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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     <p>
11     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     <br>
15 nigel 63 <ul>
16     <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17     <li><a name="TOC2" href="#SEC2">BACKSLASH</a>
18     <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>
19     <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>
20     <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>
21 nigel 75 <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a>
22 nigel 63 <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>
23     <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>
24     <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>
25     <li><a name="TOC10" href="#SEC10">SUBPATTERNS</a>
26     <li><a name="TOC11" href="#SEC11">NAMED SUBPATTERNS</a>
27     <li><a name="TOC12" href="#SEC12">REPETITION</a>
28     <li><a name="TOC13" href="#SEC13">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
29     <li><a name="TOC14" href="#SEC14">BACK REFERENCES</a>
30     <li><a name="TOC15" href="#SEC15">ASSERTIONS</a>
31     <li><a name="TOC16" href="#SEC16">CONDITIONAL SUBPATTERNS</a>
32     <li><a name="TOC17" href="#SEC17">COMMENTS</a>
33     <li><a name="TOC18" href="#SEC18">RECURSIVE PATTERNS</a>
34     <li><a name="TOC19" href="#SEC19">SUBPATTERNS AS SUBROUTINES</a>
35     <li><a name="TOC20" href="#SEC20">CALLOUTS</a>
36     </ul>
37     <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
38     <P>
39     The syntax and semantics of the regular expressions supported by PCRE are
40     described below. Regular expressions are also described in the Perl
41 nigel 75 documentation and in a number of books, some of which have copious examples.
42     Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
43     regular expressions in great detail. This description of PCRE's regular
44     expressions is intended as reference material.
45 nigel 63 </P>
46     <P>
47 nigel 75 The original operation of PCRE was on strings of one-byte characters. However,
48     there is now also support for UTF-8 character strings. To use this, you must
49     build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
50     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
51     places below. There is also a summary of UTF-8 features in the
52 nigel 63 <a href="pcre.html#utf8support">section on UTF-8 support</a>
53     in the main
54     <a href="pcre.html"><b>pcre</b></a>
55     page.
56     </P>
57     <P>
58 nigel 77 The remainder of this document discusses the patterns that are supported by
59     PCRE when its main matching function, <b>pcre_exec()</b>, is used.
60     From release 6.0, PCRE offers a second matching function,
61     <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
62     Perl-compatible. The advantages and disadvantages of the alternative function,
63     and how it differs from the normal function, are discussed in the
64     <a href="pcrematching.html"><b>pcrematching</b></a>
65     page.
66     </P>
67     <P>
68 nigel 63 A regular expression is a pattern that is matched against a subject string from
69     left to right. Most characters stand for themselves in a pattern, and match the
70     corresponding characters in the subject. As a trivial example, the pattern
71     <pre>
72     The quick brown fox
73 nigel 75 </pre>
74 nigel 77 matches a portion of a subject string that is identical to itself. When
75     caseless matching is specified (the PCRE_CASELESS option), letters are matched
76     independently of case. In UTF-8 mode, PCRE always understands the concept of
77     case for characters whose values are less than 128, so caseless matching is
78     always possible. For characters with higher values, the concept of case is
79     supported if PCRE is compiled with Unicode property support, but not otherwise.
80     If you want to use caseless matching for characters 128 and above, you must
81     ensure that PCRE is compiled with Unicode property support as well as with
82     UTF-8 support.
83     </P>
84     <P>
85     The power of regular expressions comes from the ability to include alternatives
86     and repetitions in the pattern. These are encoded in the pattern by the use of
87 nigel 75 <i>metacharacters</i>, which do not stand for themselves but instead are
88 nigel 63 interpreted in some special way.
89     </P>
90     <P>
91 nigel 75 There are two different sets of metacharacters: those that are recognized
92 nigel 63 anywhere in the pattern except within square brackets, and those that are
93 nigel 75 recognized in square brackets. Outside square brackets, the metacharacters are
94 nigel 63 as follows:
95     <pre>
96     \ general escape character with several uses
97     ^ assert start of string (or line, in multiline mode)
98     $ assert end of string (or line, in multiline mode)
99     . match any character except newline (by default)
100     [ start character class definition
101     | start of alternative branch
102     ( start subpattern
103     ) end subpattern
104     ? extends the meaning of (
105     also 0 or 1 quantifier
106     also quantifier minimizer
107     * 0 or more quantifier
108     + 1 or more quantifier
109     also "possessive quantifier"
110     { start min/max quantifier
111 nigel 75 </pre>
112 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
113 nigel 75 a character class the only metacharacters are:
114 nigel 63 <pre>
115     \ general escape character
116     ^ negate the class, but only if the first character
117     - indicates character range
118 nigel 75 [ POSIX character class (only if followed by POSIX syntax)
119 nigel 63 ] terminates the character class
120 nigel 75 </pre>
121     The following sections describe the use of each of the metacharacters.
122 nigel 63 </P>
123     <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>
124     <P>
125     The backslash character has several uses. Firstly, if it is followed by a
126 nigel 75 non-alphanumeric character, it takes away any special meaning that character may
127 nigel 63 have. This use of backslash as an escape character applies both inside and
128     outside character classes.
129     </P>
130     <P>
131     For example, if you want to match a * character, you write \* in the pattern.
132     This escaping action applies whether or not the following character would
133 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
134     non-alphanumeric with backslash to specify that it stands for itself. In
135 nigel 63 particular, if you want to match a backslash, you write \\.
136     </P>
137     <P>
138     If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
139     pattern (other than in a character class) and characters between a # outside
140     a character class and the next newline character are ignored. An escaping
141     backslash can be used to include a whitespace or # character as part of the
142     pattern.
143     </P>
144     <P>
145     If you want to remove the special meaning from a sequence of characters, you
146     can do so by putting them between \Q and \E. This is different from Perl in
147     that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
148     Perl, $ and @ cause variable interpolation. Note the following examples:
149     <pre>
150     Pattern PCRE matches Perl matches
151 nigel 75
152     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
153 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
154     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
155 nigel 75 </pre>
156 nigel 63 The \Q...\E sequence is recognized both inside and outside character classes.
157 nigel 75 <a name="digitsafterbackslash"></a></P>
158     <br><b>
159     Non-printing characters
160     </b><br>
161 nigel 63 <P>
162     A second use of backslash provides a way of encoding non-printing characters
163     in patterns in a visible manner. There is no restriction on the appearance of
164     non-printing characters, apart from the binary zero that terminates a pattern,
165     but when a pattern is being prepared by text editing, it is usually easier to
166     use one of the following escape sequences than the binary character it
167     represents:
168     <pre>
169     \a alarm, that is, the BEL character (hex 07)
170     \cx "control-x", where x is any character
171     \e escape (hex 1B)
172     \f formfeed (hex 0C)
173     \n newline (hex 0A)
174     \r carriage return (hex 0D)
175     \t tab (hex 09)
176     \ddd character with octal code ddd, or backreference
177     \xhh character with hex code hh
178 nigel 87 \x{hhh..} character with hex code hhh..
179 nigel 75 </pre>
180 nigel 63 The precise effect of \cx is as follows: if x is a lower case letter, it
181     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
182     Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
183     7B.
184     </P>
185     <P>
186     After \x, from zero to two hexadecimal digits are read (letters can be in
187 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \x{
188     and }, but the value of the character code must be less than 256 in non-UTF-8
189     mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
190     is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{
191     and }, or if there is no terminating }, this form of escape is not recognized.
192     Instead, the initial \x will be interpreted as a basic hexadecimal escape,
193     with no following digits, giving a character whose value is zero.
194 nigel 63 </P>
195     <P>
196     Characters whose value is less than 256 can be defined by either of the two
197 nigel 87 syntaxes for \x. There is no difference in the way they are handled. For
198     example, \xdc is exactly the same as \x{dc}.
