3 <title>pcrepattern specification</title>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepattern man page</h1>
8 Return to the <a href="index.html">PCRE index page</a>.
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 <li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a>
18 <li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
19 <li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
20 <li><a name="TOC5" href="#SEC5">BACKSLASH</a>
21 <li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
22 <li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
23 <li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a>
24 <li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a>
25 <li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
26 <li><a name="TOC11" href="#SEC11">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a>
27 <li><a name="TOC12" href="#SEC12">VERTICAL BAR</a>
28 <li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a>
29 <li><a name="TOC14" href="#SEC14">SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">DUPLICATE SUBPATTERN NUMBERS</a>
31 <li><a name="TOC16" href="#SEC16">NAMED SUBPATTERNS</a>
32 <li><a name="TOC17" href="#SEC17">REPETITION</a>
33 <li><a name="TOC18" href="#SEC18">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
34 <li><a name="TOC19" href="#SEC19">BACK REFERENCES</a>
35 <li><a name="TOC20" href="#SEC20">ASSERTIONS</a>
36 <li><a name="TOC21" href="#SEC21">CONDITIONAL SUBPATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">COMMENTS</a>
38 <li><a name="TOC23" href="#SEC23">RECURSIVE PATTERNS</a>
39 <li><a name="TOC24" href="#SEC24">SUBPATTERNS AS SUBROUTINES</a>
40 <li><a name="TOC25" href="#SEC25">ONIGURUMA SUBROUTINE SYNTAX</a>
41 <li><a name="TOC26" href="#SEC26">CALLOUTS</a>
42 <li><a name="TOC27" href="#SEC27">BACKTRACKING CONTROL</a>
43 <li><a name="TOC28" href="#SEC28">SEE ALSO</a>
44 <li><a name="TOC29" href="#SEC29">AUTHOR</a>
45 <li><a name="TOC30" href="#SEC30">REVISION</a>
47 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
49 The syntax and semantics of the regular expressions that are supported by PCRE
50 are described in detail below. There is a quick-reference syntax summary in the
51 <a href="pcresyntax.html"><b>pcresyntax</b></a>
52 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
53 also supports some alternative regular expression syntax (which does not
54 conflict with the Perl syntax) in order to provide some compatibility with
55 regular expressions in Python, .NET, and Oniguruma.
58 Perl's regular expressions are described in its own documentation, and
59 regular expressions in general are covered in a number of books, some of which
60 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
61 published by O'Reilly, covers regular expressions in great detail. This
62 description of PCRE's regular expressions is intended as reference material.
65 This document discusses the patterns that are supported by PCRE when one its
66 main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b>
67 (16- or 32-bit), is used. PCRE also has alternative matching functions,
68 <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, which match using a
69 different algorithm that is not Perl-compatible. Some of the features discussed
70 below are not available when DFA matching is used. The advantages and
71 disadvantages of the alternative functions, and how they differ from the normal
72 functions, are discussed in the
73 <a href="pcrematching.html"><b>pcrematching</b></a>
76 <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
78 A number of options that can be passed to <b>pcre_compile()</b> can also be set
79 by special items at the start of a pattern. These are not Perl-compatible, but
80 are provided to make these options accessible to pattern writers who are not
81 able to change the program that processes the pattern. Any number of these
82 items may appear, but they must all be together right at the start of the
83 pattern string, and the letters must be in upper case.
89 The original operation of PCRE was on strings of one-byte characters. However,
90 there is now also support for UTF-8 strings in the original library, an
91 extra library that supports 16-bit and UTF-16 character strings, and a
92 third library that supports 32-bit and UTF-32 character strings. To use these
93 features, PCRE must be built to include appropriate support. When using UTF
94 strings you must either call the compiling function with the PCRE_UTF8,
95 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
96 these special sequences:
103 (*UTF) is a generic sequence that can be used with any of the libraries.
104 Starting a pattern with such a sequence is equivalent to setting the relevant
105 option. How setting a UTF mode affects pattern matching is mentioned in several
106 places below. There is also a summary of features in the
107 <a href="pcreunicode.html"><b>pcreunicode</b></a>
111 Some applications that allow their users to supply patterns may wish to
112 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
113 option is set at compile time, (*UTF) etc. are not allowed, and their
114 appearance causes an error.
117 Unicode property support
120 Another special sequence that may appear at the start of a pattern is (*UCP).
121 This has the same effect as setting the PCRE_UCP option: it causes sequences
122 such as \d and \w to use Unicode properties to determine character types,
123 instead of recognizing only characters with codes less than 128 via a lookup
127 Disabling auto-possessification
130 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
131 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
132 quantifiers possessive when what follows cannot match the repeated item. For
133 example, by default a+b is treated as a++b. For more details, see the
134 <a href="pcreapi.html"><b>pcreapi</b></a>
138 Disabling start-up optimizations
141 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
142 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
143 several optimizations for quickly reaching "no match" results. For more
145 <a href="pcreapi.html"><b>pcreapi</b></a>
147 <a name="newlines"></a></P>
152 PCRE supports five different conventions for indicating line breaks in
153 strings: a single CR (carriage return) character, a single LF (linefeed)
154 character, the two-character sequence CRLF, any of the three preceding, or any
155 Unicode newline sequence. The
156 <a href="pcreapi.html"><b>pcreapi</b></a>
158 <a href="pcreapi.html#newlines">further discussion</a>
159 about newlines, and shows how to set the newline convention in the
160 <i>options</i> arguments for the compiling and matching functions.
163 It is also possible to specify a newline convention by starting a pattern
164 string with one of the following five sequences:
166 (*CR) carriage return
168 (*CRLF) carriage return, followed by linefeed
169 (*ANYCRLF) any of the three above
170 (*ANY) all Unicode newline sequences
172 These override the default and the options given to the compiling function. For
173 example, on a Unix system where LF is the default newline sequence, the pattern
177 changes the convention to CR. That pattern matches "a\nb" because LF is no
178 longer a newline. If more than one of these settings is present, the last one
182 The newline convention affects where the circumflex and dollar assertions are
183 true. It also affects the interpretation of the dot metacharacter when
184 PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect
185 what the \R escape sequence matches. By default, this is any Unicode newline
186 sequence, for Perl compatibility. However, this can be changed; see the
187 description of \R in the section entitled
188 <a href="#newlineseq">"Newline sequences"</a>
189 below. A change of \R setting can be combined with a change of newline
193 Setting match and recursion limits
196 The caller of <b>pcre_exec()</b> can set a limit on the number of times the
197 internal <b>match()</b> function is called and on the maximum depth of
198 recursive calls. These facilities are provided to catch runaway matches that
199 are provoked by patterns with huge matching trees (a typical example is a
200 pattern with nested unlimited repeats) and to avoid running out of system stack
201 by too much recursion. When one of these limits is reached, <b>pcre_exec()</b>
202 gives an error return. The limits can also be set by items at the start of the
208 where d is any number of decimal digits. However, the value of the setting must
209 be less than the value set (or defaulted) by the caller of <b>pcre_exec()</b>
210 for it to have any effect. In other words, the pattern writer can lower the
211 limits set by the programmer, but not raise them. If there is more than one
212 setting of one of these limits, the lower value is used.
214 <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
216 PCRE can be compiled to run in an environment that uses EBCDIC as its character
217 code rather than ASCII or Unicode (typically a mainframe system). In the
218 sections below, character code values are ASCII or Unicode; in an EBCDIC
219 environment these characters may have different code values, and there are no
220 code points greater than 255.
222 <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
224 A regular expression is a pattern that is matched against a subject string from
225 left to right. Most characters stand for themselves in a pattern, and match the
226 corresponding characters in the subject. As a trivial example, the pattern
230 matches a portion of a subject string that is identical to itself. When
231 caseless matching is specified (the PCRE_CASELESS option), letters are matched
232 independently of case. In a UTF mode, PCRE always understands the concept of
233 case for characters whose values are less than 128, so caseless matching is
234 always possible. For characters with higher values, the concept of case is
235 supported if PCRE is compiled with Unicode property support, but not otherwise.
236 If you want to use caseless matching for characters 128 and above, you must
237 ensure that PCRE is compiled with Unicode property support as well as with
241 The power of regular expressions comes from the ability to include alternatives
242 and repetitions in the pattern. These are encoded in the pattern by the use of
243 <i>metacharacters</i>, which do not stand for themselves but instead are
244 interpreted in some special way.
247 There are two different sets of metacharacters: those that are recognized
248 anywhere in the pattern except within square brackets, and those that are
249 recognized within square brackets. Outside square brackets, the metacharacters
252 \ general escape character with several uses
253 ^ assert start of string (or line, in multiline mode)
254 $ assert end of string (or line, in multiline mode)
255 . match any character except newline (by default)
256 [ start character class definition
257 | start of alternative branch
260 ? extends the meaning of (
261 also 0 or 1 quantifier
262 also quantifier minimizer
263 * 0 or more quantifier
264 + 1 or more quantifier
265 also "possessive quantifier"
266 { start min/max quantifier
268 Part of a pattern that is in square brackets is called a "character class". In
269 a character class the only metacharacters are:
271 \ general escape character
272 ^ negate the class, but only if the first character
273 - indicates character range
274 [ POSIX character class (only if followed by POSIX syntax)
275 ] terminates the character class
277 The following sections describe the use of each of the metacharacters.
279 <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
281 The backslash character has several uses. Firstly, if it is followed by a
282 character that is not a number or a letter, it takes away any special meaning
283 that character may have. This use of backslash as an escape character applies
284 both inside and outside character classes.
287 For example, if you want to match a * character, you write \* in the pattern.
288 This escaping action applies whether or not the following character would
289 otherwise be interpreted as a metacharacter, so it is always safe to precede a
290 non-alphanumeric with backslash to specify that it stands for itself. In
291 particular, if you want to match a backslash, you write \\.
294 In a UTF mode, only ASCII numbers and letters have any special meaning after a
295 backslash. All other characters (in particular, those whose codepoints are
296 greater than 127) are treated as literals.
299 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
300 pattern (other than in a character class), and characters between a # outside a
301 character class and the next newline, inclusive, are ignored. An escaping
302 backslash can be used to include a white space or # character as part of the
306 If you want to remove the special meaning from a sequence of characters, you
307 can do so by putting them between \Q and \E. This is different from Perl in
308 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
309 Perl, $ and @ cause variable interpolation. Note the following examples:
311 Pattern PCRE matches Perl matches
313 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
314 \Qabc\$xyz\E abc\$xyz abc\$xyz
315 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
317 The \Q...\E sequence is recognized both inside and outside character classes.
318 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
319 by \E later in the pattern, the literal interpretation continues to the end of
320 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
321 a character class, this causes an error, because the character class is not
323 <a name="digitsafterbackslash"></a></P>
325 Non-printing characters
328 A second use of backslash provides a way of encoding non-printing characters
329 in patterns in a visible manner. There is no restriction on the appearance of
330 non-printing characters, apart from the binary zero that terminates a pattern,
331 but when a pattern is being prepared by text editing, it is often easier to use
332 one of the following escape sequences than the binary character it represents:
334 \a alarm, that is, the BEL character (hex 07)
335 \cx "control-x", where x is any ASCII character
337 \f form feed (hex 0C)
339 \r carriage return (hex 0D)
341 \0dd character with octal code 0dd
342 \ddd character with octal code ddd, or back reference
343 \o{ddd..} character with octal code ddd..
344 \xhh character with hex code hh
345 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
346 \uhhhh character with hex code hhhh (JavaScript mode only)
348 The precise effect of \cx on ASCII characters is as follows: if x is a lower
349 case letter, it is converted to upper case. Then bit 6 of the character (hex
350 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
351 but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
352 data item (byte or 16-bit value) following \c has a value greater than 127, a
353 compile-time error occurs. This locks out non-ASCII characters in all modes.
356 The \c facility was designed for use with ASCII characters, but with the
357 extension to Unicode it is even less useful than it once was. It is, however,
358 recognized when PCRE is compiled in EBCDIC mode, where data items are always
359 bytes. In this mode, all values are valid after \c. If the next character is a
360 lower case letter, it is converted to upper case. Then the 0xc0 bits of the
361 byte are inverted. Thus \cA becomes hex 01, as in ASCII (A is C1), but because
362 the EBCDIC letters are disjoint, \cZ becomes hex 29 (Z is E9), and other
363 characters also generate different values.
366 After \0 up to two further octal digits are read. If there are fewer than two
367 digits, just those that are present are used. Thus the sequence \0\x\07
368 specifies two binary zeros followed by a BEL character (code value 7). Make
369 sure you supply two digits after the initial zero if the pattern character that
370 follows is itself an octal digit.
