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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "10 September 2012" "PCRE 8.31"
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"
# Line 9  are described in detail below. There is Line 9  are described in detail below. There is
9  .\" HREF  .\" HREF
10  \fBpcresyntax\fP  \fBpcresyntax\fP
11  .\"  .\"
12  page. Perl's regular expressions are described in its own documentation, and  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18  regular expressions in general are covered in a number of books, some of which  regular expressions in general are covered in a number of books, some of which
19  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 strings in the original library, and a
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  second library that supports 16-bit and UTF-16 character strings. To use these
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  features, PCRE must be built to include appropriate support. When using UTF
27  places below. There is also a summary of UTF-8 features in the  strings you must either call the compiling function with the PCRE_UTF8 or
28  .\" HTML <a href="pcre.html#utf8support">  PCRE_UTF16 option, or the pattern must start with one of these special
29  .\" </a>  sequences:
30  section on UTF-8 support  .sp
31  .\"    (*UTF8)
32  in the main    (*UTF16)
33    .sp
34    Starting a pattern with such a sequence is equivalent to setting the relevant
35    option. This feature is not Perl-compatible. How setting a UTF mode affects
36    pattern matching is mentioned in several places below. There is also a summary
37    of features in the
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43    Another special sequence that may appear at the start of a pattern or in
44    combination with (*UTF8) or (*UTF16) is:
45    .sp
46      (*UCP)
47    .sp
48    This has the same effect as setting the PCRE_UCP option: it causes sequences
49    such as \ed and \ew to use Unicode properties to determine character types,
50    instead of recognizing only characters with codes less than 128 via a lookup
51    table.
52    .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .P
58  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
59  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60  From release 6.0, PCRE offers a second matching function,  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62  Perl-compatible. Some of the features discussed below are not available when  a different algorithm that is not Perl-compatible. Some of the features
63  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  discussed below are not available when DFA matching is used. The advantages and
64  alternative function, and how it differs from the normal function, are  disadvantages of the alternative functions, and how they differ from the normal
65  discussed in the  functions, are discussed in the
66  .\" HREF  .\" HREF
67  \fBpcrematching\fP  \fBpcrematching\fP
68  .\"  .\"
69  page.  page.
70  .  .
71  .  .
72    .SH "EBCDIC CHARACTER CODES"
73    .rs
74    .sp
75    PCRE can be compiled to run in an environment that uses EBCDIC as its character
76    code rather than ASCII or Unicode (typically a mainframe system). In the
77    sections below, character code values are ASCII or Unicode; in an EBCDIC
78    environment these characters may have different code values, and there are no
79    code points greater than 255.
80    .
81    .
82    .\" HTML <a name="newlines"></a>
83    .SH "NEWLINE CONVENTIONS"
84    .rs
85    .sp
86    PCRE supports five different conventions for indicating line breaks in
87    strings: a single CR (carriage return) character, a single LF (linefeed)
88    character, the two-character sequence CRLF, any of the three preceding, or any
89    Unicode newline sequence. The
90    .\" HREF
91    \fBpcreapi\fP
92    .\"
93    page has
94    .\" HTML <a href="pcreapi.html#newlines">
95    .\" </a>
96    further discussion
97    .\"
98    about newlines, and shows how to set the newline convention in the
99    \fIoptions\fP arguments for the compiling and matching functions.
100    .P
101    It is also possible to specify a newline convention by starting a pattern
102    string with one of the following five sequences:
103    .sp
104      (*CR)        carriage return
105      (*LF)        linefeed
106      (*CRLF)      carriage return, followed by linefeed
107      (*ANYCRLF)   any of the three above
108      (*ANY)       all Unicode newline sequences
109    .sp
110    These override the default and the options given to the compiling function. For
111    example, on a Unix system where LF is the default newline sequence, the pattern
112    .sp
113      (*CR)a.b
114    .sp
115    changes the convention to CR. That pattern matches "a\enb" because LF is no
116    longer a newline. Note that these special settings, which are not
117    Perl-compatible, are recognized only at the very start of a pattern, and that
118    they must be in upper case. If more than one of them is present, the last one
119    is used.
120    .P
121    The newline convention affects the interpretation of the dot metacharacter when
122    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
123    affect what the \eR escape sequence matches. By default, this is any Unicode
124    newline sequence, for Perl compatibility. However, this can be changed; see the
125    description of \eR in the section entitled
126    .\" HTML <a href="#newlineseq">
127    .\" </a>
128    "Newline sequences"
129    .\"
130    below. A change of \eR setting can be combined with a change of newline
131    convention.
132    .
133    .
134  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
135  .rs  .rs
136  .sp  .sp
# Line 55  corresponding characters in the subject. Line 142  corresponding characters in the subject.
142  .sp  .sp
143  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
144  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
145  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
146  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
147  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
148  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
149  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
150  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
151  UTF-8 support.  UTF support.
152  .P  .P
153  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
154  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 107  The following sections describe the use Line 194  The following sections describe the use
194  .rs  .rs
195  .sp  .sp
196  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
197  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
198  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
199  outside character classes.  both inside and outside character classes.
200  .P  .P
201  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
202  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 117  otherwise be interpreted as a metacharac Line 204  otherwise be interpreted as a metacharac
204  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
205  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
206  .P  .P
207  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  In a UTF mode, only ASCII numbers and letters have any special meaning after a
208    backslash. All other characters (in particular, those whose codepoints are
209    greater than 127) are treated as literals.
210    .P
211    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
212  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
213  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
214  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
215  .P  .P
216  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
217  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 136  Perl, $ and @ cause variable interpolati Line 227  Perl, $ and @ cause variable interpolati
227    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
228  .sp  .sp
229  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
230    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
231    by \eE later in the pattern, the literal interpretation continues to the end of
232    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
233    a character class, this causes an error, because the character class is not
234    terminated.
235  .  .
236  .  .
237  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 145  The \eQ...\eE sequence is recognized bot Line 241  The \eQ...\eE sequence is recognized bot
241  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
242  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
243  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
244  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 often easier to use
245  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
246  .sp  .sp
247    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
248    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
249    \ee        escape (hex 1B)    \ee        escape (hex 1B)
250    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
251    \en        newline (hex 0A)    \en        linefeed (hex 0A)
252    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
253    \et        tab (hex 09)    \et        tab (hex 09)
254    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
255    \exhh      character with hex code hh    \exhh      character with hex code hh
256    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
257      \euhhhh    character with hex code hhhh (JavaScript mode only)
258  .sp  .sp
259  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
260  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
261  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
262  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
263  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
264  After \ex, from zero to two hexadecimal digits are read (letters can be in  compile-time error occurs. This locks out non-ASCII characters in all modes.
265  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
266  and }, but the value of the character code must be less than 256 in non-UTF-8  The \ec facility was designed for use with ASCII characters, but with the
267  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  extension to Unicode it is even less useful than it once was. It is, however,
268  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  recognized when PCRE is compiled in EBCDIC mode, where data items are always
269  point, which is 10FFFF.  bytes. In this mode, all values are valid after \ec. If the next character is a
270    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
271    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
272    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
273    characters also generate different values.
274    .P
275    By default, after \ex, from zero to two hexadecimal digits are read (letters
276    can be in upper or lower case). Any number of hexadecimal digits may appear
277    between \ex{ and }, but the character code is constrained as follows:
278    .sp
279      8-bit non-UTF mode    less than 0x100
280      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
281      16-bit non-UTF mode   less than 0x10000
282      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
283    .sp
284    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
285    "surrogate" codepoints).
286  .P  .P
287  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
288  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
289  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
290  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
291  .P  .P
292    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
293    as just described only when it is followed by two hexadecimal digits.
294    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
295    code points greater than 256 is provided by \eu, which must be followed by
296    four hexadecimal digits; otherwise it matches a literal "u" character.
297    Character codes specified by \eu in JavaScript mode are constrained in the same
298    was as those specified by \ex in non-JavaScript mode.
299    .P
300  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
301  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
302  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
303    \eu00dc in JavaScript mode).
304  .P  .P
305  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
306  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 205  parenthesized subpatterns. Line 326  parenthesized subpatterns.
