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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 ph10 333 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13     also supports some alternative regular expression syntax (which does not
14     conflict with the Perl syntax) in order to provide some compatibility with
15     regular expressions in Python, .NET, and Oniguruma.
16     .P
17     Perl's regular expressions are described in its own documentation, and
18 ph10 208 regular expressions in general are covered in a number of books, some of which
19     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20     published by O'Reilly, covers regular expressions in great detail. This
21     description of PCRE's regular expressions is intended as reference material.
22 nigel 75 .P
23     The original operation of PCRE was on strings of one-byte characters. However,
24     there is now also support for UTF-8 character strings. To use this, you must
25     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
26     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
27     places below. There is also a summary of UTF-8 features in the
28 nigel 63 .\" HTML <a href="pcre.html#utf8support">
29     .\" </a>
30     section on UTF-8 support
31     .\"
32     in the main
33     .\" HREF
34 nigel 75 \fBpcre\fP
35 nigel 63 .\"
36     page.
37 nigel 75 .P
38 nigel 77 The remainder of this document discusses the patterns that are supported by
39     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
40     From release 6.0, PCRE offers a second matching function,
41     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
42 ph10 172 Perl-compatible. Some of the features discussed below are not available when
43 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
44     alternative function, and how it differs from the normal function, are
45     discussed in the
46 nigel 77 .\" HREF
47     \fBpcrematching\fP
48     .\"
49     page.
50 nigel 93 .
51     .
52 ph10 227 .SH "NEWLINE CONVENTIONS"
53     .rs
54     .sp
55     PCRE supports five different conventions for indicating line breaks in
56     strings: a single CR (carriage return) character, a single LF (linefeed)
57     character, the two-character sequence CRLF, any of the three preceding, or any
58     Unicode newline sequence. The
59     .\" HREF
60     \fBpcreapi\fP
61     .\"
62     page has
63     .\" HTML <a href="pcreapi.html#newlines">
64     .\" </a>
65     further discussion
66     .\"
67     about newlines, and shows how to set the newline convention in the
68     \fIoptions\fP arguments for the compiling and matching functions.
69     .P
70     It is also possible to specify a newline convention by starting a pattern
71     string with one of the following five sequences:
72     .sp
73     (*CR) carriage return
74     (*LF) linefeed
75     (*CRLF) carriage return, followed by linefeed
76     (*ANYCRLF) any of the three above
77     (*ANY) all Unicode newline sequences
78     .sp
79     These override the default and the options given to \fBpcre_compile()\fP. For
80     example, on a Unix system where LF is the default newline sequence, the pattern
81     .sp
82     (*CR)a.b
83     .sp
84     changes the convention to CR. That pattern matches "a\enb" because LF is no
85     longer a newline. Note that these special settings, which are not
86     Perl-compatible, are recognized only at the very start of a pattern, and that
87 ph10 231 they must be in upper case. If more than one of them is present, the last one
88     is used.
89     .P
90     The newline convention does not affect what the \eR escape sequence matches. By
91     default, this is any Unicode newline sequence, for Perl compatibility. However,
92     this can be changed; see the description of \eR in the section entitled
93     .\" HTML <a href="#newlineseq">
94     .\" </a>
95     "Newline sequences"
96     .\"
97 ph10 247 below. A change of \eR setting can be combined with a change of newline
98 ph10 246 convention.
99 ph10 227 .
100     .
101 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
102     .rs
103     .sp
104 nigel 63 A regular expression is a pattern that is matched against a subject string from
105     left to right. Most characters stand for themselves in a pattern, and match the
106     corresponding characters in the subject. As a trivial example, the pattern
107 nigel 75 .sp
108 nigel 63 The quick brown fox
109 nigel 75 .sp
110 nigel 77 matches a portion of a subject string that is identical to itself. When
111     caseless matching is specified (the PCRE_CASELESS option), letters are matched
112     independently of case. In UTF-8 mode, PCRE always understands the concept of
113     case for characters whose values are less than 128, so caseless matching is
114     always possible. For characters with higher values, the concept of case is
115     supported if PCRE is compiled with Unicode property support, but not otherwise.
116     If you want to use caseless matching for characters 128 and above, you must
117     ensure that PCRE is compiled with Unicode property support as well as with
118     UTF-8 support.
119     .P
120     The power of regular expressions comes from the ability to include alternatives
121     and repetitions in the pattern. These are encoded in the pattern by the use of
122 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
123 nigel 63 interpreted in some special way.
124 nigel 75 .P
125     There are two different sets of metacharacters: those that are recognized
126 nigel 63 anywhere in the pattern except within square brackets, and those that are
127 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
128     are as follows:
129 nigel 75 .sp
130     \e general escape character with several uses
131 nigel 63 ^ assert start of string (or line, in multiline mode)
132     $ assert end of string (or line, in multiline mode)
133     . match any character except newline (by default)
134     [ start character class definition
135     | start of alternative branch
136     ( start subpattern
137     ) end subpattern
138     ? extends the meaning of (
139     also 0 or 1 quantifier
140     also quantifier minimizer
141     * 0 or more quantifier
142     + 1 or more quantifier
143     also "possessive quantifier"
144     { start min/max quantifier
145 nigel 75 .sp
146 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
147 nigel 75 a character class the only metacharacters are:
148     .sp
149     \e general escape character
150 nigel 63 ^ negate the class, but only if the first character
151     - indicates character range
152 nigel 75 .\" JOIN
153 nigel 63 [ POSIX character class (only if followed by POSIX
154     syntax)
155     ] terminates the character class
156 nigel 75 .sp
157     The following sections describe the use of each of the metacharacters.
158     .
159 nigel 93 .
160 nigel 63 .SH BACKSLASH
161     .rs
162     .sp
163     The backslash character has several uses. Firstly, if it is followed by a
164 nigel 91 non-alphanumeric character, it takes away any special meaning that character
165     may have. This use of backslash as an escape character applies both inside and
166 nigel 63 outside character classes.
167 nigel 75 .P
168     For example, if you want to match a * character, you write \e* in the pattern.
169 nigel 63 This escaping action applies whether or not the following character would
170 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
171     non-alphanumeric with backslash to specify that it stands for itself. In
172     particular, if you want to match a backslash, you write \e\e.
173     .P
174 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
175     pattern (other than in a character class) and characters between a # outside
176 nigel 91 a character class and the next newline are ignored. An escaping backslash can
177     be used to include a whitespace or # character as part of the pattern.
178 nigel 75 .P
179 nigel 63 If you want to remove the special meaning from a sequence of characters, you
180 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
181     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
182 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
183 nigel 75 .sp
184 nigel 63 Pattern PCRE matches Perl matches
185 nigel 75 .sp
186     .\" JOIN
187     \eQabc$xyz\eE abc$xyz abc followed by the
188 nigel 63 contents of $xyz
189 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
190     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
191     .sp
192     The \eQ...\eE sequence is recognized both inside and outside character classes.
193     .
194     .
195     .\" HTML <a name="digitsafterbackslash"></a>
196     .SS "Non-printing characters"
197     .rs
198     .sp
199 nigel 63 A second use of backslash provides a way of encoding non-printing characters
200     in patterns in a visible manner. There is no restriction on the appearance of
201     non-printing characters, apart from the binary zero that terminates a pattern,
202     but when a pattern is being prepared by text editing, it is usually easier to
203     use one of the following escape sequences than the binary character it
204     represents:
205 nigel 75 .sp
206     \ea alarm, that is, the BEL character (hex 07)
207     \ecx "control-x", where x is any character
208     \ee escape (hex 1B)
209     \ef formfeed (hex 0C)
210 ph10 227 \en linefeed (hex 0A)
211 nigel 75 \er carriage return (hex 0D)
212     \et tab (hex 09)
213     \eddd character with octal code ddd, or backreference
214     \exhh character with hex code hh
215 nigel 87 \ex{hhh..} character with hex code hhh..
216 nigel 75 .sp
217     The precise effect of \ecx is as follows: if x is a lower case letter, it
218 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
219 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
220 nigel 63 7B.
221 nigel 75 .P
222     After \ex, from zero to two hexadecimal digits are read (letters can be in
223 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
224     and }, but the value of the character code must be less than 256 in non-UTF-8
225 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
226     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
227     point, which is 10FFFF.
228 nigel 75 .P
229 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
230     there is no terminating }, this form of escape is not recognized. Instead, the
231     initial \ex will be interpreted as a basic hexadecimal escape, with no
232     following digits, giving a character whose value is zero.
233     .P
234 nigel 63 Characters whose value is less than 256 can be defined by either of the two
235 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
236     example, \exdc is exactly the same as \ex{dc}.
237 nigel 75 .P
238 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
239     digits, just those that are present are used. Thus the sequence \e0\ex\e07
240     specifies two binary zeros followed by a BEL character (code value 7). Make
241     sure you supply two digits after the initial zero if the pattern character that
242     follows is itself an octal digit.
243 nigel 75 .P
244 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
245     Outside a character class, PCRE reads it and any following digits as a decimal
246     number. If the number is less than 10, or if there have been at least that many
247     previous capturing left parentheses in the expression, the entire sequence is
248 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
249     .\" HTML <a href="#backreferences">
250     .\" </a>
251     later,
252     .\"
253     following the discussion of
254     .\" HTML <a href="#subpattern">
255     .\" </a>
256     parenthesized subpatterns.
257     .\"
258     .P
259 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
260     have not been that many capturing subpatterns, PCRE re-reads up to three octal
261 nigel 93 digits following the backslash, and uses them to generate a data character. Any
262 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
263     character specified in octal must be less than \e400. In UTF-8 mode, values up
264     to \e777 are permitted. For example:
265 nigel 75 .sp
266     \e040 is another way of writing a space
267     .\" JOIN
268     \e40 is the same, provided there are fewer than 40
269 nigel 63 previous capturing subpatterns
270 nigel 75 \e7 is always a back reference
271     .\" JOIN
272     \e11 might be a back reference, or another way of
273 nigel 63 writing a tab
274 nigel 75 \e011 is always a tab
275     \e0113 is a tab followed by the character "3"
276     .\" JOIN
277     \e113 might be a back reference, otherwise the
278 nigel 63 character with octal code 113
279 nigel 75 .\" JOIN
280     \e377 might be a back reference, otherwise
281 nigel 63 the byte consisting entirely of 1 bits
282 nigel 75 .\" JOIN
283     \e81 is either a back reference, or a binary zero
284 nigel 63 followed by the two characters "8" and "1"
285 nigel 75 .sp
286 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
287     zero, because no more than three octal digits are ever read.
