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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"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  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
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    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, an
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  extra library that supports 16-bit and UTF-16 character strings, and an
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  extra library that supports 32-bit and UTF-32 character strings. To use these
27  places below. There is also a summary of UTF-8 features in the  features, PCRE must be built to include appropriate support. When using UTF
28  .\" HTML <a href="pcre.html#utf8support">  strings you must either call the compiling function with the PCRE_UTF8,
29  .\" </a>  PCRE_UTF16 or PCRE_UTF32 option, or the pattern must start with one of
30  section on UTF-8 support  these special sequences:
31  .\"  .sp
32  in the main    (*UTF8)
33      (*UTF16)
34      (*UTF32)
35    .sp
36    Starting a pattern with such a sequence is equivalent to setting the relevant
37    option. This feature is not Perl-compatible. How setting a UTF mode affects
38    pattern matching is mentioned in several places below. There is also a summary
39    of features in the
40  .\" HREF  .\" HREF
41  \fBpcre\fP  \fBpcreunicode\fP
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) or (*UTF16) or (*UTF32) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
61  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
62  From release 6.0, PCRE offers a second matching function,  \fBpcre[16|32]_exec()\fP (16- or 32-bit), is used. PCRE also has alternative
63  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  matching functions, \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP,
64  Perl-compatible. The advantages and disadvantages of the alternative function,  which match using a different algorithm that is not Perl-compatible. Some of
65  and how it differs from the normal function, are discussed in the  the features discussed below are not available when DFA matching is used. The
66    advantages and disadvantages of the alternative functions, and how they differ
67    from the normal functions, are discussed in the
68  .\" HREF  .\" HREF
69  \fBpcrematching\fP  \fBpcrematching\fP
70  .\"  .\"
71  page.  page.
72  .  .
73  .  .
74    .SH "EBCDIC CHARACTER CODES"
75    .rs
76    .sp
77    PCRE can be compiled to run in an environment that uses EBCDIC as its character
78    code rather than ASCII or Unicode (typically a mainframe system). In the
79    sections below, character code values are ASCII or Unicode; in an EBCDIC
80    environment these characters may have different code values, and there are no
81    code points greater than 255.
82    .
83    .
84    .\" HTML <a name="newlines"></a>
85    .SH "NEWLINE CONVENTIONS"
86    .rs
87    .sp
88    PCRE supports five different conventions for indicating line breaks in
89    strings: a single CR (carriage return) character, a single LF (linefeed)
90    character, the two-character sequence CRLF, any of the three preceding, or any
91    Unicode newline sequence. The
92    .\" HREF
93    \fBpcreapi\fP
94    .\"
95    page has
96    .\" HTML <a href="pcreapi.html#newlines">
97    .\" </a>
98    further discussion
99    .\"
100    about newlines, and shows how to set the newline convention in the
101    \fIoptions\fP arguments for the compiling and matching functions.
102    .P
103    It is also possible to specify a newline convention by starting a pattern
104    string with one of the following five sequences:
105    .sp
106      (*CR)        carriage return
107      (*LF)        linefeed
108      (*CRLF)      carriage return, followed by linefeed
109      (*ANYCRLF)   any of the three above
110      (*ANY)       all Unicode newline sequences
111    .sp
112    These override the default and the options given to the compiling function. For
113    example, on a Unix system where LF is the default newline sequence, the pattern
114    .sp
115      (*CR)a.b
116    .sp
117    changes the convention to CR. That pattern matches "a\enb" because LF is no
118    longer a newline. Note that these special settings, which are not
119    Perl-compatible, are recognized only at the very start of a pattern, and that
120    they must be in upper case. If more than one of them is present, the last one
121    is used.
122    .P
123    The newline convention affects the interpretation of the dot metacharacter when
124    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
125    affect what the \eR escape sequence matches. By default, this is any Unicode
126    newline sequence, for Perl compatibility. However, this can be changed; see the
127    description of \eR in the section entitled
128    .\" HTML <a href="#newlineseq">
129    .\" </a>
130    "Newline sequences"
131    .\"
132    below. A change of \eR setting can be combined with a change of newline
133    convention.
134    .
135    .
136  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
137  .rs  .rs
138  .sp  .sp
# Line 49  corresponding characters in the subject. Line 144  corresponding characters in the subject.
144  .sp  .sp
145  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
146  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
147  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
148  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
149  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
150  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
151  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
152  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
153  UTF-8 support.  UTF support.
154  .P  .P
155  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
156  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 101  The following sections describe the use Line 196  The following sections describe the use
196  .rs  .rs
197  .sp  .sp
198  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
199  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
200  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
201  outside character classes.  both inside and outside character classes.
202  .P  .P
203  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.
204  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 111  otherwise be interpreted as a metacharac Line 206  otherwise be interpreted as a metacharac
206  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
207  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
208  .P  .P
209  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
210    backslash. All other characters (in particular, those whose codepoints are
211    greater than 127) are treated as literals.
212    .P
213    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
214  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
215  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
216  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.
217  .P  .P
218  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
219  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 130  Perl, $ and @ cause variable interpolati Line 229  Perl, $ and @ cause variable interpolati
229    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
230  .sp  .sp
231  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
232    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
233    by \eE later in the pattern, the literal interpretation continues to the end of
234    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
235    a character class, this causes an error, because the character class is not
236    terminated.
237  .  .
238  .  .
239  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 139  The \eQ...\eE sequence is recognized bot Line 243  The \eQ...\eE sequence is recognized bot
243  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
244  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
245  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
246  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
247  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:  
248  .sp  .sp
249    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
250    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
251    \ee        escape (hex 1B)    \ee        escape (hex 1B)
252    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
253    \en        newline (hex 0A)    \en        linefeed (hex 0A)
254    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
255    \et        tab (hex 09)    \et        tab (hex 09)
256    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
257    \exhh      character with hex code hh    \exhh      character with hex code hh
258    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
259      \euhhhh    character with hex code hhhh (JavaScript mode only)
260  .sp  .sp
261  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
262  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
263  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),
264  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
265  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
266  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.
267  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
268  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
269  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  extension to Unicode it is even less useful than it once was. It is, however,
270  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  recognized when PCRE is compiled in EBCDIC mode, where data items are always
271  and }, or if there is no terminating }, this form of escape is not recognized.  bytes. In this mode, all values are valid after \ec. If the next character is a
272  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
273  with no following digits, giving a character whose value is zero.  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
274    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
275    characters also generate different values.
276    .P
277    By default, after \ex, from zero to two hexadecimal digits are read (letters
278    can be in upper or lower case). Any number of hexadecimal digits may appear
279    between \ex{ and }, but the character code is constrained as follows:
280    .sp
281      8-bit non-UTF mode    less than 0x100
282      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
283      16-bit non-UTF mode   less than 0x10000
284      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
285      32-bit non-UTF mode   less than 0x80000000
286      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
287    .sp
288    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
289    "surrogate" codepoints), and 0xffef.
290    .P
291    If characters other than hexadecimal digits appear between \ex{ and }, or if
292    there is no terminating }, this form of escape is not recognized. Instead, the
293    initial \ex will be interpreted as a basic hexadecimal escape, with no
294    following digits, giving a character whose value is zero.
295    .P
296    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
297    as just described only when it is followed by two hexadecimal digits.
298    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
299    code points greater than 256 is provided by \eu, which must be followed by
300    four hexadecimal digits; otherwise it matches a literal "u" character.
301    Character codes specified by \eu in JavaScript mode are constrained in the same
302    was as those specified by \ex in non-JavaScript mode.
303  .P  .P
304  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
305  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
306  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
307    \eu00dc in JavaScript mode).
308  .P  .P
309  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
310  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 196  parenthesized subpatterns. Line 330  parenthesized subpatterns.
330  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
331  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
332  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
333  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
334  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.
335  to \e777 are permitted. For example:  For example:
336  .sp  .sp
337    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
338  .\" JOIN  .\" JOIN
339    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
340              previous capturing subpatterns              previous capturing subpatterns
# Line 215  to \e777 are permitted. For example: Line 349  to \e777 are permitted. For example:
349              character with octal code 113              character with octal code 113
350  .\" JOIN  .\" JOIN
351    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
352              the byte consisting entirely of 1 bits              the value 255 (decimal)
353  .\" JOIN  .\" JOIN
354    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
355              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 224  Note that octal values of 100 or greater Line 358  Note that octal values of 100 or greater
358  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
359  .P  .P
360  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
361  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
362  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
363  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
364  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
365  meanings  inside a character class. Like other unrecognized escape sequences, they are
366  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
367  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
368  (see below).  sequences have different meanings.
369  .\"  .
370    .
