Annotation of embedaddon/pcre/doc/pcrepattern.3, revision 1.1.1.4
1.1.1.4 ! misho 1: .TH PCREPATTERN 3 "26 April 2013" "PCRE 8.33"
1.1 misho 2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "PCRE REGULAR EXPRESSION DETAILS"
5: .rs
6: .sp
7: The syntax and semantics of the regular expressions that are supported by PCRE
8: are described in detail below. There is a quick-reference syntax summary in the
9: .\" HREF
10: \fBpcresyntax\fP
11: .\"
12: page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13: also supports some alternative regular expression syntax (which does not
14: conflict with the Perl syntax) in order to provide some compatibility with
15: regular expressions in Python, .NET, and Oniguruma.
16: .P
17: Perl's regular expressions are described in its own documentation, and
18: regular expressions in general are covered in a number of books, some of which
19: have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20: published by O'Reilly, covers regular expressions in great detail. This
21: description of PCRE's regular expressions is intended as reference material.
22: .P
1.1.1.4 ! misho 23: This document discusses the patterns that are supported by PCRE when one its
! 24: main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
! 25: (16- or 32-bit), is used. PCRE also has alternative matching functions,
! 26: \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
! 27: different algorithm that is not Perl-compatible. Some of the features discussed
! 28: below are not available when DFA matching is used. The advantages and
! 29: disadvantages of the alternative functions, and how they differ from the normal
! 30: functions, are discussed in the
! 31: .\" HREF
! 32: \fBpcrematching\fP
! 33: .\"
! 34: page.
! 35: .
! 36: .
! 37: .SH "SPECIAL START-OF-PATTERN ITEMS"
! 38: .rs
! 39: .sp
! 40: A number of options that can be passed to \fBpcre_compile()\fP can also be set
! 41: by special items at the start of a pattern. These are not Perl-compatible, but
! 42: are provided to make these options accessible to pattern writers who are not
! 43: able to change the program that processes the pattern. Any number of these
! 44: items may appear, but they must all be together right at the start of the
! 45: pattern string, and the letters must be in upper case.
! 46: .
! 47: .
! 48: .SS "UTF support"
! 49: .rs
! 50: .sp
1.1 misho 51: The original operation of PCRE was on strings of one-byte characters. However,
1.1.1.4 ! misho 52: there is now also support for UTF-8 strings in the original library, an
! 53: extra library that supports 16-bit and UTF-16 character strings, and a
! 54: third library that supports 32-bit and UTF-32 character strings. To use these
1.1.1.2 misho 55: features, PCRE must be built to include appropriate support. When using UTF
1.1.1.4 ! misho 56: strings you must either call the compiling function with the PCRE_UTF8,
! 57: PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
! 58: these special sequences:
1.1 misho 59: .sp
60: (*UTF8)
1.1.1.2 misho 61: (*UTF16)
1.1.1.4 ! misho 62: (*UTF32)
! 63: (*UTF)
1.1 misho 64: .sp
1.1.1.4 ! misho 65: (*UTF) is a generic sequence that can be used with any of the libraries.
1.1.1.2 misho 66: Starting a pattern with such a sequence is equivalent to setting the relevant
1.1.1.4 ! misho 67: option. How setting a UTF mode affects pattern matching is mentioned in several
! 68: places below. There is also a summary of features in the
1.1 misho 69: .\" HREF
70: \fBpcreunicode\fP
71: .\"
72: page.
73: .P
1.1.1.4 ! misho 74: Some applications that allow their users to supply patterns may wish to
! 75: restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
! 76: option is set at compile time, (*UTF) etc. are not allowed, and their
! 77: appearance causes an error.
! 78: .
! 79: .
! 80: .SS "Unicode property support"
! 81: .rs
! 82: .sp
! 83: Another special sequence that may appear at the start of a pattern is
1.1 misho 84: .sp
85: (*UCP)
86: .sp
87: This has the same effect as setting the PCRE_UCP option: it causes sequences
88: such as \ed and \ew to use Unicode properties to determine character types,
89: instead of recognizing only characters with codes less than 128 via a lookup
90: table.
1.1.1.4 ! misho 91: .
! 92: .
! 93: .SS "Disabling start-up optimizations"
! 94: .rs
! 95: .sp
1.1 misho 96: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
1.1.1.4 ! misho 97: PCRE_NO_START_OPTIMIZE option either at compile or matching time.
1.1 misho 98: .
99: .
100: .\" HTML <a name="newlines"></a>
1.1.1.4 ! misho 101: .SS "Newline conventions"
1.1 misho 102: .rs
103: .sp
104: PCRE supports five different conventions for indicating line breaks in
105: strings: a single CR (carriage return) character, a single LF (linefeed)
106: character, the two-character sequence CRLF, any of the three preceding, or any
107: Unicode newline sequence. The
108: .\" HREF
109: \fBpcreapi\fP
110: .\"
111: page has
112: .\" HTML <a href="pcreapi.html#newlines">
113: .\" </a>
114: further discussion
115: .\"
116: about newlines, and shows how to set the newline convention in the
117: \fIoptions\fP arguments for the compiling and matching functions.
118: .P
119: It is also possible to specify a newline convention by starting a pattern
120: string with one of the following five sequences:
121: .sp
122: (*CR) carriage return
123: (*LF) linefeed
124: (*CRLF) carriage return, followed by linefeed
125: (*ANYCRLF) any of the three above
126: (*ANY) all Unicode newline sequences
127: .sp
1.1.1.2 misho 128: These override the default and the options given to the compiling function. For
129: example, on a Unix system where LF is the default newline sequence, the pattern
1.1 misho 130: .sp
131: (*CR)a.b
132: .sp
133: changes the convention to CR. That pattern matches "a\enb" because LF is no
1.1.1.4 ! misho 134: longer a newline. If more than one of these settings is present, the last one
1.1 misho 135: is used.
136: .P
1.1.1.4 ! misho 137: The newline convention affects where the circumflex and dollar assertions are
! 138: true. It also affects the interpretation of the dot metacharacter when
! 139: PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
! 140: what the \eR escape sequence matches. By default, this is any Unicode newline
! 141: sequence, for Perl compatibility. However, this can be changed; see the
1.1 misho 142: description of \eR in the section entitled
143: .\" HTML <a href="#newlineseq">
144: .\" </a>
145: "Newline sequences"
146: .\"
147: below. A change of \eR setting can be combined with a change of newline
148: convention.
149: .
150: .
1.1.1.4 ! misho 151: .SS "Setting match and recursion limits"
! 152: .rs
! 153: .sp
! 154: The caller of \fBpcre_exec()\fP can set a limit on the number of times the
! 155: internal \fBmatch()\fP function is called and on the maximum depth of
! 156: recursive calls. These facilities are provided to catch runaway matches that
! 157: are provoked by patterns with huge matching trees (a typical example is a
! 158: pattern with nested unlimited repeats) and to avoid running out of system stack
! 159: by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
! 160: gives an error return. The limits can also be set by items at the start of the
! 161: pattern of the form
! 162: .sp
! 163: (*LIMIT_MATCH=d)
! 164: (*LIMIT_RECURSION=d)
! 165: .sp
! 166: where d is any number of decimal digits. However, the value of the setting must
! 167: be less than the value set by the caller of \fBpcre_exec()\fP for it to have
! 168: any effect. In other words, the pattern writer can lower the limit set by the
! 169: programmer, but not raise it. If there is more than one setting of one of these
! 170: limits, the lower value is used.
! 171: .
! 172: .
! 173: .SH "EBCDIC CHARACTER CODES"
! 174: .rs
! 175: .sp
! 176: PCRE can be compiled to run in an environment that uses EBCDIC as its character
! 177: code rather than ASCII or Unicode (typically a mainframe system). In the
! 178: sections below, character code values are ASCII or Unicode; in an EBCDIC
! 179: environment these characters may have different code values, and there are no
! 180: code points greater than 255.
! 181: .
! 182: .
1.1 misho 183: .SH "CHARACTERS AND METACHARACTERS"
184: .rs
185: .sp
186: A regular expression is a pattern that is matched against a subject string from
187: left to right. Most characters stand for themselves in a pattern, and match the
188: corresponding characters in the subject. As a trivial example, the pattern
189: .sp
190: The quick brown fox
191: .sp
192: matches a portion of a subject string that is identical to itself. When
193: caseless matching is specified (the PCRE_CASELESS option), letters are matched
1.1.1.2 misho 194: independently of case. In a UTF mode, PCRE always understands the concept of
1.1 misho 195: case for characters whose values are less than 128, so caseless matching is
196: always possible. For characters with higher values, the concept of case is
197: supported if PCRE is compiled with Unicode property support, but not otherwise.
198: If you want to use caseless matching for characters 128 and above, you must
199: ensure that PCRE is compiled with Unicode property support as well as with
1.1.1.2 misho 200: UTF support.
1.1 misho 201: .P
202: The power of regular expressions comes from the ability to include alternatives
203: and repetitions in the pattern. These are encoded in the pattern by the use of
204: \fImetacharacters\fP, which do not stand for themselves but instead are
205: interpreted in some special way.
206: .P
207: There are two different sets of metacharacters: those that are recognized
208: anywhere in the pattern except within square brackets, and those that are
209: recognized within square brackets. Outside square brackets, the metacharacters
210: are as follows:
211: .sp
212: \e general escape character with several uses
213: ^ assert start of string (or line, in multiline mode)
214: $ assert end of string (or line, in multiline mode)
215: . match any character except newline (by default)
216: [ start character class definition
217: | start of alternative branch
218: ( start subpattern
219: ) end subpattern
220: ? extends the meaning of (
221: also 0 or 1 quantifier
222: also quantifier minimizer
223: * 0 or more quantifier
224: + 1 or more quantifier
225: also "possessive quantifier"
226: { start min/max quantifier
227: .sp
228: Part of a pattern that is in square brackets is called a "character class". In
229: a character class the only metacharacters are:
230: .sp
231: \e general escape character
232: ^ negate the class, but only if the first character
233: - indicates character range
234: .\" JOIN
235: [ POSIX character class (only if followed by POSIX
236: syntax)
237: ] terminates the character class
238: .sp
239: The following sections describe the use of each of the metacharacters.
240: .
241: .
242: .SH BACKSLASH
243: .rs
244: .sp
245: The backslash character has several uses. Firstly, if it is followed by a
246: character that is not a number or a letter, it takes away any special meaning
247: that character may have. This use of backslash as an escape character applies
248: both inside and outside character classes.
249: .P
250: For example, if you want to match a * character, you write \e* in the pattern.
251: This escaping action applies whether or not the following character would
252: otherwise be interpreted as a metacharacter, so it is always safe to precede a
253: non-alphanumeric with backslash to specify that it stands for itself. In
254: particular, if you want to match a backslash, you write \e\e.
255: .P
1.1.1.2 misho 256: In a UTF mode, only ASCII numbers and letters have any special meaning after a
1.1 misho 257: backslash. All other characters (in particular, those whose codepoints are
258: greater than 127) are treated as literals.
259: .P
1.1.1.3 misho 260: If a pattern is compiled with the PCRE_EXTENDED option, white space in the
1.1 misho 261: pattern (other than in a character class) and characters between a # outside
262: a character class and the next newline are ignored. An escaping backslash can
1.1.1.3 misho 263: be used to include a white space or # character as part of the pattern.
1.1 misho 264: .P
265: If you want to remove the special meaning from a sequence of characters, you
266: can do so by putting them between \eQ and \eE. This is different from Perl in
267: that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
268: Perl, $ and @ cause variable interpolation. Note the following examples:
269: .sp
270: Pattern PCRE matches Perl matches
271: .sp
272: .\" JOIN
273: \eQabc$xyz\eE abc$xyz abc followed by the
274: contents of $xyz
275: \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
276: \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
277: .sp
278: The \eQ...\eE sequence is recognized both inside and outside character classes.
279: An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
280: by \eE later in the pattern, the literal interpretation continues to the end of
281: the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
282: a character class, this causes an error, because the character class is not
283: terminated.
284: .
285: .
286: .\" HTML <a name="digitsafterbackslash"></a>
287: .SS "Non-printing characters"
288: .rs
289: .sp
290: A second use of backslash provides a way of encoding non-printing characters
291: in patterns in a visible manner. There is no restriction on the appearance of
292: non-printing characters, apart from the binary zero that terminates a pattern,
293: but when a pattern is being prepared by text editing, it is often easier to use
294: one of the following escape sequences than the binary character it represents:
295: .sp
296: \ea alarm, that is, the BEL character (hex 07)
297: \ecx "control-x", where x is any ASCII character
298: \ee escape (hex 1B)
1.1.1.3 misho 299: \ef form feed (hex 0C)
1.1 misho 300: \en linefeed (hex 0A)
301: \er carriage return (hex 0D)
302: \et tab (hex 09)
303: \eddd character with octal code ddd, or back reference
304: \exhh character with hex code hh
305: \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
306: \euhhhh character with hex code hhhh (JavaScript mode only)
307: .sp
1.1.1.4 ! misho 308: The precise effect of \ecx on ASCII characters is as follows: if x is a lower
! 309: case letter, it is converted to upper case. Then bit 6 of the character (hex
! 310: 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
! 311: but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
! 312: data item (byte or 16-bit value) following \ec has a value greater than 127, a
! 313: compile-time error occurs. This locks out non-ASCII characters in all modes.
! 314: .P
! 315: The \ec facility was designed for use with ASCII characters, but with the
! 316: extension to Unicode it is even less useful than it once was. It is, however,
! 317: recognized when PCRE is compiled in EBCDIC mode, where data items are always
! 318: bytes. In this mode, all values are valid after \ec. If the next character is a
! 319: lower case letter, it is converted to upper case. Then the 0xc0 bits of the
! 320: byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
! 321: the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
! 322: characters also generate different values.
1.1 misho 323: .P
324: By default, after \ex, from zero to two hexadecimal digits are read (letters
325: can be in upper or lower case). Any number of hexadecimal digits may appear
1.1.1.2 misho 326: between \ex{ and }, but the character code is constrained as follows:
327: .sp
328: 8-bit non-UTF mode less than 0x100
329: 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
330: 16-bit non-UTF mode less than 0x10000
331: 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
1.1.1.4 ! misho 332: 32-bit non-UTF mode less than 0x80000000
! 333: 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
1.1.1.2 misho 334: .sp
335: Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
1.1.1.4 ! misho 336: "surrogate" codepoints), and 0xffef.
1.1 misho 337: .P
338: If characters other than hexadecimal digits appear between \ex{ and }, or if
339: there is no terminating }, this form of escape is not recognized. Instead, the
340: initial \ex will be interpreted as a basic hexadecimal escape, with no
341: following digits, giving a character whose value is zero.
342: .P
343: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
344: as just described only when it is followed by two hexadecimal digits.
345: Otherwise, it matches a literal "x" character. In JavaScript mode, support for
346: code points greater than 256 is provided by \eu, which must be followed by
347: four hexadecimal digits; otherwise it matches a literal "u" character.
1.1.1.3 misho 348: Character codes specified by \eu in JavaScript mode are constrained in the same
349: was as those specified by \ex in non-JavaScript mode.
1.1 misho 350: .P
351: Characters whose value is less than 256 can be defined by either of the two
352: syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
353: way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
354: \eu00dc in JavaScript mode).
355: .P
356: After \e0 up to two further octal digits are read. If there are fewer than two
357: digits, just those that are present are used. Thus the sequence \e0\ex\e07
358: specifies two binary zeros followed by a BEL character (code value 7). Make
359: sure you supply two digits after the initial zero if the pattern character that
360: follows is itself an octal digit.
361: .P
362: The handling of a backslash followed by a digit other than 0 is complicated.
363: Outside a character class, PCRE reads it and any following digits as a decimal
364: number. If the number is less than 10, or if there have been at least that many
365: previous capturing left parentheses in the expression, the entire sequence is
366: taken as a \fIback reference\fP. A description of how this works is given
367: .\" HTML <a href="#backreferences">
368: .\" </a>
369: later,
370: .\"
371: following the discussion of
372: .\" HTML <a href="#subpattern">
373: .\" </a>
374: parenthesized subpatterns.