199 nigel 63 </P>
200     <P>
201     After \0 up to two further octal digits are read. In both cases, if there
202     are fewer than two digits, just those that are present are used. Thus the
203     sequence \0\x\07 specifies two binary zeros followed by a BEL character
204     (code value 7). Make sure you supply two digits after the initial zero if the
205 nigel 75 pattern character that follows is itself an octal digit.
206 nigel 63 </P>
207     <P>
208     The handling of a backslash followed by a digit other than 0 is complicated.
209     Outside a character class, PCRE reads it and any following digits as a decimal
210     number. If the number is less than 10, or if there have been at least that many
211     previous capturing left parentheses in the expression, the entire sequence is
212     taken as a <i>back reference</i>. A description of how this works is given
213 nigel 75 <a href="#backreferences">later,</a>
214     following the discussion of
215     <a href="#subpattern">parenthesized subpatterns.</a>
216 nigel 63 </P>
217     <P>
218     Inside a character class, or if the decimal number is greater than 9 and there
219     have not been that many capturing subpatterns, PCRE re-reads up to three octal
220     digits following the backslash, and generates a single byte from the least
221     significant 8 bits of the value. Any subsequent digits stand for themselves.
222     For example:
223     <pre>
224     \040 is another way of writing a space
225 nigel 75 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
226 nigel 63 \7 is always a back reference
227 nigel 75 \11 might be a back reference, or another way of writing a tab
228 nigel 63 \011 is always a tab
229     \0113 is a tab followed by the character "3"
230 nigel 75 \113 might be a back reference, otherwise the character with octal code 113
231     \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
232     \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
233     </pre>
234 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
235     zero, because no more than three octal digits are ever read.
236     </P>
237     <P>
238     All the sequences that define a single byte value or a single UTF-8 character
239     (in UTF-8 mode) can be used both inside and outside character classes. In
240     addition, inside a character class, the sequence \b is interpreted as the
241 nigel 75 backspace character (hex 08), and the sequence \X is interpreted as the
242     character "X". Outside a character class, these sequences have different
243     meanings
244     <a href="#uniextseq">(see below).</a>
245 nigel 63 </P>
246 nigel 75 <br><b>
247     Generic character types
248     </b><br>
249 nigel 63 <P>
250 nigel 75 The third use of backslash is for specifying generic character types. The
251     following are always recognized:
252 nigel 63 <pre>
253     \d any decimal digit
254     \D any character that is not a decimal digit
255     \s any whitespace character
256     \S any character that is not a whitespace character
257     \w any "word" character
258     \W any "non-word" character
259 nigel 75 </pre>
260 nigel 63 Each pair of escape sequences partitions the complete set of characters into
261     two disjoint sets. Any given character matches one, and only one, of each pair.
262     </P>
263     <P>
264 nigel 75 These character type sequences can appear both inside and outside character
265     classes. They each match one character of the appropriate type. If the current
266     matching point is at the end of the subject string, all of them fail, since
267     there is no character to match.
268 nigel 63 </P>
269     <P>
270     For compatibility with Perl, \s does not match the VT character (code 11).
271     This makes it different from the the POSIX "space" class. The \s characters
272     are HT (9), LF (10), FF (12), CR (13), and space (32).
273     </P>
274     <P>
275 nigel 75 A "word" character is an underscore or any character less than 256 that is a
276     letter or digit. The definition of letters and digits is controlled by PCRE's
277     low-valued character tables, and may vary if locale-specific matching is taking
278     place (see
279 nigel 63 <a href="pcreapi.html#localesupport">"Locale support"</a>
280     in the
281     <a href="pcreapi.html"><b>pcreapi</b></a>
282 nigel 75 page). For example, in the "fr_FR" (French) locale, some character codes
283     greater than 128 are used for accented letters, and these are matched by \w.
284 nigel 63 </P>
285     <P>
286 nigel 75 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
287     \w, and always match \D, \S, and \W. This is true even when Unicode
288 nigel 87 character property support is available. The use of locales with Unicode is
289     discouraged.
290 nigel 75 <a name="uniextseq"></a></P>
291     <br><b>
292     Unicode character properties
293     </b><br>
294     <P>
295     When PCRE is built with Unicode character property support, three additional
296 nigel 87 escape sequences to match character properties are available when UTF-8 mode
297 nigel 75 is selected. They are:
298     <pre>
299 nigel 87 \p{<i>xx</i>} a character with the <i>xx</i> property
300     \P{<i>xx</i>} a character without the <i>xx</i> property
301     \X an extended Unicode sequence
302 nigel 75 </pre>
303 nigel 87 The property names represented by <i>xx</i> above are limited to the Unicode
304     script names, the general category properties, and "Any", which matches any
305     character (including newline). Other properties such as "InMusicalSymbols" are
306     not currently supported by PCRE. Note that \P{Any} does not match any
307     characters, so always causes a match failure.
308 nigel 63 </P>
309     <P>
310 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
311     character from one of these sets can be matched using a script name. For
312     example:
313 nigel 75 <pre>
314 nigel 87 \p{Greek}
315     \P{Han}
316     </pre>
317     Those that are not part of an identified script are lumped together as
318     "Common". The current list of scripts is:
319     </P>
320     <P>
321     Arabic,
322     Armenian,
323     Bengali,
324     Bopomofo,
325     Braille,
326     Buginese,
327     Buhid,
328     Canadian_Aboriginal,
329     Cherokee,
330     Common,
331     Coptic,
332     Cypriot,
333     Cyrillic,
334     Deseret,
335     Devanagari,
336     Ethiopic,
337     Georgian,
338     Glagolitic,
339     Gothic,
340     Greek,
341     Gujarati,
342     Gurmukhi,
343     Han,
344     Hangul,
345     Hanunoo,
346     Hebrew,
347     Hiragana,
348     Inherited,
349     Kannada,
350     Katakana,
351     Kharoshthi,
352     Khmer,
353     Lao,
354     Latin,
355     Limbu,
356     Linear_B,
357     Malayalam,
358     Mongolian,
359     Myanmar,
360     New_Tai_Lue,
361     Ogham,
362     Old_Italic,
363     Old_Persian,
364     Oriya,
365     Osmanya,
366     Runic,
367     Shavian,
368     Sinhala,
369     Syloti_Nagri,
370     Syriac,
371     Tagalog,
372     Tagbanwa,
373     Tai_Le,
374     Tamil,
375     Telugu,
376     Thaana,
377     Thai,
378     Tibetan,
379     Tifinagh,
380     Ugaritic,
381     Yi.
382     </P>
383     <P>
384     Each character has exactly one general category property, specified by a
385     two-letter abbreviation. For compatibility with Perl, negation can be specified
386     by including a circumflex between the opening brace and the property name. For
387     example, \p{^Lu} is the same as \P{Lu}.
388     </P>
389     <P>
390     If only one letter is specified with \p or \P, it includes all the general
391     category properties that start with that letter. In this case, in the absence
392     of negation, the curly brackets in the escape sequence are optional; these two
393     examples have the same effect:
394     <pre>
395 nigel 75 \p{L}
396     \pL
397     </pre>
398 nigel 87 The following general category property codes are supported:
399 nigel 75 <pre>
400     C Other
401     Cc Control
402     Cf Format
403     Cn Unassigned
404     Co Private use
405     Cs Surrogate
406    
407     L Letter
408     Ll Lower case letter
409     Lm Modifier letter
410     Lo Other letter
411     Lt Title case letter
412     Lu Upper case letter
413    
414     M Mark
415     Mc Spacing mark
416     Me Enclosing mark
417     Mn Non-spacing mark
418    
419     N Number
420     Nd Decimal number
421     Nl Letter number
422     No Other number
423    
424     P Punctuation
425     Pc Connector punctuation
426     Pd Dash punctuation
427     Pe Close punctuation
428     Pf Final punctuation
429     Pi Initial punctuation
430     Po Other punctuation
431     Ps Open punctuation
432    
433     S Symbol
434     Sc Currency symbol
435     Sk Modifier symbol
436     Sm Mathematical symbol
437     So Other symbol
438    
439     Z Separator
440     Zl Line separator
441     Zp Paragraph separator
442     Zs Space separator
443     </pre>
444 nigel 87 The special property L& is also supported: it matches a character that has
445     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
446     a modifier or "other".