288 nigel 75 .P
289 nigel 91 All the sequences that define a single character value can be used both inside
290     and outside character classes. In addition, inside a character class, the
291     sequence \eb is interpreted as the backspace character (hex 08), and the
292 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
293     respectively. Outside a character class, these sequences have different
294     meanings
295 nigel 75 .\" HTML <a href="#uniextseq">
296     .\" </a>
297     (see below).
298     .\"
299     .
300     .
301 nigel 93 .SS "Absolute and relative back references"
302     .rs
303     .sp
304 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
305     enclosed in braces, is an absolute or relative back reference. A named back
306     reference can be coded as \eg{name}. Back references are discussed
307 nigel 93 .\" HTML <a href="#backreferences">
308     .\" </a>
309     later,
310     .\"
311     following the discussion of
312     .\" HTML <a href="#subpattern">
313     .\" </a>
314     parenthesized subpatterns.
315     .\"
316     .
317     .
318 ph10 333 .SS "Absolute and relative subroutine calls"
319     .rs
320     .sp
321 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
322     a number enclosed either in angle brackets or single quotes, is an alternative
323     syntax for referencing a subpattern as a "subroutine". Details are discussed
324 ph10 333 .\" HTML <a href="#onigurumasubroutines">
325     .\" </a>
326     later.
327     .\"
328 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
329 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
330     .
331     .
332 nigel 75 .SS "Generic character types"
333     .rs
334     .sp
335 nigel 93 Another use of backslash is for specifying generic character types. The
336 nigel 75 following are always recognized:
337     .sp
338 ph10 182 \ed any decimal digit
339 nigel 75 \eD any character that is not a decimal digit
340 ph10 178 \eh any horizontal whitespace character
341 ph10 182 \eH any character that is not a horizontal whitespace character
342 nigel 75 \es any whitespace character
343     \eS any character that is not a whitespace character
344 ph10 178 \ev any vertical whitespace character
345 ph10 182 \eV any character that is not a vertical whitespace character
346 nigel 75 \ew any "word" character
347     \eW any "non-word" character
348     .sp
349 nigel 63 Each pair of escape sequences partitions the complete set of characters into
350     two disjoint sets. Any given character matches one, and only one, of each pair.
351 nigel 75 .P
352     These character type sequences can appear both inside and outside character
353     classes. They each match one character of the appropriate type. If the current
354     matching point is at the end of the subject string, all of them fail, since
355     there is no character to match.
356     .P
357     For compatibility with Perl, \es does not match the VT character (code 11).
358     This makes it different from the the POSIX "space" class. The \es characters
359 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
360 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
361 ph10 178 does.
362 nigel 75 .P
363 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
364     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
365 ph10 182 character property support is available. These sequences retain their original
366     meanings from before UTF-8 support was available, mainly for efficiency
367 ph10 394 reasons. Note that this also affects \eb, because it is defined in terms of \ew
368     and \eW.
369 ph10 178 .P
370 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
371 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
372     The horizontal space characters are:
373     .sp
374     U+0009 Horizontal tab
375     U+0020 Space
376     U+00A0 Non-break space
377     U+1680 Ogham space mark
378     U+180E Mongolian vowel separator
379     U+2000 En quad
380     U+2001 Em quad
381     U+2002 En space
382     U+2003 Em space
383     U+2004 Three-per-em space
384     U+2005 Four-per-em space
385     U+2006 Six-per-em space
386     U+2007 Figure space
387     U+2008 Punctuation space
388     U+2009 Thin space
389     U+200A Hair space
390     U+202F Narrow no-break space
391     U+205F Medium mathematical space
392     U+3000 Ideographic space
393     .sp
394     The vertical space characters are:
395     .sp
396     U+000A Linefeed
397     U+000B Vertical tab
398     U+000C Formfeed
399     U+000D Carriage return
400     U+0085 Next line
401     U+2028 Line separator
402     U+2029 Paragraph separator
403     .P
404 nigel 75 A "word" character is an underscore or any character less than 256 that is a
405     letter or digit. The definition of letters and digits is controlled by PCRE's
406     low-valued character tables, and may vary if locale-specific matching is taking
407     place (see
408 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
409     .\" </a>
410     "Locale support"
411     .\"
412     in the
413     .\" HREF
414 nigel 75 \fBpcreapi\fP
415 nigel 63 .\"
416 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
417     or "french" in Windows, some character codes greater than 128 are used for
418 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
419     is discouraged.
420 nigel 75 .
421     .
422 ph10 231 .\" HTML <a name="newlineseq"></a>
423 nigel 93 .SS "Newline sequences"
424     .rs
425     .sp
426 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
427     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
428     equivalent to the following:
429 nigel 93 .sp
430     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
431     .sp
432     This is an example of an "atomic group", details of which are given
433     .\" HTML <a href="#atomicgroup">
434     .\" </a>
435     below.
436     .\"
437     This particular group matches either the two-character sequence CR followed by
438     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
439     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
440     line, U+0085). The two-character sequence is treated as a single unit that
441     cannot be split.
442     .P
443     In UTF-8 mode, two additional characters whose codepoints are greater than 255
444     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
445     Unicode character property support is not needed for these characters to be
446     recognized.
447     .P
448 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
449     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
450 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
451 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is
452     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
453     It is also possible to specify these settings by starting a pattern string with
454     one of the following sequences:
455 ph10 231 .sp
456     (*BSR_ANYCRLF) CR, LF, or CRLF only
457     (*BSR_UNICODE) any Unicode newline sequence
458     .sp
459     These override the default and the options given to \fBpcre_compile()\fP, but
460     they can be overridden by options given to \fBpcre_exec()\fP. Note that these
461     special settings, which are not Perl-compatible, are recognized only at the
462     very start of a pattern, and that they must be in upper case. If more than one
463 ph10 247 of them is present, the last one is used. They can be combined with a change of
464 ph10 246 newline convention, for example, a pattern can start with:
465     .sp
466     (*ANY)(*BSR_ANYCRLF)
467     .sp
468 nigel 93 Inside a character class, \eR matches the letter "R".
469     .
470     .
471 nigel 75 .\" HTML <a name="uniextseq"></a>
472     .SS Unicode character properties
473     .rs
474     .sp
475     When PCRE is built with Unicode character property support, three additional
476 ph10 184 escape sequences that match characters with specific properties are available.
477     When not in UTF-8 mode, these sequences are of course limited to testing
478     characters whose codepoints are less than 256, but they do work in this mode.
479     The extra escape sequences are:
480 nigel 75 .sp
481 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
482     \eP{\fIxx\fP} a character without the \fIxx\fP property
483     \eX an extended Unicode sequence
484 nigel 75 .sp
485 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
486     script names, the general category properties, and "Any", which matches any
487     character (including newline). Other properties such as "InMusicalSymbols" are
488     not currently supported by PCRE. Note that \eP{Any} does not match any
489     characters, so always causes a match failure.
490 nigel 75 .P
491 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
492     character from one of these sets can be matched using a script name. For
493     example:
494 nigel 75 .sp
495 nigel 87 \ep{Greek}
496     \eP{Han}
497     .sp
498     Those that are not part of an identified script are lumped together as
499     "Common". The current list of scripts is:
500     .P
501     Arabic,
502     Armenian,
503 nigel 93 Balinese,
504 nigel 87 Bengali,
505     Bopomofo,
506     Braille,
507     Buginese,
508     Buhid,
509     Canadian_Aboriginal,
510     Cherokee,
511     Common,
512     Coptic,
513 nigel 93 Cuneiform,
514 nigel 87 Cypriot,
515     Cyrillic,
516     Deseret,
517     Devanagari,
518     Ethiopic,
519     Georgian,
520     Glagolitic,
521     Gothic,
522     Greek,
523     Gujarati,
524     Gurmukhi,
525     Han,
526     Hangul,
527     Hanunoo,
528     Hebrew,
529     Hiragana,
530     Inherited,
531     Kannada,
532     Katakana,
533     Kharoshthi,
534     Khmer,
535     Lao,
536     Latin,
537     Limbu,
538     Linear_B,
539     Malayalam,
540     Mongolian,
541     Myanmar,
542     New_Tai_Lue,
543 nigel 93 Nko,
544 nigel 87 Ogham,
545     Old_Italic,
546     Old_Persian,
547     Oriya,
548     Osmanya,
549 nigel 93 Phags_Pa,
550     Phoenician,
551 nigel 87 Runic,
552     Shavian,
553     Sinhala,
554     Syloti_Nagri,
555     Syriac,
556     Tagalog,
557     Tagbanwa,
558     Tai_Le,
559     Tamil,
560     Telugu,
561     Thaana,
562     Thai,
563     Tibetan,
564     Tifinagh,
565     Ugaritic,
566     Yi.
567     .P
568     Each character has exactly one general category property, specified by a
569     two-letter abbreviation. For compatibility with Perl, negation can be specified
570     by including a circumflex between the opening brace and the property name. For
571     example, \ep{^Lu} is the same as \eP{Lu}.
