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
21 Expressions", published by O'Reilly, covers regular expressions in
22 great detail. This description of PCRE's regular expressions is
23 intended as reference material.
24 The original operation of PCRE was on strings of one-byte characters.
26 However, there is now also support for UTF-8 strings in the original
27 library, an extra library that supports 16-bit and UTF-16 character
28 strings, and a third library that supports 32-bit and UTF-32 character
29 strings. To use these features, PCRE must be built to include appropri‐
30 ate support. When using UTF strings you must either call the compiling
31 function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the
32 pattern must start with one of these special sequences:
33 (*UTF8)
35 (*UTF16)
36 (*UTF32)
37 (*UTF)
38 (*UTF) is a generic sequence that can be used with any of the
40 libraries. Starting a pattern with such a sequence is equivalent to
41 setting the relevant option. This feature is not Perl-compatible. How
42 setting a UTF mode affects pattern matching is mentioned in several
43 places below. There is also a summary of features in the pcreunicode
44 page.
45 Another special sequence that may appear at the start of a pattern or
47 in combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) is:
48 (*UCP)
50 This has the same effect as setting the PCRE_UCP option: it causes
52 sequences such as \d and \w to use Unicode properties to determine
53 character types, instead of recognizing only characters with codes less
54 than 128 via a lookup table.
55 If a pattern starts with (*NO_START_OPT), it has the same effect as
57 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
58 time. There are also some more of these special sequences that are con‐
59 cerned with the handling of newlines; they are described below.
60 The remainder of this document discusses the patterns that are sup‐
62 ported by PCRE when one its main matching functions, pcre_exec()
63 (8-bit) or pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has
64 alternative matching functions, pcre_dfa_exec() and
65 pcre[16|32_dfa_exec(), which match using a different algorithm that is
66 not Perl-compatible. Some of the features discussed below are not
67 available when DFA matching is used. The advantages and disadvantages
68 of the alternative functions, and how they differ from the normal func‐
69 tions, are discussed in the pcrematching page.
70
72 PCRE can be compiled to run in an environment that uses EBCDIC as its
74 character code rather than ASCII or Unicode (typically a mainframe sys‐
75 tem). In the sections below, character code values are ASCII or Uni‐
76 code; in an EBCDIC environment these characters may have different code
77 values, and there are no code points greater than 255.
78
80 PCRE supports five different conventions for indicating line breaks in
82 strings: a single CR (carriage return) character, a single LF (line‐
83 feed) character, the two-character sequence CRLF, any of the three pre‐
84 ceding, or any Unicode newline sequence. The pcreapi page has further
85 discussion about newlines, and shows how to set the newline convention
86 in the options arguments for the compiling and matching functions.
87 It is also possible to specify a newline convention by starting a pat‐
89 tern string with one of the following five sequences:
90 (*CR) carriage return
92 (*LF) linefeed
93 (*CRLF) carriage return, followed by linefeed
94 (*ANYCRLF) any of the three above
95 (*ANY) all Unicode newline sequences
96 These override the default and the options given to the compiling func‐
98 tion. For example, on a Unix system where LF is the default newline
99 sequence, the pattern
100 (*CR)a.b
102 changes the convention to CR. That pattern matches "a\nb" because LF is
104 no longer a newline. Note that these special settings, which are not
105 Perl-compatible, are recognized only at the very start of a pattern,
106 and that they must be in upper case. If more than one of them is
107 present, the last one is used.
108 The newline convention affects where the circumflex and dollar asser‐
110 tions are true. It also affects the interpretation of the dot metachar‐
111 acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it
112 does not affect what the \R escape sequence matches. By default, this
113 is any Unicode newline sequence, for Perl compatibility. However, this
114 can be changed; see the description of \R in the section entitled "New‐
115 line sequences" below. A change of \R setting can be combined with a
116 change of newline convention.
117
119 A regular expression is a pattern that is matched against a subject
121 string from left to right. Most characters stand for themselves in a
122 pattern, and match the corresponding characters in the subject. As a
123 trivial example, the pattern
124 The quick brown fox
126 matches a portion of a subject string that is identical to itself. When
128 caseless matching is specified (the PCRE_CASELESS option), letters are
129 matched independently of case. In a UTF mode, PCRE always understands
130 the concept of case for characters whose values are less than 128, so
131 caseless matching is always possible. For characters with higher val‐
132 ues, the concept of case is supported if PCRE is compiled with Unicode
133 property support, but not otherwise. If you want to use caseless
134 matching for characters 128 and above, you must ensure that PCRE is
135 compiled with Unicode property support as well as with UTF support.
136 The power of regular expressions comes from the ability to include
138 alternatives and repetitions in the pattern. These are encoded in the
139 pattern by the use of metacharacters, which do not stand for themselves
140 but instead are interpreted in some special way.
141 There are two different sets of metacharacters: those that are recog‐
143 nized anywhere in the pattern except within square brackets, and those
144 that are recognized within square brackets. Outside square brackets,
145 the metacharacters are as follows:
146 \ general escape character with several uses
148 ^ assert start of string (or line, in multiline mode)
149 $ assert end of string (or line, in multiline mode)
150 . match any character except newline (by default)
151 [ start character class definition
152 | start of alternative branch
153 ( start subpattern
154 ) end subpattern
155 ? extends the meaning of (
156 also 0 or 1 quantifier
157 also quantifier minimizer
158 * 0 or more quantifier
159 + 1 or more quantifier
160 also "possessive quantifier"
161 { start min/max quantifier
162 Part of a pattern that is in square brackets is called a "character
164 class". In a character class the only metacharacters are:
165 \ general escape character
167 ^ negate the class, but only if the first character
168 - indicates character range
169 [ POSIX character class (only if followed by POSIX
170 syntax)
171 ] terminates the character class
172 The following sections describe the use of each of the metacharacters.
174
176 The backslash character has several uses. Firstly, if it is followed by
178 a character that is not a number or a letter, it takes away any special
179 meaning that character may have. This use of backslash as an escape
180 character applies both inside and outside character classes.
181 For example, if you want to match a * character, you write \* in the
183 pattern. This escaping action applies whether or not the following
184 character would otherwise be interpreted as a metacharacter, so it is
185 always safe to precede a non-alphanumeric with backslash to specify
186 that it stands for itself. In particular, if you want to match a back‐
187 slash, you write \\.
188 In a UTF mode, only ASCII numbers and letters have any special meaning
190 after a backslash. All other characters (in particular, those whose
191 codepoints are greater than 127) are treated as literals.
192 If a pattern is compiled with the PCRE_EXTENDED option, white space in
194 the pattern (other than in a character class) and characters between a
195 # outside a character class and the next newline are ignored. An escap‐
196 ing backslash can be used to include a white space or # character as
197 part of the pattern.
198 If you want to remove the special meaning from a sequence of charac‐
200 ters, you can do so by putting them between \Q and \E. This is differ‐
201 ent from Perl in that $ and @ are handled as literals in \Q...\E
202 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola‐
203 tion. Note the following examples:
204 Pattern PCRE matches Perl matches
206 \Qabc$xyz\E abc$xyz abc followed by the
208 contents of $xyz
209 \Qabc\$xyz\E abc\$xyz abc\$xyz
210 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
211 The \Q...\E sequence is recognized both inside and outside character
213 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
214 is not followed by \E later in the pattern, the literal interpretation
215 continues to the end of the pattern (that is, \E is assumed at the
216 end). If the isolated \Q is inside a character class, this causes an
217 error, because the character class is not terminated.
218 Non-printing characters
220 A second use of backslash provides a way of encoding non-printing char‐
222 acters in patterns in a visible manner. There is no restriction on the
223 appearance of non-printing characters, apart from the binary zero that
224 terminates a pattern, but when a pattern is being prepared by text
225 editing, it is often easier to use one of the following escape
226 sequences than the binary character it represents:
227 \a alarm, that is, the BEL character (hex 07)
229 \cx "control-x", where x is any ASCII character
230 \e escape (hex 1B)
231 \f form feed (hex 0C)
232 \n linefeed (hex 0A)
233 \r carriage return (hex 0D)
234 \t tab (hex 09)
235 \ddd character with octal code ddd, or back reference
236 \xhh character with hex code hh
237 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
238 \uhhhh character with hex code hhhh (JavaScript mode only)
239 The precise effect of \cx on ASCII characters is as follows: if x is a
241 lower case letter, it is converted to upper case. Then bit 6 of the
242 character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
243 (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
244 hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c
245 has a value greater than 127, a compile-time error occurs. This locks
246 out non-ASCII characters in all modes.
247 The \c facility was designed for use with ASCII characters, but with
249 the extension to Unicode it is even less useful than it once was. It
250 is, however, recognized when PCRE is compiled in EBCDIC mode, where
251 data items are always bytes. In this mode, all values are valid after
252 \c. If the next character is a lower case letter, it is converted to
253 upper case. Then the 0xc0 bits of the byte are inverted. Thus \cA
254 becomes hex 01, as in ASCII (A is C1), but because the EBCDIC letters
255 are disjoint, \cZ becomes hex 29 (Z is E9), and other characters also
256 generate different values.
257 By default, after \x, from zero to two hexadecimal digits are read
259 (letters can be in upper or lower case). Any number of hexadecimal dig‐
260 its may appear between \x{ and }, but the character code is constrained
261 as follows:
262 8-bit non-UTF mode less than 0x100
264 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
265 16-bit non-UTF mode less than 0x10000
266 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
267 32-bit non-UTF mode less than 0x80000000
268 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
269 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
271 called "surrogate" codepoints), and 0xffef.
272 If characters other than hexadecimal digits appear between \x{ and },
274 or if there is no terminating }, this form of escape is not recognized.
275 Instead, the initial \x will be interpreted as a basic hexadecimal
276 escape, with no following digits, giving a character whose value is
277 zero.
278 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
280 is as just described only when it is followed by two hexadecimal dig‐
281 its. Otherwise, it matches a literal "x" character. In JavaScript
282 mode, support for code points greater than 256 is provided by \u, which
283 must be followed by four hexadecimal digits; otherwise it matches a
284 literal "u" character. Character codes specified by \u in JavaScript
285 mode are constrained in the same was as those specified by \x in non-
286 JavaScript mode.
287 Characters whose value is less than 256 can be defined by either of the
289 two syntaxes for \x (or by \u in JavaScript mode). There is no differ‐
290 ence in the way they are handled. For example, \xdc is exactly the same
291 as \x{dc} (or \u00dc in JavaScript mode).
292 After \0 up to two further octal digits are read. If there are fewer
294 than two digits, just those that are present are used. Thus the
295 sequence \0\x\07 specifies two binary zeros followed by a BEL character
296 (code value 7). Make sure you supply two digits after the initial zero
297 if the pattern character that follows is itself an octal digit.
298 The handling of a backslash followed by a digit other than 0 is compli‐
300 cated. Outside a character class, PCRE reads it and any following dig‐
301 its as a decimal number. If the number is less than 10, or if there
302 have been at least that many previous capturing left parentheses in the
303 expression, the entire sequence is taken as a back reference. A
304 description of how this works is given later, following the discussion
305 of parenthesized subpatterns.
306 Inside a character class, or if the decimal number is greater than 9
308 and there have not been that many capturing subpatterns, PCRE re-reads
309 up to three octal digits following the backslash, and uses them to gen‐
310 erate a data character. Any subsequent digits stand for themselves. The
311 value of the character is constrained in the same way as characters
312 specified in hexadecimal. For example:
313 \040 is another way of writing an ASCII space
315 \40 is the same, provided there are fewer than 40
316 previous capturing subpatterns
317 \7 is always a back reference
318 \11 might be a back reference, or another way of
319 writing a tab
320 \011 is always a tab
321 \0113 is a tab followed by the character "3"
322 \113 might be a back reference, otherwise the
323 character with octal code 113
324 \377 might be a back reference, otherwise
325 the value 255 (decimal)
326 \81 is either a back reference, or a binary zero
327 followed by the two characters "8" and "1"
328 Note that octal values of 100 or greater must not be introduced by a
330 leading zero, because no more than three octal digits are ever read.
331 All the sequences that define a single character value can be used both
333 inside and outside character classes. In addition, inside a character
334 class, \b is interpreted as the backspace character (hex 08).
335 \N is not allowed in a character class. \B, \R, and \X are not special
337 inside a character class. Like other unrecognized escape sequences,
338 they are treated as the literal characters "B", "R", and "X" by
339 default, but cause an error if the PCRE_EXTRA option is set. Outside a
340 character class, these sequences have different meanings.
341 Unsupported escape sequences
343 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
345 handler and used to modify the case of following characters. By
346 default, PCRE does not support these escape sequences. However, if the
347 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
348 \u can be used to define a character by code point, as described in the
349 previous section.
