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1 nigel 63 <html>
2     <head>
3     <title>pcrepattern specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcrepattern man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14 ph10 111 <br>
15 nigel 63 <ul>
16     <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 nigel 93 <li><a name="TOC2" href="#SEC2">CHARACTERS AND METACHARACTERS</a>
18     <li><a name="TOC3" href="#SEC3">BACKSLASH</a>
19     <li><a name="TOC4" href="#SEC4">CIRCUMFLEX AND DOLLAR</a>
20     <li><a name="TOC5" href="#SEC5">FULL STOP (PERIOD, DOT)</a>
21     <li><a name="TOC6" href="#SEC6">MATCHING A SINGLE BYTE</a>
22     <li><a name="TOC7" href="#SEC7">SQUARE BRACKETS AND CHARACTER CLASSES</a>
23     <li><a name="TOC8" href="#SEC8">POSIX CHARACTER CLASSES</a>
24     <li><a name="TOC9" href="#SEC9">VERTICAL BAR</a>
25     <li><a name="TOC10" href="#SEC10">INTERNAL OPTION SETTING</a>
26     <li><a name="TOC11" href="#SEC11">SUBPATTERNS</a>
27     <li><a name="TOC12" href="#SEC12">NAMED SUBPATTERNS</a>
28     <li><a name="TOC13" href="#SEC13">REPETITION</a>
30     <li><a name="TOC15" href="#SEC15">BACK REFERENCES</a>
31     <li><a name="TOC16" href="#SEC16">ASSERTIONS</a>
32     <li><a name="TOC17" href="#SEC17">CONDITIONAL SUBPATTERNS</a>
33     <li><a name="TOC18" href="#SEC18">COMMENTS</a>
34     <li><a name="TOC19" href="#SEC19">RECURSIVE PATTERNS</a>
35     <li><a name="TOC20" href="#SEC20">SUBPATTERNS AS SUBROUTINES</a>
36     <li><a name="TOC21" href="#SEC21">CALLOUTS</a>
37     <li><a name="TOC22" href="#SEC22">SEE ALSO</a>
38 ph10 99 <li><a name="TOC23" href="#SEC23">AUTHOR</a>
39     <li><a name="TOC24" href="#SEC24">REVISION</a>
40 nigel 63 </ul>
41     <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
42     <P>
43     The syntax and semantics of the regular expressions supported by PCRE are
44     described below. Regular expressions are also described in the Perl
45 nigel 75 documentation and in a number of books, some of which have copious examples.
46     Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
47     regular expressions in great detail. This description of PCRE's regular
48     expressions is intended as reference material.
49 nigel 63 </P>
50     <P>
51 nigel 75 The original operation of PCRE was on strings of one-byte characters. However,
52     there is now also support for UTF-8 character strings. To use this, you must
53     build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
54     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
55     places below. There is also a summary of UTF-8 features in the
56 nigel 63 <a href="pcre.html#utf8support">section on UTF-8 support</a>
57     in the main
58     <a href="pcre.html"><b>pcre</b></a>
59     page.
60     </P>
61     <P>
62 nigel 77 The remainder of this document discusses the patterns that are supported by
63     PCRE when its main matching function, <b>pcre_exec()</b>, is used.
64     From release 6.0, PCRE offers a second matching function,
65     <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
66     Perl-compatible. The advantages and disadvantages of the alternative function,
67     and how it differs from the normal function, are discussed in the
68     <a href="pcrematching.html"><b>pcrematching</b></a>
69     page.
70     </P>
71 nigel 93 <br><a name="SEC2" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
72 nigel 77 <P>
73 nigel 63 A regular expression is a pattern that is matched against a subject string from
74     left to right. Most characters stand for themselves in a pattern, and match the
75     corresponding characters in the subject. As a trivial example, the pattern
76     <pre>
77     The quick brown fox
78 nigel 75 </pre>
79 nigel 77 matches a portion of a subject string that is identical to itself. When
80     caseless matching is specified (the PCRE_CASELESS option), letters are matched
81     independently of case. In UTF-8 mode, PCRE always understands the concept of
82     case for characters whose values are less than 128, so caseless matching is
83     always possible. For characters with higher values, the concept of case is
84     supported if PCRE is compiled with Unicode property support, but not otherwise.
85     If you want to use caseless matching for characters 128 and above, you must
86     ensure that PCRE is compiled with Unicode property support as well as with
87     UTF-8 support.
88     </P>
89     <P>
90     The power of regular expressions comes from the ability to include alternatives
91     and repetitions in the pattern. These are encoded in the pattern by the use of
92 nigel 75 <i>metacharacters</i>, which do not stand for themselves but instead are
93 nigel 63 interpreted in some special way.
94     </P>
95     <P>
96 nigel 75 There are two different sets of metacharacters: those that are recognized
97 nigel 63 anywhere in the pattern except within square brackets, and those that are
98 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
99     are as follows:
100 nigel 63 <pre>
101     \ general escape character with several uses
102     ^ assert start of string (or line, in multiline mode)
103     $ assert end of string (or line, in multiline mode)
104     . match any character except newline (by default)
105     [ start character class definition
106     | start of alternative branch
107     ( start subpattern
108     ) end subpattern
109     ? extends the meaning of (
110     also 0 or 1 quantifier
111     also quantifier minimizer
112     * 0 or more quantifier
113     + 1 or more quantifier
114     also "possessive quantifier"
115     { start min/max quantifier
116 nigel 75 </pre>
117 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
118 nigel 75 a character class the only metacharacters are:
119 nigel 63 <pre>
120     \ general escape character
121     ^ negate the class, but only if the first character
122     - indicates character range
123 nigel 75 [ POSIX character class (only if followed by POSIX syntax)
124 nigel 63 ] terminates the character class
125 nigel 75 </pre>
126     The following sections describe the use of each of the metacharacters.
127 nigel 63 </P>
128 nigel 93 <br><a name="SEC3" href="#TOC1">BACKSLASH</a><br>
129 nigel 63 <P>
130     The backslash character has several uses. Firstly, if it is followed by a
131 nigel 91 non-alphanumeric character, it takes away any special meaning that character
132     may have. This use of backslash as an escape character applies both inside and
133 nigel 63 outside character classes.
134     </P>
135     <P>
136     For example, if you want to match a * character, you write \* in the pattern.
137     This escaping action applies whether or not the following character would
138 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
139     non-alphanumeric with backslash to specify that it stands for itself. In
140 nigel 63 particular, if you want to match a backslash, you write \\.
141     </P>
142     <P>
143     If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
144     pattern (other than in a character class) and characters between a # outside
145 nigel 91 a character class and the next newline are ignored. An escaping backslash can
146     be used to include a whitespace or # character as part of the pattern.
147 nigel 63 </P>
148     <P>
149     If you want to remove the special meaning from a sequence of characters, you
150     can do so by putting them between \Q and \E. This is different from Perl in
151     that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
152     Perl, $ and @ cause variable interpolation. Note the following examples:
153     <pre>
154     Pattern PCRE matches Perl matches
155 nigel 75
156     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
157 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
158     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
159 nigel 75 </pre>
160 nigel 63 The \Q...\E sequence is recognized both inside and outside character classes.
161 nigel 75 <a name="digitsafterbackslash"></a></P>
162     <br><b>
163     Non-printing characters
164     </b><br>
165 nigel 63 <P>
166     A second use of backslash provides a way of encoding non-printing characters
167     in patterns in a visible manner. There is no restriction on the appearance of
168     non-printing characters, apart from the binary zero that terminates a pattern,
169     but when a pattern is being prepared by text editing, it is usually easier to
170     use one of the following escape sequences than the binary character it
171     represents:
172     <pre>
173     \a alarm, that is, the BEL character (hex 07)
174     \cx "control-x", where x is any character
175     \e escape (hex 1B)
176     \f formfeed (hex 0C)
177     \n newline (hex 0A)
178     \r carriage return (hex 0D)
179     \t tab (hex 09)
180     \ddd character with octal code ddd, or backreference
181     \xhh character with hex code hh
182 nigel 87 \x{hhh..} character with hex code hhh..
183 nigel 75 </pre>
184 nigel 63 The precise effect of \cx is as follows: if x is a lower case letter, it
185     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
186     Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
187     7B.
188     </P>
189     <P>
190     After \x, from zero to two hexadecimal digits are read (letters can be in
191 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \x{
192     and }, but the value of the character code must be less than 256 in non-UTF-8
193     mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
194     is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{
195     and }, or if there is no terminating }, this form of escape is not recognized.
196     Instead, the initial \x will be interpreted as a basic hexadecimal escape,
197     with no following digits, giving a character whose value is zero.
198 nigel 63 </P>
199     <P>
200     Characters whose value is less than 256 can be defined by either of the two
201 nigel 87 syntaxes for \x. There is no difference in the way they are handled. For
202     example, \xdc is exactly the same as \x{dc}.
203 nigel 63 </P>
204     <P>
205 nigel 91 After \0 up to two further octal digits are read. If there are fewer than two
206     digits, just those that are present are used. Thus the sequence \0\x\07
207     specifies two binary zeros followed by a BEL character (code value 7). Make
208     sure you supply two digits after the initial zero if the pattern character that
209     follows is itself an octal digit.