371    .SS "Unsupported escape sequences"
372    .rs
373    .sp
374    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
375    handler and used to modify the case of following characters. By default, PCRE
376    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
377    option is set, \eU matches a "U" character, and \eu can be used to define a
378    character by code point, as described in the previous section.
379  .  .
380  .  .
381  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
382  .rs  .rs
383  .sp  .sp
384  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
385  in braces, is an absolute or relative back reference. Back references are  enclosed in braces, is an absolute or relative back reference. A named back
386  discussed  reference can be coded as \eg{name}. Back references are discussed
387  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
388  .\" </a>  .\" </a>
389  later,  later,
# Line 252  parenthesized subpatterns. Line 395  parenthesized subpatterns.
395  .\"  .\"
396  .  .
397  .  .
398    .SS "Absolute and relative subroutine calls"
399    .rs
400    .sp
401    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
402    a number enclosed either in angle brackets or single quotes, is an alternative
403    syntax for referencing a subpattern as a "subroutine". Details are discussed
404    .\" HTML <a href="#onigurumasubroutines">
405    .\" </a>
406    later.
407    .\"
408    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
409    synonymous. The former is a back reference; the latter is a
410    .\" HTML <a href="#subpatternsassubroutines">
411    .\" </a>
412    subroutine
413    .\"
414    call.
415    .
416    .
417    .\" HTML <a name="genericchartypes"></a>
418  .SS "Generic character types"  .SS "Generic character types"
419  .rs  .rs
420  .sp  .sp
421  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:  
422  .sp  .sp
423    \ed     any decimal digit    \ed     any decimal digit
424    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
425    \es     any whitespace character    \eh     any horizontal white space character
426    \eS     any character that is not a whitespace character    \eH     any character that is not a horizontal white space character
427      \es     any white space character
428      \eS     any character that is not a white space character
429      \ev     any vertical white space character
430      \eV     any character that is not a vertical white space character
431    \ew     any "word" character    \ew     any "word" character
432    \eW     any "non-word" character    \eW     any "non-word" character
433  .sp  .sp
434  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.
435  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
436    .\" HTML <a href="#fullstopdot">
437    .\" </a>
438    the "." metacharacter
439    .\"
440    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
441    PCRE does not support this.
442  .P  .P
443  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
444    of characters into two disjoint sets. Any given character matches one, and only
445    one, of each pair. The sequences can appear both inside and outside character
446  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
447  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
448  there is no character to match.  there is no character to match.
449  .P  .P
450  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).
451  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
452  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
453  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
454  does.)  does.
455  .P  .P
456  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
457  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
458  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
459  place (see  place (see
460  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 293  in the Line 467  in the
467  .\"  .\"
468  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
469  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
470  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
471    Unicode is discouraged.
472  .P  .P
473  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in a UTF mode, characters with values greater than 128 never match
474  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
475  character property support is available. The use of locales with Unicode is  their original meanings from before UTF support was available, mainly for
476  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
477    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
478    properties are used to determine character types, as follows:
479    .sp
480      \ed  any character that \ep{Nd} matches (decimal digit)
481      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
482      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
483    .sp
484    The upper case escapes match the inverse sets of characters. Note that \ed
485    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
486    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
487    \eB because they are defined in terms of \ew and \eW. Matching these sequences
488    is noticeably slower when PCRE_UCP is set.
489    .P
490    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
491    release 5.10. In contrast to the other sequences, which match only ASCII
492    characters by default, these always match certain high-valued codepoints,
493    whether or not PCRE_UCP is set. The horizontal space characters are:
494    .sp
495      U+0009     Horizontal tab (HT)
496      U+0020     Space
497      U+00A0     Non-break space
498      U+1680     Ogham space mark
499      U+180E     Mongolian vowel separator
500      U+2000     En quad
501      U+2001     Em quad
502      U+2002     En space
503      U+2003     Em space
504      U+2004     Three-per-em space
505      U+2005     Four-per-em space
506      U+2006     Six-per-em space
507      U+2007     Figure space
508      U+2008     Punctuation space
509      U+2009     Thin space
510      U+200A     Hair space
511      U+202F     Narrow no-break space
512      U+205F     Medium mathematical space
513      U+3000     Ideographic space
514    .sp
515    The vertical space characters are:
516    .sp
517      U+000A     Linefeed (LF)
518      U+000B     Vertical tab (VT)
519      U+000C     Form feed (FF)
520      U+000D     Carriage return (CR)
521      U+0085     Next line (NEL)
522      U+2028     Line separator
523      U+2029     Paragraph separator
524    .sp
525    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
526    relevant.
527  .  .
528  .  .
529    .\" HTML <a name="newlineseq"></a>
530  .SS "Newline sequences"  .SS "Newline sequences"
531  .rs  .rs
532  .sp  .sp
533  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
534  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
535  the following:  following:
536  .sp  .sp
537    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
538  .sp  .sp
# Line 317  below. Line 543  below.
543  .\"  .\"
544  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
545  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,
546  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
547  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
548  cannot be split.  cannot be split.
549  .P  .P
550  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
551  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).
552  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
553  recognized.  recognized.
554  .P  .P
555  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
556    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
557    either at compile time or when the pattern is matched. (BSR is an abbrevation
558    for "backslash R".) This can be made the default when PCRE is built; if this is
559    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
560    It is also possible to specify these settings by starting a pattern string with
561    one of the following sequences:
562    .sp
563      (*BSR_ANYCRLF)   CR, LF, or CRLF only
564      (*BSR_UNICODE)   any Unicode newline sequence
565    .sp
566    These override the default and the options given to the compiling function, but
567    they can themselves be overridden by options given to a matching function. Note
568    that these special settings, which are not Perl-compatible, are recognized only
569    at the very start of a pattern, and that they must be in upper case. If more
570    than one of them is present, the last one is used. They can be combined with a
571    change of newline convention; for example, a pattern can start with:
572    .sp
573      (*ANY)(*BSR_ANYCRLF)
574    .sp
575    They can also be combined with the (*UTF8), (*UTF16), (*UTF32) or (*UCP) special
576    sequences. Inside a character class, \eR is treated as an unrecognized escape
577    sequence, and so matches the letter "R" by default, but causes an error if
578    PCRE_EXTRA is set.
579  .  .
580  .  .
581  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 334  Inside a character class, \eR matches th Line 583  Inside a character class, \eR matches th
583  .rs  .rs
584  .sp  .sp
585  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
586  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
587  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
588    characters whose codepoints are less than 256, but they do work in this mode.
589    The extra escape sequences are:
590  .sp  .sp
591    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
592    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
593    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
594  .sp  .sp
595  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
596  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
597  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
598  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
599  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
600    .\" </a>
601    next section).
602    .\"
603    Other Perl properties such as "InMusicalSymbols" are not currently supported by
604    PCRE. Note that \eP{Any} does not match any characters, so always causes a
605    match failure.
606  .P  .P
607  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
608  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 359  Those that are not part of an identified Line 616  Those that are not part of an identified