375: .\"
376: .P
377: Inside a character class, or if the decimal number is greater than 9 and there
378: have not been that many capturing subpatterns, PCRE re-reads up to three octal
379: digits following the backslash, and uses them to generate a data character. Any
1.1.1.2 misho 380: subsequent digits stand for themselves. The value of the character is
381: constrained in the same way as characters specified in hexadecimal.
382: For example:
1.1 misho 383: .sp
1.1.1.4 ! misho 384: \e040 is another way of writing an ASCII space
1.1 misho 385: .\" JOIN
386: \e40 is the same, provided there are fewer than 40
387: previous capturing subpatterns
388: \e7 is always a back reference
389: .\" JOIN
390: \e11 might be a back reference, or another way of
391: writing a tab
392: \e011 is always a tab
393: \e0113 is a tab followed by the character "3"
394: .\" JOIN
395: \e113 might be a back reference, otherwise the
396: character with octal code 113
397: .\" JOIN
398: \e377 might be a back reference, otherwise
1.1.1.2 misho 399: the value 255 (decimal)
1.1 misho 400: .\" JOIN
401: \e81 is either a back reference, or a binary zero
402: followed by the two characters "8" and "1"
403: .sp
404: Note that octal values of 100 or greater must not be introduced by a leading
405: zero, because no more than three octal digits are ever read.
406: .P
407: All the sequences that define a single character value can be used both inside
408: and outside character classes. In addition, inside a character class, \eb is
409: interpreted as the backspace character (hex 08).
410: .P
411: \eN is not allowed in a character class. \eB, \eR, and \eX are not special
412: inside a character class. Like other unrecognized escape sequences, they are
413: treated as the literal characters "B", "R", and "X" by default, but cause an
414: error if the PCRE_EXTRA option is set. Outside a character class, these
415: sequences have different meanings.
416: .
417: .
418: .SS "Unsupported escape sequences"
419: .rs
420: .sp
421: In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
422: handler and used to modify the case of following characters. By default, PCRE
423: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
424: option is set, \eU matches a "U" character, and \eu can be used to define a
425: character by code point, as described in the previous section.
426: .
427: .
428: .SS "Absolute and relative back references"
429: .rs
430: .sp
431: The sequence \eg followed by an unsigned or a negative number, optionally
432: enclosed in braces, is an absolute or relative back reference. A named back
433: reference can be coded as \eg{name}. Back references are discussed
434: .\" HTML <a href="#backreferences">
435: .\" </a>
436: later,
437: .\"
438: following the discussion of
439: .\" HTML <a href="#subpattern">
440: .\" </a>
441: parenthesized subpatterns.
442: .\"
443: .
444: .
445: .SS "Absolute and relative subroutine calls"
446: .rs
447: .sp
448: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
449: a number enclosed either in angle brackets or single quotes, is an alternative
450: syntax for referencing a subpattern as a "subroutine". Details are discussed
451: .\" HTML <a href="#onigurumasubroutines">
452: .\" </a>
453: later.
454: .\"
455: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
456: synonymous. The former is a back reference; the latter is a
457: .\" HTML <a href="#subpatternsassubroutines">
458: .\" </a>
459: subroutine
460: .\"
461: call.
462: .
463: .
464: .\" HTML <a name="genericchartypes"></a>
465: .SS "Generic character types"
466: .rs
467: .sp
468: Another use of backslash is for specifying generic character types:
469: .sp
470: \ed any decimal digit
471: \eD any character that is not a decimal digit
1.1.1.3 misho 472: \eh any horizontal white space character
473: \eH any character that is not a horizontal white space character
474: \es any white space character
475: \eS any character that is not a white space character
476: \ev any vertical white space character
477: \eV any character that is not a vertical white space character
1.1 misho 478: \ew any "word" character
479: \eW any "non-word" character
480: .sp
481: There is also the single sequence \eN, which matches a non-newline character.
482: This is the same as
483: .\" HTML <a href="#fullstopdot">
484: .\" </a>
485: the "." metacharacter
486: .\"
487: when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
488: PCRE does not support this.
489: .P
490: Each pair of lower and upper case escape sequences partitions the complete set
491: of characters into two disjoint sets. Any given character matches one, and only
492: one, of each pair. The sequences can appear both inside and outside character
493: classes. They each match one character of the appropriate type. If the current
494: matching point is at the end of the subject string, all of them fail, because
495: there is no character to match.
496: .P
497: For compatibility with Perl, \es does not match the VT character (code 11).
498: This makes it different from the the POSIX "space" class. The \es characters
499: are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
500: included in a Perl script, \es may match the VT character. In PCRE, it never
501: does.
502: .P
503: A "word" character is an underscore or any character that is a letter or digit.
504: By default, the definition of letters and digits is controlled by PCRE's
505: low-valued character tables, and may vary if locale-specific matching is taking
506: place (see
507: .\" HTML <a href="pcreapi.html#localesupport">
508: .\" </a>
509: "Locale support"
510: .\"
511: in the
512: .\" HREF
513: \fBpcreapi\fP
514: .\"
515: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
516: or "french" in Windows, some character codes greater than 128 are used for
517: accented letters, and these are then matched by \ew. The use of locales with
518: Unicode is discouraged.
519: .P
1.1.1.2 misho 520: By default, in a UTF mode, characters with values greater than 128 never match
1.1 misho 521: \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
1.1.1.2 misho 522: their original meanings from before UTF support was available, mainly for
1.1 misho 523: efficiency reasons. However, if PCRE is compiled with Unicode property support,
524: and the PCRE_UCP option is set, the behaviour is changed so that Unicode
525: properties are used to determine character types, as follows:
526: .sp
527: \ed any character that \ep{Nd} matches (decimal digit)
528: \es any character that \ep{Z} matches, plus HT, LF, FF, CR
529: \ew any character that \ep{L} or \ep{N} matches, plus underscore
530: .sp
531: The upper case escapes match the inverse sets of characters. Note that \ed
532: matches only decimal digits, whereas \ew matches any Unicode digit, as well as
533: any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
534: \eB because they are defined in terms of \ew and \eW. Matching these sequences
535: is noticeably slower when PCRE_UCP is set.
536: .P
537: The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
538: release 5.10. In contrast to the other sequences, which match only ASCII
1.1.1.2 misho 539: characters by default, these always match certain high-valued codepoints,
540: whether or not PCRE_UCP is set. The horizontal space characters are:
1.1 misho 541: .sp
1.1.1.4 ! misho 542: U+0009 Horizontal tab (HT)
1.1 misho 543: U+0020 Space
544: U+00A0 Non-break space
545: U+1680 Ogham space mark
546: U+180E Mongolian vowel separator
547: U+2000 En quad
548: U+2001 Em quad
549: U+2002 En space
550: U+2003 Em space
551: U+2004 Three-per-em space
552: U+2005 Four-per-em space
553: U+2006 Six-per-em space
554: U+2007 Figure space
555: U+2008 Punctuation space
556: U+2009 Thin space
557: U+200A Hair space
558: U+202F Narrow no-break space
559: U+205F Medium mathematical space
560: U+3000 Ideographic space
561: .sp
562: The vertical space characters are:
563: .sp
1.1.1.4 ! misho 564: U+000A Linefeed (LF)
! 565: U+000B Vertical tab (VT)
! 566: U+000C Form feed (FF)
! 567: U+000D Carriage return (CR)
! 568: U+0085 Next line (NEL)
1.1 misho 569: U+2028 Line separator
570: U+2029 Paragraph separator
1.1.1.2 misho 571: .sp
572: In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
573: relevant.
1.1 misho 574: .
575: .
576: .\" HTML <a name="newlineseq"></a>
577: .SS "Newline sequences"
578: .rs
579: .sp
580: Outside a character class, by default, the escape sequence \eR matches any
1.1.1.2 misho 581: Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
582: following:
1.1 misho 583: .sp
584: (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
585: .sp
586: This is an example of an "atomic group", details of which are given
587: .\" HTML <a href="#atomicgroup">
588: .\" </a>
589: below.
590: .\"
591: This particular group matches either the two-character sequence CR followed by
592: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
1.1.1.3 misho 593: U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
1.1 misho 594: line, U+0085). The two-character sequence is treated as a single unit that
595: cannot be split.
596: .P
1.1.1.2 misho 597: In other modes, two additional characters whose codepoints are greater than 255
1.1 misho 598: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
599: Unicode character property support is not needed for these characters to be
600: recognized.
601: .P
602: It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
603: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
604: either at compile time or when the pattern is matched. (BSR is an abbrevation
605: for "backslash R".) This can be made the default when PCRE is built; if this is
606: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
607: It is also possible to specify these settings by starting a pattern string with
608: one of the following sequences:
609: .sp
610: (*BSR_ANYCRLF) CR, LF, or CRLF only
611: (*BSR_UNICODE) any Unicode newline sequence
612: .sp
1.1.1.2 misho 613: These override the default and the options given to the compiling function, but
614: they can themselves be overridden by options given to a matching function. Note
615: that these special settings, which are not Perl-compatible, are recognized only
616: at the very start of a pattern, and that they must be in upper case. If more
617: than one of them is present, the last one is used. They can be combined with a
618: change of newline convention; for example, a pattern can start with:
1.1 misho 619: .sp
620: (*ANY)(*BSR_ANYCRLF)
621: .sp
1.1.1.4 ! misho 622: They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
! 623: (*UCP) special sequences. Inside a character class, \eR is treated as an
! 624: unrecognized escape sequence, and so matches the letter "R" by default, but
! 625: causes an error if PCRE_EXTRA is set.
1.1 misho 626: .
627: .
628: .\" HTML <a name="uniextseq"></a>
629: .SS Unicode character properties
630: .rs
631: .sp
632: When PCRE is built with Unicode character property support, three additional
633: escape sequences that match characters with specific properties are available.
1.1.1.2 misho 634: When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
1.1 misho 635: characters whose codepoints are less than 256, but they do work in this mode.
636: The extra escape sequences are:
637: .sp
638: \ep{\fIxx\fP} a character with the \fIxx\fP property
639: \eP{\fIxx\fP} a character without the \fIxx\fP property
1.1.1.4 ! misho 640: \eX a Unicode extended grapheme cluster
1.1 misho 641: .sp
642: The property names represented by \fIxx\fP above are limited to the Unicode
643: script names, the general category properties, "Any", which matches any
644: character (including newline), and some special PCRE properties (described
645: in the
646: .\" HTML <a href="#extraprops">
647: .\" </a>
648: next section).
649: .\"
650: Other Perl properties such as "InMusicalSymbols" are not currently supported by
651: PCRE. Note that \eP{Any} does not match any characters, so always causes a
652: match failure.
653: .P
654: Sets of Unicode characters are defined as belonging to certain scripts. A
655: character from one of these sets can be matched using a script name. For
656: example:
657: .sp
658: \ep{Greek}
659: \eP{Han}
660: .sp
661: Those that are not part of an identified script are lumped together as
662: "Common". The current list of scripts is:
663: .P
664: Arabic,
665: Armenian,
666: Avestan,
667: Balinese,
668: Bamum,
1.1.1.3 misho 669: Batak,
1.1 misho 670: Bengali,
671: Bopomofo,
1.1.1.3 misho 672: Brahmi,
1.1 misho 673: Braille,
674: Buginese,
675: Buhid,
676: Canadian_Aboriginal,
677: Carian,
1.1.1.3 misho 678: Chakma,
1.1 misho 679: Cham,
680: Cherokee,
681: Common,
682: Coptic,
683: Cuneiform,
684: Cypriot,
685: Cyrillic,
686: Deseret,
687: Devanagari,
688: Egyptian_Hieroglyphs,
689: Ethiopic,
690: Georgian,
691: Glagolitic,
692: Gothic,
693: Greek,
694: Gujarati,
695: Gurmukhi,
696: Han,
697: Hangul,
698: Hanunoo,
699: Hebrew,
700: Hiragana,
701: Imperial_Aramaic,
702: Inherited,
703: Inscriptional_Pahlavi,
704: Inscriptional_Parthian,
705: Javanese,
706: Kaithi,
707: Kannada,
708: Katakana,
709: Kayah_Li,
710: Kharoshthi,
711: Khmer,
712: Lao,
713: Latin,
714: Lepcha,
715: Limbu,
716: Linear_B,
717: Lisu,
718: Lycian,
719: Lydian,
720: Malayalam,
1.1.1.3 misho 721: Mandaic,
1.1 misho 722: Meetei_Mayek,
1.1.1.3 misho 723: Meroitic_Cursive,
724: Meroitic_Hieroglyphs,
725: Miao,
1.1 misho 726: Mongolian,
727: Myanmar,
728: New_Tai_Lue,
729: Nko,
730: Ogham,
731: Old_Italic,
732: Old_Persian,
733: Old_South_Arabian,
734: Old_Turkic,
735: Ol_Chiki,
736: Oriya,
737: Osmanya,
738: Phags_Pa,
739: Phoenician,
740: Rejang,
741: Runic,
742: Samaritan,
743: Saurashtra,
1.1.1.3 misho 744: Sharada,
1.1 misho 745: Shavian,
746: Sinhala,
1.1.1.3 misho 747: Sora_Sompeng,
1.1 misho 748: Sundanese,
749: Syloti_Nagri,
750: Syriac,
751: Tagalog,
752: Tagbanwa,
753: Tai_Le,
754: Tai_Tham,
755: Tai_Viet,
1.1.1.3 misho 756: Takri,
1.1 misho 757: Tamil,
758: Telugu,
759: Thaana,
760: Thai,
761: Tibetan,
762: Tifinagh,
763: Ugaritic,
764: Vai,
765: Yi.
766: .P
767: Each character has exactly one Unicode general category property, specified by
768: a two-letter abbreviation. For compatibility with Perl, negation can be
769: specified by including a circumflex between the opening brace and the property
770: name. For example, \ep{^Lu} is the same as \eP{Lu}.
771: .P
772: If only one letter is specified with \ep or \eP, it includes all the general
773: category properties that start with that letter. In this case, in the absence
774: of negation, the curly brackets in the escape sequence are optional; these two
775: examples have the same effect:
776: .sp
777: \ep{L}
778: \epL
779: .sp
780: The following general category property codes are supported:
781: .sp
782: C Other
783: Cc Control
784: Cf Format
785: Cn Unassigned
786: Co Private use
787: Cs Surrogate
788: .sp
789: L Letter
790: Ll Lower case letter
791: Lm Modifier letter
792: Lo Other letter
793: Lt Title case letter
794: Lu Upper case letter
795: .sp
796: M Mark
797: Mc Spacing mark
798: Me Enclosing mark
799: Mn Non-spacing mark
800: .sp
801: N Number
802: Nd Decimal number
803: Nl Letter number
804: No Other number
805: .sp
806: P Punctuation
807: Pc Connector punctuation
808: Pd Dash punctuation
809: Pe Close punctuation
810: Pf Final punctuation
811: Pi Initial punctuation
812: Po Other punctuation
813: Ps Open punctuation
814: .sp
815: S Symbol
816: Sc Currency symbol
817: Sk Modifier symbol
818: Sm Mathematical symbol
819: So Other symbol
820: .sp
821: Z Separator
822: Zl Line separator
823: Zp Paragraph separator
824: Zs Space separator
825: .sp
826: The special property L& is also supported: it matches a character that has
827: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
828: a modifier or "other".
829: .P
830: The Cs (Surrogate) property applies only to characters in the range U+D800 to
1.1.1.2 misho 831: U+DFFF. Such characters are not valid in Unicode strings and so
832: cannot be tested by PCRE, unless UTF validity checking has been turned off
1.1.1.4 ! misho 833: (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
! 834: PCRE_NO_UTF32_CHECK in the
1.1 misho 835: .\" HREF
836: \fBpcreapi\fP
837: .\"
838: page). Perl does not support the Cs property.
839: .P
840: The long synonyms for property names that Perl supports (such as \ep{Letter})
841: are not supported by PCRE, nor is it permitted to prefix any of these
842: properties with "Is".
843: .P
844: No character that is in the Unicode table has the Cn (unassigned) property.
