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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      \eh     any horizontal whitespace character
266      \eH     any character that is not a horizontal whitespace character
267      \es     any whitespace character
268      \eS     any character that is not a whitespace character
269      \ev     any vertical whitespace character
270      \eV     any character that is not a vertical whitespace character
271      \ew     any "word" character
272      \eW     any "non-word" character
273    .sp
274  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
275  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.
276    .P
277  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
278  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
279    matching point is at the end of the subject string, all of them fail, since
280  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
281  This makes it different from the the POSIX "space" class. The \\s characters  .P
282  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).
283    This makes it different from the the POSIX "space" class. The \es characters
284  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
285  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
286  digits is controlled by PCRE's character tables, and may vary if locale-  does.
287  specific matching is taking place (see  .P
288    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
289    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
290    character property support is available. These sequences retain their original
291    meanings from before UTF-8 support was available, mainly for efficiency
292    reasons.
293    .P
294    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
295    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
296    The horizontal space characters are:
297    .sp
298      U+0009     Horizontal tab
299      U+0020     Space
300      U+00A0     Non-break space
301      U+1680     Ogham space mark
302      U+180E     Mongolian vowel separator
303      U+2000     En quad
304      U+2001     Em quad
305      U+2002     En space
306      U+2003     Em space
307      U+2004     Three-per-em space
308      U+2005     Four-per-em space
309      U+2006     Six-per-em space
310      U+2007     Figure space
311      U+2008     Punctuation space
312      U+2009     Thin space
313      U+200A     Hair space
314      U+202F     Narrow no-break space
315      U+205F     Medium mathematical space
316      U+3000     Ideographic space
317    .sp
318    The vertical space characters are:
319    .sp
320      U+000A     Linefeed
321      U+000B     Vertical tab
322      U+000C     Formfeed
323      U+000D     Carriage return
324      U+0085     Next line
325      U+2028     Line separator
326      U+2029     Paragraph separator
327    .P
328    A "word" character is an underscore or any character less than 256 that is a
329    letter or digit. The definition of letters and digits is controlled by PCRE's
330    low-valued character tables, and may vary if locale-specific matching is taking
331    place (see
332  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
333  .\" </a>  .\" </a>
334  "Locale support"  "Locale support"
335  .\"  .\"
336  in the  in the
337  .\" HREF  .\" HREF
338  \fBpcreapi\fR  \fBpcreapi\fP
339  .\"  .\"
340  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,
341  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
342    accented letters, and these are matched by \ew. The use of locales with Unicode
343  These character type sequences can appear both inside and outside character  is discouraged.
344  classes. They each match one character of the appropriate type. If the current  .
345  matching point is at the end of the subject string, all of them fail, since  .
346  there is no character to match.  .SS "Newline sequences"
347    .rs
348  The fourth use of backslash is for certain simple assertions. An assertion  .sp
349    Outside a character class, the escape sequence \eR matches any Unicode newline
350    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
351    the following:
352    .sp
353      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
354    .sp
355    This is an example of an "atomic group", details of which are given
356    .\" HTML <a href="#atomicgroup">
357    .\" </a>
358    below.
359    .\"
360    This particular group matches either the two-character sequence CR followed by
361    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
362    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
363    line, U+0085). The two-character sequence is treated as a single unit that
364    cannot be split.
365    .P
366    In UTF-8 mode, two additional characters whose codepoints are greater than 255
367    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
368    Unicode character property support is not needed for these characters to be
369    recognized.
370    .P
371    Inside a character class, \eR matches the letter "R".
372    .
373    .
374    .\" HTML <a name="uniextseq"></a>
375    .SS Unicode character properties
376    .rs
377    .sp
378    When PCRE is built with Unicode character property support, three additional
379    escape sequences to match character properties are available when UTF-8 mode
380    is selected. They are:
381    .sp
382      \ep{\fIxx\fP}   a character with the \fIxx\fP property
383      \eP{\fIxx\fP}   a character without the \fIxx\fP property
384      \eX       an extended Unicode sequence
385    .sp
386    The property names represented by \fIxx\fP above are limited to the Unicode
387    script names, the general category properties, and "Any", which matches any
388    character (including newline). Other properties such as "InMusicalSymbols" are
389    not currently supported by PCRE. Note that \eP{Any} does not match any
390    characters, so always causes a match failure.
391    .P
392    Sets of Unicode characters are defined as belonging to certain scripts. A
393    character from one of these sets can be matched using a script name. For
394    example:
395    .sp
396      \ep{Greek}
397      \eP{Han}
398    .sp
399    Those that are not part of an identified script are lumped together as
400    "Common". The current list of scripts is:
401    .P
402    Arabic,
403    Armenian,
404    Balinese,
405    Bengali,
406    Bopomofo,
407    Braille,
408    Buginese,
409    Buhid,
410    Canadian_Aboriginal,
411    Cherokee,
412    Common,
413    Coptic,
414    Cuneiform,
415    Cypriot,
416    Cyrillic,
417    Deseret,
418    Devanagari,
419    Ethiopic,
420    Georgian,
421    Glagolitic,
422    Gothic,
423    Greek,
424    Gujarati,
425    Gurmukhi,
426    Han,
427    Hangul,
428    Hanunoo,
429    Hebrew,
430    Hiragana,
431    Inherited,
432    Kannada,
433    Katakana,
434    Kharoshthi,
435    Khmer,
436    Lao,
437    Latin,
438    Limbu,
439    Linear_B,
440    Malayalam,
441    Mongolian,
442    Myanmar,
443    New_Tai_Lue,
444    Nko,
445    Ogham,
446    Old_Italic,
447    Old_Persian,
448    Oriya,
449    Osmanya,
450    Phags_Pa,
451    Phoenician,
452    Runic,
453    Shavian,
454    Sinhala,
455    Syloti_Nagri,
456    Syriac,
457    Tagalog,
458    Tagbanwa,
459    Tai_Le,
460    Tamil,
461    Telugu,
462    Thaana,
463    Thai,
464    Tibetan,
465    Tifinagh,
466    Ugaritic,
467    Yi.
468    .P
469    Each character has exactly one general category property, specified by a
470    two-letter abbreviation. For compatibility with Perl, negation can be specified
471    by including a circumflex between the opening brace and the property name. For
472    example, \ep{^Lu} is the same as \eP{Lu}.
