicode newline sequence
6311:
6312:
6313: CALLOUTS
6314:
6315: (?C) callout
6316: (?Cn) callout with data n
6317:
6318:
6319: SEE ALSO
6320:
6321: pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
6322:
6323:
6324: AUTHOR
6325:
6326: Philip Hazel
6327: University Computing Service
6328: Cambridge CB2 3QH, England.
6329:
6330:
6331: REVISION
6332:
6333: Last updated: 21 November 2010
6334: Copyright (c) 1997-2010 University of Cambridge.
6335: ------------------------------------------------------------------------------
6336:
6337:
6338: PCREUNICODE(3) PCREUNICODE(3)
6339:
6340:
6341: NAME
6342: PCRE - Perl-compatible regular expressions
6343:
6344:
6345: UTF-8 AND UNICODE PROPERTY SUPPORT
6346:
6347: In order process UTF-8 strings, you must build PCRE to include UTF-8
6348: support in the code, and, in addition, you must call pcre_compile()
6349: with the PCRE_UTF8 option flag, or the pattern must start with the
6350: sequence (*UTF8). When either of these is the case, both the pattern
6351: and any subject strings that are matched against it are treated as
6352: UTF-8 strings instead of strings of 1-byte characters. PCRE does not
6353: support any other formats (in particular, it does not support UTF-16).
6354:
6355: If you compile PCRE with UTF-8 support, but do not use it at run time,
6356: the library will be a bit bigger, but the additional run time overhead
6357: is limited to testing the PCRE_UTF8 flag occasionally, so should not be
6358: very big.
6359:
6360: If PCRE is built with Unicode character property support (which implies
6361: UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
6362: ported. The available properties that can be tested are limited to the
6363: general category properties such as Lu for an upper case letter or Nd
6364: for a decimal number, the Unicode script names such as Arabic or Han,
6365: and the derived properties Any and L&. A full list is given in the
6366: pcrepattern documentation. Only the short names for properties are sup-
6367: ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
6368: ter}, is not supported. Furthermore, in Perl, many properties may
6369: optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
6370: does not support this.
6371:
6372: Validity of UTF-8 strings
6373:
6374: When you set the PCRE_UTF8 flag, the strings passed as patterns and
6375: subjects are (by default) checked for validity on entry to the relevant
6376: functions. From release 7.3 of PCRE, the check is according the rules
6377: of RFC 3629, which are themselves derived from the Unicode specifica-
6378: tion. Earlier releases of PCRE followed the rules of RFC 2279, which
6379: allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
6380: check allows only values in the range U+0 to U+10FFFF, excluding U+D800
6381: to U+DFFF.
6382:
6383: The excluded code points are the "Low Surrogate Area" of Unicode, of
6384: which the Unicode Standard says this: "The Low Surrogate Area does not
6385: contain any character assignments, consequently no character code
6386: charts or namelists are provided for this area. Surrogates are reserved
6387: for use with UTF-16 and then must be used in pairs." The code points
6388: that are encoded by UTF-16 pairs are available as independent code
6389: points in the UTF-8 encoding. (In other words, the whole surrogate
6390: thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
6391:
6392: If an invalid UTF-8 string is passed to PCRE, an error return is given.
6393: At compile time, the only additional information is the offset to the
6394: first byte of the failing character. The runtime functions pcre_exec()
6395: and pcre_dfa_exec() also pass back this information, as well as a more
6396: detailed reason code if the caller has provided memory in which to do
6397: this.
6398:
6399: In some situations, you may already know that your strings are valid,
6400: and therefore want to skip these checks in order to improve perfor-
6401: mance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run
6402: time, PCRE assumes that the pattern or subject it is given (respec-
6403: tively) contains only valid UTF-8 codes. In this case, it does not
6404: diagnose an invalid UTF-8 string.
6405:
6406: If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
6407: what happens depends on why the string is invalid. If the string con-
6408: forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
6409: string of characters in the range 0 to 0x7FFFFFFF by pcre_dfa_exec()
6410: and the interpreted version of pcre_exec(). In other words, apart from
6411: the initial validity test, these functions (when in UTF-8 mode) handle
6412: strings according to the more liberal rules of RFC 2279. However, the
6413: just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629.
6414: If you are using JIT optimization, or if the string does not even con-
6415: form to RFC 2279, the result is undefined. Your program may crash.
6416:
6417: If you want to process strings of values in the full range 0 to
6418: 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
6419: set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
6420: this situation, you will have to apply your own validity check, and
6421: avoid the use of JIT optimization.
6422:
6423: General comments about UTF-8 mode
6424:
6425: 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
6426: two-byte UTF-8 character if the value is greater than 127.
6427:
6428: 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
6429: characters for values greater than \177.
6430:
6431: 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
6432: vidual bytes, for example: \x{100}{3}.
6433:
6434: 4. The dot metacharacter matches one UTF-8 character instead of a sin-
6435: gle byte.
6436:
6437: 5. The escape sequence \C can be used to match a single byte in UTF-8
6438: mode, but its use can lead to some strange effects because it breaks up
6439: multibyte characters (see the description of \C in the pcrepattern doc-
6440: umentation). The use of \C is not supported in the alternative matching
6441: function pcre_dfa_exec(), nor is it supported in UTF-8 mode by the JIT
6442: optimization of pcre_exec(). If JIT optimization is requested for a
6443: UTF-8 pattern that contains \C, it will not succeed, and so the match-
6444: ing will be carried out by the normal interpretive function.
6445:
6446: 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
6447: test characters of any code value, but, by default, the characters that
6448: PCRE recognizes as digits, spaces, or word characters remain the same
6449: set as before, all with values less than 256. This remains true even
6450: when PCRE is built to include Unicode property support, because to do
6451: otherwise would slow down PCRE in many common cases. Note in particular
6452: that this applies to \b and \B, because they are defined in terms of \w
6453: and \W. If you really want to test for a wider sense of, say, "digit",
6454: you can use explicit Unicode property tests such as \p{Nd}. Alterna-
6455: tively, if you set the PCRE_UCP option, the way that the character
6456: escapes work is changed so that Unicode properties are used to deter-
6457: mine which characters match. There are more details in the section on
6458: generic character types in the pcrepattern documentation.
6459:
6460: 7. Similarly, characters that match the POSIX named character classes
6461: are all low-valued characters, unless the PCRE_UCP option is set.
6462:
6463: 8. However, the horizontal and vertical whitespace matching escapes
6464: (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
6465: whether or not PCRE_UCP is set.
6466:
6467: 9. Case-insensitive matching applies only to characters whose values
6468: are less than 128, unless PCRE is built with Unicode property support.
6469: Even when Unicode property support is available, PCRE still uses its
6470: own character tables when checking the case of low-valued characters,
6471: so as not to degrade performance. The Unicode property information is
6472: used only for characters with higher values. Furthermore, PCRE supports
6473: case-insensitive matching only when there is a one-to-one mapping
6474: between a letter's cases. There are a small number of many-to-one map-
6475: pings in Unicode; these are not supported by PCRE.
6476:
6477:
6478: AUTHOR
6479:
6480: Philip Hazel
6481: University Computing Service
6482: Cambridge CB2 3QH, England.
6483:
6484:
6485: REVISION
6486:
6487: Last updated: 19 October 2011
6488: Copyright (c) 1997-2011 University of Cambridge.
6489: ------------------------------------------------------------------------------
6490:
6491:
6492: PCREJIT(3) PCREJIT(3)
6493:
6494:
6495: NAME
6496: PCRE - Perl-compatible regular expressions
6497:
6498:
6499: PCRE JUST-IN-TIME COMPILER SUPPORT
6500:
6501: Just-in-time compiling is a heavyweight optimization that can greatly
6502: speed up pattern matching. However, it comes at the cost of extra pro-
6503: cessing before the match is performed. Therefore, it is of most benefit
6504: when the same pattern is going to be matched many times. This does not
6505: necessarily mean many calls of pcre_exec(); if the pattern is not
6506: anchored, matching attempts may take place many times at various posi-
6507: tions in the subject, even for a single call to pcre_exec(). If the
6508: subject string is very long, it may still pay to use JIT for one-off
6509: matches.
6510:
6511: JIT support applies only to the traditional matching function,
6512: pcre_exec(). It does not apply when pcre_dfa_exec() is being used. The
6513: code for this support was written by Zoltan Herczeg.
6514:
6515:
6516: AVAILABILITY OF JIT SUPPORT
6517:
6518: JIT support is an optional feature of PCRE. The "configure" option
6519: --enable-jit (or equivalent CMake option) must be set when PCRE is
6520: built if you want to use JIT. The support is limited to the following
6521: hardware platforms:
6522:
6523: ARM v5, v7, and Thumb2
6524: Intel x86 32-bit and 64-bit
6525: MIPS 32-bit
6526: Power PC 32-bit and 64-bit (experimental)
6527:
6528: The Power PC support is designated as experimental because it has not
6529: been fully tested. If --enable-jit is set on an unsupported platform,
6530: compilation fails.
6531:
6532: A program that is linked with PCRE 8.20 or later can tell if JIT sup-
6533: port is available by calling pcre_config() with the PCRE_CONFIG_JIT
6534: option. The result is 1 when JIT is available, and 0 otherwise. How-
6535: ever, a simple program does not need to check this in order to use JIT.
6536: The API is implemented in a way that falls back to the ordinary PCRE
6537: code if JIT is not available.
6538:
6539: If your program may sometimes be linked with versions of PCRE that are
6540: older than 8.20, but you want to use JIT when it is available, you can
6541: test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT
6542: macro such as PCRE_CONFIG_JIT, for compile-time control of your code.
6543:
6544:
6545: SIMPLE USE OF JIT
6546:
6547: You have to do two things to make use of the JIT support in the sim-
6548: plest way:
6549:
6550: (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
6551: each compiled pattern, and pass the resulting pcre_extra block to
6552: pcre_exec().
6553:
6554: (2) Use pcre_free_study() to free the pcre_extra block when it is
6555: no longer needed instead of just freeing it yourself. This
6556: ensures that any JIT data is also freed.
6557:
6558: For a program that may be linked with pre-8.20 versions of PCRE, you
6559: can insert
6560:
6561: #ifndef PCRE_STUDY_JIT_COMPILE
6562: #define PCRE_STUDY_JIT_COMPILE 0
6563: #endif
6564:
6565: so that no option is passed to pcre_study(), and then use something
6566: like this to free the study data:
6567:
6568: #ifdef PCRE_CONFIG_JIT
6569: pcre_free_study(study_ptr);
6570: #else
6571: pcre_free(study_ptr);
6572: #endif
6573:
6574: In some circumstances you may need to call additional functions. These
6575: are described in the section entitled "Controlling the JIT stack"
6576: below.