447 nigel 75 </P>
448     <P>
449 nigel 87 The long synonyms for these properties that Perl supports (such as \p{Letter})
450     are not supported by PCRE. Nor is is permitted to prefix any of these
451     properties with "Is".
452     </P>
453     <P>
454     No character that is in the Unicode table has the Cn (unassigned) property.
455     Instead, this property is assumed for any code point that is not in the
456     Unicode table.
457     </P>
458     <P>
459 nigel 75 Specifying caseless matching does not affect these escape sequences. For
460     example, \p{Lu} always matches only upper case letters.
461     </P>
462     <P>
463     The \X escape matches any number of Unicode characters that form an extended
464     Unicode sequence. \X is equivalent to
465     <pre>
466     (?&#62;\PM\pM*)
467     </pre>
468     That is, it matches a character without the "mark" property, followed by zero
469     or more characters with the "mark" property, and treats the sequence as an
470     atomic group
471     <a href="#atomicgroup">(see below).</a>
472     Characters with the "mark" property are typically accents that affect the
473     preceding character.
474     </P>
475     <P>
476     Matching characters by Unicode property is not fast, because PCRE has to search
477     a structure that contains data for over fifteen thousand characters. That is
478     why the traditional escape sequences such as \d and \w do not use Unicode
479     properties in PCRE.
480     <a name="smallassertions"></a></P>
481     <br><b>
482     Simple assertions
483     </b><br>
484     <P>
485 nigel 63 The fourth use of backslash is for certain simple assertions. An assertion
486     specifies a condition that has to be met at a particular point in a match,
487     without consuming any characters from the subject string. The use of
488 nigel 75 subpatterns for more complicated assertions is described
489     <a href="#bigassertions">below.</a>
490     The backslashed
491     assertions are:
492 nigel 63 <pre>
493     \b matches at a word boundary
494     \B matches when not at a word boundary
495     \A matches at start of subject
496     \Z matches at end of subject or before newline at end
497     \z matches at end of subject
498     \G matches at first matching position in subject
499 nigel 75 </pre>
500 nigel 63 These assertions may not appear in character classes (but note that \b has a
501     different meaning, namely the backspace character, inside a character class).
502     </P>
503     <P>
504     A word boundary is a position in the subject string where the current character
505     and the previous character do not both match \w or \W (i.e. one matches
506     \w and the other matches \W), or the start or end of the string if the
507     first or last character matches \w, respectively.
508     </P>
509     <P>
510     The \A, \Z, and \z assertions differ from the traditional circumflex and
511 nigel 75 dollar (described in the next section) in that they only ever match at the very
512     start and end of the subject string, whatever options are set. Thus, they are
513     independent of multiline mode. These three assertions are not affected by the
514     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
515     circumflex and dollar metacharacters. However, if the <i>startoffset</i>
516     argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
517     at a point other than the beginning of the subject, \A can never match. The
518     difference between \Z and \z is that \Z matches before a newline that is the
519     last character of the string as well as at the end of the string, whereas \z
520     matches only at the end.
521 nigel 63 </P>
522     <P>
523     The \G assertion is true only when the current matching position is at the
524     start point of the match, as specified by the <i>startoffset</i> argument of
525     <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
526     non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
527     arguments, you can mimic Perl's /g option, and it is in this kind of
528     implementation where \G can be useful.
529     </P>
530     <P>
531     Note, however, that PCRE's interpretation of \G, as the start of the current
532     match, is subtly different from Perl's, which defines it as the end of the
533     previous match. In Perl, these can be different when the previously matched
534     string was empty. Because PCRE does just one match at a time, it cannot
535     reproduce this behaviour.
536     </P>
537     <P>
538     If all the alternatives of a pattern begin with \G, the expression is anchored
539     to the starting match position, and the "anchored" flag is set in the compiled
540     regular expression.
541     </P>
542     <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
543     <P>
544     Outside a character class, in the default matching mode, the circumflex
545 nigel 75 character is an assertion that is true only if the current matching point is
546 nigel 63 at the start of the subject string. If the <i>startoffset</i> argument of
547     <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
548     option is unset. Inside a character class, circumflex has an entirely different
549 nigel 75 meaning
550     <a href="#characterclass">(see below).</a>
551 nigel 63 </P>
552     <P>
553     Circumflex need not be the first character of the pattern if a number of
554     alternatives are involved, but it should be the first thing in each alternative
555     in which it appears if the pattern is ever to match that branch. If all
556     possible alternatives start with a circumflex, that is, if the pattern is
557     constrained to match only at the start of the subject, it is said to be an
558     "anchored" pattern. (There are also other constructs that can cause a pattern
559     to be anchored.)
560     </P>
561     <P>
562 nigel 75 A dollar character is an assertion that is true only if the current matching
563 nigel 63 point is at the end of the subject string, or immediately before a newline
564     character that is the last character in the string (by default). Dollar need
565     not be the last character of the pattern if a number of alternatives are
566     involved, but it should be the last item in any branch in which it appears.
567     Dollar has no special meaning in a character class.
568     </P>
569     <P>
570     The meaning of dollar can be changed so that it matches only at the very end of
571     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
572     does not affect the \Z assertion.
573     </P>
574     <P>
575     The meanings of the circumflex and dollar characters are changed if the
576     PCRE_MULTILINE option is set. When this is the case, they match immediately
577     after and immediately before an internal newline character, respectively, in
578     addition to matching at the start and end of the subject string. For example,
579 nigel 75 the pattern /^abc$/ matches the subject string "def\nabc" (where \n
580     represents a newline character) in multiline mode, but not otherwise.
581     Consequently, patterns that are anchored in single line mode because all
582     branches start with ^ are not anchored in multiline mode, and a match for
583     circumflex is possible when the <i>startoffset</i> argument of <b>pcre_exec()</b>
584     is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
585     set.
586 nigel 63 </P>
587     <P>
588     Note that the sequences \A, \Z, and \z can be used to match the start and
589     end of the subject in both modes, and if all branches of a pattern start with
590     \A it is always anchored, whether PCRE_MULTILINE is set or not.
591     </P>
592     <br><a name="SEC4" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
593     <P>
594     Outside a character class, a dot in the pattern matches any one character in
595     the subject, including a non-printing character, but not (by default) newline.
596     In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
597 nigel 75 byte long, except (by default) newline. If the PCRE_DOTALL option is set,
598 nigel 63 dots match newlines as well. The handling of dot is entirely independent of the
599     handling of circumflex and dollar, the only relationship being that they both
600     involve newline characters. Dot has no special meaning in a character class.
601     </P>
602     <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
603     <P>
604     Outside a character class, the escape sequence \C matches any one byte, both
605 nigel 75 in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
606     provided in Perl in order to match individual bytes in UTF-8 mode. Because it
607     breaks up UTF-8 characters into individual bytes, what remains in the string
608     may be a malformed UTF-8 string. For this reason, the \C escape sequence is
609     best avoided.
610 nigel 63 </P>
611     <P>
612 nigel 75 PCRE does not allow \C to appear in lookbehind assertions
613     <a href="#lookbehind">(described below),</a>
614     because in UTF-8 mode this would make it impossible to calculate the length of
615     the lookbehind.