326  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
327  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
328  digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
329  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. The value of the character is
330  character specified in octal must be less than \e400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
331  to \e777 are permitted. For example:  For example:
332  .sp  .sp
333    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
334  .\" JOIN  .\" JOIN
335    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
336              previous capturing subpatterns              previous capturing subpatterns
# Line 224  to \e777 are permitted. For example: Line 345  to \e777 are permitted. For example:
345              character with octal code 113              character with octal code 113
346  .\" JOIN  .\" JOIN
347    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
348              the byte consisting entirely of 1 bits              the value 255 (decimal)
349  .\" JOIN  .\" JOIN
350    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
351              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 233  Note that octal values of 100 or greater Line 354  Note that octal values of 100 or greater
354  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
355  .P  .P
356  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
357  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
358  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
359  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
360  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
361  meanings  inside a character class. Like other unrecognized escape sequences, they are
362  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
363  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
364  (see below).  sequences have different meanings.
365  .\"  .
366    .
367    .SS "Unsupported escape sequences"
368    .rs
369    .sp
370    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
371    handler and used to modify the case of following characters. By default, PCRE
372    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
373    option is set, \eU matches a "U" character, and \eu can be used to define a
374    character by code point, as described in the previous section.
375  .  .
376  .  .
377  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 261  parenthesized subpatterns. Line 391  parenthesized subpatterns.
391  .\"  .\"
392  .  .
393  .  .
394    .SS "Absolute and relative subroutine calls"
395    .rs
396    .sp
397    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
398    a number enclosed either in angle brackets or single quotes, is an alternative
399    syntax for referencing a subpattern as a "subroutine". Details are discussed
400    .\" HTML <a href="#onigurumasubroutines">
401    .\" </a>
402    later.
403    .\"
404    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
405    synonymous. The former is a back reference; the latter is a
406    .\" HTML <a href="#subpatternsassubroutines">
407    .\" </a>
408    subroutine
409    .\"
410    call.
411    .
412    .
413    .\" HTML <a name="genericchartypes"></a>
414  .SS "Generic character types"  .SS "Generic character types"
415  .rs  .rs
416  .sp  .sp
417  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
418  .sp  .sp
419    \ed     any decimal digit    \ed     any decimal digit
420    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
421    \eh     any horizontal whitespace character    \eh     any horizontal white space character
422    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
423    \es     any whitespace character    \es     any white space character
424    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
425    \ev     any vertical whitespace character    \ev     any vertical white space character
426    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
427    \ew     any "word" character    \ew     any "word" character
428    \eW     any "non-word" character    \eW     any "non-word" character
429  .sp  .sp
430  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
431  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
432  .P  .\" HTML <a href="#fullstopdot">
433  These character type sequences can appear both inside and outside character  .\" </a>
434    the "." metacharacter
435    .\"
436    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
437    PCRE does not support this.
438    .P
439    Each pair of lower and upper case escape sequences partitions the complete set
440    of characters into two disjoint sets. Any given character matches one, and only
441    one, of each pair. The sequences can appear both inside and outside character
442  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
443  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
444  there is no character to match.  there is no character to match.
445  .P  .P
446  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 292  are HT (9), LF (10), FF (12), CR (13), a Line 449  are HT (9), LF (10), FF (12), CR (13), a
449  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
450  does.  does.
451  .P  .P
452  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
453  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
454  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
455  meanings from before UTF-8 support was available, mainly for efficiency  place (see
456  reasons.  .\" HTML <a href="pcreapi.html#localesupport">
457  .P  .\" </a>
458  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  "Locale support"
459  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\"
460  The horizontal space characters are:  in the
461    .\" HREF
462    \fBpcreapi\fP
463    .\"
464    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
465    or "french" in Windows, some character codes greater than 128 are used for
466    accented letters, and these are then matched by \ew. The use of locales with
467    Unicode is discouraged.
468    .P
469    By default, in a UTF mode, characters with values greater than 128 never match
470    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
471    their original meanings from before UTF support was available, mainly for
472    efficiency reasons. However, if PCRE is compiled with Unicode property support,
473    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
474    properties are used to determine character types, as follows:
475    .sp
476      \ed  any character that \ep{Nd} matches (decimal digit)
477      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
478      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
479    .sp
480    The upper case escapes match the inverse sets of characters. Note that \ed
481    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
482    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
483    \eB because they are defined in terms of \ew and \eW. Matching these sequences
484    is noticeably slower when PCRE_UCP is set.
485    .P
486    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
487    release 5.10. In contrast to the other sequences, which match only ASCII
488    characters by default, these always match certain high-valued codepoints,
489    whether or not PCRE_UCP is set. The horizontal space characters are:
490  .sp  .sp
491    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
492    U+0020     Space    U+0020     Space
493    U+00A0     Non-break space    U+00A0     Non-break space
494    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 324  The horizontal space characters are: Line 510  The horizontal space characters are:
510  .sp  .sp
511  The vertical space characters are:  The vertical space characters are:
512  .sp  .sp
513    U+000A     Linefeed    U+000A     Linefeed (LF)
514    U+000B     Vertical tab    U+000B     Vertical tab (VT)
515    U+000C     Formfeed    U+000C     Form feed (FF)
516    U+000D     Carriage return    U+000D     Carriage return (CR)
517    U+0085     Next line    U+0085     Next line (NEL)
518    U+2028     Line separator    U+2028     Line separator
519    U+2029     Paragraph separator    U+2029     Paragraph separator
520  .P  .sp
521  A "word" character is an underscore or any character less than 256 that is a  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
522  letter or digit. The definition of letters and digits is controlled by PCRE's  relevant.
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
523  .  .
524  .  .
525    .\" HTML <a name="newlineseq"></a>
526  .SS "Newline sequences"  .SS "Newline sequences"
527  .rs  .rs
528  .sp  .sp
529  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
530  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
531  the following:  following:
532  .sp  .sp
533    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
534  .sp  .sp
# Line 366  below. Line 539  below.
539  .\"  .\"
540  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
541  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
542  U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next  U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
543  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
544  cannot be split.  cannot be split.
545  .P  .P
546  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
547  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
548  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
549  recognized.  recognized.
550  .P  .P
551  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
552    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
553    either at compile time or when the pattern is matched. (BSR is an abbrevation
554    for "backslash R".) This can be made the default when PCRE is built; if this is
555    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
556    It is also possible to specify these settings by starting a pattern string with
557    one of the following sequences:
558    .sp
559      (*BSR_ANYCRLF)   CR, LF, or CRLF only
560      (*BSR_UNICODE)   any Unicode newline sequence
561    .sp
562    These override the default and the options given to the compiling function, but
563    they can themselves be overridden by options given to a matching function. Note
564    that these special settings, which are not Perl-compatible, are recognized only
565    at the very start of a pattern, and that they must be in upper case. If more
566    than one of them is present, the last one is used. They can be combined with a
567    change of newline convention; for example, a pattern can start with:
568    .sp
569      (*ANY)(*BSR_ANYCRLF)
570    .sp
571    They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
572    sequences. Inside a character class, \eR is treated as an unrecognized escape
573    sequence, and so matches the letter "R" by default, but causes an error if
574    PCRE_EXTRA is set.
575  .  .
576  .  .
577  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 384  Inside a character class, \eR matches th Line 580  Inside a character class, \eR matches th
580  .sp  .sp
581  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
582  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
583  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
584  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
585  The extra escape sequences are:  The extra escape sequences are:
586  .sp  .sp
587    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
588    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
589    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
590  .sp  .sp
591  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
592  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
593  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
594  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
595  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
596    .\" </a>
597    next section).
598    .\"
599    Other Perl properties such as "InMusicalSymbols" are not currently supported by
600    PCRE. Note that \eP{Any} does not match any characters, so always causes a
601    match failure.
