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

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