6577:
6578: If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and
6579: no JIT data is set up. Otherwise, the compiled pattern is passed to the
6580: JIT compiler, which turns it into machine code that executes much
6581: faster than the normal interpretive code. When pcre_exec() is passed a
6582: pcre_extra block containing a pointer to JIT code, it obeys that
6583: instead of the normal code. The result is identical, but the code runs
6584: much faster.
6585:
6586: There are some pcre_exec() options that are not supported for JIT exe-
6587: cution. There are also some pattern items that JIT cannot handle.
6588: Details are given below. In both cases, execution automatically falls
6589: back to the interpretive code.
6590:
6591: If the JIT compiler finds an unsupported item, no JIT data is gener-
6592: ated. You can find out if JIT execution is available after studying a
6593: pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A
6594: result of 1 means that JIT compilation was successful. A result of 0
6595: means that JIT support is not available, or the pattern was not studied
6596: with PCRE_STUDY_JIT_COMPILE, or the JIT compiler was not able to handle
6597: the pattern.
6598:
6599: Once a pattern has been studied, with or without JIT, it can be used as
6600: many times as you like for matching different subject strings.
6601:
6602:
6603: UNSUPPORTED OPTIONS AND PATTERN ITEMS
6604:
6605: The only pcre_exec() options that are supported for JIT execution are
6606: PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
6607: PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
6608: supported.
6609:
6610: The unsupported pattern items are:
6611:
6612: \C match a single byte; not supported in UTF-8 mode
6613: (?Cn) callouts
6614: (*COMMIT) )
6615: (*MARK) )
6616: (*PRUNE) ) the backtracking control verbs
6617: (*SKIP) )
6618: (*THEN) )
6619:
6620: Support for some of these may be added in future.
6621:
6622:
6623: RETURN VALUES FROM JIT EXECUTION
6624:
6625: When a pattern is matched using JIT execution, the return values are
6626: the same as those given by the interpretive pcre_exec() code, with the
6627: addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means
6628: that the memory used for the JIT stack was insufficient. See "Control-
6629: ling the JIT stack" below for a discussion of JIT stack usage. For com-
6630: patibility with the interpretive pcre_exec() code, no more than two-
6631: thirds of the ovector argument is used for passing back captured sub-
6632: strings.
6633:
6634: The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if
6635: searching a very large pattern tree goes on for too long, as it is in
6636: the same circumstance when JIT is not used, but the details of exactly
6637: what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error
6638: code is never returned by JIT execution.
6639:
6640:
6641: SAVING AND RESTORING COMPILED PATTERNS
6642:
6643: The code that is generated by the JIT compiler is architecture-spe-
6644: cific, and is also position dependent. For those reasons it cannot be
6645: saved (in a file or database) and restored later like the bytecode and
6646: other data of a compiled pattern. Saving and restoring compiled pat-
6647: terns is not something many people do. More detail about this facility
6648: is given in the pcreprecompile documentation. It should be possible to
6649: run pcre_study() on a saved and restored pattern, and thereby recreate
6650: the JIT data, but because JIT compilation uses significant resources,
6651: it is probably not worth doing this; you might as well recompile the
6652: original pattern.
6653:
6654:
6655: CONTROLLING THE JIT STACK
6656:
6657: When the compiled JIT code runs, it needs a block of memory to use as a
6658: stack. By default, it uses 32K on the machine stack. However, some
6659: large or complicated patterns need more than this. The error
6660: PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
6661: Three functions are provided for managing blocks of memory for use as
6662: JIT stacks. There is further discussion about the use of JIT stacks in
6663: the section entitled "JIT stack FAQ" below.
6664:
6665: The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments
6666: are a starting size and a maximum size, and it returns a pointer to an
6667: opaque structure of type pcre_jit_stack, or NULL if there is an error.
6668: The pcre_jit_stack_free() function can be used to free a stack that is
6669: no longer needed. (For the technically minded: the address space is
6670: allocated by mmap or VirtualAlloc.)
6671:
6672: JIT uses far less memory for recursion than the interpretive code, and
6673: a maximum stack size of 512K to 1M should be more than enough for any
6674: pattern.
6675:
6676: The pcre_assign_jit_stack() function specifies which stack JIT code
6677: should use. Its arguments are as follows:
6678:
6679: pcre_extra *extra
6680: pcre_jit_callback callback
6681: void *data
6682:
6683: The extra argument must be the result of studying a pattern with
6684: PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the
6685: other two options:
6686:
6687: (1) If callback is NULL and data is NULL, an internal 32K block
6688: on the machine stack is used.
6689:
6690: (2) If callback is NULL and data is not NULL, data must be
6691: a valid JIT stack, the result of calling pcre_jit_stack_alloc().
6692:
6693: (3) If callback not NULL, it must point to a function that is called
6694: with data as an argument at the start of matching, in order to
6695: set up a JIT stack. If the result is NULL, the internal 32K stack
6696: is used; otherwise the return value must be a valid JIT stack,
6697: the result of calling pcre_jit_stack_alloc().
6698:
6699: You may safely assign the same JIT stack to more than one pattern, as
6700: long as they are all matched sequentially in the same thread. In a mul-
6701: tithread application, each thread must use its own JIT stack.
6702:
6703: Strictly speaking, even more is allowed. You can assign the same stack
6704: to any number of patterns as long as they are not used for matching by
6705: multiple threads at the same time. For example, you can assign the same
6706: stack to all compiled patterns, and use a global mutex in the callback
6707: to wait until the stack is available for use. However, this is an inef-
6708: ficient solution, and not recommended.
6709:
6710: This is a suggestion for how a typical multithreaded program might
6711: operate:
6712:
6713: During thread initalization
6714: thread_local_var = pcre_jit_stack_alloc(...)
6715:
6716: During thread exit
6717: pcre_jit_stack_free(thread_local_var)
6718:
6719: Use a one-line callback function
6720: return thread_local_var
6721:
6722: All the functions described in this section do nothing if JIT is not
6723: available, and pcre_assign_jit_stack() does nothing unless the extra
6724: argument is non-NULL and points to a pcre_extra block that is the
6725: result of a successful study with PCRE_STUDY_JIT_COMPILE.
6726:
6727:
6728: JIT STACK FAQ
6729:
6730: (1) Why do we need JIT stacks?
6731:
6732: PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack
6733: where the local data of the current node is pushed before checking its
6734: child nodes. Allocating real machine stack on some platforms is diffi-
6735: cult. For example, the stack chain needs to be updated every time if we
6736: extend the stack on PowerPC. Although it is possible, its updating
6737: time overhead decreases performance. So we do the recursion in memory.
6738:
6739: (2) Why don't we simply allocate blocks of memory with malloc()?
6740:
6741: Modern operating systems have a nice feature: they can reserve an
6742: address space instead of allocating memory. We can safely allocate mem-
6743: ory pages inside this address space, so the stack could grow without
6744: moving memory data (this is important because of pointers). Thus we can
6745: allocate 1M address space, and use only a single memory page (usually
6746: 4K) if that is enough. However, we can still grow up to 1M anytime if
6747: needed.
6748:
6749: (3) Who "owns" a JIT stack?
6750:
6751: The owner of the stack is the user program, not the JIT studied pattern
6752: or anything else. The user program must ensure that if a stack is used
6753: by pcre_exec(), (that is, it is assigned to the pattern currently run-
6754: ning), that stack must not be used by any other threads (to avoid over-
6755: writing the same memory area). The best practice for multithreaded pro-
6756: grams is to allocate a stack for each thread, and return this stack
6757: through the JIT callback function.
6758:
6759: (4) When should a JIT stack be freed?
6760:
6761: You can free a JIT stack at any time, as long as it will not be used by
6762: pcre_exec() again. When you assign the stack to a pattern, only a
6763: pointer is set. There is no reference counting or any other magic. You
6764: can free the patterns and stacks in any order, anytime. Just do not
6765: call pcre_exec() with a pattern pointing to an already freed stack, as
6766: that will cause SEGFAULT. (Also, do not free a stack currently used by
6767: pcre_exec() in another thread). You can also replace the stack for a
6768: pattern at any time. You can even free the previous stack before
6769: assigning a replacement.
6770:
6771: (5) Should I allocate/free a stack every time before/after calling
6772: pcre_exec()?
6773:
6774: No, because this is too costly in terms of resources. However, you
6775: could implement some clever idea which release the stack if it is not
6776: used in let's say two minutes. The JIT callback can help to achive this
6777: without keeping a list of the currently JIT studied patterns.
6778:
6779: (6) OK, the stack is for long term memory allocation. But what happens
6780: if a pattern causes stack overflow with a stack of 1M? Is that 1M kept
6781: until the stack is freed?
6782:
6783: Especially on embedded sytems, it might be a good idea to release mem-
6784: ory sometimes without freeing the stack. There is no API for this at
6785: the moment. Probably a function call which returns with the currently
6786: allocated memory for any stack and another which allows releasing mem-
6787: ory (shrinking the stack) would be a good idea if someone needs this.
6788:
6789: (7) This is too much of a headache. Isn't there any better solution for
6790: JIT stack handling?
6791:
6792: No, thanks to Windows. If POSIX threads were used everywhere, we could
6793: throw out this complicated API.
6794:
6795:
6796: EXAMPLE CODE
6797:
6798: This is a single-threaded example that specifies a JIT stack without
6799: using a callback.
6800:
6801: int rc;
6802: int ovector[30];
6803: pcre *re;
6804: pcre_extra *extra;
6805: pcre_jit_stack *jit_stack;
6806:
6807: re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
6808: /* Check for errors */
6809: extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
6810: jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
6811: /* Check for error (NULL) */
6812: pcre_assign_jit_stack(extra, NULL, jit_stack);
6813: rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
6814: /* Check results */
6815: pcre_free(re);
6816: pcre_free_study(extra);
6817: pcre_jit_stack_free(jit_stack);
6818:
6819:
6820: SEE ALSO
6821:
6822: pcreapi(3)
6823:
6824:
6825: AUTHOR
6826:
6827: Philip Hazel (FAQ by Zoltan Herczeg)
6828: University Computing Service
6829: Cambridge CB2 3QH, England.
6830:
6831:
6832: REVISION
6833:
6834: Last updated: 26 November 2011
6835: Copyright (c) 1997-2011 University of Cambridge.
6836: ------------------------------------------------------------------------------
6837:
6838:
6839: PCREPARTIAL(3) PCREPARTIAL(3)
6840:
6841:
6842: NAME
6843: PCRE - Perl-compatible regular expressions
6844:
6845:
6846: PARTIAL MATCHING IN PCRE
6847:
6848: In normal use of PCRE, if the subject string that is passed to
6849: pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
6850: short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
6851: There are circumstances where it might be helpful to distinguish this
6852: case from other cases in which there is no match.