210 nigel 63 </P>
211     <P>
212     The handling of a backslash followed by a digit other than 0 is complicated.
213     Outside a character class, PCRE reads it and any following digits as a decimal
214     number. If the number is less than 10, or if there have been at least that many
215     previous capturing left parentheses in the expression, the entire sequence is
216     taken as a <i>back reference</i>. A description of how this works is given
217 nigel 75 <a href="#backreferences">later,</a>
218     following the discussion of
219     <a href="#subpattern">parenthesized subpatterns.</a>
220 nigel 63 </P>
221     <P>
222     Inside a character class, or if the decimal number is greater than 9 and there
223     have not been that many capturing subpatterns, PCRE re-reads up to three octal
224 nigel 93 digits following the backslash, and uses them to generate a data character. Any
225 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
226     character specified in octal must be less than \400. In UTF-8 mode, values up
227     to \777 are permitted. For example:
228 nigel 63 <pre>
229     \040 is another way of writing a space
230 nigel 75 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
231 nigel 63 \7 is always a back reference
232 nigel 75 \11 might be a back reference, or another way of writing a tab
233 nigel 63 \011 is always a tab
234     \0113 is a tab followed by the character "3"
235 nigel 75 \113 might be a back reference, otherwise the character with octal code 113
236     \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
237     \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
238     </pre>
239 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
240     zero, because no more than three octal digits are ever read.
241     </P>
242     <P>
243 nigel 91 All the sequences that define a single character value can be used both inside
244     and outside character classes. In addition, inside a character class, the
245     sequence \b is interpreted as the backspace character (hex 08), and the
246 nigel 93 sequences \R and \X are interpreted as the characters "R" and "X",
247     respectively. Outside a character class, these sequences have different
248     meanings
249 nigel 75 <a href="#uniextseq">(see below).</a>
250 nigel 63 </P>
251 nigel 75 <br><b>
252 nigel 93 Absolute and relative back references
253     </b><br>
254     <P>
255     The sequence \g followed by a positive or negative number, optionally enclosed
256     in braces, is an absolute or relative back reference. Back references are
257     discussed
258     <a href="#backreferences">later,</a>
259     following the discussion of
260     <a href="#subpattern">parenthesized subpatterns.</a>
261     </P>
262     <br><b>
263 nigel 75 Generic character types
264     </b><br>
265 nigel 63 <P>
266 nigel 93 Another use of backslash is for specifying generic character types. The
267 nigel 75 following are always recognized:
268 nigel 63 <pre>
269     \d any decimal digit
270     \D any character that is not a decimal digit
271     \s any whitespace character
272     \S any character that is not a whitespace character
273     \w any "word" character
274     \W any "non-word" character
275 nigel 75 </pre>
276 nigel 63 Each pair of escape sequences partitions the complete set of characters into
277     two disjoint sets. Any given character matches one, and only one, of each pair.
278     </P>
279     <P>
280 nigel 75 These character type sequences can appear both inside and outside character
281     classes. They each match one character of the appropriate type. If the current
282     matching point is at the end of the subject string, all of them fail, since
283     there is no character to match.
284 nigel 63 </P>
285     <P>
286     For compatibility with Perl, \s does not match the VT character (code 11).
287     This makes it different from the the POSIX "space" class. The \s characters
288 nigel 91 are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
289     included in a Perl script, \s may match the VT character. In PCRE, it never
290     does.)
291 nigel 63 </P>
292     <P>
293 nigel 75 A "word" character is an underscore or any character less than 256 that is a
294     letter or digit. The definition of letters and digits is controlled by PCRE's
295     low-valued character tables, and may vary if locale-specific matching is taking
296     place (see
297 nigel 63 <a href="pcreapi.html#localesupport">"Locale support"</a>
298     in the
299     <a href="pcreapi.html"><b>pcreapi</b></a>
300 nigel 75 page). For example, in the "fr_FR" (French) locale, some character codes
301     greater than 128 are used for accented letters, and these are matched by \w.
302 nigel 63 </P>
303     <P>
304 nigel 75 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
305     \w, and always match \D, \S, and \W. This is true even when Unicode
306 nigel 87 character property support is available. The use of locales with Unicode is
307     discouraged.
308 nigel 93 </P>
309     <br><b>
310     Newline sequences
311     </b><br>
312     <P>
313     Outside a character class, the escape sequence \R matches any Unicode newline
314     sequence. This is an extension to Perl. In non-UTF-8 mode \R is equivalent to
315     the following:
316     <pre>
317     (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
318     </pre>
319     This is an example of an "atomic group", details of which are given
320     <a href="#atomicgroup">below.</a>
321     This particular group matches either the two-character sequence CR followed by
322     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
323     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
324     line, U+0085). The two-character sequence is treated as a single unit that
325     cannot be split.
326     </P>
327     <P>
328     In UTF-8 mode, two additional characters whose codepoints are greater than 255
329     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
330     Unicode character property support is not needed for these characters to be
331     recognized.
332     </P>
333     <P>
334     Inside a character class, \R matches the letter "R".
335 nigel 75 <a name="uniextseq"></a></P>
336     <br><b>
337     Unicode character properties
338     </b><br>
339     <P>
340     When PCRE is built with Unicode character property support, three additional
341 nigel 87 escape sequences to match character properties are available when UTF-8 mode
342 nigel 75 is selected. They are:
343     <pre>
344 nigel 87 \p{<i>xx</i>} a character with the <i>xx</i> property
345     \P{<i>xx</i>} a character without the <i>xx</i> property
346     \X an extended Unicode sequence
347 nigel 75 </pre>
348 nigel 87 The property names represented by <i>xx</i> above are limited to the Unicode
349     script names, the general category properties, and "Any", which matches any
350     character (including newline). Other properties such as "InMusicalSymbols" are
351     not currently supported by PCRE. Note that \P{Any} does not match any
352     characters, so always causes a match failure.
353 nigel 63 </P>
354     <P>
355 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
356     character from one of these sets can be matched using a script name. For
357     example:
358 nigel 75 <pre>
359 nigel 87 \p{Greek}
360     \P{Han}
361     </pre>
362     Those that are not part of an identified script are lumped together as
363     "Common". The current list of scripts is:
364     </P>
365     <P>
366     Arabic,
367     Armenian,
368 nigel 93 Balinese,
369 nigel 87 Bengali,
370     Bopomofo,
371     Braille,
372     Buginese,
373     Buhid,
374     Canadian_Aboriginal,
375     Cherokee,
376     Common,
377     Coptic,
378 nigel 93 Cuneiform,
379 nigel 87 Cypriot,
380     Cyrillic,
381     Deseret,
382     Devanagari,
383     Ethiopic,
384     Georgian,
385     Glagolitic,
386     Gothic,
387     Greek,
388     Gujarati,
389     Gurmukhi,
390     Han,
391     Hangul,
392     Hanunoo,
393     Hebrew,
394     Hiragana,
395     Inherited,
396     Kannada,
397     Katakana,
398     Kharoshthi,
399     Khmer,
400     Lao,
401     Latin,
402     Limbu,
403     Linear_B,
404     Malayalam,
405     Mongolian,
406     Myanmar,
407     New_Tai_Lue,
408 nigel 93 Nko,
409 nigel 87 Ogham,
410     Old_Italic,
411     Old_Persian,
412     Oriya,
413     Osmanya,
414 nigel 93 Phags_Pa,
415     Phoenician,
416 nigel 87 Runic,
417     Shavian,
418     Sinhala,
419     Syloti_Nagri,
420     Syriac,
421     Tagalog,
422     Tagbanwa,
423     Tai_Le,
424     Tamil,
425     Telugu,
426     Thaana,
427     Thai,
428     Tibetan,
429     Tifinagh,
430     Ugaritic,
431     Yi.
432     </P>
433     <P>
434     Each character has exactly one general category property, specified by a
435     two-letter abbreviation. For compatibility with Perl, negation can be specified
436     by including a circumflex between the opening brace and the property name. For
437     example, \p{^Lu} is the same as \P{Lu}.
438     </P>
439     <P>
440     If only one letter is specified with \p or \P, it includes all the general
441     category properties that start with that letter. In this case, in the absence
442     of negation, the curly brackets in the escape sequence are optional; these two
443     examples have the same effect:
444     <pre>
445 nigel 75 \p{L}
446     \pL
447     </pre>
448 nigel 87 The following general category property codes are supported:
449 nigel 75 <pre>
450     C Other
451     Cc Control
452     Cf Format
453     Cn Unassigned
454     Co Private use
455     Cs Surrogate
457     L Letter
458     Ll Lower case letter
459     Lm Modifier letter
460     Lo Other letter
461     Lt Title case letter
462     Lu Upper case letter
464     M Mark
465     Mc Spacing mark
466     Me Enclosing mark
467     Mn Non-spacing mark
469     N Number
470     Nd Decimal number
471     Nl Letter number
472     No Other number
474     P Punctuation
475     Pc Connector punctuation
476     Pd Dash punctuation
477     Pe Close punctuation
478     Pf Final punctuation
479     Pi Initial punctuation
480     Po Other punctuation
481     Ps Open punctuation
483     S Symbol
484     Sc Currency symbol
485     Sk Modifier symbol
486     Sm Mathematical symbol
487     So Other symbol
489     Z Separator
490     Zl Line separator
491     Zp Paragraph separator
492     Zs Space separator
493     </pre>
494 nigel 87 The special property L& is also supported: it matches a character that has
495     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
496     a modifier or "other".