373 The escape \o must be followed by a sequence of octal digits, enclosed in
374 braces. An error occurs if this is not the case. This escape is a recent
375 addition to Perl; it provides way of specifying character code points as octal
376 numbers greater than 0777, and it also allows octal numbers and back references
377 to be unambiguously specified.
380 For greater clarity and unambiguity, it is best to avoid following \ by a
381 digit greater than zero. Instead, use \o{} or \x{} to specify character
382 numbers, and \g{} to specify back references. The following paragraphs
383 describe the old, ambiguous syntax.
386 The handling of a backslash followed by a digit other than 0 is complicated,
387 and Perl has changed in recent releases, causing PCRE also to change. Outside a
388 character class, PCRE reads the digit and any following digits as a decimal
389 number. If the number is less than 8, or if there have been at least that many
390 previous capturing left parentheses in the expression, the entire sequence is
391 taken as a <i>back reference</i>. A description of how this works is given
392 <a href="#backreferences">later,</a>
393 following the discussion of
394 <a href="#subpattern">parenthesized subpatterns.</a>
397 Inside a character class, or if the decimal number following \ is greater than
398 7 and there have not been that many capturing subpatterns, PCRE handles \8 and
399 \9 as the literal characters "8" and "9", and otherwise re-reads up to three
400 octal digits following the backslash, using them to generate a data character.
401 Any subsequent digits stand for themselves. For example:
403 \040 is another way of writing an ASCII space
404 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
405 \7 is always a back reference
406 \11 might be a back reference, or another way of writing a tab
408 \0113 is a tab followed by the character "3"
409 \113 might be a back reference, otherwise the character with octal code 113
410 \377 might be a back reference, otherwise the value 255 (decimal)
411 \81 is either a back reference, or the two characters "8" and "1"
413 Note that octal values of 100 or greater that are specified using this syntax
414 must not be introduced by a leading zero, because no more than three octal
415 digits are ever read.
418 By default, after \x that is not followed by {, from zero to two hexadecimal
419 digits are read (letters can be in upper or lower case). Any number of
420 hexadecimal digits may appear between \x{ and }. If a character other than
421 a hexadecimal digit appears between \x{ and }, or if there is no terminating
425 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
426 as just described only when it is followed by two hexadecimal digits.
427 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
428 code points greater than 256 is provided by \u, which must be followed by
429 four hexadecimal digits; otherwise it matches a literal "u" character.
432 Characters whose value is less than 256 can be defined by either of the two
433 syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
434 way they are handled. For example, \xdc is exactly the same as \x{dc} (or
435 \u00dc in JavaScript mode).
438 Constraints on character values
441 Characters that are specified using octal or hexadecimal numbers are
442 limited to certain values, as follows:
444 8-bit non-UTF mode less than 0x100
445 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
446 16-bit non-UTF mode less than 0x10000
447 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
448 32-bit non-UTF mode less than 0x100000000
449 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
451 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
452 "surrogate" codepoints), and 0xffef.
455 Escape sequences in character classes
458 All the sequences that define a single character value can be used both inside
459 and outside character classes. In addition, inside a character class, \b is
460 interpreted as the backspace character (hex 08).
463 \N is not allowed in a character class. \B, \R, and \X are not special
464 inside a character class. Like other unrecognized escape sequences, they are
465 treated as the literal characters "B", "R", and "X" by default, but cause an
466 error if the PCRE_EXTRA option is set. Outside a character class, these
467 sequences have different meanings.
470 Unsupported escape sequences
473 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
474 handler and used to modify the case of following characters. By default, PCRE
475 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
476 option is set, \U matches a "U" character, and \u can be used to define a
477 character by code point, as described in the previous section.
480 Absolute and relative back references
483 The sequence \g followed by an unsigned or a negative number, optionally
484 enclosed in braces, is an absolute or relative back reference. A named back
485 reference can be coded as \g{name}. Back references are discussed
486 <a href="#backreferences">later,</a>
487 following the discussion of
488 <a href="#subpattern">parenthesized subpatterns.</a>
491 Absolute and relative subroutine calls
494 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
495 a number enclosed either in angle brackets or single quotes, is an alternative
496 syntax for referencing a subpattern as a "subroutine". Details are discussed
497 <a href="#onigurumasubroutines">later.</a>
498 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
499 synonymous. The former is a back reference; the latter is a
500 <a href="#subpatternsassubroutines">subroutine</a>
502 <a name="genericchartypes"></a></P>
504 Generic character types
507 Another use of backslash is for specifying generic character types:
510 \D any character that is not a decimal digit
511 \h any horizontal white space character
512 \H any character that is not a horizontal white space character
513 \s any white space character
514 \S any character that is not a white space character
515 \v any vertical white space character
516 \V any character that is not a vertical white space character
517 \w any "word" character
518 \W any "non-word" character
520 There is also the single sequence \N, which matches a non-newline character.
522 <a href="#fullstopdot">the "." metacharacter</a>
523 when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
524 PCRE does not support this.
527 Each pair of lower and upper case escape sequences partitions the complete set
528 of characters into two disjoint sets. Any given character matches one, and only
529 one, of each pair. The sequences can appear both inside and outside character
530 classes. They each match one character of the appropriate type. If the current
531 matching point is at the end of the subject string, all of them fail, because
532 there is no character to match.
535 For compatibility with Perl, \s did not used to match the VT character (code
536 11), which made it different from the the POSIX "space" class. However, Perl
537 added VT at release 5.18, and PCRE followed suit at release 8.34. The default
538 \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
539 (32), which are defined as white space in the "C" locale. This list may vary if
540 locale-specific matching is taking place. For example, in some locales the
541 "non-breaking space" character (\xA0) is recognized as white space, and in
542 others the VT character is not.
545 A "word" character is an underscore or any character that is a letter or digit.
546 By default, the definition of letters and digits is controlled by PCRE's
547 low-valued character tables, and may vary if locale-specific matching is taking
549 <a href="pcreapi.html#localesupport">"Locale support"</a>
551 <a href="pcreapi.html"><b>pcreapi</b></a>
552 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
553 or "french" in Windows, some character codes greater than 127 are used for
554 accented letters, and these are then matched by \w. The use of locales with
555 Unicode is discouraged.
558 By default, characters whose code points are greater than 127 never match \d,
559 \s, or \w, and always match \D, \S, and \W, although this may vary for
560 characters in the range 128-255 when locale-specific matching is happening.
561 These escape sequences retain their original meanings from before Unicode
562 support was available, mainly for efficiency reasons. If PCRE is compiled with
563 Unicode property support, and the PCRE_UCP option is set, the behaviour is
564 changed so that Unicode properties are used to determine character types, as
567 \d any character that matches \p{Nd} (decimal digit)
568 \s any character that matches \p{Z} or \h or \v
569 \w any character that matches \p{L} or \p{N}, plus underscore
571 The upper case escapes match the inverse sets of characters. Note that \d
572 matches only decimal digits, whereas \w matches any Unicode digit, as well as
573 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
574 \B because they are defined in terms of \w and \W. Matching these sequences
575 is noticeably slower when PCRE_UCP is set.
578 The sequences \h, \H, \v, and \V are features that were added to Perl at
579 release 5.10. In contrast to the other sequences, which match only ASCII
580 characters by default, these always match certain high-valued code points,
581 whether or not PCRE_UCP is set. The horizontal space characters are:
583 U+0009 Horizontal tab (HT)
585 U+00A0 Non-break space
586 U+1680 Ogham space mark
587 U+180E Mongolian vowel separator
592 U+2004 Three-per-em space
593 U+2005 Four-per-em space
594 U+2006 Six-per-em space
596 U+2008 Punctuation space
599 U+202F Narrow no-break space
600 U+205F Medium mathematical space
601 U+3000 Ideographic space
603 The vertical space characters are:
606 U+000B Vertical tab (VT)
607 U+000C Form feed (FF)
608 U+000D Carriage return (CR)
609 U+0085 Next line (NEL)
610 U+2028 Line separator
611 U+2029 Paragraph separator
613 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
615 <a name="newlineseq"></a></P>
620 Outside a character class, by default, the escape sequence \R matches any
621 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
624 (?>\r\n|\n|\x0b|\f|\r|\x85)
626 This is an example of an "atomic group", details of which are given
627 <a href="#atomicgroup">below.</a>
628 This particular group matches either the two-character sequence CR followed by
629 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
630 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
631 line, U+0085). The two-character sequence is treated as a single unit that
635 In other modes, two additional characters whose codepoints are greater than 255
636 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
637 Unicode character property support is not needed for these characters to be
641 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
642 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
643 either at compile time or when the pattern is matched. (BSR is an abbrevation
644 for "backslash R".) This can be made the default when PCRE is built; if this is
645 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
646 It is also possible to specify these settings by starting a pattern string with
647 one of the following sequences:
649 (*BSR_ANYCRLF) CR, LF, or CRLF only
650 (*BSR_UNICODE) any Unicode newline sequence
652 These override the default and the options given to the compiling function, but
653 they can themselves be overridden by options given to a matching function. Note
654 that these special settings, which are not Perl-compatible, are recognized only
655 at the very start of a pattern, and that they must be in upper case. If more
656 than one of them is present, the last one is used. They can be combined with a
657 change of newline convention; for example, a pattern can start with:
661 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
662 (*UCP) special sequences. Inside a character class, \R is treated as an
663 unrecognized escape sequence, and so matches the letter "R" by default, but
664 causes an error if PCRE_EXTRA is set.
665 <a name="uniextseq"></a></P>
667 Unicode character properties
670 When PCRE is built with Unicode character property support, three additional
671 escape sequences that match characters with specific properties are available.
672 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
673 characters whose codepoints are less than 256, but they do work in this mode.
674 The extra escape sequences are:
676 \p{<i>xx</i>} a character with the <i>xx</i> property
677 \P{<i>xx</i>} a character without the <i>xx</i> property
678 \X a Unicode extended grapheme cluster
680 The property names represented by <i>xx</i> above are limited to the Unicode
681 script names, the general category properties, "Any", which matches any
682 character (including newline), and some special PCRE properties (described
684 <a href="#extraprops">next section).</a>
685 Other Perl properties such as "InMusicalSymbols" are not currently supported by
686 PCRE. Note that \P{Any} does not match any characters, so always causes a
690 Sets of Unicode characters are defined as belonging to certain scripts. A
691 character from one of these sets can be matched using a script name. For
697 Those that are not part of an identified script are lumped together as
698 "Common". The current list of scripts is:
725 Egyptian_Hieroglyphs,
740 Inscriptional_Pahlavi,
741 Inscriptional_Parthian,
761 Meroitic_Hieroglyphs,
805 Each character has exactly one Unicode general category property, specified by
806 a two-letter abbreviation. For compatibility with Perl, negation can be
807 specified by including a circumflex between the opening brace and the property
808 name. For example, \p{^Lu} is the same as \P{Lu}.
811 If only one letter is specified with \p or \P, it includes all the general
812 category properties that start with that letter. In this case, in the absence
813 of negation, the curly brackets in the escape sequence are optional; these two
814 examples have the same effect:
819 The following general category property codes are supported:
846 Pc Connector punctuation
850 Pi Initial punctuation
857 Sm Mathematical symbol
862 Zp Paragraph separator
865 The special property L& is also supported: it matches a character that has
866 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
867 a modifier or "other".
870 The Cs (Surrogate) property applies only to characters in the range U+D800 to
871 U+DFFF. Such characters are not valid in Unicode strings and so
872 cannot be tested by PCRE, unless UTF validity checking has been turned off
873 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
874 PCRE_NO_UTF32_CHECK in the
875 <a href="pcreapi.html"><b>pcreapi</b></a>
876 page). Perl does not support the Cs property.
879 The long synonyms for property names that Perl supports (such as \p{Letter})
880 are not supported by PCRE, nor is it permitted to prefix any of these
881 properties with "Is".
884 No character that is in the Unicode table has the Cn (unassigned) property.
885 Instead, this property is assumed for any code point that is not in the
889 Specifying caseless matching does not affect these escape sequences. For
890 example, \p{Lu} always matches only upper case letters. This is different from
891 the behaviour of current versions of Perl.