350 Absolute and relative back references
352 The sequence \g followed by an unsigned or a negative number, option‐
354 ally enclosed in braces, is an absolute or relative back reference. A
355 named back reference can be coded as \g{name}. Back references are dis‐
356 cussed later, following the discussion of parenthesized subpatterns.
357 Absolute and relative subroutine calls
359 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
361 name or a number enclosed either in angle brackets or single quotes, is
362 an alternative syntax for referencing a subpattern as a "subroutine".
363 Details are discussed later. Note that \g{...} (Perl syntax) and
364 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
365 reference; the latter is a subroutine call.
366 Generic character types
368 Another use of backslash is for specifying generic character types:
370 \d any decimal digit
372 \D any character that is not a decimal digit
373 \h any horizontal white space character
374 \H any character that is not a horizontal white space character
375 \s any white space character
376 \S any character that is not a white space character
377 \v any vertical white space character
378 \V any character that is not a vertical white space character
379 \w any "word" character
380 \W any "non-word" character
381 There is also the single sequence \N, which matches a non-newline char‐
383 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
384 not set. Perl also uses \N to match characters by name; PCRE does not
385 support this.
386 Each pair of lower and upper case escape sequences partitions the com‐
388 plete set of characters into two disjoint sets. Any given character
389 matches one, and only one, of each pair. The sequences can appear both
390 inside and outside character classes. They each match one character of
391 the appropriate type. If the current matching point is at the end of
392 the subject string, all of them fail, because there is no character to
393 match.
394 For compatibility with Perl, \s does not match the VT character (code
396 11). This makes it different from the the POSIX "space" class. The \s
397 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
398 "use locale;" is included in a Perl script, \s may match the VT charac‐
399 ter. In PCRE, it never does.
400 A "word" character is an underscore or any character that is a letter
402 or digit. By default, the definition of letters and digits is con‐
403 trolled by PCRE's low-valued character tables, and may vary if locale-
404 specific matching is taking place (see "Locale support" in the pcreapi
405 page). For example, in a French locale such as "fr_FR" in Unix-like
406 systems, or "french" in Windows, some character codes greater than 128
407 are used for accented letters, and these are then matched by \w. The
408 use of locales with Unicode is discouraged.
409 By default, in a UTF mode, characters with values greater than 128
411 never match \d, \s, or \w, and always match \D, \S, and \W. These
412 sequences retain their original meanings from before UTF support was
413 available, mainly for efficiency reasons. However, if PCRE is compiled
414 with Unicode property support, and the PCRE_UCP option is set, the be‐
415 haviour is changed so that Unicode properties are used to determine
416 character types, as follows:
417 \d any character that \p{Nd} matches (decimal digit)
419 \s any character that \p{Z} matches, plus HT, LF, FF, CR
420 \w any character that \p{L} or \p{N} matches, plus underscore
421 The upper case escapes match the inverse sets of characters. Note that
423 \d matches only decimal digits, whereas \w matches any Unicode digit,
424 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
425 affects \b, and \B because they are defined in terms of \w and \W.
426 Matching these sequences is noticeably slower when PCRE_UCP is set.
427 The sequences \h, \H, \v, and \V are features that were added to Perl
429 at release 5.10. In contrast to the other sequences, which match only
430 ASCII characters by default, these always match certain high-valued
431 codepoints, whether or not PCRE_UCP is set. The horizontal space char‐
432 acters are:
433 U+0009 Horizontal tab (HT)
435 U+0020 Space
436 U+00A0 Non-break space
437 U+1680 Ogham space mark
438 U+180E Mongolian vowel separator
439 U+2000 En quad
440 U+2001 Em quad
441 U+2002 En space
442 U+2003 Em space
443 U+2004 Three-per-em space
444 U+2005 Four-per-em space
445 U+2006 Six-per-em space
446 U+2007 Figure space
447 U+2008 Punctuation space
448 U+2009 Thin space
449 U+200A Hair space
450 U+202F Narrow no-break space
451 U+205F Medium mathematical space
452 U+3000 Ideographic space
453 The vertical space characters are:
455 U+000A Linefeed (LF)
457 U+000B Vertical tab (VT)
458 U+000C Form feed (FF)
459 U+000D Carriage return (CR)
460 U+0085 Next line (NEL)
461 U+2028 Line separator
462 U+2029 Paragraph separator
463 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
465 256 are relevant.
466 Newline sequences
468 Outside a character class, by default, the escape sequence \R matches
470 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
471 to the following:
472 (?>\r\n|\n|\x0b|\f|\r|\x85)
474 This is an example of an "atomic group", details of which are given
476 below. This particular group matches either the two-character sequence
477 CR followed by LF, or one of the single characters LF (linefeed,
478 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car‐
479 riage return, U+000D), or NEL (next line, U+0085). The two-character
480 sequence is treated as a single unit that cannot be split.
481 In other modes, two additional characters whose codepoints are greater
483 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa‐
484 rator, U+2029). Unicode character property support is not needed for
485 these characters to be recognized.
486 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
488 the complete set of Unicode line endings) by setting the option
489 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
490 (BSR is an abbrevation for "backslash R".) This can be made the default
491 when PCRE is built; if this is the case, the other behaviour can be
492 requested via the PCRE_BSR_UNICODE option. It is also possible to
493 specify these settings by starting a pattern string with one of the
494 following sequences:
495 (*BSR_ANYCRLF) CR, LF, or CRLF only
497 (*BSR_UNICODE) any Unicode newline sequence
498 These override the default and the options given to the compiling func‐
500 tion, but they can themselves be overridden by options given to a
501 matching function. Note that these special settings, which are not
502 Perl-compatible, are recognized only at the very start of a pattern,
503 and that they must be in upper case. If more than one of them is
504 present, the last one is used. They can be combined with a change of
505 newline convention; for example, a pattern can start with:
506 (*ANY)(*BSR_ANYCRLF)
508 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF)
510 or (*UCP) special sequences. Inside a character class, \R is treated as
511 an unrecognized escape sequence, and so matches the letter "R" by
512 default, but causes an error if PCRE_EXTRA is set.
513 Unicode character properties
515 When PCRE is built with Unicode character property support, three addi‐
517 tional escape sequences that match characters with specific properties
518 are available. When in 8-bit non-UTF-8 mode, these sequences are of
519 course limited to testing characters whose codepoints are less than
520 256, but they do work in this mode. The extra escape sequences are:
521 \p{xx} a character with the xx property
523 \P{xx} a character without the xx property
524 \X a Unicode extended grapheme cluster
525 The property names represented by xx above are limited to the Unicode
527 script names, the general category properties, "Any", which matches any
528 character (including newline), and some special PCRE properties
529 (described in the next section). Other Perl properties such as "InMu‐
530 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
531 does not match any characters, so always causes a match failure.
532 Sets of Unicode characters are defined as belonging to certain scripts.
534 A character from one of these sets can be matched using a script name.
535 For example:
536 \p{Greek}
538 \P{Han}
539 Those that are not part of an identified script are lumped together as
541 "Common". The current list of scripts is:
542 Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
544 Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
545 Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
546 Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic,
547 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira‐
548 gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip‐
549 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
550 Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
551 Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive,
552 Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko,
553 Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic,
554 Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari‐
555 tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese,
556 Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
557 Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
558 Yi.
559 Each character has exactly one Unicode general category property, spec‐
561 ified by a two-letter abbreviation. For compatibility with Perl, nega‐
562 tion can be specified by including a circumflex between the opening
563 brace and the property name. For example, \p{^Lu} is the same as
564 \P{Lu}.
565 If only one letter is specified with \p or \P, it includes all the gen‐
567 eral category properties that start with that letter. In this case, in
568 the absence of negation, the curly brackets in the escape sequence are
569 optional; these two examples have the same effect:
570 \p{L}
572 \pL
573 The following general category property codes are supported:
575 C Other
577 Cc Control
578 Cf Format
579 Cn Unassigned
580 Co Private use
581 Cs Surrogate
582 L Letter
584 Ll Lower case letter
585 Lm Modifier letter
586 Lo Other letter
587 Lt Title case letter
588 Lu Upper case letter
589 M Mark
591 Mc Spacing mark
592 Me Enclosing mark
593 Mn Non-spacing mark
594 N Number
596 Nd Decimal number
597 Nl Letter number
598 No Other number
599 P Punctuation
601 Pc Connector punctuation
602 Pd Dash punctuation
603 Pe Close punctuation
604 Pf Final punctuation
605 Pi Initial punctuation
606 Po Other punctuation
607 Ps Open punctuation
608 S Symbol
610 Sc Currency symbol
611 Sk Modifier symbol
612 Sm Mathematical symbol
613 So Other symbol
614 Z Separator
616 Zl Line separator
617 Zp Paragraph separator
618 Zs Space separator
619 The special property L& is also supported: it matches a character that
621 has the Lu, Ll, or Lt property, in other words, a letter that is not
622 classified as a modifier or "other".
623 The Cs (Surrogate) property applies only to characters in the range
625 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
626 so cannot be tested by PCRE, unless UTF validity checking has been
627 turned off (see the discussion of PCRE_NO_UTF8_CHECK,
628 PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl
629 does not support the Cs property.
630 The long synonyms for property names that Perl supports (such as
632 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
633 any of these properties with "Is".
634 No character that is in the Unicode table has the Cn (unassigned) prop‐
636 erty. Instead, this property is assumed for any code point that is not
637 in the Unicode table.
638 Specifying caseless matching does not affect these escape sequences.
640 For example, \p{Lu} always matches only upper case letters.
641 Matching characters by Unicode property is not fast, because PCRE has
643 to do a multistage table lookup in order to find a character's prop‐
644 erty. That is why the traditional escape sequences such as \d and \w do
645 not use Unicode properties in PCRE by default, though you can make them
646 do so by setting the PCRE_UCP option or by starting the pattern with
647 (*UCP).
648 Extended grapheme clusters
650 The \X escape matches any number of Unicode characters that form an
652 "extended grapheme cluster", and treats the sequence as an atomic group
653 (see below). Up to and including release 8.31, PCRE matched an ear‐
654 lier, simpler definition that was equivalent to
655 (?>\PM\pM*)
657 That is, it matched a character without the "mark" property, followed
659 by zero or more characters with the "mark" property. Characters with
660 the "mark" property are typically non-spacing accents that affect the
661 preceding character.
662 This simple definition was extended in Unicode to include more compli‐
664 cated kinds of composite character by giving each character a grapheme
665 breaking property, and creating rules that use these properties to
666 define the boundaries of extended grapheme clusters. In releases of
667 PCRE later than 8.31, \X matches one of these clusters.
668 \X always matches at least one character. Then it decides whether to
670 add additional characters according to the following rules for ending a
671 cluster:
672 1. End at the end of the subject string.
674 2. Do not end between CR and LF; otherwise end after any control char‐
676 acter.
677 3. Do not break Hangul (a Korean script) syllable sequences. Hangul
679 characters are of five types: L, V, T, LV, and LVT. An L character may
680 be followed by an L, V, LV, or LVT character; an LV or V character may
681 be followed by a V or T character; an LVT or T character may be follwed
682 only by a T character.
683 4. Do not end before extending characters or spacing marks. Characters
685 with the "mark" property always have the "extend" grapheme breaking
686 property.
687 5. Do not end after prepend characters.
689 6. Otherwise, end the cluster.
691 PCRE's additional properties
693 As well as the standard Unicode properties described above, PCRE sup‐
695 ports four more that make it possible to convert traditional escape
696 sequences such as \w and \s to use Unicode properties. PCRE uses these
697 non-standard, non-Perl properties internally when PCRE_UCP is set. How‐
698 ever, they may also be used explicitly. These properties are:
699 Xan Any alphanumeric character
701 Xps Any POSIX space character
702 Xsp Any Perl space character
703 Xwd Any Perl "word" character
704 Xan matches characters that have either the L (letter) or the N (num‐
706 ber) property. Xps matches the characters tab, linefeed, vertical tab,
707 form feed, or carriage return, and any other character that has the Z
708 (separator) property. Xsp is the same as Xps, except that vertical tab
709 is excluded. Xwd matches the :qa same characters as Xan, plus under‐
710 score.
711 Resetting the match start
713 The escape sequence \K causes any previously matched characters not to
715 be included in the final matched sequence. For example, the pattern:
716 foo\Kbar
718 matches "foobar", but reports that it has matched "bar". This feature
720 is similar to a lookbehind assertion (described below). However, in
721 this case, the part of the subject before the real match does not have
722 to be of fixed length, as lookbehind assertions do. The use of \K does
723 not interfere with the setting of captured substrings. For example,
724 when the pattern
725 (foo)\Kbar
727 matches "foobar", the first substring is still set to "foo".
729 Perl documents that the use of \K within assertions is "not well
731 defined". In PCRE, \K is acted upon when it occurs inside positive
732 assertions, but is ignored in negative assertions.
733 Simple assertions
735 The final use of backslash is for certain simple assertions. An asser‐
737 tion specifies a condition that has to be met at a particular point in
738 a match, without consuming any characters from the subject string. The
739 use of subpatterns for more complicated assertions is described below.
