1PCREPATTERN(3) Library Functions Manual PCREPATTERN(3)
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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. Perl's regular expressions are
13 described in its own documentation, and regular expressions in general
14 are covered in a number of books, some of which have copious examples.
15 Jeffrey Friedl's "Mastering Regular Expressions", published by
16 O'Reilly, covers regular expressions in great detail. This description
17 of PCRE's regular expressions is intended as reference material.
18 The original operation of PCRE was on strings of one-byte characters.
20 However, there is now also support for UTF-8 character strings. To use
21 this, you must build PCRE to include UTF-8 support, and then call
22 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
23 matching is mentioned in several places below. There is also a summary
24 of UTF-8 features in the section on UTF-8 support in the main pcre
25 page.
26 The remainder of this document discusses the patterns that are sup‐
28 ported by PCRE when its main matching function, pcre_exec(), is used.
29 From release 6.0, PCRE offers a second matching function,
30 pcre_dfa_exec(), which matches using a different algorithm that is not
31 Perl-compatible. Some of the features discussed below are not available
32 when pcre_dfa_exec() is used. The advantages and disadvantages of the
33 alternative function, and how it differs from the normal function, are
34 discussed in the pcrematching page.
35
37 PCRE supports five different conventions for indicating line breaks in
39 strings: a single CR (carriage return) character, a single LF (line‐
40 feed) character, the two-character sequence CRLF, any of the three pre‐
41 ceding, or any Unicode newline sequence. The pcreapi page has further
42 discussion about newlines, and shows how to set the newline convention
43 in the options arguments for the compiling and matching functions.
44 It is also possible to specify a newline convention by starting a pat‐
46 tern string with one of the following five sequences:
47 (*CR) carriage return
49 (*LF) linefeed
50 (*CRLF) carriage return, followed by linefeed
51 (*ANYCRLF) any of the three above
52 (*ANY) all Unicode newline sequences
53 These override the default and the options given to pcre_compile(). For
55 example, on a Unix system where LF is the default newline sequence, the
56 pattern
57 (*CR)a.b
59 changes the convention to CR. That pattern matches "a\nb" because LF is
61 no longer a newline. Note that these special settings, which are not
62 Perl-compatible, are recognized only at the very start of a pattern,
63 and that they must be in upper case.
64
66 A regular expression is a pattern that is matched against a subject
68 string from left to right. Most characters stand for themselves in a
69 pattern, and match the corresponding characters in the subject. As a
70 trivial example, the pattern
71 The quick brown fox
73 matches a portion of a subject string that is identical to itself. When
75 caseless matching is specified (the PCRE_CASELESS option), letters are
76 matched independently of case. In UTF-8 mode, PCRE always understands
77 the concept of case for characters whose values are less than 128, so
78 caseless matching is always possible. For characters with higher val‐
79 ues, the concept of case is supported if PCRE is compiled with Unicode
80 property support, but not otherwise. If you want to use caseless
81 matching for characters 128 and above, you must ensure that PCRE is
82 compiled with Unicode property support as well as with UTF-8 support.
83 The power of regular expressions comes from the ability to include
85 alternatives and repetitions in the pattern. These are encoded in the
86 pattern by the use of metacharacters, which do not stand for themselves
87 but instead are interpreted in some special way.
88 There are two different sets of metacharacters: those that are recog‐
90 nized anywhere in the pattern except within square brackets, and those
91 that are recognized within square brackets. Outside square brackets,
92 the metacharacters are as follows:
93 \ general escape character with several uses
95 ^ assert start of string (or line, in multiline mode)
96 $ assert end of string (or line, in multiline mode)
97 . match any character except newline (by default)
98 [ start character class definition
99 | start of alternative branch
100 ( start subpattern
101 ) end subpattern
102 ? extends the meaning of (
103 also 0 or 1 quantifier
104 also quantifier minimizer
105 * 0 or more quantifier
106 + 1 or more quantifier
107 also "possessive quantifier"
108 { start min/max quantifier
109 Part of a pattern that is in square brackets is called a "character
111 class". In a character class the only metacharacters are:
112 \ general escape character
114 ^ negate the class, but only if the first character
115 - indicates character range
116 [ POSIX character class (only if followed by POSIX
117 syntax)
118 ] terminates the character class
119 The following sections describe the use of each of the metacharacters.
121
123 The backslash character has several uses. Firstly, if it is followed by
125 a non-alphanumeric character, it takes away any special meaning that
126 character may have. This use of backslash as an escape character
127 applies both inside and outside character classes.
128 For example, if you want to match a * character, you write \* in the
130 pattern. This escaping action applies whether or not the following
131 character would otherwise be interpreted as a metacharacter, so it is
132 always safe to precede a non-alphanumeric with backslash to specify
133 that it stands for itself. In particular, if you want to match a back‐
134 slash, you write \\.
135 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
137 the pattern (other than in a character class) and characters between a
138 # outside a character class and the next newline are ignored. An escap‐
139 ing backslash can be used to include a whitespace or # character as
140 part of the pattern.
141 If you want to remove the special meaning from a sequence of charac‐
143 ters, you can do so by putting them between \Q and \E. This is differ‐
144 ent from Perl in that $ and @ are handled as literals in \Q...\E
145 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola‐
146 tion. Note the following examples:
147 Pattern PCRE matches Perl matches
149 \Qabc$xyz\E abc$xyz abc followed by the
151 contents of $xyz
152 \Qabc\$xyz\E abc\$xyz abc\$xyz
153 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
154 The \Q...\E sequence is recognized both inside and outside character
156 classes.
157 Non-printing characters
159 A second use of backslash provides a way of encoding non-printing char‐
161 acters in patterns in a visible manner. There is no restriction on the
162 appearance of non-printing characters, apart from the binary zero that
163 terminates a pattern, but when a pattern is being prepared by text
164 editing, it is usually easier to use one of the following escape
165 sequences than the binary character it represents:
166 \a alarm, that is, the BEL character (hex 07)
168 \cx "control-x", where x is any character
169 \e escape (hex 1B)
170 \f formfeed (hex 0C)
171 \n linefeed (hex 0A)
172 \r carriage return (hex 0D)
173 \t tab (hex 09)
174 \ddd character with octal code ddd, or backreference
175 \xhh character with hex code hh
176 \x{hhh..} character with hex code hhh..
177 The precise effect of \cx is as follows: if x is a lower case letter,
179 it is converted to upper case. Then bit 6 of the character (hex 40) is
180 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
181 becomes hex 7B.
182 After \x, from zero to two hexadecimal digits are read (letters can be
184 in upper or lower case). Any number of hexadecimal digits may appear
185 between \x{ and }, but the value of the character code must be less
186 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
187 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
188 than the largest Unicode code point, which is 10FFFF.
189 If characters other than hexadecimal digits appear between \x{ and },
191 or if there is no terminating }, this form of escape is not recognized.
192 Instead, the initial \x will be interpreted as a basic hexadecimal
193 escape, with no following digits, giving a character whose value is
194 zero.
195 Characters whose value is less than 256 can be defined by either of the
197 two syntaxes for \x. There is no difference in the way they are han‐
198 dled. For example, \xdc is exactly the same as \x{dc}.
199 After \0 up to two further octal digits are read. If there are fewer
201 than two digits, just those that are present are used. Thus the
202 sequence \0\x\07 specifies two binary zeros followed by a BEL character
203 (code value 7). Make sure you supply two digits after the initial zero
204 if the pattern character that follows is itself an octal digit.
205 The handling of a backslash followed by a digit other than 0 is compli‐
207 cated. Outside a character class, PCRE reads it and any following dig‐
208 its as a decimal number. If the number is less than 10, or if there
209 have been at least that many previous capturing left parentheses in the
210 expression, the entire sequence is taken as a back reference. A
211 description of how this works is given later, following the discussion
212 of parenthesized subpatterns.
213 Inside a character class, or if the decimal number is greater than 9
215 and there have not been that many capturing subpatterns, PCRE re-reads
216 up to three octal digits following the backslash, and uses them to gen‐
217 erate a data character. Any subsequent digits stand for themselves. In
218 non-UTF-8 mode, the value of a character specified in octal must be
219 less than \400. In UTF-8 mode, values up to \777 are permitted. For
220 example:
221 \040 is another way of writing a space
223 \40 is the same, provided there are fewer than 40
224 previous capturing subpatterns
225 \7 is always a back reference
226 \11 might be a back reference, or another way of
227 writing a tab
228 \011 is always a tab
229 \0113 is a tab followed by the character "3"
230 \113 might be a back reference, otherwise the
231 character with octal code 113
232 \377 might be a back reference, otherwise
233 the byte consisting entirely of 1 bits
234 \81 is either a back reference, or a binary zero
235 followed by the two characters "8" and "1"
236 Note that octal values of 100 or greater must not be introduced by a
238 leading zero, because no more than three octal digits are ever read.
239 All the sequences that define a single character value can be used both
241 inside and outside character classes. In addition, inside a character
242 class, the sequence \b is interpreted as the backspace character (hex
243 08), and the sequences \R and \X are interpreted as the characters "R"
244 and "X", respectively. Outside a character class, these sequences have
245 different meanings (see below).
