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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE REGULAR EXPRESSION DETAILS  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions supported by PCRE are
8  described below. Regular expressions are also described in the Perl  described below. Regular expressions are also described in the Perl
9  documentation and in a number of other books, some of which have copious  documentation and in a number of books, some of which have copious examples.
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in great detail. This description of PCRE's regular
12  reference documentation.  expressions is intended as reference material.
13    .P
14  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
15  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
19  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
20  .\" </a>  .\" </a>
21  section on UTF-8 support  section on UTF-8 support
22  .\"  .\"
23  in the main  in the main
24  .\" HREF  .\" HREF
25  \fBpcre\fR  \fBpcre\fP
26  .\"  .\"
27  page.  page.
28    .P
29    The remainder of this document discusses the patterns that are supported by
30    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31    From release 6.0, PCRE offers a second matching function,
32    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33    Perl-compatible. Some of the features discussed below are not available when
34    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
35    alternative function, and how it differs from the normal function, are
36    discussed in the
37    .\" HREF
38    \fBpcrematching\fP
39    .\"
40    page.
41    .
42    .
43    .SH "CHARACTERS AND METACHARACTERS"
44    .rs
45    .sp
46  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
47  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
48  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
49    .sp
50    The quick brown fox    The quick brown fox
51    .sp
52  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
53  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
54  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
55  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
56    always possible. For characters with higher values, the concept of case is
57    supported if PCRE is compiled with Unicode property support, but not otherwise.
58    If you want to use caseless matching for characters 128 and above, you must
59    ensure that PCRE is compiled with Unicode property support as well as with
60    UTF-8 support.
61    .P
62    The power of regular expressions comes from the ability to include alternatives
63    and repetitions in the pattern. These are encoded in the pattern by the use of
64    \fImetacharacters\fP, which do not stand for themselves but instead are
65  interpreted in some special way.  interpreted in some special way.
66    .P
67  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
68  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
69  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
70  as follows:  are as follows:
71    .sp
72    \\      general escape character with several uses    \e      general escape character with several uses
73    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
74    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
75    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 84  as follows:
84    +      1 or more quantifier    +      1 or more quantifier
85           also "possessive quantifier"           also "possessive quantifier"
86    {      start min/max quantifier    {      start min/max quantifier
87    .sp
88  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
89  a character class the only meta-characters are:  a character class the only metacharacters are:
90    .sp
91    \\      general escape character    \e      general escape character
92    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
93    -      indicates character range    -      indicates character range
94    .\" JOIN
95    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
96             syntax)             syntax)
97    ]      terminates the character class    ]      terminates the character class
98    .sp
99  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
100    .
101    .
102  .SH BACKSLASH  .SH BACKSLASH
103  .rs  .rs
104  .sp  .sp
105  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
106  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
107  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
108  outside character classes.  outside character classes.
109    .P
110  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
111  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
112  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
113  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
114  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
115    .P
116  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
117  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
118  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
119  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
120  pattern.  .P
   
121  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
122  can do so by putting them between \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
123  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
124  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
125    .sp
126    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
127    .sp
128    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
129      \eQabc$xyz\eE        abc$xyz        abc followed by the
130                                        contents of $xyz                                        contents of $xyz
131    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
132    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
133    .sp
134  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
135    .
136    .
137    .\" HTML <a name="digitsafterbackslash"></a>
138    .SS "Non-printing characters"
139    .rs
140    .sp
141  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
142  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
143  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
144  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is usually easier to
145  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
146  represents:  represents:
147    .sp
148    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
149    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
150    \\e        escape (hex 1B)    \ee        escape (hex 1B)
151    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
152    \\n        newline (hex 0A)    \en        newline (hex 0A)
153    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
154    \\t        tab (hex 09)    \et        tab (hex 09)
155    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
156    \\xhh      character with hex code hh    \exhh      character with hex code hh
157    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
158    .sp
159  The precise effect of \\cx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
160  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
161  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
162  7B.  7B.
163    .P
164  After \\x, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
165  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
166  appear between \\x{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
167  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
168  other than hexadecimal digits appear between \\x{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
169  terminating }, this form of escape is not recognized. Instead, the initial  and }, or if there is no terminating }, this form of escape is not recognized.
170  \\x will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
171  digits, giving a byte whose value is zero.  with no following digits, giving a character whose value is zero.
172    .P
173  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
174  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
175  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
176    .P
177  After \\0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
178  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
179  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
180  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
181  character that follows is itself an octal digit.  follows is itself an octal digit.
182    .P
183  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
184  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
185  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
186  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
187  taken as a \fIback reference\fR. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
188  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
189    .\" </a>
190    later,
191    .\"
192    following the discussion of
193    .\" HTML <a href="#subpattern">
194    .\" </a>
195    parenthesized subpatterns.
196    .\"
197    .P
198  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
199  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
200  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
201  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
202  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
203    to \e777 are permitted. For example:
204    \\040   is another way of writing a space  .sp
205    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
206    .\" JOIN
207      \e40    is the same, provided there are fewer than 40
208              previous capturing subpatterns              previous capturing subpatterns
209    \\7     is always a back reference    \e7     is always a back reference
210    \\11    might be a back reference, or another way of  .\" JOIN
211      \e11    might be a back reference, or another way of
212              writing a tab              writing a tab
213    \\011   is always a tab    \e011   is always a tab
214    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
215    \\113   might be a back reference, otherwise the  .\" JOIN
216      \e113   might be a back reference, otherwise the
217              character with octal code 113              character with octal code 113
218    \\377   might be a back reference, otherwise  .\" JOIN
219      \e377   might be a back reference, otherwise
220              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
221    \\81    is either a back reference, or a binary zero  .\" JOIN
222      \e81    is either a back reference, or a binary zero
223              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
224    .sp
225  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
226  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
227    .P
228  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
229  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
230  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
231  backspace character (hex 08). Outside a character class it has a different  sequences \eR and \eX are interpreted as the characters "R" and "X",
232  meaning (see below).  respectively. Outside a character class, these sequences have different
233    meanings
234  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
235    .\" </a>
236    \\d     any decimal digit  (see below).
237    \\D     any character that is not a decimal digit  .\"
238    \\s     any whitespace character  .
239    \\S     any character that is not a whitespace character  .
240    \\w     any "word" character  .SS "Absolute and relative back references"
241    \\W     any "non-word" character  .rs
242    .sp
243    The sequence \eg followed by a positive or negative number, optionally enclosed
244    in braces, is an absolute or relative back reference. A named back reference
245    can be coded as \eg{name}. Back references are discussed
246    .\" HTML <a href="#backreferences">
247    .\" </a>
248    later,
249    .\"
250    following the discussion of
251    .\" HTML <a href="#subpattern">
252    .\" </a>
253    parenthesized subpatterns.
254    .\"
255    .
256    .