473    .P
474    If only one letter is specified with \ep or \eP, it includes all the general
475    category properties that start with that letter. In this case, in the absence
476    of negation, the curly brackets in the escape sequence are optional; these two
477    examples have the same effect:
478    .sp
479      \ep{L}
480      \epL
481    .sp
482    The following general category property codes are supported:
483    .sp
484      C     Other
485      Cc    Control
486      Cf    Format
487      Cn    Unassigned
488      Co    Private use
489      Cs    Surrogate
490    .sp
491      L     Letter
492      Ll    Lower case letter
493      Lm    Modifier letter
494      Lo    Other letter
495      Lt    Title case letter
496      Lu    Upper case letter
497    .sp
498      M     Mark
499      Mc    Spacing mark
500      Me    Enclosing mark
501      Mn    Non-spacing mark
502    .sp
503      N     Number
504      Nd    Decimal number
505      Nl    Letter number
506      No    Other number
507    .sp
508      P     Punctuation
509      Pc    Connector punctuation
510      Pd    Dash punctuation
511      Pe    Close punctuation
512      Pf    Final punctuation
513      Pi    Initial punctuation
514      Po    Other punctuation
515      Ps    Open punctuation
516    .sp
517      S     Symbol
518      Sc    Currency symbol
519      Sk    Modifier symbol
520      Sm    Mathematical symbol
521      So    Other symbol
522    .sp
523      Z     Separator
524      Zl    Line separator
525      Zp    Paragraph separator
526      Zs    Space separator
527    .sp
528    The special property L& is also supported: it matches a character that has
529    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
530    a modifier or "other".
531    .P
532    The long synonyms for these properties that Perl supports (such as \ep{Letter})
533    are not supported by PCRE, nor is it permitted to prefix any of these
534    properties with "Is".
535    .P
536    No character that is in the Unicode table has the Cn (unassigned) property.
537    Instead, this property is assumed for any code point that is not in the
538    Unicode table.
539    .P
540    Specifying caseless matching does not affect these escape sequences. For
541    example, \ep{Lu} always matches only upper case letters.
542    .P
543    The \eX escape matches any number of Unicode characters that form an extended
544    Unicode sequence. \eX is equivalent to
545    .sp
546      (?>\ePM\epM*)
547    .sp
548    That is, it matches a character without the "mark" property, followed by zero
549    or more characters with the "mark" property, and treats the sequence as an
550    atomic group
551    .\" HTML <a href="#atomicgroup">
552    .\" </a>
553    (see below).
554    .\"
555    Characters with the "mark" property are typically accents that affect the
556    preceding character.
557    .P
558    Matching characters by Unicode property is not fast, because PCRE has to search
559    a structure that contains data for over fifteen thousand characters. That is
560    why the traditional escape sequences such as \ed and \ew do not use Unicode
561    properties in PCRE.
562    .
563    .
564    .\" HTML <a name="resetmatchstart"></a>
565    .SS "Resetting the match start"
566    .rs
567    .sp
568    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
569    matched characters not to be included in the final matched sequence. For
570    example, the pattern:
571    .sp
572      foo\eKbar
573    .sp
574    matches "foobar", but reports that it has matched "bar". This feature is
575    similar to a lookbehind assertion
576    .\" HTML <a href="#lookbehind">
577    .\" </a>
578    (described below).
579    .\"
580    However, in this case, the part of the subject before the real match does not
581    have to be of fixed length, as lookbehind assertions do. The use of \eK does
582    not interfere with the setting of
583    .\" HTML <a href="#subpattern">
584    .\" </a>
585    captured substrings.
586    .\"
587    For example, when the pattern
588    .sp
589      (foo)\eKbar
590    .sp
591    matches "foobar", the first substring is still set to "foo".
592    .
593    .
594    .\" HTML <a name="smallassertions"></a>
595    .SS "Simple assertions"
596    .rs
597    .sp
598    The final use of backslash is for certain simple assertions. An assertion
599  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,
600  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
601  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
602  assertions are  .\" HTML <a href="#bigassertions">
603    .\" </a>
604    \\b     matches at a word boundary  below.
605    \\B     matches when not at a word boundary  .\"
606    \\A     matches at start of subject  The backslashed assertions are:
607    \\Z     matches at end of subject or before newline at end  .sp
608    \\z     matches at end of subject    \eb     matches at a word boundary
609    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
610      \eA     matches at the start of the subject
611  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
612              also matches before a newline at the end of the subject
613      \ez     matches only at the end of the subject
614      \eG     matches at the first matching position in the subject
615    .sp
616    These assertions may not appear in character classes (but note that \eb has a
617  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
618    .P
619  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
620  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
621  \\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
622  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
623    .P
624  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
625  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
626  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
627  multiline mode.  independent of multiline mode. These three assertions are not affected by the
628    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
629  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
630  \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
631  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
632  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
633  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.
634  string, whereas \\z matches only at the end.  .P
635    The \eG assertion is true only when the current matching position is at the
636  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
637  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
638  \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  
639  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
640  implementation where \\G can be useful.  implementation where \eG can be useful.