845: Instead, this property is assumed for any code point that is not in the
846: Unicode table.
847: .P
848: Specifying caseless matching does not affect these escape sequences. For
1.1.1.4 ! misho 849: example, \ep{Lu} always matches only upper case letters. This is different from
! 850: the behaviour of current versions of Perl.
1.1 misho 851: .P
1.1.1.4 ! misho 852: Matching characters by Unicode property is not fast, because PCRE has to do a
! 853: multistage table lookup in order to find a character's property. That is why
! 854: the traditional escape sequences such as \ed and \ew do not use Unicode
! 855: properties in PCRE by default, though you can make them do so by setting the
! 856: PCRE_UCP option or by starting the pattern with (*UCP).
! 857: .
! 858: .
! 859: .SS Extended grapheme clusters
! 860: .rs
1.1 misho 861: .sp
1.1.1.4 ! misho 862: The \eX escape matches any number of Unicode characters that form an "extended
! 863: grapheme cluster", and treats the sequence as an atomic group
1.1 misho 864: .\" HTML <a href="#atomicgroup">
865: .\" </a>
866: (see below).
867: .\"
1.1.1.4 ! misho 868: Up to and including release 8.31, PCRE matched an earlier, simpler definition
! 869: that was equivalent to
! 870: .sp
! 871: (?>\ePM\epM*)
! 872: .sp
! 873: That is, it matched a character without the "mark" property, followed by zero
! 874: or more characters with the "mark" property. Characters with the "mark"
! 875: property are typically non-spacing accents that affect the preceding character.
! 876: .P
! 877: This simple definition was extended in Unicode to include more complicated
! 878: kinds of composite character by giving each character a grapheme breaking
! 879: property, and creating rules that use these properties to define the boundaries
! 880: of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
! 881: one of these clusters.
! 882: .P
! 883: \eX always matches at least one character. Then it decides whether to add
! 884: additional characters according to the following rules for ending a cluster:
! 885: .P
! 886: 1. End at the end of the subject string.
! 887: .P
! 888: 2. Do not end between CR and LF; otherwise end after any control character.
! 889: .P
! 890: 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
! 891: are of five types: L, V, T, LV, and LVT. An L character may be followed by an
! 892: L, V, LV, or LVT character; an LV or V character may be followed by a V or T
! 893: character; an LVT or T character may be follwed only by a T character.
! 894: .P
! 895: 4. Do not end before extending characters or spacing marks. Characters with
! 896: the "mark" property always have the "extend" grapheme breaking property.
! 897: .P
! 898: 5. Do not end after prepend characters.
! 899: .P
! 900: 6. Otherwise, end the cluster.
1.1 misho 901: .
902: .
903: .\" HTML <a name="extraprops"></a>
904: .SS PCRE's additional properties
905: .rs
906: .sp
1.1.1.4 ! misho 907: As well as the standard Unicode properties described above, PCRE supports four
! 908: more that make it possible to convert traditional escape sequences such as \ew
! 909: and \es and POSIX character classes to use Unicode properties. PCRE uses these
! 910: non-standard, non-Perl properties internally when PCRE_UCP is set. However,
! 911: they may also be used explicitly. These properties are:
1.1 misho 912: .sp
913: Xan Any alphanumeric character
914: Xps Any POSIX space character
915: Xsp Any Perl space character
916: Xwd Any Perl "word" character
917: .sp
918: Xan matches characters that have either the L (letter) or the N (number)
1.1.1.3 misho 919: property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
1.1 misho 920: carriage return, and any other character that has the Z (separator) property.
921: Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
922: same characters as Xan, plus underscore.
1.1.1.4 ! misho 923: .P
! 924: There is another non-standard property, Xuc, which matches any character that
! 925: can be represented by a Universal Character Name in C++ and other programming
! 926: languages. These are the characters $, @, ` (grave accent), and all characters
! 927: with Unicode code points greater than or equal to U+00A0, except for the
! 928: surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
! 929: excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
! 930: where H is a hexadecimal digit. Note that the Xuc property does not match these
! 931: sequences but the characters that they represent.)
1.1 misho 932: .
933: .
934: .\" HTML <a name="resetmatchstart"></a>
935: .SS "Resetting the match start"
936: .rs
937: .sp
938: The escape sequence \eK causes any previously matched characters not to be
939: included in the final matched sequence. For example, the pattern:
940: .sp
941: foo\eKbar
942: .sp
943: matches "foobar", but reports that it has matched "bar". This feature is
944: similar to a lookbehind assertion
945: .\" HTML <a href="#lookbehind">
946: .\" </a>
947: (described below).
948: .\"
949: However, in this case, the part of the subject before the real match does not
950: have to be of fixed length, as lookbehind assertions do. The use of \eK does
951: not interfere with the setting of
952: .\" HTML <a href="#subpattern">
953: .\" </a>
954: captured substrings.
955: .\"
956: For example, when the pattern
957: .sp
958: (foo)\eKbar
959: .sp
960: matches "foobar", the first substring is still set to "foo".
961: .P
962: Perl documents that the use of \eK within assertions is "not well defined". In
963: PCRE, \eK is acted upon when it occurs inside positive assertions, but is
964: ignored in negative assertions.
965: .
966: .
967: .\" HTML <a name="smallassertions"></a>
968: .SS "Simple assertions"
969: .rs
970: .sp
971: The final use of backslash is for certain simple assertions. An assertion
972: specifies a condition that has to be met at a particular point in a match,
973: without consuming any characters from the subject string. The use of
974: subpatterns for more complicated assertions is described
975: .\" HTML <a href="#bigassertions">
976: .\" </a>
977: below.
978: .\"
979: The backslashed assertions are:
980: .sp
981: \eb matches at a word boundary
982: \eB matches when not at a word boundary
983: \eA matches at the start of the subject
984: \eZ matches at the end of the subject
985: also matches before a newline at the end of the subject
986: \ez matches only at the end of the subject
987: \eG matches at the first matching position in the subject
988: .sp
989: Inside a character class, \eb has a different meaning; it matches the backspace
990: character. If any other of these assertions appears in a character class, by
991: default it matches the corresponding literal character (for example, \eB
992: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
993: escape sequence" error is generated instead.
994: .P
995: A word boundary is a position in the subject string where the current character
996: and the previous character do not both match \ew or \eW (i.e. one matches
997: \ew and the other matches \eW), or the start or end of the string if the
1.1.1.2 misho 998: first or last character matches \ew, respectively. In a UTF mode, the meanings
1.1 misho 999: of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1000: done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1001: of word" or "end of word" metasequence. However, whatever follows \eb normally
1002: determines which it is. For example, the fragment \eba matches "a" at the start
1003: of a word.
1004: .P
1005: The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1006: dollar (described in the next section) in that they only ever match at the very
1007: start and end of the subject string, whatever options are set. Thus, they are
1008: independent of multiline mode. These three assertions are not affected by the
1009: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1010: circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
1011: argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
1012: at a point other than the beginning of the subject, \eA can never match. The
1013: difference between \eZ and \ez is that \eZ matches before a newline at the end
1014: of the string as well as at the very end, whereas \ez matches only at the end.
1015: .P
1016: The \eG assertion is true only when the current matching position is at the
1017: start point of the match, as specified by the \fIstartoffset\fP argument of
1018: \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
1019: non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
1020: arguments, you can mimic Perl's /g option, and it is in this kind of
1021: implementation where \eG can be useful.
1022: .P
1023: Note, however, that PCRE's interpretation of \eG, as the start of the current
1024: match, is subtly different from Perl's, which defines it as the end of the
1025: previous match. In Perl, these can be different when the previously matched
1026: string was empty. Because PCRE does just one match at a time, it cannot
1027: reproduce this behaviour.
1028: .P
1029: If all the alternatives of a pattern begin with \eG, the expression is anchored
1030: to the starting match position, and the "anchored" flag is set in the compiled
1031: regular expression.
1032: .
1033: .
1034: .SH "CIRCUMFLEX AND DOLLAR"
1035: .rs
1036: .sp
1.1.1.4 ! misho 1037: The circumflex and dollar metacharacters are zero-width assertions. That is,
! 1038: they test for a particular condition being true without consuming any
! 1039: characters from the subject string.
! 1040: .P
1.1 misho 1041: Outside a character class, in the default matching mode, the circumflex
1.1.1.4 ! misho 1042: character is an assertion that is true only if the current matching point is at
! 1043: the start of the subject string. If the \fIstartoffset\fP argument of
1.1 misho 1044: \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1045: option is unset. Inside a character class, circumflex has an entirely different
1046: meaning
1047: .\" HTML <a href="#characterclass">
1048: .\" </a>
1049: (see below).
1050: .\"
1051: .P
1052: Circumflex need not be the first character of the pattern if a number of
1053: alternatives are involved, but it should be the first thing in each alternative
1054: in which it appears if the pattern is ever to match that branch. If all
1055: possible alternatives start with a circumflex, that is, if the pattern is
1056: constrained to match only at the start of the subject, it is said to be an
1057: "anchored" pattern. (There are also other constructs that can cause a pattern
1058: to be anchored.)
1059: .P
1.1.1.4 ! misho 1060: The dollar character is an assertion that is true only if the current matching
! 1061: point is at the end of the subject string, or immediately before a newline at
! 1062: the end of the string (by default). Note, however, that it does not actually
! 1063: match the newline. Dollar need not be the last character of the pattern if a
! 1064: number of alternatives are involved, but it should be the last item in any
! 1065: branch in which it appears. Dollar has no special meaning in a character class.
1.1 misho 1066: .P
1067: The meaning of dollar can be changed so that it matches only at the very end of
1068: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1069: does not affect the \eZ assertion.
1070: .P
1071: The meanings of the circumflex and dollar characters are changed if the
1072: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1073: immediately after internal newlines as well as at the start of the subject
1074: string. It does not match after a newline that ends the string. A dollar
1075: matches before any newlines in the string, as well as at the very end, when
1076: PCRE_MULTILINE is set. When newline is specified as the two-character
1077: sequence CRLF, isolated CR and LF characters do not indicate newlines.
1078: .P
1079: For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
1080: \en represents a newline) in multiline mode, but not otherwise. Consequently,
1081: patterns that are anchored in single line mode because all branches start with
1082: ^ are not anchored in multiline mode, and a match for circumflex is possible
1083: when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
1084: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1085: .P
1086: Note that the sequences \eA, \eZ, and \ez can be used to match the start and
1087: end of the subject in both modes, and if all branches of a pattern start with
1088: \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1089: .
1090: .
1091: .\" HTML <a name="fullstopdot"></a>
1092: .SH "FULL STOP (PERIOD, DOT) AND \eN"
1093: .rs
1094: .sp
1095: Outside a character class, a dot in the pattern matches any one character in
1096: the subject string except (by default) a character that signifies the end of a
1.1.1.2 misho 1097: line.
1.1 misho 1098: .P
1099: When a line ending is defined as a single character, dot never matches that
1100: character; when the two-character sequence CRLF is used, dot does not match CR
1101: if it is immediately followed by LF, but otherwise it matches all characters
1102: (including isolated CRs and LFs). When any Unicode line endings are being
1103: recognized, dot does not match CR or LF or any of the other line ending
1104: characters.
1105: .P
1106: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1107: option is set, a dot matches any one character, without exception. If the
1108: two-character sequence CRLF is present in the subject string, it takes two dots
1109: to match it.
1110: .P
1111: The handling of dot is entirely independent of the handling of circumflex and
1112: dollar, the only relationship being that they both involve newlines. Dot has no
1113: special meaning in a character class.
1114: .P
1115: The escape sequence \eN behaves like a dot, except that it is not affected by
1116: the PCRE_DOTALL option. In other words, it matches any character except one
1117: that signifies the end of a line. Perl also uses \eN to match characters by
1118: name; PCRE does not support this.
1119: .
1120: .
1.1.1.2 misho 1121: .SH "MATCHING A SINGLE DATA UNIT"
1.1 misho 1122: .rs
1123: .sp
1.1.1.2 misho 1124: Outside a character class, the escape sequence \eC matches any one data unit,
1125: whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1.1.1.4 ! misho 1126: byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
! 1127: a 32-bit unit. Unlike a dot, \eC always
1.1.1.2 misho 1128: matches line-ending characters. The feature is provided in Perl in order to
1129: match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1130: used. Because \eC breaks up characters into individual data units, matching one
1131: unit with \eC in a UTF mode means that the rest of the string may start with a
1132: malformed UTF character. This has undefined results, because PCRE assumes that
1133: it is dealing with valid UTF strings (and by default it checks this at the
1.1.1.4 ! misho 1134: start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
! 1135: PCRE_NO_UTF32_CHECK option is used).
1.1 misho 1136: .P
1137: PCRE does not allow \eC to appear in lookbehind assertions
1138: .\" HTML <a href="#lookbehind">
1139: .\" </a>
1140: (described below)
1141: .\"
1.1.1.2 misho 1142: in a UTF mode, because this would make it impossible to calculate the length of
1.1 misho 1143: the lookbehind.
1144: .P
1.1.1.2 misho 1145: In general, the \eC escape sequence is best avoided. However, one
1146: way of using it that avoids the problem of malformed UTF characters is to use a
1147: lookahead to check the length of the next character, as in this pattern, which
1148: could be used with a UTF-8 string (ignore white space and line breaks):
1.1 misho 1149: .sp
1150: (?| (?=[\ex00-\ex7f])(\eC) |
1151: (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1152: (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1153: (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1154: .sp
1155: A group that starts with (?| resets the capturing parentheses numbers in each
1156: alternative (see
1157: .\" HTML <a href="#dupsubpatternnumber">
1158: .\" </a>
1159: "Duplicate Subpattern Numbers"
1160: .\"
1161: below). The assertions at the start of each branch check the next UTF-8
1162: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1163: character's individual bytes are then captured by the appropriate number of
1164: groups.
1165: .
1166: .
1167: .\" HTML <a name="characterclass"></a>
1168: .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1169: .rs
1170: .sp
1171: An opening square bracket introduces a character class, terminated by a closing
1172: square bracket. A closing square bracket on its own is not special by default.
1173: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1174: bracket causes a compile-time error. If a closing square bracket is required as
1175: a member of the class, it should be the first data character in the class
1176: (after an initial circumflex, if present) or escaped with a backslash.
1177: .P
1.1.1.2 misho 1178: A character class matches a single character in the subject. In a UTF mode, the
1179: character may be more than one data unit long. A matched character must be in
1180: the set of characters defined by the class, unless the first character in the
1181: class definition is a circumflex, in which case the subject character must not
1182: be in the set defined by the class. If a circumflex is actually required as a
1183: member of the class, ensure it is not the first character, or escape it with a
1.1 misho 1184: backslash.
1185: .P
1186: For example, the character class [aeiou] matches any lower case vowel, while
1187: [^aeiou] matches any character that is not a lower case vowel. Note that a
1188: circumflex is just a convenient notation for specifying the characters that
1189: are in the class by enumerating those that are not. A class that starts with a
1190: circumflex is not an assertion; it still consumes a character from the subject
1191: string, and therefore it fails if the current pointer is at the end of the
1192: string.
1193: .P
1.1.1.4 ! misho 1194: In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
! 1195: can be included in a class as a literal string of data units, or by using the
! 1196: \ex{ escaping mechanism.
1.1 misho 1197: .P
1198: When caseless matching is set, any letters in a class represent both their
1199: upper case and lower case versions, so for example, a caseless [aeiou] matches
1200: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1.1.1.2 misho 1201: caseful version would. In a UTF mode, PCRE always understands the concept of
1.1 misho 1202: case for characters whose values are less than 128, so caseless matching is
1203: always possible. For characters with higher values, the concept of case is
1204: supported if PCRE is compiled with Unicode property support, but not otherwise.
1.1.1.2 misho 1205: If you want to use caseless matching in a UTF mode for characters 128 and
1206: above, you must ensure that PCRE is compiled with Unicode property support as
1207: well as with UTF support.
1.1 misho 1208: .P
1209: Characters that might indicate line breaks are never treated in any special way
1210: when matching character classes, whatever line-ending sequence is in use, and
1211: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1212: such as [^a] always matches one of these characters.