572     .P
573     If only one letter is specified with \ep or \eP, it includes all the general
574     category properties that start with that letter. In this case, in the absence
575     of negation, the curly brackets in the escape sequence are optional; these two
576     examples have the same effect:
577     .sp
578 nigel 75 \ep{L}
579     \epL
580     .sp
581 nigel 87 The following general category property codes are supported:
582 nigel 75 .sp
583     C Other
584     Cc Control
585     Cf Format
586     Cn Unassigned
587     Co Private use
588     Cs Surrogate
589     .sp
590     L Letter
591     Ll Lower case letter
592     Lm Modifier letter
593     Lo Other letter
594     Lt Title case letter
595     Lu Upper case letter
596     .sp
597     M Mark
598     Mc Spacing mark
599     Me Enclosing mark
600     Mn Non-spacing mark
601     .sp
602     N Number
603     Nd Decimal number
604     Nl Letter number
605     No Other number
606     .sp
607     P Punctuation
608     Pc Connector punctuation
609     Pd Dash punctuation
610     Pe Close punctuation
611     Pf Final punctuation
612     Pi Initial punctuation
613     Po Other punctuation
614     Ps Open punctuation
615     .sp
616     S Symbol
617     Sc Currency symbol
618     Sk Modifier symbol
619     Sm Mathematical symbol
620     So Other symbol
621     .sp
622     Z Separator
623     Zl Line separator
624     Zp Paragraph separator
625     Zs Space separator
626     .sp
627 nigel 87 The special property L& is also supported: it matches a character that has
628     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
629     a modifier or "other".
630 nigel 75 .P
631 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
632     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
633     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
634     (see the discussion of PCRE_NO_UTF8_CHECK in the
635     .\" HREF
636     \fBpcreapi\fP
637     .\"
638     page).
639     .P
640 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
641 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
642 nigel 87 properties with "Is".
643     .P
644     No character that is in the Unicode table has the Cn (unassigned) property.
645     Instead, this property is assumed for any code point that is not in the
646     Unicode table.
647     .P
648 nigel 75 Specifying caseless matching does not affect these escape sequences. For
649     example, \ep{Lu} always matches only upper case letters.
650     .P
651     The \eX escape matches any number of Unicode characters that form an extended
652     Unicode sequence. \eX is equivalent to
653     .sp
654     (?>\ePM\epM*)
655     .sp
656     That is, it matches a character without the "mark" property, followed by zero
657     or more characters with the "mark" property, and treats the sequence as an
658     atomic group
659     .\" HTML <a href="#atomicgroup">
660     .\" </a>
661     (see below).
662     .\"
663     Characters with the "mark" property are typically accents that affect the
664 ph10 185 preceding character. None of them have codepoints less than 256, so in
665 ph10 184 non-UTF-8 mode \eX matches any one character.
666 nigel 75 .P
667     Matching characters by Unicode property is not fast, because PCRE has to search
668     a structure that contains data for over fifteen thousand characters. That is
669     why the traditional escape sequences such as \ed and \ew do not use Unicode
670     properties in PCRE.
671     .
672     .
673 ph10 168 .\" HTML <a name="resetmatchstart"></a>
674     .SS "Resetting the match start"
675     .rs
676     .sp
677 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
678     matched characters not to be included in the final matched sequence. For
679 ph10 168 example, the pattern:
680     .sp
681     foo\eKbar
682     .sp
683 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
684 ph10 168 similar to a lookbehind assertion
685     .\" HTML <a href="#lookbehind">
686     .\" </a>
687     (described below).
688     .\"
689 ph10 172 However, in this case, the part of the subject before the real match does not
690     have to be of fixed length, as lookbehind assertions do. The use of \eK does
691 ph10 168 not interfere with the setting of
692     .\" HTML <a href="#subpattern">
693     .\" </a>
694     captured substrings.
695 ph10 172 .\"
696 ph10 168 For example, when the pattern
697     .sp
698     (foo)\eKbar
699     .sp
700 ph10 172 matches "foobar", the first substring is still set to "foo".
701 ph10 168 .
702     .
703 nigel 75 .\" HTML <a name="smallassertions"></a>
704     .SS "Simple assertions"
705     .rs
706     .sp
707 nigel 93 The final use of backslash is for certain simple assertions. An assertion
708 nigel 63 specifies a condition that has to be met at a particular point in a match,
709     without consuming any characters from the subject string. The use of
710 nigel 75 subpatterns for more complicated assertions is described
711     .\" HTML <a href="#bigassertions">
712     .\" </a>
713     below.
714     .\"
715 nigel 91 The backslashed assertions are:
716 nigel 75 .sp
717     \eb matches at a word boundary
718     \eB matches when not at a word boundary
719 nigel 93 \eA matches at the start of the subject
720     \eZ matches at the end of the subject
721     also matches before a newline at the end of the subject
722     \ez matches only at the end of the subject
723     \eG matches at the first matching position in the subject
724 nigel 75 .sp
725     These assertions may not appear in character classes (but note that \eb has a
726 nigel 63 different meaning, namely the backspace character, inside a character class).
727 nigel 75 .P
728 nigel 63 A word boundary is a position in the subject string where the current character
729 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
730     \ew and the other matches \eW), or the start or end of the string if the
731     first or last character matches \ew, respectively.
732     .P
733     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
734     dollar (described in the next section) in that they only ever match at the very
735     start and end of the subject string, whatever options are set. Thus, they are
736     independent of multiline mode. These three assertions are not affected by the
737     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
738     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
739     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
740     at a point other than the beginning of the subject, \eA can never match. The
741 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
742     of the string as well as at the very end, whereas \ez matches only at the end.
743 nigel 75 .P
744     The \eG assertion is true only when the current matching position is at the
745     start point of the match, as specified by the \fIstartoffset\fP argument of
746     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
747     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
748 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
749 nigel 75 implementation where \eG can be useful.
750     .P
751     Note, however, that PCRE's interpretation of \eG, as the start of the current
752 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
753     previous match. In Perl, these can be different when the previously matched
754     string was empty. Because PCRE does just one match at a time, it cannot
755     reproduce this behaviour.
756 nigel 75 .P
757     If all the alternatives of a pattern begin with \eG, the expression is anchored
758 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
759     regular expression.
760 nigel 75 .
761     .
762     .SH "CIRCUMFLEX AND DOLLAR"
763 nigel 63 .rs
764     .sp
765     Outside a character class, in the default matching mode, the circumflex
766 nigel 75 character is an assertion that is true only if the current matching point is
767     at the start of the subject string. If the \fIstartoffset\fP argument of
768     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
769 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
770 nigel 75 meaning
771     .\" HTML <a href="#characterclass">
772     .\" </a>
773     (see below).
774     .\"
775     .P
776 nigel 63 Circumflex need not be the first character of the pattern if a number of
777     alternatives are involved, but it should be the first thing in each alternative
778     in which it appears if the pattern is ever to match that branch. If all
779     possible alternatives start with a circumflex, that is, if the pattern is
780     constrained to match only at the start of the subject, it is said to be an
781     "anchored" pattern. (There are also other constructs that can cause a pattern
782     to be anchored.)
783 nigel 75 .P
784     A dollar character is an assertion that is true only if the current matching
785 nigel 63 point is at the end of the subject string, or immediately before a newline
786 nigel 91 at the end of the string (by default). Dollar need not be the last character of
787     the pattern if a number of alternatives are involved, but it should be the last
788     item in any branch in which it appears. Dollar has no special meaning in a
789     character class.
790 nigel 75 .P
791 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
792     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
793 nigel 75 does not affect the \eZ assertion.
794     .P
795 nigel 63 The meanings of the circumflex and dollar characters are changed if the
796 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
797     immediately after internal newlines as well as at the start of the subject
798     string. It does not match after a newline that ends the string. A dollar
799     matches before any newlines in the string, as well as at the very end, when
800     PCRE_MULTILINE is set. When newline is specified as the two-character
801     sequence CRLF, isolated CR and LF characters do not indicate newlines.
802 nigel 75 .P
803 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
804     \en represents a newline) in multiline mode, but not otherwise. Consequently,
805     patterns that are anchored in single line mode because all branches start with
806     ^ are not anchored in multiline mode, and a match for circumflex is possible
807     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
808     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
809     .P
810 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
811 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
812 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
813 nigel 75 .
814     .
815     .SH "FULL STOP (PERIOD, DOT)"
816 nigel 63 .rs
817     .sp
818     Outside a character class, a dot in the pattern matches any one character in
819 nigel 91 the subject string except (by default) a character that signifies the end of a
820 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
821 nigel 91 .P
822 nigel 93 When a line ending is defined as a single character, dot never matches that
823     character; when the two-character sequence CRLF is used, dot does not match CR
824     if it is immediately followed by LF, but otherwise it matches all characters
825     (including isolated CRs and LFs). When any Unicode line endings are being
826     recognized, dot does not match CR or LF or any of the other line ending
827     characters.
828     .P
829 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
830 nigel 93 option is set, a dot matches any one character, without exception. If the
831     two-character sequence CRLF is present in the subject string, it takes two dots
832     to match it.
833 nigel 91 .P
834     The handling of dot is entirely independent of the handling of circumflex and
835     dollar, the only relationship being that they both involve newlines. Dot has no
836     special meaning in a character class.
837 nigel 75 .
838     .
839     .SH "MATCHING A SINGLE BYTE"
840 nigel 63 .rs
841     .sp
842 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
843 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
844     characters. The feature is provided in Perl in order to match individual bytes
845     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
846     what remains in the string may be a malformed UTF-8 string. For this reason,
847     the \eC escape sequence is best avoided.
848 nigel 75 .P
849     PCRE does not allow \eC to appear in lookbehind assertions
850     .\" HTML <a href="#lookbehind">
851     .\" </a>
852     (described below),
853     .\"
854     because in UTF-8 mode this would make it impossible to calculate the length of
855     the lookbehind.
856     .
857     .
858     .\" HTML <a name="characterclass"></a>
859     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
860 nigel 63 .rs
861     .sp
862     An opening square bracket introduces a character class, terminated by a closing
863     square bracket. A closing square bracket on its own is not special. If a
864     closing square bracket is required as a member of the class, it should be the
865     first data character in the class (after an initial circumflex, if present) or
866     escaped with a backslash.