246 Absolute and relative back references
248 The sequence \g followed by an unsigned or a negative number, option‐
250 ally enclosed in braces, is an absolute or relative back reference. A
251 named back reference can be coded as \g{name}. Back references are dis‐
252 cussed later, following the discussion of parenthesized subpatterns.
253 Generic character types
255 Another use of backslash is for specifying generic character types. The
257 following are always recognized:
258 \d any decimal digit
260 \D any character that is not a decimal digit
261 \h any horizontal whitespace character
262 \H any character that is not a horizontal whitespace character
263 \s any whitespace character
264 \S any character that is not a whitespace character
265 \v any vertical whitespace character
266 \V any character that is not a vertical whitespace character
267 \w any "word" character
268 \W any "non-word" character
269 Each pair of escape sequences partitions the complete set of characters
271 into two disjoint sets. Any given character matches one, and only one,
272 of each pair.
273 These character type sequences can appear both inside and outside char‐
275 acter classes. They each match one character of the appropriate type.
276 If the current matching point is at the end of the subject string, all
277 of them fail, since there is no character to match.
278 For compatibility with Perl, \s does not match the VT character (code
280 11). This makes it different from the the POSIX "space" class. The \s
281 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
282 "use locale;" is included in a Perl script, \s may match the VT charac‐
283 ter. In PCRE, it never does.
284 In UTF-8 mode, characters with values greater than 128 never match \d,
286 \s, or \w, and always match \D, \S, and \W. This is true even when Uni‐
287 code character property support is available. These sequences retain
288 their original meanings from before UTF-8 support was available, mainly
289 for efficiency reasons.
290 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
292 the other sequences, these do match certain high-valued codepoints in
293 UTF-8 mode. The horizontal space characters are:
294 U+0009 Horizontal tab
296 U+0020 Space
297 U+00A0 Non-break space
298 U+1680 Ogham space mark
299 U+180E Mongolian vowel separator
300 U+2000 En quad
301 U+2001 Em quad
302 U+2002 En space
303 U+2003 Em space
304 U+2004 Three-per-em space
305 U+2005 Four-per-em space
306 U+2006 Six-per-em space
307 U+2007 Figure space
308 U+2008 Punctuation space
309 U+2009 Thin space
310 U+200A Hair space
311 U+202F Narrow no-break space
312 U+205F Medium mathematical space
313 U+3000 Ideographic space
314 The vertical space characters are:
316 U+000A Linefeed
318 U+000B Vertical tab
319 U+000C Formfeed
320 U+000D Carriage return
321 U+0085 Next line
322 U+2028 Line separator
323 U+2029 Paragraph separator
324 A "word" character is an underscore or any character less than 256 that
326 is a letter or digit. The definition of letters and digits is con‐
327 trolled by PCRE's low-valued character tables, and may vary if locale-
328 specific matching is taking place (see "Locale support" in the pcreapi
329 page). For example, in a French locale such as "fr_FR" in Unix-like
330 systems, or "french" in Windows, some character codes greater than 128
331 are used for accented letters, and these are matched by \w. The use of
332 locales with Unicode is discouraged.
333 Newline sequences
335 Outside a character class, the escape sequence \R matches any Unicode
337 newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
338 equivalent to the following:
339 (?>\r\n|\n|\x0b|\f|\r|\x85)
341 This is an example of an "atomic group", details of which are given
343 below. This particular group matches either the two-character sequence
344 CR followed by LF, or one of the single characters LF (linefeed,
345 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
346 return, U+000D), or NEL (next line, U+0085). The two-character sequence
347 is treated as a single unit that cannot be split.
348 In UTF-8 mode, two additional characters whose codepoints are greater
350 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa‐
351 rator, U+2029). Unicode character property support is not needed for
352 these characters to be recognized.
353 Inside a character class, \R matches the letter "R".
355 Unicode character properties
357 When PCRE is built with Unicode character property support, three addi‐
359 tional escape sequences that match characters with specific properties
360 are available. When not in UTF-8 mode, these sequences are of course
361 limited to testing characters whose codepoints are less than 256, but
362 they do work in this mode. The extra escape sequences are:
363 \p{xx} a character with the xx property
365 \P{xx} a character without the xx property
366 \X an extended Unicode sequence
367 The property names represented by xx above are limited to the Unicode
369 script names, the general category properties, and "Any", which matches
370 any character (including newline). Other properties such as "InMusical‐
371 Symbols" are not currently supported by PCRE. Note that \P{Any} does
372 not match any characters, so always causes a match failure.
373 Sets of Unicode characters are defined as belonging to certain scripts.
375 A character from one of these sets can be matched using a script name.
376 For example:
377 \p{Greek}
379 \P{Han}
380 Those that are not part of an identified script are lumped together as
382 "Common". The current list of scripts is:
383 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
385 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
386 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
387 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira‐
388 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
389 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
390 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
391 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
392 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
393 Each character has exactly one general category property, specified by
395 a two-letter abbreviation. For compatibility with Perl, negation can be
396 specified by including a circumflex between the opening brace and the
397 property name. For example, \p{^Lu} is the same as \P{Lu}.
398 If only one letter is specified with \p or \P, it includes all the gen‐
400 eral category properties that start with that letter. In this case, in
401 the absence of negation, the curly brackets in the escape sequence are
402 optional; these two examples have the same effect:
403 \p{L}
405 \pL
406 The following general category property codes are supported:
408 C Other
410 Cc Control
411 Cf Format
412 Cn Unassigned
413 Co Private use
414 Cs Surrogate
415 L Letter
417 Ll Lower case letter
418 Lm Modifier letter
419 Lo Other letter
420 Lt Title case letter
421 Lu Upper case letter
422 M Mark
424 Mc Spacing mark
425 Me Enclosing mark
426 Mn Non-spacing mark
427 N Number
429 Nd Decimal number
430 Nl Letter number
431 No Other number
432 P Punctuation
434 Pc Connector punctuation
435 Pd Dash punctuation
436 Pe Close punctuation
437 Pf Final punctuation
438 Pi Initial punctuation
439 Po Other punctuation
440 Ps Open punctuation
441 S Symbol
443 Sc Currency symbol
444 Sk Modifier symbol
445 Sm Mathematical symbol
446 So Other symbol
447 Z Separator
449 Zl Line separator
450 Zp Paragraph separator
451 Zs Space separator
452 The special property L& is also supported: it matches a character that
454 has the Lu, Ll, or Lt property, in other words, a letter that is not
455 classified as a modifier or "other".
456 The Cs (Surrogate) property applies only to characters in the range
458 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
459 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check‐
460 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
461 the pcreapi page).
462 The long synonyms for these properties that Perl supports (such as
464 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
465 any of these properties with "Is".
466 No character that is in the Unicode table has the Cn (unassigned) prop‐
468 erty. Instead, this property is assumed for any code point that is not
469 in the Unicode table.
470 Specifying caseless matching does not affect these escape sequences.
472 For example, \p{Lu} always matches only upper case letters.
473 The \X escape matches any number of Unicode characters that form an
475 extended Unicode sequence. \X is equivalent to
476 (?>\PM\pM*)
478 That is, it matches a character without the "mark" property, followed
480 by zero or more characters with the "mark" property, and treats the
481 sequence as an atomic group (see below). Characters with the "mark"
482 property are typically accents that affect the preceding character.
483 None of them have codepoints less than 256, so in non-UTF-8 mode \X
484 matches any one character.
485 Matching characters by Unicode property is not fast, because PCRE has
487 to search a structure that contains data for over fifteen thousand
488 characters. That is why the traditional escape sequences such as \d and
489 \w do not use Unicode properties in PCRE.
490 Resetting the match start
492 The escape sequence \K, which is a Perl 5.10 feature, causes any previ‐
494 ously matched characters not to be included in the final matched
495 sequence. For example, the pattern:
496 foo\Kbar
498 matches "foobar", but reports that it has matched "bar". This feature
500 is similar to a lookbehind assertion (described below). However, in
501 this case, the part of the subject before the real match does not have
502 to be of fixed length, as lookbehind assertions do. The use of \K does
503 not interfere with the setting of captured substrings. For example,
504 when the pattern
505 (foo)\Kbar
507 matches "foobar", the first substring is still set to "foo".
509 Simple assertions
511 The final use of backslash is for certain simple assertions. An asser‐
513 tion specifies a condition that has to be met at a particular point in
514 a match, without consuming any characters from the subject string. The
515 use of subpatterns for more complicated assertions is described below.
516 The backslashed assertions are:
517 \b matches at a word boundary
519 \B matches when not at a word boundary
520 \A matches at the start of the subject
521 \Z matches at the end of the subject
522 also matches before a newline at the end of the subject
523 \z matches only at the end of the subject
524 \G matches at the first matching position in the subject
525 These assertions may not appear in character classes (but note that \b
527 has a different meaning, namely the backspace character, inside a char‐
528 acter class).
529 A word boundary is a position in the subject string where the current
531 character and the previous character do not both match \w or \W (i.e.
