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1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
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     .
47 ph10 227 .SH "NEWLINE CONVENTIONS"
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 ph10 247 below. A change of \eR setting can be combined with a change of newline
93 ph10 246 convention.
94 ph10 227 .
95     .
96 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
97     .rs
98     .sp
99 nigel 63 A regular expression is a pattern that is matched against a subject string from
100     left to right. Most characters stand for themselves in a pattern, and match the
101     corresponding characters in the subject. As a trivial example, the pattern
102 nigel 75 .sp
103 nigel 63 The quick brown fox
104 nigel 75 .sp
105 nigel 77 matches a portion of a subject string that is identical to itself. When
106     caseless matching is specified (the PCRE_CASELESS option), letters are matched
107     independently of case. In UTF-8 mode, PCRE always understands the concept of
108     case for characters whose values are less than 128, so caseless matching is
109     always possible. For characters with higher values, the concept of case is
110     supported if PCRE is compiled with Unicode property support, but not otherwise.
111     If you want to use caseless matching for characters 128 and above, you must
112     ensure that PCRE is compiled with Unicode property support as well as with
113     UTF-8 support.
114     .P
115     The power of regular expressions comes from the ability to include alternatives
116     and repetitions in the pattern. These are encoded in the pattern by the use of
117 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
118 nigel 63 interpreted in some special way.
119 nigel 75 .P
120     There are two different sets of metacharacters: those that are recognized
121 nigel 63 anywhere in the pattern except within square brackets, and those that are
122 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
123     are as follows:
124 nigel 75 .sp
125     \e general escape character with several uses
126 nigel 63 ^ assert start of string (or line, in multiline mode)
127     $ assert end of string (or line, in multiline mode)
128     . match any character except newline (by default)
129     [ start character class definition
130     | start of alternative branch
131     ( start subpattern
132     ) end subpattern
133     ? extends the meaning of (
134     also 0 or 1 quantifier
135     also quantifier minimizer
136     * 0 or more quantifier
137     + 1 or more quantifier
138     also "possessive quantifier"
139     { start min/max quantifier
140 nigel 75 .sp
141 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
142 nigel 75 a character class the only metacharacters are:
143     .sp
144     \e general escape character
145 nigel 63 ^ negate the class, but only if the first character
146     - indicates character range
147 nigel 75 .\" JOIN
148 nigel 63 [ POSIX character class (only if followed by POSIX
149     syntax)
150     ] terminates the character class
151 nigel 75 .sp
152     The following sections describe the use of each of the metacharacters.
153     .
154 nigel 93 .
155 nigel 63 .SH BACKSLASH
156     .rs
157     .sp
158     The backslash character has several uses. Firstly, if it is followed by a
159 nigel 91 non-alphanumeric character, it takes away any special meaning that character
160     may have. This use of backslash as an escape character applies both inside and
161 nigel 63 outside character classes.
162 nigel 75 .P
163     For example, if you want to match a * character, you write \e* in the pattern.
164 nigel 63 This escaping action applies whether or not the following character would
165 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
166     non-alphanumeric with backslash to specify that it stands for itself. In
167     particular, if you want to match a backslash, you write \e\e.
168     .P
169 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
170     pattern (other than in a character class) and characters between a # outside
171 nigel 91 a character class and the next newline are ignored. An escaping backslash can
172     be used to include a whitespace or # character as part of the pattern.
173 nigel 75 .P
174 nigel 63 If you want to remove the special meaning from a sequence of characters, you
175 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
176     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
177 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
178 nigel 75 .sp
179 nigel 63 Pattern PCRE matches Perl matches
180 nigel 75 .sp
181     .\" JOIN
182     \eQabc$xyz\eE abc$xyz abc followed by the
183 nigel 63 contents of $xyz
184 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
185     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
186     .sp
187     The \eQ...\eE sequence is recognized both inside and outside character classes.
188     .
189     .
190     .\" HTML <a name="digitsafterbackslash"></a>
191     .SS "Non-printing characters"
192     .rs
193     .sp
194 nigel 63 A second use of backslash provides a way of encoding non-printing characters
195     in patterns in a visible manner. There is no restriction on the appearance of
196     non-printing characters, apart from the binary zero that terminates a pattern,
197     but when a pattern is being prepared by text editing, it is usually easier to
198     use one of the following escape sequences than the binary character it
199     represents:
200 nigel 75 .sp
201     \ea alarm, that is, the BEL character (hex 07)
202     \ecx "control-x", where x is any character
203     \ee escape (hex 1B)
204     \ef formfeed (hex 0C)
205 ph10 227 \en linefeed (hex 0A)
206 nigel 75 \er carriage return (hex 0D)
207     \et tab (hex 09)
208     \eddd character with octal code ddd, or backreference
209     \exhh character with hex code hh
210 nigel 87 \ex{hhh..} character with hex code hhh..
211 nigel 75 .sp
212     The precise effect of \ecx is as follows: if x is a lower case letter, it
213 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
214 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
215 nigel 63 7B.
216 nigel 75 .P
217     After \ex, from zero to two hexadecimal digits are read (letters can be in
218 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
219     and }, but the value of the character code must be less than 256 in non-UTF-8
220 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
221     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
222     point, which is 10FFFF.
223 nigel 75 .P
224 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
225     there is no terminating }, this form of escape is not recognized. Instead, the
226     initial \ex will be interpreted as a basic hexadecimal escape, with no
227     following digits, giving a character whose value is zero.
228     .P
229 nigel 63 Characters whose value is less than 256 can be defined by either of the two
230 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
231     example, \exdc is exactly the same as \ex{dc}.
232 nigel 75 .P
233 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
234     digits, just those that are present are used. Thus the sequence \e0\ex\e07
235     specifies two binary zeros followed by a BEL character (code value 7). Make
236     sure you supply two digits after the initial zero if the pattern character that
237     follows is itself an octal digit.
238 nigel 75 .P
239 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
240     Outside a character class, PCRE reads it and any following digits as a decimal
241     number. If the number is less than 10, or if there have been at least that many
242     previous capturing left parentheses in the expression, the entire sequence is
243 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
244     .\" HTML <a href="#backreferences">
245     .\" </a>
246     later,
247     .\"
248     following the discussion of
249     .\" HTML <a href="#subpattern">
250     .\" </a>
251     parenthesized subpatterns.
252     .\"
253     .P
254 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
255     have not been that many capturing subpatterns, PCRE re-reads up to three octal
256 nigel 93 digits following the backslash, and uses them to generate a data character. Any
257 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
258     character specified in octal must be less than \e400. In UTF-8 mode, values up
259     to \e777 are permitted. For example:
260 nigel 75 .sp
261     \e040 is another way of writing a space
262     .\" JOIN
263     \e40 is the same, provided there are fewer than 40
264 nigel 63 previous capturing subpatterns
265 nigel 75 \e7 is always a back reference
266     .\" JOIN
267     \e11 might be a back reference, or another way of
268 nigel 63 writing a tab
269 nigel 75 \e011 is always a tab
270     \e0113 is a tab followed by the character "3"
271     .\" JOIN
272     \e113 might be a back reference, otherwise the
273 nigel 63 character with octal code 113
274 nigel 75 .\" JOIN
275     \e377 might be a back reference, otherwise
276 nigel 63 the byte consisting entirely of 1 bits
277 nigel 75 .\" JOIN
278     \e81 is either a back reference, or a binary zero
279 nigel 63 followed by the two characters "8" and "1"
280 nigel 75 .sp
281 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
282     zero, because no more than three octal digits are ever read.
283 nigel 75 .P
284 nigel 91 All the sequences that define a single character value can be used both inside
285     and outside character classes. In addition, inside a character class, the
286     sequence \eb is interpreted as the backspace character (hex 08), and the
287 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
288     respectively. Outside a character class, these sequences have different
289     meanings
290 nigel 75 .\" HTML <a href="#uniextseq">
291     .\" </a>
292     (see below).
293     .\"
294     .
295     .
296 nigel 93 .SS "Absolute and relative back references"
297     .rs
298     .sp
299 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
300     enclosed in braces, is an absolute or relative back reference. A named back
301     reference can be coded as \eg{name}. Back references are discussed
302 nigel 93 .\" HTML <a href="#backreferences">
303     .\" </a>
304     later,
305     .\"
306     following the discussion of
307     .\" HTML <a href="#subpattern">
308     .\" </a>
309     parenthesized subpatterns.
310     .\"
311     .
312     .
313 nigel 75 .SS "Generic character types"
314     .rs
315     .sp
316 nigel 93 Another use of backslash is for specifying generic character types. The
317 nigel 75 following are always recognized:
318     .sp
319 ph10 182 \ed any decimal digit
320 nigel 75 \eD any character that is not a decimal digit
321 ph10 178 \eh any horizontal whitespace character
322 ph10 182 \eH any character that is not a horizontal whitespace character
323 nigel 75 \es any whitespace character
324     \eS any character that is not a whitespace character
325 ph10 178 \ev any vertical whitespace character
326 ph10 182 \eV any character that is not a vertical whitespace character
327 nigel 75 \ew any "word" character
328     \eW any "non-word" character
329     .sp
330 nigel 63 Each pair of escape sequences partitions the complete set of characters into
331     two disjoint sets. Any given character matches one, and only one, of each pair.
332 nigel 75 .P
333     These character type sequences can appear both inside and outside character
334     classes. They each match one character of the appropriate type. If the current
335     matching point is at the end of the subject string, all of them fail, since
336     there is no character to match.
