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Revision 231 - (hide annotations) (download)
Tue Sep 11 11:15:33 2007 UTC (7 years, 6 months ago) by ph10
File size: 88821 byte(s)
Add facility to make \R match only CR, LF, or CRLF.

1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
5 nigel 63 .rs
6     .sp
7 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
8     are described in detail below. There is a quick-reference syntax summary in the
9     .\" HREF
10     \fBpcresyntax\fP
11     .\"
12     page. Perl's regular expressions are described in its own documentation, and
13     regular expressions in general are covered in a number of books, some of which
14     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15     published by O'Reilly, covers regular expressions in great detail. This
16     description of PCRE's regular expressions is intended as reference material.
17 nigel 75 .P
18     The original operation of PCRE was on strings of one-byte characters. However,
19     there is now also support for UTF-8 character strings. To use this, you must
20     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
21     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
22     places below. There is also a summary of UTF-8 features in the
23 nigel 63 .\" HTML <a href="pcre.html#utf8support">
24     .\" </a>
25     section on UTF-8 support
26     .\"
27     in the main
28     .\" HREF
29 nigel 75 \fBpcre\fP
30 nigel 63 .\"
31     page.
32 nigel 75 .P
33 nigel 77 The remainder of this document discusses the patterns that are supported by
34     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
35     From release 6.0, PCRE offers a second matching function,
36     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
37 ph10 172 Perl-compatible. Some of the features discussed below are not available when
38 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
39     alternative function, and how it differs from the normal function, are
40     discussed in the
41 nigel 77 .\" HREF
42     \fBpcrematching\fP
43     .\"
44     page.
45 nigel 93 .
46     .
48     .rs
49     .sp
50     PCRE supports five different conventions for indicating line breaks in
51     strings: a single CR (carriage return) character, a single LF (linefeed)
52     character, the two-character sequence CRLF, any of the three preceding, or any
53     Unicode newline sequence. The
54     .\" HREF
55     \fBpcreapi\fP
56     .\"
57     page has
58     .\" HTML <a href="pcreapi.html#newlines">
59     .\" </a>
60     further discussion
61     .\"
62     about newlines, and shows how to set the newline convention in the
63     \fIoptions\fP arguments for the compiling and matching functions.
64     .P
65     It is also possible to specify a newline convention by starting a pattern
66     string with one of the following five sequences:
67     .sp
68     (*CR) carriage return
69     (*LF) linefeed
70     (*CRLF) carriage return, followed by linefeed
71     (*ANYCRLF) any of the three above
72     (*ANY) all Unicode newline sequences
73     .sp
74     These override the default and the options given to \fBpcre_compile()\fP. For
75     example, on a Unix system where LF is the default newline sequence, the pattern
76     .sp
77     (*CR)a.b
78     .sp
79     changes the convention to CR. That pattern matches "a\enb" because LF is no
80     longer a newline. Note that these special settings, which are not
81     Perl-compatible, are recognized only at the very start of a pattern, and that
82 ph10 231 they must be in upper case. If more than one of them is present, the last one
83     is used.
84     .P
85     The newline convention does not affect what the \eR escape sequence matches. By
86     default, this is any Unicode newline sequence, for Perl compatibility. However,
87     this can be changed; see the description of \eR in the section entitled
88     .\" HTML <a href="#newlineseq">
89     .\" </a>
90     "Newline sequences"
91     .\"
92     below.
93 ph10 227 .
94     .
96     .rs
97     .sp
98 nigel 63 A regular expression is a pattern that is matched against a subject string from
99     left to right. Most characters stand for themselves in a pattern, and match the
100     corresponding characters in the subject. As a trivial example, the pattern
101 nigel 75 .sp
102 nigel 63 The quick brown fox
103 nigel 75 .sp
104 nigel 77 matches a portion of a subject string that is identical to itself. When
105     caseless matching is specified (the PCRE_CASELESS option), letters are matched
106     independently of case. In UTF-8 mode, PCRE always understands the concept of
107     case for characters whose values are less than 128, so caseless matching is
108     always possible. For characters with higher values, the concept of case is
109     supported if PCRE is compiled with Unicode property support, but not otherwise.
110     If you want to use caseless matching for characters 128 and above, you must
111     ensure that PCRE is compiled with Unicode property support as well as with
112     UTF-8 support.
113     .P
114     The power of regular expressions comes from the ability to include alternatives
115     and repetitions in the pattern. These are encoded in the pattern by the use of
116 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
117 nigel 63 interpreted in some special way.
118 nigel 75 .P
119     There are two different sets of metacharacters: those that are recognized
120 nigel 63 anywhere in the pattern except within square brackets, and those that are
121 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
122     are as follows:
123 nigel 75 .sp
124     \e general escape character with several uses
125 nigel 63 ^ assert start of string (or line, in multiline mode)
126     $ assert end of string (or line, in multiline mode)
127     . match any character except newline (by default)
128     [ start character class definition
129     | start of alternative branch
130     ( start subpattern
131     ) end subpattern
132     ? extends the meaning of (
133     also 0 or 1 quantifier
134     also quantifier minimizer
135     * 0 or more quantifier
136     + 1 or more quantifier
137     also "possessive quantifier"
138     { start min/max quantifier
139 nigel 75 .sp
140 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
141 nigel 75 a character class the only metacharacters are:
142     .sp
143     \e general escape character
144 nigel 63 ^ negate the class, but only if the first character
145     - indicates character range
146 nigel 75 .\" JOIN
147 nigel 63 [ POSIX character class (only if followed by POSIX
148     syntax)
149     ] terminates the character class
150 nigel 75 .sp
151     The following sections describe the use of each of the metacharacters.
152     .
153 nigel 93 .
154 nigel 63 .SH BACKSLASH
155     .rs
156     .sp
157     The backslash character has several uses. Firstly, if it is followed by a
158 nigel 91 non-alphanumeric character, it takes away any special meaning that character
159     may have. This use of backslash as an escape character applies both inside and
160 nigel 63 outside character classes.
161 nigel 75 .P
162     For example, if you want to match a * character, you write \e* in the pattern.
163 nigel 63 This escaping action applies whether or not the following character would
164 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
165     non-alphanumeric with backslash to specify that it stands for itself. In
166     particular, if you want to match a backslash, you write \e\e.
167     .P
168 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
169     pattern (other than in a character class) and characters between a # outside
170 nigel 91 a character class and the next newline are ignored. An escaping backslash can
171     be used to include a whitespace or # character as part of the pattern.
172 nigel 75 .P
173 nigel 63 If you want to remove the special meaning from a sequence of characters, you
174 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
175     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
176 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
177 nigel 75 .sp
178 nigel 63 Pattern PCRE matches Perl matches
179 nigel 75 .sp
180     .\" JOIN
181     \eQabc$xyz\eE abc$xyz abc followed by the
182 nigel 63 contents of $xyz
183 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
184     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
185     .sp
186     The \eQ...\eE sequence is recognized both inside and outside character classes.
187     .
188     .
189     .\" HTML <a name="digitsafterbackslash"></a>
190     .SS "Non-printing characters"
191     .rs
192     .sp
193 nigel 63 A second use of backslash provides a way of encoding non-printing characters
194     in patterns in a visible manner. There is no restriction on the appearance of
195     non-printing characters, apart from the binary zero that terminates a pattern,
196     but when a pattern is being prepared by text editing, it is usually easier to
197     use one of the following escape sequences than the binary character it
198     represents:
199 nigel 75 .sp
200     \ea alarm, that is, the BEL character (hex 07)
201     \ecx "control-x", where x is any character
202     \ee escape (hex 1B)
203     \ef formfeed (hex 0C)
204 ph10 227 \en linefeed (hex 0A)
205 nigel 75 \er carriage return (hex 0D)
206     \et tab (hex 09)
207     \eddd character with octal code ddd, or backreference
208     \exhh character with hex code hh
209 nigel 87 \ex{hhh..} character with hex code hhh..
210 nigel 75 .sp
211     The precise effect of \ecx is as follows: if x is a lower case letter, it
212 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
213 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
214 nigel 63 7B.
215 nigel 75 .P
216     After \ex, from zero to two hexadecimal digits are read (letters can be in
217 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
218     and }, but the value of the character code must be less than 256 in non-UTF-8
219 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
220     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
221     point, which is 10FFFF.
222 nigel 75 .P
223 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
224     there is no terminating }, this form of escape is not recognized. Instead, the
225     initial \ex will be interpreted as a basic hexadecimal escape, with no
226     following digits, giving a character whose value is zero.
227     .P
228 nigel 63 Characters whose value is less than 256 can be defined by either of the two
229 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
230     example, \exdc is exactly the same as \ex{dc}.
231 nigel 75 .P
232 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
233     digits, just those that are present are used. Thus the sequence \e0\ex\e07
234     specifies two binary zeros followed by a BEL character (code value 7). Make
235     sure you supply two digits after the initial zero if the pattern character that
236     follows is itself an octal digit.
237 nigel 75 .P
238 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
239     Outside a character class, PCRE reads it and any following digits as a decimal
240     number. If the number is less than 10, or if there have been at least that many
241     previous capturing left parentheses in the expression, the entire sequence is
242 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
243     .\" HTML <a href="#backreferences">
244     .\" </a>
245     later,
246     .\"
247     following the discussion of
248     .\" HTML <a href="#subpattern">
249     .\" </a>
250     parenthesized subpatterns.