532 one matches \w and the other matches \W), or the start or end of the
533 string if the first or last character matches \w, respectively.
534 The \A, \Z, and \z assertions differ from the traditional circumflex
536 and dollar (described in the next section) in that they only ever match
537 at the very start and end of the subject string, whatever options are
538 set. Thus, they are independent of multiline mode. These three asser‐
539 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
540 affect only the behaviour of the circumflex and dollar metacharacters.
541 However, if the startoffset argument of pcre_exec() is non-zero, indi‐
542 cating that matching is to start at a point other than the beginning of
543 the subject, \A can never match. The difference between \Z and \z is
544 that \Z matches before a newline at the end of the string as well as at
545 the very end, whereas \z matches only at the end.
546 The \G assertion is true only when the current matching position is at
548 the start point of the match, as specified by the startoffset argument
549 of pcre_exec(). It differs from \A when the value of startoffset is
550 non-zero. By calling pcre_exec() multiple times with appropriate argu‐
551 ments, you can mimic Perl's /g option, and it is in this kind of imple‐
552 mentation where \G can be useful.
553 Note, however, that PCRE's interpretation of \G, as the start of the
555 current match, is subtly different from Perl's, which defines it as the
556 end of the previous match. In Perl, these can be different when the
557 previously matched string was empty. Because PCRE does just one match
558 at a time, it cannot reproduce this behaviour.
559 If all the alternatives of a pattern begin with \G, the expression is
561 anchored to the starting match position, and the "anchored" flag is set
562 in the compiled regular expression.
563
565 Outside a character class, in the default matching mode, the circumflex
567 character is an assertion that is true only if the current matching
568 point is at the start of the subject string. If the startoffset argu‐
569 ment of pcre_exec() is non-zero, circumflex can never match if the
570 PCRE_MULTILINE option is unset. Inside a character class, circumflex
571 has an entirely different meaning (see below).
572 Circumflex need not be the first character of the pattern if a number
574 of alternatives are involved, but it should be the first thing in each
575 alternative in which it appears if the pattern is ever to match that
576 branch. If all possible alternatives start with a circumflex, that is,
577 if the pattern is constrained to match only at the start of the sub‐
578 ject, it is said to be an "anchored" pattern. (There are also other
579 constructs that can cause a pattern to be anchored.)
580 A dollar character is an assertion that is true only if the current
582 matching point is at the end of the subject string, or immediately
583 before a newline at the end of the string (by default). Dollar need not
584 be the last character of the pattern if a number of alternatives are
585 involved, but it should be the last item in any branch in which it
586 appears. Dollar has no special meaning in a character class.
587 The meaning of dollar can be changed so that it matches only at the
589 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
590 compile time. This does not affect the \Z assertion.
591 The meanings of the circumflex and dollar characters are changed if the
593 PCRE_MULTILINE option is set. When this is the case, a circumflex
594 matches immediately after internal newlines as well as at the start of
595 the subject string. It does not match after a newline that ends the
596 string. A dollar matches before any newlines in the string, as well as
597 at the very end, when PCRE_MULTILINE is set. When newline is specified
598 as the two-character sequence CRLF, isolated CR and LF characters do
599 not indicate newlines.
600 For example, the pattern /^abc$/ matches the subject string "def\nabc"
602 (where \n represents a newline) in multiline mode, but not otherwise.
603 Consequently, patterns that are anchored in single line mode because
604 all branches start with ^ are not anchored in multiline mode, and a
605 match for circumflex is possible when the startoffset argument of
606 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
607 PCRE_MULTILINE is set.
608 Note that the sequences \A, \Z, and \z can be used to match the start
610 and end of the subject in both modes, and if all branches of a pattern
611 start with \A it is always anchored, whether or not PCRE_MULTILINE is
612 set.
613
615 Outside a character class, a dot in the pattern matches any one charac‐
617 ter in the subject string except (by default) a character that signi‐
618 fies the end of a line. In UTF-8 mode, the matched character may be
619 more than one byte long.
620 When a line ending is defined as a single character, dot never matches
622 that character; when the two-character sequence CRLF is used, dot does
623 not match CR if it is immediately followed by LF, but otherwise it
624 matches all characters (including isolated CRs and LFs). When any Uni‐
625 code line endings are being recognized, dot does not match CR or LF or
626 any of the other line ending characters.
627 The behaviour of dot with regard to newlines can be changed. If the
629 PCRE_DOTALL option is set, a dot matches any one character, without
630 exception. If the two-character sequence CRLF is present in the subject
631 string, it takes two dots to match it.
632 The handling of dot is entirely independent of the handling of circum‐
634 flex and dollar, the only relationship being that they both involve
635 newlines. Dot has no special meaning in a character class.
636
638 Outside a character class, the escape sequence \C matches any one byte,
640 both in and out of UTF-8 mode. Unlike a dot, it always matches any
641 line-ending characters. The feature is provided in Perl in order to
642 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char‐
643 acters into individual bytes, what remains in the string may be a mal‐
644 formed UTF-8 string. For this reason, the \C escape sequence is best
645 avoided.
646 PCRE does not allow \C to appear in lookbehind assertions (described
648 below), because in UTF-8 mode this would make it impossible to calcu‐
649 late the length of the lookbehind.
650
652 An opening square bracket introduces a character class, terminated by a
654 closing square bracket. A closing square bracket on its own is not spe‐
655 cial. If a closing square bracket is required as a member of the class,
656 it should be the first data character in the class (after an initial
657 circumflex, if present) or escaped with a backslash.
658 A character class matches a single character in the subject. In UTF-8
660 mode, the character may occupy more than one byte. A matched character
661 must be in the set of characters defined by the class, unless the first
662 character in the class definition is a circumflex, in which case the
663 subject character must not be in the set defined by the class. If a
664 circumflex is actually required as a member of the class, ensure it is
665 not the first character, or escape it with a backslash.
666 For example, the character class [aeiou] matches any lower case vowel,
668 while [^aeiou] matches any character that is not a lower case vowel.
669 Note that a circumflex is just a convenient notation for specifying the
670 characters that are in the class by enumerating those that are not. A
671 class that starts with a circumflex is not an assertion: it still con‐
672 sumes a character from the subject string, and therefore it fails if
673 the current pointer is at the end of the string.
674 In UTF-8 mode, characters with values greater than 255 can be included
676 in a class as a literal string of bytes, or by using the \x{ escaping
677 mechanism.
678 When caseless matching is set, any letters in a class represent both
680 their upper case and lower case versions, so for example, a caseless
681 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
682 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
683 understands the concept of case for characters whose values are less
684 than 128, so caseless matching is always possible. For characters with
685 higher values, the concept of case is supported if PCRE is compiled
686 with Unicode property support, but not otherwise. If you want to use
687 caseless matching for characters 128 and above, you must ensure that
688 PCRE is compiled with Unicode property support as well as with UTF-8
689 support.
690 Characters that might indicate line breaks are never treated in any
692 special way when matching character classes, whatever line-ending
693 sequence is in use, and whatever setting of the PCRE_DOTALL and
694 PCRE_MULTILINE options is used. A class such as [^a] always matches one
695 of these characters.
696 The minus (hyphen) character can be used to specify a range of charac‐
698 ters in a character class. For example, [d-m] matches any letter
699 between d and m, inclusive. If a minus character is required in a
700 class, it must be escaped with a backslash or appear in a position
701 where it cannot be interpreted as indicating a range, typically as the
702 first or last character in the class.
703 It is not possible to have the literal character "]" as the end charac‐
705 ter of a range. A pattern such as [W-]46] is interpreted as a class of
706 two characters ("W" and "-") followed by a literal string "46]", so it
707 would match "W46]" or "-46]". However, if the "]" is escaped with a
708 backslash it is interpreted as the end of range, so [W-\]46] is inter‐
709 preted as a class containing a range followed by two other characters.
710 The octal or hexadecimal representation of "]" can also be used to end
711 a range.
712 Ranges operate in the collating sequence of character values. They can
714 also be used for characters specified numerically, for example
715 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
716 are greater than 255, for example [\x{100}-\x{2ff}].
717 If a range that includes letters is used when caseless matching is set,
719 it matches the letters in either case. For example, [W-c] is equivalent
720 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
721 character tables for a French locale are in use, [\xc8-\xcb] matches
722 accented E characters in both cases. In UTF-8 mode, PCRE supports the
723 concept of case for characters with values greater than 128 only when
724 it is compiled with Unicode property support.
725 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
727 in a character class, and add the characters that they match to the
728 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum‐
729 flex can conveniently be used with the upper case character types to
730 specify a more restricted set of characters than the matching lower
731 case type. For example, the class [^\W_] matches any letter or digit,
732 but not underscore.
733 The only metacharacters that are recognized in character classes are
735 backslash, hyphen (only where it can be interpreted as specifying a
736 range), circumflex (only at the start), opening square bracket (only
737 when it can be interpreted as introducing a POSIX class name - see the
738 next section), and the terminating closing square bracket. However,
739 escaping other non-alphanumeric characters does no harm.