6853:
6854: Consider, for example, an application where a human is required to type
6855: in data for a field with specific formatting requirements. An example
6856: might be a date in the form ddmmmyy, defined by this pattern:
6857:
6858: ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
6859:
6860: If the application sees the user's keystrokes one by one, and can check
6861: that what has been typed so far is potentially valid, it is able to
6862: raise an error as soon as a mistake is made, by beeping and not
6863: reflecting the character that has been typed, for example. This immedi-
6864: ate feedback is likely to be a better user interface than a check that
6865: is delayed until the entire string has been entered. Partial matching
6866: can also be useful when the subject string is very long and is not all
6867: available at once.
6868:
6869: PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
6870: PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or
6871: pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym
6872: for PCRE_PARTIAL_SOFT. The essential difference between the two options
6873: is whether or not a partial match is preferred to an alternative com-
6874: plete match, though the details differ between the two matching func-
6875: tions. If both options are set, PCRE_PARTIAL_HARD takes precedence.
6876:
6877: Setting a partial matching option for pcre_exec() disables the use of
6878: any just-in-time code that was set up by calling pcre_study() with the
6879: PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
6880: optimizations. PCRE remembers the last literal byte in a pattern, and
6881: abandons matching immediately if such a byte is not present in the sub-
6882: ject string. This optimization cannot be used for a subject string that
6883: might match only partially. If the pattern was studied, PCRE knows the
6884: minimum length of a matching string, and does not bother to run the
6885: matching function on shorter strings. This optimization is also dis-
6886: abled for partial matching.
6887:
6888:
6889: PARTIAL MATCHING USING pcre_exec()
6890:
6891: A partial match occurs during a call to pcre_exec() when the end of the
6892: subject string is reached successfully, but matching cannot continue
6893: because more characters are needed. However, at least one character in
6894: the subject must have been inspected. This character need not form part
6895: of the final matched string; lookbehind assertions and the \K escape
6896: sequence provide ways of inspecting characters before the start of a
6897: matched substring. The requirement for inspecting at least one charac-
6898: ter exists because an empty string can always be matched; without such
6899: a restriction there would always be a partial match of an empty string
6900: at the end of the subject.
6901:
6902: If there are at least two slots in the offsets vector when pcre_exec()
6903: returns with a partial match, the first slot is set to the offset of
6904: the earliest character that was inspected when the partial match was
6905: found. For convenience, the second offset points to the end of the sub-
6906: ject so that a substring can easily be identified.
6907:
6908: For the majority of patterns, the first offset identifies the start of
6909: the partially matched string. However, for patterns that contain look-
6910: behind assertions, or \K, or begin with \b or \B, earlier characters
6911: have been inspected while carrying out the match. For example:
6912:
6913: /(?<=abc)123/
6914:
6915: This pattern matches "123", but only if it is preceded by "abc". If the
6916: subject string is "xyzabc12", the offsets after a partial match are for
6917: the substring "abc12", because all these characters are needed if
6918: another match is tried with extra characters added to the subject.
6919:
6920: What happens when a partial match is identified depends on which of the
6921: two partial matching options are set.
6922:
6923: PCRE_PARTIAL_SOFT with pcre_exec()
6924:
6925: If PCRE_PARTIAL_SOFT is set when pcre_exec() identifies a partial
6926: match, the partial match is remembered, but matching continues as nor-
6927: mal, and other alternatives in the pattern are tried. If no complete
6928: match can be found, pcre_exec() returns PCRE_ERROR_PARTIAL instead of
6929: PCRE_ERROR_NOMATCH.
6930:
6931: This option is "soft" because it prefers a complete match over a par-
6932: tial match. All the various matching items in a pattern behave as if
6933: the subject string is potentially complete. For example, \z, \Z, and $
6934: match at the end of the subject, as normal, and for \b and \B the end
6935: of the subject is treated as a non-alphanumeric.
6936:
6937: If there is more than one partial match, the first one that was found
6938: provides the data that is returned. Consider this pattern:
6939:
6940: /123\w+X|dogY/
6941:
6942: If this is matched against the subject string "abc123dog", both alter-
6943: natives fail to match, but the end of the subject is reached during
6944: matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3
6945: and 9, identifying "123dog" as the first partial match that was found.
6946: (In this example, there are two partial matches, because "dog" on its
6947: own partially matches the second alternative.)
6948:
6949: PCRE_PARTIAL_HARD with pcre_exec()
6950:
6951: If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR-
6952: TIAL as soon as a partial match is found, without continuing to search
6953: for possible complete matches. This option is "hard" because it prefers
6954: an earlier partial match over a later complete match. For this reason,
6955: the assumption is made that the end of the supplied subject string may
6956: not be the true end of the available data, and so, if \z, \Z, \b, \B,
6957: or $ are encountered at the end of the subject, the result is
6958: PCRE_ERROR_PARTIAL.
6959:
6960: Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks UTF-8
6961: subject strings for validity. Normally, an invalid UTF-8 sequence
6962: causes the error PCRE_ERROR_BADUTF8. However, in the special case of a
6963: truncated UTF-8 character at the end of the subject, PCRE_ERROR_SHORT-
6964: UTF8 is returned when PCRE_PARTIAL_HARD is set.
6965:
6966: Comparing hard and soft partial matching
6967:
6968: The difference between the two partial matching options can be illus-
6969: trated by a pattern such as:
6970:
6971: /dog(sbody)?/
6972:
6973: This matches either "dog" or "dogsbody", greedily (that is, it prefers
6974: the longer string if possible). If it is matched against the string
6975: "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog".
6976: However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL.
6977: On the other hand, if the pattern is made ungreedy the result is dif-
6978: ferent:
6979:
6980: /dog(sbody)??/
6981:
6982: In this case the result is always a complete match because pcre_exec()
6983: finds that first, and it never continues after finding a match. It
6984: might be easier to follow this explanation by thinking of the two pat-
6985: terns like this:
6986:
6987: /dog(sbody)?/ is the same as /dogsbody|dog/
6988: /dog(sbody)??/ is the same as /dog|dogsbody/
6989:
6990: The second pattern will never match "dogsbody" when pcre_exec() is
6991: used, because it will always find the shorter match first.
6992:
6993:
6994: PARTIAL MATCHING USING pcre_dfa_exec()
6995:
6996: The pcre_dfa_exec() function moves along the subject string character
6997: by character, without backtracking, searching for all possible matches
6998: simultaneously. If the end of the subject is reached before the end of
6999: the pattern, there is the possibility of a partial match, again pro-
7000: vided that at least one character has been inspected.
7001:
7002: When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if
7003: there have been no complete matches. Otherwise, the complete matches
7004: are returned. However, if PCRE_PARTIAL_HARD is set, a partial match
7005: takes precedence over any complete matches. The portion of the string
7006: that was inspected when the longest partial match was found is set as
7007: the first matching string, provided there are at least two slots in the
7008: offsets vector.
7009:
7010: Because pcre_dfa_exec() always searches for all possible matches, and
7011: there is no difference between greedy and ungreedy repetition, its be-
7012: haviour is different from pcre_exec when PCRE_PARTIAL_HARD is set. Con-
7013: sider the string "dog" matched against the ungreedy pattern shown
7014: above:
7015:
7016: /dog(sbody)??/
7017:
7018: Whereas pcre_exec() stops as soon as it finds the complete match for
7019: "dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and
7020: so returns that when PCRE_PARTIAL_HARD is set.
7021:
7022:
7023: PARTIAL MATCHING AND WORD BOUNDARIES
7024:
7025: If a pattern ends with one of sequences \b or \B, which test for word
7026: boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-
7027: intuitive results. Consider this pattern:
7028:
7029: /\bcat\b/
7030:
7031: This matches "cat", provided there is a word boundary at either end. If
7032: the subject string is "the cat", the comparison of the final "t" with a
7033: following character cannot take place, so a partial match is found.
7034: However, pcre_exec() carries on with normal matching, which matches \b
7035: at the end of the subject when the last character is a letter, thus
7036: finding a complete match. The result, therefore, is not PCRE_ERROR_PAR-
7037: TIAL. The same thing happens with pcre_dfa_exec(), because it also
7038: finds the complete match.
7039:
7040: Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL,
7041: because then the partial match takes precedence.
7042:
7043:
7044: FORMERLY RESTRICTED PATTERNS
7045:
7046: For releases of PCRE prior to 8.00, because of the way certain internal
7047: optimizations were implemented in the pcre_exec() function, the
7048: PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be
7049: used with all patterns. From release 8.00 onwards, the restrictions no
7050: longer apply, and partial matching with pcre_exec() can be requested
7051: for any pattern.
7052:
7053: Items that were formerly restricted were repeated single characters and
7054: repeated metasequences. If PCRE_PARTIAL was set for a pattern that did
7055: not conform to the restrictions, pcre_exec() returned the error code
7056: PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
7057: PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled
7058: pattern can be used for partial matching now always returns 1.
7059:
7060:
7061: EXAMPLE OF PARTIAL MATCHING USING PCRETEST
7062:
7063: If the escape sequence \P is present in a pcretest data line, the
7064: PCRE_PARTIAL_SOFT option is used for the match. Here is a run of
7065: pcretest that uses the date example quoted above:
7066:
7067: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
7068: data> 25jun04\P
7069: 0: 25jun04
7070: 1: jun
7071: data> 25dec3\P
7072: Partial match: 23dec3
7073: data> 3ju\P
7074: Partial match: 3ju
7075: data> 3juj\P
7076: No match
7077: data> j\P
7078: No match
7079:
7080: The first data string is matched completely, so pcretest shows the
7081: matched substrings. The remaining four strings do not match the com-
7082: plete pattern, but the first two are partial matches. Similar output is
7083: obtained when pcre_dfa_exec() is used.
7084:
7085: If the escape sequence \P is present more than once in a pcretest data
7086: line, the PCRE_PARTIAL_HARD option is set for the match.
7087:
7088:
7089: MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
7090:
7091: When a partial match has been found using pcre_dfa_exec(), it is possi-
7092: ble to continue the match by providing additional subject data and
7093: calling pcre_dfa_exec() again with the same compiled regular expres-
7094: sion, this time setting the PCRE_DFA_RESTART option. You must pass the
7095: same working space as before, because this is where details of the pre-
7096: vious partial match are stored. Here is an example using pcretest,
7097: using the \R escape sequence to set the PCRE_DFA_RESTART option (\D
7098: specifies the use of pcre_dfa_exec()):
7099:
7100: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
7101: data> 23ja\P\D
7102: Partial match: 23ja
7103: data> n05\R\D
7104: 0: n05
7105:
7106: The first call has "23ja" as the subject, and requests partial match-
7107: ing; the second call has "n05" as the subject for the continued
7108: (restarted) match. Notice that when the match is complete, only the
7109: last part is shown; PCRE does not retain the previously partially-
7110: matched string. It is up to the calling program to do that if it needs
7111: to.
7112:
7113: You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
7114: PCRE_DFA_RESTART to continue partial matching over multiple segments.