616  .P  .P
617  Arabic,  Arabic,
618  Armenian,  Armenian,
619    Avestan,
620  Balinese,  Balinese,
621    Bamum,
622    Batak,
623  Bengali,  Bengali,
624  Bopomofo,  Bopomofo,
625    Brahmi,
626  Braille,  Braille,
627  Buginese,  Buginese,
628  Buhid,  Buhid,
629  Canadian_Aboriginal,  Canadian_Aboriginal,
630    Carian,
631    Chakma,
632    Cham,
633  Cherokee,  Cherokee,
634  Common,  Common,
635  Coptic,  Coptic,
# Line 374  Cypriot, Line 638  Cypriot,
638  Cyrillic,  Cyrillic,
639  Deseret,  Deseret,
640  Devanagari,  Devanagari,
641    Egyptian_Hieroglyphs,
642  Ethiopic,  Ethiopic,
643  Georgian,  Georgian,
644  Glagolitic,  Glagolitic,
# Line 386  Hangul, Line 651  Hangul,
651  Hanunoo,  Hanunoo,
652  Hebrew,  Hebrew,
653  Hiragana,  Hiragana,
654    Imperial_Aramaic,
655  Inherited,  Inherited,
656    Inscriptional_Pahlavi,
657    Inscriptional_Parthian,
658    Javanese,
659    Kaithi,
660  Kannada,  Kannada,
661  Katakana,  Katakana,
662    Kayah_Li,
663  Kharoshthi,  Kharoshthi,
664  Khmer,  Khmer,
665  Lao,  Lao,
666  Latin,  Latin,
667    Lepcha,
668  Limbu,  Limbu,
669  Linear_B,  Linear_B,
670    Lisu,
671    Lycian,
672    Lydian,
673  Malayalam,  Malayalam,
674    Mandaic,
675    Meetei_Mayek,
676    Meroitic_Cursive,
677    Meroitic_Hieroglyphs,
678    Miao,
679  Mongolian,  Mongolian,
680  Myanmar,  Myanmar,
681  New_Tai_Lue,  New_Tai_Lue,
# Line 403  Nko, Line 683  Nko,
683  Ogham,  Ogham,
684  Old_Italic,  Old_Italic,
685  Old_Persian,  Old_Persian,
686    Old_South_Arabian,
687    Old_Turkic,
688    Ol_Chiki,
689  Oriya,  Oriya,
690  Osmanya,  Osmanya,
691  Phags_Pa,  Phags_Pa,
692  Phoenician,  Phoenician,
693    Rejang,
694  Runic,  Runic,
695    Samaritan,
696    Saurashtra,
697    Sharada,
698  Shavian,  Shavian,
699  Sinhala,  Sinhala,
700    Sora_Sompeng,
701    Sundanese,
702  Syloti_Nagri,  Syloti_Nagri,
703  Syriac,  Syriac,
704  Tagalog,  Tagalog,
705  Tagbanwa,  Tagbanwa,
706  Tai_Le,  Tai_Le,
707    Tai_Tham,
708    Tai_Viet,
709    Takri,
710  Tamil,  Tamil,
711  Telugu,  Telugu,
712  Thaana,  Thaana,
# Line 422  Thai, Line 714  Thai,
714  Tibetan,  Tibetan,
715  Tifinagh,  Tifinagh,
716  Ugaritic,  Ugaritic,
717    Vai,
718  Yi.  Yi.
719  .P  .P
720  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
721  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
722  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
723  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
724  .P  .P
725  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
726  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 487  The special property L& is also supporte Line 780  The special property L& is also supporte
780  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
781  a modifier or "other".  a modifier or "other".
782  .P  .P
783  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
784    U+DFFF. Such characters are not valid in Unicode strings and so
785    cannot be tested by PCRE, unless UTF validity checking has been turned off
786    (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
787    PCRE_NO_UTF32_CHECK in the
788    .\" HREF
789    \fBpcreapi\fP
790    .\"
791    page). Perl does not support the Cs property.
792    .P
793    The long synonyms for property names that Perl supports (such as \ep{Letter})
794  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
795  properties with "Is".  properties with "Is".
796  .P  .P
# Line 498  Unicode table. Line 801  Unicode table.
801  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
802  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
803  .P  .P
804  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
805  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
806  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
807    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
808    PCRE_UCP option or by starting the pattern with (*UCP).
809    .
810    .
811    .SS Extended grapheme clusters
812    .rs
813  .sp  .sp
814  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
815  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  
816  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
817  .\" </a>  .\" </a>
818  (see below).  (see below).
819  .\"  .\"
820  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
821  preceding character.  that was equivalent to
822    .sp
823      (?>\ePM\epM*)
824    .sp
825    That is, it matched a character without the "mark" property, followed by zero
826    or more characters with the "mark" property. Characters with the "mark"
827    property are typically non-spacing accents that affect the preceding character.
828    .P
829    This simple definition was extended in Unicode to include more complicated
830    kinds of composite character by giving each character a grapheme breaking
831    property, and creating rules that use these properties to define the boundaries
832    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
833    one of these clusters.
834    .P
835    \eX always matches at least one character. Then it decides whether to add
836    additional characters according to the following rules for ending a cluster:
837    .P
838    1. End at the end of the subject string.
839  .P  .P
840  Matching characters by Unicode property is not fast, because PCRE has to search  2. Do not end between CR and LF; otherwise end after any control character.
841  a structure that contains data for over fifteen thousand characters. That is  .P
842  why the traditional escape sequences such as \ed and \ew do not use Unicode  3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
843  properties in PCRE.  are of five types: L, V, T, LV, and LVT. An L character may be followed by an
844    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
845    character; an LVT or T character may be follwed only by a T character.
846    .P
847    4. Do not end before extending characters or spacing marks. Characters with
848    the "mark" property always have the "extend" grapheme breaking property.
849    .P
850    5. Do not end after prepend characters.
851    .P
852    6. Otherwise, end the cluster.
853    .
854    .
855    .\" HTML <a name="extraprops"></a>
856    .SS PCRE's additional properties
857    .rs
858    .sp
859    As well as the standard Unicode properties described above, PCRE supports four
860    more that make it possible to convert traditional escape sequences such as \ew
861    and \es and POSIX character classes to use Unicode properties. PCRE uses these
862    non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
863    .sp
864      Xan   Any alphanumeric character
865      Xps   Any POSIX space character
866      Xsp   Any Perl space character
867      Xwd   Any Perl "word" character
868    .sp
869    Xan matches characters that have either the L (letter) or the N (number)
870    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
871    carriage return, and any other character that has the Z (separator) property.
872    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
873    same characters as Xan, plus underscore.
874    .
875    .
876    .\" HTML <a name="resetmatchstart"></a>
877    .SS "Resetting the match start"
878    .rs
879    .sp
880    The escape sequence \eK causes any previously matched characters not to be
881    included in the final matched sequence. For example, the pattern:
882    .sp
883      foo\eKbar
884    .sp
885    matches "foobar", but reports that it has matched "bar". This feature is
886    similar to a lookbehind assertion
887    .\" HTML <a href="#lookbehind">
888    .\" </a>
889    (described below).
890    .\"
891    However, in this case, the part of the subject before the real match does not
892    have to be of fixed length, as lookbehind assertions do. The use of \eK does
893    not interfere with the setting of
894    .\" HTML <a href="#subpattern">
895    .\" </a>
896    captured substrings.
897    .\"
898    For example, when the pattern
899    .sp
900      (foo)\eKbar
901    .sp
902    matches "foobar", the first substring is still set to "foo".
903    .P
904    Perl documents that the use of \eK within assertions is "not well defined". In
905    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
906    ignored in negative assertions.
907  .  .
908  .  .
909  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 541  The backslashed assertions are: Line 928  The backslashed assertions are:
928    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
929    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
930  .sp  .sp
931  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
932  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
933    default it matches the corresponding literal character (for example, \eB
934    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
935    escape sequence" error is generated instead.
936  .P  .P
937  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
938  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
939  \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
940  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
941    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
942    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
943    of word" or "end of word" metasequence. However, whatever follows \eb normally
944    determines which it is. For example, the fragment \eba matches "a" at the start
945    of a word.
946  .P  .P
947  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
948  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 631  end of the subject in both modes, and if Line 1026  end of the subject in both modes, and if
1026  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1027  .  .
1028  .  .
1029  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1030    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1031  .rs  .rs
1032  .sp  .sp
1033  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
1034  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
1035  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1036  .P  .P
1037  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
1038  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 653  to match it. Line 1049  to match it.
1049  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
1050  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
1051  special meaning in a character class.  special meaning in a character class.
1052  .  .P
1053  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1054  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1055  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1056  .sp  name; PCRE does not support this.
1057  Outside a character class, the escape sequence \eC matches any one byte, both  .
1058  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1059  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1060  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1061  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1062  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1063    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1064    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1065    a 32-bit unit. Unlike a dot, \eC always
1066    matches line-ending characters. The feature is provided in Perl in order to
1067    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1068    used. Because \eC breaks up characters into individual data units, matching one
1069    unit with \eC in a UTF mode means that the rest of the string may start with a
1070    malformed UTF character. This has undefined results, because PCRE assumes that
1071    it is dealing with valid UTF strings (and by default it checks this at the
1072    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1073    PCRE_NO_UTF32_CHECK option is used).
1074  .P  .P
1075  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1076  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1077  .\" </a>  .\" </a>
1078  (described below),  (described below)
1079  .\"  .\"
1080  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
1081  the lookbehind.  the lookbehind.
1082    .P
1083    In general, the \eC escape sequence is best avoided. However, one
1084    way of using it that avoids the problem of malformed UTF characters is to use a
1085    lookahead to check the length of the next character, as in this pattern, which
1086    could be used with a UTF-8 string (ignore white space and line breaks):
1087    .sp
1088      (?| (?=[\ex00-\ex7f])(\eC) |
1089          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1090          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1091          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1092    .sp
1093    A group that starts with (?| resets the capturing parentheses numbers in each
1094    alternative (see
1095    .\" HTML <a href="#dupsubpatternnumber">
1096    .\" </a>
1097    "Duplicate Subpattern Numbers"
1098    .\"
1099    below). The assertions at the start of each branch check the next UTF-8
1100    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1101    character's individual bytes are then captured by the appropriate number of
1102    groups.
