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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 ph10 416 the PCRE_UTF8 option. There is also a special sequence that can be given at the
27 ph10 412 start of a pattern:
28     .sp
29     (*UTF8)
30 ph10 416 .sp
31 ph10 412 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32     option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33     pattern matching is mentioned in several places below. There is also a summary
34     of UTF-8 features in the
35 nigel 63 .\" HTML <a href="pcre.html#utf8support">
36     .\" </a>
37     section on UTF-8 support
38     .\"
39     in the main
40     .\" HREF
41 nigel 75 \fBpcre\fP
42 nigel 63 .\"
43     page.
44 nigel 75 .P
45 nigel 77 The remainder of this document discusses the patterns that are supported by
46     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47     From release 6.0, PCRE offers a second matching function,
48     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49 ph10 172 Perl-compatible. Some of the features discussed below are not available when
50 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51     alternative function, and how it differs from the normal function, are
52     discussed in the
53 nigel 77 .\" HREF
54     \fBpcrematching\fP
55     .\"
56     page.
57 nigel 93 .
58     .
59 ph10 227 .SH "NEWLINE CONVENTIONS"
60     .rs
61     .sp
62     PCRE supports five different conventions for indicating line breaks in
63     strings: a single CR (carriage return) character, a single LF (linefeed)
64     character, the two-character sequence CRLF, any of the three preceding, or any
65     Unicode newline sequence. The
66     .\" HREF
67     \fBpcreapi\fP
68     .\"
69     page has
70     .\" HTML <a href="pcreapi.html#newlines">
71     .\" </a>
72     further discussion
73     .\"
74     about newlines, and shows how to set the newline convention in the
75     \fIoptions\fP arguments for the compiling and matching functions.
76     .P
77     It is also possible to specify a newline convention by starting a pattern
78     string with one of the following five sequences:
79     .sp
80     (*CR) carriage return
81     (*LF) linefeed
82     (*CRLF) carriage return, followed by linefeed
83     (*ANYCRLF) any of the three above
84     (*ANY) all Unicode newline sequences
85     .sp
86     These override the default and the options given to \fBpcre_compile()\fP. For
87     example, on a Unix system where LF is the default newline sequence, the pattern
88     .sp
89     (*CR)a.b
90     .sp
91     changes the convention to CR. That pattern matches "a\enb" because LF is no
92     longer a newline. Note that these special settings, which are not
93     Perl-compatible, are recognized only at the very start of a pattern, and that
94 ph10 231 they must be in upper case. If more than one of them is present, the last one
95     is used.
96     .P
97     The newline convention does not affect what the \eR escape sequence matches. By
98     default, this is any Unicode newline sequence, for Perl compatibility. However,
99     this can be changed; see the description of \eR in the section entitled
100     .\" HTML <a href="#newlineseq">
101     .\" </a>
102     "Newline sequences"
103     .\"
104 ph10 247 below. A change of \eR setting can be combined with a change of newline
105 ph10 246 convention.
106 ph10 227 .
107     .
108 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
109     .rs
110     .sp
111 nigel 63 A regular expression is a pattern that is matched against a subject string from
112     left to right. Most characters stand for themselves in a pattern, and match the
113     corresponding characters in the subject. As a trivial example, the pattern
114 nigel 75 .sp
115 nigel 63 The quick brown fox
116 nigel 75 .sp
117 nigel 77 matches a portion of a subject string that is identical to itself. When
118     caseless matching is specified (the PCRE_CASELESS option), letters are matched
119     independently of case. In UTF-8 mode, PCRE always understands the concept of
120     case for characters whose values are less than 128, so caseless matching is
121     always possible. For characters with higher values, the concept of case is
122     supported if PCRE is compiled with Unicode property support, but not otherwise.
123     If you want to use caseless matching for characters 128 and above, you must
124     ensure that PCRE is compiled with Unicode property support as well as with
125     UTF-8 support.
126     .P
127     The power of regular expressions comes from the ability to include alternatives
128     and repetitions in the pattern. These are encoded in the pattern by the use of
129 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
130 nigel 63 interpreted in some special way.
131 nigel 75 .P
132     There are two different sets of metacharacters: those that are recognized
133 nigel 63 anywhere in the pattern except within square brackets, and those that are
134 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
135     are as follows:
136 nigel 75 .sp
137     \e general escape character with several uses
138 nigel 63 ^ assert start of string (or line, in multiline mode)
139     $ assert end of string (or line, in multiline mode)
140     . match any character except newline (by default)
141     [ start character class definition
142     | start of alternative branch
143     ( start subpattern
144     ) end subpattern
145     ? extends the meaning of (
146     also 0 or 1 quantifier
147     also quantifier minimizer
148     * 0 or more quantifier
149     + 1 or more quantifier
150     also "possessive quantifier"
151     { start min/max quantifier
152 nigel 75 .sp
153 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
154 nigel 75 a character class the only metacharacters are:
155     .sp
156     \e general escape character
157 nigel 63 ^ negate the class, but only if the first character
158     - indicates character range
159 nigel 75 .\" JOIN
160 nigel 63 [ POSIX character class (only if followed by POSIX
161     syntax)
162     ] terminates the character class
163 nigel 75 .sp
164     The following sections describe the use of each of the metacharacters.
165     .
166 nigel 93 .
167 nigel 63 .SH BACKSLASH
168     .rs
169     .sp
170     The backslash character has several uses. Firstly, if it is followed by a
171 nigel 91 non-alphanumeric character, it takes away any special meaning that character
172     may have. This use of backslash as an escape character applies both inside and
173 nigel 63 outside character classes.
174 nigel 75 .P
175     For example, if you want to match a * character, you write \e* in the pattern.
176 nigel 63 This escaping action applies whether or not the following character would
177 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
178     non-alphanumeric with backslash to specify that it stands for itself. In
179     particular, if you want to match a backslash, you write \e\e.
180     .P
181 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
182     pattern (other than in a character class) and characters between a # outside
183 nigel 91 a character class and the next newline are ignored. An escaping backslash can
184     be used to include a whitespace or # character as part of the pattern.
185 nigel 75 .P
186 nigel 63 If you want to remove the special meaning from a sequence of characters, you
187 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
188     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
189 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
190 nigel 75 .sp
191 nigel 63 Pattern PCRE matches Perl matches
192 nigel 75 .sp
193     .\" JOIN
194     \eQabc$xyz\eE abc$xyz abc followed by the
195 nigel 63 contents of $xyz
196 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
197     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
198     .sp
199     The \eQ...\eE sequence is recognized both inside and outside character classes.
200     .
201     .
202     .\" HTML <a name="digitsafterbackslash"></a>
203     .SS "Non-printing characters"
204     .rs
205     .sp
206 nigel 63 A second use of backslash provides a way of encoding non-printing characters
207     in patterns in a visible manner. There is no restriction on the appearance of
208     non-printing characters, apart from the binary zero that terminates a pattern,
209     but when a pattern is being prepared by text editing, it is usually easier to
210     use one of the following escape sequences than the binary character it
211     represents:
212 nigel 75 .sp
213     \ea alarm, that is, the BEL character (hex 07)
214     \ecx "control-x", where x is any character
215     \ee escape (hex 1B)
216     \ef formfeed (hex 0C)
217 ph10 227 \en linefeed (hex 0A)
218 nigel 75 \er carriage return (hex 0D)
219     \et tab (hex 09)
220     \eddd character with octal code ddd, or backreference
221     \exhh character with hex code hh
222 nigel 87 \ex{hhh..} character with hex code hhh..
223 nigel 75 .sp
224     The precise effect of \ecx is as follows: if x is a lower case letter, it
225 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
226 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
227 nigel 63 7B.
228 nigel 75 .P
229     After \ex, from zero to two hexadecimal digits are read (letters can be in
230 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
231     and }, but the value of the character code must be less than 256 in non-UTF-8
232 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234     point, which is 10FFFF.
235 nigel 75 .P
236 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
237     there is no terminating }, this form of escape is not recognized. Instead, the
238     initial \ex will be interpreted as a basic hexadecimal escape, with no
239     following digits, giving a character whose value is zero.
240     .P
241 nigel 63 Characters whose value is less than 256 can be defined by either of the two
242 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
243     example, \exdc is exactly the same as \ex{dc}.
244 nigel 75 .P
245 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
246     digits, just those that are present are used. Thus the sequence \e0\ex\e07
247     specifies two binary zeros followed by a BEL character (code value 7). Make
248     sure you supply two digits after the initial zero if the pattern character that
249     follows is itself an octal digit.
250 nigel 75 .P
251 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
252     Outside a character class, PCRE reads it and any following digits as a decimal
253     number. If the number is less than 10, or if there have been at least that many
254     previous capturing left parentheses in the expression, the entire sequence is
255 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
256     .\" HTML <a href="#backreferences">
257     .\" </a>
258     later,
259     .\"
260     following the discussion of
261     .\" HTML <a href="#subpattern">
262     .\" </a>
263     parenthesized subpatterns.
264     .\"
265     .P
266 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
267     have not been that many capturing subpatterns, PCRE re-reads up to three octal
268 nigel 93 digits following the backslash, and uses them to generate a data character. Any
269 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270     character specified in octal must be less than \e400. In UTF-8 mode, values up
271     to \e777 are permitted. For example:
272 nigel 75 .sp
273     \e040 is another way of writing a space
274     .\" JOIN
275     \e40 is the same, provided there are fewer than 40
276 nigel 63 previous capturing subpatterns
277 nigel 75 \e7 is always a back reference
278     .\" JOIN
279     \e11 might be a back reference, or another way of
280 nigel 63 writing a tab
281 nigel 75 \e011 is always a tab
282     \e0113 is a tab followed by the character "3"
283     .\" JOIN
284     \e113 might be a back reference, otherwise the
285 nigel 63 character with octal code 113
286 nigel 75 .\" JOIN
287     \e377 might be a back reference, otherwise
288 nigel 63 the byte consisting entirely of 1 bits
289 nigel 75 .\" JOIN
290     \e81 is either a back reference, or a binary zero
291 nigel 63 followed by the two characters "8" and "1"
292 nigel 75 .sp
293 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
294     zero, because no more than three octal digits are ever read.