251     .\"
252     .P
253 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
254     have not been that many capturing subpatterns, PCRE re-reads up to three octal
255 nigel 93 digits following the backslash, and uses them to generate a data character. Any
256 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
257     character specified in octal must be less than \e400. In UTF-8 mode, values up
258     to \e777 are permitted. For example:
259 nigel 75 .sp
260     \e040 is another way of writing a space
261     .\" JOIN
262     \e40 is the same, provided there are fewer than 40
263 nigel 63 previous capturing subpatterns
264 nigel 75 \e7 is always a back reference
265     .\" JOIN
266     \e11 might be a back reference, or another way of
267 nigel 63 writing a tab
268 nigel 75 \e011 is always a tab
269     \e0113 is a tab followed by the character "3"
270     .\" JOIN
271     \e113 might be a back reference, otherwise the
272 nigel 63 character with octal code 113
273 nigel 75 .\" JOIN
274     \e377 might be a back reference, otherwise
275 nigel 63 the byte consisting entirely of 1 bits
276 nigel 75 .\" JOIN
277     \e81 is either a back reference, or a binary zero
278 nigel 63 followed by the two characters "8" and "1"
279 nigel 75 .sp
280 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
281     zero, because no more than three octal digits are ever read.
282 nigel 75 .P
283 nigel 91 All the sequences that define a single character value can be used both inside
284     and outside character classes. In addition, inside a character class, the
285     sequence \eb is interpreted as the backspace character (hex 08), and the
286 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
287     respectively. Outside a character class, these sequences have different
288     meanings
289 nigel 75 .\" HTML <a href="#uniextseq">
290     .\" </a>
291     (see below).
292     .\"
293     .
294     .
295 nigel 93 .SS "Absolute and relative back references"
296     .rs
297     .sp
298 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
299     enclosed in braces, is an absolute or relative back reference. A named back
300     reference can be coded as \eg{name}. Back references are discussed
301 nigel 93 .\" HTML <a href="#backreferences">
302     .\" </a>
303     later,
304     .\"
305     following the discussion of
306     .\" HTML <a href="#subpattern">
307     .\" </a>
308     parenthesized subpatterns.
309     .\"
310     .
311     .
312 nigel 75 .SS "Generic character types"
313     .rs
314     .sp
315 nigel 93 Another use of backslash is for specifying generic character types. The
316 nigel 75 following are always recognized:
317     .sp
318 ph10 182 \ed any decimal digit
319 nigel 75 \eD any character that is not a decimal digit
320 ph10 178 \eh any horizontal whitespace character
321 ph10 182 \eH any character that is not a horizontal whitespace character
322 nigel 75 \es any whitespace character
323     \eS any character that is not a whitespace character
324 ph10 178 \ev any vertical whitespace character
325 ph10 182 \eV any character that is not a vertical whitespace character
326 nigel 75 \ew any "word" character
327     \eW any "non-word" character
328     .sp
329 nigel 63 Each pair of escape sequences partitions the complete set of characters into
330     two disjoint sets. Any given character matches one, and only one, of each pair.
331 nigel 75 .P
332     These character type sequences can appear both inside and outside character
333     classes. They each match one character of the appropriate type. If the current
334     matching point is at the end of the subject string, all of them fail, since
335     there is no character to match.
336     .P
337     For compatibility with Perl, \es does not match the VT character (code 11).
338     This makes it different from the the POSIX "space" class. The \es characters
339 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
340 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
341 ph10 178 does.
342 nigel 75 .P
343 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
344     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
345 ph10 182 character property support is available. These sequences retain their original
346     meanings from before UTF-8 support was available, mainly for efficiency
347 ph10 178 reasons.
348     .P
349 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
350 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
351     The horizontal space characters are:
352     .sp
353     U+0009 Horizontal tab
354     U+0020 Space
355     U+00A0 Non-break space
356     U+1680 Ogham space mark
357     U+180E Mongolian vowel separator
358     U+2000 En quad
359     U+2001 Em quad
360     U+2002 En space
361     U+2003 Em space
362     U+2004 Three-per-em space
363     U+2005 Four-per-em space
364     U+2006 Six-per-em space
365     U+2007 Figure space
366     U+2008 Punctuation space
367     U+2009 Thin space
368     U+200A Hair space
369     U+202F Narrow no-break space
370     U+205F Medium mathematical space
371     U+3000 Ideographic space
372     .sp
373     The vertical space characters are:
374     .sp
375     U+000A Linefeed
376     U+000B Vertical tab
377     U+000C Formfeed
378     U+000D Carriage return
379     U+0085 Next line
380     U+2028 Line separator
381     U+2029 Paragraph separator
382     .P
383 nigel 75 A "word" character is an underscore or any character less than 256 that is a
384     letter or digit. The definition of letters and digits is controlled by PCRE's
385     low-valued character tables, and may vary if locale-specific matching is taking
386     place (see
387 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
388     .\" </a>
389     "Locale support"
390     .\"
391     in the
392     .\" HREF
393 nigel 75 \fBpcreapi\fP
394 nigel 63 .\"
395 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
396     or "french" in Windows, some character codes greater than 128 are used for
397 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
398     is discouraged.
399 nigel 75 .
400     .
401 ph10 231 .\" HTML <a name="newlineseq"></a>
402 nigel 93 .SS "Newline sequences"
403     .rs
404     .sp
405 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
406     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
407     equivalent to the following:
408 nigel 93 .sp
409     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
410     .sp
411     This is an example of an "atomic group", details of which are given
412     .\" HTML <a href="#atomicgroup">
413     .\" </a>
414     below.
415     .\"
416     This particular group matches either the two-character sequence CR followed by
417     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
418     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
419     line, U+0085). The two-character sequence is treated as a single unit that
420     cannot be split.
421     .P
422     In UTF-8 mode, two additional characters whose codepoints are greater than 255
423     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
424     Unicode character property support is not needed for these characters to be
425     recognized.
426     .P
427 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
428     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
429     either at compile time or when the pattern is matched. This can be made the
430     default when PCRE is built; if this is the case, the other behaviour can be
431     requested via the PCRE_BSR_UNICODE option. It is also possible to specify these
432     settings by starting a pattern string with one of the following sequences:
433     .sp
434     (*BSR_ANYCRLF) CR, LF, or CRLF only
435     (*BSR_UNICODE) any Unicode newline sequence
436     .sp
437     These override the default and the options given to \fBpcre_compile()\fP, but
438     they can be overridden by options given to \fBpcre_exec()\fP. Note that these
439     special settings, which are not Perl-compatible, are recognized only at the
440     very start of a pattern, and that they must be in upper case. If more than one
441     of them is present, the last one is used.
442     .P
443 nigel 93 Inside a character class, \eR matches the letter "R".
444     .
445     .
446 nigel 75 .\" HTML <a name="uniextseq"></a>
447     .SS Unicode character properties
448     .rs
449     .sp
450     When PCRE is built with Unicode character property support, three additional
451 ph10 184 escape sequences that match characters with specific properties are available.
452     When not in UTF-8 mode, these sequences are of course limited to testing
453     characters whose codepoints are less than 256, but they do work in this mode.
454     The extra escape sequences are:
455 nigel 75 .sp
456 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
457     \eP{\fIxx\fP} a character without the \fIxx\fP property
458     \eX an extended Unicode sequence
459 nigel 75 .sp
460 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
461     script names, the general category properties, and "Any", which matches any
462     character (including newline). Other properties such as "InMusicalSymbols" are
463     not currently supported by PCRE. Note that \eP{Any} does not match any
464     characters, so always causes a match failure.
465 nigel 75 .P
466 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
467     character from one of these sets can be matched using a script name. For
468     example:
469 nigel 75 .sp
470 nigel 87 \ep{Greek}
471     \eP{Han}
472     .sp
473     Those that are not part of an identified script are lumped together as
474     "Common". The current list of scripts is:
475     .P
476     Arabic,
477     Armenian,
478 nigel 93 Balinese,
479 nigel 87 Bengali,
480     Bopomofo,
481     Braille,
482     Buginese,
483     Buhid,
484     Canadian_Aboriginal,
485     Cherokee,
486     Common,
487     Coptic,
488 nigel 93 Cuneiform,
489 nigel 87 Cypriot,
490     Cyrillic,
491     Deseret,
492     Devanagari,
493     Ethiopic,
494     Georgian,
495     Glagolitic,
496     Gothic,
497     Greek,
498     Gujarati,
499     Gurmukhi,
500     Han,
501     Hangul,
502     Hanunoo,
503     Hebrew,
504     Hiragana,
505     Inherited,
506     Kannada,
507     Katakana,
508     Kharoshthi,
509     Khmer,
510     Lao,
511     Latin,
512     Limbu,
513     Linear_B,
514     Malayalam,
515     Mongolian,
516     Myanmar,
517     New_Tai_Lue,
518 nigel 93 Nko,
519 nigel 87 Ogham,
520     Old_Italic,
521     Old_Persian,
522     Oriya,
523     Osmanya,
524 nigel 93 Phags_Pa,
525     Phoenician,
526 nigel 87 Runic,
527     Shavian,
528     Sinhala,
529     Syloti_Nagri,
530     Syriac,
531     Tagalog,
532     Tagbanwa,
533     Tai_Le,
534     Tamil,
535     Telugu,
536     Thaana,
537     Thai,
538     Tibetan,
539     Tifinagh,
540     Ugaritic,
541     Yi.
542     .P
543     Each character has exactly one general category property, specified by a
544     two-letter abbreviation. For compatibility with Perl, negation can be specified
545     by including a circumflex between the opening brace and the property name. For
546     example, \ep{^Lu} is the same as \eP{Lu}.