867 nigel 75 .P
868 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
869     character may occupy more than one byte. A matched character must be in the set
870     of characters defined by the class, unless the first character in the class
871     definition is a circumflex, in which case the subject character must not be in
872     the set defined by the class. If a circumflex is actually required as a member
873     of the class, ensure it is not the first character, or escape it with a
874     backslash.
875 nigel 75 .P
876 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
877     [^aeiou] matches any character that is not a lower case vowel. Note that a
878 nigel 75 circumflex is just a convenient notation for specifying the characters that
879     are in the class by enumerating those that are not. A class that starts with a
880     circumflex is not an assertion: it still consumes a character from the subject
881     string, and therefore it fails if the current pointer is at the end of the
882     string.
883     .P
884 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
885 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
886     .P
887 nigel 63 When caseless matching is set, any letters in a class represent both their
888     upper case and lower case versions, so for example, a caseless [aeiou] matches
889     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
890 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
891     case for characters whose values are less than 128, so caseless matching is
892     always possible. For characters with higher values, the concept of case is
893     supported if PCRE is compiled with Unicode property support, but not otherwise.
894     If you want to use caseless matching for characters 128 and above, you must
895     ensure that PCRE is compiled with Unicode property support as well as with
896     UTF-8 support.
897 nigel 75 .P
898 nigel 93 Characters that might indicate line breaks are never treated in any special way
899     when matching character classes, whatever line-ending sequence is in use, and
900     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
901     such as [^a] always matches one of these characters.
902 nigel 75 .P
903 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
904     character class. For example, [d-m] matches any letter between d and m,
905     inclusive. If a minus character is required in a class, it must be escaped with
906     a backslash or appear in a position where it cannot be interpreted as
907     indicating a range, typically as the first or last character in the class.
908 nigel 75 .P
909 nigel 63 It is not possible to have the literal character "]" as the end character of a
910     range. A pattern such as [W-]46] is interpreted as a class of two characters
911     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
912     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
913 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
914     followed by two other characters. The octal or hexadecimal representation of
915     "]" can also be used to end a range.
916     .P
917 nigel 63 Ranges operate in the collating sequence of character values. They can also be
918 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
919 nigel 63 mode, ranges can include characters whose values are greater than 255, for
920 nigel 75 example [\ex{100}-\ex{2ff}].
921     .P
922 nigel 63 If a range that includes letters is used when caseless matching is set, it
923     matches the letters in either case. For example, [W-c] is equivalent to
924 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
925 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
926 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
927     characters with values greater than 128 only when it is compiled with Unicode
928     property support.
929     .P
930     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
931     in a character class, and add the characters that they match to the class. For
932     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
933 nigel 63 conveniently be used with the upper case character types to specify a more
934     restricted set of characters than the matching lower case type. For example,
935 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
936     .P
937     The only metacharacters that are recognized in character classes are backslash,
938     hyphen (only where it can be interpreted as specifying a range), circumflex
939     (only at the start), opening square bracket (only when it can be interpreted as
940     introducing a POSIX class name - see the next section), and the terminating
941     closing square bracket. However, escaping other non-alphanumeric characters
942     does no harm.
943     .
944     .
945     .SH "POSIX CHARACTER CLASSES"
946 nigel 63 .rs
947     .sp
948 nigel 75 Perl supports the POSIX notation for character classes. This uses names
949 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
950     this notation. For example,
951 nigel 75 .sp
952 nigel 63 [01[:alpha:]%]
953 nigel 75 .sp
954 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
955     are
956 nigel 75 .sp
957 nigel 63 alnum letters and digits
958     alpha letters
959     ascii character codes 0 - 127
960     blank space or tab only
961     cntrl control characters
962 nigel 75 digit decimal digits (same as \ed)
963 nigel 63 graph printing characters, excluding space
964     lower lower case letters
965     print printing characters, including space
966     punct printing characters, excluding letters and digits
967 nigel 75 space white space (not quite the same as \es)
968 nigel 63 upper upper case letters
969 nigel 75 word "word" characters (same as \ew)
970 nigel 63 xdigit hexadecimal digits
971 nigel 75 .sp
972 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
973     space (32). Notice that this list includes the VT character (code 11). This
974 nigel 75 makes "space" different to \es, which does not include VT (for Perl
975 nigel 63 compatibility).
976 nigel 75 .P
977 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
978     5.8. Another Perl extension is negation, which is indicated by a ^ character
979     after the colon. For example,
980 nigel 75 .sp
981 nigel 63 [12[:^digit:]]
982 nigel 75 .sp
983 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
984     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
985     supported, and an error is given if they are encountered.
986 nigel 75 .P
987     In UTF-8 mode, characters with values greater than 128 do not match any of
988 nigel 63 the POSIX character classes.
989 nigel 75 .
990     .
991     .SH "VERTICAL BAR"
992 nigel 63 .rs
993     .sp
994     Vertical bar characters are used to separate alternative patterns. For example,
995     the pattern
996 nigel 75 .sp
997 nigel 63 gilbert|sullivan
998 nigel 75 .sp
999 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1000 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1001     process tries each alternative in turn, from left to right, and the first one
1002     that succeeds is used. If the alternatives are within a subpattern
1003 nigel 75 .\" HTML <a href="#subpattern">
1004     .\" </a>
1005     (defined below),
1006     .\"
1007     "succeeds" means matching the rest of the main pattern as well as the
1008     alternative in the subpattern.
1009     .
1010     .
1011     .SH "INTERNAL OPTION SETTING"
1012 nigel 63 .rs
1013     .sp
1014     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1015 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1016     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1017     The option letters are
1018 nigel 75 .sp
1019 nigel 63 i for PCRE_CASELESS
1020     m for PCRE_MULTILINE
1021     s for PCRE_DOTALL
1022     x for PCRE_EXTENDED
1023 nigel 75 .sp
1024 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1025     unset these options by preceding the letter with a hyphen, and a combined
1026     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1027     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1028     permitted. If a letter appears both before and after the hyphen, the option is
1029     unset.
1030 nigel 75 .P
1031 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1032     changed in the same way as the Perl-compatible options by using the characters
1033     J, U and X respectively.
1034     .P
1035 nigel 63 When an option change occurs at top level (that is, not inside subpattern
1036     parentheses), the change applies to the remainder of the pattern that follows.
1037     If the change is placed right at the start of a pattern, PCRE extracts it into
1038     the global options (and it will therefore show up in data extracted by the
1039 nigel 75 \fBpcre_fullinfo()\fP function).
1040     .P
1041 nigel 93 An option change within a subpattern (see below for a description of
1042     subpatterns) affects only that part of the current pattern that follows it, so
1043 nigel 75 .sp
1044 nigel 63 (a(?i)b)c
1045 nigel 75 .sp
1046 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1047     By this means, options can be made to have different settings in different
1048     parts of the pattern. Any changes made in one alternative do carry on
1049     into subsequent branches within the same subpattern. For example,
1050 nigel 75 .sp
1051 nigel 63 (a(?i)b|c)
1052 nigel 75 .sp
1053 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1054     branch is abandoned before the option setting. This is because the effects of
1055     option settings happen at compile time. There would be some very weird
1056     behaviour otherwise.
1057 ph10 251 .P
1058     \fBNote:\fP There are other PCRE-specific options that can be set by the
1059     application when the compile or match functions are called. In some cases the
1060     pattern can contain special leading sequences to override what the application
1061     has set or what has been defaulted. Details are given in the section entitled
1062     .\" HTML <a href="#newlineseq">
1063     .\" </a>
1064     "Newline sequences"
1065     .\"
1066 ph10 259 above.
1067 nigel 75 .
1068     .
1069     .\" HTML <a name="subpattern"></a>
1070 nigel 63 .SH SUBPATTERNS
1071     .rs
1072     .sp
1073     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1074 nigel 75 Turning part of a pattern into a subpattern does two things:
1075     .sp
1076 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1077 nigel 75 .sp
1078 nigel 63 cat(aract|erpillar|)
1079 nigel 75 .sp
1080 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1081 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1082 nigel 75 .sp
1083     2. It sets up the subpattern as a capturing subpattern. This means that, when
1084     the whole pattern matches, that portion of the subject string that matched the
1085     subpattern is passed back to the caller via the \fIovector\fP argument of
1086     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1087     from 1) to obtain numbers for the capturing subpatterns.
1088     .P
1089 nigel 63 For example, if the string "the red king" is matched against the pattern
1090 nigel 75 .sp
1091 nigel 63 the ((red|white) (king|queen))
1092 nigel 75 .sp
1093 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1094     2, and 3, respectively.
1095 nigel 75 .P
1096 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1097     There are often times when a grouping subpattern is required without a
1098     capturing requirement. If an opening parenthesis is followed by a question mark
1099     and a colon, the subpattern does not do any capturing, and is not counted when
1100     computing the number of any subsequent capturing subpatterns. For example, if
1101     the string "the white queen" is matched against the pattern
1102 nigel 75 .sp
1103 nigel 63 the ((?:red|white) (king|queen))
1104 nigel 75 .sp
1105 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1106 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1107 nigel 75 .P
1108 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1109     a non-capturing subpattern, the option letters may appear between the "?" and
1110     the ":". Thus the two patterns
1111 nigel 75 .sp
1112 nigel 63 (?i:saturday|sunday)
1113     (?:(?i)saturday|sunday)
1114 nigel 75 .sp
1115 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1116     from left to right, and options are not reset until the end of the subpattern
1117     is reached, an option setting in one branch does affect subsequent branches, so
1118     the above patterns match "SUNDAY" as well as "Saturday".
1119 nigel 75 .
1120     .
1121 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1122     .rs
1123     .sp
1124 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1125     the same numbers for its capturing parentheses. Such a subpattern starts with
1126     (?| and is itself a non-capturing subpattern. For example, consider this
1127 ph10 175 pattern:
1128     .sp
1129     (?|(Sat)ur|(Sun))day
1130 ph10 182 .sp
1131     Because the two alternatives are inside a (?| group, both sets of capturing
1132     parentheses are numbered one. Thus, when the pattern matches, you can look
1133     at captured substring number one, whichever alternative matched. This construct
1134     is useful when you want to capture part, but not all, of one of a number of
1135     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1136 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1137 ph10 182 buffers that follow the subpattern start after the highest number used in any
1138     branch. The following example is taken from the Perl documentation.