616     <a name="characterclass"></a></P>
617     <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
618 nigel 63 <P>
619     An opening square bracket introduces a character class, terminated by a closing
620     square bracket. A closing square bracket on its own is not special. If a
621     closing square bracket is required as a member of the class, it should be the
622     first data character in the class (after an initial circumflex, if present) or
623     escaped with a backslash.
624     </P>
625     <P>
626     A character class matches a single character in the subject. In UTF-8 mode, the
627     character may occupy more than one byte. A matched character must be in the set
628     of characters defined by the class, unless the first character in the class
629     definition is a circumflex, in which case the subject character must not be in
630     the set defined by the class. If a circumflex is actually required as a member
631     of the class, ensure it is not the first character, or escape it with a
632     backslash.
633     </P>
634     <P>
635     For example, the character class [aeiou] matches any lower case vowel, while
636     [^aeiou] matches any character that is not a lower case vowel. Note that a
637 nigel 75 circumflex is just a convenient notation for specifying the characters that
638     are in the class by enumerating those that are not. A class that starts with a
639     circumflex is not an assertion: it still consumes a character from the subject
640     string, and therefore it fails if the current pointer is at the end of the
641     string.
642 nigel 63 </P>
643     <P>
644     In UTF-8 mode, characters with values greater than 255 can be included in a
645     class as a literal string of bytes, or by using the \x{ escaping mechanism.
646     </P>
647     <P>
648     When caseless matching is set, any letters in a class represent both their
649     upper case and lower case versions, so for example, a caseless [aeiou] matches
650     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
651 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
652     case for characters whose values are less than 128, so caseless matching is
653     always possible. For characters with higher values, the concept of case is
654     supported if PCRE is compiled with Unicode property support, but not otherwise.
655     If you want to use caseless matching for characters 128 and above, you must
656     ensure that PCRE is compiled with Unicode property support as well as with
657     UTF-8 support.
658 nigel 63 </P>
659     <P>
660     The newline character is never treated in any special way in character classes,
661     whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
662     such as [^a] will always match a newline.
663     </P>
664     <P>
665     The minus (hyphen) character can be used to specify a range of characters in a
666     character class. For example, [d-m] matches any letter between d and m,
667     inclusive. If a minus character is required in a class, it must be escaped with
668     a backslash or appear in a position where it cannot be interpreted as
669     indicating a range, typically as the first or last character in the class.
670     </P>
671     <P>
672     It is not possible to have the literal character "]" as the end character of a
673     range. A pattern such as [W-]46] is interpreted as a class of two characters
674     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
675     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
676 nigel 75 the end of range, so [W-\]46] is interpreted as a class containing a range
677     followed by two other characters. The octal or hexadecimal representation of
678     "]" can also be used to end a range.
679 nigel 63 </P>
680     <P>
681     Ranges operate in the collating sequence of character values. They can also be
682     used for characters specified numerically, for example [\000-\037]. In UTF-8
683     mode, ranges can include characters whose values are greater than 255, for
684     example [\x{100}-\x{2ff}].
685     </P>
686     <P>
687     If a range that includes letters is used when caseless matching is set, it
688     matches the letters in either case. For example, [W-c] is equivalent to
689 nigel 75 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
690     tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E
691     characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
692     characters with values greater than 128 only when it is compiled with Unicode
693     property support.
694 nigel 63 </P>
695     <P>
696 nigel 75 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
697     in a character class, and add the characters that they match to the class. For
698 nigel 63 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
699     conveniently be used with the upper case character types to specify a more
700     restricted set of characters than the matching lower case type. For example,
701     the class [^\W_] matches any letter or digit, but not underscore.
702     </P>
703     <P>
704 nigel 75 The only metacharacters that are recognized in character classes are backslash,
705     hyphen (only where it can be interpreted as specifying a range), circumflex
706     (only at the start), opening square bracket (only when it can be interpreted as
707     introducing a POSIX class name - see the next section), and the terminating
708     closing square bracket. However, escaping other non-alphanumeric characters
709     does no harm.
710 nigel 63 </P>
711     <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
712     <P>
713 nigel 75 Perl supports the POSIX notation for character classes. This uses names
714 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
715     this notation. For example,
716     <pre>
717     [01[:alpha:]%]
718 nigel 75 </pre>
719 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
720     are
721     <pre>
722     alnum letters and digits
723     alpha letters
724     ascii character codes 0 - 127
725     blank space or tab only
726     cntrl control characters
727     digit decimal digits (same as \d)
728     graph printing characters, excluding space
729     lower lower case letters
730     print printing characters, including space
731     punct printing characters, excluding letters and digits
732     space white space (not quite the same as \s)
733     upper upper case letters
734     word "word" characters (same as \w)
735     xdigit hexadecimal digits
736 nigel 75 </pre>
737 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
738     space (32). Notice that this list includes the VT character (code 11). This
739     makes "space" different to \s, which does not include VT (for Perl
740     compatibility).
741     </P>
742     <P>
743     The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
744     5.8. Another Perl extension is negation, which is indicated by a ^ character
745     after the colon. For example,
746     <pre>
747     [12[:^digit:]]
748 nigel 75 </pre>
749 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
750     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
751     supported, and an error is given if they are encountered.
752     </P>
753     <P>
754 nigel 75 In UTF-8 mode, characters with values greater than 128 do not match any of
755 nigel 63 the POSIX character classes.
756     </P>
757     <br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br>
758     <P>
759     Vertical bar characters are used to separate alternative patterns. For example,
760     the pattern
761     <pre>
762     gilbert|sullivan
763 nigel 75 </pre>
764 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
765     and an empty alternative is permitted (matching the empty string).
766     The matching process tries each alternative in turn, from left to right,
767     and the first one that succeeds is used. If the alternatives are within a
768 nigel 75 subpattern
769     <a href="#subpattern">(defined below),</a>
770     "succeeds" means matching the rest of the main pattern as well as the
771     alternative in the subpattern.
772 nigel 63 </P>
773     <br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br>
774     <P>
775     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
776     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
777     Perl option letters enclosed between "(?" and ")". The option letters are
778     <pre>
779     i for PCRE_CASELESS
780     m for PCRE_MULTILINE
781     s for PCRE_DOTALL
782     x for PCRE_EXTENDED
783 nigel 75 </pre>
784 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
785     unset these options by preceding the letter with a hyphen, and a combined
786     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
787     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
788     permitted. If a letter appears both before and after the hyphen, the option is
789     unset.
790     </P>
791     <P>
792     When an option change occurs at top level (that is, not inside subpattern
793     parentheses), the change applies to the remainder of the pattern that follows.
794     If the change is placed right at the start of a pattern, PCRE extracts it into
795     the global options (and it will therefore show up in data extracted by the
796     <b>pcre_fullinfo()</b> function).
797     </P>
798     <P>
799     An option change within a subpattern affects only that part of the current
800     pattern that follows it, so
801     <pre>
802     (a(?i)b)c
803 nigel 75 </pre>
804 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
805     By this means, options can be made to have different settings in different
806     parts of the pattern. Any changes made in one alternative do carry on
807     into subsequent branches within the same subpattern. For example,
808     <pre>
809     (a(?i)b|c)
810 nigel 75 </pre>
811 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
812     branch is abandoned before the option setting. This is because the effects of
813     option settings happen at compile time. There would be some very weird
814     behaviour otherwise.
815     </P>
816     <P>
817     The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
818     same way as the Perl-compatible options by using the characters U and X
819     respectively. The (?X) flag setting is special in that it must always occur
820     earlier in the pattern than any of the additional features it turns on, even
821 nigel 75 when it is at top level. It is best to put it at the start.
822     <a name="subpattern"></a></P>
823 nigel 63 <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>
824     <P>
825     Subpatterns are delimited by parentheses (round brackets), which can be nested.
826 nigel 75 Turning part of a pattern into a subpattern does two things:
827     <br>
828     <br>
829 nigel 63 1. It localizes a set of alternatives. For example, the pattern
830     <pre>
831     cat(aract|erpillar|)
832 nigel 75 </pre>
833 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
834     parentheses, it would match "cataract", "erpillar" or the empty string.