547     .P
548     If only one letter is specified with \ep or \eP, it includes all the general
549     category properties that start with that letter. In this case, in the absence
550     of negation, the curly brackets in the escape sequence are optional; these two
551     examples have the same effect:
552     .sp
553 nigel 75 \ep{L}
554     \epL
555     .sp
556 nigel 87 The following general category property codes are supported:
557 nigel 75 .sp
558     C Other
559     Cc Control
560     Cf Format
561     Cn Unassigned
562     Co Private use
563     Cs Surrogate
564     .sp
565     L Letter
566     Ll Lower case letter
567     Lm Modifier letter
568     Lo Other letter
569     Lt Title case letter
570     Lu Upper case letter
571     .sp
572     M Mark
573     Mc Spacing mark
574     Me Enclosing mark
575     Mn Non-spacing mark
576     .sp
577     N Number
578     Nd Decimal number
579     Nl Letter number
580     No Other number
581     .sp
582     P Punctuation
583     Pc Connector punctuation
584     Pd Dash punctuation
585     Pe Close punctuation
586     Pf Final punctuation
587     Pi Initial punctuation
588     Po Other punctuation
589     Ps Open punctuation
590     .sp
591     S Symbol
592     Sc Currency symbol
593     Sk Modifier symbol
594     Sm Mathematical symbol
595     So Other symbol
596     .sp
597     Z Separator
598     Zl Line separator
599     Zp Paragraph separator
600     Zs Space separator
601     .sp
602 nigel 87 The special property L& is also supported: it matches a character that has
603     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
604     a modifier or "other".
605 nigel 75 .P
606 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
607     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
608     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
609     (see the discussion of PCRE_NO_UTF8_CHECK in the
610     .\" HREF
611     \fBpcreapi\fP
612     .\"
613     page).
614     .P
615 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
616 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
617 nigel 87 properties with "Is".
618     .P
619     No character that is in the Unicode table has the Cn (unassigned) property.
620     Instead, this property is assumed for any code point that is not in the
621     Unicode table.
622     .P
623 nigel 75 Specifying caseless matching does not affect these escape sequences. For
624     example, \ep{Lu} always matches only upper case letters.
625     .P
626     The \eX escape matches any number of Unicode characters that form an extended
627     Unicode sequence. \eX is equivalent to
628     .sp
629     (?>\ePM\epM*)
630     .sp
631     That is, it matches a character without the "mark" property, followed by zero
632     or more characters with the "mark" property, and treats the sequence as an
633     atomic group
634     .\" HTML <a href="#atomicgroup">
635     .\" </a>
636     (see below).
637     .\"
638     Characters with the "mark" property are typically accents that affect the
639 ph10 185 preceding character. None of them have codepoints less than 256, so in
640 ph10 184 non-UTF-8 mode \eX matches any one character.
641 nigel 75 .P
642     Matching characters by Unicode property is not fast, because PCRE has to search
643     a structure that contains data for over fifteen thousand characters. That is
644     why the traditional escape sequences such as \ed and \ew do not use Unicode
645     properties in PCRE.
646     .
647     .
648 ph10 168 .\" HTML <a name="resetmatchstart"></a>
649     .SS "Resetting the match start"
650     .rs
651     .sp
652 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
653     matched characters not to be included in the final matched sequence. For
654 ph10 168 example, the pattern:
655     .sp
656     foo\eKbar
657     .sp
658 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
659 ph10 168 similar to a lookbehind assertion
660     .\" HTML <a href="#lookbehind">
661     .\" </a>
662     (described below).
663     .\"
664 ph10 172 However, in this case, the part of the subject before the real match does not
665     have to be of fixed length, as lookbehind assertions do. The use of \eK does
666 ph10 168 not interfere with the setting of
667     .\" HTML <a href="#subpattern">
668     .\" </a>
669     captured substrings.
670 ph10 172 .\"
671 ph10 168 For example, when the pattern
672     .sp
673     (foo)\eKbar
674     .sp
675 ph10 172 matches "foobar", the first substring is still set to "foo".
676 ph10 168 .
677     .
678 nigel 75 .\" HTML <a name="smallassertions"></a>
679     .SS "Simple assertions"
680     .rs
681     .sp
682 nigel 93 The final use of backslash is for certain simple assertions. An assertion
683 nigel 63 specifies a condition that has to be met at a particular point in a match,
684     without consuming any characters from the subject string. The use of
685 nigel 75 subpatterns for more complicated assertions is described
686     .\" HTML <a href="#bigassertions">
687     .\" </a>
688     below.
689     .\"
690 nigel 91 The backslashed assertions are:
691 nigel 75 .sp
692     \eb matches at a word boundary
693     \eB matches when not at a word boundary
694 nigel 93 \eA matches at the start of the subject
695     \eZ matches at the end of the subject
696     also matches before a newline at the end of the subject
697     \ez matches only at the end of the subject
698     \eG matches at the first matching position in the subject
699 nigel 75 .sp
700     These assertions may not appear in character classes (but note that \eb has a
701 nigel 63 different meaning, namely the backspace character, inside a character class).
702 nigel 75 .P
703 nigel 63 A word boundary is a position in the subject string where the current character
704 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
705     \ew and the other matches \eW), or the start or end of the string if the
706     first or last character matches \ew, respectively.
707     .P
708     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
709     dollar (described in the next section) in that they only ever match at the very
710     start and end of the subject string, whatever options are set. Thus, they are
711     independent of multiline mode. These three assertions are not affected by the
712     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
713     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
714     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
715     at a point other than the beginning of the subject, \eA can never match. The
716 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
717     of the string as well as at the very end, whereas \ez matches only at the end.
718 nigel 75 .P
719     The \eG assertion is true only when the current matching position is at the
720     start point of the match, as specified by the \fIstartoffset\fP argument of
721     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
722     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
723 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
724 nigel 75 implementation where \eG can be useful.
725     .P
726     Note, however, that PCRE's interpretation of \eG, as the start of the current
727 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
728     previous match. In Perl, these can be different when the previously matched
729     string was empty. Because PCRE does just one match at a time, it cannot
730     reproduce this behaviour.
731 nigel 75 .P
732     If all the alternatives of a pattern begin with \eG, the expression is anchored
733 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
734     regular expression.
735 nigel 75 .
736     .
738 nigel 63 .rs
739     .sp
740     Outside a character class, in the default matching mode, the circumflex
741 nigel 75 character is an assertion that is true only if the current matching point is
742     at the start of the subject string. If the \fIstartoffset\fP argument of
743     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
744 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
745 nigel 75 meaning
746     .\" HTML <a href="#characterclass">
747     .\" </a>
748     (see below).
749     .\"
750     .P
751 nigel 63 Circumflex need not be the first character of the pattern if a number of
752     alternatives are involved, but it should be the first thing in each alternative
753     in which it appears if the pattern is ever to match that branch. If all
754     possible alternatives start with a circumflex, that is, if the pattern is
755     constrained to match only at the start of the subject, it is said to be an
756     "anchored" pattern. (There are also other constructs that can cause a pattern
757     to be anchored.)
758 nigel 75 .P
759     A dollar character is an assertion that is true only if the current matching
760 nigel 63 point is at the end of the subject string, or immediately before a newline
761 nigel 91 at the end of the string (by default). Dollar need not be the last character of
762     the pattern if a number of alternatives are involved, but it should be the last
763     item in any branch in which it appears. Dollar has no special meaning in a
764     character class.
765 nigel 75 .P
766 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
767     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
768 nigel 75 does not affect the \eZ assertion.
769     .P
770 nigel 63 The meanings of the circumflex and dollar characters are changed if the
771 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
772     immediately after internal newlines as well as at the start of the subject
773     string. It does not match after a newline that ends the string. A dollar
774     matches before any newlines in the string, as well as at the very end, when
775     PCRE_MULTILINE is set. When newline is specified as the two-character
776     sequence CRLF, isolated CR and LF characters do not indicate newlines.
777 nigel 75 .P
778 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
779     \en represents a newline) in multiline mode, but not otherwise. Consequently,
780     patterns that are anchored in single line mode because all branches start with
781     ^ are not anchored in multiline mode, and a match for circumflex is possible
782     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
783     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
784     .P
785 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
786 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
787 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
788 nigel 75 .
789     .
791 nigel 63 .rs
792     .sp
793     Outside a character class, a dot in the pattern matches any one character in
794 nigel 91 the subject string except (by default) a character that signifies the end of a
795 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
796 nigel 91 .P
797 nigel 93 When a line ending is defined as a single character, dot never matches that
798     character; when the two-character sequence CRLF is used, dot does not match CR
799     if it is immediately followed by LF, but otherwise it matches all characters
800     (including isolated CRs and LFs). When any Unicode line endings are being
801     recognized, dot does not match CR or LF or any of the other line ending
802     characters.
803     .P
804 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
805 nigel 93 option is set, a dot matches any one character, without exception. If the
806     two-character sequence CRLF is present in the subject string, it takes two dots
807     to match it.
808 nigel 91 .P
809     The handling of dot is entirely independent of the handling of circumflex and
810     dollar, the only relationship being that they both involve newlines. Dot has no
811     special meaning in a character class.
812 nigel 75 .
813     .
815 nigel 63 .rs
816     .sp
817 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
818 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
819     characters. The feature is provided in Perl in order to match individual bytes
820     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
821     what remains in the string may be a malformed UTF-8 string. For this reason,
822     the \eC escape sequence is best avoided.
823 nigel 75 .P
824     PCRE does not allow \eC to appear in lookbehind assertions
825     .\" HTML <a href="#lookbehind">
826     .\" </a>
827     (described below),
828     .\"
829     because in UTF-8 mode this would make it impossible to calculate the length of
830     the lookbehind.
831     .
832     .
833     .\" HTML <a name="characterclass"></a>
835 nigel 63 .rs
836     .sp
837     An opening square bracket introduces a character class, terminated by a closing
838     square bracket. A closing square bracket on its own is not special. If a
839     closing square bracket is required as a member of the class, it should be the
840     first data character in the class (after an initial circumflex, if present) or
841     escaped with a backslash.