1213: .P
1214: The minus (hyphen) character can be used to specify a range of characters in a
1215: character class. For example, [d-m] matches any letter between d and m,
1216: inclusive. If a minus character is required in a class, it must be escaped with
1217: a backslash or appear in a position where it cannot be interpreted as
1218: indicating a range, typically as the first or last character in the class.
1219: .P
1220: It is not possible to have the literal character "]" as the end character of a
1221: range. A pattern such as [W-]46] is interpreted as a class of two characters
1222: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1223: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1224: the end of range, so [W-\e]46] is interpreted as a class containing a range
1225: followed by two other characters. The octal or hexadecimal representation of
1226: "]" can also be used to end a range.
1227: .P
1228: Ranges operate in the collating sequence of character values. They can also be
1.1.1.2 misho 1229: used for characters specified numerically, for example [\e000-\e037]. Ranges
1230: can include any characters that are valid for the current mode.
1.1 misho 1231: .P
1232: If a range that includes letters is used when caseless matching is set, it
1233: matches the letters in either case. For example, [W-c] is equivalent to
1.1.1.2 misho 1234: [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1.1 misho 1235: tables for a French locale are in use, [\exc8-\excb] matches accented E
1.1.1.2 misho 1236: characters in both cases. In UTF modes, PCRE supports the concept of case for
1.1 misho 1237: characters with values greater than 128 only when it is compiled with Unicode
1238: property support.
1239: .P
1240: The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1241: \eV, \ew, and \eW may appear in a character class, and add the characters that
1242: they match to the class. For example, [\edABCDEF] matches any hexadecimal
1.1.1.2 misho 1243: digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1.1 misho 1244: and their upper case partners, just as it does when they appear outside a
1245: character class, as described in the section entitled
1246: .\" HTML <a href="#genericchartypes">
1247: .\" </a>
1248: "Generic character types"
1249: .\"
1250: above. The escape sequence \eb has a different meaning inside a character
1251: class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1252: are not special inside a character class. Like any other unrecognized escape
1253: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1254: default, but cause an error if the PCRE_EXTRA option is set.
1255: .P
1256: A circumflex can conveniently be used with the upper case character types to
1257: specify a more restricted set of characters than the matching lower case type.
1258: For example, the class [^\eW_] matches any letter or digit, but not underscore,
1259: whereas [\ew] includes underscore. A positive character class should be read as
1260: "something OR something OR ..." and a negative class as "NOT something AND NOT
1261: something AND NOT ...".
1262: .P
1263: The only metacharacters that are recognized in character classes are backslash,
1264: hyphen (only where it can be interpreted as specifying a range), circumflex
1265: (only at the start), opening square bracket (only when it can be interpreted as
1266: introducing a POSIX class name - see the next section), and the terminating
1267: closing square bracket. However, escaping other non-alphanumeric characters
1268: does no harm.
1269: .
1270: .
1271: .SH "POSIX CHARACTER CLASSES"
1272: .rs
1273: .sp
1274: Perl supports the POSIX notation for character classes. This uses names
1275: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1276: this notation. For example,
1277: .sp
1278: [01[:alpha:]%]
1279: .sp
1280: matches "0", "1", any alphabetic character, or "%". The supported class names
1281: are:
1282: .sp
1283: alnum letters and digits
1284: alpha letters
1285: ascii character codes 0 - 127
1286: blank space or tab only
1287: cntrl control characters
1288: digit decimal digits (same as \ed)
1289: graph printing characters, excluding space
1290: lower lower case letters
1291: print printing characters, including space
1292: punct printing characters, excluding letters and digits and space
1293: space white space (not quite the same as \es)
1294: upper upper case letters
1295: word "word" characters (same as \ew)
1296: xdigit hexadecimal digits
1297: .sp
1298: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1299: space (32). Notice that this list includes the VT character (code 11). This
1300: makes "space" different to \es, which does not include VT (for Perl
1301: compatibility).
1302: .P
1303: The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1304: 5.8. Another Perl extension is negation, which is indicated by a ^ character
1305: after the colon. For example,
1306: .sp
1307: [12[:^digit:]]
1308: .sp
1309: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1310: syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1311: supported, and an error is given if they are encountered.
1312: .P
1.1.1.2 misho 1313: By default, in UTF modes, characters with values greater than 128 do not match
1.1 misho 1314: any of the POSIX character classes. However, if the PCRE_UCP option is passed
1315: to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1316: character properties are used. This is achieved by replacing the POSIX classes
1317: by other sequences, as follows:
1318: .sp
1319: [:alnum:] becomes \ep{Xan}
1320: [:alpha:] becomes \ep{L}
1321: [:blank:] becomes \eh
1322: [:digit:] becomes \ep{Nd}
1323: [:lower:] becomes \ep{Ll}
1324: [:space:] becomes \ep{Xps}
1325: [:upper:] becomes \ep{Lu}
1326: [:word:] becomes \ep{Xwd}
1327: .sp
1328: Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1329: classes are unchanged, and match only characters with code points less than
1330: 128.
1331: .
1332: .
1333: .SH "VERTICAL BAR"
1334: .rs
1335: .sp
1336: Vertical bar characters are used to separate alternative patterns. For example,
1337: the pattern
1338: .sp
1339: gilbert|sullivan
1340: .sp
1341: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1342: and an empty alternative is permitted (matching the empty string). The matching
1343: process tries each alternative in turn, from left to right, and the first one
1344: that succeeds is used. If the alternatives are within a subpattern
1345: .\" HTML <a href="#subpattern">
1346: .\" </a>
1347: (defined below),
1348: .\"
1349: "succeeds" means matching the rest of the main pattern as well as the
1350: alternative in the subpattern.
1351: .
1352: .
1353: .SH "INTERNAL OPTION SETTING"
1354: .rs
1355: .sp
1356: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1357: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1358: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1359: The option letters are
1360: .sp
1361: i for PCRE_CASELESS
1362: m for PCRE_MULTILINE
1363: s for PCRE_DOTALL
1364: x for PCRE_EXTENDED
1365: .sp
1366: For example, (?im) sets caseless, multiline matching. It is also possible to
1367: unset these options by preceding the letter with a hyphen, and a combined
1368: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1369: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1370: permitted. If a letter appears both before and after the hyphen, the option is
1371: unset.
1372: .P
1373: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1374: changed in the same way as the Perl-compatible options by using the characters
1375: J, U and X respectively.
1376: .P
1377: When one of these option changes occurs at top level (that is, not inside
1378: subpattern parentheses), the change applies to the remainder of the pattern
1379: that follows. If the change is placed right at the start of a pattern, PCRE
1380: extracts it into the global options (and it will therefore show up in data
1381: extracted by the \fBpcre_fullinfo()\fP function).
1382: .P
1383: An option change within a subpattern (see below for a description of
1384: subpatterns) affects only that part of the subpattern that follows it, so
1385: .sp
1386: (a(?i)b)c
1387: .sp
1388: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1389: By this means, options can be made to have different settings in different
1390: parts of the pattern. Any changes made in one alternative do carry on
1391: into subsequent branches within the same subpattern. For example,
1392: .sp
1393: (a(?i)b|c)
1394: .sp
1395: matches "ab", "aB", "c", and "C", even though when matching "C" the first
1396: branch is abandoned before the option setting. This is because the effects of
1397: option settings happen at compile time. There would be some very weird
1398: behaviour otherwise.
1399: .P
1400: \fBNote:\fP There are other PCRE-specific options that can be set by the
1.1.1.2 misho 1401: application when the compiling or matching functions are called. In some cases
1402: the pattern can contain special leading sequences such as (*CRLF) to override
1403: what the application has set or what has been defaulted. Details are given in
1404: the section entitled
1.1 misho 1405: .\" HTML <a href="#newlineseq">
1406: .\" </a>
1407: "Newline sequences"
1408: .\"
1.1.1.4 ! misho 1409: above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
! 1410: sequences that can be used to set UTF and Unicode property modes; they are
! 1411: equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
! 1412: options, respectively. The (*UTF) sequence is a generic version that can be
! 1413: used with any of the libraries. However, the application can set the
! 1414: PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1.1 misho 1415: .
1416: .
1417: .\" HTML <a name="subpattern"></a>
1418: .SH SUBPATTERNS
1419: .rs
1420: .sp
1421: Subpatterns are delimited by parentheses (round brackets), which can be nested.
1422: Turning part of a pattern into a subpattern does two things:
1423: .sp
1424: 1. It localizes a set of alternatives. For example, the pattern
1425: .sp
1426: cat(aract|erpillar|)
1427: .sp
1428: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1429: match "cataract", "erpillar" or an empty string.
1430: .sp
1431: 2. It sets up the subpattern as a capturing subpattern. This means that, when
1432: the whole pattern matches, that portion of the subject string that matched the
1.1.1.2 misho 1433: subpattern is passed back to the caller via the \fIovector\fP argument of the
1434: matching function. (This applies only to the traditional matching functions;
1435: the DFA matching functions do not support capturing.)
1436: .P
1437: Opening parentheses are counted from left to right (starting from 1) to obtain
1438: numbers for the capturing subpatterns. For example, if the string "the red
1439: king" is matched against the pattern
1.1 misho 1440: .sp
1441: the ((red|white) (king|queen))
1442: .sp
1443: the captured substrings are "red king", "red", and "king", and are numbered 1,
1444: 2, and 3, respectively.
1445: .P
1446: The fact that plain parentheses fulfil two functions is not always helpful.
1447: There are often times when a grouping subpattern is required without a
1448: capturing requirement. If an opening parenthesis is followed by a question mark
1449: and a colon, the subpattern does not do any capturing, and is not counted when
1450: computing the number of any subsequent capturing subpatterns. For example, if
1451: the string "the white queen" is matched against the pattern
1452: .sp
1453: the ((?:red|white) (king|queen))
1454: .sp
1455: the captured substrings are "white queen" and "queen", and are numbered 1 and
1456: 2. The maximum number of capturing subpatterns is 65535.
1457: .P
1458: As a convenient shorthand, if any option settings are required at the start of
1459: a non-capturing subpattern, the option letters may appear between the "?" and
1460: the ":". Thus the two patterns
1461: .sp
1462: (?i:saturday|sunday)
1463: (?:(?i)saturday|sunday)
1464: .sp
1465: match exactly the same set of strings. Because alternative branches are tried
1466: from left to right, and options are not reset until the end of the subpattern
1467: is reached, an option setting in one branch does affect subsequent branches, so
1468: the above patterns match "SUNDAY" as well as "Saturday".
1469: .
1470: .
1471: .\" HTML <a name="dupsubpatternnumber"></a>
1472: .SH "DUPLICATE SUBPATTERN NUMBERS"
1473: .rs
1474: .sp
1475: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1476: the same numbers for its capturing parentheses. Such a subpattern starts with
1477: (?| and is itself a non-capturing subpattern. For example, consider this
1478: pattern:
1479: .sp
1480: (?|(Sat)ur|(Sun))day
1481: .sp
1482: Because the two alternatives are inside a (?| group, both sets of capturing
1483: parentheses are numbered one. Thus, when the pattern matches, you can look
1484: at captured substring number one, whichever alternative matched. This construct
1485: is useful when you want to capture part, but not all, of one of a number of
1486: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1487: number is reset at the start of each branch. The numbers of any capturing
1488: parentheses that follow the subpattern start after the highest number used in
1489: any branch. The following example is taken from the Perl documentation. The
1490: numbers underneath show in which buffer the captured content will be stored.
1491: .sp
1492: # before ---------------branch-reset----------- after
1493: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1494: # 1 2 2 3 2 3 4
1495: .sp
1496: A back reference to a numbered subpattern uses the most recent value that is
1497: set for that number by any subpattern. The following pattern matches "abcabc"
1498: or "defdef":
1499: .sp
1500: /(?|(abc)|(def))\e1/
1501: .sp
1502: In contrast, a subroutine call to a numbered subpattern always refers to the
1503: first one in the pattern with the given number. The following pattern matches
1504: "abcabc" or "defabc":
1505: .sp
1506: /(?|(abc)|(def))(?1)/
1507: .sp
1508: If a
1509: .\" HTML <a href="#conditions">
1510: .\" </a>
1511: condition test
1512: .\"
1513: for a subpattern's having matched refers to a non-unique number, the test is
1514: true if any of the subpatterns of that number have matched.
1515: .P
1516: An alternative approach to using this "branch reset" feature is to use
1517: duplicate named subpatterns, as described in the next section.
1518: .
1519: .
1520: .SH "NAMED SUBPATTERNS"
1521: .rs
1522: .sp
1523: Identifying capturing parentheses by number is simple, but it can be very hard
1524: to keep track of the numbers in complicated regular expressions. Furthermore,
1525: if an expression is modified, the numbers may change. To help with this
1526: difficulty, PCRE supports the naming of subpatterns. This feature was not
1527: added to Perl until release 5.10. Python had the feature earlier, and PCRE
1528: introduced it at release 4.0, using the Python syntax. PCRE now supports both
1529: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1530: have different names, but PCRE does not.
1531: .P
1532: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1533: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1534: parentheses from other parts of the pattern, such as
1535: .\" HTML <a href="#backreferences">
1536: .\" </a>
1537: back references,
1538: .\"
1539: .\" HTML <a href="#recursion">
1540: .\" </a>
1541: recursion,
1542: .\"
1543: and
1544: .\" HTML <a href="#conditions">
1545: .\" </a>
1546: conditions,
1547: .\"
1548: can be made by name as well as by number.
1549: .P
1550: Names consist of up to 32 alphanumeric characters and underscores. Named
1551: capturing parentheses are still allocated numbers as well as names, exactly as
1552: if the names were not present. The PCRE API provides function calls for
1553: extracting the name-to-number translation table from a compiled pattern. There
1554: is also a convenience function for extracting a captured substring by name.
1555: .P
1556: By default, a name must be unique within a pattern, but it is possible to relax
1557: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1558: names are also always permitted for subpatterns with the same number, set up as
1559: described in the previous section.) Duplicate names can be useful for patterns
1560: where only one instance of the named parentheses can match. Suppose you want to
1561: match the name of a weekday, either as a 3-letter abbreviation or as the full
1562: name, and in both cases you want to extract the abbreviation. This pattern
1563: (ignoring the line breaks) does the job:
1564: .sp
1565: (?<DN>Mon|Fri|Sun)(?:day)?|
1566: (?<DN>Tue)(?:sday)?|
1567: (?<DN>Wed)(?:nesday)?|
1568: (?<DN>Thu)(?:rsday)?|
1569: (?<DN>Sat)(?:urday)?
1570: .sp
1571: There are five capturing substrings, but only one is ever set after a match.
1572: (An alternative way of solving this problem is to use a "branch reset"
1573: subpattern, as described in the previous section.)
1574: .P
1575: The convenience function for extracting the data by name returns the substring
1576: for the first (and in this example, the only) subpattern of that name that
1577: matched. This saves searching to find which numbered subpattern it was.
1578: .P
1579: If you make a back reference to a non-unique named subpattern from elsewhere in
1580: the pattern, the one that corresponds to the first occurrence of the name is
1581: used. In the absence of duplicate numbers (see the previous section) this is
1582: the one with the lowest number. If you use a named reference in a condition
1583: test (see the
1584: .\"
1585: .\" HTML <a href="#conditions">
1586: .\" </a>
1587: section about conditions
1588: .\"
1589: below), either to check whether a subpattern has matched, or to check for
1590: recursion, all subpatterns with the same name are tested. If the condition is
1591: true for any one of them, the overall condition is true. This is the same
1592: behaviour as testing by number. For further details of the interfaces for
1593: handling named subpatterns, see the
1594: .\" HREF
1595: \fBpcreapi\fP
1596: .\"
1597: documentation.
1598: .P
1599: \fBWarning:\fP You cannot use different names to distinguish between two
1600: subpatterns with the same number because PCRE uses only the numbers when
1601: matching. For this reason, an error is given at compile time if different names
1602: are given to subpatterns with the same number. However, you can give the same
1603: name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1604: .