257    .SS "Generic character types"
258    .rs
259    .sp
260    Another use of backslash is for specifying generic character types. The
261    following are always recognized:
262    .sp
263      \ed     any decimal digit
264      \eD     any character that is not a decimal digit
265      \es     any whitespace character
266      \eS     any character that is not a whitespace character
267      \ew     any "word" character
268      \eW     any "non-word" character
269    .sp
270  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
271  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
272    .P
273  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  These character type sequences can appear both inside and outside character
274  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
275    matching point is at the end of the subject string, all of them fail, since
276  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
277  This makes it different from the the POSIX "space" class. The \\s characters  .P
278  are HT (9), LF (10), FF (12), CR (13), and space (32).  For compatibility with Perl, \es does not match the VT character (code 11).
279    This makes it different from the the POSIX "space" class. The \es characters
280  A "word" character is any letter or digit or the underscore character, that is,  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
281  any character which can be part of a Perl "word". The definition of letters and  included in a Perl script, \es may match the VT character. In PCRE, it never
282  digits is controlled by PCRE's character tables, and may vary if locale-  does.)
283  specific matching is taking place (see  .P
284    A "word" character is an underscore or any character less than 256 that is a
285    letter or digit. The definition of letters and digits is controlled by PCRE's
286    low-valued character tables, and may vary if locale-specific matching is taking
287    place (see
288  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
289  .\" </a>  .\" </a>
290  "Locale support"  "Locale support"
291  .\"  .\"
292  in the  in the
293  .\" HREF  .\" HREF
294  \fBpcreapi\fR  \fBpcreapi\fP
295  .\"  .\"
296  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
297  than 128 are used for accented letters, and these are matched by \\w.  or "french" in Windows, some character codes greater than 128 are used for
298    accented letters, and these are matched by \ew.
299  These character type sequences can appear both inside and outside character  .P
300  classes. They each match one character of the appropriate type. If the current  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
301  matching point is at the end of the subject string, all of them fail, since  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
302  there is no character to match.  character property support is available. The use of locales with Unicode is
303    discouraged.
304  The fourth use of backslash is for certain simple assertions. An assertion  .
305    .
306    .SS "Newline sequences"
307    .rs
308    .sp
309    Outside a character class, the escape sequence \eR matches any Unicode newline
310    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
311    the following:
312    .sp
313      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
314    .sp
315    This is an example of an "atomic group", details of which are given
316    .\" HTML <a href="#atomicgroup">
317    .\" </a>
318    below.
319    .\"
320    This particular group matches either the two-character sequence CR followed by
321    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
322    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
323    line, U+0085). The two-character sequence is treated as a single unit that
324    cannot be split.
325    .P
326    In UTF-8 mode, two additional characters whose codepoints are greater than 255
327    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
328    Unicode character property support is not needed for these characters to be
329    recognized.
330    .P
331    Inside a character class, \eR matches the letter "R".
332    .
333    .
334    .\" HTML <a name="uniextseq"></a>
335    .SS Unicode character properties
336    .rs
337    .sp
338    When PCRE is built with Unicode character property support, three additional
339    escape sequences to match character properties are available when UTF-8 mode
340    is selected. They are:
341    .sp
342      \ep{\fIxx\fP}   a character with the \fIxx\fP property
343      \eP{\fIxx\fP}   a character without the \fIxx\fP property
344      \eX       an extended Unicode sequence
345    .sp
346    The property names represented by \fIxx\fP above are limited to the Unicode
347    script names, the general category properties, and "Any", which matches any
348    character (including newline). Other properties such as "InMusicalSymbols" are
349    not currently supported by PCRE. Note that \eP{Any} does not match any
350    characters, so always causes a match failure.
351    .P
352    Sets of Unicode characters are defined as belonging to certain scripts. A
353    character from one of these sets can be matched using a script name. For
354    example:
355    .sp
356      \ep{Greek}
357      \eP{Han}
358    .sp
359    Those that are not part of an identified script are lumped together as
360    "Common". The current list of scripts is:
361    .P
362    Arabic,
363    Armenian,
364    Balinese,
365    Bengali,
366    Bopomofo,
367    Braille,
368    Buginese,
369    Buhid,
370    Canadian_Aboriginal,
371    Cherokee,
372    Common,
373    Coptic,
374    Cuneiform,
375    Cypriot,
376    Cyrillic,
377    Deseret,
378    Devanagari,
379    Ethiopic,
380    Georgian,
381    Glagolitic,
382    Gothic,
383    Greek,
384    Gujarati,
385    Gurmukhi,
386    Han,
387    Hangul,
388    Hanunoo,
389    Hebrew,
390    Hiragana,
391    Inherited,
392    Kannada,
393    Katakana,
394    Kharoshthi,
395    Khmer,
396    Lao,
397    Latin,
398    Limbu,
399    Linear_B,
400    Malayalam,
401    Mongolian,
402    Myanmar,
403    New_Tai_Lue,
404    Nko,
405    Ogham,
406    Old_Italic,
407    Old_Persian,
408    Oriya,
409    Osmanya,
410    Phags_Pa,
411    Phoenician,
412    Runic,
413    Shavian,
414    Sinhala,
415    Syloti_Nagri,
416    Syriac,
417    Tagalog,
418    Tagbanwa,
419    Tai_Le,
420    Tamil,
421    Telugu,
422    Thaana,
423    Thai,
424    Tibetan,
425    Tifinagh,
426    Ugaritic,
427    Yi.
428    .P
429    Each character has exactly one general category property, specified by a
430    two-letter abbreviation. For compatibility with Perl, negation can be specified
431    by including a circumflex between the opening brace and the property name. For
432    example, \ep{^Lu} is the same as \eP{Lu}.
433    .P
434    If only one letter is specified with \ep or \eP, it includes all the general
435    category properties that start with that letter. In this case, in the absence
436    of negation, the curly brackets in the escape sequence are optional; these two
437    examples have the same effect:
438    .sp
439      \ep{L}
440      \epL
441    .sp
442    The following general category property codes are supported:
443    .sp
444      C     Other
445      Cc    Control
446      Cf    Format
447      Cn    Unassigned
448      Co    Private use
449      Cs    Surrogate
450    .sp
451      L     Letter
452      Ll    Lower case letter
453      Lm    Modifier letter
454      Lo    Other letter
455      Lt    Title case letter
456      Lu    Upper case letter
457    .sp
458      M     Mark
459      Mc    Spacing mark
460      Me    Enclosing mark
461      Mn    Non-spacing mark
462    .sp
463      N     Number
464      Nd    Decimal number
465      Nl    Letter number
466      No    Other number
467    .sp
468      P     Punctuation
469      Pc    Connector punctuation
470      Pd    Dash punctuation
471      Pe    Close punctuation
472      Pf    Final punctuation
473      Pi    Initial punctuation
474      Po    Other punctuation
475      Ps    Open punctuation
476    .sp
477      S     Symbol
478      Sc    Currency symbol
479      Sk    Modifier symbol
480      Sm    Mathematical symbol
481      So    Other symbol
482    .sp
483      Z     Separator
484      Zl    Line separator
485      Zp    Paragraph separator
486      Zs    Space separator
487    .sp
488    The special property L& is also supported: it matches a character that has
489    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
490    a modifier or "other".