740 The backslashed assertions are:
741 \b matches at a word boundary
743 \B matches when not at a word boundary
744 \A matches at the start of the subject
745 \Z matches at the end of the subject
746 also matches before a newline at the end of the subject
747 \z matches only at the end of the subject
748 \G matches at the first matching position in the subject
749 Inside a character class, \b has a different meaning; it matches the
751 backspace character. If any other of these assertions appears in a
752 character class, by default it matches the corresponding literal char‐
753 acter (for example, \B matches the letter B). However, if the
754 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener‐
755 ated instead.
756 A word boundary is a position in the subject string where the current
758 character and the previous character do not both match \w or \W (i.e.
759 one matches \w and the other matches \W), or the start or end of the
760 string if the first or last character matches \w, respectively. In a
761 UTF mode, the meanings of \w and \W can be changed by setting the
762 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
763 PCRE nor Perl has a separate "start of word" or "end of word" metase‐
764 quence. However, whatever follows \b normally determines which it is.
765 For example, the fragment \ba matches "a" at the start of a word.
766 The \A, \Z, and \z assertions differ from the traditional circumflex
768 and dollar (described in the next section) in that they only ever match
769 at the very start and end of the subject string, whatever options are
770 set. Thus, they are independent of multiline mode. These three asser‐
771 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
772 affect only the behaviour of the circumflex and dollar metacharacters.
773 However, if the startoffset argument of pcre_exec() is non-zero, indi‐
774 cating that matching is to start at a point other than the beginning of
775 the subject, \A can never match. The difference between \Z and \z is
776 that \Z matches before a newline at the end of the string as well as at
777 the very end, whereas \z matches only at the end.
778 The \G assertion is true only when the current matching position is at
780 the start point of the match, as specified by the startoffset argument
781 of pcre_exec(). It differs from \A when the value of startoffset is
782 non-zero. By calling pcre_exec() multiple times with appropriate argu‐
783 ments, you can mimic Perl's /g option, and it is in this kind of imple‐
784 mentation where \G can be useful.
785 Note, however, that PCRE's interpretation of \G, as the start of the
787 current match, is subtly different from Perl's, which defines it as the
788 end of the previous match. In Perl, these can be different when the
789 previously matched string was empty. Because PCRE does just one match
790 at a time, it cannot reproduce this behaviour.
791 If all the alternatives of a pattern begin with \G, the expression is
793 anchored to the starting match position, and the "anchored" flag is set
794 in the compiled regular expression.
795
797 The circumflex and dollar metacharacters are zero-width assertions.
799 That is, they test for a particular condition being true without con‐
800 suming any characters from the subject string.
801 Outside a character class, in the default matching mode, the circumflex
803 character is an assertion that is true only if the current matching
804 point is at the start of the subject string. If the startoffset argu‐
805 ment of pcre_exec() is non-zero, circumflex can never match if the
806 PCRE_MULTILINE option is unset. Inside a character class, circumflex
807 has an entirely different meaning (see below).
808 Circumflex need not be the first character of the pattern if a number
810 of alternatives are involved, but it should be the first thing in each
811 alternative in which it appears if the pattern is ever to match that
812 branch. If all possible alternatives start with a circumflex, that is,
813 if the pattern is constrained to match only at the start of the sub‐
814 ject, it is said to be an "anchored" pattern. (There are also other
815 constructs that can cause a pattern to be anchored.)
816 The dollar character is an assertion that is true only if the current
818 matching point is at the end of the subject string, or immediately
819 before a newline at the end of the string (by default). Note, however,
820 that it does not actually match the newline. Dollar need not be the
821 last character of the pattern if a number of alternatives are involved,
822 but it should be the last item in any branch in which it appears. Dol‐
823 lar has no special meaning in a character class.
824 The meaning of dollar can be changed so that it matches only at the
826 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
827 compile time. This does not affect the \Z assertion.
828 The meanings of the circumflex and dollar characters are changed if the
830 PCRE_MULTILINE option is set. When this is the case, a circumflex
831 matches immediately after internal newlines as well as at the start of
832 the subject string. It does not match after a newline that ends the
833 string. A dollar matches before any newlines in the string, as well as
834 at the very end, when PCRE_MULTILINE is set. When newline is specified
835 as the two-character sequence CRLF, isolated CR and LF characters do
836 not indicate newlines.
837 For example, the pattern /^abc$/ matches the subject string "def\nabc"
839 (where \n represents a newline) in multiline mode, but not otherwise.
840 Consequently, patterns that are anchored in single line mode because
841 all branches start with ^ are not anchored in multiline mode, and a
842 match for circumflex is possible when the startoffset argument of
843 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
844 PCRE_MULTILINE is set.
845 Note that the sequences \A, \Z, and \z can be used to match the start
847 and end of the subject in both modes, and if all branches of a pattern
848 start with \A it is always anchored, whether or not PCRE_MULTILINE is
849 set.
850
852 Outside a character class, a dot in the pattern matches any one charac‐
854 ter in the subject string except (by default) a character that signi‐
855 fies the end of a line.
856 When a line ending is defined as a single character, dot never matches
858 that character; when the two-character sequence CRLF is used, dot does
859 not match CR if it is immediately followed by LF, but otherwise it
860 matches all characters (including isolated CRs and LFs). When any Uni‐
861 code line endings are being recognized, dot does not match CR or LF or
862 any of the other line ending characters.
863 The behaviour of dot with regard to newlines can be changed. If the
865 PCRE_DOTALL option is set, a dot matches any one character, without
866 exception. If the two-character sequence CRLF is present in the subject
867 string, it takes two dots to match it.
868 The handling of dot is entirely independent of the handling of circum‐
870 flex and dollar, the only relationship being that they both involve
871 newlines. Dot has no special meaning in a character class.
872 The escape sequence \N behaves like a dot, except that it is not
874 affected by the PCRE_DOTALL option. In other words, it matches any
875 character except one that signifies the end of a line. Perl also uses
876 \N to match characters by name; PCRE does not support this.
877
879 Outside a character class, the escape sequence \C matches any one data
881 unit, whether or not a UTF mode is set. In the 8-bit library, one data
882 unit is one byte; in the 16-bit library it is a 16-bit unit; in the
883 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches
884 line-ending characters. The feature is provided in Perl in order to
885 match individual bytes in UTF-8 mode, but it is unclear how it can use‐
886 fully be used. Because \C breaks up characters into individual data
887 units, matching one unit with \C in a UTF mode means that the rest of
888 the string may start with a malformed UTF character. This has undefined
889 results, because PCRE assumes that it is dealing with valid UTF strings
890 (and by default it checks this at the start of processing unless the
891 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option
892 is used).
893 PCRE does not allow \C to appear in lookbehind assertions (described
895 below) in a UTF mode, because this would make it impossible to calcu‐
896 late the length of the lookbehind.
897 In general, the \C escape sequence is best avoided. However, one way of
899 using it that avoids the problem of malformed UTF characters is to use
900 a lookahead to check the length of the next character, as in this pat‐
901 tern, which could be used with a UTF-8 string (ignore white space and
902 line breaks):
903 (?| (?=[\x00-\x7f])(\C) |
905 (?=[\x80-\x{7ff}])(\C)(\C) |
906 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
907 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
908 A group that starts with (?| resets the capturing parentheses numbers
910 in each alternative (see "Duplicate Subpattern Numbers" below). The
911 assertions at the start of each branch check the next UTF-8 character
912 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
913 character's individual bytes are then captured by the appropriate num‐
914 ber of groups.
915
917 An opening square bracket introduces a character class, terminated by a
919 closing square bracket. A closing square bracket on its own is not spe‐
920 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
921 a lone closing square bracket causes a compile-time error. If a closing
922 square bracket is required as a member of the class, it should be the
923 first data character in the class (after an initial circumflex, if
924 present) or escaped with a backslash.
925 A character class matches a single character in the subject. In a UTF
927 mode, the character may be more than one data unit long. A matched
928 character must be in the set of characters defined by the class, unless
929 the first character in the class definition is a circumflex, in which
930 case the subject character must not be in the set defined by the class.
931 If a circumflex is actually required as a member of the class, ensure
932 it is not the first character, or escape it with a backslash.
933 For example, the character class [aeiou] matches any lower case vowel,
935 while [^aeiou] matches any character that is not a lower case vowel.
936 Note that a circumflex is just a convenient notation for specifying the
937 characters that are in the class by enumerating those that are not. A
938 class that starts with a circumflex is not an assertion; it still con‐
939 sumes a character from the subject string, and therefore it fails if
940 the current pointer is at the end of the string.
941 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255
943 (0xffff) can be included in a class as a literal string of data units,
944 or by using the \x{ escaping mechanism.
945 When caseless matching is set, any letters in a class represent both
947 their upper case and lower case versions, so for example, a caseless
948 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
949 match "A", whereas a caseful version would. In a UTF mode, PCRE always
950 understands the concept of case for characters whose values are less
951 than 128, so caseless matching is always possible. For characters with
952 higher values, the concept of case is supported if PCRE is compiled
953 with Unicode property support, but not otherwise. If you want to use
954 caseless matching in a UTF mode for characters 128 and above, you must
955 ensure that PCRE is compiled with Unicode property support as well as
956 with UTF support.
957 Characters that might indicate line breaks are never treated in any
959 special way when matching character classes, whatever line-ending
960 sequence is in use, and whatever setting of the PCRE_DOTALL and
961 PCRE_MULTILINE options is used. A class such as [^a] always matches one
962 of these characters.
963 The minus (hyphen) character can be used to specify a range of charac‐
965 ters in a character class. For example, [d-m] matches any letter
966 between d and m, inclusive. If a minus character is required in a
967 class, it must be escaped with a backslash or appear in a position
968 where it cannot be interpreted as indicating a range, typically as the
969 first or last character in the class.
970 It is not possible to have the literal character "]" as the end charac‐
972 ter of a range. A pattern such as [W-]46] is interpreted as a class of
973 two characters ("W" and "-") followed by a literal string "46]", so it
974 would match "W46]" or "-46]". However, if the "]" is escaped with a
975 backslash it is interpreted as the end of range, so [W-\]46] is inter‐
976 preted as a class containing a range followed by two other characters.
977 The octal or hexadecimal representation of "]" can also be used to end
978 a range.
979 Ranges operate in the collating sequence of character values. They can
981 also be used for characters specified numerically, for example
982 [\000-\037]. Ranges can include any characters that are valid for the
983 current mode.
984 If a range that includes letters is used when caseless matching is set,
986 it matches the letters in either case. For example, [W-c] is equivalent
987 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
988 character tables for a French locale are in use, [\xc8-\xcb] matches
989 accented E characters in both cases. In UTF modes, PCRE supports the
990 concept of case for characters with values greater than 128 only when
991 it is compiled with Unicode property support.
992 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
994 \w, and \W may appear in a character class, and add the characters that
995 they match to the class. For example, [\dABCDEF] matches any hexadeci‐
996 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
997 \d, \s, \w and their upper case partners, just as it does when they
998 appear outside a character class, as described in the section entitled
999 "Generic character types" above. The escape sequence \b has a different
1000 meaning inside a character class; it matches the backspace character.
1001 The sequences \B, \N, \R, and \X are not special inside a character
1002 class. Like any other unrecognized escape sequences, they are treated
1003 as the literal characters "B", "N", "R", and "X" by default, but cause
1004 an error if the PCRE_EXTRA option is set.
1005 A circumflex can conveniently be used with the upper case character
1007 types to specify a more restricted set of characters than the matching
1008 lower case type. For example, the class [^\W_] matches any letter or
1009 digit, but not underscore, whereas [\w] includes underscore. A positive
1010 character class should be read as "something OR something OR ..." and a
1011 negative class as "NOT something AND NOT something AND NOT ...".
1012 The only metacharacters that are recognized in character classes are
1014 backslash, hyphen (only where it can be interpreted as specifying a
1015 range), circumflex (only at the start), opening square bracket (only
1016 when it can be interpreted as introducing a POSIX class name - see the
1017 next section), and the terminating closing square bracket. However,
1018 escaping other non-alphanumeric characters does no harm.
1019
1021 Perl supports the POSIX notation for character classes. This uses names
1023 enclosed by [: and :] within the enclosing square brackets. PCRE also
1024 supports this notation. For example,
1025 [01[:alpha:]%]
1027 matches "0", "1", any alphabetic character, or "%". The supported class
1029 names are:
1030 alnum letters and digits
1032 alpha letters
1033 ascii character codes 0 - 127
1034 blank space or tab only
1035 cntrl control characters
1036 digit decimal digits (same as \d)
1037 graph printing characters, excluding space
1038 lower lower case letters
1039 print printing characters, including space
1040 punct printing characters, excluding letters and digits and space
1041 space white space (not quite the same as \s)
1042 upper upper case letters
1043 word "word" characters (same as \w)
1044 xdigit hexadecimal digits
1045 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1047 and space (32). Notice that this list includes the VT character (code
1048 11). This makes "space" different to \s, which does not include VT (for
1049 Perl compatibility).
1050 The name "word" is a Perl extension, and "blank" is a GNU extension
1052 from Perl 5.8. Another Perl extension is negation, which is indicated
1053 by a ^ character after the colon. For example,
1054 [12[:^digit:]]
1056 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
1058 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
1059 these are not supported, and an error is given if they are encountered.