1605: .
1606: .SH REPETITION
1607: .rs
1608: .sp
1609: Repetition is specified by quantifiers, which can follow any of the following
1610: items:
1611: .sp
1612: a literal data character
1613: the dot metacharacter
1614: the \eC escape sequence
1.1.1.2 misho 1615: the \eX escape sequence
1.1 misho 1616: the \eR escape sequence
1617: an escape such as \ed or \epL that matches a single character
1618: a character class
1619: a back reference (see next section)
1620: a parenthesized subpattern (including assertions)
1621: a subroutine call to a subpattern (recursive or otherwise)
1622: .sp
1623: The general repetition quantifier specifies a minimum and maximum number of
1624: permitted matches, by giving the two numbers in curly brackets (braces),
1625: separated by a comma. The numbers must be less than 65536, and the first must
1626: be less than or equal to the second. For example:
1627: .sp
1628: z{2,4}
1629: .sp
1630: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1631: character. If the second number is omitted, but the comma is present, there is
1632: no upper limit; if the second number and the comma are both omitted, the
1633: quantifier specifies an exact number of required matches. Thus
1634: .sp
1635: [aeiou]{3,}
1636: .sp
1637: matches at least 3 successive vowels, but may match many more, while
1638: .sp
1639: \ed{8}
1640: .sp
1641: matches exactly 8 digits. An opening curly bracket that appears in a position
1642: where a quantifier is not allowed, or one that does not match the syntax of a
1643: quantifier, is taken as a literal character. For example, {,6} is not a
1644: quantifier, but a literal string of four characters.
1645: .P
1.1.1.2 misho 1646: In UTF modes, quantifiers apply to characters rather than to individual data
1647: units. Thus, for example, \ex{100}{2} matches two characters, each of
1648: which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1.1.1.4 ! misho 1649: \eX{3} matches three Unicode extended grapheme clusters, each of which may be
! 1650: several data units long (and they may be of different lengths).
1.1 misho 1651: .P
1652: The quantifier {0} is permitted, causing the expression to behave as if the
1653: previous item and the quantifier were not present. This may be useful for
1654: subpatterns that are referenced as
1655: .\" HTML <a href="#subpatternsassubroutines">
1656: .\" </a>
1657: subroutines
1658: .\"
1659: from elsewhere in the pattern (but see also the section entitled
1660: .\" HTML <a href="#subdefine">
1661: .\" </a>
1662: "Defining subpatterns for use by reference only"
1663: .\"
1664: below). Items other than subpatterns that have a {0} quantifier are omitted
1665: from the compiled pattern.
1666: .P
1667: For convenience, the three most common quantifiers have single-character
1668: abbreviations:
1669: .sp
1670: * is equivalent to {0,}
1671: + is equivalent to {1,}
1672: ? is equivalent to {0,1}
1673: .sp
1674: It is possible to construct infinite loops by following a subpattern that can
1675: match no characters with a quantifier that has no upper limit, for example:
1676: .sp
1677: (a?)*
1678: .sp
1679: Earlier versions of Perl and PCRE used to give an error at compile time for
1680: such patterns. However, because there are cases where this can be useful, such
1681: patterns are now accepted, but if any repetition of the subpattern does in fact
1682: match no characters, the loop is forcibly broken.
1683: .P
1684: By default, the quantifiers are "greedy", that is, they match as much as
1685: possible (up to the maximum number of permitted times), without causing the
1686: rest of the pattern to fail. The classic example of where this gives problems
1687: is in trying to match comments in C programs. These appear between /* and */
1688: and within the comment, individual * and / characters may appear. An attempt to
1689: match C comments by applying the pattern
1690: .sp
1691: /\e*.*\e*/
1692: .sp
1693: to the string
1694: .sp
1695: /* first comment */ not comment /* second comment */
1696: .sp
1697: fails, because it matches the entire string owing to the greediness of the .*
1698: item.
1699: .P
1700: However, if a quantifier is followed by a question mark, it ceases to be
1701: greedy, and instead matches the minimum number of times possible, so the
1702: pattern
1703: .sp
1704: /\e*.*?\e*/
1705: .sp
1706: does the right thing with the C comments. The meaning of the various
1707: quantifiers is not otherwise changed, just the preferred number of matches.
1708: Do not confuse this use of question mark with its use as a quantifier in its
1709: own right. Because it has two uses, it can sometimes appear doubled, as in
1710: .sp
1711: \ed??\ed
1712: .sp
1713: which matches one digit by preference, but can match two if that is the only
1714: way the rest of the pattern matches.
1715: .P
1716: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1717: the quantifiers are not greedy by default, but individual ones can be made
1718: greedy by following them with a question mark. In other words, it inverts the
1719: default behaviour.
1720: .P
1721: When a parenthesized subpattern is quantified with a minimum repeat count that
1722: is greater than 1 or with a limited maximum, more memory is required for the
1723: compiled pattern, in proportion to the size of the minimum or maximum.
1724: .P
1725: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1726: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1727: implicitly anchored, because whatever follows will be tried against every
1728: character position in the subject string, so there is no point in retrying the
1729: overall match at any position after the first. PCRE normally treats such a
1730: pattern as though it were preceded by \eA.
1731: .P
1732: In cases where it is known that the subject string contains no newlines, it is
1733: worth setting PCRE_DOTALL in order to obtain this optimization, or
1734: alternatively using ^ to indicate anchoring explicitly.
1735: .P
1.1.1.4 ! misho 1736: However, there are some cases where the optimization cannot be used. When .*
1.1 misho 1737: is inside capturing parentheses that are the subject of a back reference
1738: elsewhere in the pattern, a match at the start may fail where a later one
1739: succeeds. Consider, for example:
1740: .sp
1741: (.*)abc\e1
1742: .sp
1743: If the subject is "xyz123abc123" the match point is the fourth character. For
1744: this reason, such a pattern is not implicitly anchored.
1745: .P
1.1.1.4 ! misho 1746: Another case where implicit anchoring is not applied is when the leading .* is
! 1747: inside an atomic group. Once again, a match at the start may fail where a later
! 1748: one succeeds. Consider this pattern:
! 1749: .sp
! 1750: (?>.*?a)b
! 1751: .sp
! 1752: It matches "ab" in the subject "aab". The use of the backtracking control verbs
! 1753: (*PRUNE) and (*SKIP) also disable this optimization.
! 1754: .P
1.1 misho 1755: When a capturing subpattern is repeated, the value captured is the substring
1756: that matched the final iteration. For example, after
1757: .sp
1758: (tweedle[dume]{3}\es*)+
1759: .sp
1760: has matched "tweedledum tweedledee" the value of the captured substring is
1761: "tweedledee". However, if there are nested capturing subpatterns, the
1762: corresponding captured values may have been set in previous iterations. For
1763: example, after
1764: .sp
1765: /(a|(b))+/
1766: .sp
1767: matches "aba" the value of the second captured substring is "b".
1768: .
1769: .
1770: .\" HTML <a name="atomicgroup"></a>
1771: .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1772: .rs
1773: .sp
1774: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1775: repetition, failure of what follows normally causes the repeated item to be
1776: re-evaluated to see if a different number of repeats allows the rest of the
1777: pattern to match. Sometimes it is useful to prevent this, either to change the
1778: nature of the match, or to cause it fail earlier than it otherwise might, when
1779: the author of the pattern knows there is no point in carrying on.
1780: .P
1781: Consider, for example, the pattern \ed+foo when applied to the subject line
1782: .sp
1783: 123456bar
1784: .sp
1785: After matching all 6 digits and then failing to match "foo", the normal
1786: action of the matcher is to try again with only 5 digits matching the \ed+
1787: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1788: (a term taken from Jeffrey Friedl's book) provides the means for specifying
1789: that once a subpattern has matched, it is not to be re-evaluated in this way.
1790: .P
1791: If we use atomic grouping for the previous example, the matcher gives up
1792: immediately on failing to match "foo" the first time. The notation is a kind of
1793: special parenthesis, starting with (?> as in this example:
1794: .sp
1795: (?>\ed+)foo
1796: .sp
1797: This kind of parenthesis "locks up" the part of the pattern it contains once
1798: it has matched, and a failure further into the pattern is prevented from
1799: backtracking into it. Backtracking past it to previous items, however, works as
1800: normal.
1801: .P
1802: An alternative description is that a subpattern of this type matches the string
1803: of characters that an identical standalone pattern would match, if anchored at
1804: the current point in the subject string.
1805: .P
1806: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1807: the above example can be thought of as a maximizing repeat that must swallow
1808: everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1809: number of digits they match in order to make the rest of the pattern match,
1810: (?>\ed+) can only match an entire sequence of digits.
1811: .P
1812: Atomic groups in general can of course contain arbitrarily complicated
1813: subpatterns, and can be nested. However, when the subpattern for an atomic
1814: group is just a single repeated item, as in the example above, a simpler
1815: notation, called a "possessive quantifier" can be used. This consists of an
1816: additional + character following a quantifier. Using this notation, the
1817: previous example can be rewritten as
1818: .sp
1819: \ed++foo
1820: .sp
1821: Note that a possessive quantifier can be used with an entire group, for
1822: example:
1823: .sp
1824: (abc|xyz){2,3}+
1825: .sp
1826: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1827: option is ignored. They are a convenient notation for the simpler forms of
1828: atomic group. However, there is no difference in the meaning of a possessive
1829: quantifier and the equivalent atomic group, though there may be a performance
1830: difference; possessive quantifiers should be slightly faster.
1831: .P
1832: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1833: Jeffrey Friedl originated the idea (and the name) in the first edition of his
1834: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1835: package, and PCRE copied it from there. It ultimately found its way into Perl
1836: at release 5.10.
1837: .P
1838: PCRE has an optimization that automatically "possessifies" certain simple
1839: pattern constructs. For example, the sequence A+B is treated as A++B because
1840: there is no point in backtracking into a sequence of A's when B must follow.
1841: .P
1842: When a pattern contains an unlimited repeat inside a subpattern that can itself
1843: be repeated an unlimited number of times, the use of an atomic group is the
1844: only way to avoid some failing matches taking a very long time indeed. The
1845: pattern
1846: .sp
1847: (\eD+|<\ed+>)*[!?]
1848: .sp
1849: matches an unlimited number of substrings that either consist of non-digits, or
1850: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1851: quickly. However, if it is applied to
1852: .sp
1853: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1854: .sp
1855: it takes a long time before reporting failure. This is because the string can
1856: be divided between the internal \eD+ repeat and the external * repeat in a
1857: large number of ways, and all have to be tried. (The example uses [!?] rather
1858: than a single character at the end, because both PCRE and Perl have an
1859: optimization that allows for fast failure when a single character is used. They
1860: remember the last single character that is required for a match, and fail early
1861: if it is not present in the string.) If the pattern is changed so that it uses
1862: an atomic group, like this:
1863: .sp
1864: ((?>\eD+)|<\ed+>)*[!?]
1865: .sp
1866: sequences of non-digits cannot be broken, and failure happens quickly.
1867: .
1868: .
1869: .\" HTML <a name="backreferences"></a>
1870: .SH "BACK REFERENCES"
1871: .rs
1872: .sp
1873: Outside a character class, a backslash followed by a digit greater than 0 (and
1874: possibly further digits) is a back reference to a capturing subpattern earlier
1875: (that is, to its left) in the pattern, provided there have been that many
1876: previous capturing left parentheses.
1877: .P
1878: However, if the decimal number following the backslash is less than 10, it is
1879: always taken as a back reference, and causes an error only if there are not
1880: that many capturing left parentheses in the entire pattern. In other words, the
1881: parentheses that are referenced need not be to the left of the reference for
1882: numbers less than 10. A "forward back reference" of this type can make sense
1883: when a repetition is involved and the subpattern to the right has participated
1884: in an earlier iteration.
1885: .P
1886: It is not possible to have a numerical "forward back reference" to a subpattern
1887: whose number is 10 or more using this syntax because a sequence such as \e50 is
1888: interpreted as a character defined in octal. See the subsection entitled
1889: "Non-printing characters"
1890: .\" HTML <a href="#digitsafterbackslash">
1891: .\" </a>
1892: above
1893: .\"
1894: for further details of the handling of digits following a backslash. There is
1895: no such problem when named parentheses are used. A back reference to any
1896: subpattern is possible using named parentheses (see below).
1897: .P
1898: Another way of avoiding the ambiguity inherent in the use of digits following a
1899: backslash is to use the \eg escape sequence. This escape must be followed by an
1900: unsigned number or a negative number, optionally enclosed in braces. These
1901: examples are all identical:
1902: .sp
1903: (ring), \e1
1904: (ring), \eg1
1905: (ring), \eg{1}
1906: .sp
1907: An unsigned number specifies an absolute reference without the ambiguity that
1908: is present in the older syntax. It is also useful when literal digits follow
1909: the reference. A negative number is a relative reference. Consider this
1910: example:
1911: .sp
1912: (abc(def)ghi)\eg{-1}
1913: .sp
1914: The sequence \eg{-1} is a reference to the most recently started capturing
1915: subpattern before \eg, that is, is it equivalent to \e2 in this example.
1916: Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1917: can be helpful in long patterns, and also in patterns that are created by
1918: joining together fragments that contain references within themselves.
1919: .P
1920: A back reference matches whatever actually matched the capturing subpattern in
1921: the current subject string, rather than anything matching the subpattern
1922: itself (see
1923: .\" HTML <a href="#subpatternsassubroutines">
1924: .\" </a>
1925: "Subpatterns as subroutines"
1926: .\"
1927: below for a way of doing that). So the pattern
1928: .sp
1929: (sens|respons)e and \e1ibility
1930: .sp
1931: matches "sense and sensibility" and "response and responsibility", but not
1932: "sense and responsibility". If caseful matching is in force at the time of the
1933: back reference, the case of letters is relevant. For example,
1934: .sp
1935: ((?i)rah)\es+\e1
1936: .sp
1937: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1938: capturing subpattern is matched caselessly.
1939: .P
1940: There are several different ways of writing back references to named
1941: subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1942: \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1943: back reference syntax, in which \eg can be used for both numeric and named
1944: references, is also supported. We could rewrite the above example in any of
1945: the following ways:
1946: .sp
1947: (?<p1>(?i)rah)\es+\ek<p1>
1948: (?'p1'(?i)rah)\es+\ek{p1}
1949: (?P<p1>(?i)rah)\es+(?P=p1)
1950: (?<p1>(?i)rah)\es+\eg{p1}
1951: .sp
1952: A subpattern that is referenced by name may appear in the pattern before or
1953: after the reference.
1954: .P
1955: There may be more than one back reference to the same subpattern. If a
1956: subpattern has not actually been used in a particular match, any back
1957: references to it always fail by default. For example, the pattern
1958: .sp
1959: (a|(bc))\e2
1960: .sp
1961: always fails if it starts to match "a" rather than "bc". However, if the
1962: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1963: unset value matches an empty string.
1964: .P
1965: Because there may be many capturing parentheses in a pattern, all digits
1966: following a backslash are taken as part of a potential back reference number.
1967: If the pattern continues with a digit character, some delimiter must be used to
1968: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1.1.1.3 misho 1969: white space. Otherwise, the \eg{ syntax or an empty comment (see
1.1 misho 1970: .\" HTML <a href="#comments">
1971: .\" </a>
1972: "Comments"
1973: .\"
1974: below) can be used.
1975: .
1976: .SS "Recursive back references"
1977: .rs
1978: .sp
1979: A back reference that occurs inside the parentheses to which it refers fails
1980: when the subpattern is first used, so, for example, (a\e1) never matches.
1981: However, such references can be useful inside repeated subpatterns. For
1982: example, the pattern
1983: .sp
1984: (a|b\e1)+
1985: .sp
1986: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1987: the subpattern, the back reference matches the character string corresponding
1988: to the previous iteration. In order for this to work, the pattern must be such
1989: that the first iteration does not need to match the back reference. This can be
1990: done using alternation, as in the example above, or by a quantifier with a
1991: minimum of zero.