835 nigel 75 <br>
836     <br>
837     2. It sets up the subpattern as a capturing subpattern. This means that, when
838     the whole pattern matches, that portion of the subject string that matched the
839     subpattern is passed back to the caller via the <i>ovector</i> argument of
840 nigel 63 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
841 nigel 75 from 1) to obtain numbers for the capturing subpatterns.
842 nigel 63 </P>
843     <P>
844     For example, if the string "the red king" is matched against the pattern
845     <pre>
846     the ((red|white) (king|queen))
847 nigel 75 </pre>
848 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
849     2, and 3, respectively.
850     </P>
851     <P>
852     The fact that plain parentheses fulfil two functions is not always helpful.
853     There are often times when a grouping subpattern is required without a
854     capturing requirement. If an opening parenthesis is followed by a question mark
855     and a colon, the subpattern does not do any capturing, and is not counted when
856     computing the number of any subsequent capturing subpatterns. For example, if
857     the string "the white queen" is matched against the pattern
858     <pre>
859     the ((?:red|white) (king|queen))
860 nigel 75 </pre>
861 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
862     2. The maximum number of capturing subpatterns is 65535, and the maximum depth
863     of nesting of all subpatterns, both capturing and non-capturing, is 200.
864     </P>
865     <P>
866     As a convenient shorthand, if any option settings are required at the start of
867     a non-capturing subpattern, the option letters may appear between the "?" and
868     the ":". Thus the two patterns
869     <pre>
870     (?i:saturday|sunday)
871     (?:(?i)saturday|sunday)
872 nigel 75 </pre>
873 nigel 63 match exactly the same set of strings. Because alternative branches are tried
874     from left to right, and options are not reset until the end of the subpattern
875     is reached, an option setting in one branch does affect subsequent branches, so
876     the above patterns match "SUNDAY" as well as "Saturday".
877     </P>
878     <br><a name="SEC11" href="#TOC1">NAMED SUBPATTERNS</a><br>
879     <P>
880     Identifying capturing parentheses by number is simple, but it can be very hard
881     to keep track of the numbers in complicated regular expressions. Furthermore,
882 nigel 75 if an expression is modified, the numbers may change. To help with this
883 nigel 63 difficulty, PCRE supports the naming of subpatterns, something that Perl does
884     not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of
885     alphanumeric characters and underscores, and must be unique within a pattern.
886     </P>
887     <P>
888     Named capturing parentheses are still allocated numbers as well as names. The
889     PCRE API provides function calls for extracting the name-to-number translation
890 nigel 75 table from a compiled pattern. There is also a convenience function for
891     extracting a captured substring by name. For further details see the
892 nigel 63 <a href="pcreapi.html"><b>pcreapi</b></a>
893     documentation.
894     </P>
895     <br><a name="SEC12" href="#TOC1">REPETITION</a><br>
896     <P>
897     Repetition is specified by quantifiers, which can follow any of the following
898     items:
899     <pre>
900     a literal data character
901     the . metacharacter
902     the \C escape sequence
903 nigel 75 the \X escape sequence (in UTF-8 mode with Unicode properties)
904     an escape such as \d that matches a single character
905 nigel 63 a character class
906     a back reference (see next section)
907     a parenthesized subpattern (unless it is an assertion)
908 nigel 75 </pre>
909 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
910     permitted matches, by giving the two numbers in curly brackets (braces),
911     separated by a comma. The numbers must be less than 65536, and the first must
912     be less than or equal to the second. For example:
913     <pre>
914     z{2,4}
915 nigel 75 </pre>
916 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
917     character. If the second number is omitted, but the comma is present, there is
918     no upper limit; if the second number and the comma are both omitted, the
919     quantifier specifies an exact number of required matches. Thus
920     <pre>
921     [aeiou]{3,}
922 nigel 75 </pre>
923 nigel 63 matches at least 3 successive vowels, but may match many more, while
924     <pre>
925     \d{8}
926 nigel 75 </pre>
927 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
928     where a quantifier is not allowed, or one that does not match the syntax of a
929     quantifier, is taken as a literal character. For example, {,6} is not a
930     quantifier, but a literal string of four characters.
931     </P>
932     <P>
933     In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
934     bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
935 nigel 75 which is represented by a two-byte sequence. Similarly, when Unicode property
936     support is available, \X{3} matches three Unicode extended sequences, each of
937     which may be several bytes long (and they may be of different lengths).
938 nigel 63 </P>
939     <P>
940     The quantifier {0} is permitted, causing the expression to behave as if the
941     previous item and the quantifier were not present.
942     </P>
943     <P>
944     For convenience (and historical compatibility) the three most common
945     quantifiers have single-character abbreviations:
946     <pre>
947     * is equivalent to {0,}
948     + is equivalent to {1,}
949     ? is equivalent to {0,1}
950 nigel 75 </pre>
951 nigel 63 It is possible to construct infinite loops by following a subpattern that can
952     match no characters with a quantifier that has no upper limit, for example:
953     <pre>
954     (a?)*
955 nigel 75 </pre>
956 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
957     such patterns. However, because there are cases where this can be useful, such
958     patterns are now accepted, but if any repetition of the subpattern does in fact
959     match no characters, the loop is forcibly broken.
960     </P>
961     <P>
962     By default, the quantifiers are "greedy", that is, they match as much as
963     possible (up to the maximum number of permitted times), without causing the
964     rest of the pattern to fail. The classic example of where this gives problems
965 nigel 75 is in trying to match comments in C programs. These appear between /* and */
966     and within the comment, individual * and / characters may appear. An attempt to
967     match C comments by applying the pattern
968 nigel 63 <pre>
969     /\*.*\*/
970 nigel 75 </pre>
971 nigel 63 to the string
972     <pre>
973 nigel 75 /* first comment */ not comment /* second comment */
974     </pre>
975 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
976     item.
977     </P>
978     <P>
979     However, if a quantifier is followed by a question mark, it ceases to be
980     greedy, and instead matches the minimum number of times possible, so the
981     pattern
982     <pre>
983     /\*.*?\*/
984 nigel 75 </pre>
985 nigel 63 does the right thing with the C comments. The meaning of the various
986     quantifiers is not otherwise changed, just the preferred number of matches.
987     Do not confuse this use of question mark with its use as a quantifier in its
988     own right. Because it has two uses, it can sometimes appear doubled, as in
989     <pre>
990     \d??\d
991 nigel 75 </pre>
992 nigel 63 which matches one digit by preference, but can match two if that is the only
993     way the rest of the pattern matches.
994     </P>
995     <P>
996     If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
997     the quantifiers are not greedy by default, but individual ones can be made
998     greedy by following them with a question mark. In other words, it inverts the
999     default behaviour.
1000     </P>
1001     <P>
1002     When a parenthesized subpattern is quantified with a minimum repeat count that
1003 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1004 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1005     </P>
1006     <P>
1007     If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1008     to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
1009     implicitly anchored, because whatever follows will be tried against every
1010     character position in the subject string, so there is no point in retrying the
1011     overall match at any position after the first. PCRE normally treats such a
1012     pattern as though it were preceded by \A.
1013     </P>
1014     <P>
1015     In cases where it is known that the subject string contains no newlines, it is
1016     worth setting PCRE_DOTALL in order to obtain this optimization, or
1017     alternatively using ^ to indicate anchoring explicitly.
1018     </P>
1019     <P>
1020     However, there is one situation where the optimization cannot be used. When .*
1021     is inside capturing parentheses that are the subject of a backreference
1022     elsewhere in the pattern, a match at the start may fail, and a later one
1023     succeed. Consider, for example:
1024     <pre>
1025     (.*)abc\1
1026 nigel 75 </pre>
1027 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1028     this reason, such a pattern is not implicitly anchored.