1060 By default, in UTF modes, characters with values greater than 128 do
1062 not match any of the POSIX character classes. However, if the PCRE_UCP
1063 option is passed to pcre_compile(), some of the classes are changed so
1064 that Unicode character properties are used. This is achieved by replac‐
1065 ing certain POSIX classes by other sequences, as follows:
1066 [:alnum:] becomes \p{Xan}
1068 [:alpha:] becomes \p{L}
1069 [:blank:] becomes \h
1070 [:digit:] becomes \p{Nd}
1071 [:lower:] becomes \p{Ll}
1072 [:space:] becomes \p{Xps}
1073 [:upper:] becomes \p{Lu}
1074 [:word:] becomes \p{Xwd}
1075 Negated versions, such as [:^alpha:] use \P instead of \p. Three other
1077 POSIX classes are handled specially in UCP mode:
1078 [:graph:] This matches characters that have glyphs that mark the page
1080 when printed. In Unicode property terms, it matches all char‐
1081 acters with the L, M, N, P, S, or Cf properties, except for:
1082 U+061C Arabic Letter Mark
1084 U+180E Mongolian Vowel Separator
1085 U+2066 - U+2069 Various "isolate"s
1086 [:print:] This matches the same characters as [:graph:] plus space
1089 characters that are not controls, that is, characters with
1090 the Zs property.
1091 [:punct:] This matches all characters that have the Unicode P (punctua‐
1093 tion) property, plus those characters whose code points are
1094 less than 128 that have the S (Symbol) property.
1095 The other POSIX classes are unchanged, and match only characters with
1097 code points less than 128.
1098
1100 Vertical bar characters are used to separate alternative patterns. For
1102 example, the pattern
1103 gilbert|sullivan
1105 matches either "gilbert" or "sullivan". Any number of alternatives may
1107 appear, and an empty alternative is permitted (matching the empty
1108 string). The matching process tries each alternative in turn, from left
1109 to right, and the first one that succeeds is used. If the alternatives
1110 are within a subpattern (defined below), "succeeds" means matching the
1111 rest of the main pattern as well as the alternative in the subpattern.
1112
1114 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1116 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
1117 within the pattern by a sequence of Perl option letters enclosed
1118 between "(?" and ")". The option letters are
1119 i for PCRE_CASELESS
1121 m for PCRE_MULTILINE
1122 s for PCRE_DOTALL
1123 x for PCRE_EXTENDED
1124 For example, (?im) sets caseless, multiline matching. It is also possi‐
1126 ble to unset these options by preceding the letter with a hyphen, and a
1127 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE‐
1128 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
1129 is also permitted. If a letter appears both before and after the
1130 hyphen, the option is unset.
1131 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
1133 can be changed in the same way as the Perl-compatible options by using
1134 the characters J, U and X respectively.
1135 When one of these option changes occurs at top level (that is, not
1137 inside subpattern parentheses), the change applies to the remainder of
1138 the pattern that follows. If the change is placed right at the start of
1139 a pattern, PCRE extracts it into the global options (and it will there‐
1140 fore show up in data extracted by the pcre_fullinfo() function).
1141 An option change within a subpattern (see below for a description of
1143 subpatterns) affects only that part of the subpattern that follows it,
1144 so
1145 (a(?i)b)c
1147 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
1149 used). By this means, options can be made to have different settings
1150 in different parts of the pattern. Any changes made in one alternative
1151 do carry on into subsequent branches within the same subpattern. For
1152 example,
1153 (a(?i)b|c)
1155 matches "ab", "aB", "c", and "C", even though when matching "C" the
1157 first branch is abandoned before the option setting. This is because
1158 the effects of option settings happen at compile time. There would be
1159 some very weird behaviour otherwise.
1160 Note: There are other PCRE-specific options that can be set by the
1162 application when the compiling or matching functions are called. In
1163 some cases the pattern can contain special leading sequences such as
1164 (*CRLF) to override what the application has set or what has been
1165 defaulted. Details are given in the section entitled "Newline
1166 sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and
1167 (*UCP) leading sequences that can be used to set UTF and Unicode prop‐
1168 erty modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16,
1169 PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF) sequence
1170 is a generic version that can be used with any of the libraries.
1171
1173 Subpatterns are delimited by parentheses (round brackets), which can be
1175 nested. Turning part of a pattern into a subpattern does two things:
1176 1. It localizes a set of alternatives. For example, the pattern
1178 cat(aract|erpillar|)
1180 matches "cataract", "caterpillar", or "cat". Without the parentheses,
1182 it would match "cataract", "erpillar" or an empty string.
1183 2. It sets up the subpattern as a capturing subpattern. This means
1185 that, when the whole pattern matches, that portion of the subject
1186 string that matched the subpattern is passed back to the caller via the
1187 ovector argument of the matching function. (This applies only to the
1188 traditional matching functions; the DFA matching functions do not sup‐
1189 port capturing.)
1190 Opening parentheses are counted from left to right (starting from 1) to
1192 obtain numbers for the capturing subpatterns. For example, if the
1193 string "the red king" is matched against the pattern
1194 the ((red|white) (king|queen))
1196 the captured substrings are "red king", "red", and "king", and are num‐
1198 bered 1, 2, and 3, respectively.
1199 The fact that plain parentheses fulfil two functions is not always
1201 helpful. There are often times when a grouping subpattern is required
1202 without a capturing requirement. If an opening parenthesis is followed
1203 by a question mark and a colon, the subpattern does not do any captur‐
1204 ing, and is not counted when computing the number of any subsequent
1205 capturing subpatterns. For example, if the string "the white queen" is
1206 matched against the pattern
1207 the ((?:red|white) (king|queen))
1209 the captured substrings are "white queen" and "queen", and are numbered
1211 1 and 2. The maximum number of capturing subpatterns is 65535.
1212 As a convenient shorthand, if any option settings are required at the
1214 start of a non-capturing subpattern, the option letters may appear
1215 between the "?" and the ":". Thus the two patterns
1216 (?i:saturday|sunday)
1218 (?:(?i)saturday|sunday)
1219 match exactly the same set of strings. Because alternative branches are
1221 tried from left to right, and options are not reset until the end of
1222 the subpattern is reached, an option setting in one branch does affect
1223 subsequent branches, so the above patterns match "SUNDAY" as well as
1224 "Saturday".
1225
1227 Perl 5.10 introduced a feature whereby each alternative in a subpattern
1229 uses the same numbers for its capturing parentheses. Such a subpattern
1230 starts with (?| and is itself a non-capturing subpattern. For example,
1231 consider this pattern:
1232 (?|(Sat)ur|(Sun))day
1234 Because the two alternatives are inside a (?| group, both sets of cap‐
1236 turing parentheses are numbered one. Thus, when the pattern matches,
1237 you can look at captured substring number one, whichever alternative
1238 matched. This construct is useful when you want to capture part, but
1239 not all, of one of a number of alternatives. Inside a (?| group, paren‐
1240 theses are numbered as usual, but the number is reset at the start of
1241 each branch. The numbers of any capturing parentheses that follow the
1242 subpattern start after the highest number used in any branch. The fol‐
1243 lowing example is taken from the Perl documentation. The numbers under‐
1244 neath show in which buffer the captured content will be stored.
1245 # before ---------------branch-reset----------- after
1247 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1248 # 1 2 2 3 2 3 4
1249 A back reference to a numbered subpattern uses the most recent value
1251 that is set for that number by any subpattern. The following pattern
1252 matches "abcabc" or "defdef":
1253 /(?|(abc)|(def))\1/
1255 In contrast, a subroutine call to a numbered subpattern always refers
1257 to the first one in the pattern with the given number. The following
1258 pattern matches "abcabc" or "defabc":
1259 /(?|(abc)|(def))(?1)/
1261 If a condition test for a subpattern's having matched refers to a non-
1263 unique number, the test is true if any of the subpatterns of that num‐
1264 ber have matched.
1265 An alternative approach to using this "branch reset" feature is to use
1267 duplicate named subpatterns, as described in the next section.
1268
1270 Identifying capturing parentheses by number is simple, but it can be
1272 very hard to keep track of the numbers in complicated regular expres‐
1273 sions. Furthermore, if an expression is modified, the numbers may
1274 change. To help with this difficulty, PCRE supports the naming of sub‐
1275 patterns. This feature was not added to Perl until release 5.10. Python
1276 had the feature earlier, and PCRE introduced it at release 4.0, using
1277 the Python syntax. PCRE now supports both the Perl and the Python syn‐
1278 tax. Perl allows identically numbered subpatterns to have different
1279 names, but PCRE does not.
1280 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
1282 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
1283 to capturing parentheses from other parts of the pattern, such as back
1284 references, recursion, and conditions, can be made by name as well as
1285 by number.
1286 Names consist of up to 32 alphanumeric characters and underscores.
1288 Named capturing parentheses are still allocated numbers as well as
1289 names, exactly as if the names were not present. The PCRE API provides
1290 function calls for extracting the name-to-number translation table from
1291 a compiled pattern. There is also a convenience function for extracting
1292 a captured substring by name.
1293 By default, a name must be unique within a pattern, but it is possible
1295 to relax this constraint by setting the PCRE_DUPNAMES option at compile
1296 time. (Duplicate names are also always permitted for subpatterns with
1297 the same number, set up as described in the previous section.) Dupli‐
1298 cate names can be useful for patterns where only one instance of the
1299 named parentheses can match. Suppose you want to match the name of a
1300 weekday, either as a 3-letter abbreviation or as the full name, and in
1301 both cases you want to extract the abbreviation. This pattern (ignoring
1302 the line breaks) does the job:
1303 (?<DN>Mon|Fri|Sun)(?:day)?|
1305 (?<DN>Tue)(?:sday)?|
1306 (?<DN>Wed)(?:nesday)?|
1307 (?<DN>Thu)(?:rsday)?|
1308 (?<DN>Sat)(?:urday)?
1309 There are five capturing substrings, but only one is ever set after a
1311 match. (An alternative way of solving this problem is to use a "branch
1312 reset" subpattern, as described in the previous section.)
1313 The convenience function for extracting the data by name returns the
1315 substring for the first (and in this example, the only) subpattern of
1316 that name that matched. This saves searching to find which numbered
1317 subpattern it was.
1318 If you make a back reference to a non-unique named subpattern from
1320 elsewhere in the pattern, the one that corresponds to the first occur‐
1321 rence of the name is used. In the absence of duplicate numbers (see the
1322 previous section) this is the one with the lowest number. If you use a
1323 named reference in a condition test (see the section about conditions
1324 below), either to check whether a subpattern has matched, or to check
1325 for recursion, all subpatterns with the same name are tested. If the
1326 condition is true for any one of them, the overall condition is true.
1327 This is the same behaviour as testing by number. For further details of
1328 the interfaces for handling named subpatterns, see the pcreapi documen‐
1329 tation.
1330 Warning: You cannot use different names to distinguish between two sub‐
1332 patterns with the same number because PCRE uses only the numbers when
1333 matching. For this reason, an error is given at compile time if differ‐
1334 ent names are given to subpatterns with the same number. However, you
1335 can give the same name to subpatterns with the same number, even when
1336 PCRE_DUPNAMES is not set.
1337
1339 Repetition is specified by quantifiers, which can follow any of the
1341 following items:
1342 a literal data character
1344 the dot metacharacter
1345 the \C escape sequence
1346 the \X escape sequence
1347 the \R escape sequence
1348 an escape such as \d or \pL that matches a single character
1349 a character class
1350 a back reference (see next section)
1351 a parenthesized subpattern (including assertions)
1352 a subroutine call to a subpattern (recursive or otherwise)
1353 The general repetition quantifier specifies a minimum and maximum num‐
1355 ber of permitted matches, by giving the two numbers in curly brackets
1356 (braces), separated by a comma. The numbers must be less than 65536,
1357 and the first must be less than or equal to the second. For example:
1358 z{2,4}
1360 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
1362 special character. If the second number is omitted, but the comma is
1363 present, there is no upper limit; if the second number and the comma
1364 are both omitted, the quantifier specifies an exact number of required
1365 matches. Thus
1366 [aeiou]{3,}
1368 matches at least 3 successive vowels, but may match many more, while
1370 \d{8}
1372 matches exactly 8 digits. An opening curly bracket that appears in a
1374 position where a quantifier is not allowed, or one that does not match
1375 the syntax of a quantifier, is taken as a literal character. For exam‐
1376 ple, {,6} is not a quantifier, but a literal string of four characters.