491    .P
492    The long synonyms for these properties that Perl supports (such as \ep{Letter})
493    are not supported by PCRE, nor is it permitted to prefix any of these
494    properties with "Is".
495    .P
496    No character that is in the Unicode table has the Cn (unassigned) property.
497    Instead, this property is assumed for any code point that is not in the
498    Unicode table.
499    .P
500    Specifying caseless matching does not affect these escape sequences. For
501    example, \ep{Lu} always matches only upper case letters.
502    .P
503    The \eX escape matches any number of Unicode characters that form an extended
504    Unicode sequence. \eX is equivalent to
505    .sp
506      (?>\ePM\epM*)
507    .sp
508    That is, it matches a character without the "mark" property, followed by zero
509    or more characters with the "mark" property, and treats the sequence as an
510    atomic group
511    .\" HTML <a href="#atomicgroup">
512    .\" </a>
513    (see below).
514    .\"
515    Characters with the "mark" property are typically accents that affect the
516    preceding character.
517    .P
518    Matching characters by Unicode property is not fast, because PCRE has to search
519    a structure that contains data for over fifteen thousand characters. That is
520    why the traditional escape sequences such as \ed and \ew do not use Unicode
521    properties in PCRE.
522    .
523    .
524    .\" HTML <a name="resetmatchstart"></a>
525    .SS "Resetting the match start"
526    .rs
527    .sp
528    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
529    matched characters not to be included in the final matched sequence. For
530    example, the pattern:
531    .sp
532      foo\eKbar
533    .sp
534    matches "foobar", but reports that it has matched "bar". This feature is
535    similar to a lookbehind assertion
536    .\" HTML <a href="#lookbehind">
537    .\" </a>
538    (described below).
539    .\"
540    However, in this case, the part of the subject before the real match does not
541    have to be of fixed length, as lookbehind assertions do. The use of \eK does
542    not interfere with the setting of
543    .\" HTML <a href="#subpattern">
544    .\" </a>
545    captured substrings.
546    .\"
547    For example, when the pattern
548    .sp
549      (foo)\eKbar
550    .sp
551    matches "foobar", the first substring is still set to "foo".
552    .
553    .
554    .\" HTML <a name="smallassertions"></a>
555    .SS "Simple assertions"
556    .rs
557    .sp
558    The final use of backslash is for certain simple assertions. An assertion
559  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
560  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
561  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
562  assertions are  .\" HTML <a href="#bigassertions">
563    .\" </a>
564    \\b     matches at a word boundary  below.
565    \\B     matches when not at a word boundary  .\"
566    \\A     matches at start of subject  The backslashed assertions are:
567    \\Z     matches at end of subject or before newline at end  .sp
568    \\z     matches at end of subject    \eb     matches at a word boundary
569    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
570      \eA     matches at the start of the subject
571  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
572              also matches before a newline at the end of the subject
573      \ez     matches only at the end of the subject
574      \eG     matches at the first matching position in the subject
575    .sp
576    These assertions may not appear in character classes (but note that \eb has a
577  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
578    .P
579  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
580  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
581  \\w and the other matches \\W), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
582  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
583    .P
584  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
585  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
586  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
587  multiline mode.  independent of multiline mode. These three assertions are not affected by the
588    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
589  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
590  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
591  matching is to start at a point other than the beginning of the subject, \\A  at a point other than the beginning of the subject, \eA can never match. The
592  can never match. The difference between \\Z and \\z is that \\Z matches before  difference between \eZ and \ez is that \eZ matches before a newline at the end
593  a newline that is the last character of the string as well as at the end of the  of the string as well as at the very end, whereas \ez matches only at the end.
594  string, whereas \\z matches only at the end.  .P
595    The \eG assertion is true only when the current matching position is at the
596  The \\G assertion is true only when the current matching position is at the  start point of the match, as specified by the \fIstartoffset\fP argument of
597  start point of the match, as specified by the \fIstartoffset\fR argument of  \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
598  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
 non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  
599  arguments, you can mimic Perl's /g option, and it is in this kind of  arguments, you can mimic Perl's /g option, and it is in this kind of
600  implementation where \\G can be useful.  implementation where \eG can be useful.
601    .P
602  Note, however, that PCRE's interpretation of \\G, as the start of the current  Note, however, that PCRE's interpretation of \eG, as the start of the current
603  match, is subtly different from Perl's, which defines it as the end of the  match, is subtly different from Perl's, which defines it as the end of the
604  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
605  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
606  reproduce this behaviour.  reproduce this behaviour.
607    .P
608  If all the alternatives of a pattern begin with \\G, the expression is anchored  If all the alternatives of a pattern begin with \eG, the expression is anchored
609  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
610  regular expression.  regular expression.
611    .
612  .SH CIRCUMFLEX AND DOLLAR  .
613    .SH "CIRCUMFLEX AND DOLLAR"
614  .rs  .rs
615  .sp  .sp
616  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
617  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
618  at the start of the subject string. If the \fIstartoffset\fR argument of  at the start of the subject string. If the \fIstartoffset\fP argument of
619  \fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
620  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
621  meaning (see below).  meaning
622    .\" HTML <a href="#characterclass">
623    .\" </a>
624    (see below).
625    .\"
626    .P
627  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
628  alternatives are involved, but it should be the first thing in each alternative  alternatives are involved, but it should be the first thing in each alternative
629  in which it appears if the pattern is ever to match that branch. If all  in which it appears if the pattern is ever to match that branch. If all
# Line 290  possible alternatives start with a circu Line 631  possible alternatives start with a circu
631  constrained to match only at the start of the subject, it is said to be an  constrained to match only at the start of the subject, it is said to be an
632  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
633  to be anchored.)  to be anchored.)
634    .P
635  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
636  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
637  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
638  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
639  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
640  Dollar has no special meaning in a character class.  character class.
641    .P
642  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
643  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
644  does not affect the \\Z assertion.  does not affect the \eZ assertion.
645    .P
646  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
647  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
648  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
649  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
650  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
651  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
652  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
653  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
654  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
655  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
656    patterns that are anchored in single line mode because all branches start with
657  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  ^ are not anchored in multiline mode, and a match for circumflex is possible
658    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
659    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
660    .P
661    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
662  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
663  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
664    .
665  .SH FULL STOP (PERIOD, DOT)  .
666    .SH "FULL STOP (PERIOD, DOT)"
667  .rs  .rs
668  .sp  .sp
669  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
670  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
671  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
672  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
673  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
674  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
675  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
676    (including isolated CRs and LFs). When any Unicode line endings are being
677  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
678  .rs  characters.
679  .sp  .P
680  Outside a character class, the escape sequence \\C matches any one byte, both  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
681  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  option is set, a dot matches any one character, without exception. If the
682  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  two-character sequence CRLF is present in the subject string, it takes two dots
683  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
684  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
685    The handling of dot is entirely independent of the handling of circumflex and
686  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  dollar, the only relationship being that they both involve newlines. Dot has no
687  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
688    .
