1PCREPATTERN(3) Library Functions Manual PCREPATTERN(3)
2
6 PCRE - Perl-compatible regular expressions
7
9 The syntax and semantics of the regular expressions that are supported
11 by PCRE are described in detail below. There is a quick-reference syn‐
12 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
13 semantics as closely as it can. PCRE also supports some alternative
14 regular expression syntax (which does not conflict with the Perl syn‐
15 tax) in order to provide some compatibility with regular expressions in
16 Python, .NET, and Oniguruma.
17 Perl's regular expressions are described in its own documentation, and
19 regular expressions in general are covered in a number of books, some
20 of which have copious examples. Jeffrey Friedl's "Mastering Regular Ex‐
21 pressions", published by O'Reilly, covers regular expressions in great
22 detail. This description of PCRE's regular expressions is intended as
23 reference material.
24 This document discusses the patterns that are supported by PCRE when
26 one its main matching functions, pcre_exec() (8-bit) or
27 pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has alternative
28 matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which
29 match using a different algorithm that is not Perl-compatible. Some of
30 the features discussed below are not available when DFA matching is
31 used. The advantages and disadvantages of the alternative functions,
32 and how they differ from the normal functions, are discussed in the
33 pcrematching page.
34
36 A number of options that can be passed to pcre_compile() can also be
38 set by special items at the start of a pattern. These are not Perl-com‐
39 patible, but are provided to make these options accessible to pattern
40 writers who are not able to change the program that processes the pat‐
41 tern. Any number of these items may appear, but they must all be to‐
42 gether right at the start of the pattern string, and the letters must
43 be in upper case.
44 UTF support
46 The original operation of PCRE was on strings of one-byte characters.
48 However, there is now also support for UTF-8 strings in the original
49 library, an extra library that supports 16-bit and UTF-16 character
50 strings, and a third library that supports 32-bit and UTF-32 character
51 strings. To use these features, PCRE must be built to include appropri‐
52 ate support. When using UTF strings you must either call the compiling
53 function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the
54 pattern must start with one of these special sequences:
55 (*UTF8)
57 (*UTF16)
58 (*UTF32)
59 (*UTF)
60 (*UTF) is a generic sequence that can be used with any of the li‐
62 braries. Starting a pattern with such a sequence is equivalent to set‐
63 ting the relevant option. How setting a UTF mode affects pattern match‐
64 ing is mentioned in several places below. There is also a summary of
65 features in the pcreunicode page.
66 Some applications that allow their users to supply patterns may wish to
68 restrict them to non-UTF data for security reasons. If the
69 PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not al‐
70 lowed, and their appearance causes an error.
71 Unicode property support
73 Another special sequence that may appear at the start of a pattern is
75 (*UCP). This has the same effect as setting the PCRE_UCP option: it
76 causes sequences such as \d and \w to use Unicode properties to deter‐
77 mine character types, instead of recognizing only characters with codes
78 less than 128 via a lookup table.
79 Disabling auto-possessification
81 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
83 setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops
84 PCRE from making quantifiers possessive when what follows cannot match
85 the repeated item. For example, by default a+b is treated as a++b. For
86 more details, see the pcreapi documentation.
87 Disabling start-up optimizations
89 If a pattern starts with (*NO_START_OPT), it has the same effect as
91 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
92 time. This disables several optimizations for quickly reaching "no
93 match" results. For more details, see the pcreapi documentation.
94 Newline conventions
96 PCRE supports five different conventions for indicating line breaks in
98 strings: a single CR (carriage return) character, a single LF (line‐
99 feed) character, the two-character sequence CRLF, any of the three pre‐
100 ceding, or any Unicode newline sequence. The pcreapi page has further
101 discussion about newlines, and shows how to set the newline convention
102 in the options arguments for the compiling and matching functions.
103 It is also possible to specify a newline convention by starting a pat‐
105 tern string with one of the following five sequences:
106 (*CR) carriage return
108 (*LF) linefeed
109 (*CRLF) carriage return, followed by linefeed
110 (*ANYCRLF) any of the three above
111 (*ANY) all Unicode newline sequences
112 These override the default and the options given to the compiling func‐
114 tion. For example, on a Unix system where LF is the default newline se‐
115 quence, the pattern
116 (*CR)a.b
118 changes the convention to CR. That pattern matches "a\nb" because LF is
120 no longer a newline. If more than one of these settings is present, the
121 last one is used.
122 The newline convention affects where the circumflex and dollar asser‐
124 tions are true. It also affects the interpretation of the dot metachar‐
125 acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it
126 does not affect what the \R escape sequence matches. By default, this
127 is any Unicode newline sequence, for Perl compatibility. However, this
128 can be changed; see the description of \R in the section entitled "New‐
129 line sequences" below. A change of \R setting can be combined with a
130 change of newline convention.
131 Setting match and recursion limits
133 The caller of pcre_exec() can set a limit on the number of times the
135 internal match() function is called and on the maximum depth of recur‐
136 sive calls. These facilities are provided to catch runaway matches that
137 are provoked by patterns with huge matching trees (a typical example is
138 a pattern with nested unlimited repeats) and to avoid running out of
139 system stack by too much recursion. When one of these limits is
140 reached, pcre_exec() gives an error return. The limits can also be set
141 by items at the start of the pattern of the form
142 (*LIMIT_MATCH=d)
144 (*LIMIT_RECURSION=d)
145 where d is any number of decimal digits. However, the value of the set‐
147 ting must be less than the value set (or defaulted) by the caller of
148 pcre_exec() for it to have any effect. In other words, the pattern
149 writer can lower the limits set by the programmer, but not raise them.
150 If there is more than one setting of one of these limits, the lower
151 value is used.
152
154 PCRE can be compiled to run in an environment that uses EBCDIC as its
156 character code rather than ASCII or Unicode (typically a mainframe sys‐
157 tem). In the sections below, character code values are ASCII or Uni‐
158 code; in an EBCDIC environment these characters may have different code
159 values, and there are no code points greater than 255.
160
162 A regular expression is a pattern that is matched against a subject
164 string from left to right. Most characters stand for themselves in a
165 pattern, and match the corresponding characters in the subject. As a
166 trivial example, the pattern
167 The quick brown fox
169 matches a portion of a subject string that is identical to itself. When
171 caseless matching is specified (the PCRE_CASELESS option), letters are
172 matched independently of case. In a UTF mode, PCRE always understands
173 the concept of case for characters whose values are less than 128, so
174 caseless matching is always possible. For characters with higher val‐
175 ues, the concept of case is supported if PCRE is compiled with Unicode
176 property support, but not otherwise. If you want to use caseless
177 matching for characters 128 and above, you must ensure that PCRE is
178 compiled with Unicode property support as well as with UTF support.
179 The power of regular expressions comes from the ability to include al‐
181 ternatives and repetitions in the pattern. These are encoded in the
182 pattern by the use of metacharacters, which do not stand for themselves
183 but instead are interpreted in some special way.
184 There are two different sets of metacharacters: those that are recog‐
186 nized anywhere in the pattern except within square brackets, and those
187 that are recognized within square brackets. Outside square brackets,
188 the metacharacters are as follows:
189 \ general escape character with several uses
191 ^ assert start of string (or line, in multiline mode)
192 $ assert end of string (or line, in multiline mode)
193 . match any character except newline (by default)
194 [ start character class definition
195 | start of alternative branch
196 ( start subpattern
197 ) end subpattern
198 ? extends the meaning of (
199 also 0 or 1 quantifier
200 also quantifier minimizer
201 * 0 or more quantifier
202 + 1 or more quantifier
203 also "possessive quantifier"
204 { start min/max quantifier
205 Part of a pattern that is in square brackets is called a "character
207 class". In a character class the only metacharacters are:
208 \ general escape character
210 ^ negate the class, but only if the first character
211 - indicates character range
212 [ POSIX character class (only if followed by POSIX
213 syntax)
214 ] terminates the character class
215 The following sections describe the use of each of the metacharacters.
217
219 The backslash character has several uses. Firstly, if it is followed by
221 a character that is not a number or a letter, it takes away any special
222 meaning that character may have. This use of backslash as an escape
223 character applies both inside and outside character classes.
224 For example, if you want to match a * character, you write \* in the
226 pattern. This escaping action applies whether or not the following
227 character would otherwise be interpreted as a metacharacter, so it is
228 always safe to precede a non-alphanumeric with backslash to specify
229 that it stands for itself. In particular, if you want to match a back‐
230 slash, you write \\.
231 In a UTF mode, only ASCII numbers and letters have any special meaning
233 after a backslash. All other characters (in particular, those whose
234 codepoints are greater than 127) are treated as literals.
235 If a pattern is compiled with the PCRE_EXTENDED option, most white
237 space in the pattern (other than in a character class), and characters
238 between a # outside a character class and the next newline, inclusive,
239 are ignored. An escaping backslash can be used to include a white space
240 or # character as part of the pattern.
241 If you want to remove the special meaning from a sequence of charac‐
243 ters, you can do so by putting them between \Q and \E. This is differ‐
244 ent from Perl in that $ and @ are handled as literals in \Q...\E se‐
245 quences in PCRE, whereas in Perl, $ and @ cause variable interpolation.
246 Note the following examples:
247 Pattern PCRE matches Perl matches
249 \Qabc$xyz\E abc$xyz abc followed by the
251 contents of $xyz
252 \Qabc\$xyz\E abc\$xyz abc\$xyz
253 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
254 The \Q...\E sequence is recognized both inside and outside character
256 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
257 is not followed by \E later in the pattern, the literal interpretation
258 continues to the end of the pattern (that is, \E is assumed at the
259 end). If the isolated \Q is inside a character class, this causes an
260 error, because the character class is not terminated.
261 Non-printing characters
263 A second use of backslash provides a way of encoding non-printing char‐
265 acters in patterns in a visible manner. There is no restriction on the
266 appearance of non-printing characters, apart from the binary zero that
267 terminates a pattern, but when a pattern is being prepared by text
268 editing, it is often easier to use one of the following escape se‐
269 quences than the binary character it represents. In an ASCII or Uni‐
270 code environment, these escapes are as follows:
271 \a alarm, that is, the BEL character (hex 07)
273 \cx "control-x", where x is any ASCII character
274 \e escape (hex 1B)
275 \f form feed (hex 0C)
276 \n linefeed (hex 0A)
277 \r carriage return (hex 0D)
278 \t tab (hex 09)
279 \0dd character with octal code 0dd
280 \ddd character with octal code ddd, or back reference
281 \o{ddd..} character with octal code ddd..
282 \xhh character with hex code hh
283 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
284 \uhhhh character with hex code hhhh (JavaScript mode only)
285 The precise effect of \cx on ASCII characters is as follows: if x is a
287 lower case letter, it is converted to upper case. Then bit 6 of the
288 character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
289 (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
290 hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c
291 has a value greater than 127, a compile-time error occurs. This locks
292 out non-ASCII characters in all modes.
293 When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t gener‐
295 ate the appropriate EBCDIC code values. The \c escape is processed as
296 specified for Perl in the perlebcdic document. The only characters that
297 are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?.
298 Any other character provokes a compile-time error. The sequence \c@ en‐
299 codes character code 0; after \c the letters (in either case) encode
300 characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters
301 27-31 (hex 1B to hex 1F), and \c? becomes either 255 (hex FF) or 95
302 (hex 5F).
303 Thus, apart from \c?, these escapes generate the same character code
305 values as they do in an ASCII environment, though the meanings of the
306 values mostly differ. For example, \cG always generates code value 7,
307 which is BEL in ASCII but DEL in EBCDIC.
308 The sequence \c? generates DEL (127, hex 7F) in an ASCII environment,
310 but because 127 is not a control character in EBCDIC, Perl makes it
311 generate the APC character. Unfortunately, there are several variants
312 of EBCDIC. In most of them the APC character has the value 255 (hex
313 FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If
314 certain other characters have POSIX-BC values, PCRE makes \c? generate
315 95; otherwise it generates 255.
316 After \0 up to two further octal digits are read. If there are fewer
318 than two digits, just those that are present are used. Thus the se‐
319 quence \0\x\015 specifies two binary zeros followed by a CR character
320 (code value 13). Make sure you supply two digits after the initial zero
321 if the pattern character that follows is itself an octal digit.
322 The escape \o must be followed by a sequence of octal digits, enclosed
324 in braces. An error occurs if this is not the case. This escape is a
325 recent addition to Perl; it provides way of specifying character code
326 points as octal numbers greater than 0777, and it also allows octal
327 numbers and back references to be unambiguously specified.
328 For greater clarity and unambiguity, it is best to avoid following \ by
330 a digit greater than zero. Instead, use \o{} or \x{} to specify charac‐
331 ter numbers, and \g{} to specify back references. The following para‐
332 graphs describe the old, ambiguous syntax.
333 The handling of a backslash followed by a digit other than 0 is compli‐
335 cated, and Perl has changed in recent releases, causing PCRE also to
336 change. Outside a character class, PCRE reads the digit and any follow‐
337 ing digits as a decimal number. If the number is less than 8, or if
338 there have been at least that many previous capturing left parentheses
339 in the expression, the entire sequence is taken as a back reference. A
340 description of how this works is given later, following the discussion
341 of parenthesized subpatterns.
342 Inside a character class, or if the decimal number following \ is
344 greater than 7 and there have not been that many capturing subpatterns,
345 PCRE handles \8 and \9 as the literal characters "8" and "9", and oth‐
346 erwise re-reads up to three octal digits following the backslash, using
347 them to generate a data character. Any subsequent digits stand for
348 themselves. For example:
349 \040 is another way of writing an ASCII space
351 \40 is the same, provided there are fewer than 40
352 previous capturing subpatterns
353 \7 is always a back reference
354 \11 might be a back reference, or another way of
355 writing a tab
356 \011 is always a tab
357 \0113 is a tab followed by the character "3"
358 \113 might be a back reference, otherwise the
359 character with octal code 113
360 \377 might be a back reference, otherwise
361 the value 255 (decimal)
362 \81 is either a back reference, or the two
363 characters "8" and "1"
364 Note that octal values of 100 or greater that are specified using this
366 syntax must not be introduced by a leading zero, because no more than
367 three octal digits are ever read.