641    .P
642  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
643  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
644  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
645  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
646  reproduce this behaviour.  reproduce this behaviour.
647    .P
648  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
649  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
650  regular expression.  regular expression.
651    .
652  .SH CIRCUMFLEX AND DOLLAR  .
653    .SH "CIRCUMFLEX AND DOLLAR"
654  .rs  .rs
655  .sp  .sp
656  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
657  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
658  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
659  \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
660  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
661  meaning (see below).  meaning
662    .\" HTML <a href="#characterclass">
663    .\" </a>
664    (see below).
665    .\"
666    .P
667  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
668  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
669  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 671  possible alternatives start with a circu
671  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
672  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
673  to be anchored.)  to be anchored.)
674    .P
675  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
676  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
677  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
678  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
679  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
680  Dollar has no special meaning in a character class.  character class.
681    .P
682  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
683  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
684  does not affect the \\Z assertion.  does not affect the \eZ assertion.
685    .P
686  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
687  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
688  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
689  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
690  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
691  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
692  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
693  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
694  \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
695  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
696    patterns that are anchored in single line mode because all branches start with
697  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
698    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
699    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
700    .P
701    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
702  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
703  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
704    .
705  .SH FULL STOP (PERIOD, DOT)  .
706    .SH "FULL STOP (PERIOD, DOT)"
707  .rs  .rs
708  .sp  .sp
709  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
710  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
711  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.
712  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
713  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
714  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
715  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
716    (including isolated CRs and LFs). When any Unicode line endings are being
717  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
718  .rs  characters.
719  .sp  .P
720  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
721  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
722  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
723  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
724  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
725    The handling of dot is entirely independent of the handling of circumflex and
726  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
727  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
728    .
729  .SH SQUARE BRACKETS  .
730    .SH "MATCHING A SINGLE BYTE"
731    .rs
732    .sp
733    Outside a character class, the escape sequence \eC matches any one byte, both
734    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
735    characters. The feature is provided in Perl in order to match individual bytes
736    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
737    what remains in the string may be a malformed UTF-8 string. For this reason,
738    the \eC escape sequence is best avoided.
739    .P
740    PCRE does not allow \eC to appear in lookbehind assertions
741    .\" HTML <a href="#lookbehind">
742    .\" </a>
743    (described below),
744    .\"
745    because in UTF-8 mode this would make it impossible to calculate the length of
746    the lookbehind.
747    .
748    .
749    .\" HTML <a name="characterclass"></a>
750    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
751  .rs  .rs
752  .sp  .sp
753  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 755  square bracket. A closing square bracket
755  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
756  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
757  escaped with a backslash.  escaped with a backslash.
758    .P
759  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
760  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
761  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 763  definition is a circumflex, in which cas
763  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
764  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
765  backslash.  backslash.
766    .P
767  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
768  [^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
769  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
770  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
771  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
772  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
773    string.
774    .P
775  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
776  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.
777    .P
778  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
779  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
780  "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
781  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
782  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
783    always possible. For characters with higher values, the concept of case is
784  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.
785  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
786  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
787    UTF-8 support.
788    .P
789    Characters that might indicate line breaks are never treated in any special way
790    when matching character classes, whatever line-ending sequence is in use, and
791    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
792    such as [^a] always matches one of these characters.
793    .P
794  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
795  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,
796  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
797  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
798  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.
799    .P
800  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
801  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
802  ("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
803  "-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
804  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
805  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
806  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
807    .P
808  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
809  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
810  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
811  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
812    .P
813  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
814  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
815  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
816  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
817    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
818  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
819  character class, and add the characters that they match to the class. For  property support.
820  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
821    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
822    in a character class, and add the characters that they match to the class. For
823    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
824  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
825  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
826  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
827    .P
828  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
829  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
830  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
831    introducing a POSIX class name - see the next section), and the terminating
832  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
833    does no harm.
834    .
835    .
836    .SH "POSIX CHARACTER CLASSES"
837  .rs  .rs
838  .sp  .sp
839  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
840  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
841  this notation. For example,  this notation. For example,
842    .sp
843    [01[:alpha:]%]    [01[:alpha:]%]
844    .sp
845  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
846  are  are
847    .sp
848    alnum    letters and digits    alnum    letters and digits
849    alpha    letters    alpha    letters
850    ascii    character codes 0 - 127    ascii    character codes 0 - 127
851    blank    space or tab only    blank    space or tab only
852    cntrl    control characters    cntrl    control characters
853    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
854    graph    printing characters, excluding space    graph    printing characters, excluding space
855    lower    lower case letters    lower    lower case letters
856    print    printing characters, including space    print    printing characters, including space
857    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
858    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
859    upper    upper case letters    upper    upper case letters
860    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
861    xdigit   hexadecimal digits    xdigit   hexadecimal digits
862    .sp
863  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
864  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
865  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
866  compatibility).  compatibility).
867    .P
868  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
869  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
870  after the colon. For example,  after the colon. For example,
871    .sp
872    [12[:^digit:]]    [12[:^digit:]]
873    .sp
874  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
875  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
876  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
877    .P
878  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
879  the POSIX character classes.  the POSIX character classes.
880    .
881  .SH VERTICAL BAR  .
882    .SH "VERTICAL BAR"
883  .rs  .rs
884  .sp  .sp
885  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
886  the pattern  the pattern
887    .sp
888    gilbert|sullivan    gilbert|sullivan
889    .sp
890  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
891  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
892  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
893  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
894  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
895  pattern as well as the alternative in the subpattern.  .\" </a>
896    (defined below),
897  .SH INTERNAL OPTION SETTING  .\"
898    "succeeds" means matching the rest of the main pattern as well as the
899    alternative in the subpattern.