497 nigel 75 </P>
498     <P>
499 nigel 87 The long synonyms for these properties that Perl supports (such as \p{Letter})
500 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
501 nigel 87 properties with "Is".
502     </P>
503     <P>
504     No character that is in the Unicode table has the Cn (unassigned) property.
505     Instead, this property is assumed for any code point that is not in the
506     Unicode table.
507     </P>
508     <P>
509 nigel 75 Specifying caseless matching does not affect these escape sequences. For
510     example, \p{Lu} always matches only upper case letters.
511     </P>
512     <P>
513     The \X escape matches any number of Unicode characters that form an extended
514     Unicode sequence. \X is equivalent to
515     <pre>
516     (?&#62;\PM\pM*)
517     </pre>
518     That is, it matches a character without the "mark" property, followed by zero
519     or more characters with the "mark" property, and treats the sequence as an
520     atomic group
521     <a href="#atomicgroup">(see below).</a>
522     Characters with the "mark" property are typically accents that affect the
523     preceding character.
524     </P>
525     <P>
526     Matching characters by Unicode property is not fast, because PCRE has to search
527     a structure that contains data for over fifteen thousand characters. That is
528     why the traditional escape sequences such as \d and \w do not use Unicode
529     properties in PCRE.
530     <a name="smallassertions"></a></P>
531     <br><b>
532     Simple assertions
533     </b><br>
534     <P>
535 nigel 93 The final use of backslash is for certain simple assertions. An assertion
536 nigel 63 specifies a condition that has to be met at a particular point in a match,
537     without consuming any characters from the subject string. The use of
538 nigel 75 subpatterns for more complicated assertions is described
539     <a href="#bigassertions">below.</a>
540 nigel 91 The backslashed assertions are:
541 nigel 63 <pre>
542     \b matches at a word boundary
543     \B matches when not at a word boundary
544 nigel 93 \A matches at the start of the subject
545     \Z matches at the end of the subject
546     also matches before a newline at the end of the subject
547     \z matches only at the end of the subject
548     \G matches at the first matching position in the subject
549 nigel 75 </pre>
550 nigel 63 These assertions may not appear in character classes (but note that \b has a
551     different meaning, namely the backspace character, inside a character class).
552     </P>
553     <P>
554     A word boundary is a position in the subject string where the current character
555     and the previous character do not both match \w or \W (i.e. one matches
556     \w and the other matches \W), or the start or end of the string if the
557     first or last character matches \w, respectively.
558     </P>
559     <P>
560     The \A, \Z, and \z assertions differ from the traditional circumflex and
561 nigel 75 dollar (described in the next section) in that they only ever match at the very
562     start and end of the subject string, whatever options are set. Thus, they are
563     independent of multiline mode. These three assertions are not affected by the
564     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
565     circumflex and dollar metacharacters. However, if the <i>startoffset</i>
566     argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
567     at a point other than the beginning of the subject, \A can never match. The
568 nigel 91 difference between \Z and \z is that \Z matches before a newline at the end
569     of the string as well as at the very end, whereas \z matches only at the end.
570 nigel 63 </P>
571     <P>
572     The \G assertion is true only when the current matching position is at the
573     start point of the match, as specified by the <i>startoffset</i> argument of
574     <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
575     non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
576     arguments, you can mimic Perl's /g option, and it is in this kind of
577     implementation where \G can be useful.
578     </P>
579     <P>
580     Note, however, that PCRE's interpretation of \G, as the start of the current
581     match, is subtly different from Perl's, which defines it as the end of the
582     previous match. In Perl, these can be different when the previously matched
583     string was empty. Because PCRE does just one match at a time, it cannot
584     reproduce this behaviour.
585     </P>
586     <P>
587     If all the alternatives of a pattern begin with \G, the expression is anchored
588     to the starting match position, and the "anchored" flag is set in the compiled
589     regular expression.
590     </P>
591 nigel 93 <br><a name="SEC4" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
592 nigel 63 <P>
593     Outside a character class, in the default matching mode, the circumflex
594 nigel 75 character is an assertion that is true only if the current matching point is
595 nigel 63 at the start of the subject string. If the <i>startoffset</i> argument of
596     <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
597     option is unset. Inside a character class, circumflex has an entirely different
598 nigel 75 meaning
599     <a href="#characterclass">(see below).</a>
600 nigel 63 </P>
601     <P>
602     Circumflex need not be the first character of the pattern if a number of
603     alternatives are involved, but it should be the first thing in each alternative
604     in which it appears if the pattern is ever to match that branch. If all
605     possible alternatives start with a circumflex, that is, if the pattern is
606     constrained to match only at the start of the subject, it is said to be an
607     "anchored" pattern. (There are also other constructs that can cause a pattern
608     to be anchored.)
609     </P>
610     <P>
611 nigel 75 A dollar character is an assertion that is true only if the current matching
612 nigel 63 point is at the end of the subject string, or immediately before a newline
613 nigel 91 at the end of the string (by default). Dollar need not be the last character of
614     the pattern if a number of alternatives are involved, but it should be the last
615     item in any branch in which it appears. Dollar has no special meaning in a
616     character class.
617 nigel 63 </P>
618     <P>
619     The meaning of dollar can be changed so that it matches only at the very end of
620     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
621     does not affect the \Z assertion.
622     </P>
623     <P>
624     The meanings of the circumflex and dollar characters are changed if the
625 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
626     immediately after internal newlines as well as at the start of the subject
627     string. It does not match after a newline that ends the string. A dollar
628     matches before any newlines in the string, as well as at the very end, when
629     PCRE_MULTILINE is set. When newline is specified as the two-character
630     sequence CRLF, isolated CR and LF characters do not indicate newlines.
631 nigel 63 </P>
632     <P>
633 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
634     \n represents a newline) in multiline mode, but not otherwise. Consequently,
635     patterns that are anchored in single line mode because all branches start with
636     ^ are not anchored in multiline mode, and a match for circumflex is possible
637     when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
638     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
639     </P>
640     <P>
641 nigel 63 Note that the sequences \A, \Z, and \z can be used to match the start and
642     end of the subject in both modes, and if all branches of a pattern start with
643 nigel 91 \A it is always anchored, whether or not PCRE_MULTILINE is set.
644 nigel 63 </P>
645 nigel 93 <br><a name="SEC5" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
646 nigel 63 <P>
647     Outside a character class, a dot in the pattern matches any one character in
648 nigel 91 the subject string except (by default) a character that signifies the end of a
649 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
650 nigel 63 </P>
651 nigel 91 <P>
652 nigel 93 When a line ending is defined as a single character, dot never matches that
653     character; when the two-character sequence CRLF is used, dot does not match CR
654     if it is immediately followed by LF, but otherwise it matches all characters
655     (including isolated CRs and LFs). When any Unicode line endings are being
656     recognized, dot does not match CR or LF or any of the other line ending
657     characters.
658     </P>
659     <P>
660 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
661 nigel 93 option is set, a dot matches any one character, without exception. If the
662     two-character sequence CRLF is present in the subject string, it takes two dots
663     to match it.
664 nigel 91 </P>
665     <P>
666     The handling of dot is entirely independent of the handling of circumflex and
667     dollar, the only relationship being that they both involve newlines. Dot has no
668     special meaning in a character class.
669     </P>
670 nigel 93 <br><a name="SEC6" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
671 nigel 63 <P>
672     Outside a character class, the escape sequence \C matches any one byte, both
673 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
674     characters. The feature is provided in Perl in order to match individual bytes
675     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
676     what remains in the string may be a malformed UTF-8 string. For this reason,
677     the \C escape sequence is best avoided.
678 nigel 63 </P>
679     <P>
680 nigel 75 PCRE does not allow \C to appear in lookbehind assertions
681     <a href="#lookbehind">(described below),</a>
682     because in UTF-8 mode this would make it impossible to calculate the length of
683     the lookbehind.
684     <a name="characterclass"></a></P>
685 nigel 93 <br><a name="SEC7" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
686 nigel 63 <P>
687     An opening square bracket introduces a character class, terminated by a closing
688     square bracket. A closing square bracket on its own is not special. If a
689     closing square bracket is required as a member of the class, it should be the
690     first data character in the class (after an initial circumflex, if present) or
691     escaped with a backslash.
692     </P>
693     <P>
694     A character class matches a single character in the subject. In UTF-8 mode, the
695     character may occupy more than one byte. A matched character must be in the set
696     of characters defined by the class, unless the first character in the class
697     definition is a circumflex, in which case the subject character must not be in
698     the set defined by the class. If a circumflex is actually required as a member
699     of the class, ensure it is not the first character, or escape it with a
700     backslash.