1992: .P
1993: Back references of this type cause the group that they reference to be treated
1994: as an
1995: .\" HTML <a href="#atomicgroup">
1996: .\" </a>
1997: atomic group.
1998: .\"
1999: Once the whole group has been matched, a subsequent matching failure cannot
2000: cause backtracking into the middle of the group.
2001: .
2002: .
2003: .\" HTML <a name="bigassertions"></a>
2004: .SH ASSERTIONS
2005: .rs
2006: .sp
2007: An assertion is a test on the characters following or preceding the current
2008: matching point that does not actually consume any characters. The simple
2009: assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
2010: .\" HTML <a href="#smallassertions">
2011: .\" </a>
2012: above.
2013: .\"
2014: .P
2015: More complicated assertions are coded as subpatterns. There are two kinds:
2016: those that look ahead of the current position in the subject string, and those
2017: that look behind it. An assertion subpattern is matched in the normal way,
2018: except that it does not cause the current matching position to be changed.
2019: .P
2020: Assertion subpatterns are not capturing subpatterns. If such an assertion
2021: contains capturing subpatterns within it, these are counted for the purposes of
2022: numbering the capturing subpatterns in the whole pattern. However, substring
1.1.1.4 ! misho 2023: capturing is carried out only for positive assertions. (Perl sometimes, but not
! 2024: always, does do capturing in negative assertions.)
1.1 misho 2025: .P
2026: For compatibility with Perl, assertion subpatterns may be repeated; though
2027: it makes no sense to assert the same thing several times, the side effect of
2028: capturing parentheses may occasionally be useful. In practice, there only three
2029: cases:
2030: .sp
2031: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2032: However, it may contain internal capturing parenthesized groups that are called
2033: from elsewhere via the
2034: .\" HTML <a href="#subpatternsassubroutines">
2035: .\" </a>
2036: subroutine mechanism.
2037: .\"
2038: .sp
2039: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2040: were {0,1}. At run time, the rest of the pattern match is tried with and
2041: without the assertion, the order depending on the greediness of the quantifier.
2042: .sp
2043: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2044: The assertion is obeyed just once when encountered during matching.
2045: .
2046: .
2047: .SS "Lookahead assertions"
2048: .rs
2049: .sp
2050: Lookahead assertions start with (?= for positive assertions and (?! for
2051: negative assertions. For example,
2052: .sp
2053: \ew+(?=;)
2054: .sp
2055: matches a word followed by a semicolon, but does not include the semicolon in
2056: the match, and
2057: .sp
2058: foo(?!bar)
2059: .sp
2060: matches any occurrence of "foo" that is not followed by "bar". Note that the
2061: apparently similar pattern
2062: .sp
2063: (?!foo)bar
2064: .sp
2065: does not find an occurrence of "bar" that is preceded by something other than
2066: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2067: (?!foo) is always true when the next three characters are "bar". A
2068: lookbehind assertion is needed to achieve the other effect.
2069: .P
2070: If you want to force a matching failure at some point in a pattern, the most
2071: convenient way to do it is with (?!) because an empty string always matches, so
2072: an assertion that requires there not to be an empty string must always fail.
2073: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2074: .
2075: .
2076: .\" HTML <a name="lookbehind"></a>
2077: .SS "Lookbehind assertions"
2078: .rs
2079: .sp
2080: Lookbehind assertions start with (?<= for positive assertions and (?<! for
2081: negative assertions. For example,
2082: .sp
2083: (?<!foo)bar
2084: .sp
2085: does find an occurrence of "bar" that is not preceded by "foo". The contents of
2086: a lookbehind assertion are restricted such that all the strings it matches must
2087: have a fixed length. However, if there are several top-level alternatives, they
2088: do not all have to have the same fixed length. Thus
2089: .sp
2090: (?<=bullock|donkey)
2091: .sp
2092: is permitted, but
2093: .sp
2094: (?<!dogs?|cats?)
2095: .sp
2096: causes an error at compile time. Branches that match different length strings
2097: are permitted only at the top level of a lookbehind assertion. This is an
2098: extension compared with Perl, which requires all branches to match the same
2099: length of string. An assertion such as
2100: .sp
2101: (?<=ab(c|de))
2102: .sp
2103: is not permitted, because its single top-level branch can match two different
2104: lengths, but it is acceptable to PCRE if rewritten to use two top-level
2105: branches:
2106: .sp
2107: (?<=abc|abde)
2108: .sp
2109: In some cases, the escape sequence \eK
2110: .\" HTML <a href="#resetmatchstart">
2111: .\" </a>
2112: (see above)
2113: .\"
2114: can be used instead of a lookbehind assertion to get round the fixed-length
2115: restriction.
2116: .P
2117: The implementation of lookbehind assertions is, for each alternative, to
2118: temporarily move the current position back by the fixed length and then try to
2119: match. If there are insufficient characters before the current position, the
2120: assertion fails.
2121: .P
1.1.1.2 misho 2122: In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2123: unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2124: it impossible to calculate the length of the lookbehind. The \eX and \eR
2125: escapes, which can match different numbers of data units, are also not
2126: permitted.
1.1 misho 2127: .P
2128: .\" HTML <a href="#subpatternsassubroutines">
2129: .\" </a>
2130: "Subroutine"
2131: .\"
2132: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2133: as the subpattern matches a fixed-length string.
2134: .\" HTML <a href="#recursion">
2135: .\" </a>
2136: Recursion,
2137: .\"
2138: however, is not supported.
2139: .P
2140: Possessive quantifiers can be used in conjunction with lookbehind assertions to
2141: specify efficient matching of fixed-length strings at the end of subject
2142: strings. Consider a simple pattern such as
2143: .sp
2144: abcd$
2145: .sp
2146: when applied to a long string that does not match. Because matching proceeds
2147: from left to right, PCRE will look for each "a" in the subject and then see if
2148: what follows matches the rest of the pattern. If the pattern is specified as
2149: .sp
2150: ^.*abcd$
2151: .sp
2152: the initial .* matches the entire string at first, but when this fails (because
2153: there is no following "a"), it backtracks to match all but the last character,
2154: then all but the last two characters, and so on. Once again the search for "a"
2155: covers the entire string, from right to left, so we are no better off. However,
2156: if the pattern is written as
2157: .sp
2158: ^.*+(?<=abcd)
2159: .sp
2160: there can be no backtracking for the .*+ item; it can match only the entire
2161: string. The subsequent lookbehind assertion does a single test on the last four
2162: characters. If it fails, the match fails immediately. For long strings, this
2163: approach makes a significant difference to the processing time.
2164: .
2165: .
2166: .SS "Using multiple assertions"
2167: .rs
2168: .sp
2169: Several assertions (of any sort) may occur in succession. For example,
2170: .sp
2171: (?<=\ed{3})(?<!999)foo
2172: .sp
2173: matches "foo" preceded by three digits that are not "999". Notice that each of
2174: the assertions is applied independently at the same point in the subject
2175: string. First there is a check that the previous three characters are all
2176: digits, and then there is a check that the same three characters are not "999".
2177: This pattern does \fInot\fP match "foo" preceded by six characters, the first
2178: of which are digits and the last three of which are not "999". For example, it
2179: doesn't match "123abcfoo". A pattern to do that is
2180: .sp
2181: (?<=\ed{3}...)(?<!999)foo
2182: .sp
2183: This time the first assertion looks at the preceding six characters, checking
2184: that the first three are digits, and then the second assertion checks that the
2185: preceding three characters are not "999".
2186: .P
2187: Assertions can be nested in any combination. For example,
2188: .sp
2189: (?<=(?<!foo)bar)baz
2190: .sp
2191: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2192: preceded by "foo", while
2193: .sp
2194: (?<=\ed{3}(?!999)...)foo
2195: .sp
2196: is another pattern that matches "foo" preceded by three digits and any three
2197: characters that are not "999".
2198: .
2199: .
2200: .\" HTML <a name="conditions"></a>
2201: .SH "CONDITIONAL SUBPATTERNS"
2202: .rs
2203: .sp
2204: It is possible to cause the matching process to obey a subpattern
2205: conditionally or to choose between two alternative subpatterns, depending on
2206: the result of an assertion, or whether a specific capturing subpattern has
2207: already been matched. The two possible forms of conditional subpattern are:
2208: .sp
2209: (?(condition)yes-pattern)
2210: (?(condition)yes-pattern|no-pattern)
2211: .sp
2212: If the condition is satisfied, the yes-pattern is used; otherwise the
2213: no-pattern (if present) is used. If there are more than two alternatives in the
2214: subpattern, a compile-time error occurs. Each of the two alternatives may
2215: itself contain nested subpatterns of any form, including conditional
2216: subpatterns; the restriction to two alternatives applies only at the level of
2217: the condition. This pattern fragment is an example where the alternatives are
2218: complex:
2219: .sp
2220: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2221: .sp
2222: .P
2223: There are four kinds of condition: references to subpatterns, references to
2224: recursion, a pseudo-condition called DEFINE, and assertions.
2225: .
2226: .SS "Checking for a used subpattern by number"
2227: .rs
2228: .sp
2229: If the text between the parentheses consists of a sequence of digits, the
2230: condition is true if a capturing subpattern of that number has previously
2231: matched. If there is more than one capturing subpattern with the same number
2232: (see the earlier
2233: .\"
2234: .\" HTML <a href="#recursion">
2235: .\" </a>
2236: section about duplicate subpattern numbers),
2237: .\"
2238: the condition is true if any of them have matched. An alternative notation is
2239: to precede the digits with a plus or minus sign. In this case, the subpattern
2240: number is relative rather than absolute. The most recently opened parentheses
2241: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2242: loops it can also make sense to refer to subsequent groups. The next
2243: parentheses to be opened can be referenced as (?(+1), and so on. (The value
2244: zero in any of these forms is not used; it provokes a compile-time error.)
2245: .P
2246: Consider the following pattern, which contains non-significant white space to
2247: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2248: three parts for ease of discussion:
2249: .sp
2250: ( \e( )? [^()]+ (?(1) \e) )
2251: .sp
2252: The first part matches an optional opening parenthesis, and if that
2253: character is present, sets it as the first captured substring. The second part
2254: matches one or more characters that are not parentheses. The third part is a
2255: conditional subpattern that tests whether or not the first set of parentheses
2256: matched. If they did, that is, if subject started with an opening parenthesis,
2257: the condition is true, and so the yes-pattern is executed and a closing
2258: parenthesis is required. Otherwise, since no-pattern is not present, the
2259: subpattern matches nothing. In other words, this pattern matches a sequence of
2260: non-parentheses, optionally enclosed in parentheses.
2261: .P
2262: If you were embedding this pattern in a larger one, you could use a relative
2263: reference:
2264: .sp
2265: ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2266: .sp
2267: This makes the fragment independent of the parentheses in the larger pattern.
2268: .
2269: .SS "Checking for a used subpattern by name"
2270: .rs
2271: .sp
2272: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2273: subpattern by name. For compatibility with earlier versions of PCRE, which had
2274: this facility before Perl, the syntax (?(name)...) is also recognized. However,
2275: there is a possible ambiguity with this syntax, because subpattern names may
2276: consist entirely of digits. PCRE looks first for a named subpattern; if it
2277: cannot find one and the name consists entirely of digits, PCRE looks for a
2278: subpattern of that number, which must be greater than zero. Using subpattern
2279: names that consist entirely of digits is not recommended.
2280: .P
2281: Rewriting the above example to use a named subpattern gives this:
2282: .sp
2283: (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2284: .sp
2285: If the name used in a condition of this kind is a duplicate, the test is
2286: applied to all subpatterns of the same name, and is true if any one of them has
2287: matched.
2288: .
2289: .SS "Checking for pattern recursion"
2290: .rs
2291: .sp
2292: If the condition is the string (R), and there is no subpattern with the name R,
2293: the condition is true if a recursive call to the whole pattern or any
2294: subpattern has been made. If digits or a name preceded by ampersand follow the
2295: letter R, for example:
2296: .sp
2297: (?(R3)...) or (?(R&name)...)
2298: .sp
2299: the condition is true if the most recent recursion is into a subpattern whose
2300: number or name is given. This condition does not check the entire recursion
2301: stack. If the name used in a condition of this kind is a duplicate, the test is
2302: applied to all subpatterns of the same name, and is true if any one of them is
2303: the most recent recursion.
2304: .P
2305: At "top level", all these recursion test conditions are false.
2306: .\" HTML <a href="#recursion">
2307: .\" </a>
2308: The syntax for recursive patterns
2309: .\"
2310: is described below.
2311: .
2312: .\" HTML <a name="subdefine"></a>
2313: .SS "Defining subpatterns for use by reference only"
2314: .rs
2315: .sp
2316: If the condition is the string (DEFINE), and there is no subpattern with the
2317: name DEFINE, the condition is always false. In this case, there may be only one
2318: alternative in the subpattern. It is always skipped if control reaches this
2319: point in the pattern; the idea of DEFINE is that it can be used to define
2320: subroutines that can be referenced from elsewhere. (The use of
2321: .\" HTML <a href="#subpatternsassubroutines">
2322: .\" </a>
2323: subroutines
2324: .\"
2325: is described below.) For example, a pattern to match an IPv4 address such as
1.1.1.3 misho 2326: "192.168.23.245" could be written like this (ignore white space and line
1.1 misho 2327: breaks):
2328: .sp
2329: (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2330: \eb (?&byte) (\e.(?&byte)){3} \eb
2331: .sp
2332: The first part of the pattern is a DEFINE group inside which a another group
2333: named "byte" is defined. This matches an individual component of an IPv4
2334: address (a number less than 256). When matching takes place, this part of the
2335: pattern is skipped because DEFINE acts like a false condition. The rest of the
2336: pattern uses references to the named group to match the four dot-separated
2337: components of an IPv4 address, insisting on a word boundary at each end.
2338: .
2339: .SS "Assertion conditions"
2340: .rs
2341: .sp
2342: If the condition is not in any of the above formats, it must be an assertion.
2343: This may be a positive or negative lookahead or lookbehind assertion. Consider
2344: this pattern, again containing non-significant white space, and with the two
2345: alternatives on the second line:
2346: .sp
2347: (?(?=[^a-z]*[a-z])
2348: \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2349: .sp
2350: The condition is a positive lookahead assertion that matches an optional
2351: sequence of non-letters followed by a letter. In other words, it tests for the
2352: presence of at least one letter in the subject. If a letter is found, the
2353: subject is matched against the first alternative; otherwise it is matched
2354: against the second. This pattern matches strings in one of the two forms
2355: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2356: .
2357: .
2358: .\" HTML <a name="comments"></a>
2359: .SH COMMENTS
2360: .rs
2361: .sp
2362: There are two ways of including comments in patterns that are processed by
2363: PCRE. In both cases, the start of the comment must not be in a character class,
2364: nor in the middle of any other sequence of related characters such as (?: or a
2365: subpattern name or number. The characters that make up a comment play no part
2366: in the pattern matching.
2367: .P
2368: The sequence (?# marks the start of a comment that continues up to the next
2369: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2370: option is set, an unescaped # character also introduces a comment, which in
2371: this case continues to immediately after the next newline character or
2372: character sequence in the pattern. Which characters are interpreted as newlines
1.1.1.2 misho 2373: is controlled by the options passed to a compiling function or by a special
1.1 misho 2374: sequence at the start of the pattern, as described in the section entitled
2375: .\" HTML <a href="#newlines">
2376: .\" </a>
2377: "Newline conventions"
2378: .\"
2379: above. Note that the end of this type of comment is a literal newline sequence
2380: in the pattern; escape sequences that happen to represent a newline do not
2381: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2382: default newline convention is in force:
2383: .sp
2384: abc #comment \en still comment
2385: .sp
2386: On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2387: a newline in the pattern. The sequence \en is still literal at this stage, so
2388: it does not terminate the comment. Only an actual character with the code value
2389: 0x0a (the default newline) does so.
2390: .
2391: .
2392: .\" HTML <a name="recursion"></a>
2393: .SH "RECURSIVE PATTERNS"
2394: .rs
2395: .sp
2396: Consider the problem of matching a string in parentheses, allowing for
2397: unlimited nested parentheses. Without the use of recursion, the best that can
2398: be done is to use a pattern that matches up to some fixed depth of nesting. It
2399: is not possible to handle an arbitrary nesting depth.