740
742 Perl supports the POSIX notation for character classes. This uses names
744 enclosed by [: and :] within the enclosing square brackets. PCRE also
745 supports this notation. For example,
746 [01[:alpha:]%]
748 matches "0", "1", any alphabetic character, or "%". The supported class
750 names are
751 alnum letters and digits
753 alpha letters
754 ascii character codes 0 - 127
755 blank space or tab only
756 cntrl control characters
757 digit decimal digits (same as \d)
758 graph printing characters, excluding space
759 lower lower case letters
760 print printing characters, including space
761 punct printing characters, excluding letters and digits
762 space white space (not quite the same as \s)
763 upper upper case letters
764 word "word" characters (same as \w)
765 xdigit hexadecimal digits
766 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
768 and space (32). Notice that this list includes the VT character (code
769 11). This makes "space" different to \s, which does not include VT (for
770 Perl compatibility).
771 The name "word" is a Perl extension, and "blank" is a GNU extension
773 from Perl 5.8. Another Perl extension is negation, which is indicated
774 by a ^ character after the colon. For example,
775 [12[:^digit:]]
777 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
779 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
780 these are not supported, and an error is given if they are encountered.
781 In UTF-8 mode, characters with values greater than 128 do not match any
783 of the POSIX character classes.
784
786 Vertical bar characters are used to separate alternative patterns. For
788 example, the pattern
789 gilbert|sullivan
791 matches either "gilbert" or "sullivan". Any number of alternatives may
793 appear, and an empty alternative is permitted (matching the empty
794 string). The matching process tries each alternative in turn, from left
795 to right, and the first one that succeeds is used. If the alternatives
796 are within a subpattern (defined below), "succeeds" means matching the
797 rest of the main pattern as well as the alternative in the subpattern.
798
800 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
802 PCRE_EXTENDED options can be changed from within the pattern by a
803 sequence of Perl option letters enclosed between "(?" and ")". The
804 option letters are
805 i for PCRE_CASELESS
807 m for PCRE_MULTILINE
808 s for PCRE_DOTALL
809 x for PCRE_EXTENDED
810 For example, (?im) sets caseless, multiline matching. It is also possi‐
812 ble to unset these options by preceding the letter with a hyphen, and a
813 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE‐
814 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
815 is also permitted. If a letter appears both before and after the
816 hyphen, the option is unset.
817 When an option change occurs at top level (that is, not inside subpat‐
819 tern parentheses), the change applies to the remainder of the pattern
820 that follows. If the change is placed right at the start of a pattern,
821 PCRE extracts it into the global options (and it will therefore show up
822 in data extracted by the pcre_fullinfo() function).
823 An option change within a subpattern (see below for a description of
825 subpatterns) affects only that part of the current pattern that follows
826 it, so
827 (a(?i)b)c
829 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
831 used). By this means, options can be made to have different settings
832 in different parts of the pattern. Any changes made in one alternative
833 do carry on into subsequent branches within the same subpattern. For
834 example,
835 (a(?i)b|c)
837 matches "ab", "aB", "c", and "C", even though when matching "C" the
839 first branch is abandoned before the option setting. This is because
840 the effects of option settings happen at compile time. There would be
841 some very weird behaviour otherwise.
842 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
844 can be changed in the same way as the Perl-compatible options by using
845 the characters J, U and X respectively.
846
848 Subpatterns are delimited by parentheses (round brackets), which can be
850 nested. Turning part of a pattern into a subpattern does two things:
851 1. It localizes a set of alternatives. For example, the pattern
853 cat(aract|erpillar|)
855 matches one of the words "cat", "cataract", or "caterpillar". Without
857 the parentheses, it would match "cataract", "erpillar" or an empty
858 string.
859 2. It sets up the subpattern as a capturing subpattern. This means
861 that, when the whole pattern matches, that portion of the subject
862 string that matched the subpattern is passed back to the caller via the
863 ovector argument of pcre_exec(). Opening parentheses are counted from
864 left to right (starting from 1) to obtain numbers for the capturing
865 subpatterns.
866 For example, if the string "the red king" is matched against the pat‐
868 tern
869 the ((red|white) (king|queen))
871 the captured substrings are "red king", "red", and "king", and are num‐
873 bered 1, 2, and 3, respectively.
874 The fact that plain parentheses fulfil two functions is not always
876 helpful. There are often times when a grouping subpattern is required
877 without a capturing requirement. If an opening parenthesis is followed
878 by a question mark and a colon, the subpattern does not do any captur‐
879 ing, and is not counted when computing the number of any subsequent
880 capturing subpatterns. For example, if the string "the white queen" is
881 matched against the pattern
882 the ((?:red|white) (king|queen))
884 the captured substrings are "white queen" and "queen", and are numbered
886 1 and 2. The maximum number of capturing subpatterns is 65535.
887 As a convenient shorthand, if any option settings are required at the
889 start of a non-capturing subpattern, the option letters may appear
890 between the "?" and the ":". Thus the two patterns
891 (?i:saturday|sunday)
893 (?:(?i)saturday|sunday)
894 match exactly the same set of strings. Because alternative branches are
896 tried from left to right, and options are not reset until the end of
897 the subpattern is reached, an option setting in one branch does affect
898 subsequent branches, so the above patterns match "SUNDAY" as well as
899 "Saturday".
900
902 Perl 5.10 introduced a feature whereby each alternative in a subpattern
904 uses the same numbers for its capturing parentheses. Such a subpattern
905 starts with (?| and is itself a non-capturing subpattern. For example,
906 consider this pattern:
907 (?|(Sat)ur|(Sun))day
909 Because the two alternatives are inside a (?| group, both sets of cap‐
911 turing parentheses are numbered one. Thus, when the pattern matches,
912 you can look at captured substring number one, whichever alternative
913 matched. This construct is useful when you want to capture part, but
914 not all, of one of a number of alternatives. Inside a (?| group, paren‐
915 theses are numbered as usual, but the number is reset at the start of
916 each branch. The numbers of any capturing buffers that follow the sub‐
917 pattern start after the highest number used in any branch. The follow‐
918 ing example is taken from the Perl documentation. The numbers under‐
919 neath show in which buffer the captured content will be stored.
920 # before ---------------branch-reset----------- after
922 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
923 # 1 2 2 3 2 3 4
924 A backreference or a recursive call to a numbered subpattern always
926 refers to the first one in the pattern with the given number.
927 An alternative approach to using this "branch reset" feature is to use
929 duplicate named subpatterns, as described in the next section.
930
932 Identifying capturing parentheses by number is simple, but it can be
934 very hard to keep track of the numbers in complicated regular expres‐
935 sions. Furthermore, if an expression is modified, the numbers may
936 change. To help with this difficulty, PCRE supports the naming of sub‐
937 patterns. This feature was not added to Perl until release 5.10. Python
938 had the feature earlier, and PCRE introduced it at release 4.0, using
939 the Python syntax. PCRE now supports both the Perl and the Python syn‐
940 tax.
941 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
943 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
944 to capturing parentheses from other parts of the pattern, such as back‐
945 references, recursion, and conditions, can be made by name as well as
946 by number.
947 Names consist of up to 32 alphanumeric characters and underscores.
949 Named capturing parentheses are still allocated numbers as well as
950 names, exactly as if the names were not present. The PCRE API provides
951 function calls for extracting the name-to-number translation table from
952 a compiled pattern. There is also a convenience function for extracting
953 a captured substring by name.
954 By default, a name must be unique within a pattern, but it is possible
956 to relax this constraint by setting the PCRE_DUPNAMES option at compile
957 time. This can be useful for patterns where only one instance of the
958 named parentheses can match. Suppose you want to match the name of a
959 weekday, either as a 3-letter abbreviation or as the full name, and in
960 both cases you want to extract the abbreviation. This pattern (ignoring
961 the line breaks) does the job:
962 (?<DN>Mon|Fri|Sun)(?:day)?|
964 (?<DN>Tue)(?:sday)?|
965 (?<DN>Wed)(?:nesday)?|
966 (?<DN>Thu)(?:rsday)?|
967 (?<DN>Sat)(?:urday)?
968 There are five capturing substrings, but only one is ever set after a
970 match. (An alternative way of solving this problem is to use a "branch
971 reset" subpattern, as described in the previous section.)
972 The convenience function for extracting the data by name returns the
974 substring for the first (and in this example, the only) subpattern of
975 that name that matched. This saves searching to find which numbered
976 subpattern it was. If you make a reference to a non-unique named sub‐
977 pattern from elsewhere in the pattern, the one that corresponds to the
978 lowest number is used. For further details of the interfaces for han‐
979 dling named subpatterns, see the pcreapi documentation.