337     .P
338     For compatibility with Perl, \es does not match the VT character (code 11).
339     This makes it different from the the POSIX "space" class. The \es characters
340 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
341 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
342 ph10 178 does.
343 nigel 75 .P
344 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
345     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
346 ph10 182 character property support is available. These sequences retain their original
347     meanings from before UTF-8 support was available, mainly for efficiency
348 ph10 178 reasons.
349     .P
350 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
351 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
352     The horizontal space characters are:
353     .sp
354     U+0009 Horizontal tab
355     U+0020 Space
356     U+00A0 Non-break space
357     U+1680 Ogham space mark
358     U+180E Mongolian vowel separator
359     U+2000 En quad
360     U+2001 Em quad
361     U+2002 En space
362     U+2003 Em space
363     U+2004 Three-per-em space
364     U+2005 Four-per-em space
365     U+2006 Six-per-em space
366     U+2007 Figure space
367     U+2008 Punctuation space
368     U+2009 Thin space
369     U+200A Hair space
370     U+202F Narrow no-break space
371     U+205F Medium mathematical space
372     U+3000 Ideographic space
373     .sp
374     The vertical space characters are:
375     .sp
376     U+000A Linefeed
377     U+000B Vertical tab
378     U+000C Formfeed
379     U+000D Carriage return
380     U+0085 Next line
381     U+2028 Line separator
382     U+2029 Paragraph separator
383     .P
384 nigel 75 A "word" character is an underscore or any character less than 256 that is a
385     letter or digit. The definition of letters and digits is controlled by PCRE's
386     low-valued character tables, and may vary if locale-specific matching is taking
387     place (see
388 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
389     .\" </a>
390     "Locale support"
391     .\"
392     in the
393     .\" HREF
394 nigel 75 \fBpcreapi\fP
395 nigel 63 .\"
396 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
397     or "french" in Windows, some character codes greater than 128 are used for
398 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
399     is discouraged.
400 nigel 75 .
401     .
402 ph10 231 .\" HTML <a name="newlineseq"></a>
403 nigel 93 .SS "Newline sequences"
404     .rs
405     .sp
406 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
407     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
408     equivalent to the following:
409 nigel 93 .sp
410     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
411     .sp
412     This is an example of an "atomic group", details of which are given
413     .\" HTML <a href="#atomicgroup">
414     .\" </a>
415     below.
416     .\"
417     This particular group matches either the two-character sequence CR followed by
418     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
419     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
420     line, U+0085). The two-character sequence is treated as a single unit that
421     cannot be split.
422     .P
423     In UTF-8 mode, two additional characters whose codepoints are greater than 255
424     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
425     Unicode character property support is not needed for these characters to be
426     recognized.
427     .P
428 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
429     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
430 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
431 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is
432     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
433     It is also possible to specify these settings by starting a pattern string with
434     one of the following sequences:
435 ph10 231 .sp
436     (*BSR_ANYCRLF) CR, LF, or CRLF only
437     (*BSR_UNICODE) any Unicode newline sequence
438     .sp
439     These override the default and the options given to \fBpcre_compile()\fP, but
440     they can be overridden by options given to \fBpcre_exec()\fP. Note that these
441     special settings, which are not Perl-compatible, are recognized only at the
442     very start of a pattern, and that they must be in upper case. If more than one
443 ph10 247 of them is present, the last one is used. They can be combined with a change of
444 ph10 246 newline convention, for example, a pattern can start with:
445     .sp
446     (*ANY)(*BSR_ANYCRLF)
447     .sp
448 nigel 93 Inside a character class, \eR matches the letter "R".
449     .
450     .
451 nigel 75 .\" HTML <a name="uniextseq"></a>
452     .SS Unicode character properties
453     .rs
454     .sp
455     When PCRE is built with Unicode character property support, three additional
456 ph10 184 escape sequences that match characters with specific properties are available.
457     When not in UTF-8 mode, these sequences are of course limited to testing
458     characters whose codepoints are less than 256, but they do work in this mode.
459     The extra escape sequences are:
460 nigel 75 .sp
461 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
462     \eP{\fIxx\fP} a character without the \fIxx\fP property
463     \eX an extended Unicode sequence
464 nigel 75 .sp
465 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
466     script names, the general category properties, and "Any", which matches any
467     character (including newline). Other properties such as "InMusicalSymbols" are
468     not currently supported by PCRE. Note that \eP{Any} does not match any
469     characters, so always causes a match failure.
470 nigel 75 .P
471 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
472     character from one of these sets can be matched using a script name. For
473     example:
474 nigel 75 .sp
475 nigel 87 \ep{Greek}
476     \eP{Han}
477     .sp
478     Those that are not part of an identified script are lumped together as
479     "Common". The current list of scripts is:
480     .P
481     Arabic,
482     Armenian,
483 nigel 93 Balinese,
484 nigel 87 Bengali,
485     Bopomofo,
486     Braille,
487     Buginese,
488     Buhid,
489     Canadian_Aboriginal,
490     Cherokee,
491     Common,
492     Coptic,
493 nigel 93 Cuneiform,
494 nigel 87 Cypriot,
495     Cyrillic,
496     Deseret,
497     Devanagari,
498     Ethiopic,
499     Georgian,
500     Glagolitic,
501     Gothic,
502     Greek,
503     Gujarati,
504     Gurmukhi,
505     Han,
506     Hangul,
507     Hanunoo,
508     Hebrew,
509     Hiragana,
510     Inherited,
511     Kannada,
512     Katakana,
513     Kharoshthi,
514     Khmer,
515     Lao,
516     Latin,
517     Limbu,
518     Linear_B,
519     Malayalam,
520     Mongolian,
521     Myanmar,
522     New_Tai_Lue,
523 nigel 93 Nko,
524 nigel 87 Ogham,
525     Old_Italic,
526     Old_Persian,
527     Oriya,
528     Osmanya,
529 nigel 93 Phags_Pa,
530     Phoenician,
531 nigel 87 Runic,
532     Shavian,
533     Sinhala,
534     Syloti_Nagri,
535     Syriac,
536     Tagalog,
537     Tagbanwa,
538     Tai_Le,
539     Tamil,
540     Telugu,
541     Thaana,
542     Thai,
543     Tibetan,
544     Tifinagh,
545     Ugaritic,
546     Yi.
547     .P
548     Each character has exactly one general category property, specified by a
549     two-letter abbreviation. For compatibility with Perl, negation can be specified
550     by including a circumflex between the opening brace and the property name. For
551     example, \ep{^Lu} is the same as \eP{Lu}.
552     .P
553     If only one letter is specified with \ep or \eP, it includes all the general
554     category properties that start with that letter. In this case, in the absence
555     of negation, the curly brackets in the escape sequence are optional; these two
556     examples have the same effect:
557     .sp
558 nigel 75 \ep{L}
559     \epL
560     .sp
561 nigel 87 The following general category property codes are supported:
562 nigel 75 .sp
563     C Other
564     Cc Control
565     Cf Format
566     Cn Unassigned
567     Co Private use
568     Cs Surrogate
569     .sp
570     L Letter
571     Ll Lower case letter
572     Lm Modifier letter
573     Lo Other letter
574     Lt Title case letter
575     Lu Upper case letter
576     .sp
577     M Mark
578     Mc Spacing mark
579     Me Enclosing mark
580     Mn Non-spacing mark
581     .sp
582     N Number
583     Nd Decimal number
584     Nl Letter number
585     No Other number
586     .sp
587     P Punctuation
588     Pc Connector punctuation
589     Pd Dash punctuation
590     Pe Close punctuation
591     Pf Final punctuation
592     Pi Initial punctuation
593     Po Other punctuation
594     Ps Open punctuation
595     .sp
596     S Symbol
597     Sc Currency symbol
598     Sk Modifier symbol
599     Sm Mathematical symbol
600     So Other symbol
601     .sp
602     Z Separator
603     Zl Line separator
604     Zp Paragraph separator
605     Zs Space separator
606     .sp
607 nigel 87 The special property L& is also supported: it matches a character that has
608     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
609     a modifier or "other".
610 nigel 75 .P
611 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
612     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
613     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
614     (see the discussion of PCRE_NO_UTF8_CHECK in the
615     .\" HREF
616     \fBpcreapi\fP
617     .\"
618     page).
619     .P
620 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
621 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
622 nigel 87 properties with "Is".
623     .P
624     No character that is in the Unicode table has the Cn (unassigned) property.
625     Instead, this property is assumed for any code point that is not in the
626     Unicode table.
627     .P
628 nigel 75 Specifying caseless matching does not affect these escape sequences. For
629     example, \ep{Lu} always matches only upper case letters.
630     .P
631     The \eX escape matches any number of Unicode characters that form an extended
632     Unicode sequence. \eX is equivalent to
633     .sp
634     (?>\ePM\epM*)
635     .sp
636     That is, it matches a character without the "mark" property, followed by zero
637     or more characters with the "mark" property, and treats the sequence as an
638     atomic group
639     .\" HTML <a href="#atomicgroup">
640     .\" </a>
641     (see below).
642     .\"
643     Characters with the "mark" property are typically accents that affect the
644 ph10 185 preceding character. None of them have codepoints less than 256, so in
645 ph10 184 non-UTF-8 mode \eX matches any one character.