1139 ph10 175 The numbers underneath show in which buffer the captured content will be
1140     stored.
1141     .sp
1142     # before ---------------branch-reset----------- after
1143     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1144     # 1 2 2 3 2 3 4
1145 ph10 182 .sp
1146 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1147     the first one in the pattern with the given number.
1148     .P
1149     An alternative approach to using this "branch reset" feature is to use
1150     duplicate named subpatterns, as described in the next section.
1151     .
1152     .
1153 nigel 75 .SH "NAMED SUBPATTERNS"
1154 nigel 63 .rs
1155     .sp
1156     Identifying capturing parentheses by number is simple, but it can be very hard
1157     to keep track of the numbers in complicated regular expressions. Furthermore,
1158 nigel 75 if an expression is modified, the numbers may change. To help with this
1159 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1160     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1161     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1162     the Perl and the Python syntax.
1163     .P
1164     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1165     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1166 nigel 91 parentheses from other parts of the pattern, such as
1167     .\" HTML <a href="#backreferences">
1168     .\" </a>
1169     backreferences,
1170     .\"
1171     .\" HTML <a href="#recursion">
1172     .\" </a>
1173     recursion,
1174     .\"
1175     and
1176     .\" HTML <a href="#conditions">
1177     .\" </a>
1178     conditions,
1179     .\"
1180     can be made by name as well as by number.
1181 nigel 75 .P
1182 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1183 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1184     if the names were not present. The PCRE API provides function calls for
1185     extracting the name-to-number translation table from a compiled pattern. There
1186     is also a convenience function for extracting a captured substring by name.
1187 nigel 91 .P
1188     By default, a name must be unique within a pattern, but it is possible to relax
1189     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1190     be useful for patterns where only one instance of the named parentheses can
1191     match. Suppose you want to match the name of a weekday, either as a 3-letter
1192     abbreviation or as the full name, and in both cases you want to extract the
1193     abbreviation. This pattern (ignoring the line breaks) does the job:
1194     .sp
1195 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1196     (?<DN>Tue)(?:sday)?|
1197     (?<DN>Wed)(?:nesday)?|
1198     (?<DN>Thu)(?:rsday)?|
1199     (?<DN>Sat)(?:urday)?
1200 nigel 91 .sp
1201     There are five capturing substrings, but only one is ever set after a match.
1202 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1203 ph10 175 subpattern, as described in the previous section.)
1204     .P
1205 nigel 91 The convenience function for extracting the data by name returns the substring
1206 nigel 93 for the first (and in this example, the only) subpattern of that name that
1207 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1208     make a reference to a non-unique named subpattern from elsewhere in the
1209     pattern, the one that corresponds to the lowest number is used. For further
1210     details of the interfaces for handling named subpatterns, see the
1211 nigel 63 .\" HREF
1212 nigel 75 \fBpcreapi\fP
1213 nigel 63 .\"
1214     documentation.
1215 ph10 385 .P
1216     \fBWarning:\fP You cannot use different names to distinguish between two
1217     subpatterns with the same number (see the previous section) because PCRE uses
1218     only the numbers when matching.
1219 nigel 75 .
1220     .
1221 nigel 63 .SH REPETITION
1222     .rs
1223     .sp
1224     Repetition is specified by quantifiers, which can follow any of the following
1225     items:
1226 nigel 75 .sp
1227 nigel 63 a literal data character
1228 nigel 93 the dot metacharacter
1229 nigel 75 the \eC escape sequence
1230     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1231 nigel 93 the \eR escape sequence
1232 nigel 75 an escape such as \ed that matches a single character
1233 nigel 63 a character class
1234     a back reference (see next section)
1235     a parenthesized subpattern (unless it is an assertion)
1236 nigel 75 .sp
1237 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1238     permitted matches, by giving the two numbers in curly brackets (braces),
1239     separated by a comma. The numbers must be less than 65536, and the first must
1240     be less than or equal to the second. For example:
1241 nigel 75 .sp
1242 nigel 63 z{2,4}
1243 nigel 75 .sp
1244 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1245     character. If the second number is omitted, but the comma is present, there is
1246     no upper limit; if the second number and the comma are both omitted, the
1247     quantifier specifies an exact number of required matches. Thus
1248 nigel 75 .sp
1249 nigel 63 [aeiou]{3,}
1250 nigel 75 .sp
1251 nigel 63 matches at least 3 successive vowels, but may match many more, while
1252 nigel 75 .sp
1253     \ed{8}
1254     .sp
1255 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1256     where a quantifier is not allowed, or one that does not match the syntax of a
1257     quantifier, is taken as a literal character. For example, {,6} is not a
1258     quantifier, but a literal string of four characters.
1259 nigel 75 .P
1260 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1261 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1262     which is represented by a two-byte sequence. Similarly, when Unicode property
1263     support is available, \eX{3} matches three Unicode extended sequences, each of
1264     which may be several bytes long (and they may be of different lengths).
1265     .P
1266 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1267 ph10 345 previous item and the quantifier were not present. This may be useful for
1268     subpatterns that are referenced as
1269 ph10 335 .\" HTML <a href="#subpatternsassubroutines">
1270     .\" </a>
1271     subroutines
1272     .\"
1273 ph10 345 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1274 ph10 335 quantifier are omitted from the compiled pattern.
1275 nigel 75 .P
1276 nigel 93 For convenience, the three most common quantifiers have single-character
1277     abbreviations:
1278 nigel 75 .sp
1279 nigel 63 * is equivalent to {0,}
1280     + is equivalent to {1,}
1281     ? is equivalent to {0,1}
1282 nigel 75 .sp
1283 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1284     match no characters with a quantifier that has no upper limit, for example:
1285 nigel 75 .sp
1286 nigel 63 (a?)*
1287 nigel 75 .sp
1288 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1289     such patterns. However, because there are cases where this can be useful, such
1290     patterns are now accepted, but if any repetition of the subpattern does in fact
1291     match no characters, the loop is forcibly broken.
1292 nigel 75 .P
1293 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1294     possible (up to the maximum number of permitted times), without causing the
1295     rest of the pattern to fail. The classic example of where this gives problems
1296 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1297     and within the comment, individual * and / characters may appear. An attempt to
1298     match C comments by applying the pattern
1299     .sp
1300     /\e*.*\e*/
1301     .sp
1302 nigel 63 to the string
1303 nigel 75 .sp
1304     /* first comment */ not comment /* second comment */
1305     .sp
1306 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1307     item.
1308 nigel 75 .P
1309 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1310     greedy, and instead matches the minimum number of times possible, so the
1311     pattern
1312 nigel 75 .sp
1313     /\e*.*?\e*/
1314     .sp
1315 nigel 63 does the right thing with the C comments. The meaning of the various
1316     quantifiers is not otherwise changed, just the preferred number of matches.
1317     Do not confuse this use of question mark with its use as a quantifier in its
1318     own right. Because it has two uses, it can sometimes appear doubled, as in
1319 nigel 75 .sp
1320     \ed??\ed
1321     .sp
1322 nigel 63 which matches one digit by preference, but can match two if that is the only
1323     way the rest of the pattern matches.
1324 nigel 75 .P
1325 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1326 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1327     greedy by following them with a question mark. In other words, it inverts the
1328     default behaviour.
1329 nigel 75 .P
1330 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1331 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1332 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1333 nigel 75 .P
1334 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1335 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1336 nigel 63 implicitly anchored, because whatever follows will be tried against every
1337     character position in the subject string, so there is no point in retrying the
1338     overall match at any position after the first. PCRE normally treats such a
1339 nigel 75 pattern as though it were preceded by \eA.
1340     .P
1341 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1342     worth setting PCRE_DOTALL in order to obtain this optimization, or
1343     alternatively using ^ to indicate anchoring explicitly.
1344 nigel 75 .P
1345 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1346     is inside capturing parentheses that are the subject of a backreference
1347 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1348     succeeds. Consider, for example:
1349 nigel 75 .sp
1350     (.*)abc\e1
1351     .sp
1352 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1353     this reason, such a pattern is not implicitly anchored.
1354 nigel 75 .P
1355 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1356     that matched the final iteration. For example, after
1357 nigel 75 .sp
1358     (tweedle[dume]{3}\es*)+
1359     .sp
1360 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1361     "tweedledee". However, if there are nested capturing subpatterns, the
1362     corresponding captured values may have been set in previous iterations. For
1363     example, after
1364 nigel 75 .sp
1365 nigel 63 /(a|(b))+/
1366 nigel 75 .sp
1367 nigel 63 matches "aba" the value of the second captured substring is "b".
1368 nigel 75 .
1369     .
1370     .\" HTML <a name="atomicgroup"></a>
1371     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1372 nigel 63 .rs
1373     .sp
1374 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1375     repetition, failure of what follows normally causes the repeated item to be
1376     re-evaluated to see if a different number of repeats allows the rest of the
1377     pattern to match. Sometimes it is useful to prevent this, either to change the
1378     nature of the match, or to cause it fail earlier than it otherwise might, when
1379     the author of the pattern knows there is no point in carrying on.
1380 nigel 75 .P
1381     Consider, for example, the pattern \ed+foo when applied to the subject line
1382     .sp
1383 nigel 63 123456bar
1384 nigel 75 .sp
1385 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1386 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1387 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1388     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1389     that once a subpattern has matched, it is not to be re-evaluated in this way.
1390 nigel 75 .P
1391 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1392 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1393     special parenthesis, starting with (?> as in this example:
1394 nigel 75 .sp
1395     (?>\ed+)foo
1396     .sp
1397 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1398     it has matched, and a failure further into the pattern is prevented from
1399     backtracking into it. Backtracking past it to previous items, however, works as
1400     normal.
1401 nigel 75 .P
1402 nigel 63 An alternative description is that a subpattern of this type matches the string
1403     of characters that an identical standalone pattern would match, if anchored at
1404     the current point in the subject string.