602  .P  .P
603  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
604  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 410  Those that are not part of an identified Line 612  Those that are not part of an identified
612  .P  .P
613  Arabic,  Arabic,
614  Armenian,  Armenian,
615    Avestan,
616  Balinese,  Balinese,
617    Bamum,
618    Batak,
619  Bengali,  Bengali,
620  Bopomofo,  Bopomofo,
621    Brahmi,
622  Braille,  Braille,
623  Buginese,  Buginese,
624  Buhid,  Buhid,
625  Canadian_Aboriginal,  Canadian_Aboriginal,
626    Carian,
627    Chakma,
628    Cham,
629  Cherokee,  Cherokee,
630  Common,  Common,
631  Coptic,  Coptic,
# Line 425  Cypriot, Line 634  Cypriot,
634  Cyrillic,  Cyrillic,
635  Deseret,  Deseret,
636  Devanagari,  Devanagari,
637    Egyptian_Hieroglyphs,
638  Ethiopic,  Ethiopic,
639  Georgian,  Georgian,
640  Glagolitic,  Glagolitic,
# Line 437  Hangul, Line 647  Hangul,
647  Hanunoo,  Hanunoo,
648  Hebrew,  Hebrew,
649  Hiragana,  Hiragana,
650    Imperial_Aramaic,
651  Inherited,  Inherited,
652    Inscriptional_Pahlavi,
653    Inscriptional_Parthian,
654    Javanese,
655    Kaithi,
656  Kannada,  Kannada,
657  Katakana,  Katakana,
658    Kayah_Li,
659  Kharoshthi,  Kharoshthi,
660  Khmer,  Khmer,
661  Lao,  Lao,
662  Latin,  Latin,
663    Lepcha,
664  Limbu,  Limbu,
665  Linear_B,  Linear_B,
666    Lisu,
667    Lycian,
668    Lydian,
669  Malayalam,  Malayalam,
670    Mandaic,
671    Meetei_Mayek,
672    Meroitic_Cursive,
673    Meroitic_Hieroglyphs,
674    Miao,
675  Mongolian,  Mongolian,
676  Myanmar,  Myanmar,
677  New_Tai_Lue,  New_Tai_Lue,
# Line 454  Nko, Line 679  Nko,
679  Ogham,  Ogham,
680  Old_Italic,  Old_Italic,
681  Old_Persian,  Old_Persian,
682    Old_South_Arabian,
683    Old_Turkic,
684    Ol_Chiki,
685  Oriya,  Oriya,
686  Osmanya,  Osmanya,
687  Phags_Pa,  Phags_Pa,
688  Phoenician,  Phoenician,
689    Rejang,
690  Runic,  Runic,
691    Samaritan,
692    Saurashtra,
693    Sharada,
694  Shavian,  Shavian,
695  Sinhala,  Sinhala,
696    Sora_Sompeng,
697    Sundanese,
698  Syloti_Nagri,  Syloti_Nagri,
699  Syriac,  Syriac,
700  Tagalog,  Tagalog,
701  Tagbanwa,  Tagbanwa,
702  Tai_Le,  Tai_Le,
703    Tai_Tham,
704    Tai_Viet,
705    Takri,
706  Tamil,  Tamil,
707  Telugu,  Telugu,
708  Thaana,  Thaana,
# Line 473  Thai, Line 710  Thai,
710  Tibetan,  Tibetan,
711  Tifinagh,  Tifinagh,
712  Ugaritic,  Ugaritic,
713    Vai,
714  Yi.  Yi.
715  .P  .P
716  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
717  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
718  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
719  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
720  .P  .P
721  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
722  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 539  the Lu, Ll, or Lt property, in other wor Line 777  the Lu, Ll, or Lt property, in other wor
777  a modifier or "other".  a modifier or "other".
778  .P  .P
779  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
780  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
781  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
782  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
783  .\" HREF  .\" HREF
784  \fBpcreapi\fP  \fBpcreapi\fP
785  .\"  .\"
786  page).  page). Perl does not support the Cs property.
787  .P  .P
788  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
789  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
790  properties with "Is".  properties with "Is".
791  .P  .P
# Line 558  Unicode table. Line 796  Unicode table.
796  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
797  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
798  .P  .P
799  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
800  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
801  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
802    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
803    PCRE_UCP option or by starting the pattern with (*UCP).
804    .
805    .
806    .SS Extended grapheme clusters
807    .rs
808  .sp  .sp
809  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
810  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
811  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
812  .\" </a>  .\" </a>
813  (see below).  (see below).
814  .\"  .\"
815  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
816  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
817  non-UTF-8 mode \eX matches any one character.  .sp
818  .P    (?>\ePM\epM*)
819  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
820  a structure that contains data for over fifteen thousand characters. That is  That is, it matched a character without the "mark" property, followed by zero
821  why the traditional escape sequences such as \ed and \ew do not use Unicode  or more characters with the "mark" property. Characters with the "mark"
822  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
823    .P
824    This simple definition was extended in Unicode to include more complicated
825    kinds of composite character by giving each character a grapheme breaking
826    property, and creating rules that use these properties to define the boundaries
827    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
828    one of these clusters.
829    .P
830    \eX always matches at least one character. Then it decides whether to add
831    additional characters according to the following rules for ending a cluster:
832    .P
833    1. End at the end of the subject string.
834    .P
835    2. Do not end between CR and LF; otherwise end after any control character.
836    .P
837    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
838    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
839    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
840    character; an LVT or T character may be follwed only by a T character.
841    .P
842    4. Do not end before extending characters or spacing marks. Characters with
843    the "mark" property always have the "extend" grapheme breaking property.
844    .P
845    5. Do not end after prepend characters.
846    .P
847    6. Otherwise, end the cluster.
848    .
849    .
850    .\" HTML <a name="extraprops"></a>
851    .SS PCRE's additional properties
852    .rs
853    .sp
854    As well as the standard Unicode properties described above, PCRE supports four
855    more that make it possible to convert traditional escape sequences such as \ew
856    and \es and POSIX character classes to use Unicode properties. PCRE uses these
857    non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
858    .sp
859      Xan   Any alphanumeric character
860      Xps   Any POSIX space character
861      Xsp   Any Perl space character
862      Xwd   Any Perl "word" character
863    .sp
864    Xan matches characters that have either the L (letter) or the N (number)
865    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
866    carriage return, and any other character that has the Z (separator) property.
867    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
868    same characters as Xan, plus underscore.
869  .  .
870  .  .
871  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
872  .SS "Resetting the match start"  .SS "Resetting the match start"
873  .rs  .rs
874  .sp  .sp
875  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
876  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
877  .sp  .sp
878    foo\eKbar    foo\eKbar
879  .sp  .sp
# Line 608  For example, when the pattern Line 895  For example, when the pattern
895    (foo)\eKbar    (foo)\eKbar
896  .sp  .sp
897  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
898    .P
899    Perl documents that the use of \eK within assertions is "not well defined". In
900    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
901    ignored in negative assertions.
902  .  .
903  .  .
904  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 632  The backslashed assertions are: Line 923  The backslashed assertions are:
923    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
924    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
925  .sp  .sp
926  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
927  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
928    default it matches the corresponding literal character (for example, \eB
929    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
930    escape sequence" error is generated instead.
931  .P  .P
932  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
933  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
934  \ew and the other matches \eW), 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
935  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
936    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
937    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
938    of word" or "end of word" metasequence. However, whatever follows \eb normally
939    determines which it is. For example, the fragment \eba matches "a" at the start
940    of a word.
941  .P  .P
942  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
943  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 722  end of the subject in both modes, and if Line 1021  end of the subject in both modes, and if
1021  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1022  .  .
1023  .  .
1024  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1025    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1026  .rs  .rs
1027  .sp  .sp
1028  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
1029  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
1030  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1031  .P  .P
1032  When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
1033  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 744  to match it. Line 1044  to match it.
1044  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
1045  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
1046  special meaning in a character class.  special meaning in a character class.
1047  .  .P
1048  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1049  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1050  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1051  .sp  name; PCRE does not support this.
1052  Outside a character class, the escape sequence \eC matches any one byte, both  .
1053  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1054  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1055  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1056  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1057  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1058    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1059    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1060    matches line-ending characters. The feature is provided in Perl in order to
1061    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1062    used. Because \eC breaks up characters into individual data units, matching one
1063    unit with \eC in a UTF mode means that the rest of the string may start with a
1064    malformed UTF character. This has undefined results, because PCRE assumes that
1065    it is dealing with valid UTF strings (and by default it checks this at the
1066    start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1067    is used).
1068  .P  .P
1069  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1070  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1071  .\" </a>  .\" </a>
1072  (described below),  (described below)
1073  .\"  .\"
1074  because in UTF-8 mode this would make it impossible to calculate the length of  in a UTF mode, because this would make it impossible to calculate the length of
1075  the lookbehind.  the lookbehind.