1103  .  .
1104  .  .
1105  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 679  the lookbehind. Line 1107  the lookbehind.
1107  .rs  .rs
1108  .sp  .sp
1109  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1110  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.
1111  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
1112  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
1113  escaped with a backslash.  a member of the class, it should be the first data character in the class
1114  .P  (after an initial circumflex, if present) or escaped with a backslash.
1115  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1116  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
1117  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
1118  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
1119  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
1120  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
1121    member of the class, ensure it is not the first character, or escape it with a
1122  backslash.  backslash.
1123  .P  .P
1124  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1125  [^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
1126  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1127  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
1128  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
1129  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
1130  string.  string.
1131  .P  .P
1132  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1133  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  can be included in a class as a literal string of data units, or by using the
1134    \ex{ escaping mechanism.
1135  .P  .P
1136  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
1137  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
1138  "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
1139  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
1140  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
1141  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1142  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1143  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
1144  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
1145  UTF-8 support.  well as with UTF support.
1146  .P  .P
1147  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
1148  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 734  followed by two other characters. The oc Line 1164  followed by two other characters. The oc
1164  "]" can also be used to end a range.  "]" can also be used to end a range.
1165  .P  .P
1166  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
1167  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1168  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}].  
1169  .P  .P
1170  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
1171  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
1172  [][\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
1173  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
1174  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
1175  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
1176  property support.  property support.
1177  .P  .P
1178  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,
1179  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
1180  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1181  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
1182  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
1183  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1184    .\" HTML <a href="#genericchartypes">
1185    .\" </a>
1186    "Generic character types"
1187    .\"
1188    above. The escape sequence \eb has a different meaning inside a character
1189    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1190    are not special inside a character class. Like any other unrecognized escape
1191    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1192    default, but cause an error if the PCRE_EXTRA option is set.
1193    .P
1194    A circumflex can conveniently be used with the upper case character types to
1195    specify a more restricted set of characters than the matching lower case type.
1196    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1197    whereas [\ew] includes underscore. A positive character class should be read as
1198    "something OR something OR ..." and a negative class as "NOT something AND NOT
1199    something AND NOT ...".
1200  .P  .P
1201  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1202  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 771  this notation. For example, Line 1216  this notation. For example,
1216    [01[:alpha:]%]    [01[:alpha:]%]
1217  .sp  .sp
1218  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1219  are  are:
1220  .sp  .sp
1221    alnum    letters and digits    alnum    letters and digits
1222    alpha    letters    alpha    letters
# Line 782  are Line 1227  are
1227    graph    printing characters, excluding space    graph    printing characters, excluding space
1228    lower    lower case letters    lower    lower case letters
1229    print    printing characters, including space    print    printing characters, including space
1230    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1231    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1232    upper    upper case letters    upper    upper case letters
1233    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 803  matches "1", "2", or any non-digit. PCRE Line 1248  matches "1", "2", or any non-digit. PCRE
1248  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
1249  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1250  .P  .P
1251  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
1252  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1253    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1254    character properties are used. This is achieved by replacing the POSIX classes
1255    by other sequences, as follows:
1256    .sp
1257      [:alnum:]  becomes  \ep{Xan}
1258      [:alpha:]  becomes  \ep{L}
1259      [:blank:]  becomes  \eh
1260      [:digit:]  becomes  \ep{Nd}
1261      [:lower:]  becomes  \ep{Ll}
1262      [:space:]  becomes  \ep{Xps}
1263      [:upper:]  becomes  \ep{Lu}
1264      [:word:]   becomes  \ep{Xwd}
1265    .sp
1266    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1267    classes are unchanged, and match only characters with code points less than
1268    128.
1269  .  .
1270  .  .
1271  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 831  alternative in the subpattern. Line 1292  alternative in the subpattern.
1292  .rs  .rs
1293  .sp  .sp
1294  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1295  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
1296  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1297    The option letters are
1298  .sp  .sp
1299    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1300    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1308  PCRE_MULTILINE while unsetting PCRE_DOTA
1308  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
1309  unset.  unset.
1310  .P  .P
1311  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
1312  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
1313  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1314  the global options (and it will therefore show up in data extracted by the  .P
1315  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1316    subpattern parentheses), the change applies to the remainder of the pattern
1317    that follows. If the change is placed right at the start of a pattern, PCRE
1318    extracts it into the global options (and it will therefore show up in data
1319    extracted by the \fBpcre_fullinfo()\fP function).
1320  .P  .P
1321  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1322  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
1323  .sp  .sp
1324    (a(?i)b)c    (a(?i)b)c
1325  .sp  .sp
# Line 869  branch is abandoned before the option se Line 1335  branch is abandoned before the option se
1335  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1336  behaviour otherwise.  behaviour otherwise.
1337  .P  .P
1338  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
1339  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
1340  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1341    what the application has set or what has been defaulted. Details are given in
1342    the section entitled
1343    .\" HTML <a href="#newlineseq">
1344    .\" </a>
1345    "Newline sequences"
1346    .\"
1347    above. There are also the (*UTF8), (*UTF16),(*UTF32) and (*UCP) leading
1348    sequences that can be used to set UTF and Unicode property modes; they are
1349    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1350    options, respectively.
1351  .  .
1352  .  .
1353  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 885  Turning part of a pattern into a subpatt Line 1361  Turning part of a pattern into a subpatt
1361  .sp  .sp
1362    cat(aract|erpillar|)    cat(aract|erpillar|)
1363  .sp  .sp
1364  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1365  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1366  .sp  .sp
1367  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
1368  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
1369  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
1370  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1371  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1372  .P  .P
1373  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
1374    numbers for the capturing subpatterns. For example, if the string "the red
1375    king" is matched against the pattern
1376  .sp  .sp
1377    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1378  .sp  .sp
# Line 926  is reached, an option setting in one bra Line 1404  is reached, an option setting in one bra
1404  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1405  .  .
1406  .  .
1407    .\" HTML <a name="dupsubpatternnumber"></a>
1408    .SH "DUPLICATE SUBPATTERN NUMBERS"
1409    .rs
1410    .sp
1411    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1412    the same numbers for its capturing parentheses. Such a subpattern starts with
1413    (?| and is itself a non-capturing subpattern. For example, consider this
1414    pattern:
1415    .sp
1416      (?|(Sat)ur|(Sun))day
1417    .sp
1418    Because the two alternatives are inside a (?| group, both sets of capturing
1419    parentheses are numbered one. Thus, when the pattern matches, you can look
1420    at captured substring number one, whichever alternative matched. This construct
1421    is useful when you want to capture part, but not all, of one of a number of
1422    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1423    number is reset at the start of each branch. The numbers of any capturing
1424    parentheses that follow the subpattern start after the highest number used in
1425    any branch. The following example is taken from the Perl documentation. The
1426    numbers underneath show in which buffer the captured content will be stored.
1427    .sp
1428      # before  ---------------branch-reset----------- after
1429      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1430      # 1            2         2  3        2     3     4
1431    .sp
1432    A back reference to a numbered subpattern uses the most recent value that is
1433    set for that number by any subpattern. The following pattern matches "abcabc"
1434    or "defdef":
1435    .sp
1436      /(?|(abc)|(def))\e1/
1437    .sp
1438    In contrast, a subroutine call to a numbered subpattern always refers to the
1439    first one in the pattern with the given number. The following pattern matches
1440    "abcabc" or "defabc":
1441    .sp
1442      /(?|(abc)|(def))(?1)/
1443    .sp
1444    If a
1445    .\" HTML <a href="#conditions">
1446    .\" </a>
1447    condition test
1448    .\"
1449    for a subpattern's having matched refers to a non-unique number, the test is
1450    true if any of the subpatterns of that number have matched.
1451    .P
1452    An alternative approach to using this "branch reset" feature is to use
1453    duplicate named subpatterns, as described in the next section.
1454    .
1455    .
1456  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1457  .rs  .rs
1458  .sp  .sp
# Line 935  if an expression is modified, the number Line 1462  if an expression is modified, the number
1462  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1463  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
1464  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
1465  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1466    have different names, but PCRE does not.
1467  .P  .P
1468  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
1469  (?'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
1470  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1471  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1472  .\" </a>  .\" </a>
1473  backreferences,  back references,
1474  .\"  .\"
1475  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1476  .\" </a>  .\" </a>
# Line 962  extracting the name-to-number translatio Line 1490  extracting the name-to-number translatio
1490  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1491  .P  .P
1492  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
1493  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
1494  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
1495  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
1496  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
1497  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
1498    name, and in both cases you want to extract the abbreviation. This pattern
1499    (ignoring the line breaks) does the job:
1500  .sp  .sp
1501    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1502    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 975  abbreviation. This pattern (ignoring the Line 1505  abbreviation. This pattern (ignoring the
1505    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1506  .sp  .sp
1507  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1508    (An alternative way of solving this problem is to use a "branch reset"
1509    subpattern, as described in the previous section.)