295 nigel 75 .P
296 nigel 91 All the sequences that define a single character value can be used both inside
297     and outside character classes. In addition, inside a character class, the
298     sequence \eb is interpreted as the backspace character (hex 08), and the
299 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
300     respectively. Outside a character class, these sequences have different
301     meanings
302 nigel 75 .\" HTML <a href="#uniextseq">
303     .\" </a>
304     (see below).
305     .\"
306     .
307     .
308 nigel 93 .SS "Absolute and relative back references"
309     .rs
310     .sp
311 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
312     enclosed in braces, is an absolute or relative back reference. A named back
313     reference can be coded as \eg{name}. Back references are discussed
314 nigel 93 .\" HTML <a href="#backreferences">
315     .\" </a>
316     later,
317     .\"
318     following the discussion of
319     .\" HTML <a href="#subpattern">
320     .\" </a>
321     parenthesized subpatterns.
322     .\"
323     .
324     .
325 ph10 333 .SS "Absolute and relative subroutine calls"
326     .rs
327     .sp
328 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329     a number enclosed either in angle brackets or single quotes, is an alternative
330     syntax for referencing a subpattern as a "subroutine". Details are discussed
331 ph10 333 .\" HTML <a href="#onigurumasubroutines">
332     .\" </a>
333     later.
334     .\"
335 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
337     .
338     .
339 nigel 75 .SS "Generic character types"
340     .rs
341     .sp
342 nigel 93 Another use of backslash is for specifying generic character types. The
343 nigel 75 following are always recognized:
344     .sp
345 ph10 182 \ed any decimal digit
346 nigel 75 \eD any character that is not a decimal digit
347 ph10 178 \eh any horizontal whitespace character
348 ph10 182 \eH any character that is not a horizontal whitespace character
349 nigel 75 \es any whitespace character
350     \eS any character that is not a whitespace character
351 ph10 178 \ev any vertical whitespace character
352 ph10 182 \eV any character that is not a vertical whitespace character
353 nigel 75 \ew any "word" character
354     \eW any "non-word" character
355     .sp
356 nigel 63 Each pair of escape sequences partitions the complete set of characters into
357     two disjoint sets. Any given character matches one, and only one, of each pair.
358 nigel 75 .P
359     These character type sequences can appear both inside and outside character
360     classes. They each match one character of the appropriate type. If the current
361     matching point is at the end of the subject string, all of them fail, since
362     there is no character to match.
363     .P
364     For compatibility with Perl, \es does not match the VT character (code 11).
365     This makes it different from the the POSIX "space" class. The \es characters
366 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
367 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
368 ph10 178 does.
369 nigel 75 .P
370 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
371     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
372 ph10 182 character property support is available. These sequences retain their original
373     meanings from before UTF-8 support was available, mainly for efficiency
374 ph10 406 reasons. Note that this also affects \eb, because it is defined in terms of \ew
375 ph10 394 and \eW.
376 ph10 178 .P
377 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
378 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
379     The horizontal space characters are:
380     .sp
381     U+0009 Horizontal tab
382     U+0020 Space
383     U+00A0 Non-break space
384     U+1680 Ogham space mark
385     U+180E Mongolian vowel separator
386     U+2000 En quad
387     U+2001 Em quad
388     U+2002 En space
389     U+2003 Em space
390     U+2004 Three-per-em space
391     U+2005 Four-per-em space
392     U+2006 Six-per-em space
393     U+2007 Figure space
394     U+2008 Punctuation space
395     U+2009 Thin space
396     U+200A Hair space
397     U+202F Narrow no-break space
398     U+205F Medium mathematical space
399     U+3000 Ideographic space
400     .sp
401     The vertical space characters are:
402     .sp
403     U+000A Linefeed
404     U+000B Vertical tab
405     U+000C Formfeed
406     U+000D Carriage return
407     U+0085 Next line
408     U+2028 Line separator
409     U+2029 Paragraph separator
410     .P
411 nigel 75 A "word" character is an underscore or any character less than 256 that is a
412     letter or digit. The definition of letters and digits is controlled by PCRE's
413     low-valued character tables, and may vary if locale-specific matching is taking
414     place (see
415 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
416     .\" </a>
417     "Locale support"
418     .\"
419     in the
420     .\" HREF
421 nigel 75 \fBpcreapi\fP
422 nigel 63 .\"
423 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
424     or "french" in Windows, some character codes greater than 128 are used for
425 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
426     is discouraged.
427 nigel 75 .
428     .
429 ph10 231 .\" HTML <a name="newlineseq"></a>
430 nigel 93 .SS "Newline sequences"
431     .rs
432     .sp
433 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
434     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
435     equivalent to the following:
436 nigel 93 .sp
437     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
438     .sp
439     This is an example of an "atomic group", details of which are given
440     .\" HTML <a href="#atomicgroup">
441     .\" </a>
442     below.
443     .\"
444     This particular group matches either the two-character sequence CR followed by
445     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
446     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
447     line, U+0085). The two-character sequence is treated as a single unit that
448     cannot be split.
449     .P
450     In UTF-8 mode, two additional characters whose codepoints are greater than 255
451     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
452     Unicode character property support is not needed for these characters to be
453     recognized.
454     .P
455 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
456     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
457 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
458 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is
459     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
460     It is also possible to specify these settings by starting a pattern string with
461     one of the following sequences:
462 ph10 231 .sp
463     (*BSR_ANYCRLF) CR, LF, or CRLF only
464     (*BSR_UNICODE) any Unicode newline sequence
465     .sp
466     These override the default and the options given to \fBpcre_compile()\fP, but
467     they can be overridden by options given to \fBpcre_exec()\fP. Note that these
468     special settings, which are not Perl-compatible, are recognized only at the
469     very start of a pattern, and that they must be in upper case. If more than one
470 ph10 247 of them is present, the last one is used. They can be combined with a change of
471 ph10 246 newline convention, for example, a pattern can start with:
472     .sp
473     (*ANY)(*BSR_ANYCRLF)
474     .sp
475 nigel 93 Inside a character class, \eR matches the letter "R".
476     .
477     .
478 nigel 75 .\" HTML <a name="uniextseq"></a>
479     .SS Unicode character properties
480     .rs
481     .sp
482     When PCRE is built with Unicode character property support, three additional
483 ph10 184 escape sequences that match characters with specific properties are available.
484     When not in UTF-8 mode, these sequences are of course limited to testing
485     characters whose codepoints are less than 256, but they do work in this mode.
486     The extra escape sequences are:
487 nigel 75 .sp
488 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
489     \eP{\fIxx\fP} a character without the \fIxx\fP property
490     \eX an extended Unicode sequence
491 nigel 75 .sp
492 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
493     script names, the general category properties, and "Any", which matches any
494     character (including newline). Other properties such as "InMusicalSymbols" are
495     not currently supported by PCRE. Note that \eP{Any} does not match any
496     characters, so always causes a match failure.
497 nigel 75 .P
498 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
499     character from one of these sets can be matched using a script name. For
500     example:
501 nigel 75 .sp
502 nigel 87 \ep{Greek}
503     \eP{Han}
504     .sp
505     Those that are not part of an identified script are lumped together as
506     "Common". The current list of scripts is:
507     .P
508     Arabic,
509     Armenian,
510 nigel 93 Balinese,
511 nigel 87 Bengali,
512     Bopomofo,
513     Braille,
514     Buginese,
515     Buhid,
516     Canadian_Aboriginal,
517     Cherokee,
518     Common,
519     Coptic,
520 nigel 93 Cuneiform,
521 nigel 87 Cypriot,
522     Cyrillic,
523     Deseret,
524     Devanagari,
525     Ethiopic,
526     Georgian,
527     Glagolitic,
528     Gothic,
529     Greek,
530     Gujarati,
531     Gurmukhi,
532     Han,
533     Hangul,
534     Hanunoo,
535     Hebrew,
536     Hiragana,
537     Inherited,
538     Kannada,
539     Katakana,
540     Kharoshthi,
541     Khmer,
542     Lao,
543     Latin,
544     Limbu,
545     Linear_B,
546     Malayalam,
547     Mongolian,
548     Myanmar,
549     New_Tai_Lue,
550 nigel 93 Nko,
551 nigel 87 Ogham,
552     Old_Italic,
553     Old_Persian,
554     Oriya,
555     Osmanya,
556 nigel 93 Phags_Pa,
557     Phoenician,
558 nigel 87 Runic,
559     Shavian,
560     Sinhala,
561     Syloti_Nagri,
562     Syriac,
563     Tagalog,
564     Tagbanwa,
565     Tai_Le,
566     Tamil,
567     Telugu,
568     Thaana,
569     Thai,
570     Tibetan,
571     Tifinagh,
572     Ugaritic,
573     Yi.
574     .P
575     Each character has exactly one general category property, specified by a
576     two-letter abbreviation. For compatibility with Perl, negation can be specified
577     by including a circumflex between the opening brace and the property name. For
578     example, \ep{^Lu} is the same as \eP{Lu}.
579     .P
580     If only one letter is specified with \ep or \eP, it includes all the general
581     category properties that start with that letter. In this case, in the absence
582     of negation, the curly brackets in the escape sequence are optional; these two
583     examples have the same effect:
584     .sp
585 nigel 75 \ep{L}
586     \epL
587     .sp
588 nigel 87 The following general category property codes are supported:
589 nigel 75 .sp
590     C Other
591     Cc Control
592     Cf Format
593     Cn Unassigned
594     Co Private use
595     Cs Surrogate
596     .sp
597     L Letter
598     Ll Lower case letter
599     Lm Modifier letter
600     Lo Other letter
601     Lt Title case letter
602     Lu Upper case letter
603     .sp
604     M Mark
605     Mc Spacing mark
606     Me Enclosing mark
607     Mn Non-spacing mark
608     .sp
609     N Number
610     Nd Decimal number
611     Nl Letter number
612     No Other number
613     .sp
614     P Punctuation
615     Pc Connector punctuation
616     Pd Dash punctuation
617     Pe Close punctuation
618     Pf Final punctuation
619     Pi Initial punctuation
620     Po Other punctuation
621     Ps Open punctuation
622     .sp
623     S Symbol
624     Sc Currency symbol
625     Sk Modifier symbol
626     Sm Mathematical symbol
627     So Other symbol
628     .sp
629     Z Separator
630     Zl Line separator
631     Zp Paragraph separator
632     Zs Space separator
633     .sp
634 nigel 87 The special property L& is also supported: it matches a character that has
635     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
636     a modifier or "other".