1076    .P
1077    In general, the \eC escape sequence is best avoided. However, one
1078    way of using it that avoids the problem of malformed UTF characters is to use a
1079    lookahead to check the length of the next character, as in this pattern, which
1080    could be used with a UTF-8 string (ignore white space and line breaks):
1081    .sp
1082      (?| (?=[\ex00-\ex7f])(\eC) |
1083          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1084          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1085          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1086    .sp
1087    A group that starts with (?| resets the capturing parentheses numbers in each
1088    alternative (see
1089    .\" HTML <a href="#dupsubpatternnumber">
1090    .\" </a>
1091    "Duplicate Subpattern Numbers"
1092    .\"
1093    below). The assertions at the start of each branch check the next UTF-8
1094    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1095    character's individual bytes are then captured by the appropriate number of
1096    groups.
1097  .  .
1098  .  .
1099  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 770  the lookbehind. Line 1101  the lookbehind.
1101  .rs  .rs
1102  .sp  .sp
1103  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1104  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
1105  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1106  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
1107  escaped with a backslash.  a member of the class, it should be the first data character in the class
1108  .P  (after an initial circumflex, if present) or escaped with a backslash.
1109  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1110  character may occupy more than one byte. A matched character must be in the set  A character class matches a single character in the subject. In a UTF mode, the
1111  of characters defined by the class, unless the first character in the class  character may be more than one data unit long. A matched character must be in
1112  definition is a circumflex, in which case the subject character must not be in  the set of characters defined by the class, unless the first character in the
1113  the set defined by the class. If a circumflex is actually required as a member  class definition is a circumflex, in which case the subject character must not
1114  of the class, ensure it is not the first character, or escape it with a  be in the set defined by the class. If a circumflex is actually required as a
1115    member of the class, ensure it is not the first character, or escape it with a
1116  backslash.  backslash.
1117  .P  .P
1118  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1119  [^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
1120  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1121  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1122  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1123  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1124  string.  string.
1125  .P  .P
1126  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1127  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  included in a class as a literal string of data units, or by using the \ex{
1128    escaping mechanism.
1129  .P  .P
1130  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
1131  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
1132  "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
1133  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1134  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1135  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1136  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1137  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1138  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1139  UTF-8 support.  well as with UTF support.
1140  .P  .P
1141  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1142  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 825  followed by two other characters. The oc Line 1158  followed by two other characters. The oc
1158  "]" can also be used to end a range.  "]" can also be used to end a range.
1159  .P  .P
1160  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
1161  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1162  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1163  .P  .P
1164  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
1165  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
1166  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1167  tables for a French locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1168  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1169  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1170  property support.  property support.
1171  .P  .P
1172  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1173  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1174  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1175  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1176  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1177  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1178    .\" HTML <a href="#genericchartypes">
1179    .\" </a>
1180    "Generic character types"
1181    .\"
1182    above. The escape sequence \eb has a different meaning inside a character
1183    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1184    are not special inside a character class. Like any other unrecognized escape
1185    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1186    default, but cause an error if the PCRE_EXTRA option is set.
1187    .P
1188    A circumflex can conveniently be used with the upper case character types to
1189    specify a more restricted set of characters than the matching lower case type.
1190    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1191    whereas [\ew] includes underscore. A positive character class should be read as
1192    "something OR something OR ..." and a negative class as "NOT something AND NOT
1193    something AND NOT ...".
1194  .P  .P
1195  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1196  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 862  this notation. For example, Line 1210  this notation. For example,
1210    [01[:alpha:]%]    [01[:alpha:]%]
1211  .sp  .sp
1212  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1213  are  are:
1214  .sp  .sp
1215    alnum    letters and digits    alnum    letters and digits
1216    alpha    letters    alpha    letters
# Line 873  are Line 1221  are
1221    graph    printing characters, excluding space    graph    printing characters, excluding space
1222    lower    lower case letters    lower    lower case letters
1223    print    printing characters, including space    print    printing characters, including space
1224    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1225    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1226    upper    upper case letters    upper    upper case letters
1227    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 894  matches "1", "2", or any non-digit. PCRE Line 1242  matches "1", "2", or any non-digit. PCRE
1242  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
1243  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1244  .P  .P
1245  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1246  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1247    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1248    character properties are used. This is achieved by replacing the POSIX classes
1249    by other sequences, as follows:
1250    .sp
1251      [:alnum:]  becomes  \ep{Xan}
1252      [:alpha:]  becomes  \ep{L}
1253      [:blank:]  becomes  \eh
1254      [:digit:]  becomes  \ep{Nd}
1255      [:lower:]  becomes  \ep{Ll}
1256      [:space:]  becomes  \ep{Xps}
1257      [:upper:]  becomes  \ep{Lu}
1258      [:word:]   becomes  \ep{Xwd}
1259    .sp
1260    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1261    classes are unchanged, and match only characters with code points less than
1262    128.
1263  .  .
1264  .  .
1265  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 922  alternative in the subpattern. Line 1286  alternative in the subpattern.
1286  .rs  .rs
1287  .sp  .sp
1288  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1289  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1290  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1291    The option letters are
1292  .sp  .sp
1293    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1294    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 937  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1302  PCRE_MULTILINE while unsetting PCRE_DOTA
1302  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
1303  unset.  unset.
1304  .P  .P
1305  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1306  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1307  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1308  the global options (and it will therefore show up in data extracted by the  .P
1309  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1310    subpattern parentheses), the change applies to the remainder of the pattern
1311    that follows. If the change is placed right at the start of a pattern, PCRE
1312    extracts it into the global options (and it will therefore show up in data
1313    extracted by the \fBpcre_fullinfo()\fP function).
1314  .P  .P
1315  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1316  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1317  .sp  .sp
1318    (a(?i)b)c    (a(?i)b)c
1319  .sp  .sp
# Line 960  branch is abandoned before the option se Line 1329  branch is abandoned before the option se
1329  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1330  behaviour otherwise.  behaviour otherwise.
1331  .P  .P
1332  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1333  changed in the same way as the Perl-compatible options by using the characters  application when the compiling or matching functions are called. In some cases
1334  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1335    what the application has set or what has been defaulted. Details are given in
1336    the section entitled
1337    .\" HTML <a href="#newlineseq">
1338    .\" </a>
1339    "Newline sequences"
1340    .\"
1341    above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1342    can be used to set UTF and Unicode property modes; they are equivalent to
1343    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1344  .  .
1345  .  .
1346  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 976  Turning part of a pattern into a subpatt Line 1354  Turning part of a pattern into a subpatt
1354  .sp  .sp
1355    cat(aract|erpillar|)    cat(aract|erpillar|)
1356  .sp  .sp
1357  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1358  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1359  .sp  .sp
1360  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1361  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1362  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of the
1363  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1364  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1365  .P  .P
1366  For example, if the string "the red king" is matched against the pattern  Opening parentheses are counted from left to right (starting from 1) to obtain
1367    numbers for the capturing subpatterns. For example, if the string "the red
1368    king" is matched against the pattern
1369  .sp  .sp
1370    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1371  .sp  .sp
# Line 1017  is reached, an option setting in one bra Line 1397  is reached, an option setting in one bra
1397  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1398  .  .
1399  .  .
1400    .\" HTML <a name="dupsubpatternnumber"></a>
1401  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1402  .rs  .rs
1403  .sp  .sp
# Line 1033  at captured substring number one, whiche Line 1414  at captured substring number one, whiche
1414  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1415  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1416  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1417  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1418  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1419  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1420  .sp  .sp
1421    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1422    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1423    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1424  .sp  .sp
1425  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1426  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1427    or "defdef":
1428    .sp
1429      /(?|(abc)|(def))\e1/
1430    .sp
1431    In contrast, a subroutine call to a numbered subpattern always refers to the
1432    first one in the pattern with the given number. The following pattern matches
1433    "abcabc" or "defabc":
1434    .sp
1435      /(?|(abc)|(def))(?1)/
1436    .sp
1437    If a
1438    .\" HTML <a href="#conditions">
1439    .\" </a>
1440    condition test
1441    .\"
1442    for a subpattern's having matched refers to a non-unique number, the test is
1443    true if any of the subpatterns of that number have matched.