646 nigel 75 .P
647     Matching characters by Unicode property is not fast, because PCRE has to search
648     a structure that contains data for over fifteen thousand characters. That is
649     why the traditional escape sequences such as \ed and \ew do not use Unicode
650     properties in PCRE.
651     .
652     .
653 ph10 168 .\" HTML <a name="resetmatchstart"></a>
654     .SS "Resetting the match start"
655     .rs
656     .sp
657 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
658     matched characters not to be included in the final matched sequence. For
659 ph10 168 example, the pattern:
660     .sp
661     foo\eKbar
662     .sp
663 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
664 ph10 168 similar to a lookbehind assertion
665     .\" HTML <a href="#lookbehind">
666     .\" </a>
667     (described below).
668     .\"
669 ph10 172 However, in this case, the part of the subject before the real match does not
670     have to be of fixed length, as lookbehind assertions do. The use of \eK does
671 ph10 168 not interfere with the setting of
672     .\" HTML <a href="#subpattern">
673     .\" </a>
674     captured substrings.
675 ph10 172 .\"
676 ph10 168 For example, when the pattern
677     .sp
678     (foo)\eKbar
679     .sp
680 ph10 172 matches "foobar", the first substring is still set to "foo".
681 ph10 168 .
682     .
683 nigel 75 .\" HTML <a name="smallassertions"></a>
684     .SS "Simple assertions"
685     .rs
686     .sp
687 nigel 93 The final use of backslash is for certain simple assertions. An assertion
688 nigel 63 specifies a condition that has to be met at a particular point in a match,
689     without consuming any characters from the subject string. The use of
690 nigel 75 subpatterns for more complicated assertions is described
691     .\" HTML <a href="#bigassertions">
692     .\" </a>
693     below.
694     .\"
695 nigel 91 The backslashed assertions are:
696 nigel 75 .sp
697     \eb matches at a word boundary
698     \eB matches when not at a word boundary
699 nigel 93 \eA matches at the start of the subject
700     \eZ matches at the end of the subject
701     also matches before a newline at the end of the subject
702     \ez matches only at the end of the subject
703     \eG matches at the first matching position in the subject
704 nigel 75 .sp
705     These assertions may not appear in character classes (but note that \eb has a
706 nigel 63 different meaning, namely the backspace character, inside a character class).
707 nigel 75 .P
708 nigel 63 A word boundary is a position in the subject string where the current character
709 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
710     \ew and the other matches \eW), or the start or end of the string if the
711     first or last character matches \ew, respectively.
712     .P
713     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
714     dollar (described in the next section) in that they only ever match at the very
715     start and end of the subject string, whatever options are set. Thus, they are
716     independent of multiline mode. These three assertions are not affected by the
717     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
718     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
719     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
720     at a point other than the beginning of the subject, \eA can never match. The
721 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
722     of the string as well as at the very end, whereas \ez matches only at the end.
723 nigel 75 .P
724     The \eG assertion is true only when the current matching position is at the
725     start point of the match, as specified by the \fIstartoffset\fP argument of
726     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
727     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
728 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
729 nigel 75 implementation where \eG can be useful.
730     .P
731     Note, however, that PCRE's interpretation of \eG, as the start of the current
732 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
733     previous match. In Perl, these can be different when the previously matched
734     string was empty. Because PCRE does just one match at a time, it cannot
735     reproduce this behaviour.
736 nigel 75 .P
737     If all the alternatives of a pattern begin with \eG, the expression is anchored
738 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
739     regular expression.
740 nigel 75 .
741     .
742     .SH "CIRCUMFLEX AND DOLLAR"
743 nigel 63 .rs
744     .sp
745     Outside a character class, in the default matching mode, the circumflex
746 nigel 75 character is an assertion that is true only if the current matching point is
747     at the start of the subject string. If the \fIstartoffset\fP argument of
748     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
749 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
750 nigel 75 meaning
751     .\" HTML <a href="#characterclass">
752     .\" </a>
753     (see below).
754     .\"
755     .P
756 nigel 63 Circumflex need not be the first character of the pattern if a number of
757     alternatives are involved, but it should be the first thing in each alternative
758     in which it appears if the pattern is ever to match that branch. If all
759     possible alternatives start with a circumflex, that is, if the pattern is
760     constrained to match only at the start of the subject, it is said to be an
761     "anchored" pattern. (There are also other constructs that can cause a pattern
762     to be anchored.)
763 nigel 75 .P
764     A dollar character is an assertion that is true only if the current matching
765 nigel 63 point is at the end of the subject string, or immediately before a newline
766 nigel 91 at the end of the string (by default). Dollar need not be the last character of
767     the pattern if a number of alternatives are involved, but it should be the last
768     item in any branch in which it appears. Dollar has no special meaning in a
769     character class.
770 nigel 75 .P
771 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
772     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
773 nigel 75 does not affect the \eZ assertion.
774     .P
775 nigel 63 The meanings of the circumflex and dollar characters are changed if the
776 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
777     immediately after internal newlines as well as at the start of the subject
778     string. It does not match after a newline that ends the string. A dollar
779     matches before any newlines in the string, as well as at the very end, when
780     PCRE_MULTILINE is set. When newline is specified as the two-character
781     sequence CRLF, isolated CR and LF characters do not indicate newlines.
782 nigel 75 .P
783 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
784     \en represents a newline) in multiline mode, but not otherwise. Consequently,
785     patterns that are anchored in single line mode because all branches start with
786     ^ are not anchored in multiline mode, and a match for circumflex is possible
787     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
788     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
789     .P
790 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
791 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
792 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
793 nigel 75 .
794     .
795     .SH "FULL STOP (PERIOD, DOT)"
796 nigel 63 .rs
797     .sp
798     Outside a character class, a dot in the pattern matches any one character in
799 nigel 91 the subject string except (by default) a character that signifies the end of a
800 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
801 nigel 91 .P
802 nigel 93 When a line ending is defined as a single character, dot never matches that
803     character; when the two-character sequence CRLF is used, dot does not match CR
804     if it is immediately followed by LF, but otherwise it matches all characters
805     (including isolated CRs and LFs). When any Unicode line endings are being
806     recognized, dot does not match CR or LF or any of the other line ending
807     characters.
808     .P
809 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
810 nigel 93 option is set, a dot matches any one character, without exception. If the
811     two-character sequence CRLF is present in the subject string, it takes two dots
812     to match it.
813 nigel 91 .P
814     The handling of dot is entirely independent of the handling of circumflex and
815     dollar, the only relationship being that they both involve newlines. Dot has no
816     special meaning in a character class.
817 nigel 75 .
818     .
819     .SH "MATCHING A SINGLE BYTE"
820 nigel 63 .rs
821     .sp
822 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
823 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
824     characters. The feature is provided in Perl in order to match individual bytes
825     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
826     what remains in the string may be a malformed UTF-8 string. For this reason,
827     the \eC escape sequence is best avoided.
828 nigel 75 .P
829     PCRE does not allow \eC to appear in lookbehind assertions
830     .\" HTML <a href="#lookbehind">
831     .\" </a>
832     (described below),
833     .\"
834     because in UTF-8 mode this would make it impossible to calculate the length of
835     the lookbehind.
836     .
837     .
838     .\" HTML <a name="characterclass"></a>
839     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
840 nigel 63 .rs
841     .sp
842     An opening square bracket introduces a character class, terminated by a closing
843     square bracket. A closing square bracket on its own is not special. If a
844     closing square bracket is required as a member of the class, it should be the
845     first data character in the class (after an initial circumflex, if present) or
846     escaped with a backslash.
847 nigel 75 .P
848 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
849     character may occupy more than one byte. A matched character must be in the set
850     of characters defined by the class, unless the first character in the class
851     definition is a circumflex, in which case the subject character must not be in
852     the set defined by the class. If a circumflex is actually required as a member
853     of the class, ensure it is not the first character, or escape it with a
854     backslash.
855 nigel 75 .P
856 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
857     [^aeiou] matches any character that is not a lower case vowel. Note that a
858 nigel 75 circumflex is just a convenient notation for specifying the characters that
859     are in the class by enumerating those that are not. A class that starts with a
860     circumflex is not an assertion: it still consumes a character from the subject
861     string, and therefore it fails if the current pointer is at the end of the
862     string.
863     .P
864 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
865 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
866     .P
867 nigel 63 When caseless matching is set, any letters in a class represent both their
868     upper case and lower case versions, so for example, a caseless [aeiou] matches
869     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
870 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
871     case for characters whose values are less than 128, so caseless matching is
872     always possible. For characters with higher values, the concept of case is
873     supported if PCRE is compiled with Unicode property support, but not otherwise.
874     If you want to use caseless matching for characters 128 and above, you must
875     ensure that PCRE is compiled with Unicode property support as well as with
876     UTF-8 support.
877 nigel 75 .P
878 nigel 93 Characters that might indicate line breaks are never treated in any special way
879     when matching character classes, whatever line-ending sequence is in use, and
880     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
881     such as [^a] always matches one of these characters.
882 nigel 75 .P
883 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
884     character class. For example, [d-m] matches any letter between d and m,
885     inclusive. If a minus character is required in a class, it must be escaped with
886     a backslash or appear in a position where it cannot be interpreted as
887     indicating a range, typically as the first or last character in the class.
888 nigel 75 .P
889 nigel 63 It is not possible to have the literal character "]" as the end character of a
890     range. A pattern such as [W-]46] is interpreted as a class of two characters
891     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
892     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
893 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
894     followed by two other characters. The octal or hexadecimal representation of
895     "]" can also be used to end a range.