1029     </P>
1030     <P>
1031     When a capturing subpattern is repeated, the value captured is the substring
1032     that matched the final iteration. For example, after
1033     <pre>
1034     (tweedle[dume]{3}\s*)+
1035 nigel 75 </pre>
1036 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1037     "tweedledee". However, if there are nested capturing subpatterns, the
1038     corresponding captured values may have been set in previous iterations. For
1039     example, after
1040     <pre>
1041     /(a|(b))+/
1042 nigel 75 </pre>
1043 nigel 63 matches "aba" the value of the second captured substring is "b".
1044 nigel 75 <a name="atomicgroup"></a></P>
1045 nigel 63 <br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1046     <P>
1047     With both maximizing and minimizing repetition, failure of what follows
1048     normally causes the repeated item to be re-evaluated to see if a different
1049     number of repeats allows the rest of the pattern to match. Sometimes it is
1050     useful to prevent this, either to change the nature of the match, or to cause
1051     it fail earlier than it otherwise might, when the author of the pattern knows
1052     there is no point in carrying on.
1053     </P>
1054     <P>
1055     Consider, for example, the pattern \d+foo when applied to the subject line
1056     <pre>
1057     123456bar
1058 nigel 75 </pre>
1059 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1060     action of the matcher is to try again with only 5 digits matching the \d+
1061     item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1062     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1063     that once a subpattern has matched, it is not to be re-evaluated in this way.
1064     </P>
1065     <P>
1066     If we use atomic grouping for the previous example, the matcher would give up
1067     immediately on failing to match "foo" the first time. The notation is a kind of
1068     special parenthesis, starting with (?&#62; as in this example:
1069     <pre>
1070 nigel 73 (?&#62;\d+)foo
1071 nigel 75 </pre>
1072 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1073     it has matched, and a failure further into the pattern is prevented from
1074     backtracking into it. Backtracking past it to previous items, however, works as
1075     normal.
1076     </P>
1077     <P>
1078     An alternative description is that a subpattern of this type matches the string
1079     of characters that an identical standalone pattern would match, if anchored at
1080     the current point in the subject string.
1081     </P>
1082     <P>
1083     Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1084     the above example can be thought of as a maximizing repeat that must swallow
1085     everything it can. So, while both \d+ and \d+? are prepared to adjust the
1086     number of digits they match in order to make the rest of the pattern match,
1087     (?&#62;\d+) can only match an entire sequence of digits.
1088     </P>
1089     <P>
1090     Atomic groups in general can of course contain arbitrarily complicated
1091     subpatterns, and can be nested. However, when the subpattern for an atomic
1092     group is just a single repeated item, as in the example above, a simpler
1093     notation, called a "possessive quantifier" can be used. This consists of an
1094     additional + character following a quantifier. Using this notation, the
1095     previous example can be rewritten as
1096     <pre>
1097 nigel 75 \d++foo
1098     </pre>
1099 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1100     option is ignored. They are a convenient notation for the simpler forms of
1101     atomic group. However, there is no difference in the meaning or processing of a
1102     possessive quantifier and the equivalent atomic group.
1103     </P>
1104     <P>
1105     The possessive quantifier syntax is an extension to the Perl syntax. It
1106     originates in Sun's Java package.
1107     </P>
1108     <P>
1109     When a pattern contains an unlimited repeat inside a subpattern that can itself
1110     be repeated an unlimited number of times, the use of an atomic group is the
1111     only way to avoid some failing matches taking a very long time indeed. The
1112     pattern
1113     <pre>
1114     (\D+|&#60;\d+&#62;)*[!?]
1115 nigel 75 </pre>
1116 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1117     digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1118     quickly. However, if it is applied to
1119     <pre>
1120     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1121 nigel 75 </pre>
1122 nigel 63 it takes a long time before reporting failure. This is because the string can
1123 nigel 75 be divided between the internal \D+ repeat and the external * repeat in a
1124     large number of ways, and all have to be tried. (The example uses [!?] rather
1125     than a single character at the end, because both PCRE and Perl have an
1126     optimization that allows for fast failure when a single character is used. They
1127     remember the last single character that is required for a match, and fail early
1128     if it is not present in the string.) If the pattern is changed so that it uses
1129     an atomic group, like this:
1130 nigel 63 <pre>
1131     ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1132 nigel 75 </pre>
1133 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1134 nigel 75 <a name="backreferences"></a></P>
1135 nigel 63 <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>
1136     <P>
1137     Outside a character class, a backslash followed by a digit greater than 0 (and
1138     possibly further digits) is a back reference to a capturing subpattern earlier
1139     (that is, to its left) in the pattern, provided there have been that many
1140     previous capturing left parentheses.
1141     </P>
1142     <P>
1143     However, if the decimal number following the backslash is less than 10, it is
1144     always taken as a back reference, and causes an error only if there are not
1145     that many capturing left parentheses in the entire pattern. In other words, the
1146     parentheses that are referenced need not be to the left of the reference for
1147 nigel 75 numbers less than 10. See the subsection entitled "Non-printing characters"
1148     <a href="#digitsafterbackslash">above</a>
1149     for further details of the handling of digits following a backslash.
1150 nigel 63 </P>
1151     <P>
1152     A back reference matches whatever actually matched the capturing subpattern in
1153     the current subject string, rather than anything matching the subpattern
1154     itself (see
1155     <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1156     below for a way of doing that). So the pattern
1157     <pre>
1158     (sens|respons)e and \1ibility
1159 nigel 75 </pre>
1160 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1161     "sense and responsibility". If caseful matching is in force at the time of the
1162     back reference, the case of letters is relevant. For example,
1163     <pre>
1164     ((?i)rah)\s+\1
1165 nigel 75 </pre>
1166 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1167     capturing subpattern is matched caselessly.
1168     </P>
1169     <P>
1170     Back references to named subpatterns use the Python syntax (?P=name). We could
1171     rewrite the above example as follows:
1172     <pre>
1173     (?&#60;p1&#62;(?i)rah)\s+(?P=p1)
1174 nigel 75 </pre>
1175 nigel 63 There may be more than one back reference to the same subpattern. If a
1176     subpattern has not actually been used in a particular match, any back
1177     references to it always fail. For example, the pattern
1178     <pre>
1179     (a|(bc))\2
1180 nigel 75 </pre>
1181 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1182     many capturing parentheses in a pattern, all digits following the backslash are
1183     taken as part of a potential back reference number. If the pattern continues
1184     with a digit character, some delimiter must be used to terminate the back
1185     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1186 nigel 75 Otherwise an empty comment (see
1187     <a href="#comments">"Comments"</a>
1188     below) can be used.
1189 nigel 63 </P>
1190     <P>
1191     A back reference that occurs inside the parentheses to which it refers fails
1192     when the subpattern is first used, so, for example, (a\1) never matches.
1193     However, such references can be useful inside repeated subpatterns. For
1194     example, the pattern
1195     <pre>
1196     (a|b\1)+
1197 nigel 75 </pre>
1198 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1199     the subpattern, the back reference matches the character string corresponding
1200     to the previous iteration. In order for this to work, the pattern must be such
1201     that the first iteration does not need to match the back reference. This can be
1202     done using alternation, as in the example above, or by a quantifier with a
1203     minimum of zero.
1204 nigel 75 <a name="bigassertions"></a></P>
1205 nigel 63 <br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br>
1206     <P>
1207     An assertion is a test on the characters following or preceding the current
1208     matching point that does not actually consume any characters. The simple
1209 nigel 75 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1210     <a href="#smallassertions">above.</a>
1211     </P>
1212     <P>
1213 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1214     those that look ahead of the current position in the subject string, and those
1215 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1216     except that it does not cause the current matching position to be changed.