894 Matching characters by Unicode property is not fast, because PCRE has to do a
895 multistage table lookup in order to find a character's property. That is why
896 the traditional escape sequences such as \d and \w do not use Unicode
897 properties in PCRE by default, though you can make them do so by setting the
898 PCRE_UCP option or by starting the pattern with (*UCP).
901 Extended grapheme clusters
904 The \X escape matches any number of Unicode characters that form an "extended
905 grapheme cluster", and treats the sequence as an atomic group
906 <a href="#atomicgroup">(see below).</a>
907 Up to and including release 8.31, PCRE matched an earlier, simpler definition
908 that was equivalent to
912 That is, it matched a character without the "mark" property, followed by zero
913 or more characters with the "mark" property. Characters with the "mark"
914 property are typically non-spacing accents that affect the preceding character.
917 This simple definition was extended in Unicode to include more complicated
918 kinds of composite character by giving each character a grapheme breaking
919 property, and creating rules that use these properties to define the boundaries
920 of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches
921 one of these clusters.
924 \X always matches at least one character. Then it decides whether to add
925 additional characters according to the following rules for ending a cluster:
928 1. End at the end of the subject string.
931 2. Do not end between CR and LF; otherwise end after any control character.
934 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
935 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
936 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
937 character; an LVT or T character may be follwed only by a T character.
940 4. Do not end before extending characters or spacing marks. Characters with
941 the "mark" property always have the "extend" grapheme breaking property.
944 5. Do not end after prepend characters.
947 6. Otherwise, end the cluster.
948 <a name="extraprops"></a></P>
950 PCRE's additional properties
953 As well as the standard Unicode properties described above, PCRE supports four
954 more that make it possible to convert traditional escape sequences such as \w
955 and \s to use Unicode properties. PCRE uses these non-standard, non-Perl
956 properties internally when PCRE_UCP is set. However, they may also be used
957 explicitly. These properties are:
959 Xan Any alphanumeric character
960 Xps Any POSIX space character
961 Xsp Any Perl space character
962 Xwd Any Perl "word" character
964 Xan matches characters that have either the L (letter) or the N (number)
965 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
966 carriage return, and any other character that has the Z (separator) property.
967 Xsp is the same as Xps; it used to exclude vertical tab, for Perl
968 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
969 matches the same characters as Xan, plus underscore.
972 There is another non-standard property, Xuc, which matches any character that
973 can be represented by a Universal Character Name in C++ and other programming
974 languages. These are the characters $, @, ` (grave accent), and all characters
975 with Unicode code points greater than or equal to U+00A0, except for the
976 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
977 excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
978 where H is a hexadecimal digit. Note that the Xuc property does not match these
979 sequences but the characters that they represent.)
980 <a name="resetmatchstart"></a></P>
982 Resetting the match start
985 The escape sequence \K causes any previously matched characters not to be
986 included in the final matched sequence. For example, the pattern:
990 matches "foobar", but reports that it has matched "bar". This feature is
991 similar to a lookbehind assertion
992 <a href="#lookbehind">(described below).</a>
993 However, in this case, the part of the subject before the real match does not
994 have to be of fixed length, as lookbehind assertions do. The use of \K does
995 not interfere with the setting of
996 <a href="#subpattern">captured substrings.</a>
997 For example, when the pattern
1001 matches "foobar", the first substring is still set to "foo".
1004 Perl documents that the use of \K within assertions is "not well defined". In
1005 PCRE, \K is acted upon when it occurs inside positive assertions, but is
1006 ignored in negative assertions. Note that when a pattern such as (?=ab\K)
1007 matches, the reported start of the match can be greater than the end of the
1009 <a name="smallassertions"></a></P>
1014 The final use of backslash is for certain simple assertions. An assertion
1015 specifies a condition that has to be met at a particular point in a match,
1016 without consuming any characters from the subject string. The use of
1017 subpatterns for more complicated assertions is described
1018 <a href="#bigassertions">below.</a>
1019 The backslashed assertions are:
1021 \b matches at a word boundary
1022 \B matches when not at a word boundary
1023 \A matches at the start of the subject
1024 \Z matches at the end of the subject
1025 also matches before a newline at the end of the subject
1026 \z matches only at the end of the subject
1027 \G matches at the first matching position in the subject
1029 Inside a character class, \b has a different meaning; it matches the backspace
1030 character. If any other of these assertions appears in a character class, by
1031 default it matches the corresponding literal character (for example, \B
1032 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1033 escape sequence" error is generated instead.
1036 A word boundary is a position in the subject string where the current character
1037 and the previous character do not both match \w or \W (i.e. one matches
1038 \w and the other matches \W), or the start or end of the string if the
1039 first or last character matches \w, respectively. In a UTF mode, the meanings
1040 of \w and \W can be changed by setting the PCRE_UCP option. When this is
1041 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
1042 of word" or "end of word" metasequence. However, whatever follows \b normally
1043 determines which it is. For example, the fragment \ba matches "a" at the start
1047 The \A, \Z, and \z assertions differ from the traditional circumflex and
1048 dollar (described in the next section) in that they only ever match at the very
1049 start and end of the subject string, whatever options are set. Thus, they are
1050 independent of multiline mode. These three assertions are not affected by the
1051 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1052 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
1053 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
1054 at a point other than the beginning of the subject, \A can never match. The
1055 difference between \Z and \z is that \Z matches before a newline at the end
1056 of the string as well as at the very end, whereas \z matches only at the end.
1059 The \G assertion is true only when the current matching position is at the
1060 start point of the match, as specified by the <i>startoffset</i> argument of
1061 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
1062 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
1063 arguments, you can mimic Perl's /g option, and it is in this kind of
1064 implementation where \G can be useful.
1067 Note, however, that PCRE's interpretation of \G, as the start of the current
1068 match, is subtly different from Perl's, which defines it as the end of the
1069 previous match. In Perl, these can be different when the previously matched
1070 string was empty. Because PCRE does just one match at a time, it cannot
1071 reproduce this behaviour.
1074 If all the alternatives of a pattern begin with \G, the expression is anchored
1075 to the starting match position, and the "anchored" flag is set in the compiled
1078 <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
1080 The circumflex and dollar metacharacters are zero-width assertions. That is,
1081 they test for a particular condition being true without consuming any
1082 characters from the subject string.
1085 Outside a character class, in the default matching mode, the circumflex
1086 character is an assertion that is true only if the current matching point is at
1087 the start of the subject string. If the <i>startoffset</i> argument of
1088 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
1089 option is unset. Inside a character class, circumflex has an entirely different
1091 <a href="#characterclass">(see below).</a>
1094 Circumflex need not be the first character of the pattern if a number of
1095 alternatives are involved, but it should be the first thing in each alternative
1096 in which it appears if the pattern is ever to match that branch. If all
1097 possible alternatives start with a circumflex, that is, if the pattern is
1098 constrained to match only at the start of the subject, it is said to be an
1099 "anchored" pattern. (There are also other constructs that can cause a pattern
1103 The dollar character is an assertion that is true only if the current matching
1104 point is at the end of the subject string, or immediately before a newline at
1105 the end of the string (by default). Note, however, that it does not actually
1106 match the newline. Dollar need not be the last character of the pattern if a
1107 number of alternatives are involved, but it should be the last item in any
1108 branch in which it appears. Dollar has no special meaning in a character class.
1111 The meaning of dollar can be changed so that it matches only at the very end of
1112 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1113 does not affect the \Z assertion.
1116 The meanings of the circumflex and dollar characters are changed if the
1117 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1118 immediately after internal newlines as well as at the start of the subject
1119 string. It does not match after a newline that ends the string. A dollar
1120 matches before any newlines in the string, as well as at the very end, when
1121 PCRE_MULTILINE is set. When newline is specified as the two-character
1122 sequence CRLF, isolated CR and LF characters do not indicate newlines.
1125 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
1126 \n represents a newline) in multiline mode, but not otherwise. Consequently,
1127 patterns that are anchored in single line mode because all branches start with
1128 ^ are not anchored in multiline mode, and a match for circumflex is possible
1129 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
1130 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1133 Note that the sequences \A, \Z, and \z can be used to match the start and
1134 end of the subject in both modes, and if all branches of a pattern start with
1135 \A it is always anchored, whether or not PCRE_MULTILINE is set.
1136 <a name="fullstopdot"></a></P>
1137 <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
1139 Outside a character class, a dot in the pattern matches any one character in
1140 the subject string except (by default) a character that signifies the end of a
1144 When a line ending is defined as a single character, dot never matches that
1145 character; when the two-character sequence CRLF is used, dot does not match CR
1146 if it is immediately followed by LF, but otherwise it matches all characters
1147 (including isolated CRs and LFs). When any Unicode line endings are being
1148 recognized, dot does not match CR or LF or any of the other line ending
1152 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1153 option is set, a dot matches any one character, without exception. If the
1154 two-character sequence CRLF is present in the subject string, it takes two dots
1158 The handling of dot is entirely independent of the handling of circumflex and
1159 dollar, the only relationship being that they both involve newlines. Dot has no
1160 special meaning in a character class.
1163 The escape sequence \N behaves like a dot, except that it is not affected by
1164 the PCRE_DOTALL option. In other words, it matches any character except one
1165 that signifies the end of a line. Perl also uses \N to match characters by
1166 name; PCRE does not support this.
1168 <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1170 Outside a character class, the escape sequence \C matches any one data unit,
1171 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1172 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1173 a 32-bit unit. Unlike a dot, \C always
1174 matches line-ending characters. The feature is provided in Perl in order to
1175 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1176 used. Because \C breaks up characters into individual data units, matching one
1177 unit with \C in a UTF mode means that the rest of the string may start with a
1178 malformed UTF character. This has undefined results, because PCRE assumes that
1179 it is dealing with valid UTF strings (and by default it checks this at the
1180 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1181 PCRE_NO_UTF32_CHECK option is used).
1184 PCRE does not allow \C to appear in lookbehind assertions
1185 <a href="#lookbehind">(described below)</a>
1186 in a UTF mode, because this would make it impossible to calculate the length of
1190 In general, the \C escape sequence is best avoided. However, one
1191 way of using it that avoids the problem of malformed UTF characters is to use a
1192 lookahead to check the length of the next character, as in this pattern, which
1193 could be used with a UTF-8 string (ignore white space and line breaks):
1195 (?| (?=[\x00-\x7f])(\C) |
1196 (?=[\x80-\x{7ff}])(\C)(\C) |
1197 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1198 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1200 A group that starts with (?| resets the capturing parentheses numbers in each
1202 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1203 below). The assertions at the start of each branch check the next UTF-8
1204 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1205 character's individual bytes are then captured by the appropriate number of
1207 <a name="characterclass"></a></P>
1208 <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1210 An opening square bracket introduces a character class, terminated by a closing
1211 square bracket. A closing square bracket on its own is not special by default.
1212 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1213 bracket causes a compile-time error. If a closing square bracket is required as
1214 a member of the class, it should be the first data character in the class
1215 (after an initial circumflex, if present) or escaped with a backslash.
1218 A character class matches a single character in the subject. In a UTF mode, the
1219 character may be more than one data unit long. A matched character must be in
1220 the set of characters defined by the class, unless the first character in the
1221 class definition is a circumflex, in which case the subject character must not
1222 be in the set defined by the class. If a circumflex is actually required as a
1223 member of the class, ensure it is not the first character, or escape it with a
1227 For example, the character class [aeiou] matches any lower case vowel, while
1228 [^aeiou] matches any character that is not a lower case vowel. Note that a
1229 circumflex is just a convenient notation for specifying the characters that
1230 are in the class by enumerating those that are not. A class that starts with a
1231 circumflex is not an assertion; it still consumes a character from the subject
1232 string, and therefore it fails if the current pointer is at the end of the
1236 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1237 can be included in a class as a literal string of data units, or by using the
1238 \x{ escaping mechanism.
1241 When caseless matching is set, any letters in a class represent both their
1242 upper case and lower case versions, so for example, a caseless [aeiou] matches
1243 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1244 caseful version would. In a UTF mode, PCRE always understands the concept of
1245 case for characters whose values are less than 128, so caseless matching is
1246 always possible. For characters with higher values, the concept of case is
1247 supported if PCRE is compiled with Unicode property support, but not otherwise.
1248 If you want to use caseless matching in a UTF mode for characters 128 and
1249 above, you must ensure that PCRE is compiled with Unicode property support as
1250 well as with UTF support.
1253 Characters that might indicate line breaks are never treated in any special way
1254 when matching character classes, whatever line-ending sequence is in use, and
1255 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1256 such as [^a] always matches one of these characters.