896     .P
897 nigel 63 Ranges operate in the collating sequence of character values. They can also be
898 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
899 nigel 63 mode, ranges can include characters whose values are greater than 255, for
900 nigel 75 example [\ex{100}-\ex{2ff}].
901     .P
902 nigel 63 If a range that includes letters is used when caseless matching is set, it
903     matches the letters in either case. For example, [W-c] is equivalent to
904 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
905 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
906 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
907     characters with values greater than 128 only when it is compiled with Unicode
908     property support.
909     .P
910     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
911     in a character class, and add the characters that they match to the class. For
912     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
913 nigel 63 conveniently be used with the upper case character types to specify a more
914     restricted set of characters than the matching lower case type. For example,
915 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
916     .P
917     The only metacharacters that are recognized in character classes are backslash,
918     hyphen (only where it can be interpreted as specifying a range), circumflex
919     (only at the start), opening square bracket (only when it can be interpreted as
920     introducing a POSIX class name - see the next section), and the terminating
921     closing square bracket. However, escaping other non-alphanumeric characters
922     does no harm.
923     .
924     .
925     .SH "POSIX CHARACTER CLASSES"
926 nigel 63 .rs
927     .sp
928 nigel 75 Perl supports the POSIX notation for character classes. This uses names
929 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
930     this notation. For example,
931 nigel 75 .sp
932 nigel 63 [01[:alpha:]%]
933 nigel 75 .sp
934 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
935     are
936 nigel 75 .sp
937 nigel 63 alnum letters and digits
938     alpha letters
939     ascii character codes 0 - 127
940     blank space or tab only
941     cntrl control characters
942 nigel 75 digit decimal digits (same as \ed)
943 nigel 63 graph printing characters, excluding space
944     lower lower case letters
945     print printing characters, including space
946     punct printing characters, excluding letters and digits
947 nigel 75 space white space (not quite the same as \es)
948 nigel 63 upper upper case letters
949 nigel 75 word "word" characters (same as \ew)
950 nigel 63 xdigit hexadecimal digits
951 nigel 75 .sp
952 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
953     space (32). Notice that this list includes the VT character (code 11). This
954 nigel 75 makes "space" different to \es, which does not include VT (for Perl
955 nigel 63 compatibility).
956 nigel 75 .P
957 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
958     5.8. Another Perl extension is negation, which is indicated by a ^ character
959     after the colon. For example,
960 nigel 75 .sp
961 nigel 63 [12[:^digit:]]
962 nigel 75 .sp
963 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
964     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
965     supported, and an error is given if they are encountered.
966 nigel 75 .P
967     In UTF-8 mode, characters with values greater than 128 do not match any of
968 nigel 63 the POSIX character classes.
969 nigel 75 .
970     .
971     .SH "VERTICAL BAR"
972 nigel 63 .rs
973     .sp
974     Vertical bar characters are used to separate alternative patterns. For example,
975     the pattern
976 nigel 75 .sp
977 nigel 63 gilbert|sullivan
978 nigel 75 .sp
979 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
980 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
981     process tries each alternative in turn, from left to right, and the first one
982     that succeeds is used. If the alternatives are within a subpattern
983 nigel 75 .\" HTML <a href="#subpattern">
984     .\" </a>
985     (defined below),
986     .\"
987     "succeeds" means matching the rest of the main pattern as well as the
988     alternative in the subpattern.
989     .
990     .
991     .SH "INTERNAL OPTION SETTING"
992 nigel 63 .rs
993     .sp
994     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
995 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
996     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
997     The option letters are
998 nigel 75 .sp
999 nigel 63 i for PCRE_CASELESS
1000     m for PCRE_MULTILINE
1001     s for PCRE_DOTALL
1002     x for PCRE_EXTENDED
1003 nigel 75 .sp
1004 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1005     unset these options by preceding the letter with a hyphen, and a combined
1006     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1007     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1008     permitted. If a letter appears both before and after the hyphen, the option is
1009     unset.
1010 nigel 75 .P
1011 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1012     changed in the same way as the Perl-compatible options by using the characters
1013     J, U and X respectively.
1014     .P
1015 nigel 63 When an option change occurs at top level (that is, not inside subpattern
1016     parentheses), the change applies to the remainder of the pattern that follows.
1017     If the change is placed right at the start of a pattern, PCRE extracts it into
1018     the global options (and it will therefore show up in data extracted by the
1019 nigel 75 \fBpcre_fullinfo()\fP function).
1020     .P
1021 nigel 93 An option change within a subpattern (see below for a description of
1022     subpatterns) affects only that part of the current pattern that follows it, so
1023 nigel 75 .sp
1024 nigel 63 (a(?i)b)c
1025 nigel 75 .sp
1026 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1027     By this means, options can be made to have different settings in different
1028     parts of the pattern. Any changes made in one alternative do carry on
1029     into subsequent branches within the same subpattern. For example,
1030 nigel 75 .sp
1031 nigel 63 (a(?i)b|c)
1032 nigel 75 .sp
1033 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1034     branch is abandoned before the option setting. This is because the effects of
1035     option settings happen at compile time. There would be some very weird
1036     behaviour otherwise.
1037 ph10 251 .P
1038     \fBNote:\fP There are other PCRE-specific options that can be set by the
1039     application when the compile or match functions are called. In some cases the
1040     pattern can contain special leading sequences to override what the application
1041     has set or what has been defaulted. Details are given in the section entitled
1042     .\" HTML <a href="#newlineseq">
1043     .\" </a>
1044     "Newline sequences"
1045     .\"
1046 ph10 259 above.
1047 nigel 75 .
1048     .
1049     .\" HTML <a name="subpattern"></a>
1050 nigel 63 .SH SUBPATTERNS
1051     .rs
1052     .sp
1053     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1054 nigel 75 Turning part of a pattern into a subpattern does two things:
1055     .sp
1056 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1057 nigel 75 .sp
1058 nigel 63 cat(aract|erpillar|)
1059 nigel 75 .sp
1060 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1061 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1062 nigel 75 .sp
1063     2. It sets up the subpattern as a capturing subpattern. This means that, when
1064     the whole pattern matches, that portion of the subject string that matched the
1065     subpattern is passed back to the caller via the \fIovector\fP argument of
1066     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1067     from 1) to obtain numbers for the capturing subpatterns.
1068     .P
1069 nigel 63 For example, if the string "the red king" is matched against the pattern
1070 nigel 75 .sp
1071 nigel 63 the ((red|white) (king|queen))
1072 nigel 75 .sp
1073 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1074     2, and 3, respectively.
1075 nigel 75 .P
1076 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1077     There are often times when a grouping subpattern is required without a
1078     capturing requirement. If an opening parenthesis is followed by a question mark
1079     and a colon, the subpattern does not do any capturing, and is not counted when
1080     computing the number of any subsequent capturing subpatterns. For example, if
1081     the string "the white queen" is matched against the pattern
1082 nigel 75 .sp
1083 nigel 63 the ((?:red|white) (king|queen))
1084 nigel 75 .sp
1085 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1086 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1087 nigel 75 .P
1088 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1089     a non-capturing subpattern, the option letters may appear between the "?" and
1090     the ":". Thus the two patterns
1091 nigel 75 .sp
1092 nigel 63 (?i:saturday|sunday)
1093     (?:(?i)saturday|sunday)
1094 nigel 75 .sp
1095 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1096     from left to right, and options are not reset until the end of the subpattern
1097     is reached, an option setting in one branch does affect subsequent branches, so
1098     the above patterns match "SUNDAY" as well as "Saturday".
1099 nigel 75 .
1100     .
1101 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1102     .rs
1103     .sp
1104 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1105     the same numbers for its capturing parentheses. Such a subpattern starts with
1106     (?| and is itself a non-capturing subpattern. For example, consider this
1107 ph10 175 pattern:
1108     .sp
1109     (?|(Sat)ur|(Sun))day
1110 ph10 182 .sp
1111     Because the two alternatives are inside a (?| group, both sets of capturing
1112     parentheses are numbered one. Thus, when the pattern matches, you can look
1113     at captured substring number one, whichever alternative matched. This construct
1114     is useful when you want to capture part, but not all, of one of a number of
1115     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1116 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1117 ph10 182 buffers that follow the subpattern start after the highest number used in any
1118     branch. The following example is taken from the Perl documentation.
1119 ph10 175 The numbers underneath show in which buffer the captured content will be
1120     stored.
1121     .sp
1122     # before ---------------branch-reset----------- after
1123     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1124     # 1 2 2 3 2 3 4
1125 ph10 182 .sp
1126 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1127     the first one in the pattern with the given number.
1128     .P
1129     An alternative approach to using this "branch reset" feature is to use
1130     duplicate named subpatterns, as described in the next section.
1131     .
1132     .
1133 nigel 75 .SH "NAMED SUBPATTERNS"
1134 nigel 63 .rs
1135     .sp
1136     Identifying capturing parentheses by number is simple, but it can be very hard
1137     to keep track of the numbers in complicated regular expressions. Furthermore,
1138 nigel 75 if an expression is modified, the numbers may change. To help with this
1139 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1140     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1141     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1142     the Perl and the Python syntax.