1405 nigel 75 .P
1406 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1407     the above example can be thought of as a maximizing repeat that must swallow
1408 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1409 nigel 63 number of digits they match in order to make the rest of the pattern match,
1410 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1411     .P
1412 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1413     subpatterns, and can be nested. However, when the subpattern for an atomic
1414     group is just a single repeated item, as in the example above, a simpler
1415     notation, called a "possessive quantifier" can be used. This consists of an
1416     additional + character following a quantifier. Using this notation, the
1417     previous example can be rewritten as
1418 nigel 75 .sp
1419     \ed++foo
1420     .sp
1421 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1422     example:
1423     .sp
1424     (abc|xyz){2,3}+
1425     .sp
1426 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1427     option is ignored. They are a convenient notation for the simpler forms of
1428 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1429     quantifier and the equivalent atomic group, though there may be a performance
1430     difference; possessive quantifiers should be slightly faster.
1431 nigel 75 .P
1432 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1433     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1434     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1435     package, and PCRE copied it from there. It ultimately found its way into Perl
1436     at release 5.10.
1437 nigel 75 .P
1438 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1439     pattern constructs. For example, the sequence A+B is treated as A++B because
1440     there is no point in backtracking into a sequence of A's when B must follow.
1441     .P
1442 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1443     be repeated an unlimited number of times, the use of an atomic group is the
1444     only way to avoid some failing matches taking a very long time indeed. The
1445     pattern
1446 nigel 75 .sp
1447     (\eD+|<\ed+>)*[!?]
1448     .sp
1449 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1450     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1451     quickly. However, if it is applied to
1452 nigel 75 .sp
1453 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1454 nigel 75 .sp
1455 nigel 63 it takes a long time before reporting failure. This is because the string can
1456 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1457     large number of ways, and all have to be tried. (The example uses [!?] rather
1458     than a single character at the end, because both PCRE and Perl have an
1459     optimization that allows for fast failure when a single character is used. They
1460     remember the last single character that is required for a match, and fail early
1461     if it is not present in the string.) If the pattern is changed so that it uses
1462     an atomic group, like this:
1463     .sp
1464     ((?>\eD+)|<\ed+>)*[!?]
1465     .sp
1466 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1467 nigel 75 .
1468     .
1469     .\" HTML <a name="backreferences"></a>
1470     .SH "BACK REFERENCES"
1471 nigel 63 .rs
1472     .sp
1473     Outside a character class, a backslash followed by a digit greater than 0 (and
1474     possibly further digits) is a back reference to a capturing subpattern earlier
1475     (that is, to its left) in the pattern, provided there have been that many
1476     previous capturing left parentheses.
1477 nigel 75 .P
1478 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1479     always taken as a back reference, and causes an error only if there are not
1480     that many capturing left parentheses in the entire pattern. In other words, the
1481     parentheses that are referenced need not be to the left of the reference for
1482 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1483     when a repetition is involved and the subpattern to the right has participated
1484     in an earlier iteration.
1485     .P
1486 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1487     whose number is 10 or more using this syntax because a sequence such as \e50 is
1488     interpreted as a character defined in octal. See the subsection entitled
1489 nigel 91 "Non-printing characters"
1490 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1491     .\" </a>
1492     above
1493     .\"
1494 nigel 93 for further details of the handling of digits following a backslash. There is
1495     no such problem when named parentheses are used. A back reference to any
1496     subpattern is possible using named parentheses (see below).
1497 nigel 75 .P
1498 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1499     backslash is to use the \eg escape sequence, which is a feature introduced in
1500 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1501     number, optionally enclosed in braces. These examples are all identical:
1502 nigel 93 .sp
1503     (ring), \e1
1504     (ring), \eg1
1505     (ring), \eg{1}
1506     .sp
1507 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1508     is present in the older syntax. It is also useful when literal digits follow
1509     the reference. A negative number is a relative reference. Consider this
1510     example:
1511 nigel 93 .sp
1512     (abc(def)ghi)\eg{-1}
1513     .sp
1514     The sequence \eg{-1} is a reference to the most recently started capturing
1515     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1516     would be equivalent to \e1. The use of relative references can be helpful in
1517     long patterns, and also in patterns that are created by joining together
1518     fragments that contain references within themselves.
1519     .P
1520 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1521     the current subject string, rather than anything matching the subpattern
1522     itself (see
1523     .\" HTML <a href="#subpatternsassubroutines">
1524     .\" </a>
1525     "Subpatterns as subroutines"
1526     .\"
1527     below for a way of doing that). So the pattern
1528 nigel 75 .sp
1529     (sens|respons)e and \e1ibility
1530     .sp
1531 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1532     "sense and responsibility". If caseful matching is in force at the time of the
1533     back reference, the case of letters is relevant. For example,
1534 nigel 75 .sp
1535     ((?i)rah)\es+\e1
1536     .sp
1537 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1538     capturing subpattern is matched caselessly.
1539 nigel 75 .P
1540 ph10 171 There are several different ways of writing back references to named
1541     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1542     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1543     back reference syntax, in which \eg can be used for both numeric and named
1544     references, is also supported. We could rewrite the above example in any of
1545 nigel 93 the following ways:
1546 nigel 75 .sp
1547 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1548 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1549 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1550 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1551 nigel 75 .sp
1552 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1553     after the reference.
1554     .P
1555 nigel 63 There may be more than one back reference to the same subpattern. If a
1556     subpattern has not actually been used in a particular match, any back
1557     references to it always fail. For example, the pattern
1558 nigel 75 .sp
1559     (a|(bc))\e2
1560     .sp
1561 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1562     many capturing parentheses in a pattern, all digits following the backslash are
1563     taken as part of a potential back reference number. If the pattern continues
1564     with a digit character, some delimiter must be used to terminate the back
1565     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1566 nigel 75 Otherwise an empty comment (see
1567     .\" HTML <a href="#comments">
1568     .\" </a>
1569     "Comments"
1570     .\"
1571     below) can be used.
1572     .P
1573 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1574 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1575 nigel 63 However, such references can be useful inside repeated subpatterns. For
1576     example, the pattern
1577 nigel 75 .sp
1578     (a|b\e1)+
1579     .sp
1580 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1581     the subpattern, the back reference matches the character string corresponding
1582     to the previous iteration. In order for this to work, the pattern must be such
1583     that the first iteration does not need to match the back reference. This can be
1584     done using alternation, as in the example above, or by a quantifier with a
1585     minimum of zero.
1586 nigel 75 .
1587     .
1588     .\" HTML <a name="bigassertions"></a>
1589 nigel 63 .SH ASSERTIONS
1590     .rs
1591     .sp
1592     An assertion is a test on the characters following or preceding the current
1593     matching point that does not actually consume any characters. The simple
1594 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1595     .\" HTML <a href="#smallassertions">
1596     .\" </a>
1597     above.
1598     .\"
1599     .P
1600 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1601     those that look ahead of the current position in the subject string, and those
1602 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1603     except that it does not cause the current matching position to be changed.
1604     .P
1605     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1606     because it makes no sense to assert the same thing several times. If any kind
1607     of assertion contains capturing subpatterns within it, these are counted for
1608     the purposes of numbering the capturing subpatterns in the whole pattern.
1609     However, substring capturing is carried out only for positive assertions,
1610     because it does not make sense for negative assertions.
1611     .
1612     .
1613     .SS "Lookahead assertions"
1614     .rs
1615     .sp
1616 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1617     negative assertions. For example,
1618 nigel 75 .sp
1619     \ew+(?=;)
1620     .sp
1621 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1622     the match, and
1623 nigel 75 .sp
1624 nigel 63 foo(?!bar)
1625 nigel 75 .sp
1626 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1627     apparently similar pattern
1628 nigel 75 .sp
1629 nigel 63 (?!foo)bar
1630 nigel 75 .sp
1631 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1632     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1633     (?!foo) is always true when the next three characters are "bar". A
1634 nigel 75 lookbehind assertion is needed to achieve the other effect.
1635     .P
1636 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1637     convenient way to do it is with (?!) because an empty string always matches, so
1638     an assertion that requires there not to be an empty string must always fail.
1639 nigel 75 .
1640     .
1641     .\" HTML <a name="lookbehind"></a>
1642     .SS "Lookbehind assertions"
1643     .rs
1644     .sp
1645 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1646     negative assertions. For example,
1647 nigel 75 .sp
1648 nigel 63 (?<!foo)bar
1649 nigel 75 .sp
1650 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1651     a lookbehind assertion are restricted such that all the strings it matches must
1652 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1653     do not all have to have the same fixed length. Thus
1654 nigel 75 .sp
1655 nigel 63 (?<=bullock|donkey)
1656 nigel 75 .sp
1657 nigel 63 is permitted, but
1658 nigel 75 .sp
1659 nigel 63 (?<!dogs?|cats?)
1660 nigel 75 .sp
1661 nigel 63 causes an error at compile time. Branches that match different length strings
1662     are permitted only at the top level of a lookbehind assertion. This is an
1663     extension compared with Perl (at least for 5.8), which requires all branches to
1664     match the same length of string. An assertion such as
1665 nigel 75 .sp
1666 nigel 63 (?<=ab(c|de))
1667 nigel 75 .sp
1668 nigel 63 is not permitted, because its single top-level branch can match two different
1669     lengths, but it is acceptable if rewritten to use two top-level branches:
1670 nigel 75 .sp
1671 nigel 63 (?<=abc|abde)
1672 nigel 75 .sp
1673 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1674 ph10 168 .\" HTML <a href="#resetmatchstart">
1675     .\" </a>
1676     (see above)
1677     .\"
1678     can be used instead of a lookbehind assertion; this is not restricted to a
1679     fixed-length.
1680     .P
1681 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1682 nigel 93 temporarily move the current position back by the fixed length and then try to
1683 nigel 63 match. If there are insufficient characters before the current position, the
1684 nigel 93 assertion fails.