1377 In UTF modes, quantifiers apply to characters rather than to individual
1379 data units. Thus, for example, \x{100}{2} matches two characters, each
1380 of which is represented by a two-byte sequence in a UTF-8 string. Simi‐
1381 larly, \X{3} matches three Unicode extended grapheme clusters, each of
1382 which may be several data units long (and they may be of different
1383 lengths).
1384 The quantifier {0} is permitted, causing the expression to behave as if
1386 the previous item and the quantifier were not present. This may be use‐
1387 ful for subpatterns that are referenced as subroutines from elsewhere
1388 in the pattern (but see also the section entitled "Defining subpatterns
1389 for use by reference only" below). Items other than subpatterns that
1390 have a {0} quantifier are omitted from the compiled pattern.
1391 For convenience, the three most common quantifiers have single-charac‐
1393 ter abbreviations:
1394 * is equivalent to {0,}
1396 + is equivalent to {1,}
1397 ? is equivalent to {0,1}
1398 It is possible to construct infinite loops by following a subpattern
1400 that can match no characters with a quantifier that has no upper limit,
1401 for example:
1402 (a?)*
1404 Earlier versions of Perl and PCRE used to give an error at compile time
1406 for such patterns. However, because there are cases where this can be
1407 useful, such patterns are now accepted, but if any repetition of the
1408 subpattern does in fact match no characters, the loop is forcibly bro‐
1409 ken.
1410 By default, the quantifiers are "greedy", that is, they match as much
1412 as possible (up to the maximum number of permitted times), without
1413 causing the rest of the pattern to fail. The classic example of where
1414 this gives problems is in trying to match comments in C programs. These
1415 appear between /* and */ and within the comment, individual * and /
1416 characters may appear. An attempt to match C comments by applying the
1417 pattern
1418 /\*.*\*/
1420 to the string
1422 /* first comment */ not comment /* second comment */
1424 fails, because it matches the entire string owing to the greediness of
1426 the .* item.
1427 However, if a quantifier is followed by a question mark, it ceases to
1429 be greedy, and instead matches the minimum number of times possible, so
1430 the pattern
1431 /\*.*?\*/
1433 does the right thing with the C comments. The meaning of the various
1435 quantifiers is not otherwise changed, just the preferred number of
1436 matches. Do not confuse this use of question mark with its use as a
1437 quantifier in its own right. Because it has two uses, it can sometimes
1438 appear doubled, as in
1439 \d??\d
1441 which matches one digit by preference, but can match two if that is the
1443 only way the rest of the pattern matches.
1444 If the PCRE_UNGREEDY option is set (an option that is not available in
1446 Perl), the quantifiers are not greedy by default, but individual ones
1447 can be made greedy by following them with a question mark. In other
1448 words, it inverts the default behaviour.
1449 When a parenthesized subpattern is quantified with a minimum repeat
1451 count that is greater than 1 or with a limited maximum, more memory is
1452 required for the compiled pattern, in proportion to the size of the
1453 minimum or maximum.
1454 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv‐
1456 alent to Perl's /s) is set, thus allowing the dot to match newlines,
1457 the pattern is implicitly anchored, because whatever follows will be
1458 tried against every character position in the subject string, so there
1459 is no point in retrying the overall match at any position after the
1460 first. PCRE normally treats such a pattern as though it were preceded
1461 by \A.
1462 In cases where it is known that the subject string contains no new‐
1464 lines, it is worth setting PCRE_DOTALL in order to obtain this opti‐
1465 mization, or alternatively using ^ to indicate anchoring explicitly.
1466 However, there are some cases where the optimization cannot be used.
1468 When .* is inside capturing parentheses that are the subject of a back
1469 reference elsewhere in the pattern, a match at the start may fail where
1470 a later one succeeds. Consider, for example:
1471 (.*)abc\1
1473 If the subject is "xyz123abc123" the match point is the fourth charac‐
1475 ter. For this reason, such a pattern is not implicitly anchored.
1476 Another case where implicit anchoring is not applied is when the lead‐
1478 ing .* is inside an atomic group. Once again, a match at the start may
1479 fail where a later one succeeds. Consider this pattern:
1480 (?>.*?a)b
1482 It matches "ab" in the subject "aab". The use of the backtracking con‐
1484 trol verbs (*PRUNE) and (*SKIP) also disable this optimization.
1485 When a capturing subpattern is repeated, the value captured is the sub‐
1487 string that matched the final iteration. For example, after
1488 (tweedle[dume]{3}\s*)+
1490 has matched "tweedledum tweedledee" the value of the captured substring
1492 is "tweedledee". However, if there are nested capturing subpatterns,
1493 the corresponding captured values may have been set in previous itera‐
1494 tions. For example, after
1495 /(a|(b))+/
1497 matches "aba" the value of the second captured substring is "b".
1499
1501 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1503 repetition, failure of what follows normally causes the repeated item
1504 to be re-evaluated to see if a different number of repeats allows the
1505 rest of the pattern to match. Sometimes it is useful to prevent this,
1506 either to change the nature of the match, or to cause it fail earlier
1507 than it otherwise might, when the author of the pattern knows there is
1508 no point in carrying on.
1509 Consider, for example, the pattern \d+foo when applied to the subject
1511 line
1512 123456bar
1514 After matching all 6 digits and then failing to match "foo", the normal
1516 action of the matcher is to try again with only 5 digits matching the
1517 \d+ item, and then with 4, and so on, before ultimately failing.
1518 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
1519 the means for specifying that once a subpattern has matched, it is not
1520 to be re-evaluated in this way.
1521 If we use atomic grouping for the previous example, the matcher gives
1523 up immediately on failing to match "foo" the first time. The notation
1524 is a kind of special parenthesis, starting with (?> as in this example:
1525 (?>\d+)foo
1527 This kind of parenthesis "locks up" the part of the pattern it con‐
1529 tains once it has matched, and a failure further into the pattern is
1530 prevented from backtracking into it. Backtracking past it to previous
1531 items, however, works as normal.
1532 An alternative description is that a subpattern of this type matches
1534 the string of characters that an identical standalone pattern would
1535 match, if anchored at the current point in the subject string.
1536 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
1538 such as the above example can be thought of as a maximizing repeat that
1539 must swallow everything it can. So, while both \d+ and \d+? are pre‐
1540 pared to adjust the number of digits they match in order to make the
1541 rest of the pattern match, (?>\d+) can only match an entire sequence of
1542 digits.
1543 Atomic groups in general can of course contain arbitrarily complicated
1545 subpatterns, and can be nested. However, when the subpattern for an
1546 atomic group is just a single repeated item, as in the example above, a
1547 simpler notation, called a "possessive quantifier" can be used. This
1548 consists of an additional + character following a quantifier. Using
1549 this notation, the previous example can be rewritten as
1550 \d++foo
1552 Note that a possessive quantifier can be used with an entire group, for
1554 example:
1555 (abc|xyz){2,3}+
1557 Possessive quantifiers are always greedy; the setting of the
1559 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
1560 simpler forms of atomic group. However, there is no difference in the
1561 meaning of a possessive quantifier and the equivalent atomic group,
1562 though there may be a performance difference; possessive quantifiers
1563 should be slightly faster.
1564 The possessive quantifier syntax is an extension to the Perl 5.8 syn‐
1566 tax. Jeffrey Friedl originated the idea (and the name) in the first
1567 edition of his book. Mike McCloskey liked it, so implemented it when he
1568 built Sun's Java package, and PCRE copied it from there. It ultimately
1569 found its way into Perl at release 5.10.
1570 PCRE has an optimization that automatically "possessifies" certain sim‐
1572 ple pattern constructs. For example, the sequence A+B is treated as
1573 A++B because there is no point in backtracking into a sequence of A's
1574 when B must follow.
1575 When a pattern contains an unlimited repeat inside a subpattern that
1577 can itself be repeated an unlimited number of times, the use of an
1578 atomic group is the only way to avoid some failing matches taking a
1579 very long time indeed. The pattern
1580 (\D+|<\d+>)*[!?]
1582 matches an unlimited number of substrings that either consist of non-
1584 digits, or digits enclosed in <>, followed by either ! or ?. When it
1585 matches, it runs quickly. However, if it is applied to
1586 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1588 it takes a long time before reporting failure. This is because the
1590 string can be divided between the internal \D+ repeat and the external
1591 * repeat in a large number of ways, and all have to be tried. (The
1592 example uses [!?] rather than a single character at the end, because
1593 both PCRE and Perl have an optimization that allows for fast failure
1594 when a single character is used. They remember the last single charac‐
1595 ter that is required for a match, and fail early if it is not present
1596 in the string.) If the pattern is changed so that it uses an atomic
1597 group, like this:
1598 ((?>\D+)|<\d+>)*[!?]
1600 sequences of non-digits cannot be broken, and failure happens quickly.
1602
1604 Outside a character class, a backslash followed by a digit greater than
1606 0 (and possibly further digits) is a back reference to a capturing sub‐
1607 pattern earlier (that is, to its left) in the pattern, provided there
1608 have been that many previous capturing left parentheses.
1609 However, if the decimal number following the backslash is less than 10,
1611 it is always taken as a back reference, and causes an error only if
1612 there are not that many capturing left parentheses in the entire pat‐
1613 tern. In other words, the parentheses that are referenced need not be
1614 to the left of the reference for numbers less than 10. A "forward back
1615 reference" of this type can make sense when a repetition is involved
1616 and the subpattern to the right has participated in an earlier itera‐
1617 tion.
1618 It is not possible to have a numerical "forward back reference" to a
1620 subpattern whose number is 10 or more using this syntax because a
1621 sequence such as \50 is interpreted as a character defined in octal.
1622 See the subsection entitled "Non-printing characters" above for further
1623 details of the handling of digits following a backslash. There is no
1624 such problem when named parentheses are used. A back reference to any
1625 subpattern is possible using named parentheses (see below).
1626 Another way of avoiding the ambiguity inherent in the use of digits
1628 following a backslash is to use the \g escape sequence. This escape
1629 must be followed by an unsigned number or a negative number, optionally
1630 enclosed in braces. These examples are all identical:
1631 (ring), \1
1633 (ring), \g1
1634 (ring), \g{1}
1635 An unsigned number specifies an absolute reference without the ambigu‐
1637 ity that is present in the older syntax. It is also useful when literal
1638 digits follow the reference. A negative number is a relative reference.
1639 Consider this example:
1640 (abc(def)ghi)\g{-1}
1642 The sequence \g{-1} is a reference to the most recently started captur‐
1644 ing subpattern before \g, that is, is it equivalent to \2 in this exam‐
1645 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
1646 references can be helpful in long patterns, and also in patterns that
1647 are created by joining together fragments that contain references
1648 within themselves.
1649 A back reference matches whatever actually matched the capturing sub‐
1651 pattern in the current subject string, rather than anything matching
1652 the subpattern itself (see "Subpatterns as subroutines" below for a way
1653 of doing that). So the pattern
1654 (sens|respons)e and \1ibility
1656 matches "sense and sensibility" and "response and responsibility", but
1658 not "sense and responsibility". If caseful matching is in force at the
1659 time of the back reference, the case of letters is relevant. For exam‐
1660 ple,
1661 ((?i)rah)\s+\1
1663 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
1665 original capturing subpattern is matched caselessly.
1666 There are several different ways of writing back references to named
1668 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
1669 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
1670 unified back reference syntax, in which \g can be used for both numeric
1671 and named references, is also supported. We could rewrite the above
1672 example in any of the following ways:
1673 (?<p1>(?i)rah)\s+\k<p1>
1675 (?'p1'(?i)rah)\s+\k{p1}
1676 (?P<p1>(?i)rah)\s+(?P=p1)
1677 (?<p1>(?i)rah)\s+\g{p1}
1678 A subpattern that is referenced by name may appear in the pattern
1680 before or after the reference.
1681 There may be more than one back reference to the same subpattern. If a
1683 subpattern has not actually been used in a particular match, any back
1684 references to it always fail by default. For example, the pattern
1685 (a|(bc))\2
1687 always fails if it starts to match "a" rather than "bc". However, if
1689 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer‐
1690 ence to an unset value matches an empty string.
1691 Because there may be many capturing parentheses in a pattern, all dig‐
1693 its following a backslash are taken as part of a potential back refer‐
1694 ence number. If the pattern continues with a digit character, some
1695 delimiter must be used to terminate the back reference. If the
1696 PCRE_EXTENDED option is set, this can be white space. Otherwise, the
1697 \g{ syntax or an empty comment (see "Comments" below) can be used.