1143     .P
1144     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1145     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1146 nigel 91 parentheses from other parts of the pattern, such as
1147     .\" HTML <a href="#backreferences">
1148     .\" </a>
1149     backreferences,
1150     .\"
1151     .\" HTML <a href="#recursion">
1152     .\" </a>
1153     recursion,
1154     .\"
1155     and
1156     .\" HTML <a href="#conditions">
1157     .\" </a>
1158     conditions,
1159     .\"
1160     can be made by name as well as by number.
1161 nigel 75 .P
1162 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1163 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1164     if the names were not present. The PCRE API provides function calls for
1165     extracting the name-to-number translation table from a compiled pattern. There
1166     is also a convenience function for extracting a captured substring by name.
1167 nigel 91 .P
1168     By default, a name must be unique within a pattern, but it is possible to relax
1169     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1170     be useful for patterns where only one instance of the named parentheses can
1171     match. Suppose you want to match the name of a weekday, either as a 3-letter
1172     abbreviation or as the full name, and in both cases you want to extract the
1173     abbreviation. This pattern (ignoring the line breaks) does the job:
1174     .sp
1175 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1176     (?<DN>Tue)(?:sday)?|
1177     (?<DN>Wed)(?:nesday)?|
1178     (?<DN>Thu)(?:rsday)?|
1179     (?<DN>Sat)(?:urday)?
1180 nigel 91 .sp
1181     There are five capturing substrings, but only one is ever set after a match.
1182 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1183 ph10 175 subpattern, as described in the previous section.)
1184     .P
1185 nigel 91 The convenience function for extracting the data by name returns the substring
1186 nigel 93 for the first (and in this example, the only) subpattern of that name that
1187 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1188     make a reference to a non-unique named subpattern from elsewhere in the
1189     pattern, the one that corresponds to the lowest number is used. For further
1190     details of the interfaces for handling named subpatterns, see the
1191 nigel 63 .\" HREF
1192 nigel 75 \fBpcreapi\fP
1193 nigel 63 .\"
1194     documentation.
1195 nigel 75 .
1196     .
1197 nigel 63 .SH REPETITION
1198     .rs
1199     .sp
1200     Repetition is specified by quantifiers, which can follow any of the following
1201     items:
1202 nigel 75 .sp
1203 nigel 63 a literal data character
1204 nigel 93 the dot metacharacter
1205 nigel 75 the \eC escape sequence
1206     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1207 nigel 93 the \eR escape sequence
1208 nigel 75 an escape such as \ed that matches a single character
1209 nigel 63 a character class
1210     a back reference (see next section)
1211     a parenthesized subpattern (unless it is an assertion)
1212 nigel 75 .sp
1213 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1214     permitted matches, by giving the two numbers in curly brackets (braces),
1215     separated by a comma. The numbers must be less than 65536, and the first must
1216     be less than or equal to the second. For example:
1217 nigel 75 .sp
1218 nigel 63 z{2,4}
1219 nigel 75 .sp
1220 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1221     character. If the second number is omitted, but the comma is present, there is
1222     no upper limit; if the second number and the comma are both omitted, the
1223     quantifier specifies an exact number of required matches. Thus
1224 nigel 75 .sp
1225 nigel 63 [aeiou]{3,}
1226 nigel 75 .sp
1227 nigel 63 matches at least 3 successive vowels, but may match many more, while
1228 nigel 75 .sp
1229     \ed{8}
1230     .sp
1231 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1232     where a quantifier is not allowed, or one that does not match the syntax of a
1233     quantifier, is taken as a literal character. For example, {,6} is not a
1234     quantifier, but a literal string of four characters.
1235 nigel 75 .P
1236 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1237 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1238     which is represented by a two-byte sequence. Similarly, when Unicode property
1239     support is available, \eX{3} matches three Unicode extended sequences, each of
1240     which may be several bytes long (and they may be of different lengths).
1241     .P
1242 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1243     previous item and the quantifier were not present.
1244 nigel 75 .P
1245 nigel 93 For convenience, the three most common quantifiers have single-character
1246     abbreviations:
1247 nigel 75 .sp
1248 nigel 63 * is equivalent to {0,}
1249     + is equivalent to {1,}
1250     ? is equivalent to {0,1}
1251 nigel 75 .sp
1252 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1253     match no characters with a quantifier that has no upper limit, for example:
1254 nigel 75 .sp
1255 nigel 63 (a?)*
1256 nigel 75 .sp
1257 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1258     such patterns. However, because there are cases where this can be useful, such
1259     patterns are now accepted, but if any repetition of the subpattern does in fact
1260     match no characters, the loop is forcibly broken.
1261 nigel 75 .P
1262 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1263     possible (up to the maximum number of permitted times), without causing the
1264     rest of the pattern to fail. The classic example of where this gives problems
1265 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1266     and within the comment, individual * and / characters may appear. An attempt to
1267     match C comments by applying the pattern
1268     .sp
1269     /\e*.*\e*/
1270     .sp
1271 nigel 63 to the string
1272 nigel 75 .sp
1273     /* first comment */ not comment /* second comment */
1274     .sp
1275 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1276     item.
1277 nigel 75 .P
1278 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1279     greedy, and instead matches the minimum number of times possible, so the
1280     pattern
1281 nigel 75 .sp
1282     /\e*.*?\e*/
1283     .sp
1284 nigel 63 does the right thing with the C comments. The meaning of the various
1285     quantifiers is not otherwise changed, just the preferred number of matches.
1286     Do not confuse this use of question mark with its use as a quantifier in its
1287     own right. Because it has two uses, it can sometimes appear doubled, as in
1288 nigel 75 .sp
1289     \ed??\ed
1290     .sp
1291 nigel 63 which matches one digit by preference, but can match two if that is the only
1292     way the rest of the pattern matches.
1293 nigel 75 .P
1294 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1295 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1296     greedy by following them with a question mark. In other words, it inverts the
1297     default behaviour.
1298 nigel 75 .P
1299 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1300 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1301 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1302 nigel 75 .P
1303 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1304 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1305 nigel 63 implicitly anchored, because whatever follows will be tried against every
1306     character position in the subject string, so there is no point in retrying the
1307     overall match at any position after the first. PCRE normally treats such a
1308 nigel 75 pattern as though it were preceded by \eA.
1309     .P
1310 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1311     worth setting PCRE_DOTALL in order to obtain this optimization, or
1312     alternatively using ^ to indicate anchoring explicitly.
1313 nigel 75 .P
1314 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1315     is inside capturing parentheses that are the subject of a backreference
1316 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1317     succeeds. Consider, for example:
1318 nigel 75 .sp
1319     (.*)abc\e1
1320     .sp
1321 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1322     this reason, such a pattern is not implicitly anchored.
1323 nigel 75 .P
1324 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1325     that matched the final iteration. For example, after
1326 nigel 75 .sp
1327     (tweedle[dume]{3}\es*)+
1328     .sp
1329 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1330     "tweedledee". However, if there are nested capturing subpatterns, the
1331     corresponding captured values may have been set in previous iterations. For
1332     example, after
1333 nigel 75 .sp
1334 nigel 63 /(a|(b))+/
1335 nigel 75 .sp
1336 nigel 63 matches "aba" the value of the second captured substring is "b".
1337 nigel 75 .
1338     .
1339     .\" HTML <a name="atomicgroup"></a>
1340     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1341 nigel 63 .rs
1342     .sp
1343 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1344     repetition, failure of what follows normally causes the repeated item to be
1345     re-evaluated to see if a different number of repeats allows the rest of the
1346     pattern to match. Sometimes it is useful to prevent this, either to change the
1347     nature of the match, or to cause it fail earlier than it otherwise might, when
1348     the author of the pattern knows there is no point in carrying on.
1349 nigel 75 .P
1350     Consider, for example, the pattern \ed+foo when applied to the subject line
1351     .sp
1352 nigel 63 123456bar
1353 nigel 75 .sp
1354 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1355 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1356 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1357     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1358     that once a subpattern has matched, it is not to be re-evaluated in this way.
1359 nigel 75 .P
1360 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1361 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1362     special parenthesis, starting with (?> as in this example:
1363 nigel 75 .sp
1364     (?>\ed+)foo
1365     .sp
1366 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1367     it has matched, and a failure further into the pattern is prevented from
1368     backtracking into it. Backtracking past it to previous items, however, works as
1369     normal.
1370 nigel 75 .P
1371 nigel 63 An alternative description is that a subpattern of this type matches the string
1372     of characters that an identical standalone pattern would match, if anchored at
1373     the current point in the subject string.
1374 nigel 75 .P
1375 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1376     the above example can be thought of as a maximizing repeat that must swallow
1377 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1378 nigel 63 number of digits they match in order to make the rest of the pattern match,
1379 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1380     .P
1381 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1382     subpatterns, and can be nested. However, when the subpattern for an atomic
1383     group is just a single repeated item, as in the example above, a simpler
1384     notation, called a "possessive quantifier" can be used. This consists of an
1385     additional + character following a quantifier. Using this notation, the
1386     previous example can be rewritten as
1387 nigel 75 .sp
1388     \ed++foo
1389     .sp
1390 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1391     example:
1392     .sp
1393     (abc|xyz){2,3}+
1394     .sp
1395 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1396     option is ignored. They are a convenient notation for the simpler forms of
1397 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1398     quantifier and the equivalent atomic group, though there may be a performance
1399     difference; possessive quantifiers should be slightly faster.