1510    .P
1511  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1512  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
1513  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1514  make a reference to a non-unique named subpattern from elsewhere in the  .P
1515  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
1516  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1517    used. In the absence of duplicate numbers (see the previous section) this is
1518    the one with the lowest number. If you use a named reference in a condition
1519    test (see the
1520    .\"
1521    .\" HTML <a href="#conditions">
1522    .\" </a>
1523    section about conditions
1524    .\"
1525    below), either to check whether a subpattern has matched, or to check for
1526    recursion, all subpatterns with the same name are tested. If the condition is
1527    true for any one of them, the overall condition is true. This is the same
1528    behaviour as testing by number. For further details of the interfaces for
1529    handling named subpatterns, see the
1530  .\" HREF  .\" HREF
1531  \fBpcreapi\fP  \fBpcreapi\fP
1532  .\"  .\"
1533  documentation.  documentation.
1534    .P
1535    \fBWarning:\fP You cannot use different names to distinguish between two
1536    subpatterns with the same number because PCRE uses only the numbers when
1537    matching. For this reason, an error is given at compile time if different names
1538    are given to subpatterns with the same number. However, you can give the same
1539    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1540  .  .
1541  .  .
1542  .SH REPETITION  .SH REPETITION
# Line 996  items: Line 1548  items:
1548    a literal data character    a literal data character
1549    the dot metacharacter    the dot metacharacter
1550    the \eC escape sequence    the \eC escape sequence
1551    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1552    the \eR escape sequence    the \eR escape sequence
1553    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1554    a character class    a character class
1555    a back reference (see next section)    a back reference (see next section)
1556    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1557      a subroutine call to a subpattern (recursive or otherwise)
1558  .sp  .sp
1559  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1560  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1026  where a quantifier is not allowed, or on Line 1579  where a quantifier is not allowed, or on
1579  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
1580  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1581  .P  .P
1582  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
1583  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
1584  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,
1585  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
1586  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1587  .P  .P
1588  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
1589  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1590    subpatterns that are referenced as
1591    .\" HTML <a href="#subpatternsassubroutines">
1592    .\" </a>
1593    subroutines
1594    .\"
1595    from elsewhere in the pattern (but see also the section entitled
1596    .\" HTML <a href="#subdefine">
1597    .\" </a>
1598    "Defining subpatterns for use by reference only"
1599    .\"
1600    below). Items other than subpatterns that have a {0} quantifier are omitted
1601    from the compiled pattern.
1602  .P  .P
1603  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1604  abbreviations:  abbreviations:
# Line 1104  In cases where it is known that the subj Line 1669  In cases where it is known that the subj
1669  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1670  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1671  .P  .P
1672  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1673  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1674  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
1675  succeeds. Consider, for example:  succeeds. Consider, for example:
1676  .sp  .sp
# Line 1114  succeeds. Consider, for example: Line 1679  succeeds. Consider, for example:
1679  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
1680  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1681  .P  .P
1682    Another case where implicit anchoring is not applied is when the leading .* is
1683    inside an atomic group. Once again, a match at the start may fail where a later
1684    one succeeds. Consider this pattern:
1685    .sp
1686      (?>.*?a)b
1687    .sp
1688    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1689    (*PRUNE) and (*SKIP) also disable this optimization.
1690    .P
1691  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1692  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1693  .sp  .sp
# Line 1180  previous example can be rewritten as Line 1754  previous example can be rewritten as
1754  .sp  .sp
1755    \ed++foo    \ed++foo
1756  .sp  .sp
1757    Note that a possessive quantifier can be used with an entire group, for
1758    example:
1759    .sp
1760      (abc|xyz){2,3}+
1761    .sp
1762  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1763  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1764  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1253  no such problem when named parentheses a Line 1832  no such problem when named parentheses a
1832  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1833  .P  .P
1834  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
1835  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
1836  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1837  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1838  .sp  .sp
1839    (ring), \e1    (ring), \e1
1840    (ring), \eg1    (ring), \eg1
1841    (ring), \eg{1}    (ring), \eg{1}
1842  .sp  .sp
1843  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1844  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1845  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1846    example:
1847  .sp  .sp
1848    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1849  .sp  .sp
1850  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
1851  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.
1852  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
1853  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
1854  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1855  .P  .P
1856  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1857  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1293  back reference, the case of letters is r Line 1873  back reference, the case of letters is r
1873  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1874  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1875  .P  .P
1876  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1877  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1878    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1879    back reference syntax, in which \eg can be used for both numeric and named
1880    references, is also supported. We could rewrite the above example in any of
1881  the following ways:  the following ways:
1882  .sp  .sp
1883    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1884      (?'p1'(?i)rah)\es+\ek{p1}
1885    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1886      (?<p1>(?i)rah)\es+\eg{p1}
1887  .sp  .sp
1888  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1889  after the reference.  after the reference.
1890  .P  .P
1891  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
1892  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1893  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1894  .sp  .sp
1895    (a|(bc))\e2    (a|(bc))\e2
1896  .sp  .sp
1897  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
1898  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
1899  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1900  with a digit character, some delimiter must be used to terminate the back  .P
1901  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1902  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1903    If the pattern continues with a digit character, some delimiter must be used to
1904    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1905    white space. Otherwise, the \eg{ syntax or an empty comment (see
1906  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1907  .\" </a>  .\" </a>
1908  "Comments"  "Comments"
1909  .\"  .\"
1910  below) can be used.  below) can be used.
1911  .P  .
1912    .SS "Recursive back references"
1913    .rs
1914    .sp
1915  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
1916  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.
1917  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1334  to the previous iteration. In order for Line 1925  to the previous iteration. In order for
1925  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
1926  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
1927  minimum of zero.  minimum of zero.
1928    .P
1929    Back references of this type cause the group that they reference to be treated
1930    as an
1931    .\" HTML <a href="#atomicgroup">
1932    .\" </a>
1933    atomic group.
1934    .\"
1935    Once the whole group has been matched, a subsequent matching failure cannot
1936    cause backtracking into the middle of the group.
1937  .  .
1938  .  .
1939  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1353  those that look ahead of the current pos Line 1953  those that look ahead of the current pos
1953  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,
1954  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.
1955  .P  .P
1956  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1957  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
1958  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1959  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1960  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1961  because it does not make sense for negative assertions.  .P
1962    For compatibility with Perl, assertion subpatterns may be repeated; though
1963    it makes no sense to assert the same thing several times, the side effect of
1964    capturing parentheses may occasionally be useful. In practice, there only three
1965    cases:
1966    .sp
1967    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1968    However, it may contain internal capturing parenthesized groups that are called
1969    from elsewhere via the
1970    .\" HTML <a href="#subpatternsassubroutines">
1971    .\" </a>
1972    subroutine mechanism.
1973    .\"
1974    .sp
1975    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1976    were {0,1}. At run time, the rest of the pattern match is tried with and
1977    without the assertion, the order depending on the greediness of the quantifier.
1978    .sp
1979    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1980    The assertion is obeyed just once when encountered during matching.
1981  .  .
1982  .  .
1983  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1387  lookbehind assertion is needed to achiev Line 2006  lookbehind assertion is needed to achiev
2006  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
2007  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
2008  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.
2009    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2010  .  .
2011  .  .
2012  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1411  is permitted, but Line 2031  is permitted, but
2031  .sp  .sp
2032  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2033  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
2034  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
2035  match the same length of string. An assertion such as  length of string. An assertion such as
2036  .sp  .sp
2037    (?<=ab(c|de))    (?<=ab(c|de))
2038  .sp  .sp
2039  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
2040  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
2041    branches:
2042  .sp  .sp
2043    (?<=abc|abde)    (?<=abc|abde)
2044  .sp  .sp
2045    In some cases, the escape sequence \eK
2046    .\" HTML <a href="#resetmatchstart">
2047    .\" </a>
2048    (see above)
2049    .\"
2050    can be used instead of a lookbehind assertion to get round the fixed-length
2051    restriction.
2052    .P
2053  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2054  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
2055  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2056  assertion fails.  assertion fails.
2057  .P  .P
2058  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
2059  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
2060  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
2061  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2062    permitted.
2063    .P
2064    .\" HTML <a href="#subpatternsassubroutines">
2065    .\" </a>
2066    "Subroutine"
2067    .\"
2068    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2069    as the subpattern matches a fixed-length string.