1698 Recursive back references
1700 A back reference that occurs inside the parentheses to which it refers
1702 fails when the subpattern is first used, so, for example, (a\1) never
1703 matches. However, such references can be useful inside repeated sub‐
1704 patterns. For example, the pattern
1705 (a|b\1)+
1707 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter‐
1709 ation of the subpattern, the back reference matches the character
1710 string corresponding to the previous iteration. In order for this to
1711 work, the pattern must be such that the first iteration does not need
1712 to match the back reference. This can be done using alternation, as in
1713 the example above, or by a quantifier with a minimum of zero.
1714 Back references of this type cause the group that they reference to be
1716 treated as an atomic group. Once the whole group has been matched, a
1717 subsequent matching failure cannot cause backtracking into the middle
1718 of the group.
1719
1721 An assertion is a test on the characters following or preceding the
1723 current matching point that does not actually consume any characters.
1724 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
1725 described above.
1726 More complicated assertions are coded as subpatterns. There are two
1728 kinds: those that look ahead of the current position in the subject
1729 string, and those that look behind it. An assertion subpattern is
1730 matched in the normal way, except that it does not cause the current
1731 matching position to be changed.
1732 Assertion subpatterns are not capturing subpatterns. If such an asser‐
1734 tion contains capturing subpatterns within it, these are counted for
1735 the purposes of numbering the capturing subpatterns in the whole pat‐
1736 tern. However, substring capturing is carried out only for positive
1737 assertions, because it does not make sense for negative assertions.
1738 For compatibility with Perl, assertion subpatterns may be repeated;
1740 though it makes no sense to assert the same thing several times, the
1741 side effect of capturing parentheses may occasionally be useful. In
1742 practice, there only three cases:
1743 (1) If the quantifier is {0}, the assertion is never obeyed during
1745 matching. However, it may contain internal capturing parenthesized
1746 groups that are called from elsewhere via the subroutine mechanism.
1747 (2) If quantifier is {0,n} where n is greater than zero, it is treated
1749 as if it were {0,1}. At run time, the rest of the pattern match is
1750 tried with and without the assertion, the order depending on the greed‐
1751 iness of the quantifier.
1752 (3) If the minimum repetition is greater than zero, the quantifier is
1754 ignored. The assertion is obeyed just once when encountered during
1755 matching.
1756 Lookahead assertions
1758 Lookahead assertions start with (?= for positive assertions and (?! for
1760 negative assertions. For example,
1761 \w+(?=;)
1763 matches a word followed by a semicolon, but does not include the semi‐
1765 colon in the match, and
1766 foo(?!bar)
1768 matches any occurrence of "foo" that is not followed by "bar". Note
1770 that the apparently similar pattern
1771 (?!foo)bar
1773 does not find an occurrence of "bar" that is preceded by something
1775 other than "foo"; it finds any occurrence of "bar" whatsoever, because
1776 the assertion (?!foo) is always true when the next three characters are
1777 "bar". A lookbehind assertion is needed to achieve the other effect.
1778 If you want to force a matching failure at some point in a pattern, the
1780 most convenient way to do it is with (?!) because an empty string
1781 always matches, so an assertion that requires there not to be an empty
1782 string must always fail. The backtracking control verb (*FAIL) or (*F)
1783 is a synonym for (?!).
1784 Lookbehind assertions
1786 Lookbehind assertions start with (?<= for positive assertions and (?<!
1788 for negative assertions. For example,
1789 (?<!foo)bar
1791 does find an occurrence of "bar" that is not preceded by "foo". The
1793 contents of a lookbehind assertion are restricted such that all the
1794 strings it matches must have a fixed length. However, if there are sev‐
1795 eral top-level alternatives, they do not all have to have the same
1796 fixed length. Thus
1797 (?<=bullock|donkey)
1799 is permitted, but
1801 (?<!dogs?|cats?)
1803 causes an error at compile time. Branches that match different length
1805 strings are permitted only at the top level of a lookbehind assertion.
1806 This is an extension compared with Perl, which requires all branches to
1807 match the same length of string. An assertion such as
1808 (?<=ab(c|de))
1810 is not permitted, because its single top-level branch can match two
1812 different lengths, but it is acceptable to PCRE if rewritten to use two
1813 top-level branches:
1814 (?<=abc|abde)
1816 In some cases, the escape sequence \K (see above) can be used instead
1818 of a lookbehind assertion to get round the fixed-length restriction.
1819 The implementation of lookbehind assertions is, for each alternative,
1821 to temporarily move the current position back by the fixed length and
1822 then try to match. If there are insufficient characters before the cur‐
1823 rent position, the assertion fails.
1824 In a UTF mode, PCRE does not allow the \C escape (which matches a sin‐
1826 gle data unit even in a UTF mode) to appear in lookbehind assertions,
1827 because it makes it impossible to calculate the length of the lookbe‐
1828 hind. The \X and \R escapes, which can match different numbers of data
1829 units, are also not permitted.
1830 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
1832 lookbehinds, as long as the subpattern matches a fixed-length string.
1833 Recursion, however, is not supported.
1834 Possessive quantifiers can be used in conjunction with lookbehind
1836 assertions to specify efficient matching of fixed-length strings at the
1837 end of subject strings. Consider a simple pattern such as
1838 abcd$
1840 when applied to a long string that does not match. Because matching
1842 proceeds from left to right, PCRE will look for each "a" in the subject
1843 and then see if what follows matches the rest of the pattern. If the
1844 pattern is specified as
1845 ^.*abcd$
1847 the initial .* matches the entire string at first, but when this fails
1849 (because there is no following "a"), it backtracks to match all but the
1850 last character, then all but the last two characters, and so on. Once
1851 again the search for "a" covers the entire string, from right to left,
1852 so we are no better off. However, if the pattern is written as
1853 ^.*+(?<=abcd)
1855 there can be no backtracking for the .*+ item; it can match only the
1857 entire string. The subsequent lookbehind assertion does a single test
1858 on the last four characters. If it fails, the match fails immediately.
1859 For long strings, this approach makes a significant difference to the
1860 processing time.
1861 Using multiple assertions
1863 Several assertions (of any sort) may occur in succession. For example,
1865 (?<=\d{3})(?<!999)foo
1867 matches "foo" preceded by three digits that are not "999". Notice that
1869 each of the assertions is applied independently at the same point in
1870 the subject string. First there is a check that the previous three
1871 characters are all digits, and then there is a check that the same
1872 three characters are not "999". This pattern does not match "foo" pre‐
1873 ceded by six characters, the first of which are digits and the last
1874 three of which are not "999". For example, it doesn't match "123abc‐
1875 foo". A pattern to do that is
1876 (?<=\d{3}...)(?<!999)foo
1878 This time the first assertion looks at the preceding six characters,
1880 checking that the first three are digits, and then the second assertion
1881 checks that the preceding three characters are not "999".
1882 Assertions can be nested in any combination. For example,
1884 (?<=(?<!foo)bar)baz
1886 matches an occurrence of "baz" that is preceded by "bar" which in turn
1888 is not preceded by "foo", while
1889 (?<=\d{3}(?!999)...)foo
1891 is another pattern that matches "foo" preceded by three digits and any
1893 three characters that are not "999".
1894
1896 It is possible to cause the matching process to obey a subpattern con‐
1898 ditionally or to choose between two alternative subpatterns, depending
1899 on the result of an assertion, or whether a specific capturing subpat‐
1900 tern has already been matched. The two possible forms of conditional
1901 subpattern are:
1902 (?(condition)yes-pattern)
1904 (?(condition)yes-pattern|no-pattern)
1905 If the condition is satisfied, the yes-pattern is used; otherwise the
1907 no-pattern (if present) is used. If there are more than two alterna‐
1908 tives in the subpattern, a compile-time error occurs. Each of the two
1909 alternatives may itself contain nested subpatterns of any form, includ‐
1910 ing conditional subpatterns; the restriction to two alternatives
1911 applies only at the level of the condition. This pattern fragment is an
1912 example where the alternatives are complex:
1913 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1915 There are four kinds of condition: references to subpatterns, refer‐
1918 ences to recursion, a pseudo-condition called DEFINE, and assertions.
1919 Checking for a used subpattern by number
1921 If the text between the parentheses consists of a sequence of digits,
1923 the condition is true if a capturing subpattern of that number has pre‐
1924 viously matched. If there is more than one capturing subpattern with
1925 the same number (see the earlier section about duplicate subpattern
1926 numbers), the condition is true if any of them have matched. An alter‐
1927 native notation is to precede the digits with a plus or minus sign. In
1928 this case, the subpattern number is relative rather than absolute. The
1929 most recently opened parentheses can be referenced by (?(-1), the next
1930 most recent by (?(-2), and so on. Inside loops it can also make sense
1931 to refer to subsequent groups. The next parentheses to be opened can be
1932 referenced as (?(+1), and so on. (The value zero in any of these forms
1933 is not used; it provokes a compile-time error.)
1934 Consider the following pattern, which contains non-significant white
1936 space to make it more readable (assume the PCRE_EXTENDED option) and to
1937 divide it into three parts for ease of discussion:
1938 ( \( )? [^()]+ (?(1) \) )
1940 The first part matches an optional opening parenthesis, and if that
1942 character is present, sets it as the first captured substring. The sec‐
1943 ond part matches one or more characters that are not parentheses. The
1944 third part is a conditional subpattern that tests whether or not the
1945 first set of parentheses matched. If they did, that is, if subject
1946 started with an opening parenthesis, the condition is true, and so the
1947 yes-pattern is executed and a closing parenthesis is required. Other‐
1948 wise, since no-pattern is not present, the subpattern matches nothing.
1949 In other words, this pattern matches a sequence of non-parentheses,
1950 optionally enclosed in parentheses.
1951 If you were embedding this pattern in a larger one, you could use a
1953 relative reference:
1954 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1956 This makes the fragment independent of the parentheses in the larger
1958 pattern.
1959 Checking for a used subpattern by name
1961 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
1963 used subpattern by name. For compatibility with earlier versions of
1964 PCRE, which had this facility before Perl, the syntax (?(name)...) is
1965 also recognized. However, there is a possible ambiguity with this syn‐
1966 tax, because subpattern names may consist entirely of digits. PCRE
1967 looks first for a named subpattern; if it cannot find one and the name
1968 consists entirely of digits, PCRE looks for a subpattern of that num‐
1969 ber, which must be greater than zero. Using subpattern names that con‐
1970 sist entirely of digits is not recommended.
1971 Rewriting the above example to use a named subpattern gives this:
1973 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
1975 If the name used in a condition of this kind is a duplicate, the test
1977 is applied to all subpatterns of the same name, and is true if any one
1978 of them has matched.
1979 Checking for pattern recursion
1981 If the condition is the string (R), and there is no subpattern with the
1983 name R, the condition is true if a recursive call to the whole pattern
1984 or any subpattern has been made. If digits or a name preceded by amper‐
1985 sand follow the letter R, for example:
1986 (?(R3)...) or (?(R&name)...)
1988 the condition is true if the most recent recursion is into a subpattern
1990 whose number or name is given. This condition does not check the entire
1991 recursion stack. If the name used in a condition of this kind is a
1992 duplicate, the test is applied to all subpatterns of the same name, and
1993 is true if any one of them is the most recent recursion.
1994 At "top level", all these recursion test conditions are false. The
1996 syntax for recursive patterns is described below.
1997 Defining subpatterns for use by reference only
1999 If the condition is the string (DEFINE), and there is no subpattern
2001 with the name DEFINE, the condition is always false. In this case,
2002 there may be only one alternative in the subpattern. It is always
2003 skipped if control reaches this point in the pattern; the idea of
2004 DEFINE is that it can be used to define subroutines that can be refer‐
2005 enced from elsewhere. (The use of subroutines is described below.) For
2006 example, a pattern to match an IPv4 address such as "192.168.23.245"
2007 could be written like this (ignore white space and line breaks):
2008 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2010 \b (?&byte) (\.(?&byte)){3} \b
2011 The first part of the pattern is a DEFINE group inside which a another
2013 group named "byte" is defined. This matches an individual component of
2014 an IPv4 address (a number less than 256). When matching takes place,
2015 this part of the pattern is skipped because DEFINE acts like a false
2016 condition. The rest of the pattern uses references to the named group
2017 to match the four dot-separated components of an IPv4 address, insist‐
2018 ing on a word boundary at each end.
2019 Assertion conditions
2021 If the condition is not in any of the above formats, it must be an
2023 assertion. This may be a positive or negative lookahead or lookbehind
2024 assertion. Consider this pattern, again containing non-significant
2025 white space, and with the two alternatives on the second line:
2026 (?(?=[^a-z]*[a-z])
2028 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2029 The condition is a positive lookahead assertion that matches an
2031 optional sequence of non-letters followed by a letter. In other words,
2032 it tests for the presence of at least one letter in the subject. If a
2033 letter is found, the subject is matched against the first alternative;
2034 otherwise it is matched against the second. This pattern matches
2035 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
2036 letters and dd are digits.