637 nigel 75 .P
638 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
639     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
640     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
641     (see the discussion of PCRE_NO_UTF8_CHECK in the
642     .\" HREF
643     \fBpcreapi\fP
644     .\"
645     page).
646     .P
647 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
648 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
649 nigel 87 properties with "Is".
650     .P
651     No character that is in the Unicode table has the Cn (unassigned) property.
652     Instead, this property is assumed for any code point that is not in the
653     Unicode table.
654     .P
655 nigel 75 Specifying caseless matching does not affect these escape sequences. For
656     example, \ep{Lu} always matches only upper case letters.
657     .P
658     The \eX escape matches any number of Unicode characters that form an extended
659     Unicode sequence. \eX is equivalent to
660     .sp
661     (?>\ePM\epM*)
662     .sp
663     That is, it matches a character without the "mark" property, followed by zero
664     or more characters with the "mark" property, and treats the sequence as an
665     atomic group
666     .\" HTML <a href="#atomicgroup">
667     .\" </a>
668     (see below).
669     .\"
670     Characters with the "mark" property are typically accents that affect the
671 ph10 185 preceding character. None of them have codepoints less than 256, so in
672 ph10 184 non-UTF-8 mode \eX matches any one character.
673 nigel 75 .P
674     Matching characters by Unicode property is not fast, because PCRE has to search
675     a structure that contains data for over fifteen thousand characters. That is
676     why the traditional escape sequences such as \ed and \ew do not use Unicode
677     properties in PCRE.
678     .
679     .
680 ph10 168 .\" HTML <a name="resetmatchstart"></a>
681     .SS "Resetting the match start"
682     .rs
683     .sp
684 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
685     matched characters not to be included in the final matched sequence. For
686 ph10 168 example, the pattern:
687     .sp
688     foo\eKbar
689     .sp
690 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
691 ph10 168 similar to a lookbehind assertion
692     .\" HTML <a href="#lookbehind">
693     .\" </a>
694     (described below).
695     .\"
696 ph10 172 However, in this case, the part of the subject before the real match does not
697     have to be of fixed length, as lookbehind assertions do. The use of \eK does
698 ph10 168 not interfere with the setting of
699     .\" HTML <a href="#subpattern">
700     .\" </a>
701     captured substrings.
702 ph10 172 .\"
703 ph10 168 For example, when the pattern
704     .sp
705     (foo)\eKbar
706     .sp
707 ph10 172 matches "foobar", the first substring is still set to "foo".
708 ph10 168 .
709     .
710 nigel 75 .\" HTML <a name="smallassertions"></a>
711     .SS "Simple assertions"
712     .rs
713     .sp
714 nigel 93 The final use of backslash is for certain simple assertions. An assertion
715 nigel 63 specifies a condition that has to be met at a particular point in a match,
716     without consuming any characters from the subject string. The use of
717 nigel 75 subpatterns for more complicated assertions is described
718     .\" HTML <a href="#bigassertions">
719     .\" </a>
720     below.
721     .\"
722 nigel 91 The backslashed assertions are:
723 nigel 75 .sp
724     \eb matches at a word boundary
725     \eB matches when not at a word boundary
726 nigel 93 \eA matches at the start of the subject
727     \eZ matches at the end of the subject
728     also matches before a newline at the end of the subject
729     \ez matches only at the end of the subject
730     \eG matches at the first matching position in the subject
731 nigel 75 .sp
732     These assertions may not appear in character classes (but note that \eb has a
733 nigel 63 different meaning, namely the backspace character, inside a character class).
734 nigel 75 .P
735 nigel 63 A word boundary is a position in the subject string where the current character
736 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
737     \ew and the other matches \eW), or the start or end of the string if the
738     first or last character matches \ew, respectively.
739     .P
740     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
741     dollar (described in the next section) in that they only ever match at the very
742     start and end of the subject string, whatever options are set. Thus, they are
743     independent of multiline mode. These three assertions are not affected by the
744     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
745     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
746     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
747     at a point other than the beginning of the subject, \eA can never match. The
748 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
749     of the string as well as at the very end, whereas \ez matches only at the end.
750 nigel 75 .P
751     The \eG assertion is true only when the current matching position is at the
752     start point of the match, as specified by the \fIstartoffset\fP argument of
753     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
754     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
755 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
756 nigel 75 implementation where \eG can be useful.
757     .P
758     Note, however, that PCRE's interpretation of \eG, as the start of the current
759 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
760     previous match. In Perl, these can be different when the previously matched
761     string was empty. Because PCRE does just one match at a time, it cannot
762     reproduce this behaviour.
763 nigel 75 .P
764     If all the alternatives of a pattern begin with \eG, the expression is anchored
765 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
766     regular expression.
767 nigel 75 .
768     .
769     .SH "CIRCUMFLEX AND DOLLAR"
770 nigel 63 .rs
771     .sp
772     Outside a character class, in the default matching mode, the circumflex
773 nigel 75 character is an assertion that is true only if the current matching point is
774     at the start of the subject string. If the \fIstartoffset\fP argument of
775     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
776 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
777 nigel 75 meaning
778     .\" HTML <a href="#characterclass">
779     .\" </a>
780     (see below).
781     .\"
782     .P
783 nigel 63 Circumflex need not be the first character of the pattern if a number of
784     alternatives are involved, but it should be the first thing in each alternative
785     in which it appears if the pattern is ever to match that branch. If all
786     possible alternatives start with a circumflex, that is, if the pattern is
787     constrained to match only at the start of the subject, it is said to be an
788     "anchored" pattern. (There are also other constructs that can cause a pattern
789     to be anchored.)
790 nigel 75 .P
791     A dollar character is an assertion that is true only if the current matching
792 nigel 63 point is at the end of the subject string, or immediately before a newline
793 nigel 91 at the end of the string (by default). Dollar need not be the last character of
794     the pattern if a number of alternatives are involved, but it should be the last
795     item in any branch in which it appears. Dollar has no special meaning in a
796     character class.
797 nigel 75 .P
798 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
799     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
800 nigel 75 does not affect the \eZ assertion.
801     .P
802 nigel 63 The meanings of the circumflex and dollar characters are changed if the
803 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
804     immediately after internal newlines as well as at the start of the subject
805     string. It does not match after a newline that ends the string. A dollar
806     matches before any newlines in the string, as well as at the very end, when
807     PCRE_MULTILINE is set. When newline is specified as the two-character
808     sequence CRLF, isolated CR and LF characters do not indicate newlines.
809 nigel 75 .P
810 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
811     \en represents a newline) in multiline mode, but not otherwise. Consequently,
812     patterns that are anchored in single line mode because all branches start with
813     ^ are not anchored in multiline mode, and a match for circumflex is possible
814     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
815     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
816     .P
817 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
818 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
819 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
820 nigel 75 .
821     .
822     .SH "FULL STOP (PERIOD, DOT)"
823 nigel 63 .rs
824     .sp
825     Outside a character class, a dot in the pattern matches any one character in
826 nigel 91 the subject string except (by default) a character that signifies the end of a
827 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
828 nigel 91 .P
829 nigel 93 When a line ending is defined as a single character, dot never matches that
830     character; when the two-character sequence CRLF is used, dot does not match CR
831     if it is immediately followed by LF, but otherwise it matches all characters
832     (including isolated CRs and LFs). When any Unicode line endings are being
833     recognized, dot does not match CR or LF or any of the other line ending
834     characters.
835     .P
836 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
837 nigel 93 option is set, a dot matches any one character, without exception. If the
838     two-character sequence CRLF is present in the subject string, it takes two dots
839     to match it.
840 nigel 91 .P
841     The handling of dot is entirely independent of the handling of circumflex and
842     dollar, the only relationship being that they both involve newlines. Dot has no
843     special meaning in a character class.
844 nigel 75 .
845     .
846     .SH "MATCHING A SINGLE BYTE"
847 nigel 63 .rs
848     .sp
849 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
850 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
851     characters. The feature is provided in Perl in order to match individual bytes
852     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
853     what remains in the string may be a malformed UTF-8 string. For this reason,
854     the \eC escape sequence is best avoided.
855 nigel 75 .P
856     PCRE does not allow \eC to appear in lookbehind assertions
857     .\" HTML <a href="#lookbehind">
858     .\" </a>
859     (described below),
860     .\"
861     because in UTF-8 mode this would make it impossible to calculate the length of
862     the lookbehind.
863     .
864     .
865     .\" HTML <a name="characterclass"></a>
866     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
867 nigel 63 .rs
868     .sp
869     An opening square bracket introduces a character class, terminated by a closing
870     square bracket. A closing square bracket on its own is not special. If a
871     closing square bracket is required as a member of the class, it should be the
872     first data character in the class (after an initial circumflex, if present) or
873     escaped with a backslash.
874 nigel 75 .P
875 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
876     character may occupy more than one byte. A matched character must be in the set
877     of characters defined by the class, unless the first character in the class
878     definition is a circumflex, in which case the subject character must not be in
879     the set defined by the class. If a circumflex is actually required as a member
880     of the class, ensure it is not the first character, or escape it with a
881     backslash.
882 nigel 75 .P
883 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
884     [^aeiou] matches any character that is not a lower case vowel. Note that a
885 nigel 75 circumflex is just a convenient notation for specifying the characters that
886     are in the class by enumerating those that are not. A class that starts with a
887     circumflex is not an assertion: it still consumes a character from the subject
888     string, and therefore it fails if the current pointer is at the end of the
889     string.