980
982 Repetition is specified by quantifiers, which can follow any of the
984 following items:
985 a literal data character
987 the dot metacharacter
988 the \C escape sequence
989 the \X escape sequence (in UTF-8 mode with Unicode properties)
990 the \R escape sequence
991 an escape such as \d that matches a single character
992 a character class
993 a back reference (see next section)
994 a parenthesized subpattern (unless it is an assertion)
995 The general repetition quantifier specifies a minimum and maximum num‐
997 ber of permitted matches, by giving the two numbers in curly brackets
998 (braces), separated by a comma. The numbers must be less than 65536,
999 and the first must be less than or equal to the second. For example:
1000 z{2,4}
1002 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
1004 special character. If the second number is omitted, but the comma is
1005 present, there is no upper limit; if the second number and the comma
1006 are both omitted, the quantifier specifies an exact number of required
1007 matches. Thus
1008 [aeiou]{3,}
1010 matches at least 3 successive vowels, but may match many more, while
1012 \d{8}
1014 matches exactly 8 digits. An opening curly bracket that appears in a
1016 position where a quantifier is not allowed, or one that does not match
1017 the syntax of a quantifier, is taken as a literal character. For exam‐
1018 ple, {,6} is not a quantifier, but a literal string of four characters.
1019 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
1021 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char‐
1022 acters, each of which is represented by a two-byte sequence. Similarly,
1023 when Unicode property support is available, \X{3} matches three Unicode
1024 extended sequences, each of which may be several bytes long (and they
1025 may be of different lengths).
1026 The quantifier {0} is permitted, causing the expression to behave as if
1028 the previous item and the quantifier were not present.
1029 For convenience, the three most common quantifiers have single-charac‐
1031 ter abbreviations:
1032 * is equivalent to {0,}
1034 + is equivalent to {1,}
1035 ? is equivalent to {0,1}
1036 It is possible to construct infinite loops by following a subpattern
1038 that can match no characters with a quantifier that has no upper limit,
1039 for example:
1040 (a?)*
1042 Earlier versions of Perl and PCRE used to give an error at compile time
1044 for such patterns. However, because there are cases where this can be
1045 useful, such patterns are now accepted, but if any repetition of the
1046 subpattern does in fact match no characters, the loop is forcibly bro‐
1047 ken.
1048 By default, the quantifiers are "greedy", that is, they match as much
1050 as possible (up to the maximum number of permitted times), without
1051 causing the rest of the pattern to fail. The classic example of where
1052 this gives problems is in trying to match comments in C programs. These
1053 appear between /* and */ and within the comment, individual * and /
1054 characters may appear. An attempt to match C comments by applying the
1055 pattern
1056 /\*.*\*/
1058 to the string
1060 /* first comment */ not comment /* second comment */
1062 fails, because it matches the entire string owing to the greediness of
1064 the .* item.
1065 However, if a quantifier is followed by a question mark, it ceases to
1067 be greedy, and instead matches the minimum number of times possible, so
1068 the pattern
1069 /\*.*?\*/
1071 does the right thing with the C comments. The meaning of the various
1073 quantifiers is not otherwise changed, just the preferred number of
1074 matches. Do not confuse this use of question mark with its use as a
1075 quantifier in its own right. Because it has two uses, it can sometimes
1076 appear doubled, as in
1077 \d??\d
1079 which matches one digit by preference, but can match two if that is the
1081 only way the rest of the pattern matches.
1082 If the PCRE_UNGREEDY option is set (an option that is not available in
1084 Perl), the quantifiers are not greedy by default, but individual ones
1085 can be made greedy by following them with a question mark. In other
1086 words, it inverts the default behaviour.
1087 When a parenthesized subpattern is quantified with a minimum repeat
1089 count that is greater than 1 or with a limited maximum, more memory is
1090 required for the compiled pattern, in proportion to the size of the
1091 minimum or maximum.
1092 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv‐
1094 alent to Perl's /s) is set, thus allowing the dot to match newlines,
1095 the pattern is implicitly anchored, because whatever follows will be
1096 tried against every character position in the subject string, so there
1097 is no point in retrying the overall match at any position after the
1098 first. PCRE normally treats such a pattern as though it were preceded
1099 by \A.
1100 In cases where it is known that the subject string contains no new‐
1102 lines, it is worth setting PCRE_DOTALL in order to obtain this opti‐
1103 mization, or alternatively using ^ to indicate anchoring explicitly.
1104 However, there is one situation where the optimization cannot be used.
1106 When .* is inside capturing parentheses that are the subject of a
1107 backreference elsewhere in the pattern, a match at the start may fail
1108 where a later one succeeds. Consider, for example:
1109 (.*)abc\1
1111 If the subject is "xyz123abc123" the match point is the fourth charac‐
1113 ter. For this reason, such a pattern is not implicitly anchored.
1114 When a capturing subpattern is repeated, the value captured is the sub‐
1116 string that matched the final iteration. For example, after
1117 (tweedle[dume]{3}\s*)+
1119 has matched "tweedledum tweedledee" the value of the captured substring
1121 is "tweedledee". However, if there are nested capturing subpatterns,
1122 the corresponding captured values may have been set in previous itera‐
1123 tions. For example, after
1124 /(a|(b))+/
1126 matches "aba" the value of the second captured substring is "b".
1128
1130 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1132 repetition, failure of what follows normally causes the repeated item
1133 to be re-evaluated to see if a different number of repeats allows the
1134 rest of the pattern to match. Sometimes it is useful to prevent this,
1135 either to change the nature of the match, or to cause it fail earlier
1136 than it otherwise might, when the author of the pattern knows there is
1137 no point in carrying on.
1138 Consider, for example, the pattern \d+foo when applied to the subject
1140 line
1141 123456bar
1143 After matching all 6 digits and then failing to match "foo", the normal
1145 action of the matcher is to try again with only 5 digits matching the
1146 \d+ item, and then with 4, and so on, before ultimately failing.
1147 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
1148 the means for specifying that once a subpattern has matched, it is not
1149 to be re-evaluated in this way.
1150 If we use atomic grouping for the previous example, the matcher gives
1152 up immediately on failing to match "foo" the first time. The notation
1153 is a kind of special parenthesis, starting with (?> as in this example:
1154 (?>\d+)foo
1156 This kind of parenthesis "locks up" the part of the pattern it con‐
1158 tains once it has matched, and a failure further into the pattern is
1159 prevented from backtracking into it. Backtracking past it to previous
1160 items, however, works as normal.
1161 An alternative description is that a subpattern of this type matches
1163 the string of characters that an identical standalone pattern would
1164 match, if anchored at the current point in the subject string.
1165 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
1167 such as the above example can be thought of as a maximizing repeat that
1168 must swallow everything it can. So, while both \d+ and \d+? are pre‐
1169 pared to adjust the number of digits they match in order to make the
1170 rest of the pattern match, (?>\d+) can only match an entire sequence of
1171 digits.
1172 Atomic groups in general can of course contain arbitrarily complicated
1174 subpatterns, and can be nested. However, when the subpattern for an
1175 atomic group is just a single repeated item, as in the example above, a
1176 simpler notation, called a "possessive quantifier" can be used. This
1177 consists of an additional + character following a quantifier. Using
1178 this notation, the previous example can be rewritten as
1179 \d++foo
1181 Note that a possessive quantifier can be used with an entire group, for
1183 example:
1184 (abc|xyz){2,3}+
1186 Possessive quantifiers are always greedy; the setting of the
1188 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
1189 simpler forms of atomic group. However, there is no difference in the
1190 meaning of a possessive quantifier and the equivalent atomic group,
1191 though there may be a performance difference; possessive quantifiers
1192 should be slightly faster.
1193 The possessive quantifier syntax is an extension to the Perl 5.8 syn‐
1195 tax. Jeffrey Friedl originated the idea (and the name) in the first
1196 edition of his book. Mike McCloskey liked it, so implemented it when he
1197 built Sun's Java package, and PCRE copied it from there. It ultimately
1198 found its way into Perl at release 5.10.
1199 PCRE has an optimization that automatically "possessifies" certain sim‐
1201 ple pattern constructs. For example, the sequence A+B is treated as
1202 A++B because there is no point in backtracking into a sequence of A's
1203 when B must follow.
1204 When a pattern contains an unlimited repeat inside a subpattern that
1206 can itself be repeated an unlimited number of times, the use of an
1207 atomic group is the only way to avoid some failing matches taking a
1208 very long time indeed. The pattern
1209 (\D+|<\d+>)*[!?]
1211 matches an unlimited number of substrings that either consist of non-
1213 digits, or digits enclosed in <>, followed by either ! or ?. When it
1214 matches, it runs quickly. However, if it is applied to
1215 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1217 it takes a long time before reporting failure. This is because the
1219 string can be divided between the internal \D+ repeat and the external
1220 * repeat in a large number of ways, and all have to be tried. (The
1221 example uses [!?] rather than a single character at the end, because
1222 both PCRE and Perl have an optimization that allows for fast failure
1223 when a single character is used. They remember the last single charac‐
1224 ter that is required for a match, and fail early if it is not present
1225 in the string.) If the pattern is changed so that it uses an atomic
1226 group, like this:
1227 ((?>\D+)|<\d+>)*[!?]
1229 sequences of non-digits cannot be broken, and failure happens quickly.
1231
1233 Outside a character class, a backslash followed by a digit greater than
1235 0 (and possibly further digits) is a back reference to a capturing sub‐
1236 pattern earlier (that is, to its left) in the pattern, provided there
1237 have been that many previous capturing left parentheses.