2037
2039 There are two ways of including comments in patterns that are processed
2041 by PCRE. In both cases, the start of the comment must not be in a char‐
2042 acter class, nor in the middle of any other sequence of related charac‐
2043 ters such as (?: or a subpattern name or number. The characters that
2044 make up a comment play no part in the pattern matching.
2045 The sequence (?# marks the start of a comment that continues up to the
2047 next closing parenthesis. Nested parentheses are not permitted. If the
2048 PCRE_EXTENDED option is set, an unescaped # character also introduces a
2049 comment, which in this case continues to immediately after the next
2050 newline character or character sequence in the pattern. Which charac‐
2051 ters are interpreted as newlines is controlled by the options passed to
2052 a compiling function or by a special sequence at the start of the pat‐
2053 tern, as described in the section entitled "Newline conventions" above.
2054 Note that the end of this type of comment is a literal newline sequence
2055 in the pattern; escape sequences that happen to represent a newline do
2056 not count. For example, consider this pattern when PCRE_EXTENDED is
2057 set, and the default newline convention is in force:
2058 abc #comment \n still comment
2060 On encountering the # character, pcre_compile() skips along, looking
2062 for a newline in the pattern. The sequence \n is still literal at this
2063 stage, so it does not terminate the comment. Only an actual character
2064 with the code value 0x0a (the default newline) does so.
2065
2067 Consider the problem of matching a string in parentheses, allowing for
2069 unlimited nested parentheses. Without the use of recursion, the best
2070 that can be done is to use a pattern that matches up to some fixed
2071 depth of nesting. It is not possible to handle an arbitrary nesting
2072 depth.
2073 For some time, Perl has provided a facility that allows regular expres‐
2075 sions to recurse (amongst other things). It does this by interpolating
2076 Perl code in the expression at run time, and the code can refer to the
2077 expression itself. A Perl pattern using code interpolation to solve the
2078 parentheses problem can be created like this:
2079 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
2081 The (?p{...}) item interpolates Perl code at run time, and in this case
2083 refers recursively to the pattern in which it appears.
2084 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
2086 it supports special syntax for recursion of the entire pattern, and
2087 also for individual subpattern recursion. After its introduction in
2088 PCRE and Python, this kind of recursion was subsequently introduced
2089 into Perl at release 5.10.
2090 A special item that consists of (? followed by a number greater than
2092 zero and a closing parenthesis is a recursive subroutine call of the
2093 subpattern of the given number, provided that it occurs inside that
2094 subpattern. (If not, it is a non-recursive subroutine call, which is
2095 described in the next section.) The special item (?R) or (?0) is a
2096 recursive call of the entire regular expression.
2097 This PCRE pattern solves the nested parentheses problem (assume the
2099 PCRE_EXTENDED option is set so that white space is ignored):
2100 \( ( [^()]++ | (?R) )* \)
2102 First it matches an opening parenthesis. Then it matches any number of
2104 substrings which can either be a sequence of non-parentheses, or a
2105 recursive match of the pattern itself (that is, a correctly parenthe‐
2106 sized substring). Finally there is a closing parenthesis. Note the use
2107 of a possessive quantifier to avoid backtracking into sequences of non-
2108 parentheses.
2109 If this were part of a larger pattern, you would not want to recurse
2111 the entire pattern, so instead you could use this:
2112 ( \( ( [^()]++ | (?1) )* \) )
2114 We have put the pattern into parentheses, and caused the recursion to
2116 refer to them instead of the whole pattern.
2117 In a larger pattern, keeping track of parenthesis numbers can be
2119 tricky. This is made easier by the use of relative references. Instead
2120 of (?1) in the pattern above you can write (?-2) to refer to the second
2121 most recently opened parentheses preceding the recursion. In other
2122 words, a negative number counts capturing parentheses leftwards from
2123 the point at which it is encountered.
2124 It is also possible to refer to subsequently opened parentheses, by
2126 writing references such as (?+2). However, these cannot be recursive
2127 because the reference is not inside the parentheses that are refer‐
2128 enced. They are always non-recursive subroutine calls, as described in
2129 the next section.
2130 An alternative approach is to use named parentheses instead. The Perl
2132 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
2133 supported. We could rewrite the above example as follows:
2134 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
2136 If there is more than one subpattern with the same name, the earliest
2138 one is used.
2139 This particular example pattern that we have been looking at contains
2141 nested unlimited repeats, and so the use of a possessive quantifier for
2142 matching strings of non-parentheses is important when applying the pat‐
2143 tern to strings that do not match. For example, when this pattern is
2144 applied to
2145 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2147 it yields "no match" quickly. However, if a possessive quantifier is
2149 not used, the match runs for a very long time indeed because there are
2150 so many different ways the + and * repeats can carve up the subject,
2151 and all have to be tested before failure can be reported.
2152 At the end of a match, the values of capturing parentheses are those
2154 from the outermost level. If you want to obtain intermediate values, a
2155 callout function can be used (see below and the pcrecallout documenta‐
2156 tion). If the pattern above is matched against
2157 (ab(cd)ef)
2159 the value for the inner capturing parentheses (numbered 2) is "ef",
2161 which is the last value taken on at the top level. If a capturing sub‐
2162 pattern is not matched at the top level, its final captured value is
2163 unset, even if it was (temporarily) set at a deeper level during the
2164 matching process.
2165 If there are more than 15 capturing parentheses in a pattern, PCRE has
2167 to obtain extra memory to store data during a recursion, which it does
2168 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
2169 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2170 Do not confuse the (?R) item with the condition (R), which tests for
2172 recursion. Consider this pattern, which matches text in angle brack‐
2173 ets, allowing for arbitrary nesting. Only digits are allowed in nested
2174 brackets (that is, when recursing), whereas any characters are permit‐
2175 ted at the outer level.
2176 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
2178 In this pattern, (?(R) is the start of a conditional subpattern, with
2180 two different alternatives for the recursive and non-recursive cases.
2181 The (?R) item is the actual recursive call.
2182 Differences in recursion processing between PCRE and Perl
2184 Recursion processing in PCRE differs from Perl in two important ways.
2186 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
2187 always treated as an atomic group. That is, once it has matched some of
2188 the subject string, it is never re-entered, even if it contains untried
2189 alternatives and there is a subsequent matching failure. This can be
2190 illustrated by the following pattern, which purports to match a palin‐
2191 dromic string that contains an odd number of characters (for example,
2192 "a", "aba", "abcba", "abcdcba"):
2193 ^(.|(.)(?1)\2)$
2195 The idea is that it either matches a single character, or two identical
2197 characters surrounding a sub-palindrome. In Perl, this pattern works;
2198 in PCRE it does not if the pattern is longer than three characters.
2199 Consider the subject string "abcba":
2200 At the top level, the first character is matched, but as it is not at
2202 the end of the string, the first alternative fails; the second alterna‐
2203 tive is taken and the recursion kicks in. The recursive call to subpat‐
2204 tern 1 successfully matches the next character ("b"). (Note that the
2205 beginning and end of line tests are not part of the recursion).
2206 Back at the top level, the next character ("c") is compared with what
2208 subpattern 2 matched, which was "a". This fails. Because the recursion
2209 is treated as an atomic group, there are now no backtracking points,
2210 and so the entire match fails. (Perl is able, at this point, to re-
2211 enter the recursion and try the second alternative.) However, if the
2212 pattern is written with the alternatives in the other order, things are
2213 different:
2214 ^((.)(?1)\2|.)$
2216 This time, the recursing alternative is tried first, and continues to
2218 recurse until it runs out of characters, at which point the recursion
2219 fails. But this time we do have another alternative to try at the
2220 higher level. That is the big difference: in the previous case the
2221 remaining alternative is at a deeper recursion level, which PCRE cannot
2222 use.
2223 To change the pattern so that it matches all palindromic strings, not
2225 just those with an odd number of characters, it is tempting to change
2226 the pattern to this:
2227 ^((.)(?1)\2|.?)$
2229 Again, this works in Perl, but not in PCRE, and for the same reason.
2231 When a deeper recursion has matched a single character, it cannot be
2232 entered again in order to match an empty string. The solution is to
2233 separate the two cases, and write out the odd and even cases as alter‐
2234 natives at the higher level:
2235 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2237 If you want to match typical palindromic phrases, the pattern has to
2239 ignore all non-word characters, which can be done like this:
2240 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2242 If run with the PCRE_CASELESS option, this pattern matches phrases such
2244 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
2245 Perl. Note the use of the possessive quantifier *+ to avoid backtrack‐
2246 ing into sequences of non-word characters. Without this, PCRE takes a
2247 great deal longer (ten times or more) to match typical phrases, and
2248 Perl takes so long that you think it has gone into a loop.
2249 WARNING: The palindrome-matching patterns above work only if the sub‐
2251 ject string does not start with a palindrome that is shorter than the
2252 entire string. For example, although "abcba" is correctly matched, if
2253 the subject is "ababa", PCRE finds the palindrome "aba" at the start,
2254 then fails at top level because the end of the string does not follow.
2255 Once again, it cannot jump back into the recursion to try other alter‐
2256 natives, so the entire match fails.
2257 The second way in which PCRE and Perl differ in their recursion pro‐
2259 cessing is in the handling of captured values. In Perl, when a subpat‐
2260 tern is called recursively or as a subpattern (see the next section),
2261 it has no access to any values that were captured outside the recur‐
2262 sion, whereas in PCRE these values can be referenced. Consider this
2263 pattern:
2264 ^(.)(\1|a(?2))
2266 In PCRE, this pattern matches "bab". The first capturing parentheses
2268 match "b", then in the second group, when the back reference \1 fails
2269 to match "b", the second alternative matches "a" and then recurses. In
2270 the recursion, \1 does now match "b" and so the whole match succeeds.
2271 In Perl, the pattern fails to match because inside the recursive call
2272 \1 cannot access the externally set value.
2273
2275 If the syntax for a recursive subpattern call (either by number or by
2277 name) is used outside the parentheses to which it refers, it operates
2278 like a subroutine in a programming language. The called subpattern may
2279 be defined before or after the reference. A numbered reference can be
2280 absolute or relative, as in these examples:
2281 (...(absolute)...)...(?2)...
2283 (...(relative)...)...(?-1)...
2284 (...(?+1)...(relative)...
2285 An earlier example pointed out that the pattern
2287 (sens|respons)e and \1ibility
2289 matches "sense and sensibility" and "response and responsibility", but
2291 not "sense and responsibility". If instead the pattern
2292 (sens|respons)e and (?1)ibility
2294 is used, it does match "sense and responsibility" as well as the other
2296 two strings. Another example is given in the discussion of DEFINE
2297 above.
2298 All subroutine calls, whether recursive or not, are always treated as
2300 atomic groups. That is, once a subroutine has matched some of the sub‐
2301 ject string, it is never re-entered, even if it contains untried alter‐
2302 natives and there is a subsequent matching failure. Any capturing
2303 parentheses that are set during the subroutine call revert to their
2304 previous values afterwards.
2305 Processing options such as case-independence are fixed when a subpat‐
2307 tern is defined, so if it is used as a subroutine, such options cannot
2308 be changed for different calls. For example, consider this pattern:
2309 (abc)(?i:(?-1))
2311 It matches "abcabc". It does not match "abcABC" because the change of
2313 processing option does not affect the called subpattern.
2314
2316 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
2318 name or a number enclosed either in angle brackets or single quotes, is
2319 an alternative syntax for referencing a subpattern as a subroutine,
2320 possibly recursively. Here are two of the examples used above, rewrit‐
2321 ten using this syntax:
2322 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
2324 (sens|respons)e and \g'1'ibility
2325 PCRE supports an extension to Oniguruma: if a number is preceded by a
2327 plus or a minus sign it is taken as a relative reference. For example:
2328 (abc)(?i:\g<-1>)
2330 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
2332 synonymous. The former is a back reference; the latter is a subroutine
2333 call.
2334
2336 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
2338 Perl code to be obeyed in the middle of matching a regular expression.
2339 This makes it possible, amongst other things, to extract different sub‐
2340 strings that match the same pair of parentheses when there is a repeti‐
2341 tion.
2342 PCRE provides a similar feature, but of course it cannot obey arbitrary
2344 Perl code. The feature is called "callout". The caller of PCRE provides
2345 an external function by putting its entry point in the global variable
2346 pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit
2347 library). By default, this variable contains NULL, which disables all
2348 calling out.
2349 Within a regular expression, (?C) indicates the points at which the
2351 external function is to be called. If you want to identify different
2352 callout points, you can put a number less than 256 after the letter C.
2353 The default value is zero. For example, this pattern has two callout
2354 points:
2355 (?C1)abc(?C2)def
2357 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call‐
2359 outs are automatically installed before each item in the pattern. They
2360 are all numbered 255.