701     </P>
702     <P>
703     For example, the character class [aeiou] matches any lower case vowel, while
704     [^aeiou] matches any character that is not a lower case vowel. Note that a
705 nigel 75 circumflex is just a convenient notation for specifying the characters that
706     are in the class by enumerating those that are not. A class that starts with a
707     circumflex is not an assertion: it still consumes a character from the subject
708     string, and therefore it fails if the current pointer is at the end of the
709     string.
710 nigel 63 </P>
711     <P>
712     In UTF-8 mode, characters with values greater than 255 can be included in a
713     class as a literal string of bytes, or by using the \x{ escaping mechanism.
714     </P>
715     <P>
716     When caseless matching is set, any letters in a class represent both their
717     upper case and lower case versions, so for example, a caseless [aeiou] matches
718     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
719 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
720     case for characters whose values are less than 128, so caseless matching is
721     always possible. For characters with higher values, the concept of case is
722     supported if PCRE is compiled with Unicode property support, but not otherwise.
723     If you want to use caseless matching for characters 128 and above, you must
724     ensure that PCRE is compiled with Unicode property support as well as with
725     UTF-8 support.
726 nigel 63 </P>
727     <P>
728 nigel 93 Characters that might indicate line breaks are never treated in any special way
729     when matching character classes, whatever line-ending sequence is in use, and
730     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
731     such as [^a] always matches one of these characters.
732 nigel 63 </P>
733     <P>
734     The minus (hyphen) character can be used to specify a range of characters in a
735     character class. For example, [d-m] matches any letter between d and m,
736     inclusive. If a minus character is required in a class, it must be escaped with
737     a backslash or appear in a position where it cannot be interpreted as
738     indicating a range, typically as the first or last character in the class.
739     </P>
740     <P>
741     It is not possible to have the literal character "]" as the end character of a
742     range. A pattern such as [W-]46] is interpreted as a class of two characters
743     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
744     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
745 nigel 75 the end of range, so [W-\]46] is interpreted as a class containing a range
746     followed by two other characters. The octal or hexadecimal representation of
747     "]" can also be used to end a range.
748 nigel 63 </P>
749     <P>
750     Ranges operate in the collating sequence of character values. They can also be
751     used for characters specified numerically, for example [\000-\037]. In UTF-8
752     mode, ranges can include characters whose values are greater than 255, for
753     example [\x{100}-\x{2ff}].
754     </P>
755     <P>
756     If a range that includes letters is used when caseless matching is set, it
757     matches the letters in either case. For example, [W-c] is equivalent to
758 nigel 75 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
759     tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E
760     characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
761     characters with values greater than 128 only when it is compiled with Unicode
762     property support.
763 nigel 63 </P>
764     <P>
765 nigel 75 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
766     in a character class, and add the characters that they match to the class. For
767 nigel 63 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
768     conveniently be used with the upper case character types to specify a more
769     restricted set of characters than the matching lower case type. For example,
770     the class [^\W_] matches any letter or digit, but not underscore.
771     </P>
772     <P>
773 nigel 75 The only metacharacters that are recognized in character classes are backslash,
774     hyphen (only where it can be interpreted as specifying a range), circumflex
775     (only at the start), opening square bracket (only when it can be interpreted as
776     introducing a POSIX class name - see the next section), and the terminating
777     closing square bracket. However, escaping other non-alphanumeric characters
778     does no harm.
779 nigel 63 </P>
780 nigel 93 <br><a name="SEC8" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
781 nigel 63 <P>
782 nigel 75 Perl supports the POSIX notation for character classes. This uses names
783 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
784     this notation. For example,
785     <pre>
786     [01[:alpha:]%]
787 nigel 75 </pre>
788 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
789     are
790     <pre>
791     alnum letters and digits
792     alpha letters
793     ascii character codes 0 - 127
794     blank space or tab only
795     cntrl control characters
796     digit decimal digits (same as \d)
797     graph printing characters, excluding space
798     lower lower case letters
799     print printing characters, including space
800     punct printing characters, excluding letters and digits
801     space white space (not quite the same as \s)
802     upper upper case letters
803     word "word" characters (same as \w)
804     xdigit hexadecimal digits
805 nigel 75 </pre>
806 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
807     space (32). Notice that this list includes the VT character (code 11). This
808     makes "space" different to \s, which does not include VT (for Perl
809     compatibility).
810     </P>
811     <P>
812     The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
813     5.8. Another Perl extension is negation, which is indicated by a ^ character
814     after the colon. For example,
815     <pre>
816     [12[:^digit:]]
817 nigel 75 </pre>
818 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
819     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
820     supported, and an error is given if they are encountered.
821     </P>
822     <P>
823 nigel 75 In UTF-8 mode, characters with values greater than 128 do not match any of
824 nigel 63 the POSIX character classes.
825     </P>
826 nigel 93 <br><a name="SEC9" href="#TOC1">VERTICAL BAR</a><br>
827 nigel 63 <P>
828     Vertical bar characters are used to separate alternative patterns. For example,
829     the pattern
830     <pre>
831     gilbert|sullivan
832 nigel 75 </pre>
833 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
834 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
835     process tries each alternative in turn, from left to right, and the first one
836     that succeeds is used. If the alternatives are within a subpattern
837 nigel 75 <a href="#subpattern">(defined below),</a>
838     "succeeds" means matching the rest of the main pattern as well as the
839     alternative in the subpattern.
840 nigel 63 </P>
841 nigel 93 <br><a name="SEC10" href="#TOC1">INTERNAL OPTION SETTING</a><br>
842 nigel 63 <P>
843     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
844     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
845     Perl option letters enclosed between "(?" and ")". The option letters are
846     <pre>
847     i for PCRE_CASELESS
848     m for PCRE_MULTILINE
849     s for PCRE_DOTALL
850     x for PCRE_EXTENDED
851 nigel 75 </pre>
852 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
853     unset these options by preceding the letter with a hyphen, and a combined
854     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
855     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
856     permitted. If a letter appears both before and after the hyphen, the option is
857     unset.
858     </P>
859     <P>
860     When an option change occurs at top level (that is, not inside subpattern
861     parentheses), the change applies to the remainder of the pattern that follows.
862     If the change is placed right at the start of a pattern, PCRE extracts it into
863     the global options (and it will therefore show up in data extracted by the
864     <b>pcre_fullinfo()</b> function).
865     </P>
866     <P>
867 nigel 93 An option change within a subpattern (see below for a description of
868     subpatterns) affects only that part of the current pattern that follows it, so
869 nigel 63 <pre>
870     (a(?i)b)c
871 nigel 75 </pre>
872 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
873     By this means, options can be made to have different settings in different
874     parts of the pattern. Any changes made in one alternative do carry on
875     into subsequent branches within the same subpattern. For example,
876     <pre>
877     (a(?i)b|c)
878 nigel 75 </pre>
879 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
880     branch is abandoned before the option setting. This is because the effects of
881     option settings happen at compile time. There would be some very weird
882     behaviour otherwise.
883     </P>
884     <P>
885 nigel 91 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
886     changed in the same way as the Perl-compatible options by using the characters
887     J, U and X respectively.
888 nigel 75 <a name="subpattern"></a></P>
889 nigel 93 <br><a name="SEC11" href="#TOC1">SUBPATTERNS</a><br>
890 nigel 63 <P>
891     Subpatterns are delimited by parentheses (round brackets), which can be nested.
892 nigel 75 Turning part of a pattern into a subpattern does two things:
893     <br>
894     <br>
895 nigel 63 1. It localizes a set of alternatives. For example, the pattern
896     <pre>
897     cat(aract|erpillar|)
898 nigel 75 </pre>
899 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
900 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
901 nigel 75 <br>
902     <br>
903     2. It sets up the subpattern as a capturing subpattern. This means that, when
904     the whole pattern matches, that portion of the subject string that matched the
905     subpattern is passed back to the caller via the <i>ovector</i> argument of
906 nigel 63 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
907 nigel 75 from 1) to obtain numbers for the capturing subpatterns.
908 nigel 63 </P>
909     <P>
910     For example, if the string "the red king" is matched against the pattern
911     <pre>
912     the ((red|white) (king|queen))
913 nigel 75 </pre>
914 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
915     2, and 3, respectively.
916     </P>
917     <P>
918     The fact that plain parentheses fulfil two functions is not always helpful.
919     There are often times when a grouping subpattern is required without a
920     capturing requirement. If an opening parenthesis is followed by a question mark
921     and a colon, the subpattern does not do any capturing, and is not counted when
922     computing the number of any subsequent capturing subpatterns. For example, if
923     the string "the white queen" is matched against the pattern
924     <pre>
925     the ((?:red|white) (king|queen))
926 nigel 75 </pre>
927 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
928 nigel 93 2. The maximum number of capturing subpatterns is 65535.