368 By default, after \x that is not followed by {, from zero to two hexa‐
370 decimal digits are read (letters can be in upper or lower case). Any
371 number of hexadecimal digits may appear between \x{ and }. If a charac‐
372 ter other than a hexadecimal digit appears between \x{ and }, or if
373 there is no terminating }, an error occurs.
374 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
376 is as just described only when it is followed by two hexadecimal dig‐
377 its. Otherwise, it matches a literal "x" character. In JavaScript
378 mode, support for code points greater than 256 is provided by \u, which
379 must be followed by four hexadecimal digits; otherwise it matches a
380 literal "u" character.
381 Characters whose value is less than 256 can be defined by either of the
383 two syntaxes for \x (or by \u in JavaScript mode). There is no differ‐
384 ence in the way they are handled. For example, \xdc is exactly the same
385 as \x{dc} (or \u00dc in JavaScript mode).
386 Constraints on character values
388 Characters that are specified using octal or hexadecimal numbers are
390 limited to certain values, as follows:
391 8-bit non-UTF mode less than 0x100
393 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
394 16-bit non-UTF mode less than 0x10000
395 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
396 32-bit non-UTF mode less than 0x100000000
397 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
398 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
400 called "surrogate" codepoints), and 0xffef.
401 Escape sequences in character classes
403 All the sequences that define a single character value can be used both
405 inside and outside character classes. In addition, inside a character
406 class, \b is interpreted as the backspace character (hex 08).
407 \N is not allowed in a character class. \B, \R, and \X are not special
409 inside a character class. Like other unrecognized escape sequences,
410 they are treated as the literal characters "B", "R", and "X" by de‐
411 fault, but cause an error if the PCRE_EXTRA option is set. Outside a
412 character class, these sequences have different meanings.
413 Unsupported escape sequences
415 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
417 handler and used to modify the case of following characters. By de‐
418 fault, PCRE does not support these escape sequences. However, if the
419 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
420 \u can be used to define a character by code point, as described in the
421 previous section.
422 Absolute and relative back references
424 The sequence \g followed by an unsigned or a negative number, option‐
426 ally enclosed in braces, is an absolute or relative back reference. A
427 named back reference can be coded as \g{name}. Back references are dis‐
428 cussed later, following the discussion of parenthesized subpatterns.
429 Absolute and relative subroutine calls
431 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
433 name or a number enclosed either in angle brackets or single quotes, is
434 an alternative syntax for referencing a subpattern as a "subroutine".
435 Details are discussed later. Note that \g{...} (Perl syntax) and
436 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
437 reference; the latter is a subroutine call.
438 Generic character types
440 Another use of backslash is for specifying generic character types:
442 \d any decimal digit
444 \D any character that is not a decimal digit
445 \h any horizontal white space character
446 \H any character that is not a horizontal white space character
447 \s any white space character
448 \S any character that is not a white space character
449 \v any vertical white space character
450 \V any character that is not a vertical white space character
451 \w any "word" character
452 \W any "non-word" character
453 There is also the single sequence \N, which matches a non-newline char‐
455 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
456 not set. Perl also uses \N to match characters by name; PCRE does not
457 support this.
458 Each pair of lower and upper case escape sequences partitions the com‐
460 plete set of characters into two disjoint sets. Any given character
461 matches one, and only one, of each pair. The sequences can appear both
462 inside and outside character classes. They each match one character of
463 the appropriate type. If the current matching point is at the end of
464 the subject string, all of them fail, because there is no character to
465 match.
466 For compatibility with Perl, \s did not used to match the VT character
468 (code 11), which made it different from the the POSIX "space" class.
469 However, Perl added VT at release 5.18, and PCRE followed suit at re‐
470 lease 8.34. The default \s characters are now HT (9), LF (10), VT (11),
471 FF (12), CR (13), and space (32), which are defined as white space in
472 the "C" locale. This list may vary if locale-specific matching is tak‐
473 ing place. For example, in some locales the "non-breaking space" char‐
474 acter (\xA0) is recognized as white space, and in others the VT charac‐
475 ter is not.
476 A "word" character is an underscore or any character that is a letter
478 or digit. By default, the definition of letters and digits is con‐
479 trolled by PCRE's low-valued character tables, and may vary if locale-
480 specific matching is taking place (see "Locale support" in the pcreapi
481 page). For example, in a French locale such as "fr_FR" in Unix-like
482 systems, or "french" in Windows, some character codes greater than 127
483 are used for accented letters, and these are then matched by \w. The
484 use of locales with Unicode is discouraged.
485 By default, characters whose code points are greater than 127 never
487 match \d, \s, or \w, and always match \D, \S, and \W, although this may
488 vary for characters in the range 128-255 when locale-specific matching
489 is happening. These escape sequences retain their original meanings
490 from before Unicode support was available, mainly for efficiency rea‐
491 sons. If PCRE is compiled with Unicode property support, and the
492 PCRE_UCP option is set, the behaviour is changed so that Unicode prop‐
493 erties are used to determine character types, as follows:
494 \d any character that matches \p{Nd} (decimal digit)
496 \s any character that matches \p{Z} or \h or \v
497 \w any character that matches \p{L} or \p{N}, plus underscore
498 The upper case escapes match the inverse sets of characters. Note that
500 \d matches only decimal digits, whereas \w matches any Unicode digit,
501 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
502 affects \b, and \B because they are defined in terms of \w and \W.
503 Matching these sequences is noticeably slower when PCRE_UCP is set.
504 The sequences \h, \H, \v, and \V are features that were added to Perl
506 at release 5.10. In contrast to the other sequences, which match only
507 ASCII characters by default, these always match certain high-valued
508 code points, whether or not PCRE_UCP is set. The horizontal space char‐
509 acters are:
510 U+0009 Horizontal tab (HT)
512 U+0020 Space
513 U+00A0 Non-break space
514 U+1680 Ogham space mark
515 U+180E Mongolian vowel separator
516 U+2000 En quad
517 U+2001 Em quad
518 U+2002 En space
519 U+2003 Em space
520 U+2004 Three-per-em space
521 U+2005 Four-per-em space
522 U+2006 Six-per-em space
523 U+2007 Figure space
524 U+2008 Punctuation space
525 U+2009 Thin space
526 U+200A Hair space
527 U+202F Narrow no-break space
528 U+205F Medium mathematical space
529 U+3000 Ideographic space
530 The vertical space characters are:
532 U+000A Linefeed (LF)
534 U+000B Vertical tab (VT)
535 U+000C Form feed (FF)
536 U+000D Carriage return (CR)
537 U+0085 Next line (NEL)
538 U+2028 Line separator
539 U+2029 Paragraph separator
540 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
542 256 are relevant.
543 Newline sequences
545 Outside a character class, by default, the escape sequence \R matches
547 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
548 to the following:
549 (?>\r\n|\n|\x0b|\f|\r|\x85)
551 This is an example of an "atomic group", details of which are given be‐
553 low. This particular group matches either the two-character sequence
554 CR followed by LF, or one of the single characters LF (linefeed,
555 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car‐
556 riage return, U+000D), or NEL (next line, U+0085). The two-character
557 sequence is treated as a single unit that cannot be split.
558 In other modes, two additional characters whose codepoints are greater
560 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa‐
561 rator, U+2029). Unicode character property support is not needed for
562 these characters to be recognized.
563 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
565 the complete set of Unicode line endings) by setting the option
566 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
567 (BSR is an abbreviation for "backslash R".) This can be made the de‐
568 fault when PCRE is built; if this is the case, the other behaviour can
569 be requested via the PCRE_BSR_UNICODE option. It is also possible to
570 specify these settings by starting a pattern string with one of the
571 following sequences:
572 (*BSR_ANYCRLF) CR, LF, or CRLF only
574 (*BSR_UNICODE) any Unicode newline sequence
575 These override the default and the options given to the compiling func‐
577 tion, but they can themselves be overridden by options given to a
578 matching function. Note that these special settings, which are not
579 Perl-compatible, are recognized only at the very start of a pattern,
580 and that they must be in upper case. If more than one of them is
581 present, the last one is used. They can be combined with a change of
582 newline convention; for example, a pattern can start with:
583 (*ANY)(*BSR_ANYCRLF)
585 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF)
587 or (*UCP) special sequences. Inside a character class, \R is treated as
588 an unrecognized escape sequence, and so matches the letter "R" by de‐
589 fault, but causes an error if PCRE_EXTRA is set.
590 Unicode character properties
592 When PCRE is built with Unicode character property support, three addi‐
594 tional escape sequences that match characters with specific properties
595 are available. When in 8-bit non-UTF-8 mode, these sequences are of
596 course limited to testing characters whose codepoints are less than
597 256, but they do work in this mode. The extra escape sequences are:
598 \p{xx} a character with the xx property
600 \P{xx} a character without the xx property
601 \X a Unicode extended grapheme cluster
602 The property names represented by xx above are limited to the Unicode
604 script names, the general category properties, "Any", which matches any
605 character (including newline), and some special PCRE properties (de‐
606 scribed in the next section). Other Perl properties such as "InMusi‐
607 calSymbols" are not currently supported by PCRE. Note that \P{Any} does
608 not match any characters, so always causes a match failure.
609 Sets of Unicode characters are defined as belonging to certain scripts.
611 A character from one of these sets can be matched using a script name.
612 For example:
613 \p{Greek}
615 \P{Han}
616 Those that are not part of an identified script are lumped together as
618 "Common". The current list of scripts is:
619 Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali,
621 Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car‐
622 ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei‐
623 form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero‐
624 glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha,
625 Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Im‐
626 perial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip‐
627 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
628 Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin‐
629 ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic,
630 Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hi‐
631 eroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue,
632 Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic,
633 Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pa‐
634 hawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician,
635 Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha‐
636 vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac,
637 Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu,
638 Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi,
639 Yi.
640 Each character has exactly one Unicode general category property, spec‐
642 ified by a two-letter abbreviation. For compatibility with Perl, nega‐
643 tion can be specified by including a circumflex between the opening
644 brace and the property name. For example, \p{^Lu} is the same as
645 \P{Lu}.
646 If only one letter is specified with \p or \P, it includes all the gen‐
648 eral category properties that start with that letter. In this case, in
649 the absence of negation, the curly brackets in the escape sequence are
650 optional; these two examples have the same effect:
651 \p{L}
653 \pL
654 The following general category property codes are supported:
656 C Other
658 Cc Control
659 Cf Format
660 Cn Unassigned
661 Co Private use
662 Cs Surrogate
663 L Letter
665 Ll Lower case letter
666 Lm Modifier letter
667 Lo Other letter
668 Lt Title case letter
669 Lu Upper case letter
670 M Mark
672 Mc Spacing mark
673 Me Enclosing mark
674 Mn Non-spacing mark
675 N Number
677 Nd Decimal number
678 Nl Letter number
679 No Other number
680 P Punctuation
682 Pc Connector punctuation
683 Pd Dash punctuation
684 Pe Close punctuation
685 Pf Final punctuation
686 Pi Initial punctuation
687 Po Other punctuation
688 Ps Open punctuation
689 S Symbol
691 Sc Currency symbol
692 Sk Modifier symbol
693 Sm Mathematical symbol
694 So Other symbol
695 Z Separator
697 Zl Line separator
698 Zp Paragraph separator
699 Zs Space separator
700 The special property L& is also supported: it matches a character that
702 has the Lu, Ll, or Lt property, in other words, a letter that is not
703 classified as a modifier or "other".
704 The Cs (Surrogate) property applies only to characters in the range
706 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
707 so cannot be tested by PCRE, unless UTF validity checking has been
708 turned off (see the discussion of PCRE_NO_UTF8_CHECK,
709 PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl
710 does not support the Cs property.
711 The long synonyms for property names that Perl supports (such as
713 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
714 any of these properties with "Is".
715 No character that is in the Unicode table has the Cn (unassigned) prop‐
717 erty. Instead, this property is assumed for any code point that is not
718 in the Unicode table.
719 Specifying caseless matching does not affect these escape sequences.
721 For example, \p{Lu} always matches only upper case letters. This is
722 different from the behaviour of current versions of Perl.
723 Matching characters by Unicode property is not fast, because PCRE has
725 to do a multistage table lookup in order to find a character's prop‐
726 erty. That is why the traditional escape sequences such as \d and \w do
727 not use Unicode properties in PCRE by default, though you can make them
728 do so by setting the PCRE_UCP option or by starting the pattern with
729 (*UCP).
730 Extended grapheme clusters
732 The \X escape matches any number of Unicode characters that form an
734 "extended grapheme cluster", and treats the sequence as an atomic group
735 (see below). Up to and including release 8.31, PCRE matched an ear‐
736 lier, simpler definition that was equivalent to
737 (?>\PM\pM*)
739 That is, it matched a character without the "mark" property, followed
741 by zero or more characters with the "mark" property. Characters with
742 the "mark" property are typically non-spacing accents that affect the
743 preceding character.
744 This simple definition was extended in Unicode to include more compli‐
746 cated kinds of composite character by giving each character a grapheme
747 breaking property, and creating rules that use these properties to de‐
748 fine the boundaries of extended grapheme clusters. In releases of PCRE
749 later than 8.31, \X matches one of these clusters.
750 \X always matches at least one character. Then it decides whether to
752 add additional characters according to the following rules for ending a
753 cluster:
754 1. End at the end of the subject string.
756 2. Do not end between CR and LF; otherwise end after any control char‐
758 acter.
759 3. Do not break Hangul (a Korean script) syllable sequences. Hangul
761 characters are of five types: L, V, T, LV, and LVT. An L character may
762 be followed by an L, V, LV, or LVT character; an LV or V character may
763 be followed by a V or T character; an LVT or T character may be fol‐
764 lowed only by a T character.
765 4. Do not end before extending characters or spacing marks. Characters
767 with the "mark" property always have the "extend" grapheme breaking
768 property.
769 5. Do not end after prepend characters.
771 6. Otherwise, end the cluster.