900    .
901    .
902    .SH "INTERNAL OPTION SETTING"
903  .rs  .rs
904  .sp  .sp
905  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
906  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
907  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
908    .sp
909    i  for PCRE_CASELESS    i  for PCRE_CASELESS
910    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
911    s  for PCRE_DOTALL    s  for PCRE_DOTALL
912    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
913    .sp
914  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
915  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
916  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
917  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
918  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
919  unset.  unset.
920    .P
921  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
922  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
923  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
924  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
925  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
926    .P
927  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
928  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
929    .sp
930    (a(?i)b)c    (a(?i)b)c
931    .sp
932  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).
933  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
934  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
935  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
936    .sp
937    (a(?i)b|c)    (a(?i)b|c)
938    .sp
939  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
940  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
941  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
942  behaviour otherwise.  behaviour otherwise.
943    .P
944  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
945  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
946  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
947  earlier in the pattern than any of the additional features it turns on, even  .
948  when it is at top level. It is best put at the start.  .
949    .\" HTML <a name="subpattern"></a>
950  .SH SUBPATTERNS  .SH SUBPATTERNS
951  .rs  .rs
952  .sp  .sp
953  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
954  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
955    .sp
956  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
957    .sp
958    cat(aract|erpillar|)    cat(aract|erpillar|)
959    .sp
960  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
961  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
962    .sp
963  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
964  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
965  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
966  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
967  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
968    .P
969  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
970    .sp
971    the ((red|white) (king|queen))    the ((red|white) (king|queen))
972    .sp
973  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,
974  2, and 3, respectively.  2, and 3, respectively.
975    .P
976  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
977  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
978  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
979  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
980  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
981  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
982    .sp
983    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
984    .sp
985  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
986  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
987  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
988  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
989  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
990  the ":". Thus the two patterns  the ":". Thus the two patterns
991    .sp
992    (?i:saturday|sunday)    (?i:saturday|sunday)
993    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
994    .sp
995  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
996  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
997  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
998  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
999    .
1000  .SH NAMED SUBPATTERNS  .
1001    .SH "DUPLICATE SUBPATTERN NUMBERS"
1002    .rs
1003    .sp
1004    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1005    the same numbers for its capturing parentheses. Such a subpattern starts with
1006    (?| and is itself a non-capturing subpattern. For example, consider this
1007    pattern:
1008    .sp
1009      (?|(Sat)ur|(Sun))day
1010    .sp
1011    Because the two alternatives are inside a (?| group, both sets of capturing
1012    parentheses are numbered one. Thus, when the pattern matches, you can look
1013    at captured substring number one, whichever alternative matched. This construct
1014    is useful when you want to capture part, but not all, of one of a number of
1015    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1016    number is reset at the start of each branch. The numbers of any capturing
1017    buffers that follow the subpattern start after the highest number used in any
1018    branch. The following example is taken from the Perl documentation.
1019    The numbers underneath show in which buffer the captured content will be
1020    stored.
1021    .sp
1022      # before  ---------------branch-reset----------- after
1023      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1024      # 1            2         2  3        2     3     4
1025    .sp
1026    A backreference or a recursive call to a numbered subpattern always refers to
1027    the first one in the pattern with the given number.
1028    .P
1029    An alternative approach to using this "branch reset" feature is to use
1030    duplicate named subpatterns, as described in the next section.
1031    .
1032    .
1033    .SH "NAMED SUBPATTERNS"
1034  .rs  .rs
1035  .sp  .sp
1036  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
1037  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1038  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
1039  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1040  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
1041  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
1042    the Perl and the Python syntax.
1043  Named capturing parentheses are still allocated numbers as well as names. The  .P
1044  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
1045  table from a compiled pattern. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1046    parentheses from other parts of the pattern, such as
1047    .\" HTML <a href="#backreferences">
1048    .\" </a>
1049    backreferences,
1050    .\"
1051    .\" HTML <a href="#recursion">
1052    .\" </a>
1053    recursion,
1054    .\"
1055    and
1056    .\" HTML <a href="#conditions">
1057    .\" </a>
1058    conditions,
1059    .\"
1060    can be made by name as well as by number.
1061    .P
1062    Names consist of up to 32 alphanumeric characters and underscores. Named
1063    capturing parentheses are still allocated numbers as well as names, exactly as
1064    if the names were not present. The PCRE API provides function calls for
1065    extracting the name-to-number translation table from a compiled pattern. There
1066    is also a convenience function for extracting a captured substring by name.
1067    .P
1068    By default, a name must be unique within a pattern, but it is possible to relax
1069    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1070    be useful for patterns where only one instance of the named parentheses can
1071    match. Suppose you want to match the name of a weekday, either as a 3-letter
1072    abbreviation or as the full name, and in both cases you want to extract the
1073    abbreviation. This pattern (ignoring the line breaks) does the job:
1074    .sp
1075      (?<DN>Mon|Fri|Sun)(?:day)?|
1076      (?<DN>Tue)(?:sday)?|
1077      (?<DN>Wed)(?:nesday)?|
1078      (?<DN>Thu)(?:rsday)?|
1079      (?<DN>Sat)(?:urday)?
1080    .sp
1081    There are five capturing substrings, but only one is ever set after a match.
1082    (An alternative way of solving this problem is to use a "branch reset"
1083    subpattern, as described in the previous section.)
1084    .P
1085    The convenience function for extracting the data by name returns the substring
1086    for the first (and in this example, the only) subpattern of that name that
1087    matched. This saves searching to find which numbered subpattern it was. If you
1088    make a reference to a non-unique named subpattern from elsewhere in the
1089    pattern, the one that corresponds to the lowest number is used. For further
1090    details of the interfaces for handling named subpatterns, see the
1091  .\" HREF  .\" HREF
1092  \fBpcreapi\fR  \fBpcreapi\fP
1093  .\"  .\"
1094  documentation.  documentation.