1238 However, if the decimal number following the backslash is less than 10,
1240 it is always taken as a back reference, and causes an error only if
1241 there are not that many capturing left parentheses in the entire pat‐
1242 tern. In other words, the parentheses that are referenced need not be
1243 to the left of the reference for numbers less than 10. A "forward back
1244 reference" of this type can make sense when a repetition is involved
1245 and the subpattern to the right has participated in an earlier itera‐
1246 tion.
1247 It is not possible to have a numerical "forward back reference" to a
1249 subpattern whose number is 10 or more using this syntax because a
1250 sequence such as \50 is interpreted as a character defined in octal.
1251 See the subsection entitled "Non-printing characters" above for further
1252 details of the handling of digits following a backslash. There is no
1253 such problem when named parentheses are used. A back reference to any
1254 subpattern is possible using named parentheses (see below).
1255 Another way of avoiding the ambiguity inherent in the use of digits
1257 following a backslash is to use the \g escape sequence, which is a fea‐
1258 ture introduced in Perl 5.10. This escape must be followed by an
1259 unsigned number or a negative number, optionally enclosed in braces.
1260 These examples are all identical:
1261 (ring), \1
1263 (ring), \g1
1264 (ring), \g{1}
1265 An unsigned number specifies an absolute reference without the ambigu‐
1267 ity that is present in the older syntax. It is also useful when literal
1268 digits follow the reference. A negative number is a relative reference.
1269 Consider this example:
1270 (abc(def)ghi)\g{-1}
1272 The sequence \g{-1} is a reference to the most recently started captur‐
1274 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
1275 \g{-2} would be equivalent to \1. The use of relative references can be
1276 helpful in long patterns, and also in patterns that are created by
1277 joining together fragments that contain references within themselves.
1278 A back reference matches whatever actually matched the capturing sub‐
1280 pattern in the current subject string, rather than anything matching
1281 the subpattern itself (see "Subpatterns as subroutines" below for a way
1282 of doing that). So the pattern
1283 (sens|respons)e and \1ibility
1285 matches "sense and sensibility" and "response and responsibility", but
1287 not "sense and responsibility". If caseful matching is in force at the
1288 time of the back reference, the case of letters is relevant. For exam‐
1289 ple,
1290 ((?i)rah)\s+\1
1292 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
1294 original capturing subpattern is matched caselessly.
1295 There are several different ways of writing back references to named
1297 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
1298 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
1299 unified back reference syntax, in which \g can be used for both numeric
1300 and named references, is also supported. We could rewrite the above
1301 example in any of the following ways:
1302 (?<p1>(?i)rah)\s+\k<p1>
1304 (?'p1'(?i)rah)\s+\k{p1}
1305 (?P<p1>(?i)rah)\s+(?P=p1)
1306 (?<p1>(?i)rah)\s+\g{p1}
1307 A subpattern that is referenced by name may appear in the pattern
1309 before or after the reference.
1310 There may be more than one back reference to the same subpattern. If a
1312 subpattern has not actually been used in a particular match, any back
1313 references to it always fail. For example, the pattern
1314 (a|(bc))\2
1316 always fails if it starts to match "a" rather than "bc". Because there
1318 may be many capturing parentheses in a pattern, all digits following
1319 the backslash are taken as part of a potential back reference number.
1320 If the pattern continues with a digit character, some delimiter must be
1321 used to terminate the back reference. If the PCRE_EXTENDED option is
1322 set, this can be whitespace. Otherwise an empty comment (see "Com‐
1323 ments" below) can be used.
1324 A back reference that occurs inside the parentheses to which it refers
1326 fails when the subpattern is first used, so, for example, (a\1) never
1327 matches. However, such references can be useful inside repeated sub‐
1328 patterns. For example, the pattern
1329 (a|b\1)+
1331 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter‐
1333 ation of the subpattern, the back reference matches the character
1334 string corresponding to the previous iteration. In order for this to
1335 work, the pattern must be such that the first iteration does not need
1336 to match the back reference. This can be done using alternation, as in
1337 the example above, or by a quantifier with a minimum of zero.
1338
1340 An assertion is a test on the characters following or preceding the
1342 current matching point that does not actually consume any characters.
1343 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
1344 described above.
1345 More complicated assertions are coded as subpatterns. There are two
1347 kinds: those that look ahead of the current position in the subject
1348 string, and those that look behind it. An assertion subpattern is
1349 matched in the normal way, except that it does not cause the current
1350 matching position to be changed.
1351 Assertion subpatterns are not capturing subpatterns, and may not be
1353 repeated, because it makes no sense to assert the same thing several
1354 times. If any kind of assertion contains capturing subpatterns within
1355 it, these are counted for the purposes of numbering the capturing sub‐
1356 patterns in the whole pattern. However, substring capturing is carried
1357 out only for positive assertions, because it does not make sense for
1358 negative assertions.
1359 Lookahead assertions
1361 Lookahead assertions start with (?= for positive assertions and (?! for
1363 negative assertions. For example,
1364 \w+(?=;)
1366 matches a word followed by a semicolon, but does not include the semi‐
1368 colon in the match, and
1369 foo(?!bar)
1371 matches any occurrence of "foo" that is not followed by "bar". Note
1373 that the apparently similar pattern
1374 (?!foo)bar
1376 does not find an occurrence of "bar" that is preceded by something
1378 other than "foo"; it finds any occurrence of "bar" whatsoever, because
1379 the assertion (?!foo) is always true when the next three characters are
1380 "bar". A lookbehind assertion is needed to achieve the other effect.
1381 If you want to force a matching failure at some point in a pattern, the
1383 most convenient way to do it is with (?!) because an empty string
1384 always matches, so an assertion that requires there not to be an empty
1385 string must always fail.
1386 Lookbehind assertions
1388 Lookbehind assertions start with (?<= for positive assertions and (?<!
1390 for negative assertions. For example,
1391 (?<!foo)bar
1393 does find an occurrence of "bar" that is not preceded by "foo". The
1395 contents of a lookbehind assertion are restricted such that all the
1396 strings it matches must have a fixed length. However, if there are sev‐
1397 eral top-level alternatives, they do not all have to have the same
1398 fixed length. Thus
1399 (?<=bullock|donkey)
1401 is permitted, but
1403 (?<!dogs?|cats?)
1405 causes an error at compile time. Branches that match different length
1407 strings are permitted only at the top level of a lookbehind assertion.
1408 This is an extension compared with Perl (at least for 5.8), which
1409 requires all branches to match the same length of string. An assertion
1410 such as
1411 (?<=ab(c|de))
1413 is not permitted, because its single top-level branch can match two
1415 different lengths, but it is acceptable if rewritten to use two top-
1416 level branches:
1417 (?<=abc|abde)
1419 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
1421 instead of a lookbehind assertion; this is not restricted to a fixed-
1422 length.
1423 The implementation of lookbehind assertions is, for each alternative,
1425 to temporarily move the current position back by the fixed length and
1426 then try to match. If there are insufficient characters before the cur‐
1427 rent position, the assertion fails.
1428 PCRE does not allow the \C escape (which matches a single byte in UTF-8
1430 mode) to appear in lookbehind assertions, because it makes it impossi‐
1431 ble to calculate the length of the lookbehind. The \X and \R escapes,
1432 which can match different numbers of bytes, are also not permitted.
1433 Possessive quantifiers can be used in conjunction with lookbehind
1435 assertions to specify efficient matching at the end of the subject
1436 string. Consider a simple pattern such as
1437 abcd$
1439 when applied to a long string that does not match. Because matching
1441 proceeds from left to right, PCRE will look for each "a" in the subject
1442 and then see if what follows matches the rest of the pattern. If the
1443 pattern is specified as
1444 ^.*abcd$
1446 the initial .* matches the entire string at first, but when this fails
1448 (because there is no following "a"), it backtracks to match all but the
1449 last character, then all but the last two characters, and so on. Once
1450 again the search for "a" covers the entire string, from right to left,
1451 so we are no better off. However, if the pattern is written as
1452 ^.*+(?<=abcd)
1454 there can be no backtracking for the .*+ item; it can match only the
1456 entire string. The subsequent lookbehind assertion does a single test
1457 on the last four characters. If it fails, the match fails immediately.
1458 For long strings, this approach makes a significant difference to the
1459 processing time.
1460 Using multiple assertions
1462 Several assertions (of any sort) may occur in succession. For example,
1464 (?<=\d{3})(?<!999)foo
1466 matches "foo" preceded by three digits that are not "999". Notice that
1468 each of the assertions is applied independently at the same point in
1469 the subject string. First there is a check that the previous three
1470 characters are all digits, and then there is a check that the same
1471 three characters are not "999". This pattern does not match "foo" pre‐
1472 ceded by six characters, the first of which are digits and the last
1473 three of which are not "999". For example, it doesn't match "123abc‐
1474 foo". A pattern to do that is
1475 (?<=\d{3}...)(?<!999)foo
1477 This time the first assertion looks at the preceding six characters,
1479 checking that the first three are digits, and then the second assertion
1480 checks that the preceding three characters are not "999".