1444  .P  .P
1445  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1446  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1058  if an expression is modified, the number Line 1455  if an expression is modified, the number
1455  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1456  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1457  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1458  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1459    have different names, but PCRE does not.
1460  .P  .P
1461  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1462  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1463  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1464  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1465  .\" </a>  .\" </a>
1466  backreferences,  back references,
1467  .\"  .\"
1468  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1469  .\" </a>  .\" </a>
# Line 1085  extracting the name-to-number translatio Line 1483  extracting the name-to-number translatio
1483  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1484  .P  .P
1485  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1486  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1487  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1488  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1489  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1490  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1491    name, and in both cases you want to extract the abbreviation. This pattern
1492    (ignoring the line breaks) does the job:
1493  .sp  .sp
1494    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1495    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1103  subpattern, as described in the previous Line 1503  subpattern, as described in the previous
1503  .P  .P
1504  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1505  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1506  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1507  make a reference to a non-unique named subpattern from elsewhere in the  .P
1508  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1509  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1510    used. In the absence of duplicate numbers (see the previous section) this is
1511    the one with the lowest number. If you use a named reference in a condition
1512    test (see the
1513    .\"
1514    .\" HTML <a href="#conditions">
1515    .\" </a>
1516    section about conditions
1517    .\"
1518    below), either to check whether a subpattern has matched, or to check for
1519    recursion, all subpatterns with the same name are tested. If the condition is
1520    true for any one of them, the overall condition is true. This is the same
1521    behaviour as testing by number. For further details of the interfaces for
1522    handling named subpatterns, see the
1523  .\" HREF  .\" HREF
1524  \fBpcreapi\fP  \fBpcreapi\fP
1525  .\"  .\"
1526  documentation.  documentation.
1527    .P
1528    \fBWarning:\fP You cannot use different names to distinguish between two
1529    subpatterns with the same number because PCRE uses only the numbers when
1530    matching. For this reason, an error is given at compile time if different names
1531    are given to subpatterns with the same number. However, you can give the same
1532    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1533  .  .
1534  .  .
1535  .SH REPETITION  .SH REPETITION
# Line 1122  items: Line 1541  items:
1541    a literal data character    a literal data character
1542    the dot metacharacter    the dot metacharacter
1543    the \eC escape sequence    the \eC escape sequence
1544    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1545    the \eR escape sequence    the \eR escape sequence
1546    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1547    a character class    a character class
1548    a back reference (see next section)    a back reference (see next section)
1549    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1550      a subroutine call to a subpattern (recursive or otherwise)
1551  .sp  .sp
1552  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1553  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1152  where a quantifier is not allowed, or on Line 1572  where a quantifier is not allowed, or on
1572  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
1573  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1574  .P  .P
1575  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1576  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1577  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1578  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1579  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1580  .P  .P
1581  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
1582  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1583    subpatterns that are referenced as
1584    .\" HTML <a href="#subpatternsassubroutines">
1585    .\" </a>
1586    subroutines
1587    .\"
1588    from elsewhere in the pattern (but see also the section entitled
1589    .\" HTML <a href="#subdefine">
1590    .\" </a>
1591    "Defining subpatterns for use by reference only"
1592    .\"
1593    below). Items other than subpatterns that have a {0} quantifier are omitted
1594    from the compiled pattern.
1595  .P  .P
1596  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1597  abbreviations:  abbreviations:
# Line 1230  In cases where it is known that the subj Line 1662  In cases where it is known that the subj
1662  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1663  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1664  .P  .P
1665  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1666  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1667  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1668  succeeds. Consider, for example:  succeeds. Consider, for example:
1669  .sp  .sp
# Line 1240  succeeds. Consider, for example: Line 1672  succeeds. Consider, for example:
1672  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
1673  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1674  .P  .P
1675    Another case where implicit anchoring is not applied is when the leading .* is
1676    inside an atomic group. Once again, a match at the start may fail where a later
1677    one succeeds. Consider this pattern:
1678    .sp
1679      (?>.*?a)b
1680    .sp
1681    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1682    (*PRUNE) and (*SKIP) also disable this optimization.
1683    .P
1684  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1685  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1686  .sp  .sp
# Line 1384  no such problem when named parentheses a Line 1825  no such problem when named parentheses a
1825  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1826  .P  .P
1827  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1828  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1829  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1830  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1831  .sp  .sp
1832    (ring), \e1    (ring), \e1
1833    (ring), \eg1    (ring), \eg1
# Line 1400  example: Line 1841  example:
1841    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1842  .sp  .sp
1843  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1844  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1845  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1846  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1847  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1848  .P  .P
1849  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1850  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1442  after the reference. Line 1883  after the reference.
1883  .P  .P
1884  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
1885  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1886  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1887  .sp  .sp
1888    (a|(bc))\e2    (a|(bc))\e2
1889  .sp  .sp
1890  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". However, if the
1891  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1892  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1893  with a digit character, some delimiter must be used to terminate the back  .P
1894  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1895  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1896    If the pattern continues with a digit character, some delimiter must be used to
1897    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1898    white space. Otherwise, the \eg{ syntax or an empty comment (see
1899  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1900  .\" </a>  .\" </a>
1901  "Comments"  "Comments"
1902  .\"  .\"
1903  below) can be used.  below) can be used.
1904  .P  .
1905    .SS "Recursive back references"
1906    .rs
1907    .sp
1908  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
1909  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1910  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1471  to the previous iteration. In order for Line 1918  to the previous iteration. In order for
1918  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
1919  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
1920  minimum of zero.  minimum of zero.
1921    .P
1922    Back references of this type cause the group that they reference to be treated
1923    as an
1924    .\" HTML <a href="#atomicgroup">
1925    .\" </a>
1926    atomic group.
1927    .\"
1928    Once the whole group has been matched, a subsequent matching failure cannot
1929    cause backtracking into the middle of the group.
1930  .  .
1931  .  .
1932  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1490  those that look ahead of the current pos Line 1946  those that look ahead of the current pos
1946  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1947  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1948  .P  .P
1949  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1950  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1951  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1952  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1953  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1954  because it does not make sense for negative assertions.  .P
1955    For compatibility with Perl, assertion subpatterns may be repeated; though
1956    it makes no sense to assert the same thing several times, the side effect of
1957    capturing parentheses may occasionally be useful. In practice, there only three
1958    cases:
1959    .sp
1960    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1961    However, it may contain internal capturing parenthesized groups that are called
1962    from elsewhere via the
1963    .\" HTML <a href="#subpatternsassubroutines">
1964    .\" </a>
1965    subroutine mechanism.
1966    .\"
1967    .sp
1968    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1969    were {0,1}. At run time, the rest of the pattern match is tried with and
1970    without the assertion, the order depending on the greediness of the quantifier.
1971    .sp
1972    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1973    The assertion is obeyed just once when encountered during matching.
1974  .  .
1975  .  .
1976  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1524  lookbehind assertion is needed to achiev Line 1999  lookbehind assertion is needed to achiev
1999  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
2000  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
2001  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.
2002    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2003  .  .
2004  .  .
2005  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1548  is permitted, but Line 2024  is permitted, but
2024  .sp  .sp
2025  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2026  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
2027  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
2028  match the same length of string. An assertion such as  length of string. An assertion such as
2029  .sp  .sp
2030    (?<=ab(c|de))    (?<=ab(c|de))
2031  .sp  .sp
2032  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
2033  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
2034    branches:
2035  .sp  .sp
2036    (?<=abc|abde)    (?<=abc|abde)
2037  .sp  .sp
2038  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2039  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2040  .\" </a>  .\" </a>
2041  (see above)  (see above)
2042  .\"  .\"
2043  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
2044  fixed-length.  restriction.
2045  .P  .P
2046  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2047  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
2048  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2049  assertion fails.  assertion fails.
2050  .P  .P
2051  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2052  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2053  the length of the lookbehind. The \eX and \eR escapes, which can match  it impossible to calculate the length of the lookbehind. The \eX and \eR
2054  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2055    permitted.
2056    .P
2057    .\" HTML <a href="#subpatternsassubroutines">
2058    .\" </a>
2059    "Subroutine"
2060    .\"
2061    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2062    as the subpattern matches a fixed-length string.