1259 The minus (hyphen) character can be used to specify a range of characters in a
1260 character class. For example, [d-m] matches any letter between d and m,
1261 inclusive. If a minus character is required in a class, it must be escaped with
1262 a backslash or appear in a position where it cannot be interpreted as
1263 indicating a range, typically as the first or last character in the class, or
1264 immediately after a range. For example, [b-d-z] matches letters in the range b
1265 to d, a hyphen character, or z.
1268 It is not possible to have the literal character "]" as the end character of a
1269 range. A pattern such as [W-]46] is interpreted as a class of two characters
1270 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1271 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1272 the end of range, so [W-\]46] is interpreted as a class containing a range
1273 followed by two other characters. The octal or hexadecimal representation of
1274 "]" can also be used to end a range.
1277 An error is generated if a POSIX character class (see below) or an escape
1278 sequence other than one that defines a single character appears at a point
1279 where a range ending character is expected. For example, [z-\xff] is valid,
1280 but [A-\d] and [A-[:digit:]] are not.
1283 Ranges operate in the collating sequence of character values. They can also be
1284 used for characters specified numerically, for example [\000-\037]. Ranges
1285 can include any characters that are valid for the current mode.
1288 If a range that includes letters is used when caseless matching is set, it
1289 matches the letters in either case. For example, [W-c] is equivalent to
1290 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1291 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1292 characters in both cases. In UTF modes, PCRE supports the concept of case for
1293 characters with values greater than 128 only when it is compiled with Unicode
1297 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1298 \V, \w, and \W may appear in a character class, and add the characters that
1299 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1300 digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1301 and their upper case partners, just as it does when they appear outside a
1302 character class, as described in the section entitled
1303 <a href="#genericchartypes">"Generic character types"</a>
1304 above. The escape sequence \b has a different meaning inside a character
1305 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1306 are not special inside a character class. Like any other unrecognized escape
1307 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1308 default, but cause an error if the PCRE_EXTRA option is set.
1311 A circumflex can conveniently be used with the upper case character types to
1312 specify a more restricted set of characters than the matching lower case type.
1313 For example, the class [^\W_] matches any letter or digit, but not underscore,
1314 whereas [\w] includes underscore. A positive character class should be read as
1315 "something OR something OR ..." and a negative class as "NOT something AND NOT
1316 something AND NOT ...".
1319 The only metacharacters that are recognized in character classes are backslash,
1320 hyphen (only where it can be interpreted as specifying a range), circumflex
1321 (only at the start), opening square bracket (only when it can be interpreted as
1322 introducing a POSIX class name, or for a special compatibility feature - see
1323 the next two sections), and the terminating closing square bracket. However,
1324 escaping other non-alphanumeric characters does no harm.
1326 <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1328 Perl supports the POSIX notation for character classes. This uses names
1329 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1330 this notation. For example,
1334 matches "0", "1", any alphabetic character, or "%". The supported class names
1337 alnum letters and digits
1339 ascii character codes 0 - 127
1340 blank space or tab only
1341 cntrl control characters
1342 digit decimal digits (same as \d)
1343 graph printing characters, excluding space
1344 lower lower case letters
1345 print printing characters, including space
1346 punct printing characters, excluding letters and digits and space
1347 space white space (the same as \s from PCRE 8.34)
1348 upper upper case letters
1349 word "word" characters (same as \w)
1350 xdigit hexadecimal digits
1352 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1353 and space (32). If locale-specific matching is taking place, the list of space
1354 characters may be different; there may be fewer or more of them. "Space" used
1355 to be different to \s, which did not include VT, for Perl compatibility.
1356 However, Perl changed at release 5.18, and PCRE followed at release 8.34.
1357 "Space" and \s now match the same set of characters.
1360 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1361 5.8. Another Perl extension is negation, which is indicated by a ^ character
1362 after the colon. For example,
1366 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1367 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1368 supported, and an error is given if they are encountered.
1371 By default, characters with values greater than 128 do not match any of the
1372 POSIX character classes. However, if the PCRE_UCP option is passed to
1373 <b>pcre_compile()</b>, some of the classes are changed so that Unicode character
1374 properties are used. This is achieved by replacing certain POSIX classes by
1375 other sequences, as follows:
1377 [:alnum:] becomes \p{Xan}
1378 [:alpha:] becomes \p{L}
1379 [:blank:] becomes \h
1380 [:digit:] becomes \p{Nd}
1381 [:lower:] becomes \p{Ll}
1382 [:space:] becomes \p{Xps}
1383 [:upper:] becomes \p{Lu}
1384 [:word:] becomes \p{Xwd}
1386 Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
1387 classes are handled specially in UCP mode:
1391 This matches characters that have glyphs that mark the page when printed. In
1392 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1393 properties, except for:
1395 U+061C Arabic Letter Mark
1396 U+180E Mongolian Vowel Separator
1397 U+2066 - U+2069 Various "isolate"s
1403 This matches the same characters as [:graph:] plus space characters that are
1404 not controls, that is, characters with the Zs property.
1408 This matches all characters that have the Unicode P (punctuation) property,
1409 plus those characters whose code points are less than 128 that have the S
1413 The other POSIX classes are unchanged, and match only characters with code
1414 points less than 128.
1416 <br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br>
1418 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
1419 syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
1420 word". PCRE treats these items as follows:
1422 [[:<:]] is converted to \b(?=\w)
1423 [[:>:]] is converted to \b(?<=\w)
1425 Only these exact character sequences are recognized. A sequence such as
1426 [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
1427 not compatible with Perl. It is provided to help migrations from other
1428 environments, and is best not used in any new patterns. Note that \b matches
1429 at the start and the end of a word (see
1430 <a href="#smallassertions">"Simple assertions"</a>
1431 above), and in a Perl-style pattern the preceding or following character
1432 normally shows which is wanted, without the need for the assertions that are
1433 used above in order to give exactly the POSIX behaviour.
1435 <br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br>
1437 Vertical bar characters are used to separate alternative patterns. For example,
1442 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1443 and an empty alternative is permitted (matching the empty string). The matching
1444 process tries each alternative in turn, from left to right, and the first one
1445 that succeeds is used. If the alternatives are within a subpattern
1446 <a href="#subpattern">(defined below),</a>
1447 "succeeds" means matching the rest of the main pattern as well as the
1448 alternative in the subpattern.
1450 <br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1452 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1453 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1454 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1455 The option letters are
1458 m for PCRE_MULTILINE
1462 For example, (?im) sets caseless, multiline matching. It is also possible to
1463 unset these options by preceding the letter with a hyphen, and a combined
1464 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1465 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1466 permitted. If a letter appears both before and after the hyphen, the option is
1470 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1471 changed in the same way as the Perl-compatible options by using the characters
1472 J, U and X respectively.
1475 When one of these option changes occurs at top level (that is, not inside
1476 subpattern parentheses), the change applies to the remainder of the pattern
1477 that follows. If the change is placed right at the start of a pattern, PCRE
1478 extracts it into the global options (and it will therefore show up in data
1479 extracted by the <b>pcre_fullinfo()</b> function).
1482 An option change within a subpattern (see below for a description of
1483 subpatterns) affects only that part of the subpattern that follows it, so
1487 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1488 By this means, options can be made to have different settings in different
1489 parts of the pattern. Any changes made in one alternative do carry on
1490 into subsequent branches within the same subpattern. For example,
1494 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1495 branch is abandoned before the option setting. This is because the effects of
1496 option settings happen at compile time. There would be some very weird
1497 behaviour otherwise.
1500 <b>Note:</b> There are other PCRE-specific options that can be set by the
1501 application when the compiling or matching functions are called. In some cases
1502 the pattern can contain special leading sequences such as (*CRLF) to override
1503 what the application has set or what has been defaulted. Details are given in
1504 the section entitled
1505 <a href="#newlineseq">"Newline sequences"</a>
1506 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1507 sequences that can be used to set UTF and Unicode property modes; they are
1508 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1509 options, respectively. The (*UTF) sequence is a generic version that can be
1510 used with any of the libraries. However, the application can set the
1511 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1512 <a name="subpattern"></a></P>
1513 <br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br>
1515 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1516 Turning part of a pattern into a subpattern does two things:
1519 1. It localizes a set of alternatives. For example, the pattern
1521 cat(aract|erpillar|)
1523 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1524 match "cataract", "erpillar" or an empty string.
1527 2. It sets up the subpattern as a capturing subpattern. This means that, when
1528 the whole pattern matches, that portion of the subject string that matched the
1529 subpattern is passed back to the caller via the <i>ovector</i> argument of the
1530 matching function. (This applies only to the traditional matching functions;
1531 the DFA matching functions do not support capturing.)
1534 Opening parentheses are counted from left to right (starting from 1) to obtain
1535 numbers for the capturing subpatterns. For example, if the string "the red
1536 king" is matched against the pattern
1538 the ((red|white) (king|queen))
1540 the captured substrings are "red king", "red", and "king", and are numbered 1,
1541 2, and 3, respectively.
1544 The fact that plain parentheses fulfil two functions is not always helpful.
1545 There are often times when a grouping subpattern is required without a
1546 capturing requirement. If an opening parenthesis is followed by a question mark
1547 and a colon, the subpattern does not do any capturing, and is not counted when
1548 computing the number of any subsequent capturing subpatterns. For example, if
1549 the string "the white queen" is matched against the pattern
1551 the ((?:red|white) (king|queen))
1553 the captured substrings are "white queen" and "queen", and are numbered 1 and
1554 2. The maximum number of capturing subpatterns is 65535.
1557 As a convenient shorthand, if any option settings are required at the start of
1558 a non-capturing subpattern, the option letters may appear between the "?" and
1559 the ":". Thus the two patterns
1561 (?i:saturday|sunday)
1562 (?:(?i)saturday|sunday)
1564 match exactly the same set of strings. Because alternative branches are tried
1565 from left to right, and options are not reset until the end of the subpattern
1566 is reached, an option setting in one branch does affect subsequent branches, so
1567 the above patterns match "SUNDAY" as well as "Saturday".
1568 <a name="dupsubpatternnumber"></a></P>
1569 <br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1571 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1572 the same numbers for its capturing parentheses. Such a subpattern starts with
1573 (?| and is itself a non-capturing subpattern. For example, consider this
1576 (?|(Sat)ur|(Sun))day
1578 Because the two alternatives are inside a (?| group, both sets of capturing
1579 parentheses are numbered one. Thus, when the pattern matches, you can look
1580 at captured substring number one, whichever alternative matched. This construct
1581 is useful when you want to capture part, but not all, of one of a number of
1582 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1583 number is reset at the start of each branch. The numbers of any capturing
1584 parentheses that follow the subpattern start after the highest number used in
1585 any branch. The following example is taken from the Perl documentation. The
1586 numbers underneath show in which buffer the captured content will be stored.
1588 # before ---------------branch-reset----------- after
1589 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1592 A back reference to a numbered subpattern uses the most recent value that is
1593 set for that number by any subpattern. The following pattern matches "abcabc"
1598 In contrast, a subroutine call to a numbered subpattern always refers to the
1599 first one in the pattern with the given number. The following pattern matches
1600 "abcabc" or "defabc":
1602 /(?|(abc)|(def))(?1)/
1605 <a href="#conditions">condition test</a>
1606 for a subpattern's having matched refers to a non-unique number, the test is
1607 true if any of the subpatterns of that number have matched.
1610 An alternative approach to using this "branch reset" feature is to use
1611 duplicate named subpatterns, as described in the next section.
1613 <br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br>
1615 Identifying capturing parentheses by number is simple, but it can be very hard
1616 to keep track of the numbers in complicated regular expressions. Furthermore,
1617 if an expression is modified, the numbers may change. To help with this
1618 difficulty, PCRE supports the naming of subpatterns. This feature was not
1619 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1620 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1621 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1622 have different names, but PCRE does not.
1625 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1626 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1627 parentheses from other parts of the pattern, such as
1628 <a href="#backreferences">back references,</a>
1629 <a href="#recursion">recursion,</a>
1631 <a href="#conditions">conditions,</a>
1632 can be made by name as well as by number.
1635 Names consist of up to 32 alphanumeric characters and underscores, but must
1636 start with a non-digit. Named capturing parentheses are still allocated numbers
1637 as well as names, exactly as if the names were not present. The PCRE API
1638 provides function calls for extracting the name-to-number translation table
1639 from a compiled pattern. There is also a convenience function for extracting a
1640 captured substring by name.