2400: .P
2401: For some time, Perl has provided a facility that allows regular expressions to
2402: recurse (amongst other things). It does this by interpolating Perl code in the
2403: expression at run time, and the code can refer to the expression itself. A Perl
2404: pattern using code interpolation to solve the parentheses problem can be
2405: created like this:
2406: .sp
2407: $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2408: .sp
2409: The (?p{...}) item interpolates Perl code at run time, and in this case refers
2410: recursively to the pattern in which it appears.
2411: .P
2412: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2413: supports special syntax for recursion of the entire pattern, and also for
2414: individual subpattern recursion. After its introduction in PCRE and Python,
2415: this kind of recursion was subsequently introduced into Perl at release 5.10.
2416: .P
2417: A special item that consists of (? followed by a number greater than zero and a
2418: closing parenthesis is a recursive subroutine call of the subpattern of the
2419: given number, provided that it occurs inside that subpattern. (If not, it is a
2420: .\" HTML <a href="#subpatternsassubroutines">
2421: .\" </a>
2422: non-recursive subroutine
2423: .\"
2424: call, which is described in the next section.) The special item (?R) or (?0) is
2425: a recursive call of the entire regular expression.
2426: .P
2427: This PCRE pattern solves the nested parentheses problem (assume the
2428: PCRE_EXTENDED option is set so that white space is ignored):
2429: .sp
2430: \e( ( [^()]++ | (?R) )* \e)
2431: .sp
2432: First it matches an opening parenthesis. Then it matches any number of
2433: substrings which can either be a sequence of non-parentheses, or a recursive
2434: match of the pattern itself (that is, a correctly parenthesized substring).
2435: Finally there is a closing parenthesis. Note the use of a possessive quantifier
2436: to avoid backtracking into sequences of non-parentheses.
2437: .P
2438: If this were part of a larger pattern, you would not want to recurse the entire
2439: pattern, so instead you could use this:
2440: .sp
2441: ( \e( ( [^()]++ | (?1) )* \e) )
2442: .sp
2443: We have put the pattern into parentheses, and caused the recursion to refer to
2444: them instead of the whole pattern.
2445: .P
2446: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2447: is made easier by the use of relative references. Instead of (?1) in the
2448: pattern above you can write (?-2) to refer to the second most recently opened
2449: parentheses preceding the recursion. In other words, a negative number counts
2450: capturing parentheses leftwards from the point at which it is encountered.
2451: .P
2452: It is also possible to refer to subsequently opened parentheses, by writing
2453: references such as (?+2). However, these cannot be recursive because the
2454: reference is not inside the parentheses that are referenced. They are always
2455: .\" HTML <a href="#subpatternsassubroutines">
2456: .\" </a>
2457: non-recursive subroutine
2458: .\"
2459: calls, as described in the next section.
2460: .P
2461: An alternative approach is to use named parentheses instead. The Perl syntax
2462: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2463: could rewrite the above example as follows:
2464: .sp
2465: (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2466: .sp
2467: If there is more than one subpattern with the same name, the earliest one is
2468: used.
2469: .P
2470: This particular example pattern that we have been looking at contains nested
2471: unlimited repeats, and so the use of a possessive quantifier for matching
2472: strings of non-parentheses is important when applying the pattern to strings
2473: that do not match. For example, when this pattern is applied to
2474: .sp
2475: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2476: .sp
2477: it yields "no match" quickly. However, if a possessive quantifier is not used,
2478: the match runs for a very long time indeed because there are so many different
2479: ways the + and * repeats can carve up the subject, and all have to be tested
2480: before failure can be reported.
2481: .P
2482: At the end of a match, the values of capturing parentheses are those from
2483: the outermost level. If you want to obtain intermediate values, a callout
2484: function can be used (see below and the
2485: .\" HREF
2486: \fBpcrecallout\fP
2487: .\"
2488: documentation). If the pattern above is matched against
2489: .sp
2490: (ab(cd)ef)
2491: .sp
2492: the value for the inner capturing parentheses (numbered 2) is "ef", which is
2493: the last value taken on at the top level. If a capturing subpattern is not
2494: matched at the top level, its final captured value is unset, even if it was
2495: (temporarily) set at a deeper level during the matching process.
2496: .P
2497: If there are more than 15 capturing parentheses in a pattern, PCRE has to
2498: obtain extra memory to store data during a recursion, which it does by using
2499: \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2500: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2501: .P
2502: Do not confuse the (?R) item with the condition (R), which tests for recursion.
2503: Consider this pattern, which matches text in angle brackets, allowing for
2504: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2505: recursing), whereas any characters are permitted at the outer level.
2506: .sp
2507: < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2508: .sp
2509: In this pattern, (?(R) is the start of a conditional subpattern, with two
2510: different alternatives for the recursive and non-recursive cases. The (?R) item
2511: is the actual recursive call.
2512: .
2513: .
2514: .\" HTML <a name="recursiondifference"></a>
2515: .SS "Differences in recursion processing between PCRE and Perl"
2516: .rs
2517: .sp
2518: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2519: (like Python, but unlike Perl), a recursive subpattern call is always treated
2520: as an atomic group. That is, once it has matched some of the subject string, it
2521: is never re-entered, even if it contains untried alternatives and there is a
2522: subsequent matching failure. This can be illustrated by the following pattern,
2523: which purports to match a palindromic string that contains an odd number of
2524: characters (for example, "a", "aba", "abcba", "abcdcba"):
2525: .sp
2526: ^(.|(.)(?1)\e2)$
2527: .sp
2528: The idea is that it either matches a single character, or two identical
2529: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2530: it does not if the pattern is longer than three characters. Consider the
2531: subject string "abcba":
2532: .P
2533: At the top level, the first character is matched, but as it is not at the end
2534: of the string, the first alternative fails; the second alternative is taken
2535: and the recursion kicks in. The recursive call to subpattern 1 successfully
2536: matches the next character ("b"). (Note that the beginning and end of line
2537: tests are not part of the recursion).
2538: .P
2539: Back at the top level, the next character ("c") is compared with what
2540: subpattern 2 matched, which was "a". This fails. Because the recursion is
2541: treated as an atomic group, there are now no backtracking points, and so the
2542: entire match fails. (Perl is able, at this point, to re-enter the recursion and
2543: try the second alternative.) However, if the pattern is written with the
2544: alternatives in the other order, things are different:
2545: .sp
2546: ^((.)(?1)\e2|.)$
2547: .sp
2548: This time, the recursing alternative is tried first, and continues to recurse
2549: until it runs out of characters, at which point the recursion fails. But this
2550: time we do have another alternative to try at the higher level. That is the big
2551: difference: in the previous case the remaining alternative is at a deeper
2552: recursion level, which PCRE cannot use.
2553: .P
2554: To change the pattern so that it matches all palindromic strings, not just
2555: those with an odd number of characters, it is tempting to change the pattern to
2556: this:
2557: .sp
2558: ^((.)(?1)\e2|.?)$
2559: .sp
2560: Again, this works in Perl, but not in PCRE, and for the same reason. When a
2561: deeper recursion has matched a single character, it cannot be entered again in
2562: order to match an empty string. The solution is to separate the two cases, and
2563: write out the odd and even cases as alternatives at the higher level:
2564: .sp
2565: ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2566: .sp
2567: If you want to match typical palindromic phrases, the pattern has to ignore all
2568: non-word characters, which can be done like this:
2569: .sp
2570: ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2571: .sp
2572: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2573: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2574: the use of the possessive quantifier *+ to avoid backtracking into sequences of
2575: non-word characters. Without this, PCRE takes a great deal longer (ten times or
2576: more) to match typical phrases, and Perl takes so long that you think it has
2577: gone into a loop.
2578: .P
2579: \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2580: string does not start with a palindrome that is shorter than the entire string.
2581: For example, although "abcba" is correctly matched, if the subject is "ababa",
2582: PCRE finds the palindrome "aba" at the start, then fails at top level because
2583: the end of the string does not follow. Once again, it cannot jump back into the
2584: recursion to try other alternatives, so the entire match fails.
2585: .P
2586: The second way in which PCRE and Perl differ in their recursion processing is
2587: in the handling of captured values. In Perl, when a subpattern is called
2588: recursively or as a subpattern (see the next section), it has no access to any
2589: values that were captured outside the recursion, whereas in PCRE these values
2590: can be referenced. Consider this pattern:
2591: .sp
2592: ^(.)(\e1|a(?2))
2593: .sp
2594: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2595: then in the second group, when the back reference \e1 fails to match "b", the
2596: second alternative matches "a" and then recurses. In the recursion, \e1 does
2597: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2598: match because inside the recursive call \e1 cannot access the externally set
2599: value.
2600: .
2601: .
2602: .\" HTML <a name="subpatternsassubroutines"></a>
2603: .SH "SUBPATTERNS AS SUBROUTINES"
2604: .rs
2605: .sp
2606: If the syntax for a recursive subpattern call (either by number or by
2607: name) is used outside the parentheses to which it refers, it operates like a
2608: subroutine in a programming language. The called subpattern may be defined
2609: before or after the reference. A numbered reference can be absolute or
2610: relative, as in these examples:
2611: .sp
2612: (...(absolute)...)...(?2)...
2613: (...(relative)...)...(?-1)...
2614: (...(?+1)...(relative)...
2615: .sp
2616: An earlier example pointed out that the pattern
2617: .sp
2618: (sens|respons)e and \e1ibility
2619: .sp
2620: matches "sense and sensibility" and "response and responsibility", but not
2621: "sense and responsibility". If instead the pattern
2622: .sp
2623: (sens|respons)e and (?1)ibility
2624: .sp
2625: is used, it does match "sense and responsibility" as well as the other two
2626: strings. Another example is given in the discussion of DEFINE above.
2627: .P
2628: All subroutine calls, whether recursive or not, are always treated as atomic
2629: groups. That is, once a subroutine has matched some of the subject string, it
2630: is never re-entered, even if it contains untried alternatives and there is a
2631: subsequent matching failure. Any capturing parentheses that are set during the
2632: subroutine call revert to their previous values afterwards.
2633: .P
2634: Processing options such as case-independence are fixed when a subpattern is
2635: defined, so if it is used as a subroutine, such options cannot be changed for
2636: different calls. For example, consider this pattern:
2637: .sp
2638: (abc)(?i:(?-1))
2639: .sp
2640: It matches "abcabc". It does not match "abcABC" because the change of
2641: processing option does not affect the called subpattern.
2642: .
2643: .
2644: .\" HTML <a name="onigurumasubroutines"></a>
2645: .SH "ONIGURUMA SUBROUTINE SYNTAX"
2646: .rs
2647: .sp
2648: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2649: a number enclosed either in angle brackets or single quotes, is an alternative
2650: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2651: are two of the examples used above, rewritten using this syntax:
2652: .sp
2653: (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2654: (sens|respons)e and \eg'1'ibility
2655: .sp
2656: PCRE supports an extension to Oniguruma: if a number is preceded by a
2657: plus or a minus sign it is taken as a relative reference. For example:
2658: .sp
2659: (abc)(?i:\eg<-1>)
2660: .sp
2661: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2662: synonymous. The former is a back reference; the latter is a subroutine call.
2663: .
2664: .
2665: .SH CALLOUTS
2666: .rs
2667: .sp
2668: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2669: code to be obeyed in the middle of matching a regular expression. This makes it
2670: possible, amongst other things, to extract different substrings that match the
2671: same pair of parentheses when there is a repetition.
2672: .P
2673: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2674: code. The feature is called "callout". The caller of PCRE provides an external
1.1.1.2 misho 2675: function by putting its entry point in the global variable \fIpcre_callout\fP
1.1.1.4 ! misho 2676: (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
! 2677: By default, this variable contains NULL, which disables all calling out.
1.1 misho 2678: .P
2679: Within a regular expression, (?C) indicates the points at which the external
2680: function is to be called. If you want to identify different callout points, you
2681: can put a number less than 256 after the letter C. The default value is zero.
2682: For example, this pattern has two callout points:
2683: .sp
2684: (?C1)abc(?C2)def
2685: .sp
1.1.1.2 misho 2686: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
1.1 misho 2687: automatically installed before each item in the pattern. They are all numbered
1.1.1.4 ! misho 2688: 255. If there is a conditional group in the pattern whose condition is an
! 2689: assertion, an additional callout is inserted just before the condition. An
! 2690: explicit callout may also be set at this position, as in this example:
! 2691: .sp
! 2692: (?(?C9)(?=a)abc|def)
! 2693: .sp
! 2694: Note that this applies only to assertion conditions, not to other types of
! 2695: condition.
1.1 misho 2696: .P
1.1.1.2 misho 2697: During matching, when PCRE reaches a callout point, the external function is
2698: called. It is provided with the number of the callout, the position in the
2699: pattern, and, optionally, one item of data originally supplied by the caller of
2700: the matching function. The callout function may cause matching to proceed, to
2701: backtrack, or to fail altogether. A complete description of the interface to
2702: the callout function is given in the
1.1 misho 2703: .\" HREF
2704: \fBpcrecallout\fP
2705: .\"
2706: documentation.
2707: .
2708: .
2709: .\" HTML <a name="backtrackcontrol"></a>
2710: .SH "BACKTRACKING CONTROL"
2711: .rs
2712: .sp
2713: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1.1.1.4 ! misho 2714: are still described in the Perl documentation as "experimental and subject to
! 2715: change or removal in a future version of Perl". It goes on to say: "Their usage
! 2716: in production code should be noted to avoid problems during upgrades." The same
1.1 misho 2717: remarks apply to the PCRE features described in this section.
2718: .P
1.1.1.4 ! misho 2719: The new verbs make use of what was previously invalid syntax: an opening
! 2720: parenthesis followed by an asterisk. They are generally of the form
! 2721: (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
! 2722: differently depending on whether or not a name is present. A name is any
! 2723: sequence of characters that does not include a closing parenthesis. The maximum
! 2724: length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
! 2725: libraries. If the name is empty, that is, if the closing parenthesis
! 2726: immediately follows the colon, the effect is as if the colon were not there.
! 2727: Any number of these verbs may occur in a pattern.
! 2728: .P
1.1 misho 2729: Since these verbs are specifically related to backtracking, most of them can be
1.1.1.2 misho 2730: used only when the pattern is to be matched using one of the traditional
1.1.1.4 ! misho 2731: matching functions, because these use a backtracking algorithm. With the
! 2732: exception of (*FAIL), which behaves like a failing negative assertion, the
! 2733: backtracking control verbs cause an error if encountered by a DFA matching
! 2734: function.
1.1 misho 2735: .P
1.1.1.4 ! misho 2736: The behaviour of these verbs in
! 2737: .\" HTML <a href="#btrepeat">
! 2738: .\" </a>
! 2739: repeated groups,
! 2740: .\"
! 2741: .\" HTML <a href="#btassert">
! 2742: .\" </a>
! 2743: assertions,
! 2744: .\"
! 2745: and in
! 2746: .\" HTML <a href="#btsub">
! 2747: .\" </a>
! 2748: subpatterns called as subroutines
! 2749: .\"
! 2750: (whether or not recursively) is documented below.
1.1.1.3 misho 2751: .
2752: .
2753: .\" HTML <a name="nooptimize"></a>
2754: .SS "Optimizations that affect backtracking verbs"
2755: .rs
2756: .sp
1.1 misho 2757: PCRE contains some optimizations that are used to speed up matching by running
2758: some checks at the start of each match attempt. For example, it may know the
2759: minimum length of matching subject, or that a particular character must be
1.1.1.4 ! misho 2760: present. When one of these optimizations bypasses the running of a match, any
1.1 misho 2761: included backtracking verbs will not, of course, be processed. You can suppress
2762: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2763: when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
1.1.1.3 misho 2764: pattern with (*NO_START_OPT). There is more discussion of this option in the
2765: section entitled
2766: .\" HTML <a href="pcreapi.html#execoptions">
2767: .\" </a>
2768: "Option bits for \fBpcre_exec()\fP"
2769: .\"
2770: in the
2771: .\" HREF
2772: \fBpcreapi\fP
2773: .\"
2774: documentation.