2063    .\" HTML <a href="#recursion">
2064    .\" </a>
2065    Recursion,
2066    .\"
2067    however, is not supported.
2068  .P  .P
2069  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2070  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
2071  pattern such as  strings. Consider a simple pattern such as
2072  .sp  .sp
2073    abcd$    abcd$
2074  .sp  .sp
# Line 1642  characters that are not "999". Line 2132  characters that are not "999".
2132  .sp  .sp
2133  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2134  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2135  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2136  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2137  .sp  .sp
2138    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2139    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2140  .sp  .sp
2141  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2142  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
2143  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2144    itself contain nested subpatterns of any form, including conditional
2145    subpatterns; the restriction to two alternatives applies only at the level of
2146    the condition. This pattern fragment is an example where the alternatives are
2147    complex:
2148    .sp
2149      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2150    .sp
2151  .P  .P
2152  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2153  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1659  recursion, a pseudo-condition called DEF Line 2156  recursion, a pseudo-condition called DEF
2156  .rs  .rs
2157  .sp  .sp
2158  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2159  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2160  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2161  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2162  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2163  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2164  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2165    section about duplicate subpattern numbers),
2166    .\"
2167    the condition is true if any of them have matched. An alternative notation is
2168    to precede the digits with a plus or minus sign. In this case, the subpattern
2169    number is relative rather than absolute. The most recently opened parentheses
2170    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2171    loops it can also make sense to refer to subsequent groups. The next
2172    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2173    zero in any of these forms is not used; it provokes a compile-time error.)
2174  .P  .P
2175  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2176  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1675  three parts for ease of discussion: Line 2181  three parts for ease of discussion:
2181  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2182  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
2183  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
2184  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2185  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2186  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2187  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2188  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1705  Rewriting the above example to use a nam Line 2211  Rewriting the above example to use a nam
2211  .sp  .sp
2212    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2213  .sp  .sp
2214    If the name used in a condition of this kind is a duplicate, the test is
2215    applied to all subpatterns of the same name, and is true if any one of them has
2216    matched.
2217  .  .
2218  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2219  .rs  .rs
# Line 1716  letter R, for example: Line 2225  letter R, for example:
2225  .sp  .sp
2226    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2227  .sp  .sp
2228  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2229  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2230  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2231    applied to all subpatterns of the same name, and is true if any one of them is
2232    the most recent recursion.
2233  .P  .P
2234  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2235  patterns are described below.  .\" HTML <a href="#recursion">
2236    .\" </a>
2237    The syntax for recursive patterns
2238    .\"
2239    is described below.
2240  .  .
2241    .\" HTML <a name="subdefine"></a>
2242  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2243  .rs  .rs
2244  .sp  .sp
# Line 1730  If the condition is the string (DEFINE), Line 2246  If the condition is the string (DEFINE),
2246  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2247  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2248  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2249  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2250  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2251  written like this (ignore whitespace and line breaks):  .\" </a>
2252    subroutines
2253    .\"
2254    is described below.) For example, a pattern to match an IPv4 address such as
2255    "192.168.23.245" could be written like this (ignore white space and line
2256    breaks):
2257  .sp  .sp
2258    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2259    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1740  written like this (ignore whitespace and Line 2261  written like this (ignore whitespace and
2261  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2262  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2263  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2264  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2265  .P  pattern uses references to the named group to match the four dot-separated
2266  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2267  .  .
2268  .SS "Assertion conditions"  .SS "Assertion conditions"
2269  .rs  .rs
# Line 1769  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2288  dd-aaa-dd or dd-dd-dd, where aaa are let
2288  .SH COMMENTS  .SH COMMENTS
2289  .rs  .rs
2290  .sp  .sp
2291  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2292  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2293  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2294    subpattern name or number. The characters that make up a comment play no part
2295    in the pattern matching.
2296  .P  .P
2297  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2298  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2299  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2300    this case continues to immediately after the next newline character or
2301    character sequence in the pattern. Which characters are interpreted as newlines
2302    is controlled by the options passed to a compiling function or by a special
2303    sequence at the start of the pattern, as described in the section entitled
2304    .\" HTML <a href="#newlines">
2305    .\" </a>
2306    "Newline conventions"
2307    .\"
2308    above. Note that the end of this type of comment is a literal newline sequence
2309    in the pattern; escape sequences that happen to represent a newline do not
2310    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2311    default newline convention is in force:
2312    .sp
2313      abc #comment \en still comment
2314    .sp
2315    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2316    a newline in the pattern. The sequence \en is still literal at this stage, so
2317    it does not terminate the comment. Only an actual character with the code value
2318    0x0a (the default newline) does so.
2319  .  .
2320  .  .
2321  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1801  recursively to the pattern in which it a Line 2341  recursively to the pattern in which it a
2341  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2342  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2343  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2344  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2345  .P  .P
2346  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2347  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2348  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2349    .\" HTML <a href="#subpatternsassubroutines">
2350    .\" </a>
2351    non-recursive subroutine
2352    .\"
2353  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2354  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2355  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2356  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2357  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2358  .sp  .sp
2359    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2360  .sp  .sp
2361  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2362  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
2363  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2364  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2365    to avoid backtracking into sequences of non-parentheses.
2366  .P  .P
2367  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
2368  pattern, so instead you could use this:  pattern, so instead you could use this:
2369  .sp  .sp
2370    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2371  .sp  .sp
2372  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
2373  them instead of the whole pattern.  them instead of the whole pattern.
2374  .P  .P
2375  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2376  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. Instead of (?1) in the
2377  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2378  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2379  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2380  .P  .P
2381  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2382  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2383  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2384  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2385    .\" </a>
2386    non-recursive subroutine
2387    .\"
2388    calls, as described in the next section.
2389  .P  .P
2390  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2391  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2392  could rewrite the above example as follows:  could rewrite the above example as follows:
2393  .sp  .sp
2394    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2395  .sp  .sp
2396  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2397  used.  used.
2398  .P  .P
2399  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2400  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2401  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2402  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2403  .sp  .sp
2404    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2405  .sp  .sp
2406  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2407  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
2408  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
2409  before failure can be reported.  before failure can be reported.
2410  .P  .P
2411  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2412  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2413  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2414  .\" HREF  .\" HREF
2415  \fBpcrecallout\fP  \fBpcrecallout\fP
2416  .\"  .\"
# Line 1876  documentation). If the pattern above is Line 2418  documentation). If the pattern above is
2418  .sp  .sp
2419    (ab(cd)ef)    (ab(cd)ef)
2420  .sp  .sp
2421  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2422  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2423  .sp  matched at the top level, its final captured value is unset, even if it was
2424    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2425       ^                        ^  .P
2426       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2427  .sp  obtain extra memory to store data during a recursion, which it does by using
2428  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2429  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2430  .P  .P
2431  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.
2432  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1901  different alternatives for the recursive Line 2440  different alternatives for the recursive
2440  is the actual recursive call.  is the actual recursive call.
2441  .  .
2442  .  .
2443    .\" HTML <a name="recursiondifference"></a>
2444    .SS "Differences in recursion processing between PCRE and Perl"
2445    .rs
2446    .sp
2447    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2448    (like Python, but unlike Perl), a recursive subpattern call is always treated
2449    as an atomic group. That is, once it has matched some of the subject string, it
2450    is never re-entered, even if it contains untried alternatives and there is a
2451    subsequent matching failure. This can be illustrated by the following pattern,
2452    which purports to match a palindromic string that contains an odd number of
2453    characters (for example, "a", "aba", "abcba", "abcdcba"):
2454    .sp
2455      ^(.|(.)(?1)\e2)$
2456    .sp
2457    The idea is that it either matches a single character, or two identical
2458    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2459    it does not if the pattern is longer than three characters. Consider the
2460    subject string "abcba":
2461    .P
2462    At the top level, the first character is matched, but as it is not at the end
2463    of the string, the first alternative fails; the second alternative is taken
2464    and the recursion kicks in. The recursive call to subpattern 1 successfully
2465    matches the next character ("b"). (Note that the beginning and end of line
2466    tests are not part of the recursion).