929 nigel 63 </P>
930     <P>
931     As a convenient shorthand, if any option settings are required at the start of
932     a non-capturing subpattern, the option letters may appear between the "?" and
933     the ":". Thus the two patterns
934     <pre>
935     (?i:saturday|sunday)
936     (?:(?i)saturday|sunday)
937 nigel 75 </pre>
938 nigel 63 match exactly the same set of strings. Because alternative branches are tried
939     from left to right, and options are not reset until the end of the subpattern
940     is reached, an option setting in one branch does affect subsequent branches, so
941     the above patterns match "SUNDAY" as well as "Saturday".
942     </P>
943 nigel 93 <br><a name="SEC12" href="#TOC1">NAMED SUBPATTERNS</a><br>
944 nigel 63 <P>
945     Identifying capturing parentheses by number is simple, but it can be very hard
946     to keep track of the numbers in complicated regular expressions. Furthermore,
947 nigel 75 if an expression is modified, the numbers may change. To help with this
948 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
949     added to Perl until release 5.10. Python had the feature earlier, and PCRE
950     introduced it at release 4.0, using the Python syntax. PCRE now supports both
951     the Perl and the Python syntax.
952     </P>
953     <P>
954     In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
955     (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
956 nigel 91 parentheses from other parts of the pattern, such as
957     <a href="#backreferences">backreferences,</a>
958     <a href="#recursion">recursion,</a>
959     and
960     <a href="#conditions">conditions,</a>
961     can be made by name as well as by number.
962 nigel 63 </P>
963     <P>
964 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
965 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
966     if the names were not present. The PCRE API provides function calls for
967     extracting the name-to-number translation table from a compiled pattern. There
968     is also a convenience function for extracting a captured substring by name.
969 nigel 91 </P>
970     <P>
971     By default, a name must be unique within a pattern, but it is possible to relax
972     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
973     be useful for patterns where only one instance of the named parentheses can
974     match. Suppose you want to match the name of a weekday, either as a 3-letter
975     abbreviation or as the full name, and in both cases you want to extract the
976     abbreviation. This pattern (ignoring the line breaks) does the job:
977     <pre>
978 nigel 93 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
979     (?&#60;DN&#62;Tue)(?:sday)?|
980     (?&#60;DN&#62;Wed)(?:nesday)?|
981     (?&#60;DN&#62;Thu)(?:rsday)?|
982     (?&#60;DN&#62;Sat)(?:urday)?
983 nigel 91 </pre>
984     There are five capturing substrings, but only one is ever set after a match.
985     The convenience function for extracting the data by name returns the substring
986 nigel 93 for the first (and in this example, the only) subpattern of that name that
987 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
988     make a reference to a non-unique named subpattern from elsewhere in the
989     pattern, the one that corresponds to the lowest number is used. For further
990     details of the interfaces for handling named subpatterns, see the
991 nigel 63 <a href="pcreapi.html"><b>pcreapi</b></a>
992     documentation.
993     </P>
994 nigel 93 <br><a name="SEC13" href="#TOC1">REPETITION</a><br>
995 nigel 63 <P>
996     Repetition is specified by quantifiers, which can follow any of the following
997     items:
998     <pre>
999     a literal data character
1000 nigel 93 the dot metacharacter
1001 nigel 63 the \C escape sequence
1002 nigel 75 the \X escape sequence (in UTF-8 mode with Unicode properties)
1003 nigel 93 the \R escape sequence
1004 nigel 75 an escape such as \d that matches a single character
1005 nigel 63 a character class
1006     a back reference (see next section)
1007     a parenthesized subpattern (unless it is an assertion)
1008 nigel 75 </pre>
1009 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1010     permitted matches, by giving the two numbers in curly brackets (braces),
1011     separated by a comma. The numbers must be less than 65536, and the first must
1012     be less than or equal to the second. For example:
1013     <pre>
1014     z{2,4}
1015 nigel 75 </pre>
1016 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1017     character. If the second number is omitted, but the comma is present, there is
1018     no upper limit; if the second number and the comma are both omitted, the
1019     quantifier specifies an exact number of required matches. Thus
1020     <pre>
1021     [aeiou]{3,}
1022 nigel 75 </pre>
1023 nigel 63 matches at least 3 successive vowels, but may match many more, while
1024     <pre>
1025     \d{8}
1026 nigel 75 </pre>
1027 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1028     where a quantifier is not allowed, or one that does not match the syntax of a
1029     quantifier, is taken as a literal character. For example, {,6} is not a
1030     quantifier, but a literal string of four characters.
1031     </P>
1032     <P>
1033     In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1034     bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1035 nigel 75 which is represented by a two-byte sequence. Similarly, when Unicode property
1036     support is available, \X{3} matches three Unicode extended sequences, each of
1037     which may be several bytes long (and they may be of different lengths).
1038 nigel 63 </P>
1039     <P>
1040     The quantifier {0} is permitted, causing the expression to behave as if the
1041     previous item and the quantifier were not present.
1042     </P>
1043     <P>
1044 nigel 93 For convenience, the three most common quantifiers have single-character
1045     abbreviations:
1046 nigel 63 <pre>
1047     * is equivalent to {0,}
1048     + is equivalent to {1,}
1049     ? is equivalent to {0,1}
1050 nigel 75 </pre>
1051 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1052     match no characters with a quantifier that has no upper limit, for example:
1053     <pre>
1054     (a?)*
1055 nigel 75 </pre>
1056 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1057     such patterns. However, because there are cases where this can be useful, such
1058     patterns are now accepted, but if any repetition of the subpattern does in fact
1059     match no characters, the loop is forcibly broken.
1060     </P>
1061     <P>
1062     By default, the quantifiers are "greedy", that is, they match as much as
1063     possible (up to the maximum number of permitted times), without causing the
1064     rest of the pattern to fail. The classic example of where this gives problems
1065 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1066     and within the comment, individual * and / characters may appear. An attempt to
1067     match C comments by applying the pattern
1068 nigel 63 <pre>
1069     /\*.*\*/
1070 nigel 75 </pre>
1071 nigel 63 to the string
1072     <pre>
1073 nigel 75 /* first comment */ not comment /* second comment */
1074     </pre>
1075 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1076     item.
1077     </P>
1078     <P>
1079     However, if a quantifier is followed by a question mark, it ceases to be
1080     greedy, and instead matches the minimum number of times possible, so the
1081     pattern
1082     <pre>
1083     /\*.*?\*/
1084 nigel 75 </pre>
1085 nigel 63 does the right thing with the C comments. The meaning of the various
1086     quantifiers is not otherwise changed, just the preferred number of matches.
1087     Do not confuse this use of question mark with its use as a quantifier in its
1088     own right. Because it has two uses, it can sometimes appear doubled, as in
1089     <pre>
1090     \d??\d
1091 nigel 75 </pre>
1092 nigel 63 which matches one digit by preference, but can match two if that is the only
1093     way the rest of the pattern matches.
1094     </P>
1095     <P>
1096 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1097 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1098     greedy by following them with a question mark. In other words, it inverts the
1099     default behaviour.
1100     </P>
1101     <P>
1102     When a parenthesized subpattern is quantified with a minimum repeat count that
1103 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1104 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1105     </P>
1106     <P>
1107     If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1108 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1109 nigel 63 implicitly anchored, because whatever follows will be tried against every
1110     character position in the subject string, so there is no point in retrying the
1111     overall match at any position after the first. PCRE normally treats such a
1112     pattern as though it were preceded by \A.
1113     </P>
1114     <P>
1115     In cases where it is known that the subject string contains no newlines, it is
1116     worth setting PCRE_DOTALL in order to obtain this optimization, or
1117     alternatively using ^ to indicate anchoring explicitly.
1118     </P>
1119     <P>
1120     However, there is one situation where the optimization cannot be used. When .*
1121     is inside capturing parentheses that are the subject of a backreference
1122 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1123     succeeds. Consider, for example:
1124 nigel 63 <pre>
1125     (.*)abc\1
1126 nigel 75 </pre>
1127 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1128     this reason, such a pattern is not implicitly anchored.
1129     </P>
1130     <P>
1131     When a capturing subpattern is repeated, the value captured is the substring
1132     that matched the final iteration. For example, after
1133     <pre>
1134     (tweedle[dume]{3}\s*)+
1135 nigel 75 </pre>
1136 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1137     "tweedledee". However, if there are nested capturing subpatterns, the
1138     corresponding captured values may have been set in previous iterations. For
1139     example, after
1140     <pre>
1141     /(a|(b))+/
1142 nigel 75 </pre>
1143 nigel 63 matches "aba" the value of the second captured substring is "b".
1144 nigel 75 <a name="atomicgroup"></a></P>
1145 nigel 93 <br><a name="SEC14" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1146 nigel 63 <P>
1147 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1148     repetition, failure of what follows normally causes the repeated item to be
1149     re-evaluated to see if a different number of repeats allows the rest of the
1150     pattern to match. Sometimes it is useful to prevent this, either to change the
1151     nature of the match, or to cause it fail earlier than it otherwise might, when
1152     the author of the pattern knows there is no point in carrying on.