890     .P
891 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
892 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
893     .P
894 nigel 63 When caseless matching is set, any letters in a class represent both their
895     upper case and lower case versions, so for example, a caseless [aeiou] matches
896     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
897 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
898     case for characters whose values are less than 128, so caseless matching is
899     always possible. For characters with higher values, the concept of case is
900     supported if PCRE is compiled with Unicode property support, but not otherwise.
901     If you want to use caseless matching for characters 128 and above, you must
902     ensure that PCRE is compiled with Unicode property support as well as with
903     UTF-8 support.
904 nigel 75 .P
905 nigel 93 Characters that might indicate line breaks are never treated in any special way
906     when matching character classes, whatever line-ending sequence is in use, and
907     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
908     such as [^a] always matches one of these characters.
909 nigel 75 .P
910 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
911     character class. For example, [d-m] matches any letter between d and m,
912     inclusive. If a minus character is required in a class, it must be escaped with
913     a backslash or appear in a position where it cannot be interpreted as
914     indicating a range, typically as the first or last character in the class.
915 nigel 75 .P
916 nigel 63 It is not possible to have the literal character "]" as the end character of a
917     range. A pattern such as [W-]46] is interpreted as a class of two characters
918     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
919     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
920 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
921     followed by two other characters. The octal or hexadecimal representation of
922     "]" can also be used to end a range.
923     .P
924 nigel 63 Ranges operate in the collating sequence of character values. They can also be
925 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
926 nigel 63 mode, ranges can include characters whose values are greater than 255, for
927 nigel 75 example [\ex{100}-\ex{2ff}].
928     .P
929 nigel 63 If a range that includes letters is used when caseless matching is set, it
930     matches the letters in either case. For example, [W-c] is equivalent to
931 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
932 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
933 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
934     characters with values greater than 128 only when it is compiled with Unicode
935     property support.
936     .P
937     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
938     in a character class, and add the characters that they match to the class. For
939     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
940 nigel 63 conveniently be used with the upper case character types to specify a more
941     restricted set of characters than the matching lower case type. For example,
942 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
943     .P
944     The only metacharacters that are recognized in character classes are backslash,
945     hyphen (only where it can be interpreted as specifying a range), circumflex
946     (only at the start), opening square bracket (only when it can be interpreted as
947     introducing a POSIX class name - see the next section), and the terminating
948     closing square bracket. However, escaping other non-alphanumeric characters
949     does no harm.
950     .
951     .
952     .SH "POSIX CHARACTER CLASSES"
953 nigel 63 .rs
954     .sp
955 nigel 75 Perl supports the POSIX notation for character classes. This uses names
956 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
957     this notation. For example,
958 nigel 75 .sp
959 nigel 63 [01[:alpha:]%]
960 nigel 75 .sp
961 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
962     are
963 nigel 75 .sp
964 nigel 63 alnum letters and digits
965     alpha letters
966     ascii character codes 0 - 127
967     blank space or tab only
968     cntrl control characters
969 nigel 75 digit decimal digits (same as \ed)
970 nigel 63 graph printing characters, excluding space
971     lower lower case letters
972     print printing characters, including space
973     punct printing characters, excluding letters and digits
974 nigel 75 space white space (not quite the same as \es)
975 nigel 63 upper upper case letters
976 nigel 75 word "word" characters (same as \ew)
977 nigel 63 xdigit hexadecimal digits
978 nigel 75 .sp
979 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
980     space (32). Notice that this list includes the VT character (code 11). This
981 nigel 75 makes "space" different to \es, which does not include VT (for Perl
982 nigel 63 compatibility).
983 nigel 75 .P
984 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
985     5.8. Another Perl extension is negation, which is indicated by a ^ character
986     after the colon. For example,
987 nigel 75 .sp
988 nigel 63 [12[:^digit:]]
989 nigel 75 .sp
990 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
991     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
992     supported, and an error is given if they are encountered.
993 nigel 75 .P
994     In UTF-8 mode, characters with values greater than 128 do not match any of
995 nigel 63 the POSIX character classes.
996 nigel 75 .
997     .
998     .SH "VERTICAL BAR"
999 nigel 63 .rs
1000     .sp
1001     Vertical bar characters are used to separate alternative patterns. For example,
1002     the pattern
1003 nigel 75 .sp
1004 nigel 63 gilbert|sullivan
1005 nigel 75 .sp
1006 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1007 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1008     process tries each alternative in turn, from left to right, and the first one
1009     that succeeds is used. If the alternatives are within a subpattern
1010 nigel 75 .\" HTML <a href="#subpattern">
1011     .\" </a>
1012     (defined below),
1013     .\"
1014     "succeeds" means matching the rest of the main pattern as well as the
1015     alternative in the subpattern.
1016     .
1017     .
1018     .SH "INTERNAL OPTION SETTING"
1019 nigel 63 .rs
1020     .sp
1021     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1022 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1023     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1024     The option letters are
1025 nigel 75 .sp
1026 nigel 63 i for PCRE_CASELESS
1027     m for PCRE_MULTILINE
1028     s for PCRE_DOTALL
1029     x for PCRE_EXTENDED
1030 nigel 75 .sp
1031 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1032     unset these options by preceding the letter with a hyphen, and a combined
1033     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1034     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1035     permitted. If a letter appears both before and after the hyphen, the option is
1036     unset.
1037 nigel 75 .P
1038 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1039     changed in the same way as the Perl-compatible options by using the characters
1040     J, U and X respectively.
1041     .P
1042 ph10 412 When one of these option changes occurs at top level (that is, not inside
1043     subpattern parentheses), the change applies to the remainder of the pattern
1044     that follows. If the change is placed right at the start of a pattern, PCRE
1045     extracts it into the global options (and it will therefore show up in data
1046     extracted by the \fBpcre_fullinfo()\fP function).
1047 nigel 75 .P
1048 nigel 93 An option change within a subpattern (see below for a description of
1049     subpatterns) affects only that part of the current pattern that follows it, so
1050 nigel 75 .sp
1051 nigel 63 (a(?i)b)c
1052 nigel 75 .sp
1053 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1054     By this means, options can be made to have different settings in different
1055     parts of the pattern. Any changes made in one alternative do carry on
1056     into subsequent branches within the same subpattern. For example,
1057 nigel 75 .sp
1058 nigel 63 (a(?i)b|c)
1059 nigel 75 .sp
1060 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1061     branch is abandoned before the option setting. This is because the effects of
1062     option settings happen at compile time. There would be some very weird
1063     behaviour otherwise.
1064 ph10 251 .P
1065     \fBNote:\fP There are other PCRE-specific options that can be set by the
1066     application when the compile or match functions are called. In some cases the
1067 ph10 412 pattern can contain special leading sequences such as (*CRLF) to override what
1068     the application has set or what has been defaulted. Details are given in the
1069     section entitled
1070 ph10 251 .\" HTML <a href="#newlineseq">
1071     .\" </a>
1072     "Newline sequences"
1073     .\"
1074 ph10 416 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1075 ph10 412 mode; this is equivalent to setting the PCRE_UTF8 option.
1076 nigel 75 .
1077     .
1078     .\" HTML <a name="subpattern"></a>
1079 nigel 63 .SH SUBPATTERNS
1080     .rs
1081     .sp
1082     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1083 nigel 75 Turning part of a pattern into a subpattern does two things:
1084     .sp
1085 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1086 nigel 75 .sp
1087 nigel 63 cat(aract|erpillar|)
1088 nigel 75 .sp
1089 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1090 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1091 nigel 75 .sp
1092     2. It sets up the subpattern as a capturing subpattern. This means that, when
1093     the whole pattern matches, that portion of the subject string that matched the
1094     subpattern is passed back to the caller via the \fIovector\fP argument of
1095     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1096     from 1) to obtain numbers for the capturing subpatterns.
1097     .P
1098 nigel 63 For example, if the string "the red king" is matched against the pattern
1099 nigel 75 .sp
1100 nigel 63 the ((red|white) (king|queen))
1101 nigel 75 .sp
1102 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1103     2, and 3, respectively.
1104 nigel 75 .P
1105 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1106     There are often times when a grouping subpattern is required without a
1107     capturing requirement. If an opening parenthesis is followed by a question mark
1108     and a colon, the subpattern does not do any capturing, and is not counted when
1109     computing the number of any subsequent capturing subpatterns. For example, if
1110     the string "the white queen" is matched against the pattern
1111 nigel 75 .sp
1112 nigel 63 the ((?:red|white) (king|queen))
1113 nigel 75 .sp
1114 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1115 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1116 nigel 75 .P
1117 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1118     a non-capturing subpattern, the option letters may appear between the "?" and
1119     the ":". Thus the two patterns
1120 nigel 75 .sp
1121 nigel 63 (?i:saturday|sunday)
1122     (?:(?i)saturday|sunday)
1123 nigel 75 .sp
1124 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1125     from left to right, and options are not reset until the end of the subpattern
1126     is reached, an option setting in one branch does affect subsequent branches, so
1127     the above patterns match "SUNDAY" as well as "Saturday".
1128 nigel 75 .
1129     .
1130 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1131     .rs
1132     .sp
1133 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1134     the same numbers for its capturing parentheses. Such a subpattern starts with
1135     (?| and is itself a non-capturing subpattern. For example, consider this
1136 ph10 175 pattern:
1137     .sp
1138     (?|(Sat)ur|(Sun))day
1139 ph10 182 .sp
1140     Because the two alternatives are inside a (?| group, both sets of capturing
1141     parentheses are numbered one. Thus, when the pattern matches, you can look
1142     at captured substring number one, whichever alternative matched. This construct
1143     is useful when you want to capture part, but not all, of one of a number of
1144     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1145 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1146 ph10 182 buffers that follow the subpattern start after the highest number used in any
1147     branch. The following example is taken from the Perl documentation.
1148 ph10 175 The numbers underneath show in which buffer the captured content will be
1149     stored.