689  .SH SQUARE BRACKETS  .
690    .SH "MATCHING A SINGLE BYTE"
691    .rs
692    .sp
693    Outside a character class, the escape sequence \eC matches any one byte, both
694    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
695    characters. The feature is provided in Perl in order to match individual bytes
696    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
697    what remains in the string may be a malformed UTF-8 string. For this reason,
698    the \eC escape sequence is best avoided.
699    .P
700    PCRE does not allow \eC to appear in lookbehind assertions
701    .\" HTML <a href="#lookbehind">
702    .\" </a>
703    (described below),
704    .\"
705    because in UTF-8 mode this would make it impossible to calculate the length of
706    the lookbehind.
707    .
708    .
709    .\" HTML <a name="characterclass"></a>
710    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
711  .rs  .rs
712  .sp  .sp
713  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
# Line 348  square bracket. A closing square bracket Line 715  square bracket. A closing square bracket
715  closing square bracket is required as a member of the class, it should be the  closing square bracket is required as a member of the class, it should be the
716  first data character in the class (after an initial circumflex, if present) or  first data character in the class (after an initial circumflex, if present) or
717  escaped with a backslash.  escaped with a backslash.
718    .P
719  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
720  character may occupy more than one byte. A matched character must be in the set  character may occupy more than one byte. A matched character must be in the set
721  of characters defined by the class, unless the first character in the class  of characters defined by the class, unless the first character in the class
# Line 356  definition is a circumflex, in which cas Line 723  definition is a circumflex, in which cas
723  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
724  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
725  backslash.  backslash.
726    .P
727  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
728  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
729  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
730  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
731  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion: it still consumes a character from the subject
732  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
733    string.
734    .P
735  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
736  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  class as a literal string of bytes, or by using the \ex{ escaping mechanism.
737    .P
738  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
739  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
740  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
741  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
742  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
743    always possible. For characters with higher values, the concept of case is
744  The newline character is never treated in any special way in character classes,  supported if PCRE is compiled with Unicode property support, but not otherwise.
745  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching for characters 128 and above, you must
746  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
747    UTF-8 support.
748    .P
749    Characters that might indicate line breaks are never treated in any special way
750    when matching character classes, whatever line-ending sequence is in use, and
751    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
752    such as [^a] always matches one of these characters.
753    .P
754  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
755  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
756  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
757  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
758  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class.
759    .P
760  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
761  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
762  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
763  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
764  the end of range, so [W-\\]46] is interpreted as a single class containing a  the end of range, so [W-\e]46] is interpreted as a class containing a range
765  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
766  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
767    .P
768  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
769  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
770  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
771  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
772    .P
773  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
774  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
775  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
776  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\exc8-\excb] matches accented E
777    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
778  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a  characters with values greater than 128 only when it is compiled with Unicode
779  character class, and add the characters that they match to the class. For  property support.
780  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
781    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
782    in a character class, and add the characters that they match to the class. For
783    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
784  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
785  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
786  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
787    .P
788  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
789  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
790  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
791    introducing a POSIX class name - see the next section), and the terminating
792  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
793    does no harm.
794    .
795    .
796    .SH "POSIX CHARACTER CLASSES"
797  .rs  .rs
798  .sp  .sp
799  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
800  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
801  this notation. For example,  this notation. For example,
802    .sp
803    [01[:alpha:]%]    [01[:alpha:]%]
804    .sp
805  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
806  are  are
807    .sp
808    alnum    letters and digits    alnum    letters and digits
809    alpha    letters    alpha    letters
810    ascii    character codes 0 - 127    ascii    character codes 0 - 127
811    blank    space or tab only    blank    space or tab only
812    cntrl    control characters    cntrl    control characters
813    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
814    graph    printing characters, excluding space    graph    printing characters, excluding space
815    lower    lower case letters    lower    lower case letters
816    print    printing characters, including space    print    printing characters, including space
817    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
818    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
819    upper    upper case letters    upper    upper case letters
820    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
821    xdigit   hexadecimal digits    xdigit   hexadecimal digits
822    .sp
823  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
824  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
825  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
826  compatibility).  compatibility).
827    .P
828  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
829  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
830  after the colon. For example,  after the colon. For example,
831    .sp
832    [12[:^digit:]]    [12[:^digit:]]
833    .sp
834  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
835  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
836  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
837    .P
838  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
839  the POSIX character classes.  the POSIX character classes.
840    .
841  .SH VERTICAL BAR  .
842    .SH "VERTICAL BAR"
843  .rs  .rs
844  .sp  .sp
845  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
846  the pattern  the pattern
847    .sp
848    gilbert|sullivan    gilbert|sullivan
849    .sp
850  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
851  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
852  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
853  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
854  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
855  pattern as well as the alternative in the subpattern.  .\" </a>
856    (defined below),
857  .SH INTERNAL OPTION SETTING  .\"
858    "succeeds" means matching the rest of the main pattern as well as the
859    alternative in the subpattern.
860    .
861    .
862    .SH "INTERNAL OPTION SETTING"
863  .rs  .rs
864  .sp  .sp
865  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
866  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
867  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
868    .sp
869    i  for PCRE_CASELESS    i  for PCRE_CASELESS
870    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
871    s  for PCRE_DOTALL    s  for PCRE_DOTALL
872    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
873    .sp
874  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
875  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
876  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
877  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
878  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
879  unset.  unset.
880    .P
881  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
882  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
883  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
884  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
885  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
886    .P
887  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
888  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
889    .sp
890    (a(?i)b)c    (a(?i)b)c
891    .sp
892  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
893  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
894  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
895  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
896    .sp
897    (a(?i)b|c)    (a(?i)b|c)
898    .sp
899  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
900  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
901  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
902  behaviour otherwise.  behaviour otherwise.
903    .P
904  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
905  same way as the Perl-compatible options by using the characters U and X  changed in the same way as the Perl-compatible options by using the characters
906  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
907  earlier in the pattern than any of the additional features it turns on, even  .
908  when it is at top level. It is best put at the start.  .
909    .\" HTML <a name="subpattern"></a>
910  .SH SUBPATTERNS  .SH SUBPATTERNS
911  .rs  .rs
912  .sp  .sp
913  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
914  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
915    .sp
916  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
917    .sp
918    cat(aract|erpillar|)    cat(aract|erpillar|)
919    .sp
920  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
921  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
922    .sp
923  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
924  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
925  the subpattern is passed back to the caller via the \fIovector\fR argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
926  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
927  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
928    .P
929  For example, if the string "the red king" is matched against the pattern  For example, if the string "the red king" is matched against the pattern
930    .sp
931    the ((red|white) (king|queen))    the ((red|white) (king|queen))
932    .sp
933  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
934  2, and 3, respectively.  2, and 3, respectively.