2361 During matching, when PCRE reaches a callout point, the external func‐
2363 tion is called. It is provided with the number of the callout, the
2364 position in the pattern, and, optionally, one item of data originally
2365 supplied by the caller of the matching function. The callout function
2366 may cause matching to proceed, to backtrack, or to fail altogether. A
2367 complete description of the interface to the callout function is given
2368 in the pcrecallout documentation.
2369
2371 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
2373 which are described in the Perl documentation as "experimental and sub‐
2374 ject to change or removal in a future version of Perl". It goes on to
2375 say: "Their usage in production code should be noted to avoid problems
2376 during upgrades." The same remarks apply to the PCRE features described
2377 in this section.
2378 Since these verbs are specifically related to backtracking, most of
2380 them can be used only when the pattern is to be matched using one of
2381 the traditional matching functions, which use a backtracking algorithm.
2382 With the exception of (*FAIL), which behaves like a failing negative
2383 assertion, they cause an error if encountered by a DFA matching func‐
2384 tion.
2385 If any of these verbs are used in an assertion or in a subpattern that
2387 is called as a subroutine (whether or not recursively), their effect is
2388 confined to that subpattern; it does not extend to the surrounding pat‐
2389 tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
2390 that is encountered in a successful positive assertion is passed back
2391 when a match succeeds (compare capturing parentheses in assertions).
2392 Note that such subpatterns are processed as anchored at the point where
2393 they are tested. Note also that Perl's treatment of subroutines and
2394 assertions is different in some cases.
2395 The new verbs make use of what was previously invalid syntax: an open‐
2397 ing parenthesis followed by an asterisk. They are generally of the form
2398 (*VERB) or (*VERB:NAME). Some may take either form, with differing be‐
2399 haviour, depending on whether or not an argument is present. A name is
2400 any sequence of characters that does not include a closing parenthesis.
2401 The maximum length of name is 255 in the 8-bit library and 65535 in the
2402 16-bit and 32-bit library. If the name is empty, that is, if the clos‐
2403 ing parenthesis immediately follows the colon, the effect is as if the
2404 colon were not there. Any number of these verbs may occur in a pattern.
2405 Optimizations that affect backtracking verbs
2407 PCRE contains some optimizations that are used to speed up matching by
2409 running some checks at the start of each match attempt. For example, it
2410 may know the minimum length of matching subject, or that a particular
2411 character must be present. When one of these optimizations suppresses
2412 the running of a match, any included backtracking verbs will not, of
2413 course, be processed. You can suppress the start-of-match optimizations
2414 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com‐
2415 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
2416 There is more discussion of this option in the section entitled "Option
2417 bits for pcre_exec()" in the pcreapi documentation.
2418 Experiments with Perl suggest that it too has similar optimizations,
2420 sometimes leading to anomalous results.
2421 Verbs that act immediately
2423 The following verbs act as soon as they are encountered. They may not
2425 be followed by a name.
2426 (*ACCEPT)
2428 This verb causes the match to end successfully, skipping the remainder
2430 of the pattern. However, when it is inside a subpattern that is called
2431 as a subroutine, only that subpattern is ended successfully. Matching
2432 then continues at the outer level. If (*ACCEPT) is inside capturing
2433 parentheses, the data so far is captured. For example:
2434 A((?:A|B(*ACCEPT)|C)D)
2436 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap‐
2438 tured by the outer parentheses.
2439 (*FAIL) or (*F)
2441 This verb causes a matching failure, forcing backtracking to occur. It
2443 is equivalent to (?!) but easier to read. The Perl documentation notes
2444 that it is probably useful only when combined with (?{}) or (??{}).
2445 Those are, of course, Perl features that are not present in PCRE. The
2446 nearest equivalent is the callout feature, as for example in this pat‐
2447 tern:
2448 a+(?C)(*FAIL)
2450 A match with the string "aaaa" always fails, but the callout is taken
2452 before each backtrack happens (in this example, 10 times).
2453 Recording which path was taken
2455 There is one verb whose main purpose is to track how a match was
2457 arrived at, though it also has a secondary use in conjunction with
2458 advancing the match starting point (see (*SKIP) below).
2459 (*MARK:NAME) or (*:NAME)
2461 A name is always required with this verb. There may be as many
2463 instances of (*MARK) as you like in a pattern, and their names do not
2464 have to be unique.
2465 When a match succeeds, the name of the last-encountered (*MARK) on the
2467 matching path is passed back to the caller as described in the section
2468 entitled "Extra data for pcre_exec()" in the pcreapi documentation.
2469 Here is an example of pcretest output, where the /K modifier requests
2470 the retrieval and outputting of (*MARK) data:
2471 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2473 data> XY
2474 0: XY
2475 MK: A
2476 XZ
2477 0: XZ
2478 MK: B
2479 The (*MARK) name is tagged with "MK:" in this output, and in this exam‐
2481 ple it indicates which of the two alternatives matched. This is a more
2482 efficient way of obtaining this information than putting each alterna‐
2483 tive in its own capturing parentheses.
2484 If (*MARK) is encountered in a positive assertion, its name is recorded
2486 and passed back if it is the last-encountered. This does not happen for
2487 negative assertions.
2488 After a partial match or a failed match, the name of the last encoun‐
2490 tered (*MARK) in the entire match process is returned. For example:
2491 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2493 data> XP
2494 No match, mark = B
2495 Note that in this unanchored example the mark is retained from the
2497 match attempt that started at the letter "X" in the subject. Subsequent
2498 match attempts starting at "P" and then with an empty string do not get
2499 as far as the (*MARK) item, but nevertheless do not reset it.
2500 If you are interested in (*MARK) values after failed matches, you
2502 should probably set the PCRE_NO_START_OPTIMIZE option (see above) to
2503 ensure that the match is always attempted.
2504 Verbs that act after backtracking
2506 The following verbs do nothing when they are encountered. Matching con‐
2508 tinues with what follows, but if there is no subsequent match, causing
2509 a backtrack to the verb, a failure is forced. That is, backtracking
2510 cannot pass to the left of the verb. However, when one of these verbs
2511 appears inside an atomic group, its effect is confined to that group,
2512 because once the group has been matched, there is never any backtrack‐
2513 ing into it. In this situation, backtracking can "jump back" to the
2514 left of the entire atomic group. (Remember also, as stated above, that
2515 this localization also applies in subroutine calls and assertions.)
2516 These verbs differ in exactly what kind of failure occurs when back‐
2518 tracking reaches them.
2519 (*COMMIT)
2521 This verb, which may not be followed by a name, causes the whole match
2523 to fail outright if the rest of the pattern does not match. Even if the
2524 pattern is unanchored, no further attempts to find a match by advancing
2525 the starting point take place. Once (*COMMIT) has been passed,
2526 pcre_exec() is committed to finding a match at the current starting
2527 point, or not at all. For example:
2528 a+(*COMMIT)b
2530 This matches "xxaab" but not "aacaab". It can be thought of as a kind
2532 of dynamic anchor, or "I've started, so I must finish." The name of the
2533 most recently passed (*MARK) in the path is passed back when (*COMMIT)
2534 forces a match failure.
2535 Note that (*COMMIT) at the start of a pattern is not the same as an
2537 anchor, unless PCRE's start-of-match optimizations are turned off, as
2538 shown in this pcretest example:
2539 re> /(*COMMIT)abc/
2541 data> xyzabc
2542 0: abc
2543 xyzabc\Y
2544 No match
2545 PCRE knows that any match must start with "a", so the optimization
2547 skips along the subject to "a" before running the first match attempt,
2548 which succeeds. When the optimization is disabled by the \Y escape in
2549 the second subject, the match starts at "x" and so the (*COMMIT) causes
2550 it to fail without trying any other starting points.
2551 (*PRUNE) or (*PRUNE:NAME)
2553 This verb causes the match to fail at the current starting position in
2555 the subject if the rest of the pattern does not match. If the pattern
2556 is unanchored, the normal "bumpalong" advance to the next starting
2557 character then happens. Backtracking can occur as usual to the left of
2558 (*PRUNE), before it is reached, or when matching to the right of
2559 (*PRUNE), but if there is no match to the right, backtracking cannot
2560 cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter‐
2561 native to an atomic group or possessive quantifier, but there are some
2562 uses of (*PRUNE) that cannot be expressed in any other way. The behav‐
2563 iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2564 anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2565 (*SKIP)
2567 This verb, when given without a name, is like (*PRUNE), except that if
2569 the pattern is unanchored, the "bumpalong" advance is not to the next
2570 character, but to the position in the subject where (*SKIP) was encoun‐
2571 tered. (*SKIP) signifies that whatever text was matched leading up to
2572 it cannot be part of a successful match. Consider:
2573 a+(*SKIP)b
2575 If the subject is "aaaac...", after the first match attempt fails
2577 (starting at the first character in the string), the starting point
2578 skips on to start the next attempt at "c". Note that a possessive quan‐
2579 tifer does not have the same effect as this example; although it would
2580 suppress backtracking during the first match attempt, the second
2581 attempt would start at the second character instead of skipping on to
2582 "c".
2583 (*SKIP:NAME)
2585 When (*SKIP) has an associated name, its behaviour is modified. If the
2587 following pattern fails to match, the previous path through the pattern
2588 is searched for the most recent (*MARK) that has the same name. If one
2589 is found, the "bumpalong" advance is to the subject position that cor‐
2590 responds to that (*MARK) instead of to where (*SKIP) was encountered.
2591 If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
2592 (*THEN) or (*THEN:NAME)
2594 This verb causes a skip to the next innermost alternative if the rest
2596 of the pattern does not match. That is, it cancels pending backtrack‐
2597 ing, but only within the current alternative. Its name comes from the
2598 observation that it can be used for a pattern-based if-then-else block:
2599 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2601 If the COND1 pattern matches, FOO is tried (and possibly further items
2603 after the end of the group if FOO succeeds); on failure, the matcher
2604 skips to the second alternative and tries COND2, without backtracking
2605 into COND1. The behaviour of (*THEN:NAME) is exactly the same as
2606 (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts
2607 like (*PRUNE).
2608 Note that a subpattern that does not contain a | character is just a
2610 part of the enclosing alternative; it is not a nested alternation with
2611 only one alternative. The effect of (*THEN) extends beyond such a sub‐
2612 pattern to the enclosing alternative. Consider this pattern, where A,
2613 B, etc. are complex pattern fragments that do not contain any | charac‐
2614 ters at this level:
2615 A (B(*THEN)C) | D
2617 If A and B are matched, but there is a failure in C, matching does not
2619 backtrack into A; instead it moves to the next alternative, that is, D.
2620 However, if the subpattern containing (*THEN) is given an alternative,
2621 it behaves differently:
2622 A (B(*THEN)C | (*FAIL)) | D
2624 The effect of (*THEN) is now confined to the inner subpattern. After a
2626 failure in C, matching moves to (*FAIL), which causes the whole subpat‐
2627 tern to fail because there are no more alternatives to try. In this
2628 case, matching does now backtrack into A.
2629 Note also that a conditional subpattern is not considered as having two
2631 alternatives, because only one is ever used. In other words, the |
2632 character in a conditional subpattern has a different meaning. Ignoring
2633 white space, consider:
2634 ^.*? (?(?=a) a | b(*THEN)c )
2636 If the subject is "ba", this pattern does not match. Because .*? is
2638 ungreedy, it initially matches zero characters. The condition (?=a)
2639 then fails, the character "b" is matched, but "c" is not. At this
2640 point, matching does not backtrack to .*? as might perhaps be expected
2641 from the presence of the | character. The conditional subpattern is
2642 part of the single alternative that comprises the whole pattern, and so
2643 the match fails. (If there was a backtrack into .*?, allowing it to
2644 match "b", the match would succeed.)
2645 The verbs just described provide four different "strengths" of control
2647 when subsequent matching fails. (*THEN) is the weakest, carrying on the
2648 match at the next alternative. (*PRUNE) comes next, failing the match
2649 at the current starting position, but allowing an advance to the next
2650 character (for an unanchored pattern). (*SKIP) is similar, except that
2651 the advance may be more than one character. (*COMMIT) is the strongest,
2652 causing the entire match to fail.
2653 If more than one such verb is present in a pattern, the "strongest" one
2655 wins. For example, consider this pattern, where A, B, etc. are complex
2656 pattern fragments:
2657 (A(*COMMIT)B(*THEN)C|D)
2659 Once A has matched, PCRE is committed to this match, at the current
2661 starting position. If subsequently B matches, but C does not, the nor‐
2662 mal (*THEN) action of trying the next alternative (that is, D) does not
2663 happen because (*COMMIT) overrides.
2664
2666 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
2668 pcre16(3), pcre32(3).
2669
2671 Philip Hazel
2673 University Computing Service
2674 Cambridge CB2 3QH, England.
2675
2677 Last updated: 11 November 2012
2679 Copyright (c) 1997-2012 University of Cambridge.
2680PCRE 8.32 11 November 2012 PCREPATTERN(3)