1150     .sp
1151     # before ---------------branch-reset----------- after
1152     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1153     # 1 2 2 3 2 3 4
1154 ph10 182 .sp
1155 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1156     the first one in the pattern with the given number.
1157     .P
1158     An alternative approach to using this "branch reset" feature is to use
1159     duplicate named subpatterns, as described in the next section.
1160     .
1161     .
1162 nigel 75 .SH "NAMED SUBPATTERNS"
1163 nigel 63 .rs
1164     .sp
1165     Identifying capturing parentheses by number is simple, but it can be very hard
1166     to keep track of the numbers in complicated regular expressions. Furthermore,
1167 nigel 75 if an expression is modified, the numbers may change. To help with this
1168 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1169     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1170     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1171     the Perl and the Python syntax.
1172     .P
1173     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1174     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1175 nigel 91 parentheses from other parts of the pattern, such as
1176     .\" HTML <a href="#backreferences">
1177     .\" </a>
1178     backreferences,
1179     .\"
1180     .\" HTML <a href="#recursion">
1181     .\" </a>
1182     recursion,
1183     .\"
1184     and
1185     .\" HTML <a href="#conditions">
1186     .\" </a>
1187     conditions,
1188     .\"
1189     can be made by name as well as by number.
1190 nigel 75 .P
1191 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1192 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1193     if the names were not present. The PCRE API provides function calls for
1194     extracting the name-to-number translation table from a compiled pattern. There
1195     is also a convenience function for extracting a captured substring by name.
1196 nigel 91 .P
1197     By default, a name must be unique within a pattern, but it is possible to relax
1198     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1199     be useful for patterns where only one instance of the named parentheses can
1200     match. Suppose you want to match the name of a weekday, either as a 3-letter
1201     abbreviation or as the full name, and in both cases you want to extract the
1202     abbreviation. This pattern (ignoring the line breaks) does the job:
1203     .sp
1204 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1205     (?<DN>Tue)(?:sday)?|
1206     (?<DN>Wed)(?:nesday)?|
1207     (?<DN>Thu)(?:rsday)?|
1208     (?<DN>Sat)(?:urday)?
1209 nigel 91 .sp
1210     There are five capturing substrings, but only one is ever set after a match.
1211 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1212 ph10 175 subpattern, as described in the previous section.)
1213     .P
1214 nigel 91 The convenience function for extracting the data by name returns the substring
1215 nigel 93 for the first (and in this example, the only) subpattern of that name that
1216 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1217     make a reference to a non-unique named subpattern from elsewhere in the
1218     pattern, the one that corresponds to the lowest number is used. For further
1219     details of the interfaces for handling named subpatterns, see the
1220 nigel 63 .\" HREF
1221 nigel 75 \fBpcreapi\fP
1222 nigel 63 .\"
1223     documentation.
1224 ph10 385 .P
1225     \fBWarning:\fP You cannot use different names to distinguish between two
1226     subpatterns with the same number (see the previous section) because PCRE uses
1227     only the numbers when matching.
1228 nigel 75 .
1229     .
1230 nigel 63 .SH REPETITION
1231     .rs
1232     .sp
1233     Repetition is specified by quantifiers, which can follow any of the following
1234     items:
1235 nigel 75 .sp
1236 nigel 63 a literal data character
1237 nigel 93 the dot metacharacter
1238 nigel 75 the \eC escape sequence
1239     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1240 nigel 93 the \eR escape sequence
1241 nigel 75 an escape such as \ed that matches a single character
1242 nigel 63 a character class
1243     a back reference (see next section)
1244     a parenthesized subpattern (unless it is an assertion)
1245 nigel 75 .sp
1246 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1247     permitted matches, by giving the two numbers in curly brackets (braces),
1248     separated by a comma. The numbers must be less than 65536, and the first must
1249     be less than or equal to the second. For example:
1250 nigel 75 .sp
1251 nigel 63 z{2,4}
1252 nigel 75 .sp
1253 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1254     character. If the second number is omitted, but the comma is present, there is
1255     no upper limit; if the second number and the comma are both omitted, the
1256     quantifier specifies an exact number of required matches. Thus
1257 nigel 75 .sp
1258 nigel 63 [aeiou]{3,}
1259 nigel 75 .sp
1260 nigel 63 matches at least 3 successive vowels, but may match many more, while
1261 nigel 75 .sp
1262     \ed{8}
1263     .sp
1264 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1265     where a quantifier is not allowed, or one that does not match the syntax of a
1266     quantifier, is taken as a literal character. For example, {,6} is not a
1267     quantifier, but a literal string of four characters.
1268 nigel 75 .P
1269 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1270 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1271     which is represented by a two-byte sequence. Similarly, when Unicode property
1272     support is available, \eX{3} matches three Unicode extended sequences, each of
1273     which may be several bytes long (and they may be of different lengths).
1274     .P
1275 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1276 ph10 345 previous item and the quantifier were not present. This may be useful for
1277     subpatterns that are referenced as
1278 ph10 335 .\" HTML <a href="#subpatternsassubroutines">
1279     .\" </a>
1280     subroutines
1281     .\"
1282 ph10 345 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1283 ph10 335 quantifier are omitted from the compiled pattern.
1284 nigel 75 .P
1285 nigel 93 For convenience, the three most common quantifiers have single-character
1286     abbreviations:
1287 nigel 75 .sp
1288 nigel 63 * is equivalent to {0,}
1289     + is equivalent to {1,}
1290     ? is equivalent to {0,1}
1291 nigel 75 .sp
1292 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1293     match no characters with a quantifier that has no upper limit, for example:
1294 nigel 75 .sp
1295 nigel 63 (a?)*
1296 nigel 75 .sp
1297 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1298     such patterns. However, because there are cases where this can be useful, such
1299     patterns are now accepted, but if any repetition of the subpattern does in fact
1300     match no characters, the loop is forcibly broken.
1301 nigel 75 .P
1302 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1303     possible (up to the maximum number of permitted times), without causing the
1304     rest of the pattern to fail. The classic example of where this gives problems
1305 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1306     and within the comment, individual * and / characters may appear. An attempt to
1307     match C comments by applying the pattern
1308     .sp
1309     /\e*.*\e*/
1310     .sp
1311 nigel 63 to the string
1312 nigel 75 .sp
1313     /* first comment */ not comment /* second comment */
1314     .sp
1315 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1316     item.
1317 nigel 75 .P
1318 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1319     greedy, and instead matches the minimum number of times possible, so the
1320     pattern
1321 nigel 75 .sp
1322     /\e*.*?\e*/
1323     .sp
1324 nigel 63 does the right thing with the C comments. The meaning of the various
1325     quantifiers is not otherwise changed, just the preferred number of matches.
1326     Do not confuse this use of question mark with its use as a quantifier in its
1327     own right. Because it has two uses, it can sometimes appear doubled, as in
1328 nigel 75 .sp
1329     \ed??\ed
1330     .sp
1331 nigel 63 which matches one digit by preference, but can match two if that is the only
1332     way the rest of the pattern matches.
1333 nigel 75 .P
1334 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1335 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1336     greedy by following them with a question mark. In other words, it inverts the
1337     default behaviour.
1338 nigel 75 .P
1339 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1340 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1341 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1342 nigel 75 .P
1343 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1344 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1345 nigel 63 implicitly anchored, because whatever follows will be tried against every
1346     character position in the subject string, so there is no point in retrying the
1347     overall match at any position after the first. PCRE normally treats such a
1348 nigel 75 pattern as though it were preceded by \eA.
1349     .P
1350 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1351     worth setting PCRE_DOTALL in order to obtain this optimization, or
1352     alternatively using ^ to indicate anchoring explicitly.
1353 nigel 75 .P
1354 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1355     is inside capturing parentheses that are the subject of a backreference
1356 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1357     succeeds. Consider, for example:
1358 nigel 75 .sp
1359     (.*)abc\e1
1360     .sp
1361 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1362     this reason, such a pattern is not implicitly anchored.
1363 nigel 75 .P
1364 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1365     that matched the final iteration. For example, after
1366 nigel 75 .sp
1367     (tweedle[dume]{3}\es*)+
1368     .sp
1369 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1370     "tweedledee". However, if there are nested capturing subpatterns, the
1371     corresponding captured values may have been set in previous iterations. For
1372     example, after
1373 nigel 75 .sp
1374 nigel 63 /(a|(b))+/
1375 nigel 75 .sp
1376 nigel 63 matches "aba" the value of the second captured substring is "b".
1377 nigel 75 .
1378     .
1379     .\" HTML <a name="atomicgroup"></a>
1380     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1381 nigel 63 .rs
1382     .sp
1383 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1384     repetition, failure of what follows normally causes the repeated item to be
1385     re-evaluated to see if a different number of repeats allows the rest of the
1386     pattern to match. Sometimes it is useful to prevent this, either to change the
1387     nature of the match, or to cause it fail earlier than it otherwise might, when
1388     the author of the pattern knows there is no point in carrying on.
1389 nigel 75 .P
1390     Consider, for example, the pattern \ed+foo when applied to the subject line
1391     .sp
1392 nigel 63 123456bar
1393 nigel 75 .sp
1394 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1395 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1396 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1397     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1398     that once a subpattern has matched, it is not to be re-evaluated in this way.
1399 nigel 75 .P
1400 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1401 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1402     special parenthesis, starting with (?> as in this example:
1403 nigel 75 .sp
1404     (?>\ed+)foo
1405     .sp
1406 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1407     it has matched, and a failure further into the pattern is prevented from
1408     backtracking into it. Backtracking past it to previous items, however, works as
1409     normal.
1410 nigel 75 .P
1411 nigel 63 An alternative description is that a subpattern of this type matches the string
1412     of characters that an identical standalone pattern would match, if anchored at
1413     the current point in the subject string.