1095    .
1096    .
1097  .SH REPETITION  .SH REPETITION
1098  .rs  .rs
1099  .sp  .sp
1100  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1101  items:  items:
1102    .sp
1103    a literal data character    a literal data character
1104    the . metacharacter    the dot metacharacter
1105    the \\C escape sequence    the \eC escape sequence
1106    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1107      the \eR escape sequence
1108      an escape such as \ed that matches a single character
1109    a character class    a character class
1110    a back reference (see next section)    a back reference (see next section)
1111    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1112    .sp
1113  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1114  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1115  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
1116  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1117    .sp
1118    z{2,4}    z{2,4}
1119    .sp
1120  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
1121  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
1122  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
1123  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1124    .sp
1125    [aeiou]{3,}    [aeiou]{3,}
1126    .sp
1127  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1128    .sp
1129    \\d{8}    \ed{8}
1130    .sp
1131  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
1132  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
1133  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
1134  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1135    .P
1136  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
1137  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
1138  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1139    support is available, \eX{3} matches three Unicode extended sequences, each of
1140    which may be several bytes long (and they may be of different lengths).
1141    .P
1142  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
1143  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1144    .P
1145  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1146  quantifiers have single-character abbreviations:  abbreviations:
1147    .sp
1148    *    is equivalent to {0,}    *    is equivalent to {0,}
1149    +    is equivalent to {1,}    +    is equivalent to {1,}
1150    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1151    .sp
1152  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
1153  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:
1154    .sp
1155    (a?)*    (a?)*
1156    .sp
1157  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
1158  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
1159  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
1160  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1161    .P
1162  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
1163  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1164  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
1165  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 */
1166  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1167  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1168    .sp
1169    /\\*.*\\*/    /\e*.*\e*/
1170    .sp
1171  to the string  to the string
1172    .sp
1173    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1174    .sp
1175  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 .*
1176  item.  item.
1177    .P
1178  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
1179  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1180  pattern  pattern
1181    .sp
1182    /\\*.*?\\*/    /\e*.*?\e*/
1183    .sp
1184  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
1185  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1186  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
1187  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
1188    .sp
1189    \\d??\\d    \ed??\ed
1190    .sp
1191  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
1192  way the rest of the pattern matches.  way the rest of the pattern matches.
1193    .P
1194  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),
1195  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
1196  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
1197  default behaviour.  default behaviour.
1198    .P
1199  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1200  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
1201  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1202    .P
1203  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
1204  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
1205  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1206  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
1207  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
1208  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1209    .P
1210  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
1211  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1212  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1213    .P
1214  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1215  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1216  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
1217  succeed. Consider, for example:  succeeds. Consider, for example:
1218    .sp
1219    (.*)abc\\1    (.*)abc\e1
1220    .sp
1221  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
1222  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1223    .P
1224  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1225  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1226    .sp
1227    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1228    .sp
1229  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1230  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1231  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1232  example, after  example, after
1233    .sp
1234    /(a|(b))+/    /(a|(b))+/
1235    .sp
1236  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1237    .
1238  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1239    .\" HTML <a name="atomicgroup"></a>
1240    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1241  .rs  .rs
1242  .sp  .sp
1243  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1244  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
1245  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
1246  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
1247  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
1248  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1249    .P
1250  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
1251    .sp
1252    123456bar    123456bar
1253    .sp
1254  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
1255  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+
1256  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"
1257  (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
1258  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.
1259    .P
1260  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
1261  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
1262  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1263    .sp
1264    (?>\\d+)bar    (?>\ed+)foo
1265    .sp
1266  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
1267  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
1268  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1269  normal.  normal.
1270    .P
1271  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
1272  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1273  the current point in the subject string.  the current point in the subject string.
1274    .P
1275  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1276  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
1277  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
1278  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,
1279  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1280    .P
1281  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1282  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1283  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
1284  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1285  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1286  previous example can be rewritten as  previous example can be rewritten as
1287    .sp
1288    \\d++bar    \ed++foo
1289    .sp
1290  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1291  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
1292  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
1293  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1294    difference; possessive quantifiers should be slightly faster.
1295  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1296  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1297    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1298    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1299    package, and PCRE copied it from there. It ultimately found its way into Perl
1300    at release 5.10.
1301    .P
1302    PCRE has an optimization that automatically "possessifies" certain simple
1303    pattern constructs. For example, the sequence A+B is treated as A++B because
1304    there is no point in backtracking into a sequence of A's when B must follow.
1305    .P
1306  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
1307  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
1308  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
1309  pattern  pattern
1310    .sp
1311    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1312    .sp
1313  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
1314  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1315  quickly. However, if it is applied to  quickly. However, if it is applied to
1316    .sp
1317    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1318    .sp
1319  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
1320  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
1321  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
1322  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
1323  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
1324  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
1325  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1326    an atomic group, like this:
1327    ((?>\\D+)|<\\d+>)*[!?]  .sp
1328      ((?>\eD+)|<\ed+>)*[!?]
1329    .sp
1330  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1331    .
1332  .SH BACK REFERENCES  .
1333    .\" HTML <a name="backreferences"></a>
1334    .SH "BACK REFERENCES"
1335  .rs  .rs
1336  .sp  .sp
1337  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
1338  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1339  (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
1340  previous capturing left parentheses.  previous capturing left parentheses.
1341    .P
1342  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
1343  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
1344  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
1345  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
1346  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
1347  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1348    in an earlier iteration.