1481 Assertions can be nested in any combination. For example,
1483 (?<=(?<!foo)bar)baz
1485 matches an occurrence of "baz" that is preceded by "bar" which in turn
1487 is not preceded by "foo", while
1488 (?<=\d{3}(?!999)...)foo
1490 is another pattern that matches "foo" preceded by three digits and any
1492 three characters that are not "999".
1493
1495 It is possible to cause the matching process to obey a subpattern con‐
1497 ditionally or to choose between two alternative subpatterns, depending
1498 on the result of an assertion, or whether a previous capturing subpat‐
1499 tern matched or not. The two possible forms of conditional subpattern
1500 are
1501 (?(condition)yes-pattern)
1503 (?(condition)yes-pattern|no-pattern)
1504 If the condition is satisfied, the yes-pattern is used; otherwise the
1506 no-pattern (if present) is used. If there are more than two alterna‐
1507 tives in the subpattern, a compile-time error occurs.
1508 There are four kinds of condition: references to subpatterns, refer‐
1510 ences to recursion, a pseudo-condition called DEFINE, and assertions.
1511 Checking for a used subpattern by number
1513 If the text between the parentheses consists of a sequence of digits,
1515 the condition is true if the capturing subpattern of that number has
1516 previously matched. An alternative notation is to precede the digits
1517 with a plus or minus sign. In this case, the subpattern number is rela‐
1518 tive rather than absolute. The most recently opened parentheses can be
1519 referenced by (?(-1), the next most recent by (?(-2), and so on. In
1520 looping constructs it can also make sense to refer to subsequent groups
1521 with constructs such as (?(+2).
1522 Consider the following pattern, which contains non-significant white
1524 space to make it more readable (assume the PCRE_EXTENDED option) and to
1525 divide it into three parts for ease of discussion:
1526 ( \( )? [^()]+ (?(1) \) )
1528 The first part matches an optional opening parenthesis, and if that
1530 character is present, sets it as the first captured substring. The sec‐
1531 ond part matches one or more characters that are not parentheses. The
1532 third part is a conditional subpattern that tests whether the first set
1533 of parentheses matched or not. If they did, that is, if subject started
1534 with an opening parenthesis, the condition is true, and so the yes-pat‐
1535 tern is executed and a closing parenthesis is required. Otherwise,
1536 since no-pattern is not present, the subpattern matches nothing. In
1537 other words, this pattern matches a sequence of non-parentheses,
1538 optionally enclosed in parentheses.
1539 If you were embedding this pattern in a larger one, you could use a
1541 relative reference:
1542 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1544 This makes the fragment independent of the parentheses in the larger
1546 pattern.
1547 Checking for a used subpattern by name
1549 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
1551 used subpattern by name. For compatibility with earlier versions of
1552 PCRE, which had this facility before Perl, the syntax (?(name)...) is
1553 also recognized. However, there is a possible ambiguity with this syn‐
1554 tax, because subpattern names may consist entirely of digits. PCRE
1555 looks first for a named subpattern; if it cannot find one and the name
1556 consists entirely of digits, PCRE looks for a subpattern of that num‐
1557 ber, which must be greater than zero. Using subpattern names that con‐
1558 sist entirely of digits is not recommended.
1559 Rewriting the above example to use a named subpattern gives this:
1561 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
1563 Checking for pattern recursion
1566 If the condition is the string (R), and there is no subpattern with the
1568 name R, the condition is true if a recursive call to the whole pattern
1569 or any subpattern has been made. If digits or a name preceded by amper‐
1570 sand follow the letter R, for example:
1571 (?(R3)...) or (?(R&name)...)
1573 the condition is true if the most recent recursion is into the subpat‐
1575 tern whose number or name is given. This condition does not check the
1576 entire recursion stack.
1577 At "top level", all these recursion test conditions are false. Recur‐
1579 sive patterns are described below.
1580 Defining subpatterns for use by reference only
1582 If the condition is the string (DEFINE), and there is no subpattern
1584 with the name DEFINE, the condition is always false. In this case,
1585 there may be only one alternative in the subpattern. It is always
1586 skipped if control reaches this point in the pattern; the idea of
1587 DEFINE is that it can be used to define "subroutines" that can be ref‐
1588 erenced from elsewhere. (The use of "subroutines" is described below.)
1589 For example, a pattern to match an IPv4 address could be written like
1590 this (ignore whitespace and line breaks):
1591 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1593 \b (?&byte) (\.(?&byte)){3} \b
1594 The first part of the pattern is a DEFINE group inside which a another
1596 group named "byte" is defined. This matches an individual component of
1597 an IPv4 address (a number less than 256). When matching takes place,
1598 this part of the pattern is skipped because DEFINE acts like a false
1599 condition.
1600 The rest of the pattern uses references to the named group to match the
1602 four dot-separated components of an IPv4 address, insisting on a word
1603 boundary at each end.
1604 Assertion conditions
1606 If the condition is not in any of the above formats, it must be an
1608 assertion. This may be a positive or negative lookahead or lookbehind
1609 assertion. Consider this pattern, again containing non-significant
1610 white space, and with the two alternatives on the second line:
1611 (?(?=[^a-z]*[a-z])
1613 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1614 The condition is a positive lookahead assertion that matches an
1616 optional sequence of non-letters followed by a letter. In other words,
1617 it tests for the presence of at least one letter in the subject. If a
1618 letter is found, the subject is matched against the first alternative;
1619 otherwise it is matched against the second. This pattern matches
1620 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
1621 letters and dd are digits.
1622
1624 The sequence (?# marks the start of a comment that continues up to the
1626 next closing parenthesis. Nested parentheses are not permitted. The
1627 characters that make up a comment play no part in the pattern matching
1628 at all.
1629 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1631 character class introduces a comment that continues to immediately
1632 after the next newline in the pattern.
1633
1635 Consider the problem of matching a string in parentheses, allowing for
1637 unlimited nested parentheses. Without the use of recursion, the best
1638 that can be done is to use a pattern that matches up to some fixed
1639 depth of nesting. It is not possible to handle an arbitrary nesting
1640 depth.
1641 For some time, Perl has provided a facility that allows regular expres‐
1643 sions to recurse (amongst other things). It does this by interpolating
1644 Perl code in the expression at run time, and the code can refer to the
1645 expression itself. A Perl pattern using code interpolation to solve the
1646 parentheses problem can be created like this:
1647 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
1649 The (?p{...}) item interpolates Perl code at run time, and in this case
1651 refers recursively to the pattern in which it appears.
1652 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
1654 it supports special syntax for recursion of the entire pattern, and
1655 also for individual subpattern recursion. After its introduction in
1656 PCRE and Python, this kind of recursion was introduced into Perl at
1657 release 5.10.
1658 A special item that consists of (? followed by a number greater than
1660 zero and a closing parenthesis is a recursive call of the subpattern of
1661 the given number, provided that it occurs inside that subpattern. (If
1662 not, it is a "subroutine" call, which is described in the next sec‐
1663 tion.) The special item (?R) or (?0) is a recursive call of the entire
1664 regular expression.
1665 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
1667 always treated as an atomic group. That is, once it has matched some of
1668 the subject string, it is never re-entered, even if it contains untried
1669 alternatives and there is a subsequent matching failure.
1670 This PCRE pattern solves the nested parentheses problem (assume the
1672 PCRE_EXTENDED option is set so that white space is ignored):
1673 \( ( (?>[^()]+) | (?R) )* \)
1675 First it matches an opening parenthesis. Then it matches any number of
1677 substrings which can either be a sequence of non-parentheses, or a
1678 recursive match of the pattern itself (that is, a correctly parenthe‐
1679 sized substring). Finally there is a closing parenthesis.
1680 If this were part of a larger pattern, you would not want to recurse
1682 the entire pattern, so instead you could use this:
1683 ( \( ( (?>[^()]+) | (?1) )* \) )
1685 We have put the pattern into parentheses, and caused the recursion to
1687 refer to them instead of the whole pattern.
1688 In a larger pattern, keeping track of parenthesis numbers can be
1690 tricky. This is made easier by the use of relative references. (A Perl
1691 5.10 feature.) Instead of (?1) in the pattern above you can write
1692 (?-2) to refer to the second most recently opened parentheses preceding
1693 the recursion. In other words, a negative number counts capturing
1694 parentheses leftwards from the point at which it is encountered.
1695 It is also possible to refer to subsequently opened parentheses, by
1697 writing references such as (?+2). However, these cannot be recursive
1698 because the reference is not inside the parentheses that are refer‐
1699 enced. They are always "subroutine" calls, as described in the next
1700 section.
1701 An alternative approach is to use named parentheses instead. The Perl
1703 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
1704 supported. We could rewrite the above example as follows:
1705 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
1707 If there is more than one subpattern with the same name, the earliest
1709 one is used.