1400 nigel 75 .P
1401 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1402     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1403     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1404     package, and PCRE copied it from there. It ultimately found its way into Perl
1405     at release 5.10.
1406 nigel 75 .P
1407 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1408     pattern constructs. For example, the sequence A+B is treated as A++B because
1409     there is no point in backtracking into a sequence of A's when B must follow.
1410     .P
1411 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1412     be repeated an unlimited number of times, the use of an atomic group is the
1413     only way to avoid some failing matches taking a very long time indeed. The
1414     pattern
1415 nigel 75 .sp
1416     (\eD+|<\ed+>)*[!?]
1417     .sp
1418 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1419     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1420     quickly. However, if it is applied to
1421 nigel 75 .sp
1422 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1423 nigel 75 .sp
1424 nigel 63 it takes a long time before reporting failure. This is because the string can
1425 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1426     large number of ways, and all have to be tried. (The example uses [!?] rather
1427     than a single character at the end, because both PCRE and Perl have an
1428     optimization that allows for fast failure when a single character is used. They
1429     remember the last single character that is required for a match, and fail early
1430     if it is not present in the string.) If the pattern is changed so that it uses
1431     an atomic group, like this:
1432     .sp
1433     ((?>\eD+)|<\ed+>)*[!?]
1434     .sp
1435 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1436 nigel 75 .
1437     .
1438     .\" HTML <a name="backreferences"></a>
1439     .SH "BACK REFERENCES"
1440 nigel 63 .rs
1441     .sp
1442     Outside a character class, a backslash followed by a digit greater than 0 (and
1443     possibly further digits) is a back reference to a capturing subpattern earlier
1444     (that is, to its left) in the pattern, provided there have been that many
1445     previous capturing left parentheses.
1446 nigel 75 .P
1447 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1448     always taken as a back reference, and causes an error only if there are not
1449     that many capturing left parentheses in the entire pattern. In other words, the
1450     parentheses that are referenced need not be to the left of the reference for
1451 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1452     when a repetition is involved and the subpattern to the right has participated
1453     in an earlier iteration.
1454     .P
1455 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1456     whose number is 10 or more using this syntax because a sequence such as \e50 is
1457     interpreted as a character defined in octal. See the subsection entitled
1458 nigel 91 "Non-printing characters"
1459 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1460     .\" </a>
1461     above
1462     .\"
1463 nigel 93 for further details of the handling of digits following a backslash. There is
1464     no such problem when named parentheses are used. A back reference to any
1465     subpattern is possible using named parentheses (see below).
1466 nigel 75 .P
1467 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1468     backslash is to use the \eg escape sequence, which is a feature introduced in
1469 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1470     number, optionally enclosed in braces. These examples are all identical:
1471 nigel 93 .sp
1472     (ring), \e1
1473     (ring), \eg1
1474     (ring), \eg{1}
1475     .sp
1476 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1477     is present in the older syntax. It is also useful when literal digits follow
1478     the reference. A negative number is a relative reference. Consider this
1479     example:
1480 nigel 93 .sp
1481     (abc(def)ghi)\eg{-1}
1482     .sp
1483     The sequence \eg{-1} is a reference to the most recently started capturing
1484     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1485     would be equivalent to \e1. The use of relative references can be helpful in
1486     long patterns, and also in patterns that are created by joining together
1487     fragments that contain references within themselves.
1488     .P
1489 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1490     the current subject string, rather than anything matching the subpattern
1491     itself (see
1492     .\" HTML <a href="#subpatternsassubroutines">
1493     .\" </a>
1494     "Subpatterns as subroutines"
1495     .\"
1496     below for a way of doing that). So the pattern
1497 nigel 75 .sp
1498     (sens|respons)e and \e1ibility
1499     .sp
1500 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1501     "sense and responsibility". If caseful matching is in force at the time of the
1502     back reference, the case of letters is relevant. For example,
1503 nigel 75 .sp
1504     ((?i)rah)\es+\e1
1505     .sp
1506 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1507     capturing subpattern is matched caselessly.
1508 nigel 75 .P
1509 ph10 171 There are several different ways of writing back references to named
1510     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1511     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1512     back reference syntax, in which \eg can be used for both numeric and named
1513     references, is also supported. We could rewrite the above example in any of
1514 nigel 93 the following ways:
1515 nigel 75 .sp
1516 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1517 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1518 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1519 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1520 nigel 75 .sp
1521 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1522     after the reference.
1523     .P
1524 nigel 63 There may be more than one back reference to the same subpattern. If a
1525     subpattern has not actually been used in a particular match, any back
1526     references to it always fail. For example, the pattern
1527 nigel 75 .sp
1528     (a|(bc))\e2
1529     .sp
1530 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1531     many capturing parentheses in a pattern, all digits following the backslash are
1532     taken as part of a potential back reference number. If the pattern continues
1533     with a digit character, some delimiter must be used to terminate the back
1534     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1535 nigel 75 Otherwise an empty comment (see
1536     .\" HTML <a href="#comments">
1537     .\" </a>
1538     "Comments"
1539     .\"
1540     below) can be used.
1541     .P
1542 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1543 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1544 nigel 63 However, such references can be useful inside repeated subpatterns. For
1545     example, the pattern
1546 nigel 75 .sp
1547     (a|b\e1)+
1548     .sp
1549 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1550     the subpattern, the back reference matches the character string corresponding
1551     to the previous iteration. In order for this to work, the pattern must be such
1552     that the first iteration does not need to match the back reference. This can be
1553     done using alternation, as in the example above, or by a quantifier with a
1554     minimum of zero.
1555 nigel 75 .
1556     .
1557     .\" HTML <a name="bigassertions"></a>
1558 nigel 63 .SH ASSERTIONS
1559     .rs
1560     .sp
1561     An assertion is a test on the characters following or preceding the current
1562     matching point that does not actually consume any characters. The simple
1563 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1564     .\" HTML <a href="#smallassertions">
1565     .\" </a>
1566     above.
1567     .\"
1568     .P
1569 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1570     those that look ahead of the current position in the subject string, and those
1571 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1572     except that it does not cause the current matching position to be changed.
1573     .P
1574     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1575     because it makes no sense to assert the same thing several times. If any kind
1576     of assertion contains capturing subpatterns within it, these are counted for
1577     the purposes of numbering the capturing subpatterns in the whole pattern.
1578     However, substring capturing is carried out only for positive assertions,
1579     because it does not make sense for negative assertions.
1580     .
1581     .
1582     .SS "Lookahead assertions"
1583     .rs
1584     .sp
1585 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1586     negative assertions. For example,
1587 nigel 75 .sp
1588     \ew+(?=;)
1589     .sp
1590 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1591     the match, and
1592 nigel 75 .sp
1593 nigel 63 foo(?!bar)
1594 nigel 75 .sp
1595 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1596     apparently similar pattern
1597 nigel 75 .sp
1598 nigel 63 (?!foo)bar
1599 nigel 75 .sp
1600 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1601     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1602     (?!foo) is always true when the next three characters are "bar". A
1603 nigel 75 lookbehind assertion is needed to achieve the other effect.
1604     .P
1605 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1606     convenient way to do it is with (?!) because an empty string always matches, so
1607     an assertion that requires there not to be an empty string must always fail.
1608 nigel 75 .
1609     .
1610     .\" HTML <a name="lookbehind"></a>
1611     .SS "Lookbehind assertions"
1612     .rs
1613     .sp
1614 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1615     negative assertions. For example,
1616 nigel 75 .sp
1617 nigel 63 (?<!foo)bar
1618 nigel 75 .sp
1619 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1620     a lookbehind assertion are restricted such that all the strings it matches must
1621 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1622     do not all have to have the same fixed length. Thus
1623 nigel 75 .sp
1624 nigel 63 (?<=bullock|donkey)
1625 nigel 75 .sp
1626 nigel 63 is permitted, but
1627 nigel 75 .sp
1628 nigel 63 (?<!dogs?|cats?)
1629 nigel 75 .sp
1630 nigel 63 causes an error at compile time. Branches that match different length strings
1631     are permitted only at the top level of a lookbehind assertion. This is an
1632     extension compared with Perl (at least for 5.8), which requires all branches to
1633     match the same length of string. An assertion such as
1634 nigel 75 .sp
1635 nigel 63 (?<=ab(c|de))
1636 nigel 75 .sp
1637 nigel 63 is not permitted, because its single top-level branch can match two different
1638     lengths, but it is acceptable if rewritten to use two top-level branches:
1639 nigel 75 .sp
1640 nigel 63 (?<=abc|abde)
1641 nigel 75 .sp
1642 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1643 ph10 168 .\" HTML <a href="#resetmatchstart">
1644     .\" </a>
1645     (see above)
1646     .\"
1647     can be used instead of a lookbehind assertion; this is not restricted to a
1648     fixed-length.
1649     .P
1650 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1651 nigel 93 temporarily move the current position back by the fixed length and then try to
1652 nigel 63 match. If there are insufficient characters before the current position, the
1653 nigel 93 assertion fails.
1654 nigel 75 .P
1655     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1656 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1657 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1658     different numbers of bytes, are also not permitted.