1217 nigel 63 </P>
1218     <P>
1219 nigel 75 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1220     because it makes no sense to assert the same thing several times. If any kind
1221     of assertion contains capturing subpatterns within it, these are counted for
1222     the purposes of numbering the capturing subpatterns in the whole pattern.
1223     However, substring capturing is carried out only for positive assertions,
1224     because it does not make sense for negative assertions.
1225 nigel 63 </P>
1226 nigel 75 <br><b>
1227     Lookahead assertions
1228     </b><br>
1229 nigel 63 <P>
1230 nigel 75 Lookahead assertions start
1231     with (?= for positive assertions and (?! for negative assertions. For example,
1232 nigel 63 <pre>
1233     \w+(?=;)
1234 nigel 75 </pre>
1235 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1236     the match, and
1237     <pre>
1238     foo(?!bar)
1239 nigel 75 </pre>
1240 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1241     apparently similar pattern
1242     <pre>
1243     (?!foo)bar
1244 nigel 75 </pre>
1245 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1246     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1247     (?!foo) is always true when the next three characters are "bar". A
1248 nigel 75 lookbehind assertion is needed to achieve the other effect.
1249 nigel 63 </P>
1250     <P>
1251     If you want to force a matching failure at some point in a pattern, the most
1252     convenient way to do it is with (?!) because an empty string always matches, so
1253     an assertion that requires there not to be an empty string must always fail.
1254 nigel 75 <a name="lookbehind"></a></P>
1255     <br><b>
1256     Lookbehind assertions
1257     </b><br>
1258 nigel 63 <P>
1259     Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1260     negative assertions. For example,
1261     <pre>
1262     (?&#60;!foo)bar
1263 nigel 75 </pre>
1264 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1265     a lookbehind assertion are restricted such that all the strings it matches must
1266     have a fixed length. However, if there are several alternatives, they do not
1267     all have to have the same fixed length. Thus
1268     <pre>
1269     (?&#60;=bullock|donkey)
1270 nigel 75 </pre>
1271 nigel 63 is permitted, but
1272     <pre>
1273     (?&#60;!dogs?|cats?)
1274 nigel 75 </pre>
1275 nigel 63 causes an error at compile time. Branches that match different length strings
1276     are permitted only at the top level of a lookbehind assertion. This is an
1277     extension compared with Perl (at least for 5.8), which requires all branches to
1278     match the same length of string. An assertion such as
1279     <pre>
1280     (?&#60;=ab(c|de))
1281 nigel 75 </pre>
1282 nigel 63 is not permitted, because its single top-level branch can match two different
1283     lengths, but it is acceptable if rewritten to use two top-level branches:
1284     <pre>
1285     (?&#60;=abc|abde)
1286 nigel 75 </pre>
1287 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1288     temporarily move the current position back by the fixed width and then try to
1289     match. If there are insufficient characters before the current position, the
1290     match is deemed to fail.
1291     </P>
1292     <P>
1293     PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1294     to appear in lookbehind assertions, because it makes it impossible to calculate
1295 nigel 75 the length of the lookbehind. The \X escape, which can match different numbers
1296     of bytes, is also not permitted.
1297 nigel 63 </P>
1298     <P>
1299     Atomic groups can be used in conjunction with lookbehind assertions to specify
1300     efficient matching at the end of the subject string. Consider a simple pattern
1301     such as
1302     <pre>
1303     abcd$
1304 nigel 75 </pre>
1305 nigel 63 when applied to a long string that does not match. Because matching proceeds
1306     from left to right, PCRE will look for each "a" in the subject and then see if
1307     what follows matches the rest of the pattern. If the pattern is specified as
1308     <pre>
1309     ^.*abcd$
1310 nigel 75 </pre>
1311 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1312     there is no following "a"), it backtracks to match all but the last character,
1313     then all but the last two characters, and so on. Once again the search for "a"
1314     covers the entire string, from right to left, so we are no better off. However,
1315     if the pattern is written as
1316     <pre>
1317     ^(?&#62;.*)(?&#60;=abcd)
1318 nigel 75 </pre>
1319     or, equivalently, using the possessive quantifier syntax,
1320 nigel 63 <pre>
1321     ^.*+(?&#60;=abcd)
1322 nigel 75 </pre>
1323 nigel 63 there can be no backtracking for the .* item; it can match only the entire
1324     string. The subsequent lookbehind assertion does a single test on the last four
1325     characters. If it fails, the match fails immediately. For long strings, this
1326     approach makes a significant difference to the processing time.
1327     </P>
1328 nigel 75 <br><b>
1329     Using multiple assertions
1330     </b><br>
1331 nigel 63 <P>
1332     Several assertions (of any sort) may occur in succession. For example,
1333     <pre>
1334     (?&#60;=\d{3})(?&#60;!999)foo
1335 nigel 75 </pre>
1336 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1337     the assertions is applied independently at the same point in the subject
1338     string. First there is a check that the previous three characters are all
1339     digits, and then there is a check that the same three characters are not "999".
1340     This pattern does <i>not</i> match "foo" preceded by six characters, the first
1341     of which are digits and the last three of which are not "999". For example, it
1342     doesn't match "123abcfoo". A pattern to do that is
1343     <pre>
1344     (?&#60;=\d{3}...)(?&#60;!999)foo
1345 nigel 75 </pre>
1346 nigel 63 This time the first assertion looks at the preceding six characters, checking
1347     that the first three are digits, and then the second assertion checks that the
1348     preceding three characters are not "999".
1349     </P>
1350     <P>
1351     Assertions can be nested in any combination. For example,
1352     <pre>
1353     (?&#60;=(?&#60;!foo)bar)baz
1354 nigel 75 </pre>
1355 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1356     preceded by "foo", while
1357     <pre>
1358     (?&#60;=\d{3}(?!999)...)foo
1359 nigel 75 </pre>
1360     is another pattern that matches "foo" preceded by three digits and any three
1361 nigel 63 characters that are not "999".
1362     </P>
1363     <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1364     <P>
1365     It is possible to cause the matching process to obey a subpattern
1366     conditionally or to choose between two alternative subpatterns, depending on
1367     the result of an assertion, or whether a previous capturing subpattern matched
1368     or not. The two possible forms of conditional subpattern are
1369     <pre>
1370     (?(condition)yes-pattern)
1371     (?(condition)yes-pattern|no-pattern)
1372 nigel 75 </pre>
1373 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1374     no-pattern (if present) is used. If there are more than two alternatives in the
1375     subpattern, a compile-time error occurs.
1376     </P>
1377     <P>
1378     There are three kinds of condition. If the text between the parentheses
1379     consists of a sequence of digits, the condition is satisfied if the capturing
1380     subpattern of that number has previously matched. The number must be greater
1381     than zero. Consider the following pattern, which contains non-significant white
1382     space to make it more readable (assume the PCRE_EXTENDED option) and to divide
1383     it into three parts for ease of discussion:
1384     <pre>
1385     ( \( )? [^()]+ (?(1) \) )
1386 nigel 75 </pre>
1387 nigel 63 The first part matches an optional opening parenthesis, and if that
1388     character is present, sets it as the first captured substring. The second part
1389     matches one or more characters that are not parentheses. The third part is a
1390     conditional subpattern that tests whether the first set of parentheses matched
1391     or not. If they did, that is, if subject started with an opening parenthesis,
1392     the condition is true, and so the yes-pattern is executed and a closing
1393     parenthesis is required. Otherwise, since no-pattern is not present, the
1394     subpattern matches nothing. In other words, this pattern matches a sequence of
1395     non-parentheses, optionally enclosed in parentheses.
1396     </P>
1397     <P>
1398     If the condition is the string (R), it is satisfied if a recursive call to the
1399     pattern or subpattern has been made. At "top level", the condition is false.