7115: This facility can be used to pass very long subject strings to
7116: pcre_dfa_exec().
7117:
7118:
7119: MULTI-SEGMENT MATCHING WITH pcre_exec()
7120:
7121: From release 8.00, pcre_exec() can also be used to do multi-segment
7122: matching. Unlike pcre_dfa_exec(), it is not possible to restart the
7123: previous match with a new segment of data. Instead, new data must be
7124: added to the previous subject string, and the entire match re-run,
7125: starting from the point where the partial match occurred. Earlier data
7126: can be discarded. It is best to use PCRE_PARTIAL_HARD in this situa-
7127: tion, because it does not treat the end of a segment as the end of the
7128: subject when matching \z, \Z, \b, \B, and $. Consider an unanchored
7129: pattern that matches dates:
7130:
7131: re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
7132: data> The date is 23ja\P\P
7133: Partial match: 23ja
7134:
7135: At this stage, an application could discard the text preceding "23ja",
7136: add on text from the next segment, and call pcre_exec() again. Unlike
7137: pcre_dfa_exec(), the entire matching string must always be available,
7138: and the complete matching process occurs for each call, so more memory
7139: and more processing time is needed.
7140:
7141: Note: If the pattern contains lookbehind assertions, or \K, or starts
7142: with \b or \B, the string that is returned for a partial match will
7143: include characters that precede the partially matched string itself,
7144: because these must be retained when adding on more characters for a
7145: subsequent matching attempt.
7146:
7147:
7148: ISSUES WITH MULTI-SEGMENT MATCHING
7149:
7150: Certain types of pattern may give problems with multi-segment matching,
7151: whichever matching function is used.
7152:
7153: 1. If the pattern contains a test for the beginning of a line, you need
7154: to pass the PCRE_NOTBOL option when the subject string for any call
7155: does start at the beginning of a line. There is also a PCRE_NOTEOL
7156: option, but in practice when doing multi-segment matching you should be
7157: using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.
7158:
7159: 2. Lookbehind assertions at the start of a pattern are catered for in
7160: the offsets that are returned for a partial match. However, in theory,
7161: a lookbehind assertion later in the pattern could require even earlier
7162: characters to be inspected, and it might not have been reached when a
7163: partial match occurs. This is probably an extremely unlikely case; you
7164: could guard against it to a certain extent by always including extra
7165: characters at the start.
7166:
7167: 3. Matching a subject string that is split into multiple segments may
7168: not always produce exactly the same result as matching over one single
7169: long string, especially when PCRE_PARTIAL_SOFT is used. The section
7170: "Partial Matching and Word Boundaries" above describes an issue that
7171: arises if the pattern ends with \b or \B. Another kind of difference
7172: may occur when there are multiple matching possibilities, because (for
7173: PCRE_PARTIAL_SOFT) a partial match result is given only when there are
7174: no completed matches. This means that as soon as the shortest match has
7175: been found, continuation to a new subject segment is no longer possi-
7176: ble. Consider again this pcretest example:
7177:
7178: re> /dog(sbody)?/
7179: data> dogsb\P
7180: 0: dog
7181: data> do\P\D
7182: Partial match: do
7183: data> gsb\R\P\D
7184: 0: g
7185: data> dogsbody\D
7186: 0: dogsbody
7187: 1: dog
7188:
7189: The first data line passes the string "dogsb" to pcre_exec(), setting
7190: the PCRE_PARTIAL_SOFT option. Although the string is a partial match
7191: for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the
7192: shorter string "dog" is a complete match. Similarly, when the subject
7193: is presented to pcre_dfa_exec() in several parts ("do" and "gsb" being
7194: the first two) the match stops when "dog" has been found, and it is not
7195: possible to continue. On the other hand, if "dogsbody" is presented as
7196: a single string, pcre_dfa_exec() finds both matches.
7197:
7198: Because of these problems, it is best to use PCRE_PARTIAL_HARD when
7199: matching multi-segment data. The example above then behaves differ-
7200: ently:
7201:
7202: re> /dog(sbody)?/
7203: data> dogsb\P\P
7204: Partial match: dogsb
7205: data> do\P\D
7206: Partial match: do
7207: data> gsb\R\P\P\D
7208: Partial match: gsb
7209:
7210: 4. Patterns that contain alternatives at the top level which do not all
7211: start with the same pattern item may not work as expected when
7212: PCRE_DFA_RESTART is used with pcre_dfa_exec(). For example, consider
7213: this pattern:
7214:
7215: 1234|3789
7216:
7217: If the first part of the subject is "ABC123", a partial match of the
7218: first alternative is found at offset 3. There is no partial match for
7219: the second alternative, because such a match does not start at the same
7220: point in the subject string. Attempting to continue with the string
7221: "7890" does not yield a match because only those alternatives that
7222: match at one point in the subject are remembered. The problem arises
7223: because the start of the second alternative matches within the first
7224: alternative. There is no problem with anchored patterns or patterns
7225: such as:
7226:
7227: 1234|ABCD
7228:
7229: where no string can be a partial match for both alternatives. This is
7230: not a problem if pcre_exec() is used, because the entire match has to
7231: be rerun each time:
7232:
7233: re> /1234|3789/
7234: data> ABC123\P\P
7235: Partial match: 123
7236: data> 1237890
7237: 0: 3789
7238:
7239: Of course, instead of using PCRE_DFA_RESTART, the same technique of re-
7240: running the entire match can also be used with pcre_dfa_exec(). Another
7241: possibility is to work with two buffers. If a partial match at offset n
7242: in the first buffer is followed by "no match" when PCRE_DFA_RESTART is
7243: used on the second buffer, you can then try a new match starting at
7244: offset n+1 in the first buffer.
7245:
7246:
7247: AUTHOR
7248:
7249: Philip Hazel
7250: University Computing Service
7251: Cambridge CB2 3QH, England.
7252:
7253:
7254: REVISION
7255:
7256: Last updated: 26 August 2011
7257: Copyright (c) 1997-2011 University of Cambridge.
7258: ------------------------------------------------------------------------------
7259:
7260:
7261: PCREPRECOMPILE(3) PCREPRECOMPILE(3)
7262:
7263:
7264: NAME
7265: PCRE - Perl-compatible regular expressions
7266:
7267:
7268: SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
7269:
7270: If you are running an application that uses a large number of regular
7271: expression patterns, it may be useful to store them in a precompiled
7272: form instead of having to compile them every time the application is
7273: run. If you are not using any private character tables (see the
7274: pcre_maketables() documentation), this is relatively straightforward.
7275: If you are using private tables, it is a little bit more complicated.
7276: However, if you are using the just-in-time optimization feature of
7277: pcre_study(), it is not possible to save and reload the JIT data.
7278:
7279: If you save compiled patterns to a file, you can copy them to a differ-
7280: ent host and run them there. This works even if the new host has the
7281: opposite endianness to the one on which the patterns were compiled.
7282: There may be a small performance penalty, but it should be insignifi-
7283: cant. However, compiling regular expressions with one version of PCRE
7284: for use with a different version is not guaranteed to work and may
7285: cause crashes, and saving and restoring a compiled pattern loses any
7286: JIT optimization data.
7287:
7288:
7289: SAVING A COMPILED PATTERN
7290:
7291: The value returned by pcre_compile() points to a single block of memory
7292: that holds the compiled pattern and associated data. You can find the
7293: length of this block in bytes by calling pcre_fullinfo() with an argu-
7294: ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
7295: manner. Here is sample code that compiles a pattern and writes it to a
7296: file. It assumes that the variable fd refers to a file that is open for
7297: output:
7298:
7299: int erroroffset, rc, size;
7300: char *error;
7301: pcre *re;
7302:
7303: re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
7304: if (re == NULL) { ... handle errors ... }
7305: rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
7306: if (rc < 0) { ... handle errors ... }
7307: rc = fwrite(re, 1, size, fd);
7308: if (rc != size) { ... handle errors ... }
7309:
7310: In this example, the bytes that comprise the compiled pattern are
7311: copied exactly. Note that this is binary data that may contain any of
7312: the 256 possible byte values. On systems that make a distinction
7313: between binary and non-binary data, be sure that the file is opened for
7314: binary output.
7315:
7316: If you want to write more than one pattern to a file, you will have to
7317: devise a way of separating them. For binary data, preceding each pat-
7318: tern with its length is probably the most straightforward approach.
7319: Another possibility is to write out the data in hexadecimal instead of
7320: binary, one pattern to a line.
7321:
7322: Saving compiled patterns in a file is only one possible way of storing
7323: them for later use. They could equally well be saved in a database, or
7324: in the memory of some daemon process that passes them via sockets to
7325: the processes that want them.
7326:
7327: If the pattern has been studied, it is also possible to save the normal
7328: study data in a similar way to the compiled pattern itself. However, if
7329: the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre-
7330: ated cannot be saved because it is too dependent on the current envi-
7331: ronment. When studying generates additional information, pcre_study()
7332: returns a pointer to a pcre_extra data block. Its format is defined in
7333: the section on matching a pattern in the pcreapi documentation. The
7334: study_data field points to the binary study data, and this is what you
7335: must save (not the pcre_extra block itself). The length of the study
7336: data can be obtained by calling pcre_fullinfo() with an argument of
7337: PCRE_INFO_STUDYSIZE. Remember to check that pcre_study() did return a
7338: non-NULL value before trying to save the study data.
7339:
7340:
7341: RE-USING A PRECOMPILED PATTERN
7342:
7343: Re-using a precompiled pattern is straightforward. Having reloaded it
7344: into main memory, you pass its pointer to pcre_exec() or
7345: pcre_dfa_exec() in the usual way. This should work even on another
7346: host, and even if that host has the opposite endianness to the one
7347: where the pattern was compiled.
7348:
7349: However, if you passed a pointer to custom character tables when the
7350: pattern was compiled (the tableptr argument of pcre_compile()), you
7351: must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
7352: because the value saved with the compiled pattern will obviously be
7353: nonsense. A field in a pcre_extra() block is used to pass this data, as
7354: described in the section on matching a pattern in the pcreapi documen-
7355: tation.
7356:
7357: If you did not provide custom character tables when the pattern was
7358: compiled, the pointer in the compiled pattern is NULL, which causes
7359: pcre_exec() to use PCRE's internal tables. Thus, you do not need to
7360: take any special action at run time in this case.
7361:
7362: If you saved study data with the compiled pattern, you need to create
7363: your own pcre_extra data block and set the study_data field to point to
7364: the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
7365: bit in the flags field to indicate that study data is present. Then
7366: pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
7367: usual way. If the pattern was studied for just-in-time optimization,
7368: that data cannot be saved, and so is lost by a save/restore cycle.
7369:
7370:
7371: COMPATIBILITY WITH DIFFERENT PCRE RELEASES
7372:
7373: In general, it is safest to recompile all saved patterns when you
7374: update to a new PCRE release, though not all updates actually require
7375: this.