2467    .P
2468    Back at the top level, the next character ("c") is compared with what
2469    subpattern 2 matched, which was "a". This fails. Because the recursion is
2470    treated as an atomic group, there are now no backtracking points, and so the
2471    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2472    try the second alternative.) However, if the pattern is written with the
2473    alternatives in the other order, things are different:
2474    .sp
2475      ^((.)(?1)\e2|.)$
2476    .sp
2477    This time, the recursing alternative is tried first, and continues to recurse
2478    until it runs out of characters, at which point the recursion fails. But this
2479    time we do have another alternative to try at the higher level. That is the big
2480    difference: in the previous case the remaining alternative is at a deeper
2481    recursion level, which PCRE cannot use.
2482    .P
2483    To change the pattern so that it matches all palindromic strings, not just
2484    those with an odd number of characters, it is tempting to change the pattern to
2485    this:
2486    .sp
2487      ^((.)(?1)\e2|.?)$
2488    .sp
2489    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2490    deeper recursion has matched a single character, it cannot be entered again in
2491    order to match an empty string. The solution is to separate the two cases, and
2492    write out the odd and even cases as alternatives at the higher level:
2493    .sp
2494      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2495    .sp
2496    If you want to match typical palindromic phrases, the pattern has to ignore all
2497    non-word characters, which can be done like this:
2498    .sp
2499      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2500    .sp
2501    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2502    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2503    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2504    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2505    more) to match typical phrases, and Perl takes so long that you think it has
2506    gone into a loop.
2507    .P
2508    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2509    string does not start with a palindrome that is shorter than the entire string.
2510    For example, although "abcba" is correctly matched, if the subject is "ababa",
2511    PCRE finds the palindrome "aba" at the start, then fails at top level because
2512    the end of the string does not follow. Once again, it cannot jump back into the
2513    recursion to try other alternatives, so the entire match fails.
2514    .P
2515    The second way in which PCRE and Perl differ in their recursion processing is
2516    in the handling of captured values. In Perl, when a subpattern is called
2517    recursively or as a subpattern (see the next section), it has no access to any
2518    values that were captured outside the recursion, whereas in PCRE these values
2519    can be referenced. Consider this pattern:
2520    .sp
2521      ^(.)(\e1|a(?2))
2522    .sp
2523    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2524    then in the second group, when the back reference \e1 fails to match "b", the
2525    second alternative matches "a" and then recurses. In the recursion, \e1 does
2526    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2527    match because inside the recursive call \e1 cannot access the externally set
2528    value.
2529    .
2530    .
2531  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2532  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2533  .rs  .rs
2534  .sp  .sp
2535  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2536  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
2537  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2538  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2539  relative, as in these examples:  relative, as in these examples:
2540  .sp  .sp
# Line 1927  matches "sense and sensibility" and "res Line 2554  matches "sense and sensibility" and "res
2554  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
2555  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2556  .P  .P
2557  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2558  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2559  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2560  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2561  .P  subroutine call revert to their previous values afterwards.
2562  When a subpattern is used as a subroutine, processing options such as  .P
2563  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2564  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2565    different calls. For example, consider this pattern:
2566  .sp  .sp
2567    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2568  .sp  .sp
# Line 1942  It matches "abcabc". It does not match " Line 2570  It matches "abcabc". It does not match "
2570  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2571  .  .
2572  .  .
2573    .\" HTML <a name="onigurumasubroutines"></a>
2574    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2575    .rs
2576    .sp
2577    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2578    a number enclosed either in angle brackets or single quotes, is an alternative
2579    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2580    are two of the examples used above, rewritten using this syntax:
2581    .sp
2582      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2583      (sens|respons)e and \eg'1'ibility
2584    .sp
2585    PCRE supports an extension to Oniguruma: if a number is preceded by a
2586    plus or a minus sign it is taken as a relative reference. For example:
2587    .sp
2588      (abc)(?i:\eg<-1>)
2589    .sp
2590    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2591    synonymous. The former is a back reference; the latter is a subroutine call.
2592    .
2593    .
2594  .SH CALLOUTS  .SH CALLOUTS
2595  .rs  .rs
2596  .sp  .sp
# Line 1952  same pair of parentheses when there is a Line 2601  same pair of parentheses when there is a
2601  .P  .P
2602  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
2603  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
2604  function by putting its entry point in the global variable \fIpcre_callout\fP.  function by putting its entry point in the global variable \fIpcre_callout\fP
2605  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2606    variable contains NULL, which disables all calling out.
2607  .P  .P
2608  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2609  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
# Line 1962  For example, this pattern has two callou Line 2612  For example, this pattern has two callou
2612  .sp  .sp
2613    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2614  .sp  .sp
2615  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2616  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2617  255.  255.
2618  .P  .P
2619  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  During matching, when PCRE reaches a callout point, the external function is
2620  set), the external function is called. It is provided with the number of the  called. It is provided with the number of the callout, the position in the
2621  callout, the position in the pattern, and, optionally, one item of data  pattern, and, optionally, one item of data originally supplied by the caller of
2622  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2623  may cause matching to proceed, to backtrack, or to fail altogether. A complete  backtrack, or to fail altogether. A complete description of the interface to
2624  description of the interface to the callout function is given in the  the callout function is given in the
2625  .\" HREF  .\" HREF
2626  \fBpcrecallout\fP  \fBpcrecallout\fP
2627  .\"  .\"
2628  documentation.  documentation.
2629  .  .
2630  .  .
2631  .SH "BACTRACKING CONTROL"  .\" HTML <a name="backtrackcontrol"></a>
2632    .SH "BACKTRACKING CONTROL"
2633  .rs  .rs
2634  .sp  .sp
2635  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
# Line 1987  or removal in a future version of Perl". Line 2638  or removal in a future version of Perl".
2638  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2639  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2640  .P  .P
2641  Since these verbs are specifically related to backtracking, they can be used  Since these verbs are specifically related to backtracking, most of them can be
2642  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  used only when the pattern is to be matched using one of the traditional
2643  backtracking algorithm. They cause an error if encountered by  matching functions, which use a backtracking algorithm. With the exception of
2644  \fBpcre_dfa_exec()\fP.  (*FAIL), which behaves like a failing negative assertion, they cause an error
2645    if encountered by a DFA matching function.
2646    .P
2647    If any of these verbs are used in an assertion or in a subpattern that is
2648    called as a subroutine (whether or not recursively), their effect is confined
2649    to that subpattern; it does not extend to the surrounding pattern, with one
2650    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2651    a successful positive assertion \fIis\fP passed back when a match succeeds
2652    (compare capturing parentheses in assertions). Note that such subpatterns are
2653    processed as anchored at the point where they are tested. Note also that Perl's
2654    treatment of subroutines and assertions is different in some cases.
2655  .P  .P
2656  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2657  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2658  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2659  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2660  are two kinds:  characters that does not include a closing parenthesis. The maximum length of
2661    name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2662    is empty, that is, if the closing parenthesis immediately follows the colon,
2663    the effect is as if the colon were not there. Any number of these verbs may
2664    occur in a pattern.
2665    .
2666    .
2667    .\" HTML <a name="nooptimize"></a>
2668    .SS "Optimizations that affect backtracking verbs"
2669    .rs
2670    .sp
2671    PCRE contains some optimizations that are used to speed up matching by running
2672    some checks at the start of each match attempt. For example, it may know the
2673    minimum length of matching subject, or that a particular character must be
2674    present. When one of these optimizations suppresses the running of a match, any
2675    included backtracking verbs will not, of course, be processed. You can suppress
2676    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2677    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2678    pattern with (*NO_START_OPT). There is more discussion of this option in the
2679    section entitled
2680    .\" HTML <a href="pcreapi.html#execoptions">
2681    .\" </a>
2682    "Option bits for \fBpcre_exec()\fP"
2683    .\"
2684    in the
2685    .\" HREF
2686    \fBpcreapi\fP
2687    .\"
2688    documentation.
2689    .P
2690    Experiments with Perl suggest that it too has similar optimizations, sometimes
2691    leading to anomalous results.
2692    .
2693  .  .
2694  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2695  .rs  .rs
2696  .sp  .sp
2697  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2698    followed by a name.