1643 By default, a name must be unique within a pattern, but it is possible to relax
1644 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1645 names are also always permitted for subpatterns with the same number, set up as
1646 described in the previous section.) Duplicate names can be useful for patterns
1647 where only one instance of the named parentheses can match. Suppose you want to
1648 match the name of a weekday, either as a 3-letter abbreviation or as the full
1649 name, and in both cases you want to extract the abbreviation. This pattern
1650 (ignoring the line breaks) does the job:
1652 (?<DN>Mon|Fri|Sun)(?:day)?|
1653 (?<DN>Tue)(?:sday)?|
1654 (?<DN>Wed)(?:nesday)?|
1655 (?<DN>Thu)(?:rsday)?|
1656 (?<DN>Sat)(?:urday)?
1658 There are five capturing substrings, but only one is ever set after a match.
1659 (An alternative way of solving this problem is to use a "branch reset"
1660 subpattern, as described in the previous section.)
1663 The convenience function for extracting the data by name returns the substring
1664 for the first (and in this example, the only) subpattern of that name that
1665 matched. This saves searching to find which numbered subpattern it was.
1668 If you make a back reference to a non-unique named subpattern from elsewhere in
1669 the pattern, the subpatterns to which the name refers are checked in the order
1670 in which they appear in the overall pattern. The first one that is set is used
1671 for the reference. For example, this pattern matches both "foofoo" and
1672 "barbar" but not "foobar" or "barfoo":
1674 (?:(?<n>foo)|(?<n>bar))\k<n>
1679 If you make a subroutine call to a non-unique named subpattern, the one that
1680 corresponds to the first occurrence of the name is used. In the absence of
1681 duplicate numbers (see the previous section) this is the one with the lowest
1685 If you use a named reference in a condition
1687 <a href="#conditions">section about conditions</a>
1688 below), either to check whether a subpattern has matched, or to check for
1689 recursion, all subpatterns with the same name are tested. If the condition is
1690 true for any one of them, the overall condition is true. This is the same
1691 behaviour as testing by number. For further details of the interfaces for
1692 handling named subpatterns, see the
1693 <a href="pcreapi.html"><b>pcreapi</b></a>
1697 <b>Warning:</b> You cannot use different names to distinguish between two
1698 subpatterns with the same number because PCRE uses only the numbers when
1699 matching. For this reason, an error is given at compile time if different names
1700 are given to subpatterns with the same number. However, you can always give the
1701 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1704 <br><a name="SEC17" href="#TOC1">REPETITION</a><br>
1706 Repetition is specified by quantifiers, which can follow any of the following
1709 a literal data character
1710 the dot metacharacter
1711 the \C escape sequence
1712 the \X escape sequence
1713 the \R escape sequence
1714 an escape such as \d or \pL that matches a single character
1716 a back reference (see next section)
1717 a parenthesized subpattern (including assertions)
1718 a subroutine call to a subpattern (recursive or otherwise)
1720 The general repetition quantifier specifies a minimum and maximum number of
1721 permitted matches, by giving the two numbers in curly brackets (braces),
1722 separated by a comma. The numbers must be less than 65536, and the first must
1723 be less than or equal to the second. For example:
1727 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1728 character. If the second number is omitted, but the comma is present, there is
1729 no upper limit; if the second number and the comma are both omitted, the
1730 quantifier specifies an exact number of required matches. Thus
1734 matches at least 3 successive vowels, but may match many more, while
1738 matches exactly 8 digits. An opening curly bracket that appears in a position
1739 where a quantifier is not allowed, or one that does not match the syntax of a
1740 quantifier, is taken as a literal character. For example, {,6} is not a
1741 quantifier, but a literal string of four characters.
1744 In UTF modes, quantifiers apply to characters rather than to individual data
1745 units. Thus, for example, \x{100}{2} matches two characters, each of
1746 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1747 \X{3} matches three Unicode extended grapheme clusters, each of which may be
1748 several data units long (and they may be of different lengths).
1751 The quantifier {0} is permitted, causing the expression to behave as if the
1752 previous item and the quantifier were not present. This may be useful for
1753 subpatterns that are referenced as
1754 <a href="#subpatternsassubroutines">subroutines</a>
1755 from elsewhere in the pattern (but see also the section entitled
1756 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1757 below). Items other than subpatterns that have a {0} quantifier are omitted
1758 from the compiled pattern.
1761 For convenience, the three most common quantifiers have single-character
1764 * is equivalent to {0,}
1765 + is equivalent to {1,}
1766 ? is equivalent to {0,1}
1768 It is possible to construct infinite loops by following a subpattern that can
1769 match no characters with a quantifier that has no upper limit, for example:
1773 Earlier versions of Perl and PCRE used to give an error at compile time for
1774 such patterns. However, because there are cases where this can be useful, such
1775 patterns are now accepted, but if any repetition of the subpattern does in fact
1776 match no characters, the loop is forcibly broken.
1779 By default, the quantifiers are "greedy", that is, they match as much as
1780 possible (up to the maximum number of permitted times), without causing the
1781 rest of the pattern to fail. The classic example of where this gives problems
1782 is in trying to match comments in C programs. These appear between /* and */
1783 and within the comment, individual * and / characters may appear. An attempt to
1784 match C comments by applying the pattern
1790 /* first comment */ not comment /* second comment */
1792 fails, because it matches the entire string owing to the greediness of the .*
1796 However, if a quantifier is followed by a question mark, it ceases to be
1797 greedy, and instead matches the minimum number of times possible, so the
1802 does the right thing with the C comments. The meaning of the various
1803 quantifiers is not otherwise changed, just the preferred number of matches.
1804 Do not confuse this use of question mark with its use as a quantifier in its
1805 own right. Because it has two uses, it can sometimes appear doubled, as in
1809 which matches one digit by preference, but can match two if that is the only
1810 way the rest of the pattern matches.
1813 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1814 the quantifiers are not greedy by default, but individual ones can be made
1815 greedy by following them with a question mark. In other words, it inverts the
1819 When a parenthesized subpattern is quantified with a minimum repeat count that
1820 is greater than 1 or with a limited maximum, more memory is required for the
1821 compiled pattern, in proportion to the size of the minimum or maximum.
1824 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1825 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1826 implicitly anchored, because whatever follows will be tried against every
1827 character position in the subject string, so there is no point in retrying the
1828 overall match at any position after the first. PCRE normally treats such a
1829 pattern as though it were preceded by \A.
1832 In cases where it is known that the subject string contains no newlines, it is
1833 worth setting PCRE_DOTALL in order to obtain this optimization, or
1834 alternatively using ^ to indicate anchoring explicitly.
1837 However, there are some cases where the optimization cannot be used. When .*
1838 is inside capturing parentheses that are the subject of a back reference
1839 elsewhere in the pattern, a match at the start may fail where a later one
1840 succeeds. Consider, for example:
1844 If the subject is "xyz123abc123" the match point is the fourth character. For
1845 this reason, such a pattern is not implicitly anchored.
1848 Another case where implicit anchoring is not applied is when the leading .* is
1849 inside an atomic group. Once again, a match at the start may fail where a later
1850 one succeeds. Consider this pattern:
1854 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1855 (*PRUNE) and (*SKIP) also disable this optimization.
1858 When a capturing subpattern is repeated, the value captured is the substring
1859 that matched the final iteration. For example, after
1861 (tweedle[dume]{3}\s*)+
1863 has matched "tweedledum tweedledee" the value of the captured substring is
1864 "tweedledee". However, if there are nested capturing subpatterns, the
1865 corresponding captured values may have been set in previous iterations. For
1870 matches "aba" the value of the second captured substring is "b".
1871 <a name="atomicgroup"></a></P>
1872 <br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1874 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1875 repetition, failure of what follows normally causes the repeated item to be
1876 re-evaluated to see if a different number of repeats allows the rest of the
1877 pattern to match. Sometimes it is useful to prevent this, either to change the
1878 nature of the match, or to cause it fail earlier than it otherwise might, when
1879 the author of the pattern knows there is no point in carrying on.
1882 Consider, for example, the pattern \d+foo when applied to the subject line
1886 After matching all 6 digits and then failing to match "foo", the normal
1887 action of the matcher is to try again with only 5 digits matching the \d+
1888 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1889 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1890 that once a subpattern has matched, it is not to be re-evaluated in this way.
1893 If we use atomic grouping for the previous example, the matcher gives up
1894 immediately on failing to match "foo" the first time. The notation is a kind of
1895 special parenthesis, starting with (?> as in this example:
1899 This kind of parenthesis "locks up" the part of the pattern it contains once
1900 it has matched, and a failure further into the pattern is prevented from
1901 backtracking into it. Backtracking past it to previous items, however, works as
1905 An alternative description is that a subpattern of this type matches the string
1906 of characters that an identical standalone pattern would match, if anchored at
1907 the current point in the subject string.
1910 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1911 the above example can be thought of as a maximizing repeat that must swallow
1912 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1913 number of digits they match in order to make the rest of the pattern match,
1914 (?>\d+) can only match an entire sequence of digits.
1917 Atomic groups in general can of course contain arbitrarily complicated
1918 subpatterns, and can be nested. However, when the subpattern for an atomic
1919 group is just a single repeated item, as in the example above, a simpler
1920 notation, called a "possessive quantifier" can be used. This consists of an
1921 additional + character following a quantifier. Using this notation, the
1922 previous example can be rewritten as
1926 Note that a possessive quantifier can be used with an entire group, for
1931 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1932 option is ignored. They are a convenient notation for the simpler forms of
1933 atomic group. However, there is no difference in the meaning of a possessive
1934 quantifier and the equivalent atomic group, though there may be a performance
1935 difference; possessive quantifiers should be slightly faster.
1938 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1939 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1940 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1941 package, and PCRE copied it from there. It ultimately found its way into Perl
1945 PCRE has an optimization that automatically "possessifies" certain simple
1946 pattern constructs. For example, the sequence A+B is treated as A++B because
1947 there is no point in backtracking into a sequence of A's when B must follow.
1950 When a pattern contains an unlimited repeat inside a subpattern that can itself
1951 be repeated an unlimited number of times, the use of an atomic group is the
1952 only way to avoid some failing matches taking a very long time indeed. The
1955 (\D+|<\d+>)*[!?]
1957 matches an unlimited number of substrings that either consist of non-digits, or
1958 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1959 quickly. However, if it is applied to
1961 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1963 it takes a long time before reporting failure. This is because the string can
1964 be divided between the internal \D+ repeat and the external * repeat in a
1965 large number of ways, and all have to be tried. (The example uses [!?] rather
1966 than a single character at the end, because both PCRE and Perl have an
1967 optimization that allows for fast failure when a single character is used. They
1968 remember the last single character that is required for a match, and fail early
1969 if it is not present in the string.) If the pattern is changed so that it uses
1970 an atomic group, like this:
1972 ((?>\D+)|<\d+>)*[!?]
1974 sequences of non-digits cannot be broken, and failure happens quickly.
1975 <a name="backreferences"></a></P>
1976 <br><a name="SEC19" href="#TOC1">BACK REFERENCES</a><br>
1978 Outside a character class, a backslash followed by a digit greater than 0 (and
1979 possibly further digits) is a back reference to a capturing subpattern earlier
1980 (that is, to its left) in the pattern, provided there have been that many
1981 previous capturing left parentheses.
1984 However, if the decimal number following the backslash is less than 10, it is
1985 always taken as a back reference, and causes an error only if there are not
1986 that many capturing left parentheses in the entire pattern. In other words, the
1987 parentheses that are referenced need not be to the left of the reference for
1988 numbers less than 10. A "forward back reference" of this type can make sense
1989 when a repetition is involved and the subpattern to the right has participated
1990 in an earlier iteration.
1993 It is not possible to have a numerical "forward back reference" to a subpattern
1994 whose number is 10 or more using this syntax because a sequence such as \50 is
1995 interpreted as a character defined in octal. See the subsection entitled
1996 "Non-printing characters"
1997 <a href="#digitsafterbackslash">above</a>
1998 for further details of the handling of digits following a backslash. There is
1999 no such problem when named parentheses are used. A back reference to any
2000 subpattern is possible using named parentheses (see below).
2003 Another way of avoiding the ambiguity inherent in the use of digits following a
2004 backslash is to use the \g escape sequence. This escape must be followed by an
2005 unsigned number or a negative number, optionally enclosed in braces. These
2006 examples are all identical:
2012 An unsigned number specifies an absolute reference without the ambiguity that
2013 is present in the older syntax. It is also useful when literal digits follow
2014 the reference. A negative number is a relative reference. Consider this
2019 The sequence \g{-1} is a reference to the most recently started capturing
2020 subpattern before \g, that is, is it equivalent to \2 in this example.
2021 Similarly, \g{-2} would be equivalent to \1. The use of relative references
2022 can be helpful in long patterns, and also in patterns that are created by
2023 joining together fragments that contain references within themselves.