2070    .\" HTML <a href="#recursion">
2071    .\" </a>
2072    Recursion,
2073    .\"
2074    however, is not supported.
2075  .P  .P
2076  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2077  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
2078  pattern such as  strings. Consider a simple pattern such as
2079  .sp  .sp
2080    abcd$    abcd$
2081  .sp  .sp
# Line 1497  characters that are not "999". Line 2139  characters that are not "999".
2139  .sp  .sp
2140  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2141  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2142  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2143  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2144  .sp  .sp
2145    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2146    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2147  .sp  .sp
2148  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2149  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
2150  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2151    itself contain nested subpatterns of any form, including conditional
2152    subpatterns; the restriction to two alternatives applies only at the level of
2153    the condition. This pattern fragment is an example where the alternatives are
2154    complex:
2155    .sp
2156      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2157    .sp
2158  .P  .P
2159  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2160  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1514  recursion, a pseudo-condition called DEF Line 2163  recursion, a pseudo-condition called DEF
2163  .rs  .rs
2164  .sp  .sp
2165  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
2166  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2167  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
2168  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2169  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2170  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2171  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2172    section about duplicate subpattern numbers),
2173    .\"
2174    the condition is true if any of them have matched. An alternative notation is
2175    to precede the digits with a plus or minus sign. In this case, the subpattern
2176    number is relative rather than absolute. The most recently opened parentheses
2177    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2178    loops it can also make sense to refer to subsequent groups. The next
2179    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2180    zero in any of these forms is not used; it provokes a compile-time error.)
2181  .P  .P
2182  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2183  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 1530  three parts for ease of discussion: Line 2188  three parts for ease of discussion:
2188  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2189  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
2190  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
2191  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2192  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,
2193  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
2194  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2195  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2196  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2197  .P  .P
2198  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
2199  reference:  reference:
2200  .sp  .sp
2201    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1560  Rewriting the above example to use a nam Line 2218  Rewriting the above example to use a nam
2218  .sp  .sp
2219    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2220  .sp  .sp
2221    If the name used in a condition of this kind is a duplicate, the test is
2222    applied to all subpatterns of the same name, and is true if any one of them has
2223    matched.
2224  .  .
2225  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2226  .rs  .rs
# Line 1571  letter R, for example: Line 2232  letter R, for example:
2232  .sp  .sp
2233    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2234  .sp  .sp
2235  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
2236  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
2237  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2238    applied to all subpatterns of the same name, and is true if any one of them is
2239    the most recent recursion.
2240  .P  .P
2241  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2242  patterns are described below.  .\" HTML <a href="#recursion">
2243    .\" </a>
2244    The syntax for recursive patterns
2245    .\"
2246    is described below.
2247  .  .
2248    .\" HTML <a name="subdefine"></a>
2249  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2250  .rs  .rs
2251  .sp  .sp
# Line 1585  If the condition is the string (DEFINE), Line 2253  If the condition is the string (DEFINE),
2253  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
2254  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2255  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
2256  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2257  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2258  written like this (ignore whitespace and line breaks):  .\" </a>
2259    subroutines
2260    .\"
2261    is described below.) For example, a pattern to match an IPv4 address such as
2262    "192.168.23.245" could be written like this (ignore white space and line
2263    breaks):
2264  .sp  .sp
2265    (?(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) )
2266    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1595  written like this (ignore whitespace and Line 2268  written like this (ignore whitespace and
2268  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
2269  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2270  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
2271  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2272  .P  pattern uses references to the named group to match the four dot-separated
2273  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.  
2274  .  .
2275  .SS "Assertion conditions"  .SS "Assertion conditions"
2276  .rs  .rs
# Line 1624  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2295  dd-aaa-dd or dd-dd-dd, where aaa are let
2295  .SH COMMENTS  .SH COMMENTS
2296  .rs  .rs
2297  .sp  .sp
2298  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
2299  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,
2300  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
2301    subpattern name or number. The characters that make up a comment play no part
2302    in the pattern matching.
2303  .P  .P
2304  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
2305  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2306  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2307    this case continues to immediately after the next newline character or
2308    character sequence in the pattern. Which characters are interpreted as newlines
2309    is controlled by the options passed to a compiling function or by a special
2310    sequence at the start of the pattern, as described in the section entitled
2311    .\" HTML <a href="#newlines">
2312    .\" </a>
2313    "Newline conventions"
2314    .\"
2315    above. Note that the end of this type of comment is a literal newline sequence
2316    in the pattern; escape sequences that happen to represent a newline do not
2317    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2318    default newline convention is in force:
2319    .sp
2320      abc #comment \en still comment
2321    .sp
2322    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2323    a newline in the pattern. The sequence \en is still literal at this stage, so
2324    it does not terminate the comment. Only an actual character with the code value
2325    0x0a (the default newline) does so.
2326  .  .
2327  .  .
2328  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1656  recursively to the pattern in which it a Line 2348  recursively to the pattern in which it a
2348  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2349  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2350  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2351  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.
2352  .P  .P
2353  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
2354  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
2355  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
2356    .\" HTML <a href="#subpatternsassubroutines">
2357    .\" </a>
2358    non-recursive subroutine
2359    .\"
2360  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
2361  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2362  .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  
2363  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2364  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2365  .sp  .sp
2366    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2367  .sp  .sp
2368  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2369  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
2370  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2371  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2372    to avoid backtracking into sequences of non-parentheses.
2373  .P  .P
2374  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
2375  pattern, so instead you could use this:  pattern, so instead you could use this:
2376  .sp  .sp
2377    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2378  .sp  .sp
2379  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
2380  them instead of the whole pattern.  them instead of the whole pattern.
2381  .P  .P
2382  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
2383  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
2384  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
2385  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2386  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.  
2387  .P  .P
2388  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2389  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2390  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2391  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2392    .\" </a>
2393    non-recursive subroutine
2394    .\"
2395    calls, as described in the next section.
2396  .P  .P
2397  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2398  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
2399  could rewrite the above example as follows:  could rewrite the above example as follows:
2400  .sp  .sp
2401    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2402  .sp  .sp
2403  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
2404  used.  used.
2405  .P  .P
2406  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2407  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
2408  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
2409  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2410  .sp  .sp
2411    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2412  .sp  .sp
2413  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,
2414  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
2415  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
2416  before failure can be reported.  before failure can be reported.
2417  .P  .P
2418  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
2419  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
2420  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  
2421  .\" HREF  .\" HREF
2422  \fBpcrecallout\fP  \fBpcrecallout\fP
2423  .\"  .\"
# Line 1731  documentation). If the pattern above is Line 2425  documentation). If the pattern above is
2425  .sp  .sp
2426    (ab(cd)ef)    (ab(cd)ef)
2427  .sp  .sp
2428  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
2429  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
2430  .sp  matched at the top level, its final captured value is unset, even if it was
2431    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2432       ^                        ^  .P
2433       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2434  .sp  obtain extra memory to store data during a recursion, which it does by using
2435  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
2436  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.  
2437  .P  .P
2438  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.
2439  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1756  different alternatives for the recursive Line 2447  different alternatives for the recursive
2447  is the actual recursive call.  is the actual recursive call.
2448  .  .
2449  .  .
2450    .\" HTML <a name="recursiondifference"></a>
2451    .SS "Differences in recursion processing between PCRE and Perl"
2452    .rs
2453    .sp
2454    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2455    (like Python, but unlike Perl), a recursive subpattern call is always treated
2456    as an atomic group. That is, once it has matched some of the subject string, it
2457    is never re-entered, even if it contains untried alternatives and there is a
2458    subsequent matching failure. This can be illustrated by the following pattern,
2459    which purports to match a palindromic string that contains an odd number of
2460    characters (for example, "a", "aba", "abcba", "abcdcba"):
2461    .sp
2462      ^(.|(.)(?1)\e2)$
2463    .sp
2464    The idea is that it either matches a single character, or two identical
2465    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2466    it does not if the pattern is longer than three characters. Consider the
2467    subject string "abcba":
2468    .P
2469    At the top level, the first character is matched, but as it is not at the end
2470    of the string, the first alternative fails; the second alternative is taken
2471    and the recursion kicks in. The recursive call to subpattern 1 successfully
2472    matches the next character ("b"). (Note that the beginning and end of line
2473    tests are not part of the recursion).