773 PCRE's additional properties
775 As well as the standard Unicode properties described above, PCRE sup‐
777 ports four more that make it possible to convert traditional escape se‐
778 quences such as \w and \s to use Unicode properties. PCRE uses these
779 non-standard, non-Perl properties internally when PCRE_UCP is set. How‐
780 ever, they may also be used explicitly. These properties are:
781 Xan Any alphanumeric character
783 Xps Any POSIX space character
784 Xsp Any Perl space character
785 Xwd Any Perl "word" character
786 Xan matches characters that have either the L (letter) or the N (num‐
788 ber) property. Xps matches the characters tab, linefeed, vertical tab,
789 form feed, or carriage return, and any other character that has the Z
790 (separator) property. Xsp is the same as Xps; it used to exclude ver‐
791 tical tab, for Perl compatibility, but Perl changed, and so PCRE fol‐
792 lowed at release 8.34. Xwd matches the same characters as Xan, plus un‐
793 derscore.
794 There is another non-standard property, Xuc, which matches any charac‐
796 ter that can be represented by a Universal Character Name in C++ and
797 other programming languages. These are the characters $, @, ` (grave
798 accent), and all characters with Unicode code points greater than or
799 equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that
800 most base (ASCII) characters are excluded. (Universal Character Names
801 are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit.
802 Note that the Xuc property does not match these sequences but the char‐
803 acters that they represent.)
804 Resetting the match start
806 The escape sequence \K causes any previously matched characters not to
808 be included in the final matched sequence. For example, the pattern:
809 foo\Kbar
811 matches "foobar", but reports that it has matched "bar". This feature
813 is similar to a lookbehind assertion (described below). However, in
814 this case, the part of the subject before the real match does not have
815 to be of fixed length, as lookbehind assertions do. The use of \K does
816 not interfere with the setting of captured substrings. For example,
817 when the pattern
818 (foo)\Kbar
820 matches "foobar", the first substring is still set to "foo".
822 Perl documents that the use of \K within assertions is "not well de‐
824 fined". In PCRE, \K is acted upon when it occurs inside positive asser‐
825 tions, but is ignored in negative assertions. Note that when a pattern
826 such as (?=ab\K) matches, the reported start of the match can be
827 greater than the end of the match.
828 Simple assertions
830 The final use of backslash is for certain simple assertions. An asser‐
832 tion specifies a condition that has to be met at a particular point in
833 a match, without consuming any characters from the subject string. The
834 use of subpatterns for more complicated assertions is described below.
835 The backslashed assertions are:
836 \b matches at a word boundary
838 \B matches when not at a word boundary
839 \A matches at the start of the subject
840 \Z matches at the end of the subject
841 also matches before a newline at the end of the subject
842 \z matches only at the end of the subject
843 \G matches at the first matching position in the subject
844 Inside a character class, \b has a different meaning; it matches the
846 backspace character. If any other of these assertions appears in a
847 character class, by default it matches the corresponding literal char‐
848 acter (for example, \B matches the letter B). However, if the PCRE_EX‐
849 TRA option is set, an "invalid escape sequence" error is generated in‐
850 stead.
851 A word boundary is a position in the subject string where the current
853 character and the previous character do not both match \w or \W (i.e.
854 one matches \w and the other matches \W), or the start or end of the
855 string if the first or last character matches \w, respectively. In a
856 UTF mode, the meanings of \w and \W can be changed by setting the
857 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
858 PCRE nor Perl has a separate "start of word" or "end of word" metase‐
859 quence. However, whatever follows \b normally determines which it is.
860 For example, the fragment \ba matches "a" at the start of a word.
861 The \A, \Z, and \z assertions differ from the traditional circumflex
863 and dollar (described in the next section) in that they only ever match
864 at the very start and end of the subject string, whatever options are
865 set. Thus, they are independent of multiline mode. These three asser‐
866 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
867 affect only the behaviour of the circumflex and dollar metacharacters.
868 However, if the startoffset argument of pcre_exec() is non-zero, indi‐
869 cating that matching is to start at a point other than the beginning of
870 the subject, \A can never match. The difference between \Z and \z is
871 that \Z matches before a newline at the end of the string as well as at
872 the very end, whereas \z matches only at the end.
873 The \G assertion is true only when the current matching position is at
875 the start point of the match, as specified by the startoffset argument
876 of pcre_exec(). It differs from \A when the value of startoffset is
877 non-zero. By calling pcre_exec() multiple times with appropriate argu‐
878 ments, you can mimic Perl's /g option, and it is in this kind of imple‐
879 mentation where \G can be useful.
880 Note, however, that PCRE's interpretation of \G, as the start of the
882 current match, is subtly different from Perl's, which defines it as the
883 end of the previous match. In Perl, these can be different when the
884 previously matched string was empty. Because PCRE does just one match
885 at a time, it cannot reproduce this behaviour.
886 If all the alternatives of a pattern begin with \G, the expression is
888 anchored to the starting match position, and the "anchored" flag is set
889 in the compiled regular expression.
890
892 The circumflex and dollar metacharacters are zero-width assertions.
894 That is, they test for a particular condition being true without con‐
895 suming any characters from the subject string.
896 Outside a character class, in the default matching mode, the circumflex
898 character is an assertion that is true only if the current matching
899 point is at the start of the subject string. If the startoffset argu‐
900 ment of pcre_exec() is non-zero, circumflex can never match if the
901 PCRE_MULTILINE option is unset. Inside a character class, circumflex
902 has an entirely different meaning (see below).
903 Circumflex need not be the first character of the pattern if a number
905 of alternatives are involved, but it should be the first thing in each
906 alternative in which it appears if the pattern is ever to match that
907 branch. If all possible alternatives start with a circumflex, that is,
908 if the pattern is constrained to match only at the start of the sub‐
909 ject, it is said to be an "anchored" pattern. (There are also other
910 constructs that can cause a pattern to be anchored.)
911 The dollar character is an assertion that is true only if the current
913 matching point is at the end of the subject string, or immediately be‐
914 fore a newline at the end of the string (by default). Note, however,
915 that it does not actually match the newline. Dollar need not be the
916 last character of the pattern if a number of alternatives are involved,
917 but it should be the last item in any branch in which it appears. Dol‐
918 lar has no special meaning in a character class.
919 The meaning of dollar can be changed so that it matches only at the
921 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
922 compile time. This does not affect the \Z assertion.
923 The meanings of the circumflex and dollar characters are changed if the
925 PCRE_MULTILINE option is set. When this is the case, a circumflex
926 matches immediately after internal newlines as well as at the start of
927 the subject string. It does not match after a newline that ends the
928 string. A dollar matches before any newlines in the string, as well as
929 at the very end, when PCRE_MULTILINE is set. When newline is specified
930 as the two-character sequence CRLF, isolated CR and LF characters do
931 not indicate newlines.
932 For example, the pattern /^abc$/ matches the subject string "def\nabc"
934 (where \n represents a newline) in multiline mode, but not otherwise.
935 Consequently, patterns that are anchored in single line mode because
936 all branches start with ^ are not anchored in multiline mode, and a
937 match for circumflex is possible when the startoffset argument of
938 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
939 PCRE_MULTILINE is set.
940 Note that the sequences \A, \Z, and \z can be used to match the start
942 and end of the subject in both modes, and if all branches of a pattern
943 start with \A it is always anchored, whether or not PCRE_MULTILINE is
944 set.
945
947 Outside a character class, a dot in the pattern matches any one charac‐
949 ter in the subject string except (by default) a character that signi‐
950 fies the end of a line.
951 When a line ending is defined as a single character, dot never matches
953 that character; when the two-character sequence CRLF is used, dot does
954 not match CR if it is immediately followed by LF, but otherwise it
955 matches all characters (including isolated CRs and LFs). When any Uni‐
956 code line endings are being recognized, dot does not match CR or LF or
957 any of the other line ending characters.
958 The behaviour of dot with regard to newlines can be changed. If the
960 PCRE_DOTALL option is set, a dot matches any one character, without ex‐
961 ception. If the two-character sequence CRLF is present in the subject
962 string, it takes two dots to match it.
963 The handling of dot is entirely independent of the handling of circum‐
965 flex and dollar, the only relationship being that they both involve
966 newlines. Dot has no special meaning in a character class.
967 The escape sequence \N behaves like a dot, except that it is not af‐
969 fected by the PCRE_DOTALL option. In other words, it matches any char‐
970 acter except one that signifies the end of a line. Perl also uses \N to
971 match characters by name; PCRE does not support this.
972
974 Outside a character class, the escape sequence \C matches any one data
976 unit, whether or not a UTF mode is set. In the 8-bit library, one data
977 unit is one byte; in the 16-bit library it is a 16-bit unit; in the
978 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches
979 line-ending characters. The feature is provided in Perl in order to
980 match individual bytes in UTF-8 mode, but it is unclear how it can use‐
981 fully be used. Because \C breaks up characters into individual data
982 units, matching one unit with \C in a UTF mode means that the rest of
983 the string may start with a malformed UTF character. This has undefined
984 results, because PCRE assumes that it is dealing with valid UTF strings
985 (and by default it checks this at the start of processing unless the
986 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option
987 is used).
988 PCRE does not allow \C to appear in lookbehind assertions (described
990 below) in a UTF mode, because this would make it impossible to calcu‐
991 late the length of the lookbehind.
992 In general, the \C escape sequence is best avoided. However, one way of
994 using it that avoids the problem of malformed UTF characters is to use
995 a lookahead to check the length of the next character, as in this pat‐
996 tern, which could be used with a UTF-8 string (ignore white space and
997 line breaks):
998 (?| (?=[\x00-\x7f])(\C) |
1000 (?=[\x80-\x{7ff}])(\C)(\C) |
1001 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1002 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1003 A group that starts with (?| resets the capturing parentheses numbers
1005 in each alternative (see "Duplicate Subpattern Numbers" below). The as‐
1006 sertions at the start of each branch check the next UTF-8 character for
1007 values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The char‐
1008 acter's individual bytes are then captured by the appropriate number of
1009 groups.
1010
1012 An opening square bracket introduces a character class, terminated by a
1014 closing square bracket. A closing square bracket on its own is not spe‐
1015 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
1016 a lone closing square bracket causes a compile-time error. If a closing
1017 square bracket is required as a member of the class, it should be the
1018 first data character in the class (after an initial circumflex, if
1019 present) or escaped with a backslash.
1020 A character class matches a single character in the subject. In a UTF
1022 mode, the character may be more than one data unit long. A matched
1023 character must be in the set of characters defined by the class, unless
1024 the first character in the class definition is a circumflex, in which
1025 case the subject character must not be in the set defined by the class.
1026 If a circumflex is actually required as a member of the class, ensure
1027 it is not the first character, or escape it with a backslash.
1028 For example, the character class [aeiou] matches any lower case vowel,
1030 while [^aeiou] matches any character that is not a lower case vowel.
1031 Note that a circumflex is just a convenient notation for specifying the
1032 characters that are in the class by enumerating those that are not. A
1033 class that starts with a circumflex is not an assertion; it still con‐
1034 sumes a character from the subject string, and therefore it fails if
1035 the current pointer is at the end of the string.
1036 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255
1038 (0xffff) can be included in a class as a literal string of data units,
1039 or by using the \x{ escaping mechanism.
1040 When caseless matching is set, any letters in a class represent both
1042 their upper case and lower case versions, so for example, a caseless
1043 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
1044 match "A", whereas a caseful version would. In a UTF mode, PCRE always
1045 understands the concept of case for characters whose values are less
1046 than 128, so caseless matching is always possible. For characters with
1047 higher values, the concept of case is supported if PCRE is compiled
1048 with Unicode property support, but not otherwise. If you want to use
1049 caseless matching in a UTF mode for characters 128 and above, you must
1050 ensure that PCRE is compiled with Unicode property support as well as
1051 with UTF support.
1052 Characters that might indicate line breaks are never treated in any
1054 special way when matching character classes, whatever line-ending se‐
1055 quence is in use, and whatever setting of the PCRE_DOTALL and PCRE_MUL‐
1056 TILINE options is used. A class such as [^a] always matches one of
1057 these characters.
1058 The minus (hyphen) character can be used to specify a range of charac‐
1060 ters in a character class. For example, [d-m] matches any letter be‐
1061 tween d and m, inclusive. If a minus character is required in a class,
1062 it must be escaped with a backslash or appear in a position where it
1063 cannot be interpreted as indicating a range, typically as the first or
1064 last character in the class, or immediately after a range. For example,
1065 [b-d-z] matches letters in the range b to d, a hyphen character, or z.
1066 It is not possible to have the literal character "]" as the end charac‐
1068 ter of a range. A pattern such as [W-]46] is interpreted as a class of
1069 two characters ("W" and "-") followed by a literal string "46]", so it
1070 would match "W46]" or "-46]". However, if the "]" is escaped with a
1071 backslash it is interpreted as the end of range, so [W-\]46] is inter‐
1072 preted as a class containing a range followed by two other characters.
1073 The octal or hexadecimal representation of "]" can also be used to end
1074 a range.
1075 An error is generated if a POSIX character class (see below) or an es‐
1077 cape sequence other than one that defines a single character appears at
1078 a point where a range ending character is expected. For example,
1079 [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.
1080 Ranges operate in the collating sequence of character values. They can
1082 also be used for characters specified numerically, for example
1083 [\000-\037]. Ranges can include any characters that are valid for the
1084 current mode.
1085 If a range that includes letters is used when caseless matching is set,
1087 it matches the letters in either case. For example, [W-c] is equivalent
1088 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
1089 character tables for a French locale are in use, [\xc8-\xcb] matches
1090 accented E characters in both cases. In UTF modes, PCRE supports the
1091 concept of case for characters with values greater than 128 only when
1092 it is compiled with Unicode property support.
1093 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
1095 \w, and \W may appear in a character class, and add the characters that
1096 they match to the class. For example, [\dABCDEF] matches any hexadeci‐
1097 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
1098 \d, \s, \w and their upper case partners, just as it does when they ap‐
1099 pear outside a character class, as described in the section entitled
1100 "Generic character types" above. The escape sequence \b has a different
1101 meaning inside a character class; it matches the backspace character.
1102 The sequences \B, \N, \R, and \X are not special inside a character
1103 class. Like any other unrecognized escape sequences, they are treated
1104 as the literal characters "B", "N", "R", and "X" by default, but cause
1105 an error if the PCRE_EXTRA option is set.
1106 A circumflex can conveniently be used with the upper case character
1108 types to specify a more restricted set of characters than the matching
1109 lower case type. For example, the class [^\W_] matches any letter or
1110 digit, but not underscore, whereas [\w] includes underscore. A positive
1111 character class should be read as "something OR something OR ..." and a
1112 negative class as "NOT something AND NOT something AND NOT ...".