1349    .P
1350    It is not possible to have a numerical "forward back reference" to a subpattern
1351    whose number is 10 or more using this syntax because a sequence such as \e50 is
1352    interpreted as a character defined in octal. See the subsection entitled
1353    "Non-printing characters"
1354    .\" HTML <a href="#digitsafterbackslash">
1355    .\" </a>
1356    above
1357    .\"
1358    for further details of the handling of digits following a backslash. There is
1359    no such problem when named parentheses are used. A back reference to any
1360    subpattern is possible using named parentheses (see below).
1361    .P
1362    Another way of avoiding the ambiguity inherent in the use of digits following a
1363    backslash is to use the \eg escape sequence, which is a feature introduced in
1364    Perl 5.10. This escape must be followed by a positive or a negative number,
1365    optionally enclosed in braces. These examples are all identical:
1366    .sp
1367      (ring), \e1
1368      (ring), \eg1
1369      (ring), \eg{1}
1370    .sp
1371    A positive number specifies an absolute reference without the ambiguity that is
1372    present in the older syntax. It is also useful when literal digits follow the
1373    reference. A negative number is a relative reference. Consider this example:
1374    .sp
1375      (abc(def)ghi)\eg{-1}
1376    .sp
1377    The sequence \eg{-1} is a reference to the most recently started capturing
1378    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1379    would be equivalent to \e1. The use of relative references can be helpful in
1380    long patterns, and also in patterns that are created by joining together
1381    fragments that contain references within themselves.
1382    .P
1383  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1384  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1385  itself (see  itself (see
# Line 832  itself (see Line 1388  itself (see
1388  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1389  .\"  .\"
1390  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1391    .sp
1392    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1393    .sp
1394  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1395  "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
1396  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1397    .sp
1398    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1399    .sp
1400  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
1401  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1402    .P
1403  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1404  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1405    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1406    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1407    references, is also supported. We could rewrite the above example in any of
1408    the following ways:
1409    .sp
1410      (?<p1>(?i)rah)\es+\ek<p1>
1411      (?'p1'(?i)rah)\es+\ek{p1}
1412      (?P<p1>(?i)rah)\es+(?P=p1)
1413      (?<p1>(?i)rah)\es+\eg{p1}
1414    .sp
1415    A subpattern that is referenced by name may appear in the pattern before or
1416    after the reference.
1417    .P
1418  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
1419  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1420  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1421    .sp
1422    (a|(bc))\\2    (a|(bc))\e2
1423    .sp
1424  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
1425  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1426  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
1427  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
1428  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1429  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1430    .\" HTML <a href="#comments">
1431    .\" </a>
1432    "Comments"
1433    .\"
1434    below) can be used.
1435    .P
1436  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
1437  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.
1438  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1439  example, the pattern  example, the pattern
1440    .sp
1441    (a|b\\1)+    (a|b\e1)+
1442    .sp
1443  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
1444  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1445  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
1446  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
1447  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
1448  minimum of zero.  minimum of zero.
1449    .
1450    .
1451    .\" HTML <a name="bigassertions"></a>
1452  .SH ASSERTIONS  .SH ASSERTIONS
1453  .rs  .rs
1454  .sp  .sp
1455  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
1456  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1457  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
1458    .\" HTML <a href="#smallassertions">
1459    .\" </a>
1460    above.
1461    .\"
1462    .P
1463  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1464  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
1465  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1466    except that it does not cause the current matching position to be changed.
1467  An assertion subpattern is matched in the normal way, except that it does not  .P
1468  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1469  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
1470    of assertion contains capturing subpatterns within it, these are counted for
1471    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1472    However, substring capturing is carried out only for positive assertions,
1473    because it does not make sense for negative assertions.
1474    .
1475    .
1476    .SS "Lookahead assertions"
1477    .rs
1478    .sp
1479    Lookahead assertions start with (?= for positive assertions and (?! for
1480    negative assertions. For example,
1481    .sp
1482      \ew+(?=;)
1483    .sp
1484  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
1485  the match, and  the match, and
1486    .sp
1487    foo(?!bar)    foo(?!bar)
1488    .sp
1489  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
1490  apparently similar pattern  apparently similar pattern
1491    .sp
1492    (?!foo)bar    (?!foo)bar
1493    .sp
1494  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
1495  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1496  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1497  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1498    .P
1499  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
1500  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
1501  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.
1502    .
1503    .
1504    .\" HTML <a name="lookbehind"></a>
1505    .SS "Lookbehind assertions"
1506    .rs
1507    .sp
1508  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1509  negative assertions. For example,  negative assertions. For example,
1510    .sp
1511    (?<!foo)bar    (?<!foo)bar
1512    .sp
1513  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
1514  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
1515  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
1516  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1517    .sp
1518    (?<=bullock|donkey)    (?<=bullock|donkey)
1519    .sp
1520  is permitted, but  is permitted, but
1521    .sp
1522    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1523    .sp
1524  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1525  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
1526  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
1527  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1528    .sp
1529    (?<=ab(c|de))    (?<=ab(c|de))
1530    .sp
1531  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
1532  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:
1533    .sp
1534    (?<=abc|abde)    (?<=abc|abde)
1535    .sp
1536    In some cases, the Perl 5.10 escape sequence \eK
1537    .\" HTML <a href="#resetmatchstart">
1538    .\" </a>
1539    (see above)
1540    .\"
1541    can be used instead of a lookbehind assertion; this is not restricted to a
1542    fixed-length.
1543    .P
1544  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1545  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
1546  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1547  match is deemed to fail.  assertion fails.
1548    .P
1549  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)
1550  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1551  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1552    different numbers of bytes, are also not permitted.