2474    .P
2475    Back at the top level, the next character ("c") is compared with what
2476    subpattern 2 matched, which was "a". This fails. Because the recursion is
2477    treated as an atomic group, there are now no backtracking points, and so the
2478    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2479    try the second alternative.) However, if the pattern is written with the
2480    alternatives in the other order, things are different:
2481    .sp
2482      ^((.)(?1)\e2|.)$
2483    .sp
2484    This time, the recursing alternative is tried first, and continues to recurse
2485    until it runs out of characters, at which point the recursion fails. But this
2486    time we do have another alternative to try at the higher level. That is the big
2487    difference: in the previous case the remaining alternative is at a deeper
2488    recursion level, which PCRE cannot use.
2489    .P
2490    To change the pattern so that it matches all palindromic strings, not just
2491    those with an odd number of characters, it is tempting to change the pattern to
2492    this:
2493    .sp
2494      ^((.)(?1)\e2|.?)$
2495    .sp
2496    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2497    deeper recursion has matched a single character, it cannot be entered again in
2498    order to match an empty string. The solution is to separate the two cases, and
2499    write out the odd and even cases as alternatives at the higher level:
2500    .sp
2501      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2502    .sp
2503    If you want to match typical palindromic phrases, the pattern has to ignore all
2504    non-word characters, which can be done like this:
2505    .sp
2506      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2507    .sp
2508    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2509    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2510    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2511    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2512    more) to match typical phrases, and Perl takes so long that you think it has
2513    gone into a loop.
2514    .P
2515    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2516    string does not start with a palindrome that is shorter than the entire string.
2517    For example, although "abcba" is correctly matched, if the subject is "ababa",
2518    PCRE finds the palindrome "aba" at the start, then fails at top level because
2519    the end of the string does not follow. Once again, it cannot jump back into the
2520    recursion to try other alternatives, so the entire match fails.
2521    .P
2522    The second way in which PCRE and Perl differ in their recursion processing is
2523    in the handling of captured values. In Perl, when a subpattern is called
2524    recursively or as a subpattern (see the next section), it has no access to any
2525    values that were captured outside the recursion, whereas in PCRE these values
2526    can be referenced. Consider this pattern:
2527    .sp
2528      ^(.)(\e1|a(?2))
2529    .sp
2530    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2531    then in the second group, when the back reference \e1 fails to match "b", the
2532    second alternative matches "a" and then recurses. In the recursion, \e1 does
2533    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2534    match because inside the recursive call \e1 cannot access the externally set
2535    value.
2536    .
2537    .
2538  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2539  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2540  .rs  .rs
2541  .sp  .sp
2542  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
2543  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
2544  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2545  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2546  relative, as in these examples:  relative, as in these examples:
2547  .sp  .sp
2548    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2549    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2550    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2551  .sp  .sp
2552  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2553  .sp  .sp
# Line 1782  matches "sense and sensibility" and "res Line 2561  matches "sense and sensibility" and "res
2561  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
2562  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2563  .P  .P
2564  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2565  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
2566  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
2567  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2568  .P  subroutine call revert to their previous values afterwards.
2569  When a subpattern is used as a subroutine, processing options such as  .P
2570  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2571  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2572    different calls. For example, consider this pattern:
2573  .sp  .sp
2574    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2575  .sp  .sp
# Line 1797  It matches "abcabc". It does not match " Line 2577  It matches "abcabc". It does not match "
2577  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2578  .  .
2579  .  .
2580    .\" HTML <a name="onigurumasubroutines"></a>
2581    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2582    .rs
2583    .sp
2584    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2585    a number enclosed either in angle brackets or single quotes, is an alternative
2586    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2587    are two of the examples used above, rewritten using this syntax:
2588    .sp
2589      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2590      (sens|respons)e and \eg'1'ibility
2591    .sp
2592    PCRE supports an extension to Oniguruma: if a number is preceded by a
2593    plus or a minus sign it is taken as a relative reference. For example:
2594    .sp
2595      (abc)(?i:\eg<-1>)
2596    .sp
2597    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2598    synonymous. The former is a back reference; the latter is a subroutine call.
2599    .
2600    .
2601  .SH CALLOUTS  .SH CALLOUTS
2602  .rs  .rs
2603  .sp  .sp
# Line 1807  same pair of parentheses when there is a Line 2608  same pair of parentheses when there is a
2608  .P  .P
2609  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
2610  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
2611  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
2612    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2613  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2614  .P  .P
2615  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
# Line 1817  For example, this pattern has two callou Line 2619  For example, this pattern has two callou
2619  .sp  .sp
2620    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2621  .sp  .sp
2622  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
2623  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2624  255.  255.
2625  .P  .P
2626  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
2627  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
2628  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
2629  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2630  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
2631  description of the interface to the callout function is given in the  the callout function is given in the
2632  .\" HREF  .\" HREF
2633  \fBpcrecallout\fP  \fBpcrecallout\fP
2634  .\"  .\"
2635  documentation.  documentation.
2636  .  .
2637  .  .
2638    .\" HTML <a name="backtrackcontrol"></a>
2639    .SH "BACKTRACKING CONTROL"
2640    .rs
2641    .sp
2642    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2643    are described in the Perl documentation as "experimental and subject to change
2644    or removal in a future version of Perl". It goes on to say: "Their usage in
2645    production code should be noted to avoid problems during upgrades." The same
2646    remarks apply to the PCRE features described in this section.
2647    .P
2648    Since these verbs are specifically related to backtracking, most of them can be
2649    used only when the pattern is to be matched using one of the traditional
2650    matching functions, which use a backtracking algorithm. With the exception of
2651    (*FAIL), which behaves like a failing negative assertion, they cause an error
2652    if encountered by a DFA matching function.
2653    .P
2654    If any of these verbs are used in an assertion or in a subpattern that is
2655    called as a subroutine (whether or not recursively), their effect is confined
2656    to that subpattern; it does not extend to the surrounding pattern, with one
2657    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2658    a successful positive assertion \fIis\fP passed back when a match succeeds
2659    (compare capturing parentheses in assertions). Note that such subpatterns are
2660    processed as anchored at the point where they are tested. Note also that Perl's
2661    treatment of subroutines and assertions is different in some cases.
2662    .P
2663    The new verbs make use of what was previously invalid syntax: an opening
2664    parenthesis followed by an asterisk. They are generally of the form
2665    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2666    depending on whether or not an argument is present. A name is any sequence of
2667    characters that does not include a closing parenthesis. The maximum length of
2668    name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit library.
2669    If the name is empty, that is, if the closing parenthesis immediately follows
2670    the colon, the effect is as if the colon were not there. Any number of these
2671    verbs may occur in a pattern.
2672    .
2673    .
2674    .\" HTML <a name="nooptimize"></a>
2675    .SS "Optimizations that affect backtracking verbs"
2676    .rs
2677    .sp
2678    PCRE contains some optimizations that are used to speed up matching by running
2679    some checks at the start of each match attempt. For example, it may know the
2680    minimum length of matching subject, or that a particular character must be
2681    present. When one of these optimizations suppresses the running of a match, any
2682    included backtracking verbs will not, of course, be processed. You can suppress
2683    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2684    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2685    pattern with (*NO_START_OPT). There is more discussion of this option in the
2686    section entitled
2687    .\" HTML <a href="pcreapi.html#execoptions">
2688    .\" </a>
2689    "Option bits for \fBpcre_exec()\fP"
2690    .\"
2691    in the
2692    .\" HREF
2693    \fBpcreapi\fP
2694    .\"
2695    documentation.
2696    .P
2697    Experiments with Perl suggest that it too has similar optimizations, sometimes
2698    leading to anomalous results.
2699    .
2700    .
2701    .SS "Verbs that act immediately"
2702    .rs
2703    .sp
2704    The following verbs act as soon as they are encountered. They may not be
2705    followed by a name.
2706    .sp
2707       (*ACCEPT)
2708    .sp
2709    This verb causes the match to end successfully, skipping the remainder of the
2710    pattern. However, when it is inside a subpattern that is called as a
2711    subroutine, only that subpattern is ended successfully. Matching then continues
2712    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2713    far is captured. For example:
2714    .sp
2715      A((?:A|B(*ACCEPT)|C)D)
2716    .sp
2717    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2718    the outer parentheses.
2719    .sp
2720      (*FAIL) or (*F)
2721    .sp
2722    This verb causes a matching failure, forcing backtracking to occur. It is
2723    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2724    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2725    Perl features that are not present in PCRE. The nearest equivalent is the
2726    callout feature, as for example in this pattern:
2727    .sp
2728      a+(?C)(*FAIL)
2729    .sp
2730    A match with the string "aaaa" always fails, but the callout is taken before
2731    each backtrack happens (in this example, 10 times).
2732    .
2733    .
2734    .SS "Recording which path was taken"
2735    .rs
2736    .sp
2737    There is one verb whose main purpose is to track how a match was arrived at,
2738    though it also has a secondary use in conjunction with advancing the match
2739    starting point (see (*SKIP) below).