1113 The only metacharacters that are recognized in character classes are
1115 backslash, hyphen (only where it can be interpreted as specifying a
1116 range), circumflex (only at the start), opening square bracket (only
1117 when it can be interpreted as introducing a POSIX class name, or for a
1118 special compatibility feature - see the next two sections), and the
1119 terminating closing square bracket. However, escaping other non-al‐
1120 phanumeric characters does no harm.
1121
1123 Perl supports the POSIX notation for character classes. This uses names
1125 enclosed by [: and :] within the enclosing square brackets. PCRE also
1126 supports this notation. For example,
1127 [01[:alpha:]%]
1129 matches "0", "1", any alphabetic character, or "%". The supported class
1131 names are:
1132 alnum letters and digits
1134 alpha letters
1135 ascii character codes 0 - 127
1136 blank space or tab only
1137 cntrl control characters
1138 digit decimal digits (same as \d)
1139 graph printing characters, excluding space
1140 lower lower case letters
1141 print printing characters, including space
1142 punct printing characters, excluding letters and digits and space
1143 space white space (the same as \s from PCRE 8.34)
1144 upper upper case letters
1145 word "word" characters (same as \w)
1146 xdigit hexadecimal digits
1147 The default "space" characters are HT (9), LF (10), VT (11), FF (12),
1149 CR (13), and space (32). If locale-specific matching is taking place,
1150 the list of space characters may be different; there may be fewer or
1151 more of them. "Space" used to be different to \s, which did not include
1152 VT, for Perl compatibility. However, Perl changed at release 5.18, and
1153 PCRE followed at release 8.34. "Space" and \s now match the same set
1154 of characters.
1155 The name "word" is a Perl extension, and "blank" is a GNU extension
1157 from Perl 5.8. Another Perl extension is negation, which is indicated
1158 by a ^ character after the colon. For example,
1159 [12[:^digit:]]
1161 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
1163 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
1164 these are not supported, and an error is given if they are encountered.
1165 By default, characters with values greater than 128 do not match any of
1167 the POSIX character classes. However, if the PCRE_UCP option is passed
1168 to pcre_compile(), some of the classes are changed so that Unicode
1169 character properties are used. This is achieved by replacing certain
1170 POSIX classes by other sequences, as follows:
1171 [:alnum:] becomes \p{Xan}
1173 [:alpha:] becomes \p{L}
1174 [:blank:] becomes \h
1175 [:digit:] becomes \p{Nd}
1176 [:lower:] becomes \p{Ll}
1177 [:space:] becomes \p{Xps}
1178 [:upper:] becomes \p{Lu}
1179 [:word:] becomes \p{Xwd}
1180 Negated versions, such as [:^alpha:] use \P instead of \p. Three other
1182 POSIX classes are handled specially in UCP mode:
1183 [:graph:] This matches characters that have glyphs that mark the page
1185 when printed. In Unicode property terms, it matches all char‐
1186 acters with the L, M, N, P, S, or Cf properties, except for:
1187 U+061C Arabic Letter Mark
1189 U+180E Mongolian Vowel Separator
1190 U+2066 - U+2069 Various "isolate"s
1191 [:print:] This matches the same characters as [:graph:] plus space
1194 characters that are not controls, that is, characters with
1195 the Zs property.
1196 [:punct:] This matches all characters that have the Unicode P (punctua‐
1198 tion) property, plus those characters whose code points are
1199 less than 128 that have the S (Symbol) property.
1200 The other POSIX classes are unchanged, and match only characters with
1202 code points less than 128.
1203
1205 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the
1207 ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word"
1208 and "end of word". PCRE treats these items as follows:
1209 [[:<:]] is converted to \b(?=\w)
1211 [[:>:]] is converted to \b(?<=\w)
1212 Only these exact character sequences are recognized. A sequence such as
1214 [a[:<:]b] provokes error for an unrecognized POSIX class name. This
1215 support is not compatible with Perl. It is provided to help migrations
1216 from other environments, and is best not used in any new patterns. Note
1217 that \b matches at the start and the end of a word (see "Simple asser‐
1218 tions" above), and in a Perl-style pattern the preceding or following
1219 character normally shows which is wanted, without the need for the as‐
1220 sertions that are used above in order to give exactly the POSIX behav‐
1221 iour.
1222
1224 Vertical bar characters are used to separate alternative patterns. For
1226 example, the pattern
1227 gilbert|sullivan
1229 matches either "gilbert" or "sullivan". Any number of alternatives may
1231 appear, and an empty alternative is permitted (matching the empty
1232 string). The matching process tries each alternative in turn, from left
1233 to right, and the first one that succeeds is used. If the alternatives
1234 are within a subpattern (defined below), "succeeds" means matching the
1235 rest of the main pattern as well as the alternative in the subpattern.
1236
1238 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1240 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
1241 within the pattern by a sequence of Perl option letters enclosed be‐
1242 tween "(?" and ")". The option letters are
1243 i for PCRE_CASELESS
1245 m for PCRE_MULTILINE
1246 s for PCRE_DOTALL
1247 x for PCRE_EXTENDED
1248 For example, (?im) sets caseless, multiline matching. It is also possi‐
1250 ble to unset these options by preceding the letter with a hyphen, and a
1251 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE‐
1252 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
1253 is also permitted. If a letter appears both before and after the hy‐
1254 phen, the option is unset.
1255 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
1257 can be changed in the same way as the Perl-compatible options by using
1258 the characters J, U and X respectively.
1259 When one of these option changes occurs at top level (that is, not in‐
1261 side subpattern parentheses), the change applies to the remainder of
1262 the pattern that follows. An option change within a subpattern (see be‐
1263 low for a description of subpatterns) affects only that part of the
1264 subpattern that follows it, so
1265 (a(?i)b)c
1267 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
1269 used). By this means, options can be made to have different settings
1270 in different parts of the pattern. Any changes made in one alternative
1271 do carry on into subsequent branches within the same subpattern. For
1272 example,
1273 (a(?i)b|c)
1275 matches "ab", "aB", "c", and "C", even though when matching "C" the
1277 first branch is abandoned before the option setting. This is because
1278 the effects of option settings happen at compile time. There would be
1279 some very weird behaviour otherwise.
1280 Note: There are other PCRE-specific options that can be set by the ap‐
1282 plication when the compiling or matching functions are called. In some
1283 cases the pattern can contain special leading sequences such as (*CRLF)
1284 to override what the application has set or what has been defaulted.
1285 Details are given in the section entitled "Newline sequences" above.
1286 There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading se‐
1287 quences that can be used to set UTF and Unicode property modes; they
1288 are equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the
1289 PCRE_UCP options, respectively. The (*UTF) sequence is a generic ver‐
1290 sion that can be used with any of the libraries. However, the applica‐
1291 tion can set the PCRE_NEVER_UTF option, which locks out the use of the
1292 (*UTF) sequences.
1293
1295 Subpatterns are delimited by parentheses (round brackets), which can be
1297 nested. Turning part of a pattern into a subpattern does two things:
1298 1. It localizes a set of alternatives. For example, the pattern
1300 cat(aract|erpillar|)
1302 matches "cataract", "caterpillar", or "cat". Without the parentheses,
1304 it would match "cataract", "erpillar" or an empty string.
1305 2. It sets up the subpattern as a capturing subpattern. This means
1307 that, when the whole pattern matches, that portion of the subject
1308 string that matched the subpattern is passed back to the caller via the
1309 ovector argument of the matching function. (This applies only to the
1310 traditional matching functions; the DFA matching functions do not sup‐
1311 port capturing.)
1312 Opening parentheses are counted from left to right (starting from 1) to
1314 obtain numbers for the capturing subpatterns. For example, if the
1315 string "the red king" is matched against the pattern
1316 the ((red|white) (king|queen))
1318 the captured substrings are "red king", "red", and "king", and are num‐
1320 bered 1, 2, and 3, respectively.
1321 The fact that plain parentheses fulfil two functions is not always
1323 helpful. There are often times when a grouping subpattern is required
1324 without a capturing requirement. If an opening parenthesis is followed
1325 by a question mark and a colon, the subpattern does not do any captur‐
1326 ing, and is not counted when computing the number of any subsequent
1327 capturing subpatterns. For example, if the string "the white queen" is
1328 matched against the pattern
1329 the ((?:red|white) (king|queen))
1331 the captured substrings are "white queen" and "queen", and are numbered
1333 1 and 2. The maximum number of capturing subpatterns is 65535.
1334 As a convenient shorthand, if any option settings are required at the
1336 start of a non-capturing subpattern, the option letters may appear be‐
1337 tween the "?" and the ":". Thus the two patterns
1338 (?i:saturday|sunday)
1340 (?:(?i)saturday|sunday)
1341 match exactly the same set of strings. Because alternative branches are
1343 tried from left to right, and options are not reset until the end of
1344 the subpattern is reached, an option setting in one branch does affect
1345 subsequent branches, so the above patterns match "SUNDAY" as well as
1346 "Saturday".
1347
1349 Perl 5.10 introduced a feature whereby each alternative in a subpattern
1351 uses the same numbers for its capturing parentheses. Such a subpattern
1352 starts with (?| and is itself a non-capturing subpattern. For example,
1353 consider this pattern:
1354 (?|(Sat)ur|(Sun))day
1356 Because the two alternatives are inside a (?| group, both sets of cap‐
1358 turing parentheses are numbered one. Thus, when the pattern matches,
1359 you can look at captured substring number one, whichever alternative
1360 matched. This construct is useful when you want to capture part, but
1361 not all, of one of a number of alternatives. Inside a (?| group, paren‐
1362 theses are numbered as usual, but the number is reset at the start of
1363 each branch. The numbers of any capturing parentheses that follow the
1364 subpattern start after the highest number used in any branch. The fol‐
1365 lowing example is taken from the Perl documentation. The numbers under‐
1366 neath show in which buffer the captured content will be stored.
1367 # before ---------------branch-reset----------- after
1369 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1370 # 1 2 2 3 2 3 4
1371 A back reference to a numbered subpattern uses the most recent value
1373 that is set for that number by any subpattern. The following pattern
1374 matches "abcabc" or "defdef":
1375 /(?|(abc)|(def))\1/
1377 In contrast, a subroutine call to a numbered subpattern always refers
1379 to the first one in the pattern with the given number. The following
1380 pattern matches "abcabc" or "defabc":
1381 /(?|(abc)|(def))(?1)/
1383 If a condition test for a subpattern's having matched refers to a non-
1385 unique number, the test is true if any of the subpatterns of that num‐
1386 ber have matched.
1387 An alternative approach to using this "branch reset" feature is to use
1389 duplicate named subpatterns, as described in the next section.
1390
1392 Identifying capturing parentheses by number is simple, but it can be
1394 very hard to keep track of the numbers in complicated regular expres‐
1395 sions. Furthermore, if an expression is modified, the numbers may
1396 change. To help with this difficulty, PCRE supports the naming of sub‐
1397 patterns. This feature was not added to Perl until release 5.10. Python
1398 had the feature earlier, and PCRE introduced it at release 4.0, using
1399 the Python syntax. PCRE now supports both the Perl and the Python syn‐
1400 tax. Perl allows identically numbered subpatterns to have different
1401 names, but PCRE does not.
1402 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
1404 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
1405 to capturing parentheses from other parts of the pattern, such as back
1406 references, recursion, and conditions, can be made by name as well as
1407 by number.
1408 Names consist of up to 32 alphanumeric characters and underscores, but
1410 must start with a non-digit. Named capturing parentheses are still al‐
1411 located numbers as well as names, exactly as if the names were not
1412 present. The PCRE API provides function calls for extracting the name-
1413 to-number translation table from a compiled pattern. There is also a
1414 convenience function for extracting a captured substring by name.
1415 By default, a name must be unique within a pattern, but it is possible
1417 to relax this constraint by setting the PCRE_DUPNAMES option at compile
1418 time. (Duplicate names are also always permitted for subpatterns with
1419 the same number, set up as described in the previous section.) Dupli‐
1420 cate names can be useful for patterns where only one instance of the
1421 named parentheses can match. Suppose you want to match the name of a
1422 weekday, either as a 3-letter abbreviation or as the full name, and in
1423 both cases you want to extract the abbreviation. This pattern (ignoring
1424 the line breaks) does the job:
1425 (?<DN>Mon|Fri|Sun)(?:day)?|
1427 (?<DN>Tue)(?:sday)?|
1428 (?<DN>Wed)(?:nesday)?|
1429 (?<DN>Thu)(?:rsday)?|
1430 (?<DN>Sat)(?:urday)?
1431 There are five capturing substrings, but only one is ever set after a
1433 match. (An alternative way of solving this problem is to use a "branch
1434 reset" subpattern, as described in the previous section.)
1435 The convenience function for extracting the data by name returns the
1437 substring for the first (and in this example, the only) subpattern of
1438 that name that matched. This saves searching to find which numbered
1439 subpattern it was.
1440 If you make a back reference to a non-unique named subpattern from
1442 elsewhere in the pattern, the subpatterns to which the name refers are
1443 checked in the order in which they appear in the overall pattern. The
1444 first one that is set is used for the reference. For example, this pat‐
1445 tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":
1446 (?:(?<n>foo)|(?<n>bar))\k<n>
1448 If you make a subroutine call to a non-unique named subpattern, the one
1451 that corresponds to the first occurrence of the name is used. In the
1452 absence of duplicate numbers (see the previous section) this is the one
1453 with the lowest number.
1454 If you use a named reference in a condition test (see the section about
1456 conditions below), either to check whether a subpattern has matched, or
1457 to check for recursion, all subpatterns with the same name are tested.
1458 If the condition is true for any one of them, the overall condition is
1459 true. This is the same behaviour as testing by number. For further de‐
1460 tails of the interfaces for handling named subpatterns, see the pcreapi
1461 documentation.
1462 Warning: You cannot use different names to distinguish between two sub‐
1464 patterns with the same number because PCRE uses only the numbers when
1465 matching. For this reason, an error is given at compile time if differ‐
1466 ent names are given to subpatterns with the same number. However, you
1467 can always give the same name to subpatterns with the same number, even
1468 when PCRE_DUPNAMES is not set.