1400     This is a PCRE extension. Recursive patterns are described in the next section.
1401     </P>
1402     <P>
1403     If the condition is not a sequence of digits or (R), it must be an assertion.
1404     This may be a positive or negative lookahead or lookbehind assertion. Consider
1405     this pattern, again containing non-significant white space, and with the two
1406     alternatives on the second line:
1407     <pre>
1408     (?(?=[^a-z]*[a-z])
1409     \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1410 nigel 75 </pre>
1411 nigel 63 The condition is a positive lookahead assertion that matches an optional
1412     sequence of non-letters followed by a letter. In other words, it tests for the
1413     presence of at least one letter in the subject. If a letter is found, the
1414     subject is matched against the first alternative; otherwise it is matched
1415     against the second. This pattern matches strings in one of the two forms
1416     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1417 nigel 75 <a name="comments"></a></P>
1418 nigel 63 <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>
1419     <P>
1420 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1421 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1422     that make up a comment play no part in the pattern matching at all.
1423     </P>
1424     <P>
1425     If the PCRE_EXTENDED option is set, an unescaped # character outside a
1426     character class introduces a comment that continues up to the next newline
1427     character in the pattern.
1428     </P>
1429     <br><a name="SEC18" href="#TOC1">RECURSIVE PATTERNS</a><br>
1430     <P>
1431     Consider the problem of matching a string in parentheses, allowing for
1432     unlimited nested parentheses. Without the use of recursion, the best that can
1433     be done is to use a pattern that matches up to some fixed depth of nesting. It
1434 nigel 75 is not possible to handle an arbitrary nesting depth. Perl provides a facility
1435     that allows regular expressions to recurse (amongst other things). It does this
1436     by interpolating Perl code in the expression at run time, and the code can
1437     refer to the expression itself. A Perl pattern to solve the parentheses problem
1438     can be created like this:
1439 nigel 63 <pre>
1440     $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1441 nigel 75 </pre>
1442 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1443     recursively to the pattern in which it appears. Obviously, PCRE cannot support
1444     the interpolation of Perl code. Instead, it supports some special syntax for
1445     recursion of the entire pattern, and also for individual subpattern recursion.
1446     </P>
1447     <P>
1448     The special item that consists of (? followed by a number greater than zero and
1449     a closing parenthesis is a recursive call of the subpattern of the given
1450     number, provided that it occurs inside that subpattern. (If not, it is a
1451     "subroutine" call, which is described in the next section.) The special item
1452     (?R) is a recursive call of the entire regular expression.
1453     </P>
1454     <P>
1455 nigel 87 A recursive subpattern call is always treated as an atomic group. That is, once
1456     it has matched some of the subject string, it is never re-entered, even if
1457     it contains untried alternatives and there is a subsequent matching failure.
1458     </P>
1459     <P>
1460     This PCRE pattern solves the nested parentheses problem (assume the
1461     PCRE_EXTENDED option is set so that white space is ignored):
1462 nigel 63 <pre>
1463     \( ( (?&#62;[^()]+) | (?R) )* \)
1464 nigel 75 </pre>
1465 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1466     substrings which can either be a sequence of non-parentheses, or a recursive
1467 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1468 nigel 63 Finally there is a closing parenthesis.
1469     </P>
1470     <P>
1471     If this were part of a larger pattern, you would not want to recurse the entire
1472     pattern, so instead you could use this:
1473     <pre>
1474     ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1475 nigel 75 </pre>
1476 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1477     them instead of the whole pattern. In a larger pattern, keeping track of
1478     parenthesis numbers can be tricky. It may be more convenient to use named
1479     parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to
1480     the Python syntax that PCRE uses for named parentheses (Perl does not provide
1481     named parentheses). We could rewrite the above example as follows:
1482     <pre>
1483 nigel 73 (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )
1484 nigel 75 </pre>
1485 nigel 63 This particular example pattern contains nested unlimited repeats, and so the
1486     use of atomic grouping for matching strings of non-parentheses is important
1487     when applying the pattern to strings that do not match. For example, when this
1488     pattern is applied to
1489     <pre>
1490     (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1491 nigel 75 </pre>
1492 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1493     the match runs for a very long time indeed because there are so many different
1494     ways the + and * repeats can carve up the subject, and all have to be tested
1495     before failure can be reported.
1496     </P>
1497     <P>
1498     At the end of a match, the values set for any capturing subpatterns are those
1499     from the outermost level of the recursion at which the subpattern value is set.
1500     If you want to obtain intermediate values, a callout function can be used (see
1501 nigel 75 the next section and the
1502 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1503     documentation). If the pattern above is matched against
1504     <pre>
1505     (ab(cd)ef)
1506 nigel 75 </pre>
1507 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1508     on at the top level. If additional parentheses are added, giving
1509     <pre>
1510     \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
1511     ^ ^
1512     ^ ^
1513 nigel 75 </pre>
1514 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1515     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1516     has to obtain extra memory to store data during a recursion, which it does by
1517     using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
1518     memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1519     </P>
1520     <P>
1521     Do not confuse the (?R) item with the condition (R), which tests for recursion.
1522     Consider this pattern, which matches text in angle brackets, allowing for
1523     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1524     recursing), whereas any characters are permitted at the outer level.
1525     <pre>
1526     &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
1527 nigel 75 </pre>
1528 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1529     different alternatives for the recursive and non-recursive cases. The (?R) item
1530     is the actual recursive call.
1531 nigel 75 <a name="subpatternsassubroutines"></a></P>
1532     <br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1533 nigel 63 <P>
1534     If the syntax for a recursive subpattern reference (either by number or by
1535     name) is used outside the parentheses to which it refers, it operates like a
1536     subroutine in a programming language. An earlier example pointed out that the
1537     pattern
1538     <pre>
1539     (sens|respons)e and \1ibility
1540 nigel 75 </pre>
1541 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1542     "sense and responsibility". If instead the pattern
1543     <pre>
1544     (sens|respons)e and (?1)ibility
1545 nigel 75 </pre>
1546 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1547     strings. Such references must, however, follow the subpattern to which they
1548     refer.
1549     </P>
1550 nigel 87 <P>
1551     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1552     group. That is, once it has matched some of the subject string, it is never
1553     re-entered, even if it contains untried alternatives and there is a subsequent
1554     matching failure.
1555     </P>
1556 nigel 63 <br><a name="SEC20" href="#TOC1">CALLOUTS</a><br>
1557     <P>
1558     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1559     code to be obeyed in the middle of matching a regular expression. This makes it
1560     possible, amongst other things, to extract different substrings that match the
1561     same pair of parentheses when there is a repetition.
1562     </P>
1563     <P>
1564     PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1565     code. The feature is called "callout". The caller of PCRE provides an external
1566     function by putting its entry point in the global variable <i>pcre_callout</i>.
1567     By default, this variable contains NULL, which disables all calling out.
1568     </P>
1569     <P>
1570     Within a regular expression, (?C) indicates the points at which the external
1571     function is to be called. If you want to identify different callout points, you
1572     can put a number less than 256 after the letter C. The default value is zero.
1573     For example, this pattern has two callout points:
1574     <pre>
1575     (?C1)\dabc(?C2)def
1576 nigel 75 </pre>
1577     If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
1578     automatically installed before each item in the pattern. They are all numbered
1579     255.
1580 nigel 63 </P>
1581     <P>
1582     During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
1583     set), the external function is called. It is provided with the number of the
1584 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1585     originally supplied by the caller of <b>pcre_exec()</b>. The callout function
1586     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1587     description of the interface to the callout function is given in the
1588 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1589     documentation.
1590     </P>
1591     <P>
1592 nigel 87 Last updated: 24 January 2006
1593 nigel 63 <br>
1594 nigel 87 Copyright &copy; 1997-2006 University of Cambridge.
1595 nigel 75 <p>
1596     Return to the <a href="index.html">PCRE index page</a>.
1597     </p>

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