842 nigel 75 .P
843 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
844     character may occupy more than one byte. A matched character must be in the set
845     of characters defined by the class, unless the first character in the class
846     definition is a circumflex, in which case the subject character must not be in
847     the set defined by the class. If a circumflex is actually required as a member
848     of the class, ensure it is not the first character, or escape it with a
849     backslash.
850 nigel 75 .P
851 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
852     [^aeiou] matches any character that is not a lower case vowel. Note that a
853 nigel 75 circumflex is just a convenient notation for specifying the characters that
854     are in the class by enumerating those that are not. A class that starts with a
855     circumflex is not an assertion: it still consumes a character from the subject
856     string, and therefore it fails if the current pointer is at the end of the
857     string.
858     .P
859 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
860 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
861     .P
862 nigel 63 When caseless matching is set, any letters in a class represent both their
863     upper case and lower case versions, so for example, a caseless [aeiou] matches
864     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
865 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
866     case for characters whose values are less than 128, so caseless matching is
867     always possible. For characters with higher values, the concept of case is
868     supported if PCRE is compiled with Unicode property support, but not otherwise.
869     If you want to use caseless matching for characters 128 and above, you must
870     ensure that PCRE is compiled with Unicode property support as well as with
871     UTF-8 support.
872 nigel 75 .P
873 nigel 93 Characters that might indicate line breaks are never treated in any special way
874     when matching character classes, whatever line-ending sequence is in use, and
875     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
876     such as [^a] always matches one of these characters.
877 nigel 75 .P
878 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
879     character class. For example, [d-m] matches any letter between d and m,
880     inclusive. If a minus character is required in a class, it must be escaped with
881     a backslash or appear in a position where it cannot be interpreted as
882     indicating a range, typically as the first or last character in the class.
883 nigel 75 .P
884 nigel 63 It is not possible to have the literal character "]" as the end character of a
885     range. A pattern such as [W-]46] is interpreted as a class of two characters
886     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
887     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
888 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
889     followed by two other characters. The octal or hexadecimal representation of
890     "]" can also be used to end a range.
891     .P
892 nigel 63 Ranges operate in the collating sequence of character values. They can also be
893 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
894 nigel 63 mode, ranges can include characters whose values are greater than 255, for
895 nigel 75 example [\ex{100}-\ex{2ff}].
896     .P
897 nigel 63 If a range that includes letters is used when caseless matching is set, it
898     matches the letters in either case. For example, [W-c] is equivalent to
899 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
900 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
901 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
902     characters with values greater than 128 only when it is compiled with Unicode
903     property support.
904     .P
905     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
906     in a character class, and add the characters that they match to the class. For
907     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
908 nigel 63 conveniently be used with the upper case character types to specify a more
909     restricted set of characters than the matching lower case type. For example,
910 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
911     .P
912     The only metacharacters that are recognized in character classes are backslash,
913     hyphen (only where it can be interpreted as specifying a range), circumflex
914     (only at the start), opening square bracket (only when it can be interpreted as
915     introducing a POSIX class name - see the next section), and the terminating
916     closing square bracket. However, escaping other non-alphanumeric characters
917     does no harm.
918     .
919     .
921 nigel 63 .rs
922     .sp
923 nigel 75 Perl supports the POSIX notation for character classes. This uses names
924 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
925     this notation. For example,
926 nigel 75 .sp
927 nigel 63 [01[:alpha:]%]
928 nigel 75 .sp
929 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
930     are
931 nigel 75 .sp
932 nigel 63 alnum letters and digits
933     alpha letters
934     ascii character codes 0 - 127
935     blank space or tab only
936     cntrl control characters
937 nigel 75 digit decimal digits (same as \ed)
938 nigel 63 graph printing characters, excluding space
939     lower lower case letters
940     print printing characters, including space
941     punct printing characters, excluding letters and digits
942 nigel 75 space white space (not quite the same as \es)
943 nigel 63 upper upper case letters
944 nigel 75 word "word" characters (same as \ew)
945 nigel 63 xdigit hexadecimal digits
946 nigel 75 .sp
947 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
948     space (32). Notice that this list includes the VT character (code 11). This
949 nigel 75 makes "space" different to \es, which does not include VT (for Perl
950 nigel 63 compatibility).
951 nigel 75 .P
952 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
953     5.8. Another Perl extension is negation, which is indicated by a ^ character
954     after the colon. For example,
955 nigel 75 .sp
956 nigel 63 [12[:^digit:]]
957 nigel 75 .sp
958 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
959     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
960     supported, and an error is given if they are encountered.
961 nigel 75 .P
962     In UTF-8 mode, characters with values greater than 128 do not match any of
963 nigel 63 the POSIX character classes.
964 nigel 75 .
965     .
966     .SH "VERTICAL BAR"
967 nigel 63 .rs
968     .sp
969     Vertical bar characters are used to separate alternative patterns. For example,
970     the pattern
971 nigel 75 .sp
972 nigel 63 gilbert|sullivan
973 nigel 75 .sp
974 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
975 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
976     process tries each alternative in turn, from left to right, and the first one
977     that succeeds is used. If the alternatives are within a subpattern
978 nigel 75 .\" HTML <a href="#subpattern">
979     .\" </a>
980     (defined below),
981     .\"
982     "succeeds" means matching the rest of the main pattern as well as the
983     alternative in the subpattern.
984     .
985     .
987 nigel 63 .rs
988     .sp
989     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
990 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
991     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
992     The option letters are
993 nigel 75 .sp
994 nigel 63 i for PCRE_CASELESS
995     m for PCRE_MULTILINE
996     s for PCRE_DOTALL
997     x for PCRE_EXTENDED
998 nigel 75 .sp
999 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1000     unset these options by preceding the letter with a hyphen, and a combined
1001     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1002     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1003     permitted. If a letter appears both before and after the hyphen, the option is
1004     unset.
1005 nigel 75 .P
1006 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1007     changed in the same way as the Perl-compatible options by using the characters
1008     J, U and X respectively.
1009     .P
1010 nigel 63 When an option change occurs at top level (that is, not inside subpattern
1011     parentheses), the change applies to the remainder of the pattern that follows.
1012     If the change is placed right at the start of a pattern, PCRE extracts it into
1013     the global options (and it will therefore show up in data extracted by the
1014 nigel 75 \fBpcre_fullinfo()\fP function).
1015     .P
1016 nigel 93 An option change within a subpattern (see below for a description of
1017     subpatterns) affects only that part of the current pattern that follows it, so
1018 nigel 75 .sp
1019 nigel 63 (a(?i)b)c
1020 nigel 75 .sp
1021 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1022     By this means, options can be made to have different settings in different
1023     parts of the pattern. Any changes made in one alternative do carry on
1024     into subsequent branches within the same subpattern. For example,
1025 nigel 75 .sp
1026 nigel 63 (a(?i)b|c)
1027 nigel 75 .sp
1028 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1029     branch is abandoned before the option setting. This is because the effects of
1030     option settings happen at compile time. There would be some very weird
1031     behaviour otherwise.
1032 nigel 75 .
1033     .
1034     .\" HTML <a name="subpattern"></a>
1035 nigel 63 .SH SUBPATTERNS
1036     .rs
1037     .sp
1038     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1039 nigel 75 Turning part of a pattern into a subpattern does two things:
1040     .sp
1041 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1042 nigel 75 .sp
1043 nigel 63 cat(aract|erpillar|)
1044 nigel 75 .sp
1045 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1046 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1047 nigel 75 .sp
1048     2. It sets up the subpattern as a capturing subpattern. This means that, when
1049     the whole pattern matches, that portion of the subject string that matched the
1050     subpattern is passed back to the caller via the \fIovector\fP argument of
1051     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1052     from 1) to obtain numbers for the capturing subpatterns.
1053     .P
1054 nigel 63 For example, if the string "the red king" is matched against the pattern
1055 nigel 75 .sp
1056 nigel 63 the ((red|white) (king|queen))
1057 nigel 75 .sp
1058 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1059     2, and 3, respectively.
1060 nigel 75 .P
1061 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1062     There are often times when a grouping subpattern is required without a
1063     capturing requirement. If an opening parenthesis is followed by a question mark
1064     and a colon, the subpattern does not do any capturing, and is not counted when
1065     computing the number of any subsequent capturing subpatterns. For example, if
1066     the string "the white queen" is matched against the pattern
1067 nigel 75 .sp
1068 nigel 63 the ((?:red|white) (king|queen))
1069 nigel 75 .sp
1070 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1071 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1072 nigel 75 .P
1073 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1074     a non-capturing subpattern, the option letters may appear between the "?" and
1075     the ":". Thus the two patterns
1076 nigel 75 .sp
1077 nigel 63 (?i:saturday|sunday)
1078     (?:(?i)saturday|sunday)
1079 nigel 75 .sp
1080 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1081     from left to right, and options are not reset until the end of the subpattern
1082     is reached, an option setting in one branch does affect subsequent branches, so
1083     the above patterns match "SUNDAY" as well as "Saturday".
1084 nigel 75 .
1085     .
1087     .rs
1088     .sp
1089 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1090     the same numbers for its capturing parentheses. Such a subpattern starts with
1091     (?| and is itself a non-capturing subpattern. For example, consider this
1092 ph10 175 pattern:
1093     .sp
1094     (?|(Sat)ur|(Sun))day
1095 ph10 182 .sp
1096     Because the two alternatives are inside a (?| group, both sets of capturing
1097     parentheses are numbered one. Thus, when the pattern matches, you can look
1098     at captured substring number one, whichever alternative matched. This construct
1099     is useful when you want to capture part, but not all, of one of a number of
1100     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1101 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1102 ph10 182 buffers that follow the subpattern start after the highest number used in any
1103     branch. The following example is taken from the Perl documentation.