1.1 misho 2775: .P
2776: Experiments with Perl suggest that it too has similar optimizations, sometimes
2777: leading to anomalous results.
2778: .
2779: .
2780: .SS "Verbs that act immediately"
2781: .rs
2782: .sp
2783: The following verbs act as soon as they are encountered. They may not be
2784: followed by a name.
2785: .sp
2786: (*ACCEPT)
2787: .sp
2788: This verb causes the match to end successfully, skipping the remainder of the
2789: pattern. However, when it is inside a subpattern that is called as a
2790: subroutine, only that subpattern is ended successfully. Matching then continues
1.1.1.4 ! misho 2791: at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
! 2792: assertion succeeds; in a negative assertion, the assertion fails.
! 2793: .P
! 2794: If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
! 2795: example:
1.1 misho 2796: .sp
2797: A((?:A|B(*ACCEPT)|C)D)
2798: .sp
2799: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2800: the outer parentheses.
2801: .sp
2802: (*FAIL) or (*F)
2803: .sp
2804: This verb causes a matching failure, forcing backtracking to occur. It is
2805: equivalent to (?!) but easier to read. The Perl documentation notes that it is
2806: probably useful only when combined with (?{}) or (??{}). Those are, of course,
2807: Perl features that are not present in PCRE. The nearest equivalent is the
2808: callout feature, as for example in this pattern:
2809: .sp
2810: a+(?C)(*FAIL)
2811: .sp
2812: A match with the string "aaaa" always fails, but the callout is taken before
2813: each backtrack happens (in this example, 10 times).
2814: .
2815: .
2816: .SS "Recording which path was taken"
2817: .rs
2818: .sp
2819: There is one verb whose main purpose is to track how a match was arrived at,
2820: though it also has a secondary use in conjunction with advancing the match
2821: starting point (see (*SKIP) below).
2822: .sp
2823: (*MARK:NAME) or (*:NAME)
2824: .sp
2825: A name is always required with this verb. There may be as many instances of
2826: (*MARK) as you like in a pattern, and their names do not have to be unique.
2827: .P
1.1.1.4 ! misho 2828: When a match succeeds, the name of the last-encountered (*MARK:NAME),
! 2829: (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
! 2830: caller as described in the section entitled
1.1 misho 2831: .\" HTML <a href="pcreapi.html#extradata">
2832: .\" </a>
1.1.1.2 misho 2833: "Extra data for \fBpcre_exec()\fP"
1.1 misho 2834: .\"
2835: in the
2836: .\" HREF
2837: \fBpcreapi\fP
2838: .\"
2839: documentation. Here is an example of \fBpcretest\fP output, where the /K
2840: modifier requests the retrieval and outputting of (*MARK) data:
2841: .sp
2842: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2843: data> XY
2844: 0: XY
2845: MK: A
2846: XZ
2847: 0: XZ
2848: MK: B
2849: .sp
2850: The (*MARK) name is tagged with "MK:" in this output, and in this example it
2851: indicates which of the two alternatives matched. This is a more efficient way
2852: of obtaining this information than putting each alternative in its own
2853: capturing parentheses.
2854: .P
1.1.1.4 ! misho 2855: If a verb with a name is encountered in a positive assertion that is true, the
! 2856: name is recorded and passed back if it is the last-encountered. This does not
! 2857: happen for negative assertions or failing positive assertions.
1.1 misho 2858: .P
1.1.1.4 ! misho 2859: After a partial match or a failed match, the last encountered name in the
! 2860: entire match process is returned. For example:
1.1 misho 2861: .sp
2862: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2863: data> XP
2864: No match, mark = B
2865: .sp
2866: Note that in this unanchored example the mark is retained from the match
1.1.1.3 misho 2867: attempt that started at the letter "X" in the subject. Subsequent match
2868: attempts starting at "P" and then with an empty string do not get as far as the
2869: (*MARK) item, but nevertheless do not reset it.
2870: .P
2871: If you are interested in (*MARK) values after failed matches, you should
2872: probably set the PCRE_NO_START_OPTIMIZE option
2873: .\" HTML <a href="#nooptimize">
2874: .\" </a>
2875: (see above)
2876: .\"
2877: to ensure that the match is always attempted.
1.1 misho 2878: .
2879: .
2880: .SS "Verbs that act after backtracking"
2881: .rs
2882: .sp
2883: The following verbs do nothing when they are encountered. Matching continues
2884: with what follows, but if there is no subsequent match, causing a backtrack to
2885: the verb, a failure is forced. That is, backtracking cannot pass to the left of
1.1.1.4 ! misho 2886: the verb. However, when one of these verbs appears inside an atomic group or an
! 2887: assertion that is true, its effect is confined to that group, because once the
! 2888: group has been matched, there is never any backtracking into it. In this
! 2889: situation, backtracking can "jump back" to the left of the entire atomic group
! 2890: or assertion. (Remember also, as stated above, that this localization also
! 2891: applies in subroutine calls.)
1.1 misho 2892: .P
2893: These verbs differ in exactly what kind of failure occurs when backtracking
1.1.1.4 ! misho 2894: reaches them. The behaviour described below is what happens when the verb is
! 2895: not in a subroutine or an assertion. Subsequent sections cover these special
! 2896: cases.
1.1 misho 2897: .sp
2898: (*COMMIT)
2899: .sp
2900: This verb, which may not be followed by a name, causes the whole match to fail
1.1.1.4 ! misho 2901: outright if there is a later matching failure that causes backtracking to reach
! 2902: it. Even if the pattern is unanchored, no further attempts to find a match by
! 2903: advancing the starting point take place. If (*COMMIT) is the only backtracking
! 2904: verb that is encountered, once it has been passed \fBpcre_exec()\fP is
! 2905: committed to finding a match at the current starting point, or not at all. For
! 2906: example:
1.1 misho 2907: .sp
2908: a+(*COMMIT)b
2909: .sp
2910: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2911: dynamic anchor, or "I've started, so I must finish." The name of the most
2912: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2913: match failure.
2914: .P
1.1.1.4 ! misho 2915: If there is more than one backtracking verb in a pattern, a different one that
! 2916: follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
! 2917: match does not always guarantee that a match must be at this starting point.
! 2918: .P
1.1 misho 2919: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2920: unless PCRE's start-of-match optimizations are turned off, as shown in this
2921: \fBpcretest\fP example:
2922: .sp
2923: re> /(*COMMIT)abc/
2924: data> xyzabc
2925: 0: abc
2926: xyzabc\eY
2927: No match
2928: .sp
2929: PCRE knows that any match must start with "a", so the optimization skips along
2930: the subject to "a" before running the first match attempt, which succeeds. When
2931: the optimization is disabled by the \eY escape in the second subject, the match
2932: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2933: starting points.
2934: .sp
2935: (*PRUNE) or (*PRUNE:NAME)
2936: .sp
2937: This verb causes the match to fail at the current starting position in the
1.1.1.4 ! misho 2938: subject if there is a later matching failure that causes backtracking to reach
! 2939: it. If the pattern is unanchored, the normal "bumpalong" advance to the next
! 2940: starting character then happens. Backtracking can occur as usual to the left of
! 2941: (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
! 2942: if there is no match to the right, backtracking cannot cross (*PRUNE). In
! 2943: simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
! 2944: possessive quantifier, but there are some uses of (*PRUNE) that cannot be
! 2945: expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
! 2946: as (*COMMIT).
! 2947: .P
! 2948: The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
! 2949: It is like (*MARK:NAME) in that the name is remembered for passing back to the
! 2950: caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
1.1 misho 2951: .sp
2952: (*SKIP)
2953: .sp
2954: This verb, when given without a name, is like (*PRUNE), except that if the
2955: pattern is unanchored, the "bumpalong" advance is not to the next character,
2956: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2957: signifies that whatever text was matched leading up to it cannot be part of a
2958: successful match. Consider:
2959: .sp
2960: a+(*SKIP)b
2961: .sp
2962: If the subject is "aaaac...", after the first match attempt fails (starting at
2963: the first character in the string), the starting point skips on to start the
2964: next attempt at "c". Note that a possessive quantifer does not have the same
2965: effect as this example; although it would suppress backtracking during the
2966: first match attempt, the second attempt would start at the second character
2967: instead of skipping on to "c".
2968: .sp
2969: (*SKIP:NAME)
2970: .sp
1.1.1.4 ! misho 2971: When (*SKIP) has an associated name, its behaviour is modified. When it is
! 2972: triggered, the previous path through the pattern is searched for the most
! 2973: recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
! 2974: is to the subject position that corresponds to that (*MARK) instead of to where
! 2975: (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
! 2976: (*SKIP) is ignored.
! 2977: .P
! 2978: Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
! 2979: names that are set by (*PRUNE:NAME) or (*THEN:NAME).
1.1 misho 2980: .sp
2981: (*THEN) or (*THEN:NAME)
2982: .sp
1.1.1.4 ! misho 2983: This verb causes a skip to the next innermost alternative when backtracking
! 2984: reaches it. That is, it cancels any further backtracking within the current
! 2985: alternative. Its name comes from the observation that it can be used for a
! 2986: pattern-based if-then-else block:
1.1 misho 2987: .sp
2988: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2989: .sp
2990: If the COND1 pattern matches, FOO is tried (and possibly further items after
2991: the end of the group if FOO succeeds); on failure, the matcher skips to the
1.1.1.4 ! misho 2992: second alternative and tries COND2, without backtracking into COND1. If that
! 2993: succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
! 2994: more alternatives, so there is a backtrack to whatever came before the entire
! 2995: group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
! 2996: .P
! 2997: The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
! 2998: It is like (*MARK:NAME) in that the name is remembered for passing back to the
! 2999: caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
1.1 misho 3000: .P
1.1.1.4 ! misho 3001: A subpattern that does not contain a | character is just a part of the
! 3002: enclosing alternative; it is not a nested alternation with only one
1.1 misho 3003: alternative. The effect of (*THEN) extends beyond such a subpattern to the
3004: enclosing alternative. Consider this pattern, where A, B, etc. are complex
3005: pattern fragments that do not contain any | characters at this level:
3006: .sp
3007: A (B(*THEN)C) | D
3008: .sp
3009: If A and B are matched, but there is a failure in C, matching does not
3010: backtrack into A; instead it moves to the next alternative, that is, D.
3011: However, if the subpattern containing (*THEN) is given an alternative, it
3012: behaves differently:
3013: .sp
3014: A (B(*THEN)C | (*FAIL)) | D
3015: .sp
3016: The effect of (*THEN) is now confined to the inner subpattern. After a failure
3017: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3018: because there are no more alternatives to try. In this case, matching does now
3019: backtrack into A.
3020: .P
1.1.1.4 ! misho 3021: Note that a conditional subpattern is not considered as having two
1.1 misho 3022: alternatives, because only one is ever used. In other words, the | character in
3023: a conditional subpattern has a different meaning. Ignoring white space,
3024: consider:
3025: .sp
3026: ^.*? (?(?=a) a | b(*THEN)c )
3027: .sp
3028: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3029: it initially matches zero characters. The condition (?=a) then fails, the
3030: character "b" is matched, but "c" is not. At this point, matching does not
3031: backtrack to .*? as might perhaps be expected from the presence of the |
3032: character. The conditional subpattern is part of the single alternative that
3033: comprises the whole pattern, and so the match fails. (If there was a backtrack
3034: into .*?, allowing it to match "b", the match would succeed.)
3035: .P
3036: The verbs just described provide four different "strengths" of control when
3037: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3038: next alternative. (*PRUNE) comes next, failing the match at the current
3039: starting position, but allowing an advance to the next character (for an
3040: unanchored pattern). (*SKIP) is similar, except that the advance may be more
3041: than one character. (*COMMIT) is the strongest, causing the entire match to
3042: fail.
1.1.1.4 ! misho 3043: .
! 3044: .
! 3045: .SS "More than one backtracking verb"
! 3046: .rs
! 3047: .sp
! 3048: If more than one backtracking verb is present in a pattern, the one that is
! 3049: backtracked onto first acts. For example, consider this pattern, where A, B,
! 3050: etc. are complex pattern fragments:
! 3051: .sp
! 3052: (A(*COMMIT)B(*THEN)C|ABD)
! 3053: .sp
! 3054: If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
! 3055: fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
! 3056: the next alternative (ABD) to be tried. This behaviour is consistent, but is
! 3057: not always the same as Perl's. It means that if two or more backtracking verbs
! 3058: appear in succession, all the the last of them has no effect. Consider this
! 3059: example:
! 3060: .sp
! 3061: ...(*COMMIT)(*PRUNE)...
! 3062: .sp
! 3063: If there is a matching failure to the right, backtracking onto (*PRUNE) cases
! 3064: it to be triggered, and its action is taken. There can never be a backtrack
! 3065: onto (*COMMIT).
! 3066: .
! 3067: .
! 3068: .\" HTML <a name="btrepeat"></a>
! 3069: .SS "Backtracking verbs in repeated groups"
! 3070: .rs
! 3071: .sp
! 3072: PCRE differs from Perl in its handling of backtracking verbs in repeated
! 3073: groups. For example, consider:
! 3074: .sp
! 3075: /(a(*COMMIT)b)+ac/
! 3076: .sp
! 3077: If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
! 3078: the second repeat of the group acts.
! 3079: .
! 3080: .
! 3081: .\" HTML <a name="btassert"></a>
! 3082: .SS "Backtracking verbs in assertions"
! 3083: .rs
! 3084: .sp
! 3085: (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
! 3086: .P
! 3087: (*ACCEPT) in a positive assertion causes the assertion to succeed without any
! 3088: further processing. In a negative assertion, (*ACCEPT) causes the assertion to
! 3089: fail without any further processing.
! 3090: .P
! 3091: The other backtracking verbs are not treated specially if they appear in a
! 3092: positive assertion. In particular, (*THEN) skips to the next alternative in the
! 3093: innermost enclosing group that has alternations, whether or not this is within
! 3094: the assertion.
! 3095: .P
! 3096: Negative assertions are, however, different, in order to ensure that changing a
! 3097: positive assertion into a negative assertion changes its result. Backtracking
! 3098: into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
! 3099: without considering any further alternative branches in the assertion.
! 3100: Backtracking into (*THEN) causes it to skip to the next enclosing alternative
! 3101: within the assertion (the normal behaviour), but if the assertion does not have
! 3102: such an alternative, (*THEN) behaves like (*PRUNE).
! 3103: .
! 3104: .
! 3105: .\" HTML <a name="btsub"></a>
! 3106: .SS "Backtracking verbs in subroutines"
! 3107: .rs
! 3108: .sp
! 3109: These behaviours occur whether or not the subpattern is called recursively.
! 3110: Perl's treatment of subroutines is different in some cases.
! 3111: .P
! 3112: (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
! 3113: an immediate backtrack.
! 3114: .P
! 3115: (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
! 3116: succeed without any further processing. Matching then continues after the
! 3117: subroutine call.
! 3118: .P
! 3119: (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
! 3120: the subroutine match to fail.
1.1 misho 3121: .P
1.1.1.4 ! misho 3122: (*THEN) skips to the next alternative in the innermost enclosing group within
! 3123: the subpattern that has alternatives. If there is no such group within the
! 3124: subpattern, (*THEN) causes the subroutine match to fail.
1.1 misho 3125: .
3126: .
3127: .SH "SEE ALSO"
3128: .rs
3129: .sp
3130: \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
1.1.1.4 ! misho 3131: \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
1.1 misho 3132: .
3133: .
3134: .SH AUTHOR
3135: .rs
3136: .sp
3137: .nf
3138: Philip Hazel
3139: University Computing Service
3140: Cambridge CB2 3QH, England.
3141: .fi
3142: .
3143: .
3144: .SH REVISION
3145: .rs
3146: .sp
3147: .nf
1.1.1.4 ! misho 3148: Last updated: 26 April 2013
! 3149: Copyright (c) 1997-2013 University of Cambridge.
1.1 misho 3150: .fi
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