7376:
7377:
7378: AUTHOR
7379:
7380: Philip Hazel
7381: University Computing Service
7382: Cambridge CB2 3QH, England.
7383:
7384:
7385: REVISION
7386:
7387: Last updated: 26 August 2011
7388: Copyright (c) 1997-2011 University of Cambridge.
7389: ------------------------------------------------------------------------------
7390:
7391:
7392: PCREPERFORM(3) PCREPERFORM(3)
7393:
7394:
7395: NAME
7396: PCRE - Perl-compatible regular expressions
7397:
7398:
7399: PCRE PERFORMANCE
7400:
7401: Two aspects of performance are discussed below: memory usage and pro-
7402: cessing time. The way you express your pattern as a regular expression
7403: can affect both of them.
7404:
7405:
7406: COMPILED PATTERN MEMORY USAGE
7407:
7408: Patterns are compiled by PCRE into a reasonably efficient byte code, so
7409: that most simple patterns do not use much memory. However, there is one
7410: case where the memory usage of a compiled pattern can be unexpectedly
7411: large. If a parenthesized subpattern has a quantifier with a minimum
7412: greater than 1 and/or a limited maximum, the whole subpattern is
7413: repeated in the compiled code. For example, the pattern
7414:
7415: (abc|def){2,4}
7416:
7417: is compiled as if it were
7418:
7419: (abc|def)(abc|def)((abc|def)(abc|def)?)?
7420:
7421: (Technical aside: It is done this way so that backtrack points within
7422: each of the repetitions can be independently maintained.)
7423:
7424: For regular expressions whose quantifiers use only small numbers, this
7425: is not usually a problem. However, if the numbers are large, and par-
7426: ticularly if such repetitions are nested, the memory usage can become
7427: an embarrassment. For example, the very simple pattern
7428:
7429: ((ab){1,1000}c){1,3}
7430:
7431: uses 51K bytes when compiled. When PCRE is compiled with its default
7432: internal pointer size of two bytes, the size limit on a compiled pat-
7433: tern is 64K, and this is reached with the above pattern if the outer
7434: repetition is increased from 3 to 4. PCRE can be compiled to use larger
7435: internal pointers and thus handle larger compiled patterns, but it is
7436: better to try to rewrite your pattern to use less memory if you can.
7437:
7438: One way of reducing the memory usage for such patterns is to make use
7439: of PCRE's "subroutine" facility. Re-writing the above pattern as
7440:
7441: ((ab)(?2){0,999}c)(?1){0,2}
7442:
7443: reduces the memory requirements to 18K, and indeed it remains under 20K
7444: even with the outer repetition increased to 100. However, this pattern
7445: is not exactly equivalent, because the "subroutine" calls are treated
7446: as atomic groups into which there can be no backtracking if there is a
7447: subsequent matching failure. Therefore, PCRE cannot do this kind of
7448: rewriting automatically. Furthermore, there is a noticeable loss of
7449: speed when executing the modified pattern. Nevertheless, if the atomic
7450: grouping is not a problem and the loss of speed is acceptable, this
7451: kind of rewriting will allow you to process patterns that PCRE cannot
7452: otherwise handle.
7453:
7454:
7455: STACK USAGE AT RUN TIME
7456:
7457: When pcre_exec() is used for matching, certain kinds of pattern can
7458: cause it to use large amounts of the process stack. In some environ-
7459: ments the default process stack is quite small, and if it runs out the
7460: result is often SIGSEGV. This issue is probably the most frequently
7461: raised problem with PCRE. Rewriting your pattern can often help. The
7462: pcrestack documentation discusses this issue in detail.
7463:
7464:
7465: PROCESSING TIME
7466:
7467: Certain items in regular expression patterns are processed more effi-
7468: ciently than others. It is more efficient to use a character class like
7469: [aeiou] than a set of single-character alternatives such as
7470: (a|e|i|o|u). In general, the simplest construction that provides the
7471: required behaviour is usually the most efficient. Jeffrey Friedl's book
7472: contains a lot of useful general discussion about optimizing regular
7473: expressions for efficient performance. This document contains a few
7474: observations about PCRE.
7475:
7476: Using Unicode character properties (the \p, \P, and \X escapes) is
7477: slow, because PCRE has to scan a structure that contains data for over
7478: fifteen thousand characters whenever it needs a character's property.
7479: If you can find an alternative pattern that does not use character
7480: properties, it will probably be faster.
7481:
7482: By default, the escape sequences \b, \d, \s, and \w, and the POSIX
7483: character classes such as [:alpha:] do not use Unicode properties,
7484: partly for backwards compatibility, and partly for performance reasons.
7485: However, you can set PCRE_UCP if you want Unicode character properties
7486: to be used. This can double the matching time for items such as \d,
7487: when matched with pcre_exec(); the performance loss is less with
7488: pcre_dfa_exec(), and in both cases there is not much difference for \b.
7489:
7490: When a pattern begins with .* not in parentheses, or in parentheses
7491: that are not the subject of a backreference, and the PCRE_DOTALL option
7492: is set, the pattern is implicitly anchored by PCRE, since it can match
7493: only at the start of a subject string. However, if PCRE_DOTALL is not
7494: set, PCRE cannot make this optimization, because the . metacharacter
7495: does not then match a newline, and if the subject string contains new-
7496: lines, the pattern may match from the character immediately following
7497: one of them instead of from the very start. For example, the pattern
7498:
7499: .*second
7500:
7501: matches the subject "first\nand second" (where \n stands for a newline
7502: character), with the match starting at the seventh character. In order
7503: to do this, PCRE has to retry the match starting after every newline in
7504: the subject.
7505:
7506: If you are using such a pattern with subject strings that do not con-
7507: tain newlines, the best performance is obtained by setting PCRE_DOTALL,
7508: or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
7509: ing. That saves PCRE from having to scan along the subject looking for
7510: a newline to restart at.
7511:
7512: Beware of patterns that contain nested indefinite repeats. These can
7513: take a long time to run when applied to a string that does not match.
7514: Consider the pattern fragment
7515:
7516: ^(a+)*
7517:
7518: This can match "aaaa" in 16 different ways, and this number increases
7519: very rapidly as the string gets longer. (The * repeat can match 0, 1,
7520: 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
7521: repeats can match different numbers of times.) When the remainder of
7522: the pattern is such that the entire match is going to fail, PCRE has in
7523: principle to try every possible variation, and this can take an
7524: extremely long time, even for relatively short strings.
7525:
7526: An optimization catches some of the more simple cases such as
7527:
7528: (a+)*b
7529:
7530: where a literal character follows. Before embarking on the standard
7531: matching procedure, PCRE checks that there is a "b" later in the sub-
7532: ject string, and if there is not, it fails the match immediately. How-
7533: ever, when there is no following literal this optimization cannot be
7534: used. You can see the difference by comparing the behaviour of
7535:
7536: (a+)*\d
7537:
7538: with the pattern above. The former gives a failure almost instantly
7539: when applied to a whole line of "a" characters, whereas the latter
7540: takes an appreciable time with strings longer than about 20 characters.
7541:
7542: In many cases, the solution to this kind of performance issue is to use
7543: an atomic group or a possessive quantifier.
7544:
7545:
7546: AUTHOR
7547:
7548: Philip Hazel
7549: University Computing Service
7550: Cambridge CB2 3QH, England.
7551:
7552:
7553: REVISION
7554:
7555: Last updated: 16 May 2010
7556: Copyright (c) 1997-2010 University of Cambridge.
7557: ------------------------------------------------------------------------------
7558:
7559:
7560: PCREPOSIX(3) PCREPOSIX(3)
7561:
7562:
7563: NAME
7564: PCRE - Perl-compatible regular expressions.
7565:
7566:
7567: SYNOPSIS OF POSIX API
7568:
7569: #include <pcreposix.h>
7570:
7571: int regcomp(regex_t *preg, const char *pattern,
7572: int cflags);
7573:
7574: int regexec(regex_t *preg, const char *string,
7575: size_t nmatch, regmatch_t pmatch[], int eflags);
7576:
7577: size_t regerror(int errcode, const regex_t *preg,
7578: char *errbuf, size_t errbuf_size);
7579:
7580: void regfree(regex_t *preg);
7581:
7582:
7583: DESCRIPTION
7584:
7585: This set of functions provides a POSIX-style API to the PCRE regular
7586: expression package. See the pcreapi documentation for a description of
7587: PCRE's native API, which contains much additional functionality.
7588:
7589: The functions described here are just wrapper functions that ultimately
7590: call the PCRE native API. Their prototypes are defined in the
7591: pcreposix.h header file, and on Unix systems the library itself is
7592: called pcreposix.a, so can be accessed by adding -lpcreposix to the
7593: command for linking an application that uses them. Because the POSIX
7594: functions call the native ones, it is also necessary to add -lpcre.
7595:
7596: I have implemented only those POSIX option bits that can be reasonably
7597: mapped to PCRE native options. In addition, the option REG_EXTENDED is
7598: defined with the value zero. This has no effect, but since programs
7599: that are written to the POSIX interface often use it, this makes it
7600: easier to slot in PCRE as a replacement library. Other POSIX options
7601: are not even defined.
7602:
7603: There are also some other options that are not defined by POSIX. These
7604: have been added at the request of users who want to make use of certain
7605: PCRE-specific features via the POSIX calling interface.
7606:
7607: When PCRE is called via these functions, it is only the API that is
7608: POSIX-like in style. The syntax and semantics of the regular expres-
7609: sions themselves are still those of Perl, subject to the setting of
7610: various PCRE options, as described below. "POSIX-like in style" means
7611: that the API approximates to the POSIX definition; it is not fully
7612: POSIX-compatible, and in multi-byte encoding domains it is probably
7613: even less compatible.
7614:
7615: The header for these functions is supplied as pcreposix.h to avoid any
7616: potential clash with other POSIX libraries. It can, of course, be
7617: renamed or aliased as regex.h, which is the "correct" name. It provides
7618: two structure types, regex_t for compiled internal forms, and reg-
7619: match_t for returning captured substrings. It also defines some con-
7620: stants whose names start with "REG_"; these are used for setting
7621: options and identifying error codes.
7622:
7623:
7624: COMPILING A PATTERN
7625:
7626: The function regcomp() is called to compile a pattern into an internal
7627: form. The pattern is a C string terminated by a binary zero, and is
7628: passed in the argument pattern. The preg argument is a pointer to a
7629: regex_t structure that is used as a base for storing information about
7630: the compiled regular expression.
7631:
7632: The argument cflags is either zero, or contains one or more of the bits
7633: defined by the following macros:
7634:
7635: REG_DOTALL
7636:
7637: The PCRE_DOTALL option is set when the regular expression is passed for
7638: compilation to the native function. Note that REG_DOTALL is not part of
7639: the POSIX standard.