935    .P
936  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
937  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
938  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
939  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
940  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
941  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
942    .sp
943    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
944    .sp
945  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
946  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
947  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
948  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
949  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
950  the ":". Thus the two patterns  the ":". Thus the two patterns
951    .sp
952    (?i:saturday|sunday)    (?i:saturday|sunday)
953    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
954    .sp
955  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
956  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
957  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
958  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
959    .
960  .SH NAMED SUBPATTERNS  .
961    .SH "DUPLICATE SUBPATTERN NUMBERS"
962    .rs
963    .sp
964    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
965    the same numbers for its capturing parentheses. Such a subpattern starts with
966    (?| and is itself a non-capturing subpattern. For example, consider this
967    pattern:
968    .sp
969      (?|(Sat)ur|(Sun))day
970    .sp
971    Because the two alternatives are inside a (?| group, both sets of capturing
972    parentheses are numbered one. Thus, when the pattern matches, you can look
973    at captured substring number one, whichever alternative matched. This construct
974    is useful when you want to capture part, but not all, of one of a number of
975    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
976    number is reset at the start of each branch. The numbers of any capturing
977    buffers that follow the subpattern start after the highest number used in any
978    branch. The following example is taken from the Perl documentation.
979    The numbers underneath show in which buffer the captured content will be
980    stored.
981    .sp
982      # before  ---------------branch-reset----------- after
983      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
984      # 1            2         2  3        2     3     4
985    .sp
986    A backreference or a recursive call to a numbered subpattern always refers to
987    the first one in the pattern with the given number.
988    .P
989    An alternative approach to using this "branch reset" feature is to use
990    duplicate named subpatterns, as described in the next section.
991    .
992    .
993    .SH "NAMED SUBPATTERNS"
994  .rs  .rs
995  .sp  .sp
996  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
997  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
998  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
999  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1000  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1001  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1002    the Perl and the Python syntax.
1003  Named capturing parentheses are still allocated numbers as well as names. The  .P
1004  PCRE API provides function calls for extracting the name-to-number translation  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1005  table from a compiled pattern. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1006    parentheses from other parts of the pattern, such as
1007    .\" HTML <a href="#backreferences">
1008    .\" </a>
1009    backreferences,
1010    .\"
1011    .\" HTML <a href="#recursion">
1012    .\" </a>
1013    recursion,
1014    .\"
1015    and
1016    .\" HTML <a href="#conditions">
1017    .\" </a>
1018    conditions,
1019    .\"
1020    can be made by name as well as by number.
1021    .P
1022    Names consist of up to 32 alphanumeric characters and underscores. Named
1023    capturing parentheses are still allocated numbers as well as names, exactly as
1024    if the names were not present. The PCRE API provides function calls for
1025    extracting the name-to-number translation table from a compiled pattern. There
1026    is also a convenience function for extracting a captured substring by name.
1027    .P
1028    By default, a name must be unique within a pattern, but it is possible to relax
1029    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1030    be useful for patterns where only one instance of the named parentheses can
1031    match. Suppose you want to match the name of a weekday, either as a 3-letter
1032    abbreviation or as the full name, and in both cases you want to extract the
1033    abbreviation. This pattern (ignoring the line breaks) does the job:
1034    .sp
1035      (?<DN>Mon|Fri|Sun)(?:day)?|
1036      (?<DN>Tue)(?:sday)?|
1037      (?<DN>Wed)(?:nesday)?|
1038      (?<DN>Thu)(?:rsday)?|
1039      (?<DN>Sat)(?:urday)?
1040    .sp
1041    There are five capturing substrings, but only one is ever set after a match.
1042    (An alternative way of solving this problem is to use a "branch reset"
1043    subpattern, as described in the previous section.)
1044    .P
1045    The convenience function for extracting the data by name returns the substring
1046    for the first (and in this example, the only) subpattern of that name that
1047    matched. This saves searching to find which numbered subpattern it was. If you
1048    make a reference to a non-unique named subpattern from elsewhere in the
1049    pattern, the one that corresponds to the lowest number is used. For further
1050    details of the interfaces for handling named subpatterns, see the
1051  .\" HREF  .\" HREF
1052  \fBpcreapi\fR  \fBpcreapi\fP
1053  .\"  .\"
1054  documentation.  documentation.
1055    .
1056    .
1057  .SH REPETITION  .SH REPETITION
1058  .rs  .rs
1059  .sp  .sp
1060  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1061  items:  items:
1062    .sp
1063    a literal data character    a literal data character
1064    the . metacharacter    the dot metacharacter
1065    the \\C escape sequence    the \eC escape sequence
1066    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1067      the \eR escape sequence
1068      an escape such as \ed that matches a single character
1069    a character class    a character class
1070    a back reference (see next section)    a back reference (see next section)
1071    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1072    .sp
1073  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1074  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1075  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1076  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1077    .sp
1078    z{2,4}    z{2,4}
1079    .sp
1080  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1081  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
1082  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
1083  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1084    .sp
1085    [aeiou]{3,}    [aeiou]{3,}
1086    .sp
1087  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1088    .sp
1089    \\d{8}    \ed{8}
1090    .sp
1091  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
1092  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
1093  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1094  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1095    .P
1096  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1097  bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1098  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1099    support is available, \eX{3} matches three Unicode extended sequences, each of
1100    which may be several bytes long (and they may be of different lengths).
1101    .P
1102  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1103  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1104    .P
1105  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1106  quantifiers have single-character abbreviations:  abbreviations:
1107    .sp
1108    *    is equivalent to {0,}    *    is equivalent to {0,}
1109    +    is equivalent to {1,}    +    is equivalent to {1,}
1110    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1111    .sp
1112  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
1113  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
1114    .sp
1115    (a?)*    (a?)*
1116    .sp
1117  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
1118  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
1119  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
1120  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1121    .P
1122  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
1123  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1124  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1125  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
1126  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1127  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1128    .sp
1129    /\\*.*\\*/    /\e*.*\e*/
1130    .sp
1131  to the string  to the string
1132    .sp
1133    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1134    .sp
1135  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
1136  item.  item.
1137    .P
1138  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
1139  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1140  pattern  pattern
1141    .sp
1142    /\\*.*?\\*/    /\e*.*?\e*/
1143    .sp
1144  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
1145  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1146  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1147  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1148    .sp
1149    \\d??\\d    \ed??\ed
1150    .sp
1151  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1152  way the rest of the pattern matches.  way the rest of the pattern matches.
1153    .P
1154  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1155  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1156  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1157  default behaviour.  default behaviour.
1158    .P
1159  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1160  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
1161  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1162    .P
1163  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1164  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1165  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1166  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1167  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
1168  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1169    .P
1170  In cases where it is known that the subject string contains no newlines, it is  In cases where it is known that the subject string contains no newlines, it is
1171  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1172  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1173    .P
1174  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1175  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1176  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1177  succeed. Consider, for example:  succeeds. Consider, for example:
1178    .sp
1179    (.*)abc\\1    (.*)abc\e1
1180    .sp
1181  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1182  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1183    .P
1184  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1185  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1186    .sp
1187    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1188    .sp
1189  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1190  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1191  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1192  example, after  example, after
1193    .sp
1194    /(a|(b))+/    /(a|(b))+/
1195    .sp
1196  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1197    .