1104 ph10 175 The numbers underneath show in which buffer the captured content will be
1105     stored.
1106     .sp
1107     # before ---------------branch-reset----------- after
1108     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1109     # 1 2 2 3 2 3 4
1110 ph10 182 .sp
1111 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1112     the first one in the pattern with the given number.
1113     .P
1114     An alternative approach to using this "branch reset" feature is to use
1115     duplicate named subpatterns, as described in the next section.
1116     .
1117     .
1118 nigel 75 .SH "NAMED SUBPATTERNS"
1119 nigel 63 .rs
1120     .sp
1121     Identifying capturing parentheses by number is simple, but it can be very hard
1122     to keep track of the numbers in complicated regular expressions. Furthermore,
1123 nigel 75 if an expression is modified, the numbers may change. To help with this
1124 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1125     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1126     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1127     the Perl and the Python syntax.
1128     .P
1129     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1130     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1131 nigel 91 parentheses from other parts of the pattern, such as
1132     .\" HTML <a href="#backreferences">
1133     .\" </a>
1134     backreferences,
1135     .\"
1136     .\" HTML <a href="#recursion">
1137     .\" </a>
1138     recursion,
1139     .\"
1140     and
1141     .\" HTML <a href="#conditions">
1142     .\" </a>
1143     conditions,
1144     .\"
1145     can be made by name as well as by number.
1146 nigel 75 .P
1147 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1148 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1149     if the names were not present. The PCRE API provides function calls for
1150     extracting the name-to-number translation table from a compiled pattern. There
1151     is also a convenience function for extracting a captured substring by name.
1152 nigel 91 .P
1153     By default, a name must be unique within a pattern, but it is possible to relax
1154     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1155     be useful for patterns where only one instance of the named parentheses can
1156     match. Suppose you want to match the name of a weekday, either as a 3-letter
1157     abbreviation or as the full name, and in both cases you want to extract the
1158     abbreviation. This pattern (ignoring the line breaks) does the job:
1159     .sp
1160 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1161     (?<DN>Tue)(?:sday)?|
1162     (?<DN>Wed)(?:nesday)?|
1163     (?<DN>Thu)(?:rsday)?|
1164     (?<DN>Sat)(?:urday)?
1165 nigel 91 .sp
1166     There are five capturing substrings, but only one is ever set after a match.
1167 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1168 ph10 175 subpattern, as described in the previous section.)
1169     .P
1170 nigel 91 The convenience function for extracting the data by name returns the substring
1171 nigel 93 for the first (and in this example, the only) subpattern of that name that
1172 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1173     make a reference to a non-unique named subpattern from elsewhere in the
1174     pattern, the one that corresponds to the lowest number is used. For further
1175     details of the interfaces for handling named subpatterns, see the
1176 nigel 63 .\" HREF
1177 nigel 75 \fBpcreapi\fP
1178 nigel 63 .\"
1179     documentation.
1180 nigel 75 .
1181     .
1182 nigel 63 .SH REPETITION
1183     .rs
1184     .sp
1185     Repetition is specified by quantifiers, which can follow any of the following
1186     items:
1187 nigel 75 .sp
1188 nigel 63 a literal data character
1189 nigel 93 the dot metacharacter
1190 nigel 75 the \eC escape sequence
1191     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1192 nigel 93 the \eR escape sequence
1193 nigel 75 an escape such as \ed that matches a single character
1194 nigel 63 a character class
1195     a back reference (see next section)
1196     a parenthesized subpattern (unless it is an assertion)
1197 nigel 75 .sp
1198 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1199     permitted matches, by giving the two numbers in curly brackets (braces),
1200     separated by a comma. The numbers must be less than 65536, and the first must
1201     be less than or equal to the second. For example:
1202 nigel 75 .sp
1203 nigel 63 z{2,4}
1204 nigel 75 .sp
1205 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1206     character. If the second number is omitted, but the comma is present, there is
1207     no upper limit; if the second number and the comma are both omitted, the
1208     quantifier specifies an exact number of required matches. Thus
1209 nigel 75 .sp
1210 nigel 63 [aeiou]{3,}
1211 nigel 75 .sp
1212 nigel 63 matches at least 3 successive vowels, but may match many more, while
1213 nigel 75 .sp
1214     \ed{8}
1215     .sp
1216 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1217     where a quantifier is not allowed, or one that does not match the syntax of a
1218     quantifier, is taken as a literal character. For example, {,6} is not a
1219     quantifier, but a literal string of four characters.
1220 nigel 75 .P
1221 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1222 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1223     which is represented by a two-byte sequence. Similarly, when Unicode property
1224     support is available, \eX{3} matches three Unicode extended sequences, each of
1225     which may be several bytes long (and they may be of different lengths).
1226     .P
1227 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1228     previous item and the quantifier were not present.
1229 nigel 75 .P
1230 nigel 93 For convenience, the three most common quantifiers have single-character
1231     abbreviations:
1232 nigel 75 .sp
1233 nigel 63 * is equivalent to {0,}
1234     + is equivalent to {1,}
1235     ? is equivalent to {0,1}
1236 nigel 75 .sp
1237 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1238     match no characters with a quantifier that has no upper limit, for example:
1239 nigel 75 .sp
1240 nigel 63 (a?)*
1241 nigel 75 .sp
1242 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1243     such patterns. However, because there are cases where this can be useful, such
1244     patterns are now accepted, but if any repetition of the subpattern does in fact
1245     match no characters, the loop is forcibly broken.
1246 nigel 75 .P
1247 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1248     possible (up to the maximum number of permitted times), without causing the
1249     rest of the pattern to fail. The classic example of where this gives problems
1250 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1251     and within the comment, individual * and / characters may appear. An attempt to
1252     match C comments by applying the pattern
1253     .sp
1254     /\e*.*\e*/
1255     .sp
1256 nigel 63 to the string
1257 nigel 75 .sp
1258     /* first comment */ not comment /* second comment */
1259     .sp
1260 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1261     item.
1262 nigel 75 .P
1263 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1264     greedy, and instead matches the minimum number of times possible, so the
1265     pattern
1266 nigel 75 .sp
1267     /\e*.*?\e*/
1268     .sp
1269 nigel 63 does the right thing with the C comments. The meaning of the various
1270     quantifiers is not otherwise changed, just the preferred number of matches.
1271     Do not confuse this use of question mark with its use as a quantifier in its
1272     own right. Because it has two uses, it can sometimes appear doubled, as in
1273 nigel 75 .sp
1274     \ed??\ed
1275     .sp
1276 nigel 63 which matches one digit by preference, but can match two if that is the only
1277     way the rest of the pattern matches.
1278 nigel 75 .P
1279 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1280 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1281     greedy by following them with a question mark. In other words, it inverts the
1282     default behaviour.
1283 nigel 75 .P
1284 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1285 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1286 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1287 nigel 75 .P
1288 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1289 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1290 nigel 63 implicitly anchored, because whatever follows will be tried against every
1291     character position in the subject string, so there is no point in retrying the
1292     overall match at any position after the first. PCRE normally treats such a
1293 nigel 75 pattern as though it were preceded by \eA.
1294     .P
1295 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1296     worth setting PCRE_DOTALL in order to obtain this optimization, or
1297     alternatively using ^ to indicate anchoring explicitly.
1298 nigel 75 .P
1299 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1300     is inside capturing parentheses that are the subject of a backreference
1301 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1302     succeeds. Consider, for example:
1303 nigel 75 .sp
1304     (.*)abc\e1
1305     .sp
1306 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1307     this reason, such a pattern is not implicitly anchored.
1308 nigel 75 .P
1309 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1310     that matched the final iteration. For example, after
1311 nigel 75 .sp
1312     (tweedle[dume]{3}\es*)+
1313     .sp
1314 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1315     "tweedledee". However, if there are nested capturing subpatterns, the
1316     corresponding captured values may have been set in previous iterations. For
1317     example, after
1318 nigel 75 .sp
1319 nigel 63 /(a|(b))+/
1320 nigel 75 .sp
1321 nigel 63 matches "aba" the value of the second captured substring is "b".
1322 nigel 75 .
1323     .
1324     .\" HTML <a name="atomicgroup"></a>
1326 nigel 63 .rs
1327     .sp
1328 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1329     repetition, failure of what follows normally causes the repeated item to be
1330     re-evaluated to see if a different number of repeats allows the rest of the
1331     pattern to match. Sometimes it is useful to prevent this, either to change the
1332     nature of the match, or to cause it fail earlier than it otherwise might, when
1333     the author of the pattern knows there is no point in carrying on.
1334 nigel 75 .P
1335     Consider, for example, the pattern \ed+foo when applied to the subject line
1336     .sp
1337 nigel 63 123456bar
1338 nigel 75 .sp
1339 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1340 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1341 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1342     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1343     that once a subpattern has matched, it is not to be re-evaluated in this way.
1344 nigel 75 .P
1345 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1346 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1347     special parenthesis, starting with (?> as in this example:
1348 nigel 75 .sp
1349     (?>\ed+)foo
1350     .sp
1351 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1352     it has matched, and a failure further into the pattern is prevented from
1353     backtracking into it. Backtracking past it to previous items, however, works as
1354     normal.
1355 nigel 75 .P
1356 nigel 63 An alternative description is that a subpattern of this type matches the string
1357     of characters that an identical standalone pattern would match, if anchored at
1358     the current point in the subject string.