7640:
7641: REG_ICASE
7642:
7643: The PCRE_CASELESS option is set when the regular expression is passed
7644: for compilation to the native function.
7645:
7646: REG_NEWLINE
7647:
7648: The PCRE_MULTILINE option is set when the regular expression is passed
7649: for compilation to the native function. Note that this does not mimic
7650: the defined POSIX behaviour for REG_NEWLINE (see the following sec-
7651: tion).
7652:
7653: REG_NOSUB
7654:
7655: The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
7656: passed for compilation to the native function. In addition, when a pat-
7657: tern that is compiled with this flag is passed to regexec() for match-
7658: ing, the nmatch and pmatch arguments are ignored, and no captured
7659: strings are returned.
7660:
7661: REG_UCP
7662:
7663: The PCRE_UCP option is set when the regular expression is passed for
7664: compilation to the native function. This causes PCRE to use Unicode
7665: properties when matchine \d, \w, etc., instead of just recognizing
7666: ASCII values. Note that REG_UTF8 is not part of the POSIX standard.
7667:
7668: REG_UNGREEDY
7669:
7670: The PCRE_UNGREEDY option is set when the regular expression is passed
7671: for compilation to the native function. Note that REG_UNGREEDY is not
7672: part of the POSIX standard.
7673:
7674: REG_UTF8
7675:
7676: The PCRE_UTF8 option is set when the regular expression is passed for
7677: compilation to the native function. This causes the pattern itself and
7678: all data strings used for matching it to be treated as UTF-8 strings.
7679: Note that REG_UTF8 is not part of the POSIX standard.
7680:
7681: In the absence of these flags, no options are passed to the native
7682: function. This means the the regex is compiled with PCRE default
7683: semantics. In particular, the way it handles newline characters in the
7684: subject string is the Perl way, not the POSIX way. Note that setting
7685: PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
7686: It does not affect the way newlines are matched by . (they are not) or
7687: by a negative class such as [^a] (they are).
7688:
7689: The yield of regcomp() is zero on success, and non-zero otherwise. The
7690: preg structure is filled in on success, and one member of the structure
7691: is public: re_nsub contains the number of capturing subpatterns in the
7692: regular expression. Various error codes are defined in the header file.
7693:
7694: NOTE: If the yield of regcomp() is non-zero, you must not attempt to
7695: use the contents of the preg structure. If, for example, you pass it to
7696: regexec(), the result is undefined and your program is likely to crash.
7697:
7698:
7699: MATCHING NEWLINE CHARACTERS
7700:
7701: This area is not simple, because POSIX and Perl take different views of
7702: things. It is not possible to get PCRE to obey POSIX semantics, but
7703: then PCRE was never intended to be a POSIX engine. The following table
7704: lists the different possibilities for matching newline characters in
7705: PCRE:
7706:
7707: Default Change with
7708:
7709: . matches newline no PCRE_DOTALL
7710: newline matches [^a] yes not changeable
7711: $ matches \n at end yes PCRE_DOLLARENDONLY
7712: $ matches \n in middle no PCRE_MULTILINE
7713: ^ matches \n in middle no PCRE_MULTILINE
7714:
7715: This is the equivalent table for POSIX:
7716:
7717: Default Change with
7718:
7719: . matches newline yes REG_NEWLINE
7720: newline matches [^a] yes REG_NEWLINE
7721: $ matches \n at end no REG_NEWLINE
7722: $ matches \n in middle no REG_NEWLINE
7723: ^ matches \n in middle no REG_NEWLINE
7724:
7725: PCRE's behaviour is the same as Perl's, except that there is no equiva-
7726: lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
7727: no way to stop newline from matching [^a].
7728:
7729: The default POSIX newline handling can be obtained by setting
7730: PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
7731: behave exactly as for the REG_NEWLINE action.
7732:
7733:
7734: MATCHING A PATTERN
7735:
7736: The function regexec() is called to match a compiled pattern preg
7737: against a given string, which is by default terminated by a zero byte
7738: (but see REG_STARTEND below), subject to the options in eflags. These
7739: can be:
7740:
7741: REG_NOTBOL
7742:
7743: The PCRE_NOTBOL option is set when calling the underlying PCRE matching
7744: function.
7745:
7746: REG_NOTEMPTY
7747:
7748: The PCRE_NOTEMPTY option is set when calling the underlying PCRE match-
7749: ing function. Note that REG_NOTEMPTY is not part of the POSIX standard.
7750: However, setting this option can give more POSIX-like behaviour in some
7751: situations.
7752:
7753: REG_NOTEOL
7754:
7755: The PCRE_NOTEOL option is set when calling the underlying PCRE matching
7756: function.
7757:
7758: REG_STARTEND
7759:
7760: The string is considered to start at string + pmatch[0].rm_so and to
7761: have a terminating NUL located at string + pmatch[0].rm_eo (there need
7762: not actually be a NUL at that location), regardless of the value of
7763: nmatch. This is a BSD extension, compatible with but not specified by
7764: IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
7765: software intended to be portable to other systems. Note that a non-zero
7766: rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
7767: of the string, not how it is matched.
7768:
7769: If the pattern was compiled with the REG_NOSUB flag, no data about any
7770: matched strings is returned. The nmatch and pmatch arguments of
7771: regexec() are ignored.
7772:
7773: If the value of nmatch is zero, or if the value pmatch is NULL, no data
7774: about any matched strings is returned.
7775:
7776: Otherwise,the portion of the string that was matched, and also any cap-
7777: tured substrings, are returned via the pmatch argument, which points to
7778: an array of nmatch structures of type regmatch_t, containing the mem-
7779: bers rm_so and rm_eo. These contain the offset to the first character
7780: of each substring and the offset to the first character after the end
7781: of each substring, respectively. The 0th element of the vector relates
7782: to the entire portion of string that was matched; subsequent elements
7783: relate to the capturing subpatterns of the regular expression. Unused
7784: entries in the array have both structure members set to -1.
7785:
7786: A successful match yields a zero return; various error codes are
7787: defined in the header file, of which REG_NOMATCH is the "expected"
7788: failure code.
7789:
7790:
7791: ERROR MESSAGES
7792:
7793: The regerror() function maps a non-zero errorcode from either regcomp()
7794: or regexec() to a printable message. If preg is not NULL, the error
7795: should have arisen from the use of that structure. A message terminated
7796: by a binary zero is placed in errbuf. The length of the message,
7797: including the zero, is limited to errbuf_size. The yield of the func-
7798: tion is the size of buffer needed to hold the whole message.
7799:
7800:
7801: MEMORY USAGE
7802:
7803: Compiling a regular expression causes memory to be allocated and asso-
7804: ciated with the preg structure. The function regfree() frees all such
7805: memory, after which preg may no longer be used as a compiled expres-
7806: sion.
7807:
7808:
7809: AUTHOR
7810:
7811: Philip Hazel
7812: University Computing Service
7813: Cambridge CB2 3QH, England.
7814:
7815:
7816: REVISION
7817:
7818: Last updated: 16 May 2010
7819: Copyright (c) 1997-2010 University of Cambridge.
7820: ------------------------------------------------------------------------------
7821:
7822:
7823: PCRECPP(3) PCRECPP(3)
7824:
7825:
7826: NAME
7827: PCRE - Perl-compatible regular expressions.
7828:
7829:
7830: SYNOPSIS OF C++ WRAPPER
7831:
7832: #include <pcrecpp.h>
7833:
7834:
7835: DESCRIPTION
7836:
7837: The C++ wrapper for PCRE was provided by Google Inc. Some additional
7838: functionality was added by Giuseppe Maxia. This brief man page was con-
7839: structed from the notes in the pcrecpp.h file, which should be con-
7840: sulted for further details.
7841:
7842:
7843: MATCHING INTERFACE
7844:
7845: The "FullMatch" operation checks that supplied text matches a supplied
7846: pattern exactly. If pointer arguments are supplied, it copies matched
7847: sub-strings that match sub-patterns into them.
7848:
7849: Example: successful match
7850: pcrecpp::RE re("h.*o");
7851: re.FullMatch("hello");
7852:
7853: Example: unsuccessful match (requires full match):
7854: pcrecpp::RE re("e");
7855: !re.FullMatch("hello");
7856:
7857: Example: creating a temporary RE object:
7858: pcrecpp::RE("h.*o").FullMatch("hello");
7859:
7860: You can pass in a "const char*" or a "string" for "text". The examples
7861: below tend to use a const char*. You can, as in the different examples
7862: above, store the RE object explicitly in a variable or use a temporary
7863: RE object. The examples below use one mode or the other arbitrarily.
7864: Either could correctly be used for any of these examples.
7865:
7866: You must supply extra pointer arguments to extract matched subpieces.
7867:
7868: Example: extracts "ruby" into "s" and 1234 into "i"
7869: int i;
7870: string s;
7871: pcrecpp::RE re("(\\w+):(\\d+)");
7872: re.FullMatch("ruby:1234", &s, &i);
7873:
7874: Example: does not try to extract any extra sub-patterns
7875: re.FullMatch("ruby:1234", &s);
7876:
7877: Example: does not try to extract into NULL
7878: re.FullMatch("ruby:1234", NULL, &i);
7879:
7880: Example: integer overflow causes failure
7881: !re.FullMatch("ruby:1234567891234", NULL, &i);
7882:
7883: Example: fails because there aren't enough sub-patterns:
7884: !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
7885:
7886: Example: fails because string cannot be stored in integer
7887: !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
7888:
7889: The provided pointer arguments can be pointers to any scalar numeric
7890: type, or one of:
7891:
7892: string (matched piece is copied to string)
7893: StringPiece (StringPiece is mutated to point to matched piece)
7894: T (where "bool T::ParseFrom(const char*, int)" exists)
7895: NULL (the corresponding matched sub-pattern is not copied)
7896:
7897: The function returns true iff all of the following conditions are sat-
7898: isfied:
7899:
7900: a. "text" matches "pattern" exactly;
7901:
7902: b. The number of matched sub-patterns is >= number of supplied
7903: pointers;
7904:
7905: c. The "i"th argument has a suitable type for holding the
7906: string captured as the "i"th sub-pattern. If you pass in
7907: void * NULL for the "i"th argument, or a non-void * NULL
7908: of the correct type, or pass fewer arguments than the
7909: number of sub-patterns, "i"th captured sub-pattern is
7910: ignored.
7911:
7912: CAVEAT: An optional sub-pattern that does not exist in the matched
7913: string is assigned the empty string. Therefore, the following will
7914: return false (because the empty string is not a valid number):
7915:
7916: int number;
7917: pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
7918:
7919: The matching interface supports at most 16 arguments per call. If you
7920: need more, consider using the more general interface
7921: pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
7922:
7923: NOTE: Do not use no_arg, which is used internally to mark the end of a
7924: list of optional arguments, as a placeholder for missing arguments, as
7925: this can lead to segfaults.