1198  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1199    .\" HTML <a name="atomicgroup"></a>
1200    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1201  .rs  .rs
1202  .sp  .sp
1203  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1204  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1205  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1206  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1207  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1208  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1209    .P
1210  Consider, for example, the pattern \\d+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1211    .sp
1212    123456bar    123456bar
1213    .sp
1214  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1215  action of the matcher is to try again with only 5 digits matching the \\d+  action of the matcher is to try again with only 5 digits matching the \ed+
1216  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1217  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1218  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1219    .P
1220  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1221  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1222  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1223    .sp
1224    (?>\\d+)bar    (?>\ed+)foo
1225    .sp
1226  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1227  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1228  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1229  normal.  normal.
1230    .P
1231  An alternative description is that a subpattern of this type matches the string  An alternative description is that a subpattern of this type matches the string
1232  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1233  the current point in the subject string.  the current point in the subject string.
1234    .P
1235  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1236  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1237  everything it can. So, while both \\d+ and \\d+? are prepared to adjust the  everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1238  number of digits they match in order to make the rest of the pattern match,  number of digits they match in order to make the rest of the pattern match,
1239  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1240    .P
1241  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1242  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1243  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1244  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1245  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1246  previous example can be rewritten as  previous example can be rewritten as
1247    .sp
1248    \\d++bar    \ed++foo
1249    .sp
1250  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1251  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1252  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1253  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1254    difference; possessive quantifiers should be slightly faster.
1255  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1256  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1257    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1258    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1259    package, and PCRE copied it from there. It ultimately found its way into Perl
1260    at release 5.10.
1261    .P
1262    PCRE has an optimization that automatically "possessifies" certain simple
1263    pattern constructs. For example, the sequence A+B is treated as A++B because
1264    there is no point in backtracking into a sequence of A's when B must follow.
1265    .P
1266  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1267  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
1268  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1269  pattern  pattern
1270    .sp
1271    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1272    .sp
1273  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1274  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1275  quickly. However, if it is applied to  quickly. However, if it is applied to
1276    .sp
1277    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1278    .sp
1279  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1280  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \eD+ repeat and the external * repeat in a
1281  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1282  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1283  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1284  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1285  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1286    an atomic group, like this:
1287    ((?>\\D+)|<\\d+>)*[!?]  .sp
1288      ((?>\eD+)|<\ed+>)*[!?]
1289    .sp
1290  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1291    .
1292  .SH BACK REFERENCES  .
1293    .\" HTML <a name="backreferences"></a>
1294    .SH "BACK REFERENCES"
1295  .rs  .rs
1296  .sp  .sp
1297  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1298  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1299  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1300  previous capturing left parentheses.  previous capturing left parentheses.
1301    .P
1302  However, if the decimal number following the backslash is less than 10, it is  However, if the decimal number following the backslash is less than 10, it is
1303  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1304  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1305  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1306  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1307  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1308    in an earlier iteration.
1309    .P
1310    It is not possible to have a numerical "forward back reference" to a subpattern
1311    whose number is 10 or more using this syntax because a sequence such as \e50 is
1312    interpreted as a character defined in octal. See the subsection entitled
1313    "Non-printing characters"
1314    .\" HTML <a href="#digitsafterbackslash">
1315    .\" </a>
1316    above
1317    .\"
1318    for further details of the handling of digits following a backslash. There is
1319    no such problem when named parentheses are used. A back reference to any
1320    subpattern is possible using named parentheses (see below).
1321    .P
1322    Another way of avoiding the ambiguity inherent in the use of digits following a
1323    backslash is to use the \eg escape sequence, which is a feature introduced in
1324    Perl 5.10. This escape must be followed by a positive or a negative number,
1325    optionally enclosed in braces. These examples are all identical:
1326    .sp
1327      (ring), \e1
1328      (ring), \eg1
1329      (ring), \eg{1}
1330    .sp
1331    A positive number specifies an absolute reference without the ambiguity that is
1332    present in the older syntax. It is also useful when literal digits follow the
1333    reference. A negative number is a relative reference. Consider this example:
1334    .sp
1335      (abc(def)ghi)\eg{-1}
1336    .sp
1337    The sequence \eg{-1} is a reference to the most recently started capturing
1338    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1339    would be equivalent to \e1. The use of relative references can be helpful in
1340    long patterns, and also in patterns that are created by joining together
1341    fragments that contain references within themselves.
1342    .P
1343  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1344  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1345  itself (see  itself (see
# Line 832  itself (see Line 1348  itself (see
1348  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1349  .\"  .\"
1350  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1351    .sp
1352    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1353    .sp
1354  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1355  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1356  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1357    .sp
1358    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1359    .sp
1360  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1361  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1362    .P
1363  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1364  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1365    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1366    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1367    references, is also supported. We could rewrite the above example in any of
1368    the following ways:
1369    .sp
1370      (?<p1>(?i)rah)\es+\ek<p1>
1371      (?'p1'(?i)rah)\es+\ek{p1}
1372      (?P<p1>(?i)rah)\es+(?P=p1)
1373      (?<p1>(?i)rah)\es+\eg{p1}
1374    .sp
1375    A subpattern that is referenced by name may appear in the pattern before or
1376    after the reference.
1377    .P
1378  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1379  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1380  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1381    .sp
1382    (a|(bc))\\2    (a|(bc))\e2
1383    .sp
1384  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". Because there may be
1385  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1386  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1387  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1388  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1389  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1390    .\" HTML <a href="#comments">
1391    .\" </a>
1392    "Comments"
1393    .\"
1394    below) can be used.
1395    .P
1396  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1397  when the subpattern is first used, so, for example, (a\\1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1398  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1399  example, the pattern  example, the pattern
1400    .sp
1401    (a|b\\1)+    (a|b\e1)+
1402    .sp
1403  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1404  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1405  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1406  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1407  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1408  minimum of zero.  minimum of zero.
1409    .
1410    .
1411    .\" HTML <a name="bigassertions"></a>
1412  .SH ASSERTIONS  .SH ASSERTIONS
1413  .rs  .rs
1414  .sp  .sp
1415  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1416  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1417  assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.  assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1418    .\" HTML <a href="#smallassertions">
1419    .\" </a>
1420    above.
1421    .\"
1422    .P
1423  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1424  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1425  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1426    except that it does not cause the current matching position to be changed.
1427  An assertion subpattern is matched in the normal way, except that it does not  .P
1428  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1429  with (?= for positive assertions and (?! for negative assertions. For example,  because it makes no sense to assert the same thing several times. If any kind
1430    of assertion contains capturing subpatterns within it, these are counted for
1431    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1432    However, substring capturing is carried out only for positive assertions,
1433    because it does not make sense for negative assertions.
1434    .
1435    .