2699  .sp  .sp
2700     (*ACCEPT)     (*ACCEPT)
2701  .sp  .sp
2702  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2703  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2704  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2705  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2706  captured. For example:  far is captured. For example:
2707  .sp  .sp
2708    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2709  .sp  .sp
2710  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2711  captured.  the outer parentheses.
2712  .sp  .sp
2713    (*FAIL) or (*F)    (*FAIL) or (*F)
2714  .sp  .sp
2715  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2716  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2717  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2718  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2029  callout feature, as for example in this Line 2723  callout feature, as for example in this
2723  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2724  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2725  .  .
2726    .
2727    .SS "Recording which path was taken"
2728    .rs
2729    .sp
2730    There is one verb whose main purpose is to track how a match was arrived at,
2731    though it also has a secondary use in conjunction with advancing the match
2732    starting point (see (*SKIP) below).
2733    .sp
2734      (*MARK:NAME) or (*:NAME)
2735    .sp
2736    A name is always required with this verb. There may be as many instances of
2737    (*MARK) as you like in a pattern, and their names do not have to be unique.
2738    .P
2739    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2740    path is passed back to the caller as described in the section entitled
2741    .\" HTML <a href="pcreapi.html#extradata">
2742    .\" </a>
2743    "Extra data for \fBpcre_exec()\fP"
2744    .\"
2745    in the
2746    .\" HREF
2747    \fBpcreapi\fP
2748    .\"
2749    documentation. Here is an example of \fBpcretest\fP output, where the /K
2750    modifier requests the retrieval and outputting of (*MARK) data:
2751    .sp
2752        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2753      data> XY
2754       0: XY
2755      MK: A
2756      XZ
2757       0: XZ
2758      MK: B
2759    .sp
2760    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2761    indicates which of the two alternatives matched. This is a more efficient way
2762    of obtaining this information than putting each alternative in its own
2763    capturing parentheses.
2764    .P
2765    If (*MARK) is encountered in a positive assertion, its name is recorded and
2766    passed back if it is the last-encountered. This does not happen for negative
2767    assertions.
2768    .P
2769    After a partial match or a failed match, the name of the last encountered
2770    (*MARK) in the entire match process is returned. For example:
2771    .sp
2772        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2773      data> XP
2774      No match, mark = B
2775    .sp
2776    Note that in this unanchored example the mark is retained from the match
2777    attempt that started at the letter "X" in the subject. Subsequent match
2778    attempts starting at "P" and then with an empty string do not get as far as the
2779    (*MARK) item, but nevertheless do not reset it.
2780    .P
2781    If you are interested in (*MARK) values after failed matches, you should
2782    probably set the PCRE_NO_START_OPTIMIZE option
2783    .\" HTML <a href="#nooptimize">
2784    .\" </a>
2785    (see above)
2786    .\"
2787    to ensure that the match is always attempted.
2788    .
2789    .
2790  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2791  .rs  .rs
2792  .sp  .sp
2793  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2794  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2795  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2796    the verb. However, when one of these verbs appears inside an atomic group, its
2797    effect is confined to that group, because once the group has been matched,
2798    there is never any backtracking into it. In this situation, backtracking can
2799    "jump back" to the left of the entire atomic group. (Remember also, as stated
2800    above, that this localization also applies in subroutine calls and assertions.)
2801    .P
2802    These verbs differ in exactly what kind of failure occurs when backtracking
2803    reaches them.
2804  .sp  .sp
2805    (*COMMIT)    (*COMMIT)
2806  .sp  .sp
2807  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2808  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2809  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2810  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2811  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2812  .sp  .sp
2813    a+(*COMMIT)b    a+(*COMMIT)b
2814  .sp  .sp
2815  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2816  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2817  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2818    (*PRUNE)  match failure.
2819  .sp  .P
2820  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2821  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2822  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2823  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2824  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2825  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2826  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2827  be expressed in any other way.    xyzabc\eY
2828      No match
2829    .sp
2830    PCRE knows that any match must start with "a", so the optimization skips along
2831    the subject to "a" before running the first match attempt, which succeeds. When
2832    the optimization is disabled by the \eY escape in the second subject, the match
2833    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2834    starting points.
2835    .sp
2836      (*PRUNE) or (*PRUNE:NAME)
2837    .sp
2838    This verb causes the match to fail at the current starting position in the
2839    subject if the rest of the pattern does not match. If the pattern is
2840    unanchored, the normal "bumpalong" advance to the next starting character then
2841    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2842    reached, or when matching to the right of (*PRUNE), but if there is no match to
2843    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2844    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2845    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2846    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2847    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2848  .sp  .sp
2849    (*SKIP)    (*SKIP)
2850  .sp  .sp
2851  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2852  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2853  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2854  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2855    successful match. Consider:
2856  .sp  .sp
2857    a+(*SKIP)b    a+(*SKIP)b
2858  .sp  .sp
2859  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2860  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2861  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2862  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2863  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2864  instead of skipping on to "c".  instead of skipping on to "c".
2865  .sp  .sp
2866    (*THEN)    (*SKIP:NAME)
2867  .sp  .sp
2868  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2869  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2870  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2871  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2872    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2873    matching name is found, the (*SKIP) is ignored.
2874    .sp
2875      (*THEN) or (*THEN:NAME)
2876    .sp
2877    This verb causes a skip to the next innermost alternative if the rest of the
2878    pattern does not match. That is, it cancels pending backtracking, but only
2879    within the current alternative. Its name comes from the observation that it can
2880    be used for a pattern-based if-then-else block:
2881  .sp  .sp
2882    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2883  .sp  .sp
2884  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2885  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2886  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2887  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2888    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2889    .P
2890    Note that a subpattern that does not contain a | character is just a part of
2891    the enclosing alternative; it is not a nested alternation with only one
2892    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2893    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2894    pattern fragments that do not contain any | characters at this level:
2895    .sp
2896      A (B(*THEN)C) | D
2897    .sp
2898    If A and B are matched, but there is a failure in C, matching does not
2899    backtrack into A; instead it moves to the next alternative, that is, D.
2900    However, if the subpattern containing (*THEN) is given an alternative, it
2901    behaves differently:
2902    .sp
2903      A (B(*THEN)C | (*FAIL)) | D
2904    .sp
2905    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2906    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2907    because there are no more alternatives to try. In this case, matching does now
2908    backtrack into A.
2909    .P
2910    Note also that a conditional subpattern is not considered as having two
2911    alternatives, because only one is ever used. In other words, the | character in
2912    a conditional subpattern has a different meaning. Ignoring white space,
2913    consider:
2914    .sp
2915      ^.*? (?(?=a) a | b(*THEN)c )
2916    .sp
2917    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2918    it initially matches zero characters. The condition (?=a) then fails, the
2919    character "b" is matched, but "c" is not. At this point, matching does not
2920    backtrack to .*? as might perhaps be expected from the presence of the |
2921    character. The conditional subpattern is part of the single alternative that
2922    comprises the whole pattern, and so the match fails. (If there was a backtrack
2923    into .*?, allowing it to match "b", the match would succeed.)
2924    .P
2925    The verbs just described provide four different "strengths" of control when
2926    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2927    next alternative. (*PRUNE) comes next, failing the match at the current
2928    starting position, but allowing an advance to the next character (for an
2929    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2930    than one character. (*COMMIT) is the strongest, causing the entire match to
2931    fail.
2932    .P
2933    If more than one such verb is present in a pattern, the "strongest" one wins.
2934    For example, consider this pattern, where A, B, etc. are complex pattern
2935    fragments:
2936    .sp
2937      (A(*COMMIT)B(*THEN)C|D)
2938    .sp
2939    Once A has matched, PCRE is committed to this match, at the current starting
2940    position. If subsequently B matches, but C does not, the normal (*THEN) action
2941    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2942    overrides.
2943  .  .
2944  .  .
2945  .SH "SEE ALSO"  .SH "SEE ALSO"
2946  .rs  .rs
2947  .sp  .sp
2948  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2949    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2950  .  .
2951  .  .
2952  .SH AUTHOR  .SH AUTHOR
# Line 2111  Cambridge CB2 3QH, England. Line 2963  Cambridge CB2 3QH, England.
2963  .rs  .rs
2964  .sp  .sp
2965  .nf  .nf
2966  Last updated: 09 August 2007  Last updated: 10 September 2012
2967  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2968  .fi  .fi

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