1153 nigel 63 </P>
1154     <P>
1155     Consider, for example, the pattern \d+foo when applied to the subject line
1156     <pre>
1157     123456bar
1158 nigel 75 </pre>
1159 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1160     action of the matcher is to try again with only 5 digits matching the \d+
1161     item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1162     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1163     that once a subpattern has matched, it is not to be re-evaluated in this way.
1164     </P>
1165     <P>
1166 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1167 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1168     special parenthesis, starting with (?&#62; as in this example:
1169     <pre>
1170 nigel 73 (?&#62;\d+)foo
1171 nigel 75 </pre>
1172 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1173     it has matched, and a failure further into the pattern is prevented from
1174     backtracking into it. Backtracking past it to previous items, however, works as
1175     normal.
1176     </P>
1177     <P>
1178     An alternative description is that a subpattern of this type matches the string
1179     of characters that an identical standalone pattern would match, if anchored at
1180     the current point in the subject string.
1181     </P>
1182     <P>
1183     Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1184     the above example can be thought of as a maximizing repeat that must swallow
1185     everything it can. So, while both \d+ and \d+? are prepared to adjust the
1186     number of digits they match in order to make the rest of the pattern match,
1187     (?&#62;\d+) can only match an entire sequence of digits.
1188     </P>
1189     <P>
1190     Atomic groups in general can of course contain arbitrarily complicated
1191     subpatterns, and can be nested. However, when the subpattern for an atomic
1192     group is just a single repeated item, as in the example above, a simpler
1193     notation, called a "possessive quantifier" can be used. This consists of an
1194     additional + character following a quantifier. Using this notation, the
1195     previous example can be rewritten as
1196     <pre>
1197 nigel 75 \d++foo
1198     </pre>
1199 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1200     option is ignored. They are a convenient notation for the simpler forms of
1201 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1202     quantifier and the equivalent atomic group, though there may be a performance
1203     difference; possessive quantifiers should be slightly faster.
1204 nigel 63 </P>
1205     <P>
1206 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1207     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1208     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1209     package, and PCRE copied it from there. It ultimately found its way into Perl
1210     at release 5.10.
1211 nigel 63 </P>
1212     <P>
1213 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1214     pattern constructs. For example, the sequence A+B is treated as A++B because
1215     there is no point in backtracking into a sequence of A's when B must follow.
1216     </P>
1217     <P>
1218 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1219     be repeated an unlimited number of times, the use of an atomic group is the
1220     only way to avoid some failing matches taking a very long time indeed. The
1221     pattern
1222     <pre>
1223     (\D+|&#60;\d+&#62;)*[!?]
1224 nigel 75 </pre>
1225 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1226     digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1227     quickly. However, if it is applied to
1228     <pre>
1229     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1230 nigel 75 </pre>
1231 nigel 63 it takes a long time before reporting failure. This is because the string can
1232 nigel 75 be divided between the internal \D+ repeat and the external * repeat in a
1233     large number of ways, and all have to be tried. (The example uses [!?] rather
1234     than a single character at the end, because both PCRE and Perl have an
1235     optimization that allows for fast failure when a single character is used. They
1236     remember the last single character that is required for a match, and fail early
1237     if it is not present in the string.) If the pattern is changed so that it uses
1238     an atomic group, like this:
1239 nigel 63 <pre>
1240     ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1241 nigel 75 </pre>
1242 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1243 nigel 75 <a name="backreferences"></a></P>
1244 nigel 93 <br><a name="SEC15" href="#TOC1">BACK REFERENCES</a><br>
1245 nigel 63 <P>
1246     Outside a character class, a backslash followed by a digit greater than 0 (and
1247     possibly further digits) is a back reference to a capturing subpattern earlier
1248     (that is, to its left) in the pattern, provided there have been that many
1249     previous capturing left parentheses.
1250     </P>
1251     <P>
1252     However, if the decimal number following the backslash is less than 10, it is
1253     always taken as a back reference, and causes an error only if there are not
1254     that many capturing left parentheses in the entire pattern. In other words, the
1255     parentheses that are referenced need not be to the left of the reference for
1256 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1257     when a repetition is involved and the subpattern to the right has participated
1258     in an earlier iteration.
1259     </P>
1260     <P>
1261 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1262     whose number is 10 or more using this syntax because a sequence such as \50 is
1263     interpreted as a character defined in octal. See the subsection entitled
1264 nigel 91 "Non-printing characters"
1265 nigel 75 <a href="#digitsafterbackslash">above</a>
1266 nigel 93 for further details of the handling of digits following a backslash. There is
1267     no such problem when named parentheses are used. A back reference to any
1268     subpattern is possible using named parentheses (see below).
1269 nigel 63 </P>
1270     <P>
1271 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1272     backslash is to use the \g escape sequence, which is a feature introduced in
1273     Perl 5.10. This escape must be followed by a positive or a negative number,
1274     optionally enclosed in braces. These examples are all identical:
1275     <pre>
1276     (ring), \1
1277     (ring), \g1
1278     (ring), \g{1}
1279     </pre>
1280     A positive number specifies an absolute reference without the ambiguity that is
1281     present in the older syntax. It is also useful when literal digits follow the
1282     reference. A negative number is a relative reference. Consider this example:
1283     <pre>
1284     (abc(def)ghi)\g{-1}
1285     </pre>
1286     The sequence \g{-1} is a reference to the most recently started capturing
1287     subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1288     would be equivalent to \1. The use of relative references can be helpful in
1289     long patterns, and also in patterns that are created by joining together
1290     fragments that contain references within themselves.
1291     </P>
1292     <P>
1293 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1294     the current subject string, rather than anything matching the subpattern
1295     itself (see
1296     <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1297     below for a way of doing that). So the pattern
1298     <pre>
1299     (sens|respons)e and \1ibility
1300 nigel 75 </pre>
1301 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1302     "sense and responsibility". If caseful matching is in force at the time of the
1303     back reference, the case of letters is relevant. For example,
1304     <pre>
1305     ((?i)rah)\s+\1
1306 nigel 75 </pre>
1307 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1308     capturing subpattern is matched caselessly.
1309     </P>
1310     <P>
1311 nigel 93 Back references to named subpatterns use the Perl syntax \k&#60;name&#62; or \k'name'
1312     or the Python syntax (?P=name). We could rewrite the above example in either of
1313     the following ways:
1314 nigel 63 <pre>
1315 nigel 93 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1316 nigel 91 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1317 nigel 75 </pre>
1318 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1319     after the reference.
1320     </P>
1321     <P>
1322 nigel 63 There may be more than one back reference to the same subpattern. If a
1323     subpattern has not actually been used in a particular match, any back
1324     references to it always fail. For example, the pattern
1325     <pre>
1326     (a|(bc))\2
1327 nigel 75 </pre>
1328 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1329     many capturing parentheses in a pattern, all digits following the backslash are
1330     taken as part of a potential back reference number. If the pattern continues
1331     with a digit character, some delimiter must be used to terminate the back
1332     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1333 nigel 75 Otherwise an empty comment (see
1334     <a href="#comments">"Comments"</a>
1335     below) can be used.
1336 nigel 63 </P>
1337     <P>
1338     A back reference that occurs inside the parentheses to which it refers fails
1339     when the subpattern is first used, so, for example, (a\1) never matches.
1340     However, such references can be useful inside repeated subpatterns. For
1341     example, the pattern
1342     <pre>
1343     (a|b\1)+
1344 nigel 75 </pre>
1345 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1346     the subpattern, the back reference matches the character string corresponding
1347     to the previous iteration. In order for this to work, the pattern must be such
1348     that the first iteration does not need to match the back reference. This can be
1349     done using alternation, as in the example above, or by a quantifier with a
1350     minimum of zero.
1351 nigel 75 <a name="bigassertions"></a></P>
1352 nigel 93 <br><a name="SEC16" href="#TOC1">ASSERTIONS</a><br>
1353 nigel 63 <P>
1354     An assertion is a test on the characters following or preceding the current
1355     matching point that does not actually consume any characters. The simple
1356 nigel 75 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1357     <a href="#smallassertions">above.</a>
1358     </P>
1359     <P>
1360 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1361     those that look ahead of the current position in the subject string, and those
1362 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1363     except that it does not cause the current matching position to be changed.
1364 nigel 63 </P>
1365     <P>
1366 nigel 75 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1367     because it makes no sense to assert the same thing several times. If any kind
1368     of assertion contains capturing subpatterns within it, these are counted for
1369     the purposes of numbering the capturing subpatterns in the whole pattern.
1370     However, substring capturing is carried out only for positive assertions,
1371     because it does not make sense for negative assertions.
1372 nigel 63 </P>
1373 nigel 75 <br><b>
1374     Lookahead assertions
1375     </b><br>
1376 nigel 63 <P>
1377 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1378     negative assertions. For example,
1379 nigel 63 <pre>
1380     \w+(?=;)
1381 nigel 75 </pre>
1382 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1383     the match, and
1384     <pre>
1385     foo(?!bar)
1386 nigel 75 </pre>
1387 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1388     apparently similar pattern
1389     <pre>
1390     (?!foo)bar
1391 nigel 75 </pre>
1392 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1393     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1394     (?!foo) is always true when the next three characters are "bar". A
1395 nigel 75 lookbehind assertion is needed to achieve the other effect.