1436    .SS "Lookahead assertions"
1437    .rs
1438    .sp
1439    Lookahead assertions start with (?= for positive assertions and (?! for
1440    negative assertions. For example,
1441    .sp
1442      \ew+(?=;)
1443    .sp
1444  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1445  the match, and  the match, and
1446    .sp
1447    foo(?!bar)    foo(?!bar)
1448    .sp
1449  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1450  apparently similar pattern  apparently similar pattern
1451    .sp
1452    (?!foo)bar    (?!foo)bar
1453    .sp
1454  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1455  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1456  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1457  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1458    .P
1459  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1460  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1461  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1462    .
1463    .
1464    .\" HTML <a name="lookbehind"></a>
1465    .SS "Lookbehind assertions"
1466    .rs
1467    .sp
1468  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1469  negative assertions. For example,  negative assertions. For example,
1470    .sp
1471    (?<!foo)bar    (?<!foo)bar
1472    .sp
1473  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1474  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1475  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1476  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1477    .sp
1478    (?<=bullock|donkey)    (?<=bullock|donkey)
1479    .sp
1480  is permitted, but  is permitted, but
1481    .sp
1482    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1483    .sp
1484  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1485  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1486  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1487  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1488    .sp
1489    (?<=ab(c|de))    (?<=ab(c|de))
1490    .sp
1491  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1492  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
1493    .sp
1494    (?<=abc|abde)    (?<=abc|abde)
1495    .sp
1496    In some cases, the Perl 5.10 escape sequence \eK
1497    .\" HTML <a href="#resetmatchstart">
1498    .\" </a>
1499    (see above)
1500    .\"
1501    can be used instead of a lookbehind assertion; this is not restricted to a
1502    fixed-length.
1503    .P
1504  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1505  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1506  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1507  match is deemed to fail.  assertion fails.
1508    .P
1509  PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1510  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1511  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1512    different numbers of bytes, are also not permitted.
1513  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1514  efficient matching at the end of the subject string. Consider a simple pattern  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1515  such as  specify efficient matching at the end of the subject string. Consider a simple
1516    pattern such as
1517    .sp
1518    abcd$    abcd$
1519    .sp
1520  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1521  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1522  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
1523    .sp
1524    ^.*abcd$    ^.*abcd$
1525    .sp
1526  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1527  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1528  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1529  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1530  if the pattern is written as  if the pattern is written as
1531    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1532    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1533    .sp
1534  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1535  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1536  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1537  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1538    .
1539    .
1540    .SS "Using multiple assertions"
1541    .rs
1542    .sp
1543  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1544    .sp
1545    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1546    .sp
1547  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1548  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1549  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1550  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1551  This pattern does \fInot\fR match "foo" preceded by six characters, the first  This pattern does \fInot\fP match "foo" preceded by six characters, the first
1552  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1553  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1554    .sp
1555    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1556    .sp
1557  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1558  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1559  preceding three characters are not "999".  preceding three characters are not "999".
1560    .P
1561  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1562    .sp
1563    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1564    .sp
1565  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1566  preceded by "foo", while  preceded by "foo", while
1567    .sp
1568    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1569    .sp
1570  is another pattern which matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
1571  characters that are not "999".  characters that are not "999".
1572    .
1573  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1574  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1575  of assertion contains capturing subpatterns within it, these are counted for  .SH "CONDITIONAL SUBPATTERNS"
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
   
 .SH CONDITIONAL SUBPATTERNS  
1576  .rs  .rs
1577  .sp  .sp
1578  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1579  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1580  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1581  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1582    .sp
1583    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1584    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1585    .sp
1586  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1587  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1588  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1589    .P
1590  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1591  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1592  subpattern of that number has previously matched. The number must be greater  .
1593  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1594  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1595  it into three parts for ease of discussion:  .sp
1596    If the text between the parentheses consists of a sequence of digits, the
1597    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if the capturing subpattern of that number has previously
1598    matched. An alternative notation is to precede the digits with a plus or minus
1599    sign. In this case, the subpattern number is relative rather than absolute.
1600    The most recently opened parentheses can be referenced by (?(-1), the next most
1601    recent by (?(-2), and so on. In looping constructs it can also make sense to
1602    refer to subsequent groups with constructs such as (?(+2).
1603    .P
1604    Consider the following pattern, which contains non-significant white space to
1605    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1606    three parts for ease of discussion:
1607    .sp
1608      ( \e( )?    [^()]+    (?(1) \e) )
1609    .sp
1610  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1611  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
1612  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
# Line 1047  the condition is true, and so the yes-pa Line 1616  the condition is true, and so the yes-pa
1616  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1617  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1618  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1619    .P
1620  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
1621  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1622  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1623      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1624  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1625    This makes the fragment independent of the parentheses in the larger pattern.
1626    .
1627    .SS "Checking for a used subpattern by name"
1628    .rs
1629    .sp
1630    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1631    subpattern by name. For compatibility with earlier versions of PCRE, which had
1632    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1633    there is a possible ambiguity with this syntax, because subpattern names may
1634    consist entirely of digits. PCRE looks first for a named subpattern; if it
1635    cannot find one and the name consists entirely of digits, PCRE looks for a
1636    subpattern of that number, which must be greater than zero. Using subpattern
1637    names that consist entirely of digits is not recommended.
1638    .P
1639    Rewriting the above example to use a named subpattern gives this:
1640    .sp
1641      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1642    .sp
1643    .
1644    .SS "Checking for pattern recursion"
1645    .rs
1646    .sp
1647    If the condition is the string (R), and there is no subpattern with the name R,
1648    the condition is true if a recursive call to the whole pattern or any
1649    subpattern has been made. If digits or a name preceded by ampersand follow the
1650    letter R, for example:
1651    .sp
1652      (?(R3)...) or (?(R&name)...)
1653    .sp
1654    the condition is true if the most recent recursion is into the subpattern whose
1655    number or name is given. This condition does not check the entire recursion
1656    stack.
1657    .P
1658    At "top level", all these recursion test conditions are false. Recursive
1659    patterns are described below.
1660    .
1661    .SS "Defining subpatterns for use by reference only"
1662    .rs
1663    .sp
1664    If the condition is the string (DEFINE), and there is no subpattern with the
1665    name DEFINE, the condition is always false. In this case, there may be only one
1666    alternative in the subpattern. It is always skipped if control reaches this
1667    point in the pattern; the idea of DEFINE is that it can be used to define
1668    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1669    is described below.) For example, a pattern to match an IPv4 address could be
1670    written like this (ignore whitespace and line breaks):
1671    .sp
1672      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1673      \eb (?&byte) (\e.(?&byte)){3} \eb
1674    .sp
1675    The first part of the pattern is a DEFINE group inside which a another group
1676    named "byte" is defined. This matches an individual component of an IPv4
1677    address (a number less than 256). When matching takes place, this part of the
1678    pattern is skipped because DEFINE acts like a false condition.
1679    .P
1680    The rest of the pattern uses references to the named group to match the four
1681    dot-separated components of an IPv4 address, insisting on a word boundary at
1682    each end.
1683    .