1469
1471 Repetition is specified by quantifiers, which can follow any of the
1473 following items:
1474 a literal data character
1476 the dot metacharacter
1477 the \C escape sequence
1478 the \X escape sequence
1479 the \R escape sequence
1480 an escape such as \d or \pL that matches a single character
1481 a character class
1482 a back reference (see next section)
1483 a parenthesized subpattern (including assertions)
1484 a subroutine call to a subpattern (recursive or otherwise)
1485 The general repetition quantifier specifies a minimum and maximum num‐
1487 ber of permitted matches, by giving the two numbers in curly brackets
1488 (braces), separated by a comma. The numbers must be less than 65536,
1489 and the first must be less than or equal to the second. For example:
1490 z{2,4}
1492 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
1494 special character. If the second number is omitted, but the comma is
1495 present, there is no upper limit; if the second number and the comma
1496 are both omitted, the quantifier specifies an exact number of required
1497 matches. Thus
1498 [aeiou]{3,}
1500 matches at least 3 successive vowels, but may match many more, while
1502 \d{8}
1504 matches exactly 8 digits. An opening curly bracket that appears in a
1506 position where a quantifier is not allowed, or one that does not match
1507 the syntax of a quantifier, is taken as a literal character. For exam‐
1508 ple, {,6} is not a quantifier, but a literal string of four characters.
1509 In UTF modes, quantifiers apply to characters rather than to individual
1511 data units. Thus, for example, \x{100}{2} matches two characters, each
1512 of which is represented by a two-byte sequence in a UTF-8 string. Simi‐
1513 larly, \X{3} matches three Unicode extended grapheme clusters, each of
1514 which may be several data units long (and they may be of different
1515 lengths).
1516 The quantifier {0} is permitted, causing the expression to behave as if
1518 the previous item and the quantifier were not present. This may be use‐
1519 ful for subpatterns that are referenced as subroutines from elsewhere
1520 in the pattern (but see also the section entitled "Defining subpatterns
1521 for use by reference only" below). Items other than subpatterns that
1522 have a {0} quantifier are omitted from the compiled pattern.
1523 For convenience, the three most common quantifiers have single-charac‐
1525 ter abbreviations:
1526 * is equivalent to {0,}
1528 + is equivalent to {1,}
1529 ? is equivalent to {0,1}
1530 It is possible to construct infinite loops by following a subpattern
1532 that can match no characters with a quantifier that has no upper limit,
1533 for example:
1534 (a?)*
1536 Earlier versions of Perl and PCRE used to give an error at compile time
1538 for such patterns. However, because there are cases where this can be
1539 useful, such patterns are now accepted, but if any repetition of the
1540 subpattern does in fact match no characters, the loop is forcibly bro‐
1541 ken.
1542 By default, the quantifiers are "greedy", that is, they match as much
1544 as possible (up to the maximum number of permitted times), without
1545 causing the rest of the pattern to fail. The classic example of where
1546 this gives problems is in trying to match comments in C programs. These
1547 appear between /* and */ and within the comment, individual * and /
1548 characters may appear. An attempt to match C comments by applying the
1549 pattern
1550 /\*.*\*/
1552 to the string
1554 /* first comment */ not comment /* second comment */
1556 fails, because it matches the entire string owing to the greediness of
1558 the .* item.
1559 However, if a quantifier is followed by a question mark, it ceases to
1561 be greedy, and instead matches the minimum number of times possible, so
1562 the pattern
1563 /\*.*?\*/
1565 does the right thing with the C comments. The meaning of the various
1567 quantifiers is not otherwise changed, just the preferred number of
1568 matches. Do not confuse this use of question mark with its use as a
1569 quantifier in its own right. Because it has two uses, it can sometimes
1570 appear doubled, as in
1571 \d??\d
1573 which matches one digit by preference, but can match two if that is the
1575 only way the rest of the pattern matches.
1576 If the PCRE_UNGREEDY option is set (an option that is not available in
1578 Perl), the quantifiers are not greedy by default, but individual ones
1579 can be made greedy by following them with a question mark. In other
1580 words, it inverts the default behaviour.
1581 When a parenthesized subpattern is quantified with a minimum repeat
1583 count that is greater than 1 or with a limited maximum, more memory is
1584 required for the compiled pattern, in proportion to the size of the
1585 minimum or maximum.
1586 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv‐
1588 alent to Perl's /s) is set, thus allowing the dot to match newlines,
1589 the pattern is implicitly anchored, because whatever follows will be
1590 tried against every character position in the subject string, so there
1591 is no point in retrying the overall match at any position after the
1592 first. PCRE normally treats such a pattern as though it were preceded
1593 by \A.
1594 In cases where it is known that the subject string contains no new‐
1596 lines, it is worth setting PCRE_DOTALL in order to obtain this opti‐
1597 mization, or alternatively using ^ to indicate anchoring explicitly.
1598 However, there are some cases where the optimization cannot be used.
1600 When .* is inside capturing parentheses that are the subject of a back
1601 reference elsewhere in the pattern, a match at the start may fail where
1602 a later one succeeds. Consider, for example:
1603 (.*)abc\1
1605 If the subject is "xyz123abc123" the match point is the fourth charac‐
1607 ter. For this reason, such a pattern is not implicitly anchored.
1608 Another case where implicit anchoring is not applied is when the lead‐
1610 ing .* is inside an atomic group. Once again, a match at the start may
1611 fail where a later one succeeds. Consider this pattern:
1612 (?>.*?a)b
1614 It matches "ab" in the subject "aab". The use of the backtracking con‐
1616 trol verbs (*PRUNE) and (*SKIP) also disable this optimization.
1617 When a capturing subpattern is repeated, the value captured is the sub‐
1619 string that matched the final iteration. For example, after
1620 (tweedle[dume]{3}\s*)+
1622 has matched "tweedledum tweedledee" the value of the captured substring
1624 is "tweedledee". However, if there are nested capturing subpatterns,
1625 the corresponding captured values may have been set in previous itera‐
1626 tions. For example, after
1627 /(a|(b))+/
1629 matches "aba" the value of the second captured substring is "b".
1631
1633 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1635 repetition, failure of what follows normally causes the repeated item
1636 to be re-evaluated to see if a different number of repeats allows the
1637 rest of the pattern to match. Sometimes it is useful to prevent this,
1638 either to change the nature of the match, or to cause it fail earlier
1639 than it otherwise might, when the author of the pattern knows there is
1640 no point in carrying on.
1641 Consider, for example, the pattern \d+foo when applied to the subject
1643 line
1644 123456bar
1646 After matching all 6 digits and then failing to match "foo", the normal
1648 action of the matcher is to try again with only 5 digits matching the
1649 \d+ item, and then with 4, and so on, before ultimately failing.
1650 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
1651 the means for specifying that once a subpattern has matched, it is not
1652 to be re-evaluated in this way.
1653 If we use atomic grouping for the previous example, the matcher gives
1655 up immediately on failing to match "foo" the first time. The notation
1656 is a kind of special parenthesis, starting with (?> as in this example:
1657 (?>\d+)foo
1659 This kind of parenthesis "locks up" the part of the pattern it con‐
1661 tains once it has matched, and a failure further into the pattern is
1662 prevented from backtracking into it. Backtracking past it to previous
1663 items, however, works as normal.
1664 An alternative description is that a subpattern of this type matches
1666 the string of characters that an identical standalone pattern would
1667 match, if anchored at the current point in the subject string.
1668 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
1670 such as the above example can be thought of as a maximizing repeat that
1671 must swallow everything it can. So, while both \d+ and \d+? are pre‐
1672 pared to adjust the number of digits they match in order to make the
1673 rest of the pattern match, (?>\d+) can only match an entire sequence of
1674 digits.
1675 Atomic groups in general can of course contain arbitrarily complicated
1677 subpatterns, and can be nested. However, when the subpattern for an
1678 atomic group is just a single repeated item, as in the example above, a
1679 simpler notation, called a "possessive quantifier" can be used. This
1680 consists of an additional + character following a quantifier. Using
1681 this notation, the previous example can be rewritten as
1682 \d++foo
1684 Note that a possessive quantifier can be used with an entire group, for
1686 example:
1687 (abc|xyz){2,3}+
1689 Possessive quantifiers are always greedy; the setting of the PCRE_UN‐
1691 GREEDY option is ignored. They are a convenient notation for the sim‐
1692 pler forms of atomic group. However, there is no difference in the
1693 meaning of a possessive quantifier and the equivalent atomic group,
1694 though there may be a performance difference; possessive quantifiers
1695 should be slightly faster.
1696 The possessive quantifier syntax is an extension to the Perl 5.8 syn‐
1698 tax. Jeffrey Friedl originated the idea (and the name) in the first
1699 edition of his book. Mike McCloskey liked it, so implemented it when he
1700 built Sun's Java package, and PCRE copied it from there. It ultimately
1701 found its way into Perl at release 5.10.
1702 PCRE has an optimization that automatically "possessifies" certain sim‐
1704 ple pattern constructs. For example, the sequence A+B is treated as
1705 A++B because there is no point in backtracking into a sequence of A's
1706 when B must follow.
1707 When a pattern contains an unlimited repeat inside a subpattern that
1709 can itself be repeated an unlimited number of times, the use of an
1710 atomic group is the only way to avoid some failing matches taking a
1711 very long time indeed. The pattern
1712 (\D+|<\d+>)*[!?]
1714 matches an unlimited number of substrings that either consist of non-
1716 digits, or digits enclosed in <>, followed by either ! or ?. When it
1717 matches, it runs quickly. However, if it is applied to
1718 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1720 it takes a long time before reporting failure. This is because the
1722 string can be divided between the internal \D+ repeat and the external
1723 * repeat in a large number of ways, and all have to be tried. (The ex‐
1724 ample uses [!?] rather than a single character at the end, because both
1725 PCRE and Perl have an optimization that allows for fast failure when a
1726 single character is used. They remember the last single character that
1727 is required for a match, and fail early if it is not present in the
1728 string.) If the pattern is changed so that it uses an atomic group,
1729 like this:
1730 ((?>\D+)|<\d+>)*[!?]
1732 sequences of non-digits cannot be broken, and failure happens quickly.
1734
1736 Outside a character class, a backslash followed by a digit greater than
1738 0 (and possibly further digits) is a back reference to a capturing sub‐
1739 pattern earlier (that is, to its left) in the pattern, provided there
1740 have been that many previous capturing left parentheses.
1741 However, if the decimal number following the backslash is less than 10,
1743 it is always taken as a back reference, and causes an error only if
1744 there are not that many capturing left parentheses in the entire pat‐
1745 tern. In other words, the parentheses that are referenced need not be
1746 to the left of the reference for numbers less than 10. A "forward back
1747 reference" of this type can make sense when a repetition is involved
1748 and the subpattern to the right has participated in an earlier itera‐
1749 tion.
1750 It is not possible to have a numerical "forward back reference" to a
1752 subpattern whose number is 10 or more using this syntax because a se‐
1753 quence such as \50 is interpreted as a character defined in octal. See
1754 the subsection entitled "Non-printing characters" above for further de‐
1755 tails of the handling of digits following a backslash. There is no such
1756 problem when named parentheses are used. A back reference to any sub‐
1757 pattern is possible using named parentheses (see below).
1758 Another way of avoiding the ambiguity inherent in the use of digits
1760 following a backslash is to use the \g escape sequence. This escape
1761 must be followed by an unsigned number or a negative number, optionally
1762 enclosed in braces. These examples are all identical:
1763 (ring), \1
1765 (ring), \g1
1766 (ring), \g{1}
1767 An unsigned number specifies an absolute reference without the ambigu‐
1769 ity that is present in the older syntax. It is also useful when literal
1770 digits follow the reference. A negative number is a relative reference.
1771 Consider this example:
1772 (abc(def)ghi)\g{-1}
1774 The sequence \g{-1} is a reference to the most recently started captur‐
1776 ing subpattern before \g, that is, is it equivalent to \2 in this exam‐
1777 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
1778 references can be helpful in long patterns, and also in patterns that
1779 are created by joining together fragments that contain references
1780 within themselves.
1781 A back reference matches whatever actually matched the capturing sub‐
1783 pattern in the current subject string, rather than anything matching
1784 the subpattern itself (see "Subpatterns as subroutines" below for a way
1785 of doing that). So the pattern
1786 (sens|respons)e and \1ibility
1788 matches "sense and sensibility" and "response and responsibility", but
1790 not "sense and responsibility". If caseful matching is in force at the
1791 time of the back reference, the case of letters is relevant. For exam‐
1792 ple,
1793 ((?i)rah)\s+\1
1795 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
1797 original capturing subpattern is matched caselessly.
1798 There are several different ways of writing back references to named
1800 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
1801 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
1802 unified back reference syntax, in which \g can be used for both numeric
1803 and named references, is also supported. We could rewrite the above ex‐
1804 ample in any of the following ways:
1805 (?<p1>(?i)rah)\s+\k<p1>
1807 (?'p1'(?i)rah)\s+\k{p1}
1808 (?P<p1>(?i)rah)\s+(?P=p1)
1809 (?<p1>(?i)rah)\s+\g{p1}
1810 A subpattern that is referenced by name may appear in the pattern be‐
1812 fore or after the reference.
1813 There may be more than one back reference to the same subpattern. If a
1815 subpattern has not actually been used in a particular match, any back
1816 references to it always fail by default. For example, the pattern
1817 (a|(bc))\2
1819 always fails if it starts to match "a" rather than "bc". However, if
1821 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer‐
1822 ence to an unset value matches an empty string.
1823 Because there may be many capturing parentheses in a pattern, all dig‐
1825 its following a backslash are taken as part of a potential back refer‐
1826 ence number. If the pattern continues with a digit character, some de‐
1827 limiter must be used to terminate the back reference. If the PCRE_EX‐
1828 TENDED option is set, this can be white space. Otherwise, the \g{ syn‐
1829 tax or an empty comment (see "Comments" below) can be used.
1830 Recursive back references
1832 A back reference that occurs inside the parentheses to which it refers
1834 fails when the subpattern is first used, so, for example, (a\1) never
1835 matches. However, such references can be useful inside repeated sub‐
1836 patterns. For example, the pattern
1837 (a|b\1)+
1839 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter‐
1841 ation of the subpattern, the back reference matches the character
1842 string corresponding to the previous iteration. In order for this to
1843 work, the pattern must be such that the first iteration does not need
1844 to match the back reference. This can be done using alternation, as in
1845 the example above, or by a quantifier with a minimum of zero.