7926:
7927:
7928: QUOTING METACHARACTERS
7929:
7930: You can use the "QuoteMeta" operation to insert backslashes before all
7931: potentially meaningful characters in a string. The returned string,
7932: used as a regular expression, will exactly match the original string.
7933:
7934: Example:
7935: string quoted = RE::QuoteMeta(unquoted);
7936:
7937: Note that it's legal to escape a character even if it has no special
7938: meaning in a regular expression -- so this function does that. (This
7939: also makes it identical to the perl function of the same name; see
7940: "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
7941: "1\.5\-2\.0\?".
7942:
7943:
7944: PARTIAL MATCHES
7945:
7946: You can use the "PartialMatch" operation when you want the pattern to
7947: match any substring of the text.
7948:
7949: Example: simple search for a string:
7950: pcrecpp::RE("ell").PartialMatch("hello");
7951:
7952: Example: find first number in a string:
7953: int number;
7954: pcrecpp::RE re("(\\d+)");
7955: re.PartialMatch("x*100 + 20", &number);
7956: assert(number == 100);
7957:
7958:
7959: UTF-8 AND THE MATCHING INTERFACE
7960:
7961: By default, pattern and text are plain text, one byte per character.
7962: The UTF8 flag, passed to the constructor, causes both pattern and
7963: string to be treated as UTF-8 text, still a byte stream but potentially
7964: multiple bytes per character. In practice, the text is likelier to be
7965: UTF-8 than the pattern, but the match returned may depend on the UTF8
7966: flag, so always use it when matching UTF8 text. For example, "." will
7967: match one byte normally but with UTF8 set may match up to three bytes
7968: of a multi-byte character.
7969:
7970: Example:
7971: pcrecpp::RE_Options options;
7972: options.set_utf8();
7973: pcrecpp::RE re(utf8_pattern, options);
7974: re.FullMatch(utf8_string);
7975:
7976: Example: using the convenience function UTF8():
7977: pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
7978: re.FullMatch(utf8_string);
7979:
7980: NOTE: The UTF8 flag is ignored if pcre was not configured with the
7981: --enable-utf8 flag.
7982:
7983:
7984: PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
7985:
7986: PCRE defines some modifiers to change the behavior of the regular
7987: expression engine. The C++ wrapper defines an auxiliary class,
7988: RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
7989: rently, the following modifiers are supported:
7990:
7991: modifier description Perl corresponding
7992:
7993: PCRE_CASELESS case insensitive match /i
7994: PCRE_MULTILINE multiple lines match /m
7995: PCRE_DOTALL dot matches newlines /s
7996: PCRE_DOLLAR_ENDONLY $ matches only at end N/A
7997: PCRE_EXTRA strict escape parsing N/A
7998: PCRE_EXTENDED ignore whitespaces /x
7999: PCRE_UTF8 handles UTF8 chars built-in
8000: PCRE_UNGREEDY reverses * and *? N/A
8001: PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
8002:
8003: (*) Both Perl and PCRE allow non capturing parentheses by means of the
8004: "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
8005: ture, while (ab|cd) does.
8006:
8007: For a full account on how each modifier works, please check the PCRE
8008: API reference page.
8009:
8010: For each modifier, there are two member functions whose name is made
8011: out of the modifier in lowercase, without the "PCRE_" prefix. For
8012: instance, PCRE_CASELESS is handled by
8013:
8014: bool caseless()
8015:
8016: which returns true if the modifier is set, and
8017:
8018: RE_Options & set_caseless(bool)
8019:
8020: which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
8021: be accessed through the set_match_limit() and match_limit() member
8022: functions. Setting match_limit to a non-zero value will limit the exe-
8023: cution of pcre to keep it from doing bad things like blowing the stack
8024: or taking an eternity to return a result. A value of 5000 is good
8025: enough to stop stack blowup in a 2MB thread stack. Setting match_limit
8026: to zero disables match limiting. Alternatively, you can call
8027: match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
8028: limit how much PCRE recurses. match_limit() limits the number of
8029: matches PCRE does; match_limit_recursion() limits the depth of internal
8030: recursion, and therefore the amount of stack that is used.
8031:
8032: Normally, to pass one or more modifiers to a RE class, you declare a
8033: RE_Options object, set the appropriate options, and pass this object to
8034: a RE constructor. Example:
8035:
8036: RE_Options opt;
8037: opt.set_caseless(true);
8038: if (RE("HELLO", opt).PartialMatch("hello world")) ...
8039:
8040: RE_options has two constructors. The default constructor takes no argu-
8041: ments and creates a set of flags that are off by default. The optional
8042: parameter option_flags is to facilitate transfer of legacy code from C
8043: programs. This lets you do
8044:
8045: RE(pattern,
8046: RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
8047:
8048: However, new code is better off doing
8049:
8050: RE(pattern,
8051: RE_Options().set_caseless(true).set_multiline(true))
8052: .PartialMatch(str);
8053:
8054: If you are going to pass one of the most used modifiers, there are some
8055: convenience functions that return a RE_Options class with the appropri-
8056: ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
8057: and EXTENDED().
8058:
8059: If you need to set several options at once, and you don't want to go
8060: through the pains of declaring a RE_Options object and setting several
8061: options, there is a parallel method that give you such ability on the
8062: fly. You can concatenate several set_xxxxx() member functions, since
8063: each of them returns a reference to its class object. For example, to
8064: pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
8065: statement, you may write:
8066:
8067: RE(" ^ xyz \\s+ .* blah$",
8068: RE_Options()
8069: .set_caseless(true)
8070: .set_extended(true)
8071: .set_multiline(true)).PartialMatch(sometext);
8072:
8073:
8074: SCANNING TEXT INCREMENTALLY
8075:
8076: The "Consume" operation may be useful if you want to repeatedly match
8077: regular expressions at the front of a string and skip over them as they
8078: match. This requires use of the "StringPiece" type, which represents a
8079: sub-range of a real string. Like RE, StringPiece is defined in the
8080: pcrecpp namespace.
8081:
8082: Example: read lines of the form "var = value" from a string.
8083: string contents = ...; // Fill string somehow
8084: pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
8085:
8086: string var;
8087: int value;
8088: pcrecpp::RE re("(\\w+) = (\\d+)\n");
8089: while (re.Consume(&input, &var, &value)) {
8090: ...;
8091: }
8092:
8093: Each successful call to "Consume" will set "var/value", and also
8094: advance "input" so it points past the matched text.
8095:
8096: The "FindAndConsume" operation is similar to "Consume" but does not
8097: anchor your match at the beginning of the string. For example, you
8098: could extract all words from a string by repeatedly calling
8099:
8100: pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)
8101:
8102:
8103: PARSING HEX/OCTAL/C-RADIX NUMBERS
8104:
8105: By default, if you pass a pointer to a numeric value, the corresponding
8106: text is interpreted as a base-10 number. You can instead wrap the
8107: pointer with a call to one of the operators Hex(), Octal(), or CRadix()
8108: to interpret the text in another base. The CRadix operator interprets
8109: C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to
8110: base-10.
8111:
8112: Example:
8113: int a, b, c, d;
8114: pcrecpp::RE re("(.*) (.*) (.*) (.*)");
8115: re.FullMatch("100 40 0100 0x40",
8116: pcrecpp::Octal(&a), pcrecpp::Hex(&b),
8117: pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));
8118:
8119: will leave 64 in a, b, c, and d.
8120:
8121:
8122: REPLACING PARTS OF STRINGS
8123:
8124: You can replace the first match of "pattern" in "str" with "rewrite".
8125: Within "rewrite", backslash-escaped digits (\1 to \9) can be used to
8126: insert text matching corresponding parenthesized group from the pat-
8127: tern. \0 in "rewrite" refers to the entire matching text. For example:
8128:
8129: string s = "yabba dabba doo";
8130: pcrecpp::RE("b+").Replace("d", &s);
8131:
8132: will leave "s" containing "yada dabba doo". The result is true if the
8133: pattern matches and a replacement occurs, false otherwise.
8134:
8135: GlobalReplace is like Replace except that it replaces all occurrences
8136: of the pattern in the string with the rewrite. Replacements are not
8137: subject to re-matching. For example:
8138:
8139: string s = "yabba dabba doo";
8140: pcrecpp::RE("b+").GlobalReplace("d", &s);
8141:
8142: will leave "s" containing "yada dada doo". It returns the number of
8143: replacements made.
8144:
8145: Extract is like Replace, except that if the pattern matches, "rewrite"
8146: is copied into "out" (an additional argument) with substitutions. The
8147: non-matching portions of "text" are ignored. Returns true iff a match
8148: occurred and the extraction happened successfully; if no match occurs,
8149: the string is left unaffected.
8150:
8151:
8152: AUTHOR
8153:
8154: The C++ wrapper was contributed by Google Inc.
8155: Copyright (c) 2007 Google Inc.
8156:
8157:
8158: REVISION
8159:
8160: Last updated: 17 March 2009
8161: Minor typo fixed: 25 July 2011
8162: ------------------------------------------------------------------------------
8163:
8164:
8165: PCRESAMPLE(3) PCRESAMPLE(3)
8166:
8167:
8168: NAME
8169: PCRE - Perl-compatible regular expressions
8170:
8171:
8172: PCRE SAMPLE PROGRAM
8173:
8174: A simple, complete demonstration program, to get you started with using
8175: PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A
8176: listing of this program is given in the pcredemo documentation. If you
8177: do not have a copy of the PCRE distribution, you can save this listing
8178: to re-create pcredemo.c.
8179:
8180: The program compiles the regular expression that is its first argument,
8181: and matches it against the subject string in its second argument. No
8182: PCRE options are set, and default character tables are used. If match-
8183: ing succeeds, the program outputs the portion of the subject that
8184: matched, together with the contents of any captured substrings.
8185:
8186: If the -g option is given on the command line, the program then goes on
8187: to check for further matches of the same regular expression in the same
8188: subject string. The logic is a little bit tricky because of the possi-
8189: bility of matching an empty string. Comments in the code explain what
8190: is going on.
8191:
8192: If PCRE is installed in the standard include and library directories
8193: for your operating system, you should be able to compile the demonstra-
8194: tion program using this command:
8195:
8196: gcc -o pcredemo pcredemo.c -lpcre
8197:
8198: If PCRE is installed elsewhere, you may need to add additional options
8199: to the command line. For example, on a Unix-like system that has PCRE
8200: installed in /usr/local, you can compile the demonstration program
8201: using a command like this:
8202:
8203: gcc -o pcredemo -I/usr/local/include pcredemo.c \
8204: -L/usr/local/lib -lpcre
8205:
8206: In a Windows environment, if you want to statically link the program
8207: against a non-dll pcre.a file, you must uncomment the line that defines
8208: PCRE_STATIC before including pcre.h, because otherwise the pcre_mal-
8209: loc() and pcre_free() exported functions will be declared
8210: __declspec(dllimport), with unwanted results.