2740    .sp
2741      (*MARK:NAME) or (*:NAME)
2742    .sp
2743    A name is always required with this verb. There may be as many instances of
2744    (*MARK) as you like in a pattern, and their names do not have to be unique.
2745    .P
2746    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2747    path is passed back to the caller as described in the section entitled
2748    .\" HTML <a href="pcreapi.html#extradata">
2749    .\" </a>
2750    "Extra data for \fBpcre_exec()\fP"
2751    .\"
2752    in the
2753    .\" HREF
2754    \fBpcreapi\fP
2755    .\"
2756    documentation. Here is an example of \fBpcretest\fP output, where the /K
2757    modifier requests the retrieval and outputting of (*MARK) data:
2758    .sp
2759        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2760      data> XY
2761       0: XY
2762      MK: A
2763      XZ
2764       0: XZ
2765      MK: B
2766    .sp
2767    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2768    indicates which of the two alternatives matched. This is a more efficient way
2769    of obtaining this information than putting each alternative in its own
2770    capturing parentheses.
2771    .P
2772    If (*MARK) is encountered in a positive assertion, its name is recorded and
2773    passed back if it is the last-encountered. This does not happen for negative
2774    assertions.
2775    .P
2776    After a partial match or a failed match, the name of the last encountered
2777    (*MARK) in the entire match process is returned. For example:
2778    .sp
2779        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2780      data> XP
2781      No match, mark = B
2782    .sp
2783    Note that in this unanchored example the mark is retained from the match
2784    attempt that started at the letter "X" in the subject. Subsequent match
2785    attempts starting at "P" and then with an empty string do not get as far as the
2786    (*MARK) item, but nevertheless do not reset it.
2787    .P
2788    If you are interested in (*MARK) values after failed matches, you should
2789    probably set the PCRE_NO_START_OPTIMIZE option
2790    .\" HTML <a href="#nooptimize">
2791    .\" </a>
2792    (see above)
2793    .\"
2794    to ensure that the match is always attempted.
2795    .
2796    .
2797    .SS "Verbs that act after backtracking"
2798    .rs
2799    .sp
2800    The following verbs do nothing when they are encountered. Matching continues
2801    with what follows, but if there is no subsequent match, causing a backtrack to
2802    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2803    the verb. However, when one of these verbs appears inside an atomic group, its
2804    effect is confined to that group, because once the group has been matched,
2805    there is never any backtracking into it. In this situation, backtracking can
2806    "jump back" to the left of the entire atomic group. (Remember also, as stated
2807    above, that this localization also applies in subroutine calls and assertions.)
2808    .P
2809    These verbs differ in exactly what kind of failure occurs when backtracking
2810    reaches them.
2811    .sp
2812      (*COMMIT)
2813    .sp
2814    This verb, which may not be followed by a name, causes the whole match to fail
2815    outright if the rest of the pattern does not match. Even if the pattern is
2816    unanchored, no further attempts to find a match by advancing the starting point
2817    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2818    finding a match at the current starting point, or not at all. For example:
2819    .sp
2820      a+(*COMMIT)b
2821    .sp
2822    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2823    dynamic anchor, or "I've started, so I must finish." The name of the most
2824    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2825    match failure.
2826    .P
2827    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2828    unless PCRE's start-of-match optimizations are turned off, as shown in this
2829    \fBpcretest\fP example:
2830    .sp
2831        re> /(*COMMIT)abc/
2832      data> xyzabc
2833       0: abc
2834      xyzabc\eY
2835      No match
2836    .sp
2837    PCRE knows that any match must start with "a", so the optimization skips along
2838    the subject to "a" before running the first match attempt, which succeeds. When
2839    the optimization is disabled by the \eY escape in the second subject, the match
2840    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2841    starting points.
2842    .sp
2843      (*PRUNE) or (*PRUNE:NAME)
2844    .sp
2845    This verb causes the match to fail at the current starting position in the
2846    subject if the rest of the pattern does not match. If the pattern is
2847    unanchored, the normal "bumpalong" advance to the next starting character then
2848    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2849    reached, or when matching to the right of (*PRUNE), but if there is no match to
2850    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2851    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2852    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2853    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2854    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2855    .sp
2856      (*SKIP)
2857    .sp
2858    This verb, when given without a name, is like (*PRUNE), except that if the
2859    pattern is unanchored, the "bumpalong" advance is not to the next character,
2860    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2861    signifies that whatever text was matched leading up to it cannot be part of a
2862    successful match. Consider:
2863    .sp
2864      a+(*SKIP)b
2865    .sp
2866    If the subject is "aaaac...", after the first match attempt fails (starting at
2867    the first character in the string), the starting point skips on to start the
2868    next attempt at "c". Note that a possessive quantifer does not have the same
2869    effect as this example; although it would suppress backtracking during the
2870    first match attempt, the second attempt would start at the second character
2871    instead of skipping on to "c".
2872    .sp
2873      (*SKIP:NAME)
2874    .sp
2875    When (*SKIP) has an associated name, its behaviour is modified. If the
2876    following pattern fails to match, the previous path through the pattern is
2877    searched for the most recent (*MARK) that has the same name. If one is found,
2878    the "bumpalong" advance is to the subject position that corresponds to that
2879    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2880    matching name is found, the (*SKIP) is ignored.
2881    .sp
2882      (*THEN) or (*THEN:NAME)
2883    .sp
2884    This verb causes a skip to the next innermost alternative if the rest of the
2885    pattern does not match. That is, it cancels pending backtracking, but only
2886    within the current alternative. Its name comes from the observation that it can
2887    be used for a pattern-based if-then-else block:
2888    .sp
2889      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2890    .sp
2891    If the COND1 pattern matches, FOO is tried (and possibly further items after
2892    the end of the group if FOO succeeds); on failure, the matcher skips to the
2893    second alternative and tries COND2, without backtracking into COND1. The
2894    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2895    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2896    .P
2897    Note that a subpattern that does not contain a | character is just a part of
2898    the enclosing alternative; it is not a nested alternation with only one
2899    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2900    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2901    pattern fragments that do not contain any | characters at this level:
2902    .sp
2903      A (B(*THEN)C) | D
2904    .sp
2905    If A and B are matched, but there is a failure in C, matching does not
2906    backtrack into A; instead it moves to the next alternative, that is, D.
2907    However, if the subpattern containing (*THEN) is given an alternative, it
2908    behaves differently:
2909    .sp
2910      A (B(*THEN)C | (*FAIL)) | D
2911    .sp
2912    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2913    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2914    because there are no more alternatives to try. In this case, matching does now
2915    backtrack into A.
2916    .P
2917    Note also that a conditional subpattern is not considered as having two
2918    alternatives, because only one is ever used. In other words, the | character in
2919    a conditional subpattern has a different meaning. Ignoring white space,
2920    consider:
2921    .sp
2922      ^.*? (?(?=a) a | b(*THEN)c )
2923    .sp
2924    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2925    it initially matches zero characters. The condition (?=a) then fails, the
2926    character "b" is matched, but "c" is not. At this point, matching does not
2927    backtrack to .*? as might perhaps be expected from the presence of the |
2928    character. The conditional subpattern is part of the single alternative that
2929    comprises the whole pattern, and so the match fails. (If there was a backtrack
2930    into .*?, allowing it to match "b", the match would succeed.)
2931    .P
2932    The verbs just described provide four different "strengths" of control when
2933    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2934    next alternative. (*PRUNE) comes next, failing the match at the current
2935    starting position, but allowing an advance to the next character (for an
2936    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2937    than one character. (*COMMIT) is the strongest, causing the entire match to
2938    fail.
2939    .P
2940    If more than one such verb is present in a pattern, the "strongest" one wins.
2941    For example, consider this pattern, where A, B, etc. are complex pattern
2942    fragments:
2943    .sp
2944      (A(*COMMIT)B(*THEN)C|D)
2945    .sp
2946    Once A has matched, PCRE is committed to this match, at the current starting
2947    position. If subsequently B matches, but C does not, the normal (*THEN) action
2948    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2949    overrides.
2950    .
2951    .
2952  .SH "SEE ALSO"  .SH "SEE ALSO"
2953  .rs  .rs
2954  .sp  .sp
2955  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2956    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
2957  .  .
2958  .  .
2959  .SH AUTHOR  .SH AUTHOR
# Line 1853  Cambridge CB2 3QH, England. Line 2970  Cambridge CB2 3QH, England.
2970  .rs  .rs
2971  .sp  .sp
2972  .nf  .nf
2973  Last updated: 09 May 2007  Last updated: 10 September 2012
2974  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2975  .fi  .fi

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