1414 nigel 75 .P
1415 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1416     the above example can be thought of as a maximizing repeat that must swallow
1417 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1418 nigel 63 number of digits they match in order to make the rest of the pattern match,
1419 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1420     .P
1421 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1422     subpatterns, and can be nested. However, when the subpattern for an atomic
1423     group is just a single repeated item, as in the example above, a simpler
1424     notation, called a "possessive quantifier" can be used. This consists of an
1425     additional + character following a quantifier. Using this notation, the
1426     previous example can be rewritten as
1427 nigel 75 .sp
1428     \ed++foo
1429     .sp
1430 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1431     example:
1432     .sp
1433     (abc|xyz){2,3}+
1434     .sp
1435 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1436     option is ignored. They are a convenient notation for the simpler forms of
1437 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1438     quantifier and the equivalent atomic group, though there may be a performance
1439     difference; possessive quantifiers should be slightly faster.
1440 nigel 75 .P
1441 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1442     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1443     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1444     package, and PCRE copied it from there. It ultimately found its way into Perl
1445     at release 5.10.
1446 nigel 75 .P
1447 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1448     pattern constructs. For example, the sequence A+B is treated as A++B because
1449     there is no point in backtracking into a sequence of A's when B must follow.
1450     .P
1451 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1452     be repeated an unlimited number of times, the use of an atomic group is the
1453     only way to avoid some failing matches taking a very long time indeed. The
1454     pattern
1455 nigel 75 .sp
1456     (\eD+|<\ed+>)*[!?]
1457     .sp
1458 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1459     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1460     quickly. However, if it is applied to
1461 nigel 75 .sp
1462 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1463 nigel 75 .sp
1464 nigel 63 it takes a long time before reporting failure. This is because the string can
1465 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1466     large number of ways, and all have to be tried. (The example uses [!?] rather
1467     than a single character at the end, because both PCRE and Perl have an
1468     optimization that allows for fast failure when a single character is used. They
1469     remember the last single character that is required for a match, and fail early
1470     if it is not present in the string.) If the pattern is changed so that it uses
1471     an atomic group, like this:
1472     .sp
1473     ((?>\eD+)|<\ed+>)*[!?]
1474     .sp
1475 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1476 nigel 75 .
1477     .
1478     .\" HTML <a name="backreferences"></a>
1479     .SH "BACK REFERENCES"
1480 nigel 63 .rs
1481     .sp
1482     Outside a character class, a backslash followed by a digit greater than 0 (and
1483     possibly further digits) is a back reference to a capturing subpattern earlier
1484     (that is, to its left) in the pattern, provided there have been that many
1485     previous capturing left parentheses.
1486 nigel 75 .P
1487 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1488     always taken as a back reference, and causes an error only if there are not
1489     that many capturing left parentheses in the entire pattern. In other words, the
1490     parentheses that are referenced need not be to the left of the reference for
1491 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1492     when a repetition is involved and the subpattern to the right has participated
1493     in an earlier iteration.
1494     .P
1495 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1496     whose number is 10 or more using this syntax because a sequence such as \e50 is
1497     interpreted as a character defined in octal. See the subsection entitled
1498 nigel 91 "Non-printing characters"
1499 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1500     .\" </a>
1501     above
1502     .\"
1503 nigel 93 for further details of the handling of digits following a backslash. There is
1504     no such problem when named parentheses are used. A back reference to any
1505     subpattern is possible using named parentheses (see below).
1506 nigel 75 .P
1507 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1508     backslash is to use the \eg escape sequence, which is a feature introduced in
1509 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1510     number, optionally enclosed in braces. These examples are all identical:
1511 nigel 93 .sp
1512     (ring), \e1
1513     (ring), \eg1
1514     (ring), \eg{1}
1515     .sp
1516 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1517     is present in the older syntax. It is also useful when literal digits follow
1518     the reference. A negative number is a relative reference. Consider this
1519     example:
1520 nigel 93 .sp
1521     (abc(def)ghi)\eg{-1}
1522     .sp
1523     The sequence \eg{-1} is a reference to the most recently started capturing
1524     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1525     would be equivalent to \e1. The use of relative references can be helpful in
1526     long patterns, and also in patterns that are created by joining together
1527     fragments that contain references within themselves.
1528     .P
1529 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1530     the current subject string, rather than anything matching the subpattern
1531     itself (see
1532     .\" HTML <a href="#subpatternsassubroutines">
1533     .\" </a>
1534     "Subpatterns as subroutines"
1535     .\"
1536     below for a way of doing that). So the pattern
1537 nigel 75 .sp
1538     (sens|respons)e and \e1ibility
1539     .sp
1540 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1541     "sense and responsibility". If caseful matching is in force at the time of the
1542     back reference, the case of letters is relevant. For example,
1543 nigel 75 .sp
1544     ((?i)rah)\es+\e1
1545     .sp
1546 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1547     capturing subpattern is matched caselessly.
1548 nigel 75 .P
1549 ph10 171 There are several different ways of writing back references to named
1550     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1551     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1552     back reference syntax, in which \eg can be used for both numeric and named
1553     references, is also supported. We could rewrite the above example in any of
1554 nigel 93 the following ways:
1555 nigel 75 .sp
1556 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1557 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1558 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1559 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1560 nigel 75 .sp
1561 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1562     after the reference.
1563     .P
1564 nigel 63 There may be more than one back reference to the same subpattern. If a
1565     subpattern has not actually been used in a particular match, any back
1566     references to it always fail. For example, the pattern
1567 nigel 75 .sp
1568     (a|(bc))\e2
1569     .sp
1570 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1571     many capturing parentheses in a pattern, all digits following the backslash are
1572     taken as part of a potential back reference number. If the pattern continues
1573     with a digit character, some delimiter must be used to terminate the back
1574     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1575 nigel 75 Otherwise an empty comment (see
1576     .\" HTML <a href="#comments">
1577     .\" </a>
1578     "Comments"
1579     .\"
1580     below) can be used.
1581     .P
1582 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1583 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1584 nigel 63 However, such references can be useful inside repeated subpatterns. For
1585     example, the pattern
1586 nigel 75 .sp
1587     (a|b\e1)+
1588     .sp
1589 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1590     the subpattern, the back reference matches the character string corresponding
1591     to the previous iteration. In order for this to work, the pattern must be such
1592     that the first iteration does not need to match the back reference. This can be
1593     done using alternation, as in the example above, or by a quantifier with a
1594     minimum of zero.
1595 nigel 75 .
1596     .
1597     .\" HTML <a name="bigassertions"></a>
1598 nigel 63 .SH ASSERTIONS
1599     .rs
1600     .sp
1601     An assertion is a test on the characters following or preceding the current
1602     matching point that does not actually consume any characters. The simple
1603 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1604     .\" HTML <a href="#smallassertions">
1605     .\" </a>
1606     above.
1607     .\"
1608     .P
1609 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1610     those that look ahead of the current position in the subject string, and those
1611 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1612     except that it does not cause the current matching position to be changed.
1613     .P
1614     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1615     because it makes no sense to assert the same thing several times. If any kind
1616     of assertion contains capturing subpatterns within it, these are counted for
1617     the purposes of numbering the capturing subpatterns in the whole pattern.
1618     However, substring capturing is carried out only for positive assertions,
1619     because it does not make sense for negative assertions.
1620     .
1621     .
1622     .SS "Lookahead assertions"
1623     .rs
1624     .sp
1625 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1626     negative assertions. For example,
1627 nigel 75 .sp
1628     \ew+(?=;)
1629     .sp
1630 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1631     the match, and
1632 nigel 75 .sp
1633 nigel 63 foo(?!bar)
1634 nigel 75 .sp
1635 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1636     apparently similar pattern
1637 nigel 75 .sp
1638 nigel 63 (?!foo)bar
1639 nigel 75 .sp
1640 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1641     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1642     (?!foo) is always true when the next three characters are "bar". A
1643 nigel 75 lookbehind assertion is needed to achieve the other effect.
1644     .P
1645 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1646     convenient way to do it is with (?!) because an empty string always matches, so
1647     an assertion that requires there not to be an empty string must always fail.
1648 nigel 75 .
1649     .
1650     .\" HTML <a name="lookbehind"></a>
1651     .SS "Lookbehind assertions"
1652     .rs
1653     .sp
1654 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1655     negative assertions. For example,
1656 nigel 75 .sp
1657 nigel 63 (?<!foo)bar
1658 nigel 75 .sp
1659 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1660     a lookbehind assertion are restricted such that all the strings it matches must
1661 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1662     do not all have to have the same fixed length. Thus
1663 nigel 75 .sp
1664 nigel 63 (?<=bullock|donkey)
1665 nigel 75 .sp
1666 nigel 63 is permitted, but
1667 nigel 75 .sp
1668 nigel 63 (?<!dogs?|cats?)
1669 nigel 75 .sp
1670 nigel 63 causes an error at compile time. Branches that match different length strings
1671     are permitted only at the top level of a lookbehind assertion. This is an
1672     extension compared with Perl (at least for 5.8), which requires all branches to
1673     match the same length of string. An assertion such as
1674 nigel 75 .sp
1675 nigel 63 (?<=ab(c|de))
1676 nigel 75 .sp
1677 nigel 63 is not permitted, because its single top-level branch can match two different
1678     lengths, but it is acceptable if rewritten to use two top-level branches:
1679 nigel 75 .sp
1680 nigel 63 (?<=abc|abde)
1681 nigel 75 .sp
1682 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1683 ph10 168 .\" HTML <a href="#resetmatchstart">
1684     .\" </a>
1685     (see above)
1686     .\"
1687     can be used instead of a lookbehind assertion; this is not restricted to a
1688     fixed-length.
1689     .P
1690 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1691 nigel 93 temporarily move the current position back by the fixed length and then try to
1692 nigel 63 match. If there are insufficient characters before the current position, the
1693 nigel 93 assertion fails.
1694 nigel 75 .P
1695     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1696 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1697 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1698     different numbers of bytes, are also not permitted.