1684    .SS "Assertion conditions"
1685    .rs
1686    .sp
1687    If the condition is not in any of the above formats, it must be an assertion.
1688  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1689  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1690  alternatives on the second line:  alternatives on the second line:
1691    .sp
1692    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1693    \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )    \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
1694    .sp
1695  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1696  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1697  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1698  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1699  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1700  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1701    .
1702    .
1703    .\" HTML <a name="comments"></a>
1704  .SH COMMENTS  .SH COMMENTS
1705  .rs  .rs
1706  .sp  .sp
1707  The sequence (?# marks the start of a comment which continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1708  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1709  that make up a comment play no part in the pattern matching at all.  that make up a comment play no part in the pattern matching at all.
1710    .P
1711  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1712  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1713  character in the pattern.  next newline in the pattern.
1714    .
1715  .SH RECURSIVE PATTERNS  .
1716    .\" HTML <a name="recursion"></a>
1717    .SH "RECURSIVE PATTERNS"
1718  .rs  .rs
1719  .sp  .sp
1720  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1721  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1722  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1723  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1724  experimental facility that allows regular expressions to recurse (amongst other  .P
1725  things). It does this by interpolating Perl code in the expression at run time,  For some time, Perl has provided a facility that allows regular expressions to
1726  and the code can refer to the expression itself. A Perl pattern to solve the  recurse (amongst other things). It does this by interpolating Perl code in the
1727  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1728    pattern using code interpolation to solve the parentheses problem can be
1729    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
1730    .sp
1731      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1732    .sp
1733  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
1734  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1735  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1736  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1737    supports special syntax for recursion of the entire pattern, and also for
1738  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1739  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1740  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1741  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
1742  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1743    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1744  For example, this PCRE pattern solves the nested parentheses problem (assume  call, which is described in the next section.) The special item (?R) or (?0) is
1745  the PCRE_EXTENDED option is set so that white space is ignored):  a recursive call of the entire regular expression.
1746    .P
1747    \\( ( (?>[^()]+) | (?R) )* \\)  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1748    treated as an atomic group. That is, once it has matched some of the subject
1749    string, it is never re-entered, even if it contains untried alternatives and
1750    there is a subsequent matching failure.
1751    .P
1752    This PCRE pattern solves the nested parentheses problem (assume the
1753    PCRE_EXTENDED option is set so that white space is ignored):
1754    .sp
1755      \e( ( (?>[^()]+) | (?R) )* \e)
1756    .sp
1757  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1758  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1759  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1760  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1761    .P
1762  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
1763  pattern, so instead you could use this:  pattern, so instead you could use this:
1764    .sp
1765    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1766    .sp
1767  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
1768  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1769  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1770  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1771  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1772  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1773    most recently opened parentheses preceding the recursion. In other words, a
1774    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
1775    it is encountered.
1776  This particular example pattern contains nested unlimited repeats, and so the  .P
1777  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1778  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1779  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1780    "subroutine" calls, as described in the next section.
1781    .P
1782    An alternative approach is to use named parentheses instead. The Perl syntax
1783    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1784    could rewrite the above example as follows:
1785    .sp
1786      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1787    .sp
1788    If there is more than one subpattern with the same name, the earliest one is
1789    used.
1790    .P
1791    This particular example pattern that we have been looking at contains nested
1792    unlimited repeats, and so the use of atomic grouping for matching strings of
1793    non-parentheses is important when applying the pattern to strings that do not
1794    match. For example, when this pattern is applied to
1795    .sp
1796    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1797    .sp
1798  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1799  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
1800  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
1801  before failure can be reported.  before failure can be reported.
1802    .P
1803  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1804  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1805  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1806  below and the  below and the
1807  .\" HREF  .\" HREF
1808  \fBpcrecallout\fR  \fBpcrecallout\fP
1809  .\"  .\"
1810  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1811    .sp
1812    (ab(cd)ef)    (ab(cd)ef)
1813    .sp
1814  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1815  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1816    .sp
1817    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1818       ^                        ^       ^                        ^
1819       ^                        ^       ^                        ^
1820    .sp
1821  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1822  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1823  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
1824  using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1825  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1826    .P
1827  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
1828  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1829  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1830  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1831    .sp
1832    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1833    .sp
1834  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
1835  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1836  is the actual recursive call.  is the actual recursive call.
1837    .
1838    .
1839  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1840  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1841  .rs  .rs
1842  .sp  .sp
1843  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1844  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
1845  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1846  pattern  before or after the reference. A numbered reference can be absolute or
1847    relative, as in these examples:
1848    (sens|respons)e and \\1ibility  .sp
1849      (...(absolute)...)...(?2)...
1850      (...(relative)...)...(?-1)...
1851      (...(?+1)...(relative)...
1852    .sp
1853    An earlier example pointed out that the pattern
1854    .sp
1855      (sens|respons)e and \e1ibility
1856    .sp
1857  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1858  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1859    .sp
1860    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1861    .sp
1862  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
1863  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1864  refer.  .P
1865    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1866    group. That is, once it has matched some of the subject string, it is never
1867    re-entered, even if it contains untried alternatives and there is a subsequent
1868    matching failure.
1869    .P
1870    When a subpattern is used as a subroutine, processing options such as
1871    case-independence are fixed when the subpattern is defined. They cannot be
1872    changed for different calls. For example, consider this pattern:
1873    .sp
1874      (abc)(?i:(?-1))
1875    .sp
1876    It matches "abcabc". It does not match "abcABC" because the change of
1877    processing option does not affect the called subpattern.
1878    .
1879    .
1880  .SH CALLOUTS  .SH CALLOUTS
1881  .rs  .rs
1882  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1884  Perl has a feature whereby using the seq
1884  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
1885  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1886  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1887    .P
1888  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1889  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
1890  function by putting its entry point in the global variable \fIpcre_callout\fR.  function by putting its entry point in the global variable \fIpcre_callout\fP.
1891  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1892    .P
1893  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1894  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
1895  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1896  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1897    .sp
1898    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1899    .sp
1900  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1901    automatically installed before each item in the pattern. They are all numbered
1902    255.
1903    .P
1904    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1905  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
1906  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
1907  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1908  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1909  function is given in the  description of the interface to the callout function is given in the
1910  .\" HREF  .\" HREF
1911  \fBpcrecallout\fR  \fBpcrecallout\fP
1912  .\"  .\"
1913  documentation.  documentation.
1914    .
1915  .in 0  .
1916  Last updated: 03 February 2003  .SH "SEE ALSO"
1917  .br  .rs
1918  Copyright (c) 1997-2003 University of Cambridge.  .sp
1919    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1920    .
1921    .
1922    .SH AUTHOR
1923    .rs
1924    .sp
1925    .nf
1926    Philip Hazel
1927    University Computing Service
1928    Cambridge CB2 3QH, England.
1929    .fi
1930    .
1931    .
1932    .SH REVISION
1933    .rs
1934    .sp
1935    .nf
1936    Last updated: 11 June 2007
1937    Copyright (c) 1997-2007 University of Cambridge.
1938    .fi

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