1659 nigel 75 .P
1660 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1661     specify efficient matching at the end of the subject string. Consider a simple
1662     pattern such as
1663 nigel 75 .sp
1664 nigel 63 abcd$
1665 nigel 75 .sp
1666 nigel 63 when applied to a long string that does not match. Because matching proceeds
1667     from left to right, PCRE will look for each "a" in the subject and then see if
1668     what follows matches the rest of the pattern. If the pattern is specified as
1669 nigel 75 .sp
1670 nigel 63 ^.*abcd$
1671 nigel 75 .sp
1672 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1673     there is no following "a"), it backtracks to match all but the last character,
1674     then all but the last two characters, and so on. Once again the search for "a"
1675     covers the entire string, from right to left, so we are no better off. However,
1676     if the pattern is written as
1677 nigel 75 .sp
1678 nigel 63 ^.*+(?<=abcd)
1679 nigel 75 .sp
1680 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1681 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1682     characters. If it fails, the match fails immediately. For long strings, this
1683     approach makes a significant difference to the processing time.
1684 nigel 75 .
1685     .
1686     .SS "Using multiple assertions"
1687     .rs
1688     .sp
1689 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1690 nigel 75 .sp
1691     (?<=\ed{3})(?<!999)foo
1692     .sp
1693 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1694     the assertions is applied independently at the same point in the subject
1695     string. First there is a check that the previous three characters are all
1696     digits, and then there is a check that the same three characters are not "999".
1697 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1698 nigel 63 of which are digits and the last three of which are not "999". For example, it
1699     doesn't match "123abcfoo". A pattern to do that is
1700 nigel 75 .sp
1701     (?<=\ed{3}...)(?<!999)foo
1702     .sp
1703 nigel 63 This time the first assertion looks at the preceding six characters, checking
1704     that the first three are digits, and then the second assertion checks that the
1705     preceding three characters are not "999".
1706 nigel 75 .P
1707 nigel 63 Assertions can be nested in any combination. For example,
1708 nigel 75 .sp
1709 nigel 63 (?<=(?<!foo)bar)baz
1710 nigel 75 .sp
1711 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1712     preceded by "foo", while
1713 nigel 75 .sp
1714     (?<=\ed{3}(?!999)...)foo
1715     .sp
1716     is another pattern that matches "foo" preceded by three digits and any three
1717 nigel 63 characters that are not "999".
1718 nigel 75 .
1719     .
1720 nigel 91 .\" HTML <a name="conditions"></a>
1721 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1722 nigel 63 .rs
1723     .sp
1724     It is possible to cause the matching process to obey a subpattern
1725     conditionally or to choose between two alternative subpatterns, depending on
1726     the result of an assertion, or whether a previous capturing subpattern matched
1727     or not. The two possible forms of conditional subpattern are
1728 nigel 75 .sp
1729 nigel 63 (?(condition)yes-pattern)
1730     (?(condition)yes-pattern|no-pattern)
1731 nigel 75 .sp
1732 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1733     no-pattern (if present) is used. If there are more than two alternatives in the
1734     subpattern, a compile-time error occurs.
1735 nigel 75 .P
1736 nigel 93 There are four kinds of condition: references to subpatterns, references to
1737     recursion, a pseudo-condition called DEFINE, and assertions.
1738     .
1739     .SS "Checking for a used subpattern by number"
1740     .rs
1741     .sp
1742     If the text between the parentheses consists of a sequence of digits, the
1743     condition is true if the capturing subpattern of that number has previously
1744 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1745 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1746 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1747 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1748     refer to subsequent groups with constructs such as (?(+2).
1749 nigel 91 .P
1750     Consider the following pattern, which contains non-significant white space to
1751     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1752     three parts for ease of discussion:
1753 nigel 75 .sp
1754     ( \e( )? [^()]+ (?(1) \e) )
1755     .sp
1756 nigel 63 The first part matches an optional opening parenthesis, and if that
1757     character is present, sets it as the first captured substring. The second part
1758     matches one or more characters that are not parentheses. The third part is a
1759     conditional subpattern that tests whether the first set of parentheses matched
1760     or not. If they did, that is, if subject started with an opening parenthesis,
1761     the condition is true, and so the yes-pattern is executed and a closing
1762     parenthesis is required. Otherwise, since no-pattern is not present, the
1763     subpattern matches nothing. In other words, this pattern matches a sequence of
1764 nigel 93 non-parentheses, optionally enclosed in parentheses.
1765 ph10 167 .P
1766 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1767 ph10 167 reference:
1768     .sp
1769     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1770     .sp
1771     This makes the fragment independent of the parentheses in the larger pattern.
1772 nigel 93 .
1773     .SS "Checking for a used subpattern by name"
1774     .rs
1775 nigel 91 .sp
1776 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1777     subpattern by name. For compatibility with earlier versions of PCRE, which had
1778     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1779     there is a possible ambiguity with this syntax, because subpattern names may
1780     consist entirely of digits. PCRE looks first for a named subpattern; if it
1781     cannot find one and the name consists entirely of digits, PCRE looks for a
1782     subpattern of that number, which must be greater than zero. Using subpattern
1783     names that consist entirely of digits is not recommended.
1784     .P
1785     Rewriting the above example to use a named subpattern gives this:
1786 nigel 91 .sp
1787 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1788     .sp
1789     .
1790     .SS "Checking for pattern recursion"
1791     .rs
1792     .sp
1793 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1794 nigel 93 the condition is true if a recursive call to the whole pattern or any
1795     subpattern has been made. If digits or a name preceded by ampersand follow the
1796     letter R, for example:
1797     .sp
1798     (?(R3)...) or (?(R&name)...)
1799     .sp
1800     the condition is true if the most recent recursion is into the subpattern whose
1801     number or name is given. This condition does not check the entire recursion
1802     stack.
1803 nigel 75 .P
1804 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1805     patterns are described below.
1806     .
1807     .SS "Defining subpatterns for use by reference only"
1808     .rs
1809     .sp
1810     If the condition is the string (DEFINE), and there is no subpattern with the
1811     name DEFINE, the condition is always false. In this case, there may be only one
1812     alternative in the subpattern. It is always skipped if control reaches this
1813     point in the pattern; the idea of DEFINE is that it can be used to define
1814     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1815     is described below.) For example, a pattern to match an IPv4 address could be
1816     written like this (ignore whitespace and line breaks):
1817     .sp
1818     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1819     \eb (?&byte) (\e.(?&byte)){3} \eb
1820     .sp
1821     The first part of the pattern is a DEFINE group inside which a another group
1822     named "byte" is defined. This matches an individual component of an IPv4
1823     address (a number less than 256). When matching takes place, this part of the
1824     pattern is skipped because DEFINE acts like a false condition.
1825     .P
1826     The rest of the pattern uses references to the named group to match the four
1827     dot-separated components of an IPv4 address, insisting on a word boundary at
1828     each end.
1829     .
1830     .SS "Assertion conditions"
1831     .rs
1832     .sp
1833     If the condition is not in any of the above formats, it must be an assertion.
1834 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1835     this pattern, again containing non-significant white space, and with the two
1836     alternatives on the second line:
1837 nigel 75 .sp
1838 nigel 63 (?(?=[^a-z]*[a-z])
1839 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1840     .sp
1841 nigel 63 The condition is a positive lookahead assertion that matches an optional
1842     sequence of non-letters followed by a letter. In other words, it tests for the
1843     presence of at least one letter in the subject. If a letter is found, the
1844     subject is matched against the first alternative; otherwise it is matched
1845     against the second. This pattern matches strings in one of the two forms
1846     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1847 nigel 75 .
1848     .
1849     .\" HTML <a name="comments"></a>
1850 nigel 63 .SH COMMENTS
1851     .rs
1852     .sp
1853 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1854 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1855     that make up a comment play no part in the pattern matching at all.
1856 nigel 75 .P
1857 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1858 nigel 91 character class introduces a comment that continues to immediately after the
1859     next newline in the pattern.
1860 nigel 75 .
1861     .
1862 nigel 91 .\" HTML <a name="recursion"></a>
1863 nigel 75 .SH "RECURSIVE PATTERNS"
1864 nigel 63 .rs
1865     .sp
1866     Consider the problem of matching a string in parentheses, allowing for
1867     unlimited nested parentheses. Without the use of recursion, the best that can
1868     be done is to use a pattern that matches up to some fixed depth of nesting. It
1869 nigel 93 is not possible to handle an arbitrary nesting depth.
1870     .P
1871     For some time, Perl has provided a facility that allows regular expressions to
1872     recurse (amongst other things). It does this by interpolating Perl code in the
1873     expression at run time, and the code can refer to the expression itself. A Perl
1874     pattern using code interpolation to solve the parentheses problem can be
1875     created like this:
1876 nigel 75 .sp
1877     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1878     .sp
1879 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1880 nigel 93 recursively to the pattern in which it appears.
1881 nigel 75 .P
1882 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1883     supports special syntax for recursion of the entire pattern, and also for
1884     individual subpattern recursion. After its introduction in PCRE and Python,
1885     this kind of recursion was introduced into Perl at release 5.10.
1886 nigel 75 .P
1887 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1888     closing parenthesis is a recursive call of the subpattern of the given number,
1889     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1890     call, which is described in the next section.) The special item (?R) or (?0) is
1891     a recursive call of the entire regular expression.
1892 nigel 87 .P
1893 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1894     treated as an atomic group. That is, once it has matched some of the subject
1895     string, it is never re-entered, even if it contains untried alternatives and
1896     there is a subsequent matching failure.