1846 Back references of this type cause the group that they reference to be
1848 treated as an atomic group. Once the whole group has been matched, a
1849 subsequent matching failure cannot cause backtracking into the middle
1850 of the group.
1851
1853 An assertion is a test on the characters following or preceding the
1855 current matching point that does not actually consume any characters.
1856 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are de‐
1857 scribed above.
1858 More complicated assertions are coded as subpatterns. There are two
1860 kinds: those that look ahead of the current position in the subject
1861 string, and those that look behind it. An assertion subpattern is
1862 matched in the normal way, except that it does not cause the current
1863 matching position to be changed.
1864 Assertion subpatterns are not capturing subpatterns. If such an asser‐
1866 tion contains capturing subpatterns within it, these are counted for
1867 the purposes of numbering the capturing subpatterns in the whole pat‐
1868 tern. However, substring capturing is carried out only for positive as‐
1869 sertions. (Perl sometimes, but not always, does do capturing in nega‐
1870 tive assertions.)
1871 WARNING: If a positive assertion containing one or more capturing sub‐
1873 patterns succeeds, but failure to match later in the pattern causes
1874 backtracking over this assertion, the captures within the assertion are
1875 reset only if no higher numbered captures are already set. This is, un‐
1876 fortunately, a fundamental limitation of the current implementation,
1877 and as PCRE1 is now in maintenance-only status, it is unlikely ever to
1878 change.
1879 For compatibility with Perl, assertion subpatterns may be repeated;
1881 though it makes no sense to assert the same thing several times, the
1882 side effect of capturing parentheses may occasionally be useful. In
1883 practice, there only three cases:
1884 (1) If the quantifier is {0}, the assertion is never obeyed during
1886 matching. However, it may contain internal capturing parenthesized
1887 groups that are called from elsewhere via the subroutine mechanism.
1888 (2) If quantifier is {0,n} where n is greater than zero, it is treated
1890 as if it were {0,1}. At run time, the rest of the pattern match is
1891 tried with and without the assertion, the order depending on the greed‐
1892 iness of the quantifier.
1893 (3) If the minimum repetition is greater than zero, the quantifier is
1895 ignored. The assertion is obeyed just once when encountered during
1896 matching.
1897 Lookahead assertions
1899 Lookahead assertions start with (?= for positive assertions and (?! for
1901 negative assertions. For example,
1902 \w+(?=;)
1904 matches a word followed by a semicolon, but does not include the semi‐
1906 colon in the match, and
1907 foo(?!bar)
1909 matches any occurrence of "foo" that is not followed by "bar". Note
1911 that the apparently similar pattern
1912 (?!foo)bar
1914 does not find an occurrence of "bar" that is preceded by something
1916 other than "foo"; it finds any occurrence of "bar" whatsoever, because
1917 the assertion (?!foo) is always true when the next three characters are
1918 "bar". A lookbehind assertion is needed to achieve the other effect.
1919 If you want to force a matching failure at some point in a pattern, the
1921 most convenient way to do it is with (?!) because an empty string al‐
1922 ways matches, so an assertion that requires there not to be an empty
1923 string must always fail. The backtracking control verb (*FAIL) or (*F)
1924 is a synonym for (?!).
1925 Lookbehind assertions
1927 Lookbehind assertions start with (?<= for positive assertions and (?<!
1929 for negative assertions. For example,
1930 (?<!foo)bar
1932 does find an occurrence of "bar" that is not preceded by "foo". The
1934 contents of a lookbehind assertion are restricted such that all the
1935 strings it matches must have a fixed length. However, if there are sev‐
1936 eral top-level alternatives, they do not all have to have the same
1937 fixed length. Thus
1938 (?<=bullock|donkey)
1940 is permitted, but
1942 (?<!dogs?|cats?)
1944 causes an error at compile time. Branches that match different length
1946 strings are permitted only at the top level of a lookbehind assertion.
1947 This is an extension compared with Perl, which requires all branches to
1948 match the same length of string. An assertion such as
1949 (?<=ab(c|de))
1951 is not permitted, because its single top-level branch can match two
1953 different lengths, but it is acceptable to PCRE if rewritten to use two
1954 top-level branches:
1955 (?<=abc|abde)
1957 In some cases, the escape sequence \K (see above) can be used instead
1959 of a lookbehind assertion to get round the fixed-length restriction.
1960 The implementation of lookbehind assertions is, for each alternative,
1962 to temporarily move the current position back by the fixed length and
1963 then try to match. If there are insufficient characters before the cur‐
1964 rent position, the assertion fails.
1965 In a UTF mode, PCRE does not allow the \C escape (which matches a sin‐
1967 gle data unit even in a UTF mode) to appear in lookbehind assertions,
1968 because it makes it impossible to calculate the length of the lookbe‐
1969 hind. The \X and \R escapes, which can match different numbers of data
1970 units, are also not permitted.
1971 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
1973 lookbehinds, as long as the subpattern matches a fixed-length string.
1974 Recursion, however, is not supported.
1975 Possessive quantifiers can be used in conjunction with lookbehind as‐
1977 sertions to specify efficient matching of fixed-length strings at the
1978 end of subject strings. Consider a simple pattern such as
1979 abcd$
1981 when applied to a long string that does not match. Because matching
1983 proceeds from left to right, PCRE will look for each "a" in the subject
1984 and then see if what follows matches the rest of the pattern. If the
1985 pattern is specified as
1986 ^.*abcd$
1988 the initial .* matches the entire string at first, but when this fails
1990 (because there is no following "a"), it backtracks to match all but the
1991 last character, then all but the last two characters, and so on. Once
1992 again the search for "a" covers the entire string, from right to left,
1993 so we are no better off. However, if the pattern is written as
1994 ^.*+(?<=abcd)
1996 there can be no backtracking for the .*+ item; it can match only the
1998 entire string. The subsequent lookbehind assertion does a single test
1999 on the last four characters. If it fails, the match fails immediately.
2000 For long strings, this approach makes a significant difference to the
2001 processing time.
2002 Using multiple assertions
2004 Several assertions (of any sort) may occur in succession. For example,
2006 (?<=\d{3})(?<!999)foo
2008 matches "foo" preceded by three digits that are not "999". Notice that
2010 each of the assertions is applied independently at the same point in
2011 the subject string. First there is a check that the previous three
2012 characters are all digits, and then there is a check that the same
2013 three characters are not "999". This pattern does not match "foo" pre‐
2014 ceded by six characters, the first of which are digits and the last
2015 three of which are not "999". For example, it doesn't match "123abc‐
2016 foo". A pattern to do that is
2017 (?<=\d{3}...)(?<!999)foo
2019 This time the first assertion looks at the preceding six characters,
2021 checking that the first three are digits, and then the second assertion
2022 checks that the preceding three characters are not "999".
2023 Assertions can be nested in any combination. For example,
2025 (?<=(?<!foo)bar)baz
2027 matches an occurrence of "baz" that is preceded by "bar" which in turn
2029 is not preceded by "foo", while
2030 (?<=\d{3}(?!999)...)foo
2032 is another pattern that matches "foo" preceded by three digits and any
2034 three characters that are not "999".
2035
2037 It is possible to cause the matching process to obey a subpattern con‐
2039 ditionally or to choose between two alternative subpatterns, depending
2040 on the result of an assertion, or whether a specific capturing subpat‐
2041 tern has already been matched. The two possible forms of conditional
2042 subpattern are:
2043 (?(condition)yes-pattern)
2045 (?(condition)yes-pattern|no-pattern)
2046 If the condition is satisfied, the yes-pattern is used; otherwise the
2048 no-pattern (if present) is used. If there are more than two alterna‐
2049 tives in the subpattern, a compile-time error occurs. Each of the two
2050 alternatives may itself contain nested subpatterns of any form, includ‐
2051 ing conditional subpatterns; the restriction to two alternatives ap‐
2052 plies only at the level of the condition. This pattern fragment is an
2053 example where the alternatives are complex:
2054 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2056 There are four kinds of condition: references to subpatterns, refer‐
2059 ences to recursion, a pseudo-condition called DEFINE, and assertions.
2060 Checking for a used subpattern by number
2062 If the text between the parentheses consists of a sequence of digits,
2064 the condition is true if a capturing subpattern of that number has pre‐
2065 viously matched. If there is more than one capturing subpattern with
2066 the same number (see the earlier section about duplicate subpattern
2067 numbers), the condition is true if any of them have matched. An alter‐
2068 native notation is to precede the digits with a plus or minus sign. In
2069 this case, the subpattern number is relative rather than absolute. The
2070 most recently opened parentheses can be referenced by (?(-1), the next
2071 most recent by (?(-2), and so on. Inside loops it can also make sense
2072 to refer to subsequent groups. The next parentheses to be opened can be
2073 referenced as (?(+1), and so on. (The value zero in any of these forms
2074 is not used; it provokes a compile-time error.)
2075 Consider the following pattern, which contains non-significant white
2077 space to make it more readable (assume the PCRE_EXTENDED option) and to
2078 divide it into three parts for ease of discussion:
2079 ( \( )? [^()]+ (?(1) \) )
2081 The first part matches an optional opening parenthesis, and if that
2083 character is present, sets it as the first captured substring. The sec‐
2084 ond part matches one or more characters that are not parentheses. The
2085 third part is a conditional subpattern that tests whether or not the
2086 first set of parentheses matched. If they did, that is, if subject
2087 started with an opening parenthesis, the condition is true, and so the
2088 yes-pattern is executed and a closing parenthesis is required. Other‐
2089 wise, since no-pattern is not present, the subpattern matches nothing.
2090 In other words, this pattern matches a sequence of non-parentheses, op‐
2091 tionally enclosed in parentheses.
2092 If you were embedding this pattern in a larger one, you could use a
2094 relative reference:
2095 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2097 This makes the fragment independent of the parentheses in the larger
2099 pattern.
2100 Checking for a used subpattern by name
2102 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
2104 used subpattern by name. For compatibility with earlier versions of
2105 PCRE, which had this facility before Perl, the syntax (?(name)...) is
2106 also recognized.
2107 Rewriting the above example to use a named subpattern gives this:
2109 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
2111 If the name used in a condition of this kind is a duplicate, the test
2113 is applied to all subpatterns of the same name, and is true if any one
2114 of them has matched.
2115 Checking for pattern recursion
2117 If the condition is the string (R), and there is no subpattern with the
2119 name R, the condition is true if a recursive call to the whole pattern
2120 or any subpattern has been made. If digits or a name preceded by amper‐
2121 sand follow the letter R, for example:
2122 (?(R3)...) or (?(R&name)...)
2124 the condition is true if the most recent recursion is into a subpattern
2126 whose number or name is given. This condition does not check the entire
2127 recursion stack. If the name used in a condition of this kind is a du‐
2128 plicate, the test is applied to all subpatterns of the same name, and
2129 is true if any one of them is the most recent recursion.
2130 At "top level", all these recursion test conditions are false. The
2132 syntax for recursive patterns is described below.
2133 Defining subpatterns for use by reference only
2135 If the condition is the string (DEFINE), and there is no subpattern
2137 with the name DEFINE, the condition is always false. In this case,
2138 there may be only one alternative in the subpattern. It is always
2139 skipped if control reaches this point in the pattern; the idea of DE‐
2140 FINE is that it can be used to define subroutines that can be refer‐
2141 enced from elsewhere. (The use of subroutines is described below.) For
2142 example, a pattern to match an IPv4 address such as "192.168.23.245"
2143 could be written like this (ignore white space and line breaks):
2144 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2146 \b (?&byte) (\.(?&byte)){3} \b
2147 The first part of the pattern is a DEFINE group inside which a another
2149 group named "byte" is defined. This matches an individual component of
2150 an IPv4 address (a number less than 256). When matching takes place,
2151 this part of the pattern is skipped because DEFINE acts like a false
2152 condition. The rest of the pattern uses references to the named group
2153 to match the four dot-separated components of an IPv4 address, insist‐
2154 ing on a word boundary at each end.
2155 Assertion conditions
2157 If the condition is not in any of the above formats, it must be an as‐
2159 sertion. This may be a positive or negative lookahead or lookbehind
2160 assertion. Consider this pattern, again containing non-significant
2161 white space, and with the two alternatives on the second line:
2162 (?(?=[^a-z]*[a-z])
2164 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2165 The condition is a positive lookahead assertion that matches an op‐
2167 tional sequence of non-letters followed by a letter. In other words, it
2168 tests for the presence of at least one letter in the subject. If a let‐
2169 ter is found, the subject is matched against the first alternative;
2170 otherwise it is matched against the second. This pattern matches
2171 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
2172 letters and dd are digits.
2173
2175 There are two ways of including comments in patterns that are processed
2177 by PCRE. In both cases, the start of the comment must not be in a char‐
2178 acter class, nor in the middle of any other sequence of related charac‐
2179 ters such as (?: or a subpattern name or number. The characters that
2180 make up a comment play no part in the pattern matching.
2181 The sequence (?# marks the start of a comment that continues up to the
2183 next closing parenthesis. Nested parentheses are not permitted. If the
2184 PCRE_EXTENDED option is set, an unescaped # character also introduces a
2185 comment, which in this case continues to immediately after the next
2186 newline character or character sequence in the pattern. Which charac‐
2187 ters are interpreted as newlines is controlled by the options passed to
2188 a compiling function or by a special sequence at the start of the pat‐
2189 tern, as described in the section entitled "Newline conventions" above.
2190 Note that the end of this type of comment is a literal newline sequence
2191 in the pattern; escape sequences that happen to represent a newline do
2192 not count. For example, consider this pattern when PCRE_EXTENDED is
2193 set, and the default newline convention is in force:
2194 abc #comment \n still comment
2196 On encountering the # character, pcre_compile() skips along, looking
2198 for a newline in the pattern. The sequence \n is still literal at this
2199 stage, so it does not terminate the comment. Only an actual character
2200 with the code value 0x0a (the default newline) does so.