1685 nigel 75 .P
1686     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1687 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1688 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1689     different numbers of bytes, are also not permitted.
1690 nigel 75 .P
1691 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1692     specify efficient matching at the end of the subject string. Consider a simple
1693     pattern such as
1694 nigel 75 .sp
1695 nigel 63 abcd$
1696 nigel 75 .sp
1697 nigel 63 when applied to a long string that does not match. Because matching proceeds
1698     from left to right, PCRE will look for each "a" in the subject and then see if
1699     what follows matches the rest of the pattern. If the pattern is specified as
1700 nigel 75 .sp
1701 nigel 63 ^.*abcd$
1702 nigel 75 .sp
1703 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1704     there is no following "a"), it backtracks to match all but the last character,
1705     then all but the last two characters, and so on. Once again the search for "a"
1706     covers the entire string, from right to left, so we are no better off. However,
1707     if the pattern is written as
1708 nigel 75 .sp
1709 nigel 63 ^.*+(?<=abcd)
1710 nigel 75 .sp
1711 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1712 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1713     characters. If it fails, the match fails immediately. For long strings, this
1714     approach makes a significant difference to the processing time.
1715 nigel 75 .
1716     .
1717     .SS "Using multiple assertions"
1718     .rs
1719     .sp
1720 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1721 nigel 75 .sp
1722     (?<=\ed{3})(?<!999)foo
1723     .sp
1724 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1725     the assertions is applied independently at the same point in the subject
1726     string. First there is a check that the previous three characters are all
1727     digits, and then there is a check that the same three characters are not "999".
1728 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1729 nigel 63 of which are digits and the last three of which are not "999". For example, it
1730     doesn't match "123abcfoo". A pattern to do that is
1731 nigel 75 .sp
1732     (?<=\ed{3}...)(?<!999)foo
1733     .sp
1734 nigel 63 This time the first assertion looks at the preceding six characters, checking
1735     that the first three are digits, and then the second assertion checks that the
1736     preceding three characters are not "999".
1737 nigel 75 .P
1738 nigel 63 Assertions can be nested in any combination. For example,
1739 nigel 75 .sp
1740 nigel 63 (?<=(?<!foo)bar)baz
1741 nigel 75 .sp
1742 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1743     preceded by "foo", while
1744 nigel 75 .sp
1745     (?<=\ed{3}(?!999)...)foo
1746     .sp
1747     is another pattern that matches "foo" preceded by three digits and any three
1748 nigel 63 characters that are not "999".
1749 nigel 75 .
1750     .
1751 nigel 91 .\" HTML <a name="conditions"></a>
1752 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1753 nigel 63 .rs
1754     .sp
1755     It is possible to cause the matching process to obey a subpattern
1756     conditionally or to choose between two alternative subpatterns, depending on
1757     the result of an assertion, or whether a previous capturing subpattern matched
1758     or not. The two possible forms of conditional subpattern are
1759 nigel 75 .sp
1760 nigel 63 (?(condition)yes-pattern)
1761     (?(condition)yes-pattern|no-pattern)
1762 nigel 75 .sp
1763 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1764     no-pattern (if present) is used. If there are more than two alternatives in the
1765     subpattern, a compile-time error occurs.
1766 nigel 75 .P
1767 nigel 93 There are four kinds of condition: references to subpatterns, references to
1768     recursion, a pseudo-condition called DEFINE, and assertions.
1769     .
1770     .SS "Checking for a used subpattern by number"
1771     .rs
1772     .sp
1773     If the text between the parentheses consists of a sequence of digits, the
1774     condition is true if the capturing subpattern of that number has previously
1775 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1776 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1777 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1778 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1779     refer to subsequent groups with constructs such as (?(+2).
1780 nigel 91 .P
1781     Consider the following pattern, which contains non-significant white space to
1782     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1783     three parts for ease of discussion:
1784 nigel 75 .sp
1785     ( \e( )? [^()]+ (?(1) \e) )
1786     .sp
1787 nigel 63 The first part matches an optional opening parenthesis, and if that
1788     character is present, sets it as the first captured substring. The second part
1789     matches one or more characters that are not parentheses. The third part is a
1790     conditional subpattern that tests whether the first set of parentheses matched
1791     or not. If they did, that is, if subject started with an opening parenthesis,
1792     the condition is true, and so the yes-pattern is executed and a closing
1793     parenthesis is required. Otherwise, since no-pattern is not present, the
1794     subpattern matches nothing. In other words, this pattern matches a sequence of
1795 nigel 93 non-parentheses, optionally enclosed in parentheses.
1796 ph10 167 .P
1797 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1798 ph10 167 reference:
1799     .sp
1800     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1801     .sp
1802     This makes the fragment independent of the parentheses in the larger pattern.
1803 nigel 93 .
1804     .SS "Checking for a used subpattern by name"
1805     .rs
1806 nigel 91 .sp
1807 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1808     subpattern by name. For compatibility with earlier versions of PCRE, which had
1809     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1810     there is a possible ambiguity with this syntax, because subpattern names may
1811     consist entirely of digits. PCRE looks first for a named subpattern; if it
1812     cannot find one and the name consists entirely of digits, PCRE looks for a
1813     subpattern of that number, which must be greater than zero. Using subpattern
1814     names that consist entirely of digits is not recommended.
1815     .P
1816     Rewriting the above example to use a named subpattern gives this:
1817 nigel 91 .sp
1818 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1819     .sp
1820     .
1821     .SS "Checking for pattern recursion"
1822     .rs
1823     .sp
1824 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1825 nigel 93 the condition is true if a recursive call to the whole pattern or any
1826     subpattern has been made. If digits or a name preceded by ampersand follow the
1827     letter R, for example:
1828     .sp
1829     (?(R3)...) or (?(R&name)...)
1830     .sp
1831     the condition is true if the most recent recursion is into the subpattern whose
1832     number or name is given. This condition does not check the entire recursion
1833     stack.
1834 nigel 75 .P
1835 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1836     patterns are described below.
1837     .
1838     .SS "Defining subpatterns for use by reference only"
1839     .rs
1840     .sp
1841     If the condition is the string (DEFINE), and there is no subpattern with the
1842     name DEFINE, the condition is always false. In this case, there may be only one
1843     alternative in the subpattern. It is always skipped if control reaches this
1844     point in the pattern; the idea of DEFINE is that it can be used to define
1845     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1846     is described below.) For example, a pattern to match an IPv4 address could be
1847     written like this (ignore whitespace and line breaks):
1848     .sp
1849     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1850     \eb (?&byte) (\e.(?&byte)){3} \eb
1851     .sp
1852     The first part of the pattern is a DEFINE group inside which a another group
1853     named "byte" is defined. This matches an individual component of an IPv4
1854     address (a number less than 256). When matching takes place, this part of the
1855     pattern is skipped because DEFINE acts like a false condition.
1856     .P
1857     The rest of the pattern uses references to the named group to match the four
1858     dot-separated components of an IPv4 address, insisting on a word boundary at
1859     each end.
1860     .
1861     .SS "Assertion conditions"
1862     .rs
1863     .sp
1864     If the condition is not in any of the above formats, it must be an assertion.
1865 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1866     this pattern, again containing non-significant white space, and with the two
1867     alternatives on the second line:
1868 nigel 75 .sp
1869 nigel 63 (?(?=[^a-z]*[a-z])
1870 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1871     .sp
1872 nigel 63 The condition is a positive lookahead assertion that matches an optional
1873     sequence of non-letters followed by a letter. In other words, it tests for the
1874     presence of at least one letter in the subject. If a letter is found, the
1875     subject is matched against the first alternative; otherwise it is matched
1876     against the second. This pattern matches strings in one of the two forms
1877     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1878 nigel 75 .
1879     .
1880     .\" HTML <a name="comments"></a>
1881 nigel 63 .SH COMMENTS
1882     .rs
1883     .sp
1884 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1885 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1886     that make up a comment play no part in the pattern matching at all.
1887 nigel 75 .P
1888 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1889 nigel 91 character class introduces a comment that continues to immediately after the
1890     next newline in the pattern.
1891 nigel 75 .
1892     .
1893 nigel 91 .\" HTML <a name="recursion"></a>
1894 nigel 75 .SH "RECURSIVE PATTERNS"
1895 nigel 63 .rs
1896     .sp
1897     Consider the problem of matching a string in parentheses, allowing for
1898     unlimited nested parentheses. Without the use of recursion, the best that can
1899     be done is to use a pattern that matches up to some fixed depth of nesting. It
1900 nigel 93 is not possible to handle an arbitrary nesting depth.
1901     .P
1902     For some time, Perl has provided a facility that allows regular expressions to
1903     recurse (amongst other things). It does this by interpolating Perl code in the
1904     expression at run time, and the code can refer to the expression itself. A Perl
1905     pattern using code interpolation to solve the parentheses problem can be
1906     created like this:
1907 nigel 75 .sp
1908     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1909     .sp
1910 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1911 nigel 93 recursively to the pattern in which it appears.
1912 nigel 75 .P
1913 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1914     supports special syntax for recursion of the entire pattern, and also for
1915     individual subpattern recursion. After its introduction in PCRE and Python,
1916     this kind of recursion was introduced into Perl at release 5.10.
1917 nigel 75 .P
1918 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1919     closing parenthesis is a recursive call of the subpattern of the given number,
1920     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1921     call, which is described in the next section.) The special item (?R) or (?0) is
1922     a recursive call of the entire regular expression.
1923 nigel 87 .P
1924 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1925     treated as an atomic group. That is, once it has matched some of the subject
1926     string, it is never re-entered, even if it contains untried alternatives and
1927     there is a subsequent matching failure.
1928     .P
1929 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1930     PCRE_EXTENDED option is set so that white space is ignored):
1931 nigel 75 .sp
1932     \e( ( (?>[^()]+) | (?R) )* \e)
1933     .sp
1934 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1935     substrings which can either be a sequence of non-parentheses, or a recursive
1936 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1937 nigel 63 Finally there is a closing parenthesis.