1396 nigel 63 </P>
1397     <P>
1398     If you want to force a matching failure at some point in a pattern, the most
1399     convenient way to do it is with (?!) because an empty string always matches, so
1400     an assertion that requires there not to be an empty string must always fail.
1401 nigel 75 <a name="lookbehind"></a></P>
1402     <br><b>
1403     Lookbehind assertions
1404     </b><br>
1405 nigel 63 <P>
1406     Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1407     negative assertions. For example,
1408     <pre>
1409     (?&#60;!foo)bar
1410 nigel 75 </pre>
1411 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1412     a lookbehind assertion are restricted such that all the strings it matches must
1413 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1414     do not all have to have the same fixed length. Thus
1415 nigel 63 <pre>
1416     (?&#60;=bullock|donkey)
1417 nigel 75 </pre>
1418 nigel 63 is permitted, but
1419     <pre>
1420     (?&#60;!dogs?|cats?)
1421 nigel 75 </pre>
1422 nigel 63 causes an error at compile time. Branches that match different length strings
1423     are permitted only at the top level of a lookbehind assertion. This is an
1424     extension compared with Perl (at least for 5.8), which requires all branches to
1425     match the same length of string. An assertion such as
1426     <pre>
1427     (?&#60;=ab(c|de))
1428 nigel 75 </pre>
1429 nigel 63 is not permitted, because its single top-level branch can match two different
1430     lengths, but it is acceptable if rewritten to use two top-level branches:
1431     <pre>
1432     (?&#60;=abc|abde)
1433 nigel 75 </pre>
1434 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1435 nigel 93 temporarily move the current position back by the fixed length and then try to
1436 nigel 63 match. If there are insufficient characters before the current position, the
1437 nigel 93 assertion fails.
1438 nigel 63 </P>
1439     <P>
1440     PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1441     to appear in lookbehind assertions, because it makes it impossible to calculate
1442 nigel 93 the length of the lookbehind. The \X and \R escapes, which can match
1443     different numbers of bytes, are also not permitted.
1444 nigel 63 </P>
1445     <P>
1446 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1447     specify efficient matching at the end of the subject string. Consider a simple
1448     pattern such as
1449 nigel 63 <pre>
1450     abcd$
1451 nigel 75 </pre>
1452 nigel 63 when applied to a long string that does not match. Because matching proceeds
1453     from left to right, PCRE will look for each "a" in the subject and then see if
1454     what follows matches the rest of the pattern. If the pattern is specified as
1455     <pre>
1456     ^.*abcd$
1457 nigel 75 </pre>
1458 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1459     there is no following "a"), it backtracks to match all but the last character,
1460     then all but the last two characters, and so on. Once again the search for "a"
1461     covers the entire string, from right to left, so we are no better off. However,
1462     if the pattern is written as
1463     <pre>
1464     ^.*+(?&#60;=abcd)
1465 nigel 75 </pre>
1466 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1467 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1468     characters. If it fails, the match fails immediately. For long strings, this
1469     approach makes a significant difference to the processing time.
1470     </P>
1471 nigel 75 <br><b>
1472     Using multiple assertions
1473     </b><br>
1474 nigel 63 <P>
1475     Several assertions (of any sort) may occur in succession. For example,
1476     <pre>
1477     (?&#60;=\d{3})(?&#60;!999)foo
1478 nigel 75 </pre>
1479 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1480     the assertions is applied independently at the same point in the subject
1481     string. First there is a check that the previous three characters are all
1482     digits, and then there is a check that the same three characters are not "999".
1483     This pattern does <i>not</i> match "foo" preceded by six characters, the first
1484     of which are digits and the last three of which are not "999". For example, it
1485     doesn't match "123abcfoo". A pattern to do that is
1486     <pre>
1487     (?&#60;=\d{3}...)(?&#60;!999)foo
1488 nigel 75 </pre>
1489 nigel 63 This time the first assertion looks at the preceding six characters, checking
1490     that the first three are digits, and then the second assertion checks that the
1491     preceding three characters are not "999".
1492     </P>
1493     <P>
1494     Assertions can be nested in any combination. For example,
1495     <pre>
1496     (?&#60;=(?&#60;!foo)bar)baz
1497 nigel 75 </pre>
1498 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1499     preceded by "foo", while
1500     <pre>
1501     (?&#60;=\d{3}(?!999)...)foo
1502 nigel 75 </pre>
1503     is another pattern that matches "foo" preceded by three digits and any three
1504 nigel 63 characters that are not "999".
1505 nigel 91 <a name="conditions"></a></P>
1506 nigel 93 <br><a name="SEC17" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1507 nigel 63 <P>
1508     It is possible to cause the matching process to obey a subpattern
1509     conditionally or to choose between two alternative subpatterns, depending on
1510     the result of an assertion, or whether a previous capturing subpattern matched
1511     or not. The two possible forms of conditional subpattern are
1512     <pre>
1513     (?(condition)yes-pattern)
1514     (?(condition)yes-pattern|no-pattern)
1515 nigel 75 </pre>
1516 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1517     no-pattern (if present) is used. If there are more than two alternatives in the
1518     subpattern, a compile-time error occurs.
1519     </P>
1520     <P>
1521 nigel 93 There are four kinds of condition: references to subpatterns, references to
1522     recursion, a pseudo-condition called DEFINE, and assertions.
1523 nigel 91 </P>
1524 nigel 93 <br><b>
1525     Checking for a used subpattern by number
1526     </b><br>
1527 nigel 91 <P>
1528 nigel 93 If the text between the parentheses consists of a sequence of digits, the
1529     condition is true if the capturing subpattern of that number has previously
1530     matched.
1531     </P>
1532     <P>
1533 nigel 91 Consider the following pattern, which contains non-significant white space to
1534     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1535     three parts for ease of discussion:
1536 nigel 63 <pre>
1537     ( \( )? [^()]+ (?(1) \) )
1538 nigel 75 </pre>
1539 nigel 63 The first part matches an optional opening parenthesis, and if that
1540     character is present, sets it as the first captured substring. The second part
1541     matches one or more characters that are not parentheses. The third part is a
1542     conditional subpattern that tests whether the first set of parentheses matched
1543     or not. If they did, that is, if subject started with an opening parenthesis,
1544     the condition is true, and so the yes-pattern is executed and a closing
1545     parenthesis is required. Otherwise, since no-pattern is not present, the
1546     subpattern matches nothing. In other words, this pattern matches a sequence of
1547 nigel 93 non-parentheses, optionally enclosed in parentheses.
1548     </P>
1549     <br><b>
1550     Checking for a used subpattern by name
1551     </b><br>
1552     <P>
1553     Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1554     subpattern by name. For compatibility with earlier versions of PCRE, which had
1555     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1556     there is a possible ambiguity with this syntax, because subpattern names may
1557     consist entirely of digits. PCRE looks first for a named subpattern; if it
1558     cannot find one and the name consists entirely of digits, PCRE looks for a
1559     subpattern of that number, which must be greater than zero. Using subpattern
1560     names that consist entirely of digits is not recommended.
1561     </P>
1562     <P>
1563     Rewriting the above example to use a named subpattern gives this:
1564 nigel 91 <pre>
1565 nigel 93 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
1567     </PRE>
1568     </P>
1569     <br><b>
1570     Checking for pattern recursion
1571     </b><br>
1572     <P>
1573     If the condition is the string (R), and there is no subpattern with the name R,
1574     the condition is true if a recursive call to the whole pattern or any
1575     subpattern has been made. If digits or a name preceded by ampersand follow the
1576     letter R, for example:
1577     <pre>
1578     (?(R3)...) or (?(R&name)...)
1579 nigel 91 </pre>
1580 nigel 93 the condition is true if the most recent recursion is into the subpattern whose
1581     number or name is given. This condition does not check the entire recursion
1582     stack.
1583 nigel 63 </P>
1584     <P>
1585 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1586     patterns are described below.
1587     </P>
1588     <br><b>
1589     Defining subpatterns for use by reference only
1590     </b><br>
1591     <P>
1592     If the condition is the string (DEFINE), and there is no subpattern with the
1593     name DEFINE, the condition is always false. In this case, there may be only one
1594     alternative in the subpattern. It is always skipped if control reaches this
1595     point in the pattern; the idea of DEFINE is that it can be used to define
1596     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1597     is described below.) For example, a pattern to match an IPv4 address could be
1598     written like this (ignore whitespace and line breaks):
1599     <pre>
1600     (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1601     \b (?&byte) (\.(?&byte)){3} \b
1602     </pre>
1603     The first part of the pattern is a DEFINE group inside which a another group
1604     named "byte" is defined. This matches an individual component of an IPv4
1605     address (a number less than 256). When matching takes place, this part of the
1606     pattern is skipped because DEFINE acts like a false condition.
1607     </P>
1608     <P>
1609     The rest of the pattern uses references to the named group to match the four
1610     dot-separated components of an IPv4 address, insisting on a word boundary at
1611     each end.