1897     .P
1898 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1899     PCRE_EXTENDED option is set so that white space is ignored):
1900 nigel 75 .sp
1901     \e( ( (?>[^()]+) | (?R) )* \e)
1902     .sp
1903 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1904     substrings which can either be a sequence of non-parentheses, or a recursive
1905 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1906 nigel 63 Finally there is a closing parenthesis.
1907 nigel 75 .P
1908 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1909     pattern, so instead you could use this:
1910 nigel 75 .sp
1911     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1912     .sp
1913 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1914 ph10 172 them instead of the whole pattern.
1915 ph10 166 .P
1916     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1917     is made easier by the use of relative references. (A Perl 5.10 feature.)
1918     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1919     most recently opened parentheses preceding the recursion. In other words, a
1920     negative number counts capturing parentheses leftwards from the point at which
1921     it is encountered.
1922     .P
1923     It is also possible to refer to subsequently opened parentheses, by writing
1924     references such as (?+2). However, these cannot be recursive because the
1925     reference is not inside the parentheses that are referenced. They are always
1926     "subroutine" calls, as described in the next section.
1927     .P
1928     An alternative approach is to use named parentheses instead. The Perl syntax
1929     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1930     could rewrite the above example as follows:
1931 nigel 75 .sp
1932 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1933 nigel 75 .sp
1934 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1935 ph10 172 used.
1936 ph10 166 .P
1937     This particular example pattern that we have been looking at contains nested
1938     unlimited repeats, and so the use of atomic grouping for matching strings of
1939     non-parentheses is important when applying the pattern to strings that do not
1940     match. For example, when this pattern is applied to
1941 nigel 75 .sp
1942 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1943 nigel 75 .sp
1944 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1945     the match runs for a very long time indeed because there are so many different
1946     ways the + and * repeats can carve up the subject, and all have to be tested
1947     before failure can be reported.
1948 nigel 75 .P
1949 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1950     from the outermost level of the recursion at which the subpattern value is set.
1951     If you want to obtain intermediate values, a callout function can be used (see
1952 nigel 93 below and the
1953 nigel 63 .\" HREF
1954 nigel 75 \fBpcrecallout\fP
1955 nigel 63 .\"
1956     documentation). If the pattern above is matched against
1957 nigel 75 .sp
1958 nigel 63 (ab(cd)ef)
1959 nigel 75 .sp
1960 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1961     on at the top level. If additional parentheses are added, giving
1962 nigel 75 .sp
1963     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1964 nigel 63 ^ ^
1965     ^ ^
1966 nigel 75 .sp
1967 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1968     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1969     has to obtain extra memory to store data during a recursion, which it does by
1970 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1971 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1972 nigel 75 .P
1973 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1974     Consider this pattern, which matches text in angle brackets, allowing for
1975     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1976     recursing), whereas any characters are permitted at the outer level.
1977 nigel 75 .sp
1978     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1979     .sp
1980 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1981     different alternatives for the recursive and non-recursive cases. The (?R) item
1982     is the actual recursive call.
1983 nigel 75 .
1984     .
1985 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1986 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
1987 nigel 63 .rs
1988     .sp
1989     If the syntax for a recursive subpattern reference (either by number or by
1990     name) is used outside the parentheses to which it refers, it operates like a
1991 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
1992 ph10 166 before or after the reference. A numbered reference can be absolute or
1993     relative, as in these examples:
1994 nigel 75 .sp
1995 ph10 166 (...(absolute)...)...(?2)...
1996     (...(relative)...)...(?-1)...
1997 ph10 172 (...(?+1)...(relative)...
1998 ph10 166 .sp
1999     An earlier example pointed out that the pattern
2000     .sp
2001 nigel 75 (sens|respons)e and \e1ibility
2002     .sp
2003 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2004     "sense and responsibility". If instead the pattern
2005 nigel 75 .sp
2006 nigel 63 (sens|respons)e and (?1)ibility
2007 nigel 75 .sp
2008 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2009 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2010 nigel 87 .P
2011     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2012     group. That is, once it has matched some of the subject string, it is never
2013     re-entered, even if it contains untried alternatives and there is a subsequent
2014     matching failure.
2015 nigel 93 .P
2016     When a subpattern is used as a subroutine, processing options such as
2017     case-independence are fixed when the subpattern is defined. They cannot be
2018     changed for different calls. For example, consider this pattern:
2019     .sp
2020 ph10 166 (abc)(?i:(?-1))
2021 nigel 93 .sp
2022     It matches "abcabc". It does not match "abcABC" because the change of
2023     processing option does not affect the called subpattern.
2024 nigel 75 .
2025     .
2026 nigel 63 .SH CALLOUTS
2027     .rs
2028     .sp
2029     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2030     code to be obeyed in the middle of matching a regular expression. This makes it
2031     possible, amongst other things, to extract different substrings that match the
2032     same pair of parentheses when there is a repetition.
2033 nigel 75 .P
2034 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2035     code. The feature is called "callout". The caller of PCRE provides an external
2036 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2037 nigel 63 By default, this variable contains NULL, which disables all calling out.
2038 nigel 75 .P
2039 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2040     function is to be called. If you want to identify different callout points, you
2041     can put a number less than 256 after the letter C. The default value is zero.
2042     For example, this pattern has two callout points:
2043 nigel 75 .sp
2044 ph10 155 (?C1)abc(?C2)def
2045 nigel 75 .sp
2046     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2047     automatically installed before each item in the pattern. They are all numbered
2048     255.
2049     .P
2050     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2051 nigel 63 set), the external function is called. It is provided with the number of the
2052 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2053     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2054     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2055     description of the interface to the callout function is given in the
2056 nigel 63 .\" HREF
2057 nigel 75 \fBpcrecallout\fP
2058 nigel 63 .\"
2059     documentation.
2060 nigel 93 .
2061     .
2062 ph10 235 .SH "BACKTRACKING CONTROL"
2063 ph10 210 .rs
2064     .sp
2065 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2066 ph10 210 are described in the Perl documentation as "experimental and subject to change
2067 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2068     production code should be noted to avoid problems during upgrades." The same
2069 ph10 210 remarks apply to the PCRE features described in this section.
2070     .P
2071 ph10 211 Since these verbs are specifically related to backtracking, they can be used
2072     only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2073     backtracking algorithm. They cause an error if encountered by
2074 ph10 210 \fBpcre_dfa_exec()\fP.
2075     .P
2076 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2077 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
2078     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2079     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2080     are two kinds:
2081     .
2082     .SS "Verbs that act immediately"
2083     .rs
2084     .sp
2085     The following verbs act as soon as they are encountered:
2086     .sp
2087     (*ACCEPT)
2088     .sp
2089     This verb causes the match to end successfully, skipping the remainder of the
2090 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2091     immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2092 ph10 210 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2093     captured. For example:
2094     .sp
2095     A(A|B(*ACCEPT)|C)D
2096     .sp
2097 ph10 211 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2098 ph10 210 captured.
2099     .sp
2100     (*FAIL) or (*F)
2101     .sp
2102 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2103 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2104     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2105     Perl features that are not present in PCRE. The nearest equivalent is the
2106     callout feature, as for example in this pattern:
2107     .sp
2108     a+(?C)(*FAIL)
2109     .sp
2110 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2111     each backtrack happens (in this example, 10 times).
2112 ph10 210 .
2113     .SS "Verbs that act after backtracking"
2114     .rs
2115     .sp
2116 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2117     with what follows, but if there is no subsequent match, a failure is forced.
2118 ph10 210 The verbs differ in exactly what kind of failure occurs.
2119     .sp
2120     (*COMMIT)
2121     .sp
2122 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2123 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2124     a match by advancing the start point take place. Once (*COMMIT) has been
2125 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2126 ph10 210 starting point, or not at all. For example:
2127     .sp
2128     a+(*COMMIT)b
2129     .sp
2130 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2131 ph10 210 dynamic anchor, or "I've started, so I must finish."
2132     .sp
2133     (*PRUNE)
2134     .sp
2135 ph10 211 This verb causes the match to fail at the current position if the rest of the
2136     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2137 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2138     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2139     if there is no match to the right, backtracking cannot cross (*PRUNE).
2140 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2141 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2142     be expressed in any other way.
2143     .sp
2144     (*SKIP)
2145     .sp
2146 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2147 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2148     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2149     was matched leading up to it cannot be part of a successful match. Consider:
2150     .sp
2151     a+(*SKIP)b
2152     .sp
2153 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2154 ph10 210 the first character in the string), the starting point skips on to start the
2155 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2156 ph10 210 effect in this example; although it would suppress backtracking during the
2157     first match attempt, the second attempt would start at the second character
2158     instead of skipping on to "c".
2159     .sp
2160     (*THEN)
2161 ph10 211 .sp
2162 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2163     not match. That is, it cancels pending backtracking, but only within the
2164     current alternation. Its name comes from the observation that it can be used
2165     for a pattern-based if-then-else block:
2166     .sp
2167     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2168     .sp
2169 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2170 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2171 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2172 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2173     .
2174     .
2175 nigel 93 .SH "SEE ALSO"
2176     .rs
2177     .sp
2178     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2179 ph10 99 .
2180     .
2181     .SH AUTHOR
2182     .rs
2183     .sp
2184     .nf
2185     Philip Hazel
2186     University Computing Service
2187     Cambridge CB2 3QH, England.
2188     .fi
2189     .
2190     .
2191     .SH REVISION
2192     .rs
2193     .sp
2194     .nf
2195 ph10 251 Last updated: 17 September 2007
2196 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
2197     .fi

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