1359 nigel 75 .P
1360 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1361     the above example can be thought of as a maximizing repeat that must swallow
1362 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1363 nigel 63 number of digits they match in order to make the rest of the pattern match,
1364 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1365     .P
1366 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1367     subpatterns, and can be nested. However, when the subpattern for an atomic
1368     group is just a single repeated item, as in the example above, a simpler
1369     notation, called a "possessive quantifier" can be used. This consists of an
1370     additional + character following a quantifier. Using this notation, the
1371     previous example can be rewritten as
1372 nigel 75 .sp
1373     \ed++foo
1374     .sp
1375 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1376     example:
1377     .sp
1378     (abc|xyz){2,3}+
1379     .sp
1380 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1381     option is ignored. They are a convenient notation for the simpler forms of
1382 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1383     quantifier and the equivalent atomic group, though there may be a performance
1384     difference; possessive quantifiers should be slightly faster.
1385 nigel 75 .P
1386 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1387     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1388     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1389     package, and PCRE copied it from there. It ultimately found its way into Perl
1390     at release 5.10.
1391 nigel 75 .P
1392 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1393     pattern constructs. For example, the sequence A+B is treated as A++B because
1394     there is no point in backtracking into a sequence of A's when B must follow.
1395     .P
1396 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1397     be repeated an unlimited number of times, the use of an atomic group is the
1398     only way to avoid some failing matches taking a very long time indeed. The
1399     pattern
1400 nigel 75 .sp
1401     (\eD+|<\ed+>)*[!?]
1402     .sp
1403 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1404     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1405     quickly. However, if it is applied to
1406 nigel 75 .sp
1407 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1408 nigel 75 .sp
1409 nigel 63 it takes a long time before reporting failure. This is because the string can
1410 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1411     large number of ways, and all have to be tried. (The example uses [!?] rather
1412     than a single character at the end, because both PCRE and Perl have an
1413     optimization that allows for fast failure when a single character is used. They
1414     remember the last single character that is required for a match, and fail early
1415     if it is not present in the string.) If the pattern is changed so that it uses
1416     an atomic group, like this:
1417     .sp
1418     ((?>\eD+)|<\ed+>)*[!?]
1419     .sp
1420 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1421 nigel 75 .
1422     .
1423     .\" HTML <a name="backreferences"></a>
1425 nigel 63 .rs
1426     .sp
1427     Outside a character class, a backslash followed by a digit greater than 0 (and
1428     possibly further digits) is a back reference to a capturing subpattern earlier
1429     (that is, to its left) in the pattern, provided there have been that many
1430     previous capturing left parentheses.
1431 nigel 75 .P
1432 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1433     always taken as a back reference, and causes an error only if there are not
1434     that many capturing left parentheses in the entire pattern. In other words, the
1435     parentheses that are referenced need not be to the left of the reference for
1436 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1437     when a repetition is involved and the subpattern to the right has participated
1438     in an earlier iteration.
1439     .P
1440 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1441     whose number is 10 or more using this syntax because a sequence such as \e50 is
1442     interpreted as a character defined in octal. See the subsection entitled
1443 nigel 91 "Non-printing characters"
1444 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1445     .\" </a>
1446     above
1447     .\"
1448 nigel 93 for further details of the handling of digits following a backslash. There is
1449     no such problem when named parentheses are used. A back reference to any
1450     subpattern is possible using named parentheses (see below).
1451 nigel 75 .P
1452 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1453     backslash is to use the \eg escape sequence, which is a feature introduced in
1454 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1455     number, optionally enclosed in braces. These examples are all identical:
1456 nigel 93 .sp
1457     (ring), \e1
1458     (ring), \eg1
1459     (ring), \eg{1}
1460     .sp
1461 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1462     is present in the older syntax. It is also useful when literal digits follow
1463     the reference. A negative number is a relative reference. Consider this
1464     example:
1465 nigel 93 .sp
1466     (abc(def)ghi)\eg{-1}
1467     .sp
1468     The sequence \eg{-1} is a reference to the most recently started capturing
1469     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1470     would be equivalent to \e1. The use of relative references can be helpful in
1471     long patterns, and also in patterns that are created by joining together
1472     fragments that contain references within themselves.
1473     .P
1474 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1475     the current subject string, rather than anything matching the subpattern
1476     itself (see
1477     .\" HTML <a href="#subpatternsassubroutines">
1478     .\" </a>
1479     "Subpatterns as subroutines"
1480     .\"
1481     below for a way of doing that). So the pattern
1482 nigel 75 .sp
1483     (sens|respons)e and \e1ibility
1484     .sp
1485 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1486     "sense and responsibility". If caseful matching is in force at the time of the
1487     back reference, the case of letters is relevant. For example,
1488 nigel 75 .sp
1489     ((?i)rah)\es+\e1
1490     .sp
1491 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1492     capturing subpattern is matched caselessly.
1493 nigel 75 .P
1494 ph10 171 There are several different ways of writing back references to named
1495     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1496     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1497     back reference syntax, in which \eg can be used for both numeric and named
1498     references, is also supported. We could rewrite the above example in any of
1499 nigel 93 the following ways:
1500 nigel 75 .sp
1501 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1502 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1503 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1504 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1505 nigel 75 .sp
1506 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1507     after the reference.
1508     .P
1509 nigel 63 There may be more than one back reference to the same subpattern. If a
1510     subpattern has not actually been used in a particular match, any back
1511     references to it always fail. For example, the pattern
1512 nigel 75 .sp
1513     (a|(bc))\e2
1514     .sp
1515 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1516     many capturing parentheses in a pattern, all digits following the backslash are
1517     taken as part of a potential back reference number. If the pattern continues
1518     with a digit character, some delimiter must be used to terminate the back
1519     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1520 nigel 75 Otherwise an empty comment (see
1521     .\" HTML <a href="#comments">
1522     .\" </a>
1523     "Comments"
1524     .\"
1525     below) can be used.
1526     .P
1527 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1528 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1529 nigel 63 However, such references can be useful inside repeated subpatterns. For
1530     example, the pattern
1531 nigel 75 .sp
1532     (a|b\e1)+
1533     .sp
1534 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1535     the subpattern, the back reference matches the character string corresponding
1536     to the previous iteration. In order for this to work, the pattern must be such
1537     that the first iteration does not need to match the back reference. This can be
1538     done using alternation, as in the example above, or by a quantifier with a
1539     minimum of zero.
1540 nigel 75 .
1541     .
1542     .\" HTML <a name="bigassertions"></a>
1543 nigel 63 .SH ASSERTIONS
1544     .rs
1545     .sp
1546     An assertion is a test on the characters following or preceding the current
1547     matching point that does not actually consume any characters. The simple
1548 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1549     .\" HTML <a href="#smallassertions">
1550     .\" </a>
1551     above.
1552     .\"
1553     .P
1554 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1555     those that look ahead of the current position in the subject string, and those
1556 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1557     except that it does not cause the current matching position to be changed.
1558     .P
1559     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1560     because it makes no sense to assert the same thing several times. If any kind
1561     of assertion contains capturing subpatterns within it, these are counted for
1562     the purposes of numbering the capturing subpatterns in the whole pattern.
1563     However, substring capturing is carried out only for positive assertions,
1564     because it does not make sense for negative assertions.
1565     .
1566     .
1567     .SS "Lookahead assertions"
1568     .rs
1569     .sp
1570 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1571     negative assertions. For example,
1572 nigel 75 .sp
1573     \ew+(?=;)
1574     .sp
1575 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1576     the match, and
1577 nigel 75 .sp
1578 nigel 63 foo(?!bar)
1579 nigel 75 .sp
1580 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1581     apparently similar pattern
1582 nigel 75 .sp
1583 nigel 63 (?!foo)bar
1584 nigel 75 .sp
1585 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1586     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1587     (?!foo) is always true when the next three characters are "bar". A
1588 nigel 75 lookbehind assertion is needed to achieve the other effect.
1589     .P
1590 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1591     convenient way to do it is with (?!) because an empty string always matches, so
1592     an assertion that requires there not to be an empty string must always fail.
1593 nigel 75 .
1594     .
1595     .\" HTML <a name="lookbehind"></a>
1596     .SS "Lookbehind assertions"
1597     .rs
1598     .sp
1599 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1600     negative assertions. For example,
1601 nigel 75 .sp
1602 nigel 63 (?<!foo)bar
1603 nigel 75 .sp
1604 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1605     a lookbehind assertion are restricted such that all the strings it matches must
1606 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1607     do not all have to have the same fixed length. Thus
1608 nigel 75 .sp
1609 nigel 63 (?<=bullock|donkey)
1610 nigel 75 .sp
1611 nigel 63 is permitted, but
1612 nigel 75 .sp
1613 nigel 63 (?<!dogs?|cats?)
1614 nigel 75 .sp
1615 nigel 63 causes an error at compile time. Branches that match different length strings
1616     are permitted only at the top level of a lookbehind assertion. This is an
1617     extension compared with Perl (at least for 5.8), which requires all branches to
1618     match the same length of string. An assertion such as
1619 nigel 75 .sp
1620 nigel 63 (?<=ab(c|de))
1621 nigel 75 .sp
1622 nigel 63 is not permitted, because its single top-level branch can match two different
1623     lengths, but it is acceptable if rewritten to use two top-level branches:
1624 nigel 75 .sp
1625 nigel 63 (?<=abc|abde)
1626 nigel 75 .sp
1627 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1628 ph10 168 .\" HTML <a href="#resetmatchstart">
1629     .\" </a>
1630     (see above)
1631     .\"
1632     can be used instead of a lookbehind assertion; this is not restricted to a
1633     fixed-length.