1699 nigel 75 .P
1700 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1701     specify efficient matching at the end of the subject string. Consider a simple
1702     pattern such as
1703 nigel 75 .sp
1704 nigel 63 abcd$
1705 nigel 75 .sp
1706 nigel 63 when applied to a long string that does not match. Because matching proceeds
1707     from left to right, PCRE will look for each "a" in the subject and then see if
1708     what follows matches the rest of the pattern. If the pattern is specified as
1709 nigel 75 .sp
1710 nigel 63 ^.*abcd$
1711 nigel 75 .sp
1712 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1713     there is no following "a"), it backtracks to match all but the last character,
1714     then all but the last two characters, and so on. Once again the search for "a"
1715     covers the entire string, from right to left, so we are no better off. However,
1716     if the pattern is written as
1717 nigel 75 .sp
1718 nigel 63 ^.*+(?<=abcd)
1719 nigel 75 .sp
1720 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1721 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1722     characters. If it fails, the match fails immediately. For long strings, this
1723     approach makes a significant difference to the processing time.
1724 nigel 75 .
1725     .
1726     .SS "Using multiple assertions"
1727     .rs
1728     .sp
1729 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1730 nigel 75 .sp
1731     (?<=\ed{3})(?<!999)foo
1732     .sp
1733 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1734     the assertions is applied independently at the same point in the subject
1735     string. First there is a check that the previous three characters are all
1736     digits, and then there is a check that the same three characters are not "999".
1737 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1738 nigel 63 of which are digits and the last three of which are not "999". For example, it
1739     doesn't match "123abcfoo". A pattern to do that is
1740 nigel 75 .sp
1741     (?<=\ed{3}...)(?<!999)foo
1742     .sp
1743 nigel 63 This time the first assertion looks at the preceding six characters, checking
1744     that the first three are digits, and then the second assertion checks that the
1745     preceding three characters are not "999".
1746 nigel 75 .P
1747 nigel 63 Assertions can be nested in any combination. For example,
1748 nigel 75 .sp
1749 nigel 63 (?<=(?<!foo)bar)baz
1750 nigel 75 .sp
1751 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1752     preceded by "foo", while
1753 nigel 75 .sp
1754     (?<=\ed{3}(?!999)...)foo
1755     .sp
1756     is another pattern that matches "foo" preceded by three digits and any three
1757 nigel 63 characters that are not "999".
1758 nigel 75 .
1759     .
1760 nigel 91 .\" HTML <a name="conditions"></a>
1761 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1762 nigel 63 .rs
1763     .sp
1764     It is possible to cause the matching process to obey a subpattern
1765     conditionally or to choose between two alternative subpatterns, depending on
1766     the result of an assertion, or whether a previous capturing subpattern matched
1767     or not. The two possible forms of conditional subpattern are
1768 nigel 75 .sp
1769 nigel 63 (?(condition)yes-pattern)
1770     (?(condition)yes-pattern|no-pattern)
1771 nigel 75 .sp
1772 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1773     no-pattern (if present) is used. If there are more than two alternatives in the
1774     subpattern, a compile-time error occurs.
1775 nigel 75 .P
1776 nigel 93 There are four kinds of condition: references to subpatterns, references to
1777     recursion, a pseudo-condition called DEFINE, and assertions.
1778     .
1779     .SS "Checking for a used subpattern by number"
1780     .rs
1781     .sp
1782     If the text between the parentheses consists of a sequence of digits, the
1783     condition is true if the capturing subpattern of that number has previously
1784 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1785 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1786 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1787 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1788     refer to subsequent groups with constructs such as (?(+2).
1789 nigel 91 .P
1790     Consider the following pattern, which contains non-significant white space to
1791     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1792     three parts for ease of discussion:
1793 nigel 75 .sp
1794     ( \e( )? [^()]+ (?(1) \e) )
1795     .sp
1796 nigel 63 The first part matches an optional opening parenthesis, and if that
1797     character is present, sets it as the first captured substring. The second part
1798     matches one or more characters that are not parentheses. The third part is a
1799     conditional subpattern that tests whether the first set of parentheses matched
1800     or not. If they did, that is, if subject started with an opening parenthesis,
1801     the condition is true, and so the yes-pattern is executed and a closing
1802     parenthesis is required. Otherwise, since no-pattern is not present, the
1803     subpattern matches nothing. In other words, this pattern matches a sequence of
1804 nigel 93 non-parentheses, optionally enclosed in parentheses.
1805 ph10 167 .P
1806 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1807 ph10 167 reference:
1808     .sp
1809     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1810     .sp
1811     This makes the fragment independent of the parentheses in the larger pattern.
1812 nigel 93 .
1813     .SS "Checking for a used subpattern by name"
1814     .rs
1815 nigel 91 .sp
1816 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1817     subpattern by name. For compatibility with earlier versions of PCRE, which had
1818     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1819     there is a possible ambiguity with this syntax, because subpattern names may
1820     consist entirely of digits. PCRE looks first for a named subpattern; if it
1821     cannot find one and the name consists entirely of digits, PCRE looks for a
1822     subpattern of that number, which must be greater than zero. Using subpattern
1823     names that consist entirely of digits is not recommended.
1824     .P
1825     Rewriting the above example to use a named subpattern gives this:
1826 nigel 91 .sp
1827 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1828     .sp
1829     .
1830     .SS "Checking for pattern recursion"
1831     .rs
1832     .sp
1833 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1834 nigel 93 the condition is true if a recursive call to the whole pattern or any
1835     subpattern has been made. If digits or a name preceded by ampersand follow the
1836     letter R, for example:
1837     .sp
1838     (?(R3)...) or (?(R&name)...)
1839     .sp
1840     the condition is true if the most recent recursion is into the subpattern whose
1841     number or name is given. This condition does not check the entire recursion
1842     stack.
1843 nigel 75 .P
1844 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1845     patterns are described below.
1846     .
1847     .SS "Defining subpatterns for use by reference only"
1848     .rs
1849     .sp
1850     If the condition is the string (DEFINE), and there is no subpattern with the
1851     name DEFINE, the condition is always false. In this case, there may be only one
1852     alternative in the subpattern. It is always skipped if control reaches this
1853     point in the pattern; the idea of DEFINE is that it can be used to define
1854     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1855     is described below.) For example, a pattern to match an IPv4 address could be
1856     written like this (ignore whitespace and line breaks):
1857     .sp
1858     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1859     \eb (?&byte) (\e.(?&byte)){3} \eb
1860     .sp
1861     The first part of the pattern is a DEFINE group inside which a another group
1862     named "byte" is defined. This matches an individual component of an IPv4
1863     address (a number less than 256). When matching takes place, this part of the
1864     pattern is skipped because DEFINE acts like a false condition.
1865     .P
1866     The rest of the pattern uses references to the named group to match the four
1867     dot-separated components of an IPv4 address, insisting on a word boundary at
1868     each end.
1869     .
1870     .SS "Assertion conditions"
1871     .rs
1872     .sp
1873     If the condition is not in any of the above formats, it must be an assertion.
1874 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1875     this pattern, again containing non-significant white space, and with the two
1876     alternatives on the second line:
1877 nigel 75 .sp
1878 nigel 63 (?(?=[^a-z]*[a-z])
1879 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1880     .sp
1881 nigel 63 The condition is a positive lookahead assertion that matches an optional
1882     sequence of non-letters followed by a letter. In other words, it tests for the
1883     presence of at least one letter in the subject. If a letter is found, the
1884     subject is matched against the first alternative; otherwise it is matched
1885     against the second. This pattern matches strings in one of the two forms
1886     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1887 nigel 75 .
1888     .
1889     .\" HTML <a name="comments"></a>
1890 nigel 63 .SH COMMENTS
1891     .rs
1892     .sp
1893 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1894 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1895     that make up a comment play no part in the pattern matching at all.
1896 nigel 75 .P
1897 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1898 nigel 91 character class introduces a comment that continues to immediately after the
1899     next newline in the pattern.
1900 nigel 75 .
1901     .
1902 nigel 91 .\" HTML <a name="recursion"></a>
1903 nigel 75 .SH "RECURSIVE PATTERNS"
1904 nigel 63 .rs
1905     .sp
1906     Consider the problem of matching a string in parentheses, allowing for
1907     unlimited nested parentheses. Without the use of recursion, the best that can
1908     be done is to use a pattern that matches up to some fixed depth of nesting. It
1909 nigel 93 is not possible to handle an arbitrary nesting depth.
1910     .P
1911     For some time, Perl has provided a facility that allows regular expressions to
1912     recurse (amongst other things). It does this by interpolating Perl code in the
1913     expression at run time, and the code can refer to the expression itself. A Perl
1914     pattern using code interpolation to solve the parentheses problem can be
1915     created like this:
1916 nigel 75 .sp
1917     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1918     .sp
1919 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1920 nigel 93 recursively to the pattern in which it appears.
1921 nigel 75 .P
1922 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1923     supports special syntax for recursion of the entire pattern, and also for
1924     individual subpattern recursion. After its introduction in PCRE and Python,
1925     this kind of recursion was introduced into Perl at release 5.10.
1926 nigel 75 .P
1927 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1928     closing parenthesis is a recursive call of the subpattern of the given number,
1929     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1930     call, which is described in the next section.) The special item (?R) or (?0) is
1931     a recursive call of the entire regular expression.
1932 nigel 87 .P
1933 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1934     treated as an atomic group. That is, once it has matched some of the subject
1935     string, it is never re-entered, even if it contains untried alternatives and
1936     there is a subsequent matching failure.
1937     .P
1938 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1939     PCRE_EXTENDED option is set so that white space is ignored):
1940 nigel 75 .sp
1941     \e( ( (?>[^()]+) | (?R) )* \e)
1942     .sp
1943 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1944     substrings which can either be a sequence of non-parentheses, or a recursive
1945 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1946 nigel 63 Finally there is a closing parenthesis.
1947 nigel 75 .P
1948 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1949     pattern, so instead you could use this:
1950 nigel 75 .sp
1951     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1952     .sp
1953 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1954 ph10 172 them instead of the whole pattern.
1955 ph10 166 .P
1956     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1957     is made easier by the use of relative references. (A Perl 5.10 feature.)