1710 This particular example pattern that we have been looking at contains
1712 nested unlimited repeats, and so the use of atomic grouping for match‐
1713 ing strings of non-parentheses is important when applying the pattern
1714 to strings that do not match. For example, when this pattern is applied
1715 to
1716 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1718 it yields "no match" quickly. However, if atomic grouping is not used,
1720 the match runs for a very long time indeed because there are so many
1721 different ways the + and * repeats can carve up the subject, and all
1722 have to be tested before failure can be reported.
1723 At the end of a match, the values set for any capturing subpatterns are
1725 those from the outermost level of the recursion at which the subpattern
1726 value is set. If you want to obtain intermediate values, a callout
1727 function can be used (see below and the pcrecallout documentation). If
1728 the pattern above is matched against
1729 (ab(cd)ef)
1731 the value for the capturing parentheses is "ef", which is the last
1733 value taken on at the top level. If additional parentheses are added,
1734 giving
1735 \( ( ( (?>[^()]+) | (?R) )* ) \)
1737 ^ ^
1738 ^ ^
1739 the string they capture is "ab(cd)ef", the contents of the top level
1741 parentheses. If there are more than 15 capturing parentheses in a pat‐
1742 tern, PCRE has to obtain extra memory to store data during a recursion,
1743 which it does by using pcre_malloc, freeing it via pcre_free after‐
1744 wards. If no memory can be obtained, the match fails with the
1745 PCRE_ERROR_NOMEMORY error.
1746 Do not confuse the (?R) item with the condition (R), which tests for
1748 recursion. Consider this pattern, which matches text in angle brack‐
1749 ets, allowing for arbitrary nesting. Only digits are allowed in nested
1750 brackets (that is, when recursing), whereas any characters are permit‐
1751 ted at the outer level.
1752 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
1754 In this pattern, (?(R) is the start of a conditional subpattern, with
1756 two different alternatives for the recursive and non-recursive cases.
1757 The (?R) item is the actual recursive call.
1758
1760 If the syntax for a recursive subpattern reference (either by number or
1762 by name) is used outside the parentheses to which it refers, it oper‐
1763 ates like a subroutine in a programming language. The "called" subpat‐
1764 tern may be defined before or after the reference. A numbered reference
1765 can be absolute or relative, as in these examples:
1766 (...(absolute)...)...(?2)...
1768 (...(relative)...)...(?-1)...
1769 (...(?+1)...(relative)...
1770 An earlier example pointed out that the pattern
1772 (sens|respons)e and \1ibility
1774 matches "sense and sensibility" and "response and responsibility", but
1776 not "sense and responsibility". If instead the pattern
1777 (sens|respons)e and (?1)ibility
1779 is used, it does match "sense and responsibility" as well as the other
1781 two strings. Another example is given in the discussion of DEFINE
1782 above.
1783 Like recursive subpatterns, a "subroutine" call is always treated as an
1785 atomic group. That is, once it has matched some of the subject string,
1786 it is never re-entered, even if it contains untried alternatives and
1787 there is a subsequent matching failure.
1788 When a subpattern is used as a subroutine, processing options such as
1790 case-independence are fixed when the subpattern is defined. They cannot
1791 be changed for different calls. For example, consider this pattern:
1792 (abc)(?i:(?-1))
1794 It matches "abcabc". It does not match "abcABC" because the change of
1796 processing option does not affect the called subpattern.
1797
1799 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
1801 Perl code to be obeyed in the middle of matching a regular expression.
1802 This makes it possible, amongst other things, to extract different sub‐
1803 strings that match the same pair of parentheses when there is a repeti‐
1804 tion.
1805 PCRE provides a similar feature, but of course it cannot obey arbitrary
1807 Perl code. The feature is called "callout". The caller of PCRE provides
1808 an external function by putting its entry point in the global variable
1809 pcre_callout. By default, this variable contains NULL, which disables
1810 all calling out.
1811 Within a regular expression, (?C) indicates the points at which the
1813 external function is to be called. If you want to identify different
1814 callout points, you can put a number less than 256 after the letter C.
1815 The default value is zero. For example, this pattern has two callout
1816 points:
1817 (?C1)abc(?C2)def
1819 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
1821 automatically installed before each item in the pattern. They are all
1822 numbered 255.
1823 During matching, when PCRE reaches a callout point (and pcre_callout is
1825 set), the external function is called. It is provided with the number
1826 of the callout, the position in the pattern, and, optionally, one item
1827 of data originally supplied by the caller of pcre_exec(). The callout
1828 function may cause matching to proceed, to backtrack, or to fail alto‐
1829 gether. A complete description of the interface to the callout function
1830 is given in the pcrecallout documentation.
1831
1833 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
1835 which are described in the Perl documentation as "experimental and sub‐
1836 ject to change or removal in a future version of Perl". It goes on to
1837 say: "Their usage in production code should be noted to avoid problems
1838 during upgrades." The same remarks apply to the PCRE features described
1839 in this section.
1840 Since these verbs are specifically related to backtracking, they can be
1842 used only when the pattern is to be matched using pcre_exec(), which
1843 uses a backtracking algorithm. They cause an error if encountered by
1844 pcre_dfa_exec().
1845 The new verbs make use of what was previously invalid syntax: an open‐
1847 ing parenthesis followed by an asterisk. In Perl, they are generally of
1848 the form (*VERB:ARG) but PCRE does not support the use of arguments, so
1849 its general form is just (*VERB). Any number of these verbs may occur
1850 in a pattern. There are two kinds:
1851 Verbs that act immediately
1853 The following verbs act as soon as they are encountered:
1855 (*ACCEPT)
1857 This verb causes the match to end successfully, skipping the remainder
1859 of the pattern. When inside a recursion, only the innermost pattern is
1860 ended immediately. PCRE differs from Perl in what happens if the
1861 (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
1862 captured: in PCRE no data is captured. For example:
1863 A(A|B(*ACCEPT)|C)D
1865 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
1867 is captured.
1868 (*FAIL) or (*F)
1870 This verb causes the match to fail, forcing backtracking to occur. It
1872 is equivalent to (?!) but easier to read. The Perl documentation notes
1873 that it is probably useful only when combined with (?{}) or (??{}).
1874 Those are, of course, Perl features that are not present in PCRE. The
1875 nearest equivalent is the callout feature, as for example in this pat‐
1876 tern:
1877 a+(?C)(*FAIL)
1879 A match with the string "aaaa" always fails, but the callout is taken
1881 before each backtrack happens (in this example, 10 times).
1882 Verbs that act after backtracking
1884 The following verbs do nothing when they are encountered. Matching con‐
1886 tinues with what follows, but if there is no subsequent match, a fail‐
1887 ure is forced. The verbs differ in exactly what kind of failure
1888 occurs.
1889 (*COMMIT)
1891 This verb causes the whole match to fail outright if the rest of the
1893 pattern does not match. Even if the pattern is unanchored, no further
1894 attempts to find a match by advancing the start point take place. Once
1895 (*COMMIT) has been passed, pcre_exec() is committed to finding a match
1896 at the current starting point, or not at all. For example:
1897 a+(*COMMIT)b
1899 This matches "xxaab" but not "aacaab". It can be thought of as a kind
1901 of dynamic anchor, or "I've started, so I must finish."
1902 (*PRUNE)
1904 This verb causes the match to fail at the current position if the rest
1906 of the pattern does not match. If the pattern is unanchored, the normal
1907 "bumpalong" advance to the next starting character then happens. Back‐
1908 tracking can occur as usual to the left of (*PRUNE), or when matching
1909 to the right of (*PRUNE), but if there is no match to the right, back‐
1910 tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
1911 is just an alternative to an atomic group or possessive quantifier, but
1912 there are some uses of (*PRUNE) that cannot be expressed in any other
1913 way.
1914 (*SKIP)
1916 This verb is like (*PRUNE), except that if the pattern is unanchored,
1918 the "bumpalong" advance is not to the next character, but to the posi‐
1919 tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
1920 that whatever text was matched leading up to it cannot be part of a
1921 successful match. Consider:
1922 a+(*SKIP)b
1924 If the subject is "aaaac...", after the first match attempt fails
1926 (starting at the first character in the string), the starting point
1927 skips on to start the next attempt at "c". Note that a possessive quan‐
1928 tifer does not have the same effect in this example; although it would
1929 suppress backtracking during the first match attempt, the second
1930 attempt would start at the second character instead of skipping on to
1931 "c".
1932 (*THEN)
1934 This verb causes a skip to the next alternation if the rest of the pat‐
1936 tern does not match. That is, it cancels pending backtracking, but only
1937 within the current alternation. Its name comes from the observation
1938 that it can be used for a pattern-based if-then-else block:
1939 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
1941 If the COND1 pattern matches, FOO is tried (and possibly further items
1943 after the end of the group if FOO succeeds); on failure the matcher
1944 skips to the second alternative and tries COND2, without backtracking
1945 into COND1. If (*THEN) is used outside of any alternation, it acts
1946 exactly like (*PRUNE).
1947
1949 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
1951
1953 Philip Hazel
1955 University Computing Service
1956 Cambridge CB2 3QH, England.
1957
1959 Last updated: 21 August 2007
1961 Copyright (c) 1997-2007 University of Cambridge.
1962 PCREPATTERN(3)