1634     .P
1635 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1636 nigel 93 temporarily move the current position back by the fixed length and then try to
1637 nigel 63 match. If there are insufficient characters before the current position, the
1638 nigel 93 assertion fails.
1639 nigel 75 .P
1640     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1641 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1642 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1643     different numbers of bytes, are also not permitted.
1644 nigel 75 .P
1645 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1646     specify efficient matching at the end of the subject string. Consider a simple
1647     pattern such as
1648 nigel 75 .sp
1649 nigel 63 abcd$
1650 nigel 75 .sp
1651 nigel 63 when applied to a long string that does not match. Because matching proceeds
1652     from left to right, PCRE will look for each "a" in the subject and then see if
1653     what follows matches the rest of the pattern. If the pattern is specified as
1654 nigel 75 .sp
1655 nigel 63 ^.*abcd$
1656 nigel 75 .sp
1657 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1658     there is no following "a"), it backtracks to match all but the last character,
1659     then all but the last two characters, and so on. Once again the search for "a"
1660     covers the entire string, from right to left, so we are no better off. However,
1661     if the pattern is written as
1662 nigel 75 .sp
1663 nigel 63 ^.*+(?<=abcd)
1664 nigel 75 .sp
1665 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1666 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1667     characters. If it fails, the match fails immediately. For long strings, this
1668     approach makes a significant difference to the processing time.
1669 nigel 75 .
1670     .
1671     .SS "Using multiple assertions"
1672     .rs
1673     .sp
1674 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1675 nigel 75 .sp
1676     (?<=\ed{3})(?<!999)foo
1677     .sp
1678 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1679     the assertions is applied independently at the same point in the subject
1680     string. First there is a check that the previous three characters are all
1681     digits, and then there is a check that the same three characters are not "999".
1682 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1683 nigel 63 of which are digits and the last three of which are not "999". For example, it
1684     doesn't match "123abcfoo". A pattern to do that is
1685 nigel 75 .sp
1686     (?<=\ed{3}...)(?<!999)foo
1687     .sp
1688 nigel 63 This time the first assertion looks at the preceding six characters, checking
1689     that the first three are digits, and then the second assertion checks that the
1690     preceding three characters are not "999".
1691 nigel 75 .P
1692 nigel 63 Assertions can be nested in any combination. For example,
1693 nigel 75 .sp
1694 nigel 63 (?<=(?<!foo)bar)baz
1695 nigel 75 .sp
1696 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1697     preceded by "foo", while
1698 nigel 75 .sp
1699     (?<=\ed{3}(?!999)...)foo
1700     .sp
1701     is another pattern that matches "foo" preceded by three digits and any three
1702 nigel 63 characters that are not "999".
1703 nigel 75 .
1704     .
1705 nigel 91 .\" HTML <a name="conditions"></a>
1707 nigel 63 .rs
1708     .sp
1709     It is possible to cause the matching process to obey a subpattern
1710     conditionally or to choose between two alternative subpatterns, depending on
1711     the result of an assertion, or whether a previous capturing subpattern matched
1712     or not. The two possible forms of conditional subpattern are
1713 nigel 75 .sp
1714 nigel 63 (?(condition)yes-pattern)
1715     (?(condition)yes-pattern|no-pattern)
1716 nigel 75 .sp
1717 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1718     no-pattern (if present) is used. If there are more than two alternatives in the
1719     subpattern, a compile-time error occurs.
1720 nigel 75 .P
1721 nigel 93 There are four kinds of condition: references to subpatterns, references to
1722     recursion, a pseudo-condition called DEFINE, and assertions.
1723     .
1724     .SS "Checking for a used subpattern by number"
1725     .rs
1726     .sp
1727     If the text between the parentheses consists of a sequence of digits, the
1728     condition is true if the capturing subpattern of that number has previously
1729 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1730 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1731 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1732 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1733     refer to subsequent groups with constructs such as (?(+2).
1734 nigel 91 .P
1735     Consider the following pattern, which contains non-significant white space to
1736     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1737     three parts for ease of discussion:
1738 nigel 75 .sp
1739     ( \e( )? [^()]+ (?(1) \e) )
1740     .sp
1741 nigel 63 The first part matches an optional opening parenthesis, and if that
1742     character is present, sets it as the first captured substring. The second part
1743     matches one or more characters that are not parentheses. The third part is a
1744     conditional subpattern that tests whether the first set of parentheses matched
1745     or not. If they did, that is, if subject started with an opening parenthesis,
1746     the condition is true, and so the yes-pattern is executed and a closing
1747     parenthesis is required. Otherwise, since no-pattern is not present, the
1748     subpattern matches nothing. In other words, this pattern matches a sequence of
1749 nigel 93 non-parentheses, optionally enclosed in parentheses.
1750 ph10 167 .P
1751 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1752 ph10 167 reference:
1753     .sp
1754     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1755     .sp
1756     This makes the fragment independent of the parentheses in the larger pattern.
1757 nigel 93 .
1758     .SS "Checking for a used subpattern by name"
1759     .rs
1760 nigel 91 .sp
1761 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1762     subpattern by name. For compatibility with earlier versions of PCRE, which had
1763     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1764     there is a possible ambiguity with this syntax, because subpattern names may
1765     consist entirely of digits. PCRE looks first for a named subpattern; if it
1766     cannot find one and the name consists entirely of digits, PCRE looks for a
1767     subpattern of that number, which must be greater than zero. Using subpattern
1768     names that consist entirely of digits is not recommended.
1769     .P
1770     Rewriting the above example to use a named subpattern gives this:
1771 nigel 91 .sp
1772 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1773     .sp
1774     .
1775     .SS "Checking for pattern recursion"
1776     .rs
1777     .sp
1778 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1779 nigel 93 the condition is true if a recursive call to the whole pattern or any
1780     subpattern has been made. If digits or a name preceded by ampersand follow the
1781     letter R, for example:
1782     .sp
1783     (?(R3)...) or (?(R&name)...)
1784     .sp
1785     the condition is true if the most recent recursion is into the subpattern whose
1786     number or name is given. This condition does not check the entire recursion
1787     stack.
1788 nigel 75 .P
1789 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1790     patterns are described below.
1791     .
1792     .SS "Defining subpatterns for use by reference only"
1793     .rs
1794     .sp
1795     If the condition is the string (DEFINE), and there is no subpattern with the
1796     name DEFINE, the condition is always false. In this case, there may be only one
1797     alternative in the subpattern. It is always skipped if control reaches this
1798     point in the pattern; the idea of DEFINE is that it can be used to define
1799     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1800     is described below.) For example, a pattern to match an IPv4 address could be
1801     written like this (ignore whitespace and line breaks):
1802     .sp
1803     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1804     \eb (?&byte) (\e.(?&byte)){3} \eb
1805     .sp
1806     The first part of the pattern is a DEFINE group inside which a another group
1807     named "byte" is defined. This matches an individual component of an IPv4
1808     address (a number less than 256). When matching takes place, this part of the
1809     pattern is skipped because DEFINE acts like a false condition.
1810     .P
1811     The rest of the pattern uses references to the named group to match the four
1812     dot-separated components of an IPv4 address, insisting on a word boundary at
1813     each end.
1814     .
1815     .SS "Assertion conditions"
1816     .rs
1817     .sp
1818     If the condition is not in any of the above formats, it must be an assertion.
1819 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1820     this pattern, again containing non-significant white space, and with the two
1821     alternatives on the second line:
1822 nigel 75 .sp
1823 nigel 63 (?(?=[^a-z]*[a-z])
1824 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1825     .sp
1826 nigel 63 The condition is a positive lookahead assertion that matches an optional
1827     sequence of non-letters followed by a letter. In other words, it tests for the
1828     presence of at least one letter in the subject. If a letter is found, the
1829     subject is matched against the first alternative; otherwise it is matched
1830     against the second. This pattern matches strings in one of the two forms
1831     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1832 nigel 75 .
1833     .
1834     .\" HTML <a name="comments"></a>
1835 nigel 63 .SH COMMENTS
1836     .rs
1837     .sp
1838 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1839 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1840     that make up a comment play no part in the pattern matching at all.
1841 nigel 75 .P
1842 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1843 nigel 91 character class introduces a comment that continues to immediately after the
1844     next newline in the pattern.
1845 nigel 75 .
1846     .
1847 nigel 91 .\" HTML <a name="recursion"></a>
1848 nigel 75 .SH "RECURSIVE PATTERNS"
1849 nigel 63 .rs
1850     .sp
1851     Consider the problem of matching a string in parentheses, allowing for
1852     unlimited nested parentheses. Without the use of recursion, the best that can
1853     be done is to use a pattern that matches up to some fixed depth of nesting. It
1854 nigel 93 is not possible to handle an arbitrary nesting depth.
1855     .P
1856     For some time, Perl has provided a facility that allows regular expressions to
1857     recurse (amongst other things). It does this by interpolating Perl code in the
1858     expression at run time, and the code can refer to the expression itself. A Perl
1859     pattern using code interpolation to solve the parentheses problem can be
1860     created like this:
1861 nigel 75 .sp
1862     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1863     .sp
1864 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1865 nigel 93 recursively to the pattern in which it appears.
1866 nigel 75 .P
1867 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1868     supports special syntax for recursion of the entire pattern, and also for
1869     individual subpattern recursion. After its introduction in PCRE and Python,
1870     this kind of recursion was introduced into Perl at release 5.10.
1871 nigel 75 .P
1872 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1873     closing parenthesis is a recursive call of the subpattern of the given number,
1874     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1875     call, which is described in the next section.) The special item (?R) or (?0) is
1876     a recursive call of the entire regular expression.