2026 A back reference matches whatever actually matched the capturing subpattern in
2027 the current subject string, rather than anything matching the subpattern
2029 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
2030 below for a way of doing that). So the pattern
2032 (sens|respons)e and \1ibility
2034 matches "sense and sensibility" and "response and responsibility", but not
2035 "sense and responsibility". If caseful matching is in force at the time of the
2036 back reference, the case of letters is relevant. For example,
2040 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
2041 capturing subpattern is matched caselessly.
2044 There are several different ways of writing back references to named
2045 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
2046 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
2047 back reference syntax, in which \g can be used for both numeric and named
2048 references, is also supported. We could rewrite the above example in any of
2051 (?<p1>(?i)rah)\s+\k<p1>
2052 (?'p1'(?i)rah)\s+\k{p1}
2053 (?P<p1>(?i)rah)\s+(?P=p1)
2054 (?<p1>(?i)rah)\s+\g{p1}
2056 A subpattern that is referenced by name may appear in the pattern before or
2057 after the reference.
2060 There may be more than one back reference to the same subpattern. If a
2061 subpattern has not actually been used in a particular match, any back
2062 references to it always fail by default. For example, the pattern
2066 always fails if it starts to match "a" rather than "bc". However, if the
2067 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2068 unset value matches an empty string.
2071 Because there may be many capturing parentheses in a pattern, all digits
2072 following a backslash are taken as part of a potential back reference number.
2073 If the pattern continues with a digit character, some delimiter must be used to
2074 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2075 white space. Otherwise, the \g{ syntax or an empty comment (see
2076 <a href="#comments">"Comments"</a>
2080 Recursive back references
2083 A back reference that occurs inside the parentheses to which it refers fails
2084 when the subpattern is first used, so, for example, (a\1) never matches.
2085 However, such references can be useful inside repeated subpatterns. For
2086 example, the pattern
2090 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
2091 the subpattern, the back reference matches the character string corresponding
2092 to the previous iteration. In order for this to work, the pattern must be such
2093 that the first iteration does not need to match the back reference. This can be
2094 done using alternation, as in the example above, or by a quantifier with a
2098 Back references of this type cause the group that they reference to be treated
2100 <a href="#atomicgroup">atomic group.</a>
2101 Once the whole group has been matched, a subsequent matching failure cannot
2102 cause backtracking into the middle of the group.
2103 <a name="bigassertions"></a></P>
2104 <br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br>
2106 An assertion is a test on the characters following or preceding the current
2107 matching point that does not actually consume any characters. The simple
2108 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
2109 <a href="#smallassertions">above.</a>
2112 More complicated assertions are coded as subpatterns. There are two kinds:
2113 those that look ahead of the current position in the subject string, and those
2114 that look behind it. An assertion subpattern is matched in the normal way,
2115 except that it does not cause the current matching position to be changed.
2118 Assertion subpatterns are not capturing subpatterns. If such an assertion
2119 contains capturing subpatterns within it, these are counted for the purposes of
2120 numbering the capturing subpatterns in the whole pattern. However, substring
2121 capturing is carried out only for positive assertions. (Perl sometimes, but not
2122 always, does do capturing in negative assertions.)
2125 For compatibility with Perl, assertion subpatterns may be repeated; though
2126 it makes no sense to assert the same thing several times, the side effect of
2127 capturing parentheses may occasionally be useful. In practice, there only three
2131 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2132 However, it may contain internal capturing parenthesized groups that are called
2133 from elsewhere via the
2134 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
2137 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2138 were {0,1}. At run time, the rest of the pattern match is tried with and
2139 without the assertion, the order depending on the greediness of the quantifier.
2142 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2143 The assertion is obeyed just once when encountered during matching.
2146 Lookahead assertions
2149 Lookahead assertions start with (?= for positive assertions and (?! for
2150 negative assertions. For example,
2154 matches a word followed by a semicolon, but does not include the semicolon in
2159 matches any occurrence of "foo" that is not followed by "bar". Note that the
2160 apparently similar pattern
2164 does not find an occurrence of "bar" that is preceded by something other than
2165 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2166 (?!foo) is always true when the next three characters are "bar". A
2167 lookbehind assertion is needed to achieve the other effect.
2170 If you want to force a matching failure at some point in a pattern, the most
2171 convenient way to do it is with (?!) because an empty string always matches, so
2172 an assertion that requires there not to be an empty string must always fail.
2173 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2174 <a name="lookbehind"></a></P>
2176 Lookbehind assertions
2179 Lookbehind assertions start with (?<= for positive assertions and (?<! for
2180 negative assertions. For example,
2184 does find an occurrence of "bar" that is not preceded by "foo". The contents of
2185 a lookbehind assertion are restricted such that all the strings it matches must
2186 have a fixed length. However, if there are several top-level alternatives, they
2187 do not all have to have the same fixed length. Thus
2189 (?<=bullock|donkey)
2193 (?<!dogs?|cats?)
2195 causes an error at compile time. Branches that match different length strings
2196 are permitted only at the top level of a lookbehind assertion. This is an
2197 extension compared with Perl, which requires all branches to match the same
2198 length of string. An assertion such as
2202 is not permitted, because its single top-level branch can match two different
2203 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2208 In some cases, the escape sequence \K
2209 <a href="#resetmatchstart">(see above)</a>
2210 can be used instead of a lookbehind assertion to get round the fixed-length
2214 The implementation of lookbehind assertions is, for each alternative, to
2215 temporarily move the current position back by the fixed length and then try to
2216 match. If there are insufficient characters before the current position, the
2220 In a UTF mode, PCRE does not allow the \C escape (which matches a single data
2221 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2222 it impossible to calculate the length of the lookbehind. The \X and \R
2223 escapes, which can match different numbers of data units, are also not
2227 <a href="#subpatternsassubroutines">"Subroutine"</a>
2228 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2229 as the subpattern matches a fixed-length string.
2230 <a href="#recursion">Recursion,</a>
2231 however, is not supported.
2234 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2235 specify efficient matching of fixed-length strings at the end of subject
2236 strings. Consider a simple pattern such as
2240 when applied to a long string that does not match. Because matching proceeds
2241 from left to right, PCRE will look for each "a" in the subject and then see if
2242 what follows matches the rest of the pattern. If the pattern is specified as
2246 the initial .* matches the entire string at first, but when this fails (because
2247 there is no following "a"), it backtracks to match all but the last character,
2248 then all but the last two characters, and so on. Once again the search for "a"
2249 covers the entire string, from right to left, so we are no better off. However,
2250 if the pattern is written as
2254 there can be no backtracking for the .*+ item; it can match only the entire
2255 string. The subsequent lookbehind assertion does a single test on the last four
2256 characters. If it fails, the match fails immediately. For long strings, this
2257 approach makes a significant difference to the processing time.
2260 Using multiple assertions
2263 Several assertions (of any sort) may occur in succession. For example,
2265 (?<=\d{3})(?<!999)foo
2267 matches "foo" preceded by three digits that are not "999". Notice that each of
2268 the assertions is applied independently at the same point in the subject
2269 string. First there is a check that the previous three characters are all
2270 digits, and then there is a check that the same three characters are not "999".
2271 This pattern does <i>not</i> match "foo" preceded by six characters, the first
2272 of which are digits and the last three of which are not "999". For example, it
2273 doesn't match "123abcfoo". A pattern to do that is
2275 (?<=\d{3}...)(?<!999)foo
2277 This time the first assertion looks at the preceding six characters, checking
2278 that the first three are digits, and then the second assertion checks that the
2279 preceding three characters are not "999".
2282 Assertions can be nested in any combination. For example,
2284 (?<=(?<!foo)bar)baz
2286 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2287 preceded by "foo", while
2289 (?<=\d{3}(?!999)...)foo
2291 is another pattern that matches "foo" preceded by three digits and any three
2292 characters that are not "999".
2293 <a name="conditions"></a></P>
2294 <br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2296 It is possible to cause the matching process to obey a subpattern
2297 conditionally or to choose between two alternative subpatterns, depending on
2298 the result of an assertion, or whether a specific capturing subpattern has
2299 already been matched. The two possible forms of conditional subpattern are:
2301 (?(condition)yes-pattern)
2302 (?(condition)yes-pattern|no-pattern)
2304 If the condition is satisfied, the yes-pattern is used; otherwise the
2305 no-pattern (if present) is used. If there are more than two alternatives in the
2306 subpattern, a compile-time error occurs. Each of the two alternatives may
2307 itself contain nested subpatterns of any form, including conditional
2308 subpatterns; the restriction to two alternatives applies only at the level of
2309 the condition. This pattern fragment is an example where the alternatives are
2312 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2317 There are four kinds of condition: references to subpatterns, references to
2318 recursion, a pseudo-condition called DEFINE, and assertions.
2321 Checking for a used subpattern by number
2324 If the text between the parentheses consists of a sequence of digits, the
2325 condition is true if a capturing subpattern of that number has previously
2326 matched. If there is more than one capturing subpattern with the same number
2328 <a href="#recursion">section about duplicate subpattern numbers),</a>
2329 the condition is true if any of them have matched. An alternative notation is
2330 to precede the digits with a plus or minus sign. In this case, the subpattern
2331 number is relative rather than absolute. The most recently opened parentheses
2332 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2333 loops it can also make sense to refer to subsequent groups. The next
2334 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2335 zero in any of these forms is not used; it provokes a compile-time error.)
2338 Consider the following pattern, which contains non-significant white space to
2339 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2340 three parts for ease of discussion:
2342 ( \( )? [^()]+ (?(1) \) )
2344 The first part matches an optional opening parenthesis, and if that
2345 character is present, sets it as the first captured substring. The second part
2346 matches one or more characters that are not parentheses. The third part is a
2347 conditional subpattern that tests whether or not the first set of parentheses
2348 matched. If they did, that is, if subject started with an opening parenthesis,
2349 the condition is true, and so the yes-pattern is executed and a closing
2350 parenthesis is required. Otherwise, since no-pattern is not present, the
2351 subpattern matches nothing. In other words, this pattern matches a sequence of
2352 non-parentheses, optionally enclosed in parentheses.
2355 If you were embedding this pattern in a larger one, you could use a relative
2358 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2360 This makes the fragment independent of the parentheses in the larger pattern.
2363 Checking for a used subpattern by name
2366 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2367 subpattern by name. For compatibility with earlier versions of PCRE, which had
2368 this facility before Perl, the syntax (?(name)...) is also recognized.
2371 Rewriting the above example to use a named subpattern gives this:
2373 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
2375 If the name used in a condition of this kind is a duplicate, the test is
2376 applied to all subpatterns of the same name, and is true if any one of them has
2380 Checking for pattern recursion
2383 If the condition is the string (R), and there is no subpattern with the name R,
2384 the condition is true if a recursive call to the whole pattern or any
2385 subpattern has been made. If digits or a name preceded by ampersand follow the
2386 letter R, for example:
2388 (?(R3)...) or (?(R&name)...)
2390 the condition is true if the most recent recursion is into a subpattern whose
2391 number or name is given. This condition does not check the entire recursion
2392 stack. If the name used in a condition of this kind is a duplicate, the test is
2393 applied to all subpatterns of the same name, and is true if any one of them is
2394 the most recent recursion.
2397 At "top level", all these recursion test conditions are false.
2398 <a href="#recursion">The syntax for recursive patterns</a>
2400 <a name="subdefine"></a></P>
2402 Defining subpatterns for use by reference only
2405 If the condition is the string (DEFINE), and there is no subpattern with the
2406 name DEFINE, the condition is always false. In this case, there may be only one
2407 alternative in the subpattern. It is always skipped if control reaches this
2408 point in the pattern; the idea of DEFINE is that it can be used to define
2409 subroutines that can be referenced from elsewhere. (The use of
2410 <a href="#subpatternsassubroutines">subroutines</a>
2411 is described below.) For example, a pattern to match an IPv4 address such as
2412 "192.168.23.245" could be written like this (ignore white space and line
2415 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2416 \b (?&byte) (\.(?&byte)){3} \b
2418 The first part of the pattern is a DEFINE group inside which a another group
2419 named "byte" is defined. This matches an individual component of an IPv4
2420 address (a number less than 256). When matching takes place, this part of the
2421 pattern is skipped because DEFINE acts like a false condition. The rest of the
2422 pattern uses references to the named group to match the four dot-separated
2423 components of an IPv4 address, insisting on a word boundary at each end.