1958     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1959     most recently opened parentheses preceding the recursion. In other words, a
1960     negative number counts capturing parentheses leftwards from the point at which
1961     it is encountered.
1962     .P
1963     It is also possible to refer to subsequently opened parentheses, by writing
1964     references such as (?+2). However, these cannot be recursive because the
1965     reference is not inside the parentheses that are referenced. They are always
1966     "subroutine" calls, as described in the next section.
1967     .P
1968     An alternative approach is to use named parentheses instead. The Perl syntax
1969     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1970     could rewrite the above example as follows:
1971 nigel 75 .sp
1972 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1973 nigel 75 .sp
1974 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1975 ph10 172 used.
1976 ph10 166 .P
1977     This particular example pattern that we have been looking at contains nested
1978     unlimited repeats, and so the use of atomic grouping for matching strings of
1979     non-parentheses is important when applying the pattern to strings that do not
1980     match. For example, when this pattern is applied to
1981 nigel 75 .sp
1982 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1983 nigel 75 .sp
1984 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1985     the match runs for a very long time indeed because there are so many different
1986     ways the + and * repeats can carve up the subject, and all have to be tested
1987     before failure can be reported.
1988 nigel 75 .P
1989 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1990     from the outermost level of the recursion at which the subpattern value is set.
1991     If you want to obtain intermediate values, a callout function can be used (see
1992 nigel 93 below and the
1993 nigel 63 .\" HREF
1994 nigel 75 \fBpcrecallout\fP
1995 nigel 63 .\"
1996     documentation). If the pattern above is matched against
1997 nigel 75 .sp
1998 nigel 63 (ab(cd)ef)
1999 nigel 75 .sp
2000 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
2001     on at the top level. If additional parentheses are added, giving
2002 nigel 75 .sp
2003     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
2004 nigel 63 ^ ^
2005     ^ ^
2006 nigel 75 .sp
2007 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
2008     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
2009     has to obtain extra memory to store data during a recursion, which it does by
2010 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
2011 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2012 nigel 75 .P
2013 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2014     Consider this pattern, which matches text in angle brackets, allowing for
2015     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2016     recursing), whereas any characters are permitted at the outer level.
2017 nigel 75 .sp
2018     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2019     .sp
2020 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2021     different alternatives for the recursive and non-recursive cases. The (?R) item
2022     is the actual recursive call.
2023 nigel 75 .
2024     .
2025 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
2026 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
2027 nigel 63 .rs
2028     .sp
2029     If the syntax for a recursive subpattern reference (either by number or by
2030     name) is used outside the parentheses to which it refers, it operates like a
2031 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2032 ph10 166 before or after the reference. A numbered reference can be absolute or
2033     relative, as in these examples:
2034 nigel 75 .sp
2035 ph10 166 (...(absolute)...)...(?2)...
2036     (...(relative)...)...(?-1)...
2037 ph10 172 (...(?+1)...(relative)...
2038 ph10 166 .sp
2039     An earlier example pointed out that the pattern
2040     .sp
2041 nigel 75 (sens|respons)e and \e1ibility
2042     .sp
2043 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2044     "sense and responsibility". If instead the pattern
2045 nigel 75 .sp
2046 nigel 63 (sens|respons)e and (?1)ibility
2047 nigel 75 .sp
2048 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2049 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2050 nigel 87 .P
2051     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2052     group. That is, once it has matched some of the subject string, it is never
2053     re-entered, even if it contains untried alternatives and there is a subsequent
2054     matching failure.
2055 nigel 93 .P
2056     When a subpattern is used as a subroutine, processing options such as
2057     case-independence are fixed when the subpattern is defined. They cannot be
2058     changed for different calls. For example, consider this pattern:
2059     .sp
2060 ph10 166 (abc)(?i:(?-1))
2061 nigel 93 .sp
2062     It matches "abcabc". It does not match "abcABC" because the change of
2063     processing option does not affect the called subpattern.
2064 nigel 75 .
2065     .
2066 ph10 333 .\" HTML <a name="onigurumasubroutines"></a>
2067     .SH "ONIGURUMA SUBROUTINE SYNTAX"
2068     .rs
2069     .sp
2070 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2071     a number enclosed either in angle brackets or single quotes, is an alternative
2072     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2073 ph10 333 are two of the examples used above, rewritten using this syntax:
2074     .sp
2075     (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2076     (sens|respons)e and \eg'1'ibility
2077     .sp
2078 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a
2079 ph10 333 plus or a minus sign it is taken as a relative reference. For example:
2080     .sp
2081     (abc)(?i:\eg<-1>)
2082     .sp
2083 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2084 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
2085     .
2086     .
2087 nigel 63 .SH CALLOUTS
2088     .rs
2089     .sp
2090     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2091     code to be obeyed in the middle of matching a regular expression. This makes it
2092     possible, amongst other things, to extract different substrings that match the
2093     same pair of parentheses when there is a repetition.
2094 nigel 75 .P
2095 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2096     code. The feature is called "callout". The caller of PCRE provides an external
2097 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2098 nigel 63 By default, this variable contains NULL, which disables all calling out.
2099 nigel 75 .P
2100 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2101     function is to be called. If you want to identify different callout points, you
2102     can put a number less than 256 after the letter C. The default value is zero.
2103     For example, this pattern has two callout points:
2104 nigel 75 .sp
2105 ph10 155 (?C1)abc(?C2)def
2106 nigel 75 .sp
2107     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2108     automatically installed before each item in the pattern. They are all numbered
2109     255.
2110     .P
2111     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2112 nigel 63 set), the external function is called. It is provided with the number of the
2113 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2114     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2115     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2116     description of the interface to the callout function is given in the
2117 nigel 63 .\" HREF
2118 nigel 75 \fBpcrecallout\fP
2119 nigel 63 .\"
2120     documentation.
2121 nigel 93 .
2122     .
2123 ph10 235 .SH "BACKTRACKING CONTROL"
2124 ph10 210 .rs
2125     .sp
2126 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2127 ph10 210 are described in the Perl documentation as "experimental and subject to change
2128 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2129     production code should be noted to avoid problems during upgrades." The same
2130 ph10 210 remarks apply to the PCRE features described in this section.
2131     .P
2132 ph10 341 Since these verbs are specifically related to backtracking, most of them can be
2133     used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2134 ph10 345 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2135 ph10 341 failing negative assertion, they cause an error if encountered by
2136 ph10 210 \fBpcre_dfa_exec()\fP.
2137     .P
2138 ph10 445 If any of these verbs are used in an assertion subpattern, their effect is
2139     confined to that subpattern; it does not extend to the surrounding pattern.
2140     Note that assertion subpatterns are processed as anchored at the point where
2141     they are tested.
2142     .P
2143 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2144 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
2145     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2146     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2147     are two kinds:
2148     .
2149     .SS "Verbs that act immediately"
2150     .rs
2151     .sp
2152     The following verbs act as soon as they are encountered:
2153     .sp
2154     (*ACCEPT)
2155     .sp
2156     This verb causes the match to end successfully, skipping the remainder of the
2157 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2158 ph10 447 immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
2159     is captured. (This feature was added to PCRE at release 8.00.) For example:
2160 ph10 210 .sp
2161 ph10 447 A((?:A|B(*ACCEPT)|C)D)
2162 ph10 210 .sp
2163 ph10 447 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2164     the outer parentheses.
2165 ph10 210 .sp
2166     (*FAIL) or (*F)
2167     .sp
2168 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2169 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2170     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2171     Perl features that are not present in PCRE. The nearest equivalent is the
2172     callout feature, as for example in this pattern:
2173     .sp
2174     a+(?C)(*FAIL)
2175     .sp
2176 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2177     each backtrack happens (in this example, 10 times).
2178 ph10 210 .
2179     .SS "Verbs that act after backtracking"
2180     .rs
2181     .sp
2182 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2183     with what follows, but if there is no subsequent match, a failure is forced.
2184 ph10 210 The verbs differ in exactly what kind of failure occurs.
2185     .sp
2186     (*COMMIT)
2187     .sp
2188 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2189 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2190     a match by advancing the start point take place. Once (*COMMIT) has been
2191 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2192 ph10 210 starting point, or not at all. For example:
2193     .sp
2194     a+(*COMMIT)b
2195     .sp
2196 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2197 ph10 210 dynamic anchor, or "I've started, so I must finish."
2198     .sp
2199     (*PRUNE)
2200     .sp
2201 ph10 211 This verb causes the match to fail at the current position if the rest of the
2202     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2203 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2204     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2205     if there is no match to the right, backtracking cannot cross (*PRUNE).
2206 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2207 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2208     be expressed in any other way.
2209     .sp
2210     (*SKIP)
2211     .sp
2212 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2213 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2214     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2215     was matched leading up to it cannot be part of a successful match. Consider:
2216     .sp
2217     a+(*SKIP)b
2218     .sp
2219 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2220 ph10 210 the first character in the string), the starting point skips on to start the
2221 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2222 ph10 210 effect in this example; although it would suppress backtracking during the
2223     first match attempt, the second attempt would start at the second character
2224     instead of skipping on to "c".
2225     .sp
2226     (*THEN)
2227 ph10 211 .sp
2228 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2229     not match. That is, it cancels pending backtracking, but only within the
2230     current alternation. Its name comes from the observation that it can be used
2231     for a pattern-based if-then-else block:
2232     .sp
2233     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2234     .sp
2235 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2236 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2237 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2238 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2239     .
2240     .
2241 nigel 93 .SH "SEE ALSO"
2242     .rs
2243     .sp
2244     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2245 ph10 99 .
2246     .
2247     .SH AUTHOR
2248     .rs
2249     .sp
2250     .nf
2251     Philip Hazel
2252     University Computing Service
2253     Cambridge CB2 3QH, England.
2254     .fi
2255     .
2256     .
2257     .SH REVISION
2258     .rs
2259     .sp
2260     .nf
2261 ph10 447 Last updated: 15 September 2009
2262 ph10 385 Copyright (c) 1997-2009 University of Cambridge.
2263 ph10 99 .fi

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