1553  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1554  efficient matching at the end of the subject string. Consider a simple pattern  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1555  such as  specify efficient matching at the end of the subject string. Consider a simple
1556    pattern such as
1557    .sp
1558    abcd$    abcd$
1559    .sp
1560  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
1561  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
1562  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
1563    .sp
1564    ^.*abcd$    ^.*abcd$
1565    .sp
1566  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
1567  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,
1568  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"
1569  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,
1570  if the pattern is written as  if the pattern is written as
1571    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1572    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1573    .sp
1574  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
1575  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
1576  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1577  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1578    .
1579    .
1580    .SS "Using multiple assertions"
1581    .rs
1582    .sp
1583  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1584    .sp
1585    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1586    .sp
1587  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
1588  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1589  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
1590  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".
1591  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
1592  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
1593  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1594    .sp
1595    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1596    .sp
1597  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1598  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
1599  preceding three characters are not "999".  preceding three characters are not "999".
1600    .P
1601  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1602    .sp
1603    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1604    .sp
1605  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
1606  preceded by "foo", while  preceded by "foo", while
1607    .sp
1608    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1609    .sp
1610  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
1611  characters that are not "999".  characters that are not "999".
1612    .
1613  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1614  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1615  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  
1616  .rs  .rs
1617  .sp  .sp
1618  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1619  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1620  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1621  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1622    .sp
1623    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1624    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1625    .sp
1626  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1627  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
1628  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1629    .P
1630  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1631  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1632  subpattern of that number has previously matched. The number must be greater  .
1633  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1634  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1635  it into three parts for ease of discussion:  .sp
1636    If the text between the parentheses consists of a sequence of digits, the
1637    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if the capturing subpattern of that number has previously
1638    matched. An alternative notation is to precede the digits with a plus or minus
1639    sign. In this case, the subpattern number is relative rather than absolute.
1640    The most recently opened parentheses can be referenced by (?(-1), the next most
1641    recent by (?(-2), and so on. In looping constructs it can also make sense to
1642    refer to subsequent groups with constructs such as (?(+2).
1643    .P
1644    Consider the following pattern, which contains non-significant white space to
1645    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1646    three parts for ease of discussion:
1647    .sp
1648      ( \e( )?    [^()]+    (?(1) \e) )
1649    .sp
1650  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1651  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
1652  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 1656  the condition is true, and so the yes-pa
1656  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1657  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1658  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1659    .P
1660  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
1661  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1662  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1663      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1664  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1665    This makes the fragment independent of the parentheses in the larger pattern.
1666    .
1667    .SS "Checking for a used subpattern by name"
1668    .rs
1669    .sp
1670    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1671    subpattern by name. For compatibility with earlier versions of PCRE, which had
1672    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1673    there is a possible ambiguity with this syntax, because subpattern names may
1674    consist entirely of digits. PCRE looks first for a named subpattern; if it
1675    cannot find one and the name consists entirely of digits, PCRE looks for a
1676    subpattern of that number, which must be greater than zero. Using subpattern
1677    names that consist entirely of digits is not recommended.
1678    .P
1679    Rewriting the above example to use a named subpattern gives this:
1680    .sp
1681      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1682    .sp
1683    .
1684    .SS "Checking for pattern recursion"
1685    .rs
1686    .sp
1687    If the condition is the string (R), and there is no subpattern with the name R,
1688    the condition is true if a recursive call to the whole pattern or any
1689    subpattern has been made. If digits or a name preceded by ampersand follow the
1690    letter R, for example:
1691    .sp
1692      (?(R3)...) or (?(R&name)...)
1693    .sp
1694    the condition is true if the most recent recursion is into the subpattern whose
1695    number or name is given. This condition does not check the entire recursion
1696    stack.
1697    .P
1698    At "top level", all these recursion test conditions are false. Recursive
1699    patterns are described below.
1700    .
1701    .SS "Defining subpatterns for use by reference only"
1702    .rs
1703    .sp
1704    If the condition is the string (DEFINE), and there is no subpattern with the
1705    name DEFINE, the condition is always false. In this case, there may be only one
1706    alternative in the subpattern. It is always skipped if control reaches this
1707    point in the pattern; the idea of DEFINE is that it can be used to define
1708    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1709    is described below.) For example, a pattern to match an IPv4 address could be
1710    written like this (ignore whitespace and line breaks):
1711    .sp
1712      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1713      \eb (?&byte) (\e.(?&byte)){3} \eb
1714    .sp
1715    The first part of the pattern is a DEFINE group inside which a another group
1716    named "byte" is defined. This matches an individual component of an IPv4
1717    address (a number less than 256). When matching takes place, this part of the
1718    pattern is skipped because DEFINE acts like a false condition.
1719    .P
1720    The rest of the pattern uses references to the named group to match the four
1721    dot-separated components of an IPv4 address, insisting on a word boundary at
1722    each end.
1723    .
1724    .SS "Assertion conditions"
1725    .rs
1726    .sp
1727    If the condition is not in any of the above formats, it must be an assertion.
1728  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1729  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1730  alternatives on the second line:  alternatives on the second line:
1731    .sp
1732    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1733    \\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} )
1734    .sp
1735  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1736  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
1737  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
1738  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1739  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
1740  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.
1741    .
1742    .
1743    .\" HTML <a name="comments"></a>
1744  .SH COMMENTS  .SH COMMENTS
1745  .rs  .rs
1746  .sp  .sp
1747  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
1748  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1749  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.
1750    .P
1751  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1752  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1753  character in the pattern.  next newline in the pattern.
1754    .
1755  .SH RECURSIVE PATTERNS  .