2201
2203 Consider the problem of matching a string in parentheses, allowing for
2205 unlimited nested parentheses. Without the use of recursion, the best
2206 that can be done is to use a pattern that matches up to some fixed
2207 depth of nesting. It is not possible to handle an arbitrary nesting
2208 depth.
2209 For some time, Perl has provided a facility that allows regular expres‐
2211 sions to recurse (amongst other things). It does this by interpolating
2212 Perl code in the expression at run time, and the code can refer to the
2213 expression itself. A Perl pattern using code interpolation to solve the
2214 parentheses problem can be created like this:
2215 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
2217 The (?p{...}) item interpolates Perl code at run time, and in this case
2219 refers recursively to the pattern in which it appears.
2220 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
2222 it supports special syntax for recursion of the entire pattern, and
2223 also for individual subpattern recursion. After its introduction in
2224 PCRE and Python, this kind of recursion was subsequently introduced
2225 into Perl at release 5.10.
2226 A special item that consists of (? followed by a number greater than
2228 zero and a closing parenthesis is a recursive subroutine call of the
2229 subpattern of the given number, provided that it occurs inside that
2230 subpattern. (If not, it is a non-recursive subroutine call, which is
2231 described in the next section.) The special item (?R) or (?0) is a re‐
2232 cursive call of the entire regular expression.
2233 This PCRE pattern solves the nested parentheses problem (assume the
2235 PCRE_EXTENDED option is set so that white space is ignored):
2236 \( ( [^()]++ | (?R) )* \)
2238 First it matches an opening parenthesis. Then it matches any number of
2240 substrings which can either be a sequence of non-parentheses, or a re‐
2241 cursive match of the pattern itself (that is, a correctly parenthesized
2242 substring). Finally there is a closing parenthesis. Note the use of a
2243 possessive quantifier to avoid backtracking into sequences of non-
2244 parentheses.
2245 If this were part of a larger pattern, you would not want to recurse
2247 the entire pattern, so instead you could use this:
2248 ( \( ( [^()]++ | (?1) )* \) )
2250 We have put the pattern into parentheses, and caused the recursion to
2252 refer to them instead of the whole pattern.
2253 In a larger pattern, keeping track of parenthesis numbers can be
2255 tricky. This is made easier by the use of relative references. Instead
2256 of (?1) in the pattern above you can write (?-2) to refer to the second
2257 most recently opened parentheses preceding the recursion. In other
2258 words, a negative number counts capturing parentheses leftwards from
2259 the point at which it is encountered.
2260 It is also possible to refer to subsequently opened parentheses, by
2262 writing references such as (?+2). However, these cannot be recursive
2263 because the reference is not inside the parentheses that are refer‐
2264 enced. They are always non-recursive subroutine calls, as described in
2265 the next section.
2266 An alternative approach is to use named parentheses instead. The Perl
2268 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
2269 supported. We could rewrite the above example as follows:
2270 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
2272 If there is more than one subpattern with the same name, the earliest
2274 one is used.
2275 This particular example pattern that we have been looking at contains
2277 nested unlimited repeats, and so the use of a possessive quantifier for
2278 matching strings of non-parentheses is important when applying the pat‐
2279 tern to strings that do not match. For example, when this pattern is
2280 applied to
2281 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2283 it yields "no match" quickly. However, if a possessive quantifier is
2285 not used, the match runs for a very long time indeed because there are
2286 so many different ways the + and * repeats can carve up the subject,
2287 and all have to be tested before failure can be reported.
2288 At the end of a match, the values of capturing parentheses are those
2290 from the outermost level. If you want to obtain intermediate values, a
2291 callout function can be used (see below and the pcrecallout documenta‐
2292 tion). If the pattern above is matched against
2293 (ab(cd)ef)
2295 the value for the inner capturing parentheses (numbered 2) is "ef",
2297 which is the last value taken on at the top level. If a capturing sub‐
2298 pattern is not matched at the top level, its final captured value is
2299 unset, even if it was (temporarily) set at a deeper level during the
2300 matching process.
2301 If there are more than 15 capturing parentheses in a pattern, PCRE has
2303 to obtain extra memory to store data during a recursion, which it does
2304 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
2305 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2306 Do not confuse the (?R) item with the condition (R), which tests for
2308 recursion. Consider this pattern, which matches text in angle brack‐
2309 ets, allowing for arbitrary nesting. Only digits are allowed in nested
2310 brackets (that is, when recursing), whereas any characters are permit‐
2311 ted at the outer level.
2312 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
2314 In this pattern, (?(R) is the start of a conditional subpattern, with
2316 two different alternatives for the recursive and non-recursive cases.
2317 The (?R) item is the actual recursive call.
2318 Differences in recursion processing between PCRE and Perl
2320 Recursion processing in PCRE differs from Perl in two important ways.
2322 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
2323 always treated as an atomic group. That is, once it has matched some of
2324 the subject string, it is never re-entered, even if it contains untried
2325 alternatives and there is a subsequent matching failure. This can be
2326 illustrated by the following pattern, which purports to match a palin‐
2327 dromic string that contains an odd number of characters (for example,
2328 "a", "aba", "abcba", "abcdcba"):
2329 ^(.|(.)(?1)\2)$
2331 The idea is that it either matches a single character, or two identical
2333 characters surrounding a sub-palindrome. In Perl, this pattern works;
2334 in PCRE it does not if the pattern is longer than three characters.
2335 Consider the subject string "abcba":
2336 At the top level, the first character is matched, but as it is not at
2338 the end of the string, the first alternative fails; the second alterna‐
2339 tive is taken and the recursion kicks in. The recursive call to subpat‐
2340 tern 1 successfully matches the next character ("b"). (Note that the
2341 beginning and end of line tests are not part of the recursion).
2342 Back at the top level, the next character ("c") is compared with what
2344 subpattern 2 matched, which was "a". This fails. Because the recursion
2345 is treated as an atomic group, there are now no backtracking points,
2346 and so the entire match fails. (Perl is able, at this point, to re-en‐
2347 ter the recursion and try the second alternative.) However, if the pat‐
2348 tern is written with the alternatives in the other order, things are
2349 different:
2350 ^((.)(?1)\2|.)$
2352 This time, the recursing alternative is tried first, and continues to
2354 recurse until it runs out of characters, at which point the recursion
2355 fails. But this time we do have another alternative to try at the
2356 higher level. That is the big difference: in the previous case the re‐
2357 maining alternative is at a deeper recursion level, which PCRE cannot
2358 use.
2359 To change the pattern so that it matches all palindromic strings, not
2361 just those with an odd number of characters, it is tempting to change
2362 the pattern to this:
2363 ^((.)(?1)\2|.?)$
2365 Again, this works in Perl, but not in PCRE, and for the same reason.
2367 When a deeper recursion has matched a single character, it cannot be
2368 entered again in order to match an empty string. The solution is to
2369 separate the two cases, and write out the odd and even cases as alter‐
2370 natives at the higher level:
2371 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2373 If you want to match typical palindromic phrases, the pattern has to
2375 ignore all non-word characters, which can be done like this:
2376 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2378 If run with the PCRE_CASELESS option, this pattern matches phrases such
2380 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
2381 Perl. Note the use of the possessive quantifier *+ to avoid backtrack‐
2382 ing into sequences of non-word characters. Without this, PCRE takes a
2383 great deal longer (ten times or more) to match typical phrases, and
2384 Perl takes so long that you think it has gone into a loop.
2385 WARNING: The palindrome-matching patterns above work only if the sub‐
2387 ject string does not start with a palindrome that is shorter than the
2388 entire string. For example, although "abcba" is correctly matched, if
2389 the subject is "ababa", PCRE finds the palindrome "aba" at the start,
2390 then fails at top level because the end of the string does not follow.
2391 Once again, it cannot jump back into the recursion to try other alter‐
2392 natives, so the entire match fails.
2393 The second way in which PCRE and Perl differ in their recursion pro‐
2395 cessing is in the handling of captured values. In Perl, when a subpat‐
2396 tern is called recursively or as a subpattern (see the next section),
2397 it has no access to any values that were captured outside the recur‐
2398 sion, whereas in PCRE these values can be referenced. Consider this
2399 pattern:
2400 ^(.)(\1|a(?2))
2402 In PCRE, this pattern matches "bab". The first capturing parentheses
2404 match "b", then in the second group, when the back reference \1 fails
2405 to match "b", the second alternative matches "a" and then recurses. In
2406 the recursion, \1 does now match "b" and so the whole match succeeds.
2407 In Perl, the pattern fails to match because inside the recursive call
2408 \1 cannot access the externally set value.
2409
2411 If the syntax for a recursive subpattern call (either by number or by
2413 name) is used outside the parentheses to which it refers, it operates
2414 like a subroutine in a programming language. The called subpattern may
2415 be defined before or after the reference. A numbered reference can be
2416 absolute or relative, as in these examples:
2417 (...(absolute)...)...(?2)...
2419 (...(relative)...)...(?-1)...
2420 (...(?+1)...(relative)...
2421 An earlier example pointed out that the pattern
2423 (sens|respons)e and \1ibility
2425 matches "sense and sensibility" and "response and responsibility", but
2427 not "sense and responsibility". If instead the pattern
2428 (sens|respons)e and (?1)ibility
2430 is used, it does match "sense and responsibility" as well as the other
2432 two strings. Another example is given in the discussion of DEFINE
2433 above.
2434 All subroutine calls, whether recursive or not, are always treated as
2436 atomic groups. That is, once a subroutine has matched some of the sub‐
2437 ject string, it is never re-entered, even if it contains untried alter‐
2438 natives and there is a subsequent matching failure. Any capturing
2439 parentheses that are set during the subroutine call revert to their
2440 previous values afterwards.
2441 Processing options such as case-independence are fixed when a subpat‐
2443 tern is defined, so if it is used as a subroutine, such options cannot
2444 be changed for different calls. For example, consider this pattern:
2445 (abc)(?i:(?-1))
2447 It matches "abcabc". It does not match "abcABC" because the change of
2449 processing option does not affect the called subpattern.
2450
2452 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
2454 name or a number enclosed either in angle brackets or single quotes, is
2455 an alternative syntax for referencing a subpattern as a subroutine,
2456 possibly recursively. Here are two of the examples used above, rewrit‐
2457 ten using this syntax:
2458 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
2460 (sens|respons)e and \g'1'ibility
2461 PCRE supports an extension to Oniguruma: if a number is preceded by a
2463 plus or a minus sign it is taken as a relative reference. For example:
2464 (abc)(?i:\g<-1>)
2466 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
2468 synonymous. The former is a back reference; the latter is a subroutine
2469 call.
2470
2472 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
2474 Perl code to be obeyed in the middle of matching a regular expression.
2475 This makes it possible, amongst other things, to extract different sub‐
2476 strings that match the same pair of parentheses when there is a repeti‐
2477 tion.
2478 PCRE provides a similar feature, but of course it cannot obey arbitrary
2480 Perl code. The feature is called "callout". The caller of PCRE provides
2481 an external function by putting its entry point in the global variable
2482 pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit
2483 library). By default, this variable contains NULL, which disables all
2484 calling out.
2485 Within a regular expression, (?C) indicates the points at which the ex‐
2487 ternal function is to be called. If you want to identify different
2488 callout points, you can put a number less than 256 after the letter C.
2489 The default value is zero. For example, this pattern has two callout
2490 points:
2491 (?C1)abc(?C2)def
2493 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call‐
2495 outs are automatically installed before each item in the pattern. They
2496 are all numbered 255. If there is a conditional group in the pattern
2497 whose condition is an assertion, an additional callout is inserted just
2498 before the condition. An explicit callout may also be set at this posi‐
2499 tion, as in this example:
2500 (?(?C9)(?=a)abc|def)
2502 Note that this applies only to assertion conditions, not to other types
2504 of condition.
2505 During matching, when PCRE reaches a callout point, the external func‐
2507 tion is called. It is provided with the number of the callout, the po‐
2508 sition in the pattern, and, optionally, one item of data originally
2509 supplied by the caller of the matching function. The callout function
2510 may cause matching to proceed, to backtrack, or to fail altogether.
2511 By default, PCRE implements a number of optimizations at compile time
2513 and matching time, and one side-effect is that sometimes callouts are
2514 skipped. If you need all possible callouts to happen, you need to set
2515 options that disable the relevant optimizations. More details, and a
2516 complete description of the interface to the callout function, are
2517 given in the pcrecallout documentation.
2518
2520 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
2522 which are still described in the Perl documentation as "experimental
2523 and subject to change or removal in a future version of Perl". It goes
2524 on to say: "Their usage in production code should be noted to avoid
2525 problems during upgrades." The same remarks apply to the PCRE features
2526 described in this section.
2527 The new verbs make use of what was previously invalid syntax: an open‐
2529 ing parenthesis followed by an asterisk. They are generally of the form
2530 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2531 differently depending on whether or not a name is present. A name is
2532 any sequence of characters that does not include a closing parenthesis.
2533 The maximum length of name is 255 in the 8-bit library and 65535 in the
2534 16-bit and 32-bit libraries. If the name is empty, that is, if the
2535 closing parenthesis immediately follows the colon, the effect is as if
2536 the colon were not there. Any number of these verbs may occur in a
2537 pattern.
2538 Since these verbs are specifically related to backtracking, most of
2540 them can be used only when the pattern is to be matched using one of
2541 the traditional matching functions, because these use a backtracking
2542 algorithm. With the exception of (*FAIL), which behaves like a failing
2543 negative assertion, the backtracking control verbs cause an error if
2544 encountered by a DFA matching function.
2545 The behaviour of these verbs in repeated groups, assertions, and in
2547 subpatterns called as subroutines (whether or not recursively) is docu‐
2548 mented below.
2549 Optimizations that affect backtracking verbs
2551 PCRE contains some optimizations that are used to speed up matching by
2553 running some checks at the start of each match attempt. For example, it
2554 may know the minimum length of matching subject, or that a particular
2555 character must be present. When one of these optimizations bypasses the
2556 running of a match, any included backtracking verbs will not, of
2557 course, be processed. You can suppress the start-of-match optimizations
2558 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com‐
2559 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
2560 There is more discussion of this option in the section entitled "Option
2561 bits for pcre_exec()" in the pcreapi documentation.
2562 Experiments with Perl suggest that it too has similar optimizations,
2564 sometimes leading to anomalous results.