2426 Assertion conditions
2429 If the condition is not in any of the above formats, it must be an assertion.
2430 This may be a positive or negative lookahead or lookbehind assertion. Consider
2431 this pattern, again containing non-significant white space, and with the two
2432 alternatives on the second line:
2435 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2437 The condition is a positive lookahead assertion that matches an optional
2438 sequence of non-letters followed by a letter. In other words, it tests for the
2439 presence of at least one letter in the subject. If a letter is found, the
2440 subject is matched against the first alternative; otherwise it is matched
2441 against the second. This pattern matches strings in one of the two forms
2442 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2443 <a name="comments"></a></P>
2444 <br><a name="SEC22" href="#TOC1">COMMENTS</a><br>
2446 There are two ways of including comments in patterns that are processed by
2447 PCRE. In both cases, the start of the comment must not be in a character class,
2448 nor in the middle of any other sequence of related characters such as (?: or a
2449 subpattern name or number. The characters that make up a comment play no part
2450 in the pattern matching.
2453 The sequence (?# marks the start of a comment that continues up to the next
2454 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2455 option is set, an unescaped # character also introduces a comment, which in
2456 this case continues to immediately after the next newline character or
2457 character sequence in the pattern. Which characters are interpreted as newlines
2458 is controlled by the options passed to a compiling function or by a special
2459 sequence at the start of the pattern, as described in the section entitled
2460 <a href="#newlines">"Newline conventions"</a>
2461 above. Note that the end of this type of comment is a literal newline sequence
2462 in the pattern; escape sequences that happen to represent a newline do not
2463 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2464 default newline convention is in force:
2466 abc #comment \n still comment
2468 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2469 a newline in the pattern. The sequence \n is still literal at this stage, so
2470 it does not terminate the comment. Only an actual character with the code value
2471 0x0a (the default newline) does so.
2472 <a name="recursion"></a></P>
2473 <br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br>
2475 Consider the problem of matching a string in parentheses, allowing for
2476 unlimited nested parentheses. Without the use of recursion, the best that can
2477 be done is to use a pattern that matches up to some fixed depth of nesting. It
2478 is not possible to handle an arbitrary nesting depth.
2481 For some time, Perl has provided a facility that allows regular expressions to
2482 recurse (amongst other things). It does this by interpolating Perl code in the
2483 expression at run time, and the code can refer to the expression itself. A Perl
2484 pattern using code interpolation to solve the parentheses problem can be
2487 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
2489 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2490 recursively to the pattern in which it appears.
2493 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2494 supports special syntax for recursion of the entire pattern, and also for
2495 individual subpattern recursion. After its introduction in PCRE and Python,
2496 this kind of recursion was subsequently introduced into Perl at release 5.10.
2499 A special item that consists of (? followed by a number greater than zero and a
2500 closing parenthesis is a recursive subroutine call of the subpattern of the
2501 given number, provided that it occurs inside that subpattern. (If not, it is a
2502 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2503 call, which is described in the next section.) The special item (?R) or (?0) is
2504 a recursive call of the entire regular expression.
2507 This PCRE pattern solves the nested parentheses problem (assume the
2508 PCRE_EXTENDED option is set so that white space is ignored):
2510 \( ( [^()]++ | (?R) )* \)
2512 First it matches an opening parenthesis. Then it matches any number of
2513 substrings which can either be a sequence of non-parentheses, or a recursive
2514 match of the pattern itself (that is, a correctly parenthesized substring).
2515 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2516 to avoid backtracking into sequences of non-parentheses.
2519 If this were part of a larger pattern, you would not want to recurse the entire
2520 pattern, so instead you could use this:
2522 ( \( ( [^()]++ | (?1) )* \) )
2524 We have put the pattern into parentheses, and caused the recursion to refer to
2525 them instead of the whole pattern.
2528 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2529 is made easier by the use of relative references. Instead of (?1) in the
2530 pattern above you can write (?-2) to refer to the second most recently opened
2531 parentheses preceding the recursion. In other words, a negative number counts
2532 capturing parentheses leftwards from the point at which it is encountered.
2535 It is also possible to refer to subsequently opened parentheses, by writing
2536 references such as (?+2). However, these cannot be recursive because the
2537 reference is not inside the parentheses that are referenced. They are always
2538 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2539 calls, as described in the next section.
2542 An alternative approach is to use named parentheses instead. The Perl syntax
2543 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2544 could rewrite the above example as follows:
2546 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
2548 If there is more than one subpattern with the same name, the earliest one is
2552 This particular example pattern that we have been looking at contains nested
2553 unlimited repeats, and so the use of a possessive quantifier for matching
2554 strings of non-parentheses is important when applying the pattern to strings
2555 that do not match. For example, when this pattern is applied to
2557 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2559 it yields "no match" quickly. However, if a possessive quantifier is not used,
2560 the match runs for a very long time indeed because there are so many different
2561 ways the + and * repeats can carve up the subject, and all have to be tested
2562 before failure can be reported.
2565 At the end of a match, the values of capturing parentheses are those from
2566 the outermost level. If you want to obtain intermediate values, a callout
2567 function can be used (see below and the
2568 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2569 documentation). If the pattern above is matched against
2573 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2574 the last value taken on at the top level. If a capturing subpattern is not
2575 matched at the top level, its final captured value is unset, even if it was
2576 (temporarily) set at a deeper level during the matching process.
2579 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2580 obtain extra memory to store data during a recursion, which it does by using
2581 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2582 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2585 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2586 Consider this pattern, which matches text in angle brackets, allowing for
2587 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2588 recursing), whereas any characters are permitted at the outer level.
2590 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
2592 In this pattern, (?(R) is the start of a conditional subpattern, with two
2593 different alternatives for the recursive and non-recursive cases. The (?R) item
2594 is the actual recursive call.
2595 <a name="recursiondifference"></a></P>
2597 Differences in recursion processing between PCRE and Perl
2600 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2601 (like Python, but unlike Perl), a recursive subpattern call is always treated
2602 as an atomic group. That is, once it has matched some of the subject string, it
2603 is never re-entered, even if it contains untried alternatives and there is a
2604 subsequent matching failure. This can be illustrated by the following pattern,
2605 which purports to match a palindromic string that contains an odd number of
2606 characters (for example, "a", "aba", "abcba", "abcdcba"):
2610 The idea is that it either matches a single character, or two identical
2611 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2612 it does not if the pattern is longer than three characters. Consider the
2613 subject string "abcba":
2616 At the top level, the first character is matched, but as it is not at the end
2617 of the string, the first alternative fails; the second alternative is taken
2618 and the recursion kicks in. The recursive call to subpattern 1 successfully
2619 matches the next character ("b"). (Note that the beginning and end of line
2620 tests are not part of the recursion).
2623 Back at the top level, the next character ("c") is compared with what
2624 subpattern 2 matched, which was "a". This fails. Because the recursion is
2625 treated as an atomic group, there are now no backtracking points, and so the
2626 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2627 try the second alternative.) However, if the pattern is written with the
2628 alternatives in the other order, things are different:
2632 This time, the recursing alternative is tried first, and continues to recurse
2633 until it runs out of characters, at which point the recursion fails. But this
2634 time we do have another alternative to try at the higher level. That is the big
2635 difference: in the previous case the remaining alternative is at a deeper
2636 recursion level, which PCRE cannot use.
2639 To change the pattern so that it matches all palindromic strings, not just
2640 those with an odd number of characters, it is tempting to change the pattern to
2645 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2646 deeper recursion has matched a single character, it cannot be entered again in
2647 order to match an empty string. The solution is to separate the two cases, and
2648 write out the odd and even cases as alternatives at the higher level:
2650 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2652 If you want to match typical palindromic phrases, the pattern has to ignore all
2653 non-word characters, which can be done like this:
2655 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2657 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2658 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2659 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2660 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2661 more) to match typical phrases, and Perl takes so long that you think it has
2665 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2666 string does not start with a palindrome that is shorter than the entire string.
2667 For example, although "abcba" is correctly matched, if the subject is "ababa",
2668 PCRE finds the palindrome "aba" at the start, then fails at top level because
2669 the end of the string does not follow. Once again, it cannot jump back into the
2670 recursion to try other alternatives, so the entire match fails.
2673 The second way in which PCRE and Perl differ in their recursion processing is
2674 in the handling of captured values. In Perl, when a subpattern is called
2675 recursively or as a subpattern (see the next section), it has no access to any
2676 values that were captured outside the recursion, whereas in PCRE these values
2677 can be referenced. Consider this pattern:
2681 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2682 then in the second group, when the back reference \1 fails to match "b", the
2683 second alternative matches "a" and then recurses. In the recursion, \1 does
2684 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2685 match because inside the recursive call \1 cannot access the externally set
2687 <a name="subpatternsassubroutines"></a></P>
2688 <br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2690 If the syntax for a recursive subpattern call (either by number or by
2691 name) is used outside the parentheses to which it refers, it operates like a
2692 subroutine in a programming language. The called subpattern may be defined
2693 before or after the reference. A numbered reference can be absolute or
2694 relative, as in these examples:
2696 (...(absolute)...)...(?2)...
2697 (...(relative)...)...(?-1)...
2698 (...(?+1)...(relative)...
2700 An earlier example pointed out that the pattern
2702 (sens|respons)e and \1ibility
2704 matches "sense and sensibility" and "response and responsibility", but not
2705 "sense and responsibility". If instead the pattern
2707 (sens|respons)e and (?1)ibility
2709 is used, it does match "sense and responsibility" as well as the other two
2710 strings. Another example is given in the discussion of DEFINE above.
2713 All subroutine calls, whether recursive or not, are always treated as atomic
2714 groups. That is, once a subroutine has matched some of the subject string, it
2715 is never re-entered, even if it contains untried alternatives and there is a
2716 subsequent matching failure. Any capturing parentheses that are set during the
2717 subroutine call revert to their previous values afterwards.
2720 Processing options such as case-independence are fixed when a subpattern is
2721 defined, so if it is used as a subroutine, such options cannot be changed for
2722 different calls. For example, consider this pattern:
2726 It matches "abcabc". It does not match "abcABC" because the change of
2727 processing option does not affect the called subpattern.
2728 <a name="onigurumasubroutines"></a></P>
2729 <br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2731 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2732 a number enclosed either in angle brackets or single quotes, is an alternative
2733 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2734 are two of the examples used above, rewritten using this syntax:
2736 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
2737 (sens|respons)e and \g'1'ibility
2739 PCRE supports an extension to Oniguruma: if a number is preceded by a
2740 plus or a minus sign it is taken as a relative reference. For example:
2742 (abc)(?i:\g<-1>)
2744 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
2745 synonymous. The former is a back reference; the latter is a subroutine call.
2747 <br><a name="SEC26" href="#TOC1">CALLOUTS</a><br>
2749 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2750 code to be obeyed in the middle of matching a regular expression. This makes it
2751 possible, amongst other things, to extract different substrings that match the
2752 same pair of parentheses when there is a repetition.
2755 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2756 code. The feature is called "callout". The caller of PCRE provides an external
2757 function by putting its entry point in the global variable <i>pcre_callout</i>
2758 (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library).
2759 By default, this variable contains NULL, which disables all calling out.
2762 Within a regular expression, (?C) indicates the points at which the external
2763 function is to be called. If you want to identify different callout points, you
2764 can put a number less than 256 after the letter C. The default value is zero.
2765 For example, this pattern has two callout points:
2769 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2770 automatically installed before each item in the pattern. They are all numbered
2771 255. If there is a conditional group in the pattern whose condition is an
2772 assertion, an additional callout is inserted just before the condition. An
2773 explicit callout may also be set at this position, as in this example:
2775 (?(?C9)(?=a)abc|def)
2777 Note that this applies only to assertion conditions, not to other types of
2781 During matching, when PCRE reaches a callout point, the external function is
2782 called. It is provided with the number of the callout, the position in the
2783 pattern, and, optionally, one item of data originally supplied by the caller of
2784 the matching function. The callout function may cause matching to proceed, to
2785 backtrack, or to fail altogether.
2788 By default, PCRE implements a number of optimizations at compile time and
2789 matching time, and one side-effect is that sometimes callouts are skipped. If
2790 you need all possible callouts to happen, you need to set options that disable
2791 the relevant optimizations. More details, and a complete description of the
2792 interface to the callout function, are given in the
2793 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2795 <a name="backtrackcontrol"></a></P>
2796 <br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br>
2798 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2799 are still described in the Perl documentation as "experimental and subject to
2800 change or removal in a future version of Perl". It goes on to say: "Their usage
2801 in production code should be noted to avoid problems during upgrades." The same
2802 remarks apply to the PCRE features described in this section.