1938 nigel 75 .P
1939 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1940     pattern, so instead you could use this:
1941 nigel 75 .sp
1942     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1943     .sp
1944 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1945 ph10 172 them instead of the whole pattern.
1946 ph10 166 .P
1947     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1948     is made easier by the use of relative references. (A Perl 5.10 feature.)
1949     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1950     most recently opened parentheses preceding the recursion. In other words, a
1951     negative number counts capturing parentheses leftwards from the point at which
1952     it is encountered.
1953     .P
1954     It is also possible to refer to subsequently opened parentheses, by writing
1955     references such as (?+2). However, these cannot be recursive because the
1956     reference is not inside the parentheses that are referenced. They are always
1957     "subroutine" calls, as described in the next section.
1958     .P
1959     An alternative approach is to use named parentheses instead. The Perl syntax
1960     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1961     could rewrite the above example as follows:
1962 nigel 75 .sp
1963 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1964 nigel 75 .sp
1965 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1966 ph10 172 used.
1967 ph10 166 .P
1968     This particular example pattern that we have been looking at contains nested
1969     unlimited repeats, and so the use of atomic grouping for matching strings of
1970     non-parentheses is important when applying the pattern to strings that do not
1971     match. For example, when this pattern is applied to
1972 nigel 75 .sp
1973 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1974 nigel 75 .sp
1975 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1976     the match runs for a very long time indeed because there are so many different
1977     ways the + and * repeats can carve up the subject, and all have to be tested
1978     before failure can be reported.
1979 nigel 75 .P
1980 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1981     from the outermost level of the recursion at which the subpattern value is set.
1982     If you want to obtain intermediate values, a callout function can be used (see
1983 nigel 93 below and the
1984 nigel 63 .\" HREF
1985 nigel 75 \fBpcrecallout\fP
1986 nigel 63 .\"
1987     documentation). If the pattern above is matched against
1988 nigel 75 .sp
1989 nigel 63 (ab(cd)ef)
1990 nigel 75 .sp
1991 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1992     on at the top level. If additional parentheses are added, giving
1993 nigel 75 .sp
1994     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1995 nigel 63 ^ ^
1996     ^ ^
1997 nigel 75 .sp
1998 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1999     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
2000     has to obtain extra memory to store data during a recursion, which it does by
2001 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
2002 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2003 nigel 75 .P
2004 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2005     Consider this pattern, which matches text in angle brackets, allowing for
2006     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2007     recursing), whereas any characters are permitted at the outer level.
2008 nigel 75 .sp
2009     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2010     .sp
2011 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2012     different alternatives for the recursive and non-recursive cases. The (?R) item
2013     is the actual recursive call.
2014 nigel 75 .
2015     .
2016 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
2017 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
2018 nigel 63 .rs
2019     .sp
2020     If the syntax for a recursive subpattern reference (either by number or by
2021     name) is used outside the parentheses to which it refers, it operates like a
2022 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2023 ph10 166 before or after the reference. A numbered reference can be absolute or
2024     relative, as in these examples:
2025 nigel 75 .sp
2026 ph10 166 (...(absolute)...)...(?2)...
2027     (...(relative)...)...(?-1)...
2028 ph10 172 (...(?+1)...(relative)...
2029 ph10 166 .sp
2030     An earlier example pointed out that the pattern
2031     .sp
2032 nigel 75 (sens|respons)e and \e1ibility
2033     .sp
2034 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2035     "sense and responsibility". If instead the pattern
2036 nigel 75 .sp
2037 nigel 63 (sens|respons)e and (?1)ibility
2038 nigel 75 .sp
2039 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2040 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2041 nigel 87 .P
2042     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2043     group. That is, once it has matched some of the subject string, it is never
2044     re-entered, even if it contains untried alternatives and there is a subsequent
2045     matching failure.
2046 nigel 93 .P
2047     When a subpattern is used as a subroutine, processing options such as
2048     case-independence are fixed when the subpattern is defined. They cannot be
2049     changed for different calls. For example, consider this pattern:
2050     .sp
2051 ph10 166 (abc)(?i:(?-1))
2052 nigel 93 .sp
2053     It matches "abcabc". It does not match "abcABC" because the change of
2054     processing option does not affect the called subpattern.
2055 nigel 75 .
2056     .
2057 ph10 333 .\" HTML <a name="onigurumasubroutines"></a>
2058     .SH "ONIGURUMA SUBROUTINE SYNTAX"
2059     .rs
2060     .sp
2061 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2062     a number enclosed either in angle brackets or single quotes, is an alternative
2063     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2064 ph10 333 are two of the examples used above, rewritten using this syntax:
2065     .sp
2066     (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2067     (sens|respons)e and \eg'1'ibility
2068     .sp
2069 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a
2070 ph10 333 plus or a minus sign it is taken as a relative reference. For example:
2071     .sp
2072     (abc)(?i:\eg<-1>)
2073     .sp
2074 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2075 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
2076     .
2077     .
2078 nigel 63 .SH CALLOUTS
2079     .rs
2080     .sp
2081     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2082     code to be obeyed in the middle of matching a regular expression. This makes it
2083     possible, amongst other things, to extract different substrings that match the
2084     same pair of parentheses when there is a repetition.
2085 nigel 75 .P
2086 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2087     code. The feature is called "callout". The caller of PCRE provides an external
2088 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2089 nigel 63 By default, this variable contains NULL, which disables all calling out.
2090 nigel 75 .P
2091 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2092     function is to be called. If you want to identify different callout points, you
2093     can put a number less than 256 after the letter C. The default value is zero.
2094     For example, this pattern has two callout points:
2095 nigel 75 .sp
2096 ph10 155 (?C1)abc(?C2)def
2097 nigel 75 .sp
2098     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2099     automatically installed before each item in the pattern. They are all numbered
2100     255.
2101     .P
2102     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2103 nigel 63 set), the external function is called. It is provided with the number of the
2104 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2105     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2106     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2107     description of the interface to the callout function is given in the
2108 nigel 63 .\" HREF
2109 nigel 75 \fBpcrecallout\fP
2110 nigel 63 .\"
2111     documentation.
2112 nigel 93 .
2113     .
2114 ph10 235 .SH "BACKTRACKING CONTROL"
2115 ph10 210 .rs
2116     .sp
2117 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2118 ph10 210 are described in the Perl documentation as "experimental and subject to change
2119 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2120     production code should be noted to avoid problems during upgrades." The same
2121 ph10 210 remarks apply to the PCRE features described in this section.
2122     .P
2123 ph10 341 Since these verbs are specifically related to backtracking, most of them can be
2124     used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2125 ph10 345 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2126 ph10 341 failing negative assertion, they cause an error if encountered by
2127 ph10 210 \fBpcre_dfa_exec()\fP.
2128     .P
2129 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2130 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
2131     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2132     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2133     are two kinds:
2134     .
2135     .SS "Verbs that act immediately"
2136     .rs
2137     .sp
2138     The following verbs act as soon as they are encountered:
2139     .sp
2140     (*ACCEPT)
2141     .sp
2142     This verb causes the match to end successfully, skipping the remainder of the
2143 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2144     immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2145 ph10 210 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2146     captured. For example:
2147     .sp
2148     A(A|B(*ACCEPT)|C)D
2149     .sp
2150 ph10 211 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2151 ph10 210 captured.
2152     .sp
2153     (*FAIL) or (*F)
2154     .sp
2155 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2156 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2157     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2158     Perl features that are not present in PCRE. The nearest equivalent is the
2159     callout feature, as for example in this pattern:
2160     .sp
2161     a+(?C)(*FAIL)
2162     .sp
2163 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2164     each backtrack happens (in this example, 10 times).
2165 ph10 210 .
2166     .SS "Verbs that act after backtracking"
2167     .rs
2168     .sp
2169 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2170     with what follows, but if there is no subsequent match, a failure is forced.
2171 ph10 210 The verbs differ in exactly what kind of failure occurs.
2172     .sp
2173     (*COMMIT)
2174     .sp
2175 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2176 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2177     a match by advancing the start point take place. Once (*COMMIT) has been
2178 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2179 ph10 210 starting point, or not at all. For example:
2180     .sp
2181     a+(*COMMIT)b
2182     .sp
2183 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2184 ph10 210 dynamic anchor, or "I've started, so I must finish."
2185     .sp
2186     (*PRUNE)
2187     .sp
2188 ph10 211 This verb causes the match to fail at the current position if the rest of the
2189     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2190 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2191     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2192     if there is no match to the right, backtracking cannot cross (*PRUNE).
2193 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2194 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2195     be expressed in any other way.
2196     .sp
2197     (*SKIP)
2198     .sp
2199 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2200 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2201     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2202     was matched leading up to it cannot be part of a successful match. Consider:
2203     .sp
2204     a+(*SKIP)b
2205     .sp
2206 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2207 ph10 210 the first character in the string), the starting point skips on to start the
2208 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2209 ph10 210 effect in this example; although it would suppress backtracking during the
2210     first match attempt, the second attempt would start at the second character
2211     instead of skipping on to "c".
2212     .sp
2213     (*THEN)
2214 ph10 211 .sp
2215 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2216     not match. That is, it cancels pending backtracking, but only within the
2217     current alternation. Its name comes from the observation that it can be used
2218     for a pattern-based if-then-else block:
2219     .sp
2220     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2221     .sp
2222 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2223 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2224 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2225 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2226     .
2227     .
2228 nigel 93 .SH "SEE ALSO"
2229     .rs
2230     .sp
2231     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2232 ph10 99 .
2233     .
2234     .SH AUTHOR
2235     .rs
2236     .sp
2237     .nf
2238     Philip Hazel
2239     University Computing Service
2240     Cambridge CB2 3QH, England.
2241     .fi
2242     .
2243     .
2244     .SH REVISION
2245     .rs
2246     .sp
2247     .nf
2248 ph10 394 Last updated: 18 March 2009
2249 ph10 385 Copyright (c) 1997-2009 University of Cambridge.
2250 ph10 99 .fi

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