2565 Verbs that act immediately
2567 The following verbs act as soon as they are encountered. They may not
2569 be followed by a name.
2570 (*ACCEPT)
2572 This verb causes the match to end successfully, skipping the remainder
2574 of the pattern. However, when it is inside a subpattern that is called
2575 as a subroutine, only that subpattern is ended successfully. Matching
2576 then continues at the outer level. If (*ACCEPT) in triggered in a posi‐
2577 tive assertion, the assertion succeeds; in a negative assertion, the
2578 assertion fails.
2579 If (*ACCEPT) is inside capturing parentheses, the data so far is cap‐
2581 tured. For example:
2582 A((?:A|B(*ACCEPT)|C)D)
2584 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap‐
2586 tured by the outer parentheses.
2587 (*FAIL) or (*F)
2589 This verb causes a matching failure, forcing backtracking to occur. It
2591 is equivalent to (?!) but easier to read. The Perl documentation notes
2592 that it is probably useful only when combined with (?{}) or (??{}).
2593 Those are, of course, Perl features that are not present in PCRE. The
2594 nearest equivalent is the callout feature, as for example in this pat‐
2595 tern:
2596 a+(?C)(*FAIL)
2598 A match with the string "aaaa" always fails, but the callout is taken
2600 before each backtrack happens (in this example, 10 times).
2601 Recording which path was taken
2603 There is one verb whose main purpose is to track how a match was ar‐
2605 rived at, though it also has a secondary use in conjunction with ad‐
2606 vancing the match starting point (see (*SKIP) below).
2607 (*MARK:NAME) or (*:NAME)
2609 A name is always required with this verb. There may be as many in‐
2611 stances of (*MARK) as you like in a pattern, and their names do not
2612 have to be unique.
2613 When a match succeeds, the name of the last-encountered (*MARK:NAME),
2615 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to
2616 the caller as described in the section entitled "Extra data for
2617 pcre_exec()" in the pcreapi documentation. Here is an example of
2618 pcretest output, where the /K modifier requests the retrieval and out‐
2619 putting of (*MARK) data:
2620 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2622 data> XY
2623 0: XY
2624 MK: A
2625 XZ
2626 0: XZ
2627 MK: B
2628 The (*MARK) name is tagged with "MK:" in this output, and in this exam‐
2630 ple it indicates which of the two alternatives matched. This is a more
2631 efficient way of obtaining this information than putting each alterna‐
2632 tive in its own capturing parentheses.
2633 If a verb with a name is encountered in a positive assertion that is
2635 true, the name is recorded and passed back if it is the last-encoun‐
2636 tered. This does not happen for negative assertions or failing positive
2637 assertions.
2638 After a partial match or a failed match, the last encountered name in
2640 the entire match process is returned. For example:
2641 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2643 data> XP
2644 No match, mark = B
2645 Note that in this unanchored example the mark is retained from the
2647 match attempt that started at the letter "X" in the subject. Subsequent
2648 match attempts starting at "P" and then with an empty string do not get
2649 as far as the (*MARK) item, but nevertheless do not reset it.
2650 If you are interested in (*MARK) values after failed matches, you
2652 should probably set the PCRE_NO_START_OPTIMIZE option (see above) to
2653 ensure that the match is always attempted.
2654 Verbs that act after backtracking
2656 The following verbs do nothing when they are encountered. Matching con‐
2658 tinues with what follows, but if there is no subsequent match, causing
2659 a backtrack to the verb, a failure is forced. That is, backtracking
2660 cannot pass to the left of the verb. However, when one of these verbs
2661 appears inside an atomic group or an assertion that is true, its effect
2662 is confined to that group, because once the group has been matched,
2663 there is never any backtracking into it. In this situation, backtrack‐
2664 ing can "jump back" to the left of the entire atomic group or asser‐
2665 tion. (Remember also, as stated above, that this localization also ap‐
2666 plies in subroutine calls.)
2667 These verbs differ in exactly what kind of failure occurs when back‐
2669 tracking reaches them. The behaviour described below is what happens
2670 when the verb is not in a subroutine or an assertion. Subsequent sec‐
2671 tions cover these special cases.
2672 (*COMMIT)
2674 This verb, which may not be followed by a name, causes the whole match
2676 to fail outright if there is a later matching failure that causes back‐
2677 tracking to reach it. Even if the pattern is unanchored, no further at‐
2678 tempts to find a match by advancing the starting point take place. If
2679 (*COMMIT) is the only backtracking verb that is encountered, once it
2680 has been passed pcre_exec() is committed to finding a match at the cur‐
2681 rent starting point, or not at all. For example:
2682 a+(*COMMIT)b
2684 This matches "xxaab" but not "aacaab". It can be thought of as a kind
2686 of dynamic anchor, or "I've started, so I must finish." The name of the
2687 most recently passed (*MARK) in the path is passed back when (*COMMIT)
2688 forces a match failure.
2689 If there is more than one backtracking verb in a pattern, a different
2691 one that follows (*COMMIT) may be triggered first, so merely passing
2692 (*COMMIT) during a match does not always guarantee that a match must be
2693 at this starting point.
2694 Note that (*COMMIT) at the start of a pattern is not the same as an an‐
2696 chor, unless PCRE's start-of-match optimizations are turned off, as
2697 shown in this output from pcretest:
2698 re> /(*COMMIT)abc/
2700 data> xyzabc
2701 0: abc
2702 data> xyzabc\Y
2703 No match
2704 For this pattern, PCRE knows that any match must start with "a", so the
2706 optimization skips along the subject to "a" before applying the pattern
2707 to the first set of data. The match attempt then succeeds. In the sec‐
2708 ond set of data, the escape sequence \Y is interpreted by the pcretest
2709 program. It causes the PCRE_NO_START_OPTIMIZE option to be set when
2710 pcre_exec() is called. This disables the optimization that skips along
2711 to the first character. The pattern is now applied starting at "x", and
2712 so the (*COMMIT) causes the match to fail without trying any other
2713 starting points.
2714 (*PRUNE) or (*PRUNE:NAME)
2716 This verb causes the match to fail at the current starting position in
2718 the subject if there is a later matching failure that causes backtrack‐
2719 ing to reach it. If the pattern is unanchored, the normal "bumpalong"
2720 advance to the next starting character then happens. Backtracking can
2721 occur as usual to the left of (*PRUNE), before it is reached, or when
2722 matching to the right of (*PRUNE), but if there is no match to the
2723 right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2724 (*PRUNE) is just an alternative to an atomic group or possessive quan‐
2725 tifier, but there are some uses of (*PRUNE) that cannot be expressed in
2726 any other way. In an anchored pattern (*PRUNE) has the same effect as
2727 (*COMMIT).
2728 The behaviour of (*PRUNE:NAME) is the not the same as
2730 (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is re‐
2731 membered for passing back to the caller. However, (*SKIP:NAME) searches
2732 only for names set with (*MARK).
2733 (*SKIP)
2735 This verb, when given without a name, is like (*PRUNE), except that if
2737 the pattern is unanchored, the "bumpalong" advance is not to the next
2738 character, but to the position in the subject where (*SKIP) was encoun‐
2739 tered. (*SKIP) signifies that whatever text was matched leading up to
2740 it cannot be part of a successful match. Consider:
2741 a+(*SKIP)b
2743 If the subject is "aaaac...", after the first match attempt fails
2745 (starting at the first character in the string), the starting point
2746 skips on to start the next attempt at "c". Note that a possessive quan‐
2747 tifier does not have the same effect as this example; although it would
2748 suppress backtracking during the first match attempt, the second at‐
2749 tempt would start at the second character instead of skipping on to
2750 "c".
2751 (*SKIP:NAME)
2753 When (*SKIP) has an associated name, its behaviour is modified. When it
2755 is triggered, the previous path through the pattern is searched for the
2756 most recent (*MARK) that has the same name. If one is found, the
2757 "bumpalong" advance is to the subject position that corresponds to that
2758 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with
2759 a matching name is found, the (*SKIP) is ignored.
2760 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
2762 ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2763 (*THEN) or (*THEN:NAME)
2765 This verb causes a skip to the next innermost alternative when back‐
2767 tracking reaches it. That is, it cancels any further backtracking
2768 within the current alternative. Its name comes from the observation
2769 that it can be used for a pattern-based if-then-else block:
2770 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2772 If the COND1 pattern matches, FOO is tried (and possibly further items
2774 after the end of the group if FOO succeeds); on failure, the matcher
2775 skips to the second alternative and tries COND2, without backtracking
2776 into COND1. If that succeeds and BAR fails, COND3 is tried. If subse‐
2777 quently BAZ fails, there are no more alternatives, so there is a back‐
2778 track to whatever came before the entire group. If (*THEN) is not in‐
2779 side an alternation, it acts like (*PRUNE).
2780 The behaviour of (*THEN:NAME) is the not the same as
2782 (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is re‐
2783 membered for passing back to the caller. However, (*SKIP:NAME) searches
2784 only for names set with (*MARK).
2785 A subpattern that does not contain a | character is just a part of the
2787 enclosing alternative; it is not a nested alternation with only one al‐
2788 ternative. The effect of (*THEN) extends beyond such a subpattern to
2789 the enclosing alternative. Consider this pattern, where A, B, etc. are
2790 complex pattern fragments that do not contain any | characters at this
2791 level:
2792 A (B(*THEN)C) | D
2794 If A and B are matched, but there is a failure in C, matching does not
2796 backtrack into A; instead it moves to the next alternative, that is, D.
2797 However, if the subpattern containing (*THEN) is given an alternative,
2798 it behaves differently:
2799 A (B(*THEN)C | (*FAIL)) | D
2801 The effect of (*THEN) is now confined to the inner subpattern. After a
2803 failure in C, matching moves to (*FAIL), which causes the whole subpat‐
2804 tern to fail because there are no more alternatives to try. In this
2805 case, matching does now backtrack into A.
2806 Note that a conditional subpattern is not considered as having two al‐
2808 ternatives, because only one is ever used. In other words, the | char‐
2809 acter in a conditional subpattern has a different meaning. Ignoring
2810 white space, consider:
2811 ^.*? (?(?=a) a | b(*THEN)c )
2813 If the subject is "ba", this pattern does not match. Because .*? is un‐
2815 greedy, it initially matches zero characters. The condition (?=a) then
2816 fails, the character "b" is matched, but "c" is not. At this point,
2817 matching does not backtrack to .*? as might perhaps be expected from
2818 the presence of the | character. The conditional subpattern is part of
2819 the single alternative that comprises the whole pattern, and so the
2820 match fails. (If there was a backtrack into .*?, allowing it to match
2821 "b", the match would succeed.)
2822 The verbs just described provide four different "strengths" of control
2824 when subsequent matching fails. (*THEN) is the weakest, carrying on the
2825 match at the next alternative. (*PRUNE) comes next, failing the match
2826 at the current starting position, but allowing an advance to the next
2827 character (for an unanchored pattern). (*SKIP) is similar, except that
2828 the advance may be more than one character. (*COMMIT) is the strongest,
2829 causing the entire match to fail.
2830 More than one backtracking verb
2832 If more than one backtracking verb is present in a pattern, the one
2834 that is backtracked onto first acts. For example, consider this pat‐
2835 tern, where A, B, etc. are complex pattern fragments:
2836 (A(*COMMIT)B(*THEN)C|ABD)
2838 If A matches but B fails, the backtrack to (*COMMIT) causes the entire
2840 match to fail. However, if A and B match, but C fails, the backtrack to
2841 (*THEN) causes the next alternative (ABD) to be tried. This behaviour
2842 is consistent, but is not always the same as Perl's. It means that if
2843 two or more backtracking verbs appear in succession, all the the last
2844 of them has no effect. Consider this example:
2845 ...(*COMMIT)(*PRUNE)...
2847 If there is a matching failure to the right, backtracking onto (*PRUNE)
2849 causes it to be triggered, and its action is taken. There can never be
2850 a backtrack onto (*COMMIT).
2851 Backtracking verbs in repeated groups
2853 PCRE differs from Perl in its handling of backtracking verbs in re‐
2855 peated groups. For example, consider:
2856 /(a(*COMMIT)b)+ac/
2858 If the subject is "abac", Perl matches, but PCRE fails because the
2860 (*COMMIT) in the second repeat of the group acts.
2861 Backtracking verbs in assertions
2863 (*FAIL) in an assertion has its normal effect: it forces an immediate
2865 backtrack.
2866 (*ACCEPT) in a positive assertion causes the assertion to succeed with‐
2868 out any further processing. In a negative assertion, (*ACCEPT) causes
2869 the assertion to fail without any further processing.
2870 The other backtracking verbs are not treated specially if they appear
2872 in a positive assertion. In particular, (*THEN) skips to the next al‐
2873 ternative in the innermost enclosing group that has alternations,
2874 whether or not this is within the assertion.
2875 Negative assertions are, however, different, in order to ensure that
2877 changing a positive assertion into a negative assertion changes its re‐
2878 sult. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a nega‐
2879 tive assertion to be true, without considering any further alternative
2880 branches in the assertion. Backtracking into (*THEN) causes it to skip
2881 to the next enclosing alternative within the assertion (the normal be‐
2882 haviour), but if the assertion does not have such an alternative,
2883 (*THEN) behaves like (*PRUNE).
2884 Backtracking verbs in subroutines
2886 These behaviours occur whether or not the subpattern is called recur‐
2888 sively. Perl's treatment of subroutines is different in some cases.
2889 (*FAIL) in a subpattern called as a subroutine has its normal effect:
2891 it forces an immediate backtrack.
2892 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine
2894 match to succeed without any further processing. Matching then contin‐
2895 ues after the subroutine call.
2896 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
2898 cause the subroutine match to fail.
2899 (*THEN) skips to the next alternative in the innermost enclosing group
2901 within the subpattern that has alternatives. If there is no such group
2902 within the subpattern, (*THEN) causes the subroutine match to fail.
2903
2905 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
2907 pcre16(3), pcre32(3).
2908
2910 Philip Hazel
2912 University Computing Service
2913 Cambridge CB2 3QH, England.
2914
2916 Last updated: 23 October 2016
2918 Copyright (c) 1997-2016 University of Cambridge.
2919PCRE 8.40 23 October 2016 PCREPATTERN(3)