1756    .\" HTML <a name="recursion"></a>
1757    .SH "RECURSIVE PATTERNS"
1758  .rs  .rs
1759  .sp  .sp
1760  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1761  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1762  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
1763  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1764  experimental facility that allows regular expressions to recurse (amongst other  .P
1765  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
1766  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
1767  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1768    pattern using code interpolation to solve the parentheses problem can be
1769    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
1770    .sp
1771      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1772    .sp
1773  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
1774  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1775  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1776  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1777    supports special syntax for recursion of the entire pattern, and also for
1778  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1779  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.
1780  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1781  "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
1782  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1783    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1784  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
1785  the PCRE_EXTENDED option is set so that white space is ignored):  a recursive call of the entire regular expression.
1786    .P
1787    \\( ( (?>[^()]+) | (?R) )* \\)  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1788    treated as an atomic group. That is, once it has matched some of the subject
1789    string, it is never re-entered, even if it contains untried alternatives and
1790    there is a subsequent matching failure.
1791    .P
1792    This PCRE pattern solves the nested parentheses problem (assume the
1793    PCRE_EXTENDED option is set so that white space is ignored):
1794    .sp
1795      \e( ( (?>[^()]+) | (?R) )* \e)
1796    .sp
1797  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1798  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
1799  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1800  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1801    .P
1802  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
1803  pattern, so instead you could use this:  pattern, so instead you could use this:
1804    .sp
1805    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1806    .sp
1807  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
1808  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1809  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1810  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
1811  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.)
1812  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
1813    most recently opened parentheses preceding the recursion. In other words, a
1814    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
1815    it is encountered.
1816  This particular example pattern contains nested unlimited repeats, and so the  .P
1817  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1818  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
1819  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1820    "subroutine" calls, as described in the next section.
1821    .P
1822    An alternative approach is to use named parentheses instead. The Perl syntax
1823    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1824    could rewrite the above example as follows:
1825    .sp
1826      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1827    .sp
1828    If there is more than one subpattern with the same name, the earliest one is
1829    used.
1830    .P
1831    This particular example pattern that we have been looking at contains nested
1832    unlimited repeats, and so the use of atomic grouping for matching strings of
1833    non-parentheses is important when applying the pattern to strings that do not
1834    match. For example, when this pattern is applied to
1835    .sp
1836    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1837    .sp
1838  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1839  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
1840  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
1841  before failure can be reported.  before failure can be reported.
1842    .P
1843  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
1844  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.
1845  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
1846  below and the  below and the
1847  .\" HREF  .\" HREF
1848  \fBpcrecallout\fR  \fBpcrecallout\fP
1849  .\"  .\"
1850  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1851    .sp
1852    (ab(cd)ef)    (ab(cd)ef)
1853    .sp
1854  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
1855  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1856    .sp
1857    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1858       ^                        ^       ^                        ^
1859       ^                        ^       ^                        ^
1860    .sp
1861  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
1862  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
1863  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
1864  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
1865  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.
1866    .P
1867  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.
1868  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1869  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1870  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1871    .sp
1872    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1873    .sp
1874  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
1875  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1876  is the actual recursive call.  is the actual recursive call.
1877    .
1878    .
1879  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1880  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1881  .rs  .rs
1882  .sp  .sp
1883  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
1884  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
1885  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1886  pattern  before or after the reference. A numbered reference can be absolute or
1887    relative, as in these examples:
1888    (sens|respons)e and \\1ibility  .sp
1889      (...(absolute)...)...(?2)...
1890      (...(relative)...)...(?-1)...
1891      (...(?+1)...(relative)...
1892    .sp
1893    An earlier example pointed out that the pattern
1894    .sp
1895      (sens|respons)e and \e1ibility
1896    .sp
1897  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1898  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1899    .sp
1900    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1901    .sp
1902  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
1903  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1904  refer.  .P
1905    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1906    group. That is, once it has matched some of the subject string, it is never
1907    re-entered, even if it contains untried alternatives and there is a subsequent
1908    matching failure.
1909    .P
1910    When a subpattern is used as a subroutine, processing options such as
1911    case-independence are fixed when the subpattern is defined. They cannot be
1912    changed for different calls. For example, consider this pattern:
1913    .sp
1914      (abc)(?i:(?-1))
1915    .sp
1916    It matches "abcabc". It does not match "abcABC" because the change of
1917    processing option does not affect the called subpattern.
1918    .
1919    .
1920  .SH CALLOUTS  .SH CALLOUTS
1921  .rs  .rs
1922  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1924  Perl has a feature whereby using the seq
1924  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
1925  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1926  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1927    .P
1928  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
1929  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
1930  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.
1931  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1932    .P
1933  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1934  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
1935  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.
1936  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1937    .sp
1938    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1939    .sp
1940  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
1941    automatically installed before each item in the pattern. They are all numbered
1942    255.
1943    .P
1944    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1945  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
1946  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
1947  \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
1948  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1949  function is given in the  description of the interface to the callout function is given in the
1950  .\" HREF  .\" HREF
1951  \fBpcrecallout\fR  \fBpcrecallout\fP
1952  .\"  .\"
1953  documentation.  documentation.
1954    .
1955  .in 0  .
1956  Last updated: 03 February 2003  .SH "SEE ALSO"
1957  .br  .rs
1958  Copyright (c) 1997-2003 University of Cambridge.  .sp
1959    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1960    .
1961    .
1962    .SH AUTHOR
1963    .rs
1964    .sp
1965    .nf
1966    Philip Hazel
1967    University Computing Service
1968    Cambridge CB2 3QH, England.
1969    .fi
1970    .
1971    .
1972    .SH REVISION
1973    .rs
1974    .sp
1975    .nf
1976    Last updated: 13 June 2007
1977    Copyright (c) 1997-2007 University of Cambridge.
1978    .fi

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