8211:
8212: Once you have compiled and linked the demonstration program, you can
8213: run simple tests like this:
8214:
8215: ./pcredemo 'cat|dog' 'the cat sat on the mat'
8216: ./pcredemo -g 'cat|dog' 'the dog sat on the cat'
8217:
8218: Note that there is a much more comprehensive test program, called
8219: pcretest, which supports many more facilities for testing regular
8220: expressions and the PCRE library. The pcredemo program is provided as a
8221: simple coding example.
8222:
8223: If you try to run pcredemo when PCRE is not installed in the standard
8224: library directory, you may get an error like this on some operating
8225: systems (e.g. Solaris):
8226:
8227: ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
8228: directory
8229:
8230: This is caused by the way shared library support works on those sys-
8231: tems. You need to add
8232:
8233: -R/usr/local/lib
8234:
8235: (for example) to the compile command to get round this problem.
8236:
8237:
8238: AUTHOR
8239:
8240: Philip Hazel
8241: University Computing Service
8242: Cambridge CB2 3QH, England.
8243:
8244:
8245: REVISION
8246:
8247: Last updated: 17 November 2010
8248: Copyright (c) 1997-2010 University of Cambridge.
8249: ------------------------------------------------------------------------------
8250: PCRELIMITS(3) PCRELIMITS(3)
8251:
8252:
8253: NAME
8254: PCRE - Perl-compatible regular expressions
8255:
8256:
8257: SIZE AND OTHER LIMITATIONS
8258:
8259: There are some size limitations in PCRE but it is hoped that they will
8260: never in practice be relevant.
8261:
8262: The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
8263: is compiled with the default internal linkage size of 2. If you want to
8264: process regular expressions that are truly enormous, you can compile
8265: PCRE with an internal linkage size of 3 or 4 (see the README file in
8266: the source distribution and the pcrebuild documentation for details).
8267: In these cases the limit is substantially larger. However, the speed
8268: of execution is slower.
8269:
8270: All values in repeating quantifiers must be less than 65536.
8271:
8272: There is no limit to the number of parenthesized subpatterns, but there
8273: can be no more than 65535 capturing subpatterns.
8274:
8275: There is a limit to the number of forward references to subsequent sub-
8276: patterns of around 200,000. Repeated forward references with fixed
8277: upper limits, for example, (?2){0,100} when subpattern number 2 is to
8278: the right, are included in the count. There is no limit to the number
8279: of backward references.
8280:
8281: The maximum length of name for a named subpattern is 32 characters, and
8282: the maximum number of named subpatterns is 10000.
8283:
8284: The maximum length of a subject string is the largest positive number
8285: that an integer variable can hold. However, when using the traditional
8286: matching function, PCRE uses recursion to handle subpatterns and indef-
8287: inite repetition. This means that the available stack space may limit
8288: the size of a subject string that can be processed by certain patterns.
8289: For a discussion of stack issues, see the pcrestack documentation.
8290:
8291:
8292: AUTHOR
8293:
8294: Philip Hazel
8295: University Computing Service
8296: Cambridge CB2 3QH, England.
8297:
8298:
8299: REVISION
8300:
8301: Last updated: 30 November 2011
8302: Copyright (c) 1997-2011 University of Cambridge.
8303: ------------------------------------------------------------------------------
8304:
8305:
8306: PCRESTACK(3) PCRESTACK(3)
8307:
8308:
8309: NAME
8310: PCRE - Perl-compatible regular expressions
8311:
8312:
8313: PCRE DISCUSSION OF STACK USAGE
8314:
8315: When you call pcre_exec(), it makes use of an internal function called
8316: match(). This calls itself recursively at branch points in the pattern,
8317: in order to remember the state of the match so that it can back up and
8318: try a different alternative if the first one fails. As matching pro-
8319: ceeds deeper and deeper into the tree of possibilities, the recursion
8320: depth increases. The match() function is also called in other circum-
8321: stances, for example, whenever a parenthesized sub-pattern is entered,
8322: and in certain cases of repetition.
8323:
8324: Not all calls of match() increase the recursion depth; for an item such
8325: as a* it may be called several times at the same level, after matching
8326: different numbers of a's. Furthermore, in a number of cases where the
8327: result of the recursive call would immediately be passed back as the
8328: result of the current call (a "tail recursion"), the function is just
8329: restarted instead.
8330:
8331: The above comments apply when pcre_exec() is run in its normal inter-
8332: pretive manner. If the pattern was studied with the PCRE_STUDY_JIT_COM-
8333: PILE option, and just-in-time compiling was successful, and the options
8334: passed to pcre_exec() were not incompatible, the matching process uses
8335: the JIT-compiled code instead of the match() function. In this case,
8336: the memory requirements are handled entirely differently. See the pcre-
8337: jit documentation for details.
8338:
8339: The pcre_dfa_exec() function operates in an entirely different way, and
8340: uses recursion only when there is a regular expression recursion or
8341: subroutine call in the pattern. This includes the processing of asser-
8342: tion and "once-only" subpatterns, which are handled like subroutine
8343: calls. Normally, these are never very deep, and the limit on the com-
8344: plexity of pcre_dfa_exec() is controlled by the amount of workspace it
8345: is given. However, it is possible to write patterns with runaway infi-
8346: nite recursions; such patterns will cause pcre_dfa_exec() to run out of
8347: stack. At present, there is no protection against this.
8348:
8349: The comments that follow do NOT apply to pcre_dfa_exec(); they are rel-
8350: evant only for pcre_exec() without the JIT optimization.
8351:
8352: Reducing pcre_exec()'s stack usage
8353:
8354: Each time that match() is actually called recursively, it uses memory
8355: from the process stack. For certain kinds of pattern and data, very
8356: large amounts of stack may be needed, despite the recognition of "tail
8357: recursion". You can often reduce the amount of recursion, and there-
8358: fore the amount of stack used, by modifying the pattern that is being
8359: matched. Consider, for example, this pattern:
8360:
8361: ([^<]|<(?!inet))+
8362:
8363: It matches from wherever it starts until it encounters "<inet" or the
8364: end of the data, and is the kind of pattern that might be used when
8365: processing an XML file. Each iteration of the outer parentheses matches
8366: either one character that is not "<" or a "<" that is not followed by
8367: "inet". However, each time a parenthesis is processed, a recursion
8368: occurs, so this formulation uses a stack frame for each matched charac-
8369: ter. For a long string, a lot of stack is required. Consider now this
8370: rewritten pattern, which matches exactly the same strings:
8371:
8372: ([^<]++|<(?!inet))+
8373:
8374: This uses very much less stack, because runs of characters that do not
8375: contain "<" are "swallowed" in one item inside the parentheses. Recur-
8376: sion happens only when a "<" character that is not followed by "inet"
8377: is encountered (and we assume this is relatively rare). A possessive
8378: quantifier is used to stop any backtracking into the runs of non-"<"
8379: characters, but that is not related to stack usage.
8380:
8381: This example shows that one way of avoiding stack problems when match-
8382: ing long subject strings is to write repeated parenthesized subpatterns
8383: to match more than one character whenever possible.
8384:
8385: Compiling PCRE to use heap instead of stack for pcre_exec()
8386:
8387: In environments where stack memory is constrained, you might want to
8388: compile PCRE to use heap memory instead of stack for remembering back-
8389: up points when pcre_exec() is running. This makes it run a lot more
8390: slowly, however. Details of how to do this are given in the pcrebuild
8391: documentation. When built in this way, instead of using the stack, PCRE
8392: obtains and frees memory by calling the functions that are pointed to
8393: by the pcre_stack_malloc and pcre_stack_free variables. By default,
8394: these point to malloc() and free(), but you can replace the pointers to
8395: cause PCRE to use your own functions. Since the block sizes are always
8396: the same, and are always freed in reverse order, it may be possible to
8397: implement customized memory handlers that are more efficient than the
8398: standard functions.
8399:
8400: Limiting pcre_exec()'s stack usage
8401:
8402: You can set limits on the number of times that match() is called, both
8403: in total and recursively. If a limit is exceeded, pcre_exec() returns
8404: an error code. Setting suitable limits should prevent it from running
8405: out of stack. The default values of the limits are very large, and
8406: unlikely ever to operate. They can be changed when PCRE is built, and
8407: they can also be set when pcre_exec() is called. For details of these
8408: interfaces, see the pcrebuild documentation and the section on extra
8409: data for pcre_exec() in the pcreapi documentation.
8410:
8411: As a very rough rule of thumb, you should reckon on about 500 bytes per
8412: recursion. Thus, if you want to limit your stack usage to 8Mb, you
8413: should set the limit at 16000 recursions. A 64Mb stack, on the other
8414: hand, can support around 128000 recursions.
8415:
8416: In Unix-like environments, the pcretest test program has a command line
8417: option (-S) that can be used to increase the size of its stack. As long
8418: as the stack is large enough, another option (-M) can be used to find
8419: the smallest limits that allow a particular pattern to match a given
8420: subject string. This is done by calling pcre_exec() repeatedly with
8421: different limits.
8422:
8423: Changing stack size in Unix-like systems
8424:
8425: In Unix-like environments, there is not often a problem with the stack
8426: unless very long strings are involved, though the default limit on
8427: stack size varies from system to system. Values from 8Mb to 64Mb are
8428: common. You can find your default limit by running the command:
8429:
8430: ulimit -s
8431:
8432: Unfortunately, the effect of running out of stack is often SIGSEGV,
8433: though sometimes a more explicit error message is given. You can nor-
8434: mally increase the limit on stack size by code such as this:
8435:
8436: struct rlimit rlim;
8437: getrlimit(RLIMIT_STACK, &rlim);
8438: rlim.rlim_cur = 100*1024*1024;
8439: setrlimit(RLIMIT_STACK, &rlim);
8440:
8441: This reads the current limits (soft and hard) using getrlimit(), then
8442: attempts to increase the soft limit to 100Mb using setrlimit(). You
8443: must do this before calling pcre_exec().
8444:
8445: Changing stack size in Mac OS X
8446:
8447: Using setrlimit(), as described above, should also work on Mac OS X. It
8448: is also possible to set a stack size when linking a program. There is a
8449: discussion about stack sizes in Mac OS X at this web site:
8450: http://developer.apple.com/qa/qa2005/qa1419.html.
8451:
8452:
8453: AUTHOR
8454:
8455: Philip Hazel
8456: University Computing Service
8457: Cambridge CB2 3QH, England.
8458:
8459:
8460: REVISION
8461:
8462: Last updated: 26 August 2011
8463: Copyright (c) 1997-2011 University of Cambridge.
8464: ------------------------------------------------------------------------------
8465:
8466:
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