1612     </P>
1613     <br><b>
1614     Assertion conditions
1615     </b><br>
1616     <P>
1617     If the condition is not in any of the above formats, it must be an assertion.
1618 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1619     this pattern, again containing non-significant white space, and with the two
1620     alternatives on the second line:
1621     <pre>
1622     (?(?=[^a-z]*[a-z])
1623     \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1624 nigel 75 </pre>
1625 nigel 63 The condition is a positive lookahead assertion that matches an optional
1626     sequence of non-letters followed by a letter. In other words, it tests for the
1627     presence of at least one letter in the subject. If a letter is found, the
1628     subject is matched against the first alternative; otherwise it is matched
1629     against the second. This pattern matches strings in one of the two forms
1630     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1631 nigel 75 <a name="comments"></a></P>
1632 nigel 93 <br><a name="SEC18" href="#TOC1">COMMENTS</a><br>
1633 nigel 63 <P>
1634 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1635 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1636     that make up a comment play no part in the pattern matching at all.
1637     </P>
1638     <P>
1639     If the PCRE_EXTENDED option is set, an unescaped # character outside a
1640 nigel 91 character class introduces a comment that continues to immediately after the
1641     next newline in the pattern.
1642     <a name="recursion"></a></P>
1643 nigel 93 <br><a name="SEC19" href="#TOC1">RECURSIVE PATTERNS</a><br>
1644 nigel 63 <P>
1645     Consider the problem of matching a string in parentheses, allowing for
1646     unlimited nested parentheses. Without the use of recursion, the best that can
1647     be done is to use a pattern that matches up to some fixed depth of nesting. It
1648 nigel 93 is not possible to handle an arbitrary nesting depth.
1649     </P>
1650     <P>
1651     For some time, Perl has provided a facility that allows regular expressions to
1652     recurse (amongst other things). It does this by interpolating Perl code in the
1653     expression at run time, and the code can refer to the expression itself. A Perl
1654     pattern using code interpolation to solve the parentheses problem can be
1655     created like this:
1656 nigel 63 <pre>
1657     $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1658 nigel 75 </pre>
1659 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1660 nigel 93 recursively to the pattern in which it appears.
1661 nigel 63 </P>
1662     <P>
1663 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1664     supports special syntax for recursion of the entire pattern, and also for
1665     individual subpattern recursion. After its introduction in PCRE and Python,
1666     this kind of recursion was introduced into Perl at release 5.10.
1667 nigel 63 </P>
1668     <P>
1669 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1670     closing parenthesis is a recursive call of the subpattern of the given number,
1671     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1672     call, which is described in the next section.) The special item (?R) or (?0) is
1673     a recursive call of the entire regular expression.
1674 nigel 87 </P>
1675     <P>
1676 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1677     treated as an atomic group. That is, once it has matched some of the subject
1678     string, it is never re-entered, even if it contains untried alternatives and
1679     there is a subsequent matching failure.
1680     </P>
1681     <P>
1682 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1683     PCRE_EXTENDED option is set so that white space is ignored):
1684 nigel 63 <pre>
1685     \( ( (?&#62;[^()]+) | (?R) )* \)
1686 nigel 75 </pre>
1687 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1688     substrings which can either be a sequence of non-parentheses, or a recursive
1689 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1690 nigel 63 Finally there is a closing parenthesis.
1691     </P>
1692     <P>
1693     If this were part of a larger pattern, you would not want to recurse the entire
1694     pattern, so instead you could use this:
1695     <pre>
1696     ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1697 nigel 75 </pre>
1698 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1699     them instead of the whole pattern. In a larger pattern, keeping track of
1700     parenthesis numbers can be tricky. It may be more convenient to use named
1701 nigel 93 parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier
1702     syntax (?P&#62;name) is also supported. We could rewrite the above example as
1703     follows:
1704 nigel 63 <pre>
1705 nigel 93 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1706 nigel 75 </pre>
1707 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1708     used. This particular example pattern contains nested unlimited repeats, and so
1709     the use of atomic grouping for matching strings of non-parentheses is important
1710 nigel 63 when applying the pattern to strings that do not match. For example, when this
1711     pattern is applied to
1712     <pre>
1713     (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1714 nigel 75 </pre>
1715 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1716     the match runs for a very long time indeed because there are so many different
1717     ways the + and * repeats can carve up the subject, and all have to be tested
1718     before failure can be reported.
1719     </P>
1720     <P>
1721     At the end of a match, the values set for any capturing subpatterns are those
1722     from the outermost level of the recursion at which the subpattern value is set.
1723     If you want to obtain intermediate values, a callout function can be used (see
1724 nigel 93 below and the
1725 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1726     documentation). If the pattern above is matched against
1727     <pre>
1728     (ab(cd)ef)
1729 nigel 75 </pre>
1730 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1731     on at the top level. If additional parentheses are added, giving
1732     <pre>
1733     \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
1734     ^ ^
1735     ^ ^
1736 nigel 75 </pre>
1737 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1738     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1739     has to obtain extra memory to store data during a recursion, which it does by
1740     using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
1741     memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1742     </P>
1743     <P>
1744     Do not confuse the (?R) item with the condition (R), which tests for recursion.
1745     Consider this pattern, which matches text in angle brackets, allowing for
1746     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1747     recursing), whereas any characters are permitted at the outer level.
1748     <pre>
1749     &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
1750 nigel 75 </pre>
1751 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1752     different alternatives for the recursive and non-recursive cases. The (?R) item
1753     is the actual recursive call.
1754 nigel 75 <a name="subpatternsassubroutines"></a></P>
1755 nigel 93 <br><a name="SEC20" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1756 nigel 63 <P>
1757     If the syntax for a recursive subpattern reference (either by number or by
1758     name) is used outside the parentheses to which it refers, it operates like a
1759 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
1760     before or after the reference. An earlier example pointed out that the pattern
1761 nigel 63 <pre>
1762     (sens|respons)e and \1ibility
1763 nigel 75 </pre>
1764 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1765     "sense and responsibility". If instead the pattern
1766     <pre>
1767     (sens|respons)e and (?1)ibility
1768 nigel 75 </pre>
1769 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1770 nigel 93 strings. Another example is given in the discussion of DEFINE above.
1771 nigel 63 </P>
1772 nigel 87 <P>
1773     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1774     group. That is, once it has matched some of the subject string, it is never
1775     re-entered, even if it contains untried alternatives and there is a subsequent
1776     matching failure.
1777     </P>
1778 nigel 63 <P>
1779 nigel 93 When a subpattern is used as a subroutine, processing options such as
1780     case-independence are fixed when the subpattern is defined. They cannot be
1781     changed for different calls. For example, consider this pattern:
1782     <pre>
1783     (abc)(?i:(?1))
1784     </pre>
1785     It matches "abcabc". It does not match "abcABC" because the change of
1786     processing option does not affect the called subpattern.
1787     </P>
1788     <br><a name="SEC21" href="#TOC1">CALLOUTS</a><br>
1789     <P>
1790 nigel 63 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1791     code to be obeyed in the middle of matching a regular expression. This makes it
1792     possible, amongst other things, to extract different substrings that match the
1793     same pair of parentheses when there is a repetition.
1794     </P>
1795     <P>
1796     PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1797     code. The feature is called "callout". The caller of PCRE provides an external
1798     function by putting its entry point in the global variable <i>pcre_callout</i>.
1799     By default, this variable contains NULL, which disables all calling out.
1800     </P>
1801     <P>
1802     Within a regular expression, (?C) indicates the points at which the external
1803     function is to be called. If you want to identify different callout points, you
1804     can put a number less than 256 after the letter C. The default value is zero.
1805     For example, this pattern has two callout points:
1806     <pre>
1807     (?C1)\dabc(?C2)def
1808 nigel 75 </pre>
1809     If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
1810     automatically installed before each item in the pattern. They are all numbered
1811     255.
1812 nigel 63 </P>
1813     <P>
1814     During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
1815     set), the external function is called. It is provided with the number of the
1816 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1817     originally supplied by the caller of <b>pcre_exec()</b>. The callout function
1818     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1819     description of the interface to the callout function is given in the
1820 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1821     documentation.
1822     </P>
1823 nigel 93 <br><a name="SEC22" href="#TOC1">SEE ALSO</a><br>
1824 nigel 63 <P>
1825 nigel 93 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
1826     </P>
1827 ph10 99 <br><a name="SEC23" href="#TOC1">AUTHOR</a><br>
1828 nigel 93 <P>
1829 ph10 99 Philip Hazel
1830 nigel 63 <br>
1831 ph10 99 University Computing Service
1832     <br>
1833     Cambridge CB2 3QH, England.
1834     <br>
1835     </P>
1836     <br><a name="SEC24" href="#TOC1">REVISION</a><br>
1837     <P>
1838     Last updated: 06 March 2007
1839     <br>
1840     Copyright &copy; 1997-2007 University of Cambridge.
1841     <br>
1842 nigel 75 <p>
1843     Return to the <a href="index.html">PCRE index page</a>.
1844     </p>


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