1877 nigel 87 .P
1878 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1879     treated as an atomic group. That is, once it has matched some of the subject
1880     string, it is never re-entered, even if it contains untried alternatives and
1881     there is a subsequent matching failure.
1882     .P
1883 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1884     PCRE_EXTENDED option is set so that white space is ignored):
1885 nigel 75 .sp
1886     \e( ( (?>[^()]+) | (?R) )* \e)
1887     .sp
1888 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1889     substrings which can either be a sequence of non-parentheses, or a recursive
1890 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1891 nigel 63 Finally there is a closing parenthesis.
1892 nigel 75 .P
1893 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1894     pattern, so instead you could use this:
1895 nigel 75 .sp
1896     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1897     .sp
1898 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1899 ph10 172 them instead of the whole pattern.
1900 ph10 166 .P
1901     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1902     is made easier by the use of relative references. (A Perl 5.10 feature.)
1903     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1904     most recently opened parentheses preceding the recursion. In other words, a
1905     negative number counts capturing parentheses leftwards from the point at which
1906     it is encountered.
1907     .P
1908     It is also possible to refer to subsequently opened parentheses, by writing
1909     references such as (?+2). However, these cannot be recursive because the
1910     reference is not inside the parentheses that are referenced. They are always
1911     "subroutine" calls, as described in the next section.
1912     .P
1913     An alternative approach is to use named parentheses instead. The Perl syntax
1914     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1915     could rewrite the above example as follows:
1916 nigel 75 .sp
1917 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1918 nigel 75 .sp
1919 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1920 ph10 172 used.
1921 ph10 166 .P
1922     This particular example pattern that we have been looking at contains nested
1923     unlimited repeats, and so the use of atomic grouping for matching strings of
1924     non-parentheses is important when applying the pattern to strings that do not
1925     match. For example, when this pattern is applied to
1926 nigel 75 .sp
1927 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1928 nigel 75 .sp
1929 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1930     the match runs for a very long time indeed because there are so many different
1931     ways the + and * repeats can carve up the subject, and all have to be tested
1932     before failure can be reported.
1933 nigel 75 .P
1934 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1935     from the outermost level of the recursion at which the subpattern value is set.
1936     If you want to obtain intermediate values, a callout function can be used (see
1937 nigel 93 below and the
1938 nigel 63 .\" HREF
1939 nigel 75 \fBpcrecallout\fP
1940 nigel 63 .\"
1941     documentation). If the pattern above is matched against
1942 nigel 75 .sp
1943 nigel 63 (ab(cd)ef)
1944 nigel 75 .sp
1945 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1946     on at the top level. If additional parentheses are added, giving
1947 nigel 75 .sp
1948     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1949 nigel 63 ^ ^
1950     ^ ^
1951 nigel 75 .sp
1952 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1953     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1954     has to obtain extra memory to store data during a recursion, which it does by
1955 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1956 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1957 nigel 75 .P
1958 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1959     Consider this pattern, which matches text in angle brackets, allowing for
1960     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1961     recursing), whereas any characters are permitted at the outer level.
1962 nigel 75 .sp
1963     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1964     .sp
1965 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1966     different alternatives for the recursive and non-recursive cases. The (?R) item
1967     is the actual recursive call.
1968 nigel 75 .
1969     .
1970 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1972 nigel 63 .rs
1973     .sp
1974     If the syntax for a recursive subpattern reference (either by number or by
1975     name) is used outside the parentheses to which it refers, it operates like a
1976 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
1977 ph10 166 before or after the reference. A numbered reference can be absolute or
1978     relative, as in these examples:
1979 nigel 75 .sp
1980 ph10 166 (...(absolute)...)...(?2)...
1981     (...(relative)...)...(?-1)...
1982 ph10 172 (...(?+1)...(relative)...
1983 ph10 166 .sp
1984     An earlier example pointed out that the pattern
1985     .sp
1986 nigel 75 (sens|respons)e and \e1ibility
1987     .sp
1988 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1989     "sense and responsibility". If instead the pattern
1990 nigel 75 .sp
1991 nigel 63 (sens|respons)e and (?1)ibility
1992 nigel 75 .sp
1993 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1994 nigel 93 strings. Another example is given in the discussion of DEFINE above.
1995 nigel 87 .P
1996     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1997     group. That is, once it has matched some of the subject string, it is never
1998     re-entered, even if it contains untried alternatives and there is a subsequent
1999     matching failure.
2000 nigel 93 .P
2001     When a subpattern is used as a subroutine, processing options such as
2002     case-independence are fixed when the subpattern is defined. They cannot be
2003     changed for different calls. For example, consider this pattern:
2004     .sp
2005 ph10 166 (abc)(?i:(?-1))
2006 nigel 93 .sp
2007     It matches "abcabc". It does not match "abcABC" because the change of
2008     processing option does not affect the called subpattern.
2009 nigel 75 .
2010     .
2011 nigel 63 .SH CALLOUTS
2012     .rs
2013     .sp
2014     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2015     code to be obeyed in the middle of matching a regular expression. This makes it
2016     possible, amongst other things, to extract different substrings that match the
2017     same pair of parentheses when there is a repetition.
2018 nigel 75 .P
2019 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2020     code. The feature is called "callout". The caller of PCRE provides an external
2021 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2022 nigel 63 By default, this variable contains NULL, which disables all calling out.
2023 nigel 75 .P
2024 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2025     function is to be called. If you want to identify different callout points, you
2026     can put a number less than 256 after the letter C. The default value is zero.
2027     For example, this pattern has two callout points:
2028 nigel 75 .sp
2029 ph10 155 (?C1)abc(?C2)def
2030 nigel 75 .sp
2031     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2032     automatically installed before each item in the pattern. They are all numbered
2033     255.
2034     .P
2035     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2036 nigel 63 set), the external function is called. It is provided with the number of the
2037 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2038     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2039     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2040     description of the interface to the callout function is given in the
2041 nigel 63 .\" HREF
2042 nigel 75 \fBpcrecallout\fP
2043 nigel 63 .\"
2044     documentation.
2045 nigel 93 .
2046     .
2048     .rs
2049     .sp
2050 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2051 ph10 210 are described in the Perl documentation as "experimental and subject to change
2052 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2053     production code should be noted to avoid problems during upgrades." The same
2054 ph10 210 remarks apply to the PCRE features described in this section.
2055     .P
2056 ph10 211 Since these verbs are specifically related to backtracking, they can be used
2057     only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2058     backtracking algorithm. They cause an error if encountered by
2059 ph10 210 \fBpcre_dfa_exec()\fP.
2060     .P
2061 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2062 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
2063     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2064     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2065     are two kinds:
2066     .
2067     .SS "Verbs that act immediately"
2068     .rs
2069     .sp
2070     The following verbs act as soon as they are encountered:
2071     .sp
2072     (*ACCEPT)
2073     .sp
2074     This verb causes the match to end successfully, skipping the remainder of the
2075 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2076     immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2077 ph10 210 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2078     captured. For example:
2079     .sp
2080     A(A|B(*ACCEPT)|C)D
2081     .sp
2082 ph10 211 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2083 ph10 210 captured.
2084     .sp
2085     (*FAIL) or (*F)
2086     .sp
2087 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2088 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2089     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2090     Perl features that are not present in PCRE. The nearest equivalent is the
2091     callout feature, as for example in this pattern:
2092     .sp
2093     a+(?C)(*FAIL)
2094     .sp
2095 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2096     each backtrack happens (in this example, 10 times).
2097 ph10 210 .
2098     .SS "Verbs that act after backtracking"
2099     .rs
2100     .sp
2101 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2102     with what follows, but if there is no subsequent match, a failure is forced.
2103 ph10 210 The verbs differ in exactly what kind of failure occurs.
2104     .sp
2105     (*COMMIT)
2106     .sp
2107 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2108 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2109     a match by advancing the start point take place. Once (*COMMIT) has been
2110 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2111 ph10 210 starting point, or not at all. For example:
2112     .sp
2113     a+(*COMMIT)b
2114     .sp
2115 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2116 ph10 210 dynamic anchor, or "I've started, so I must finish."
2117     .sp
2118     (*PRUNE)
2119     .sp
2120 ph10 211 This verb causes the match to fail at the current position if the rest of the
2121     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2122 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2123     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2124     if there is no match to the right, backtracking cannot cross (*PRUNE).
2125 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2126 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2127     be expressed in any other way.
2128     .sp
2129     (*SKIP)
2130     .sp
2131 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2132 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2133     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2134     was matched leading up to it cannot be part of a successful match. Consider:
2135     .sp
2136     a+(*SKIP)b
2137     .sp
2138 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2139 ph10 210 the first character in the string), the starting point skips on to start the
2140 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2141 ph10 210 effect in this example; although it would suppress backtracking during the
2142     first match attempt, the second attempt would start at the second character
2143     instead of skipping on to "c".
2144     .sp
2145     (*THEN)
2146 ph10 211 .sp
2147 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2148     not match. That is, it cancels pending backtracking, but only within the
2149     current alternation. Its name comes from the observation that it can be used
2150     for a pattern-based if-then-else block:
2151     .sp
2152     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2153     .sp
2154 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2155 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2156 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2157 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2158     .
2159     .
2160 nigel 93 .SH "SEE ALSO"
2161     .rs
2162     .sp
2163     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2164 ph10 99 .
2165     .
2166     .SH AUTHOR
2167     .rs
2168     .sp
2169     .nf
2170     Philip Hazel
2171     University Computing Service
2172     Cambridge CB2 3QH, England.
2173     .fi
2174     .
2175     .
2176     .SH REVISION
2177     .rs
2178     .sp
2179     .nf
2180 ph10 231 Last updated: 11 September 2007
2181 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
2182     .fi


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