2805 The new verbs make use of what was previously invalid syntax: an opening
2806 parenthesis followed by an asterisk. They are generally of the form
2807 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2808 differently depending on whether or not a name is present. A name is any
2809 sequence of characters that does not include a closing parenthesis. The maximum
2810 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2811 libraries. If the name is empty, that is, if the closing parenthesis
2812 immediately follows the colon, the effect is as if the colon were not there.
2813 Any number of these verbs may occur in a pattern.
2816 Since these verbs are specifically related to backtracking, most of them can be
2817 used only when the pattern is to be matched using one of the traditional
2818 matching functions, because these use a backtracking algorithm. With the
2819 exception of (*FAIL), which behaves like a failing negative assertion, the
2820 backtracking control verbs cause an error if encountered by a DFA matching
2824 The behaviour of these verbs in
2825 <a href="#btrepeat">repeated groups,</a>
2826 <a href="#btassert">assertions,</a>
2828 <a href="#btsub">subpatterns called as subroutines</a>
2829 (whether or not recursively) is documented below.
2830 <a name="nooptimize"></a></P>
2832 Optimizations that affect backtracking verbs
2835 PCRE contains some optimizations that are used to speed up matching by running
2836 some checks at the start of each match attempt. For example, it may know the
2837 minimum length of matching subject, or that a particular character must be
2838 present. When one of these optimizations bypasses the running of a match, any
2839 included backtracking verbs will not, of course, be processed. You can suppress
2840 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2841 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2842 pattern with (*NO_START_OPT). There is more discussion of this option in the
2844 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
2846 <a href="pcreapi.html"><b>pcreapi</b></a>
2850 Experiments with Perl suggest that it too has similar optimizations, sometimes
2851 leading to anomalous results.
2854 Verbs that act immediately
2857 The following verbs act as soon as they are encountered. They may not be
2862 This verb causes the match to end successfully, skipping the remainder of the
2863 pattern. However, when it is inside a subpattern that is called as a
2864 subroutine, only that subpattern is ended successfully. Matching then continues
2865 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2866 assertion succeeds; in a negative assertion, the assertion fails.
2869 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2872 A((?:A|B(*ACCEPT)|C)D)
2874 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2875 the outer parentheses.
2879 This verb causes a matching failure, forcing backtracking to occur. It is
2880 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2881 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2882 Perl features that are not present in PCRE. The nearest equivalent is the
2883 callout feature, as for example in this pattern:
2887 A match with the string "aaaa" always fails, but the callout is taken before
2888 each backtrack happens (in this example, 10 times).
2891 Recording which path was taken
2894 There is one verb whose main purpose is to track how a match was arrived at,
2895 though it also has a secondary use in conjunction with advancing the match
2896 starting point (see (*SKIP) below).
2898 (*MARK:NAME) or (*:NAME)
2900 A name is always required with this verb. There may be as many instances of
2901 (*MARK) as you like in a pattern, and their names do not have to be unique.
2904 When a match succeeds, the name of the last-encountered (*MARK:NAME),
2905 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2906 caller as described in the section entitled
2907 <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
2909 <a href="pcreapi.html"><b>pcreapi</b></a>
2910 documentation. Here is an example of <b>pcretest</b> output, where the /K
2911 modifier requests the retrieval and outputting of (*MARK) data:
2913 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2921 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2922 indicates which of the two alternatives matched. This is a more efficient way
2923 of obtaining this information than putting each alternative in its own
2924 capturing parentheses.
2927 If a verb with a name is encountered in a positive assertion that is true, the
2928 name is recorded and passed back if it is the last-encountered. This does not
2929 happen for negative assertions or failing positive assertions.
2932 After a partial match or a failed match, the last encountered name in the
2933 entire match process is returned. For example:
2935 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2939 Note that in this unanchored example the mark is retained from the match
2940 attempt that started at the letter "X" in the subject. Subsequent match
2941 attempts starting at "P" and then with an empty string do not get as far as the
2942 (*MARK) item, but nevertheless do not reset it.
2945 If you are interested in (*MARK) values after failed matches, you should
2946 probably set the PCRE_NO_START_OPTIMIZE option
2947 <a href="#nooptimize">(see above)</a>
2948 to ensure that the match is always attempted.
2951 Verbs that act after backtracking
2954 The following verbs do nothing when they are encountered. Matching continues
2955 with what follows, but if there is no subsequent match, causing a backtrack to
2956 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2957 the verb. However, when one of these verbs appears inside an atomic group or an
2958 assertion that is true, its effect is confined to that group, because once the
2959 group has been matched, there is never any backtracking into it. In this
2960 situation, backtracking can "jump back" to the left of the entire atomic group
2961 or assertion. (Remember also, as stated above, that this localization also
2962 applies in subroutine calls.)
2965 These verbs differ in exactly what kind of failure occurs when backtracking
2966 reaches them. The behaviour described below is what happens when the verb is
2967 not in a subroutine or an assertion. Subsequent sections cover these special
2972 This verb, which may not be followed by a name, causes the whole match to fail
2973 outright if there is a later matching failure that causes backtracking to reach
2974 it. Even if the pattern is unanchored, no further attempts to find a match by
2975 advancing the starting point take place. If (*COMMIT) is the only backtracking
2976 verb that is encountered, once it has been passed <b>pcre_exec()</b> is
2977 committed to finding a match at the current starting point, or not at all. For
2982 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2983 dynamic anchor, or "I've started, so I must finish." The name of the most
2984 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2988 If there is more than one backtracking verb in a pattern, a different one that
2989 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2990 match does not always guarantee that a match must be at this starting point.
2993 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2994 unless PCRE's start-of-match optimizations are turned off, as shown in this
2995 output from <b>pcretest</b>:
2997 re> /(*COMMIT)abc/
3003 For this pattern, PCRE knows that any match must start with "a", so the
3004 optimization skips along the subject to "a" before applying the pattern to the
3005 first set of data. The match attempt then succeeds. In the second set of data,
3006 the escape sequence \Y is interpreted by the <b>pcretest</b> program. It causes
3007 the PCRE_NO_START_OPTIMIZE option to be set when <b>pcre_exec()</b> is called.
3008 This disables the optimization that skips along to the first character. The
3009 pattern is now applied starting at "x", and so the (*COMMIT) causes the match
3010 to fail without trying any other starting points.
3012 (*PRUNE) or (*PRUNE:NAME)
3014 This verb causes the match to fail at the current starting position in the
3015 subject if there is a later matching failure that causes backtracking to reach
3016 it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3017 starting character then happens. Backtracking can occur as usual to the left of
3018 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3019 if there is no match to the right, backtracking cannot cross (*PRUNE). In
3020 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3021 possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3022 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3026 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3027 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3028 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3032 This verb, when given without a name, is like (*PRUNE), except that if the
3033 pattern is unanchored, the "bumpalong" advance is not to the next character,
3034 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3035 signifies that whatever text was matched leading up to it cannot be part of a
3036 successful match. Consider:
3040 If the subject is "aaaac...", after the first match attempt fails (starting at
3041 the first character in the string), the starting point skips on to start the
3042 next attempt at "c". Note that a possessive quantifer does not have the same
3043 effect as this example; although it would suppress backtracking during the
3044 first match attempt, the second attempt would start at the second character
3045 instead of skipping on to "c".
3049 When (*SKIP) has an associated name, its behaviour is modified. When it is
3050 triggered, the previous path through the pattern is searched for the most
3051 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3052 is to the subject position that corresponds to that (*MARK) instead of to where
3053 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3057 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3058 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3060 (*THEN) or (*THEN:NAME)
3062 This verb causes a skip to the next innermost alternative when backtracking
3063 reaches it. That is, it cancels any further backtracking within the current
3064 alternative. Its name comes from the observation that it can be used for a
3065 pattern-based if-then-else block:
3067 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3069 If the COND1 pattern matches, FOO is tried (and possibly further items after
3070 the end of the group if FOO succeeds); on failure, the matcher skips to the
3071 second alternative and tries COND2, without backtracking into COND1. If that
3072 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3073 more alternatives, so there is a backtrack to whatever came before the entire
3074 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3077 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3078 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3079 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3082 A subpattern that does not contain a | character is just a part of the
3083 enclosing alternative; it is not a nested alternation with only one
3084 alternative. The effect of (*THEN) extends beyond such a subpattern to the
3085 enclosing alternative. Consider this pattern, where A, B, etc. are complex
3086 pattern fragments that do not contain any | characters at this level:
3090 If A and B are matched, but there is a failure in C, matching does not
3091 backtrack into A; instead it moves to the next alternative, that is, D.
3092 However, if the subpattern containing (*THEN) is given an alternative, it
3093 behaves differently:
3095 A (B(*THEN)C | (*FAIL)) | D
3097 The effect of (*THEN) is now confined to the inner subpattern. After a failure
3098 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3099 because there are no more alternatives to try. In this case, matching does now
3103 Note that a conditional subpattern is not considered as having two
3104 alternatives, because only one is ever used. In other words, the | character in
3105 a conditional subpattern has a different meaning. Ignoring white space,
3108 ^.*? (?(?=a) a | b(*THEN)c )
3110 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3111 it initially matches zero characters. The condition (?=a) then fails, the
3112 character "b" is matched, but "c" is not. At this point, matching does not
3113 backtrack to .*? as might perhaps be expected from the presence of the |
3114 character. The conditional subpattern is part of the single alternative that
3115 comprises the whole pattern, and so the match fails. (If there was a backtrack
3116 into .*?, allowing it to match "b", the match would succeed.)
3119 The verbs just described provide four different "strengths" of control when
3120 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3121 next alternative. (*PRUNE) comes next, failing the match at the current
3122 starting position, but allowing an advance to the next character (for an
3123 unanchored pattern). (*SKIP) is similar, except that the advance may be more
3124 than one character. (*COMMIT) is the strongest, causing the entire match to
3128 More than one backtracking verb
3131 If more than one backtracking verb is present in a pattern, the one that is
3132 backtracked onto first acts. For example, consider this pattern, where A, B,
3133 etc. are complex pattern fragments:
3135 (A(*COMMIT)B(*THEN)C|ABD)
3137 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3138 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3139 the next alternative (ABD) to be tried. This behaviour is consistent, but is
3140 not always the same as Perl's. It means that if two or more backtracking verbs
3141 appear in succession, all the the last of them has no effect. Consider this
3144 ...(*COMMIT)(*PRUNE)...
3146 If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3147 it to be triggered, and its action is taken. There can never be a backtrack
3149 <a name="btrepeat"></a></P>
3151 Backtracking verbs in repeated groups
3154 PCRE differs from Perl in its handling of backtracking verbs in repeated
3155 groups. For example, consider:
3159 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3160 the second repeat of the group acts.
3161 <a name="btassert"></a></P>
3163 Backtracking verbs in assertions
3166 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3169 (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3170 further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3171 fail without any further processing.
3174 The other backtracking verbs are not treated specially if they appear in a
3175 positive assertion. In particular, (*THEN) skips to the next alternative in the
3176 innermost enclosing group that has alternations, whether or not this is within
3180 Negative assertions are, however, different, in order to ensure that changing a
3181 positive assertion into a negative assertion changes its result. Backtracking
3182 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3183 without considering any further alternative branches in the assertion.
3184 Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3185 within the assertion (the normal behaviour), but if the assertion does not have
3186 such an alternative, (*THEN) behaves like (*PRUNE).
3187 <a name="btsub"></a></P>
3189 Backtracking verbs in subroutines
3192 These behaviours occur whether or not the subpattern is called recursively.
3193 Perl's treatment of subroutines is different in some cases.
3196 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3197 an immediate backtrack.
3200 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3201 succeed without any further processing. Matching then continues after the
3205 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3206 the subroutine match to fail.
3209 (*THEN) skips to the next alternative in the innermost enclosing group within
3210 the subpattern that has alternatives. If there is no such group within the
3211 subpattern, (*THEN) causes the subroutine match to fail.
3213 <br><a name="SEC28" href="#TOC1">SEE ALSO</a><br>
3215 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
3216 <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>.
3218 <br><a name="SEC29" href="#TOC1">AUTHOR</a><br>
3222 University Computing Service
3224 Cambridge CB2 3QH, England.
3227 <br><a name="SEC30" href="#TOC1">REVISION</a><br>
3229 Last updated: 08 January 2014
3231 Copyright © 1997-2014 University of Cambridge.
3234 Return to the <a href="index.html">PCRE index page</a>.
~matthewv / pcre3.git / blob