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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "07 November 2012" "PCRE 8.32"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 9  are described in detail below. There is Line 9  are described in detail below. There is
9  .\" HREF  .\" HREF
10  \fBpcresyntax\fP  \fBpcresyntax\fP
11  .\"  .\"
12  page. Perl's regular expressions are described in its own documentation, and  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18  regular expressions in general are covered in a number of books, some of which  regular expressions in general are covered in a number of books, some of which
19  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 strings in the original library, 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. Some of the features discussed below are not available when  which match using a different algorithm that is not Perl-compatible. Some of
65  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  the features discussed below are not available when DFA matching is used. The
66  alternative function, and how it differs from the normal function, are  advantages and disadvantages of the alternative functions, and how they differ
67  discussed in the  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 where the circumflex and dollar assertions are
124    true. It also affects the interpretation of the dot metacharacter when
125    PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
126    what the \eR escape sequence matches. By default, this is any Unicode newline
127    sequence, for Perl compatibility. However, this can be changed; see the
128    description of \eR in the section entitled
129    .\" HTML <a href="#newlineseq">
130    .\" </a>
131    "Newline sequences"
132    .\"
133    below. A change of \eR setting can be combined with a change of newline
134    convention.
135    .
136    .
137  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
138  .rs  .rs
139  .sp  .sp
# Line 55  corresponding characters in the subject. Line 145  corresponding characters in the subject.
145  .sp  .sp
146  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
147  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
148  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
149  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
150  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
151  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
152  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
153  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
154  UTF-8 support.  UTF support.
155  .P  .P
156  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
157  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 107  The following sections describe the use Line 197  The following sections describe the use
197  .rs  .rs
198  .sp  .sp
199  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
200  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
201  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
202  outside character classes.  both inside and outside character classes.
203  .P  .P
204  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.
205  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 117  otherwise be interpreted as a metacharac Line 207  otherwise be interpreted as a metacharac
207  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
208  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
209  .P  .P
210  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
211    backslash. All other characters (in particular, those whose codepoints are
212    greater than 127) are treated as literals.
213    .P
214    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
215  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
216  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
217  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.
218  .P  .P
219  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
220  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 136  Perl, $ and @ cause variable interpolati Line 230  Perl, $ and @ cause variable interpolati
230    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
231  .sp  .sp
232  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
233    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
234    by \eE later in the pattern, the literal interpretation continues to the end of
235    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
236    a character class, this causes an error, because the character class is not
237    terminated.
238  .  .
239  .  .
240  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 145  The \eQ...\eE sequence is recognized bot Line 244  The \eQ...\eE sequence is recognized bot
244  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
245  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
246  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
247  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
248  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:  
249  .sp  .sp
250    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
251    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
252    \ee        escape (hex 1B)    \ee        escape (hex 1B)
253    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
254    \en        newline (hex 0A)    \en        linefeed (hex 0A)
255    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
256    \et        tab (hex 09)    \et        tab (hex 09)
257    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
258    \exhh      character with hex code hh    \exhh      character with hex code hh
259    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
260      \euhhhh    character with hex code hhhh (JavaScript mode only)
261  .sp  .sp
262  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
263  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
264  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),
265  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
266  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
267  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.
268  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
269  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
270  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  extension to Unicode it is even less useful than it once was. It is, however,
271  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  recognized when PCRE is compiled in EBCDIC mode, where data items are always
272  point, which is 10FFFF.  bytes. In this mode, all values are valid after \ec. If the next character is a
273    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
274    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
275    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
276    characters also generate different values.
277    .P
278    By default, after \ex, from zero to two hexadecimal digits are read (letters
279    can be in upper or lower case). Any number of hexadecimal digits may appear
280    between \ex{ and }, but the character code is constrained as follows:
281    .sp
282      8-bit non-UTF mode    less than 0x100
283      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
284      16-bit non-UTF mode   less than 0x10000
285      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
286      32-bit non-UTF mode   less than 0x80000000
287      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
288    .sp
289    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
290    "surrogate" codepoints), and 0xffef.
291  .P  .P
292  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
293  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
294  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
295  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
296  .P  .P
297    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
298    as just described only when it is followed by two hexadecimal digits.
299    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
300    code points greater than 256 is provided by \eu, which must be followed by
301    four hexadecimal digits; otherwise it matches a literal "u" character.
302    Character codes specified by \eu in JavaScript mode are constrained in the same
303    was as those specified by \ex in non-JavaScript mode.
304    .P
305  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
306  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
307  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
308    \eu00dc in JavaScript mode).
309  .P  .P
310  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
311  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 205  parenthesized subpatterns. Line 331  parenthesized subpatterns.
331  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
332  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
333  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
334  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
335  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.
336  to \e777 are permitted. For example:  For example:
337  .sp  .sp
338    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
339  .\" JOIN  .\" JOIN
340    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
341              previous capturing subpatterns              previous capturing subpatterns
# Line 224  to \e777 are permitted. For example: Line 350  to \e777 are permitted. For example:
350              character with octal code 113              character with octal code 113
351  .\" JOIN  .\" JOIN
352    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
353              the byte consisting entirely of 1 bits              the value 255 (decimal)
354  .\" JOIN  .\" JOIN
355    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
356              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 233  Note that octal values of 100 or greater Line 359  Note that octal values of 100 or greater
359  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
360  .P  .P
361  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
362  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
363  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
364  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
365  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
366  meanings  inside a character class. Like other unrecognized escape sequences, they are
367  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
368  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
369  (see below).  sequences have different meanings.
370  .\"  .
371    .
372    .SS "Unsupported escape sequences"
373    .rs
374    .sp
375    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
376    handler and used to modify the case of following characters. By default, PCRE
377    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
378    option is set, \eU matches a "U" character, and \eu can be used to define a
379    character by code point, as described in the previous section.
380  .  .
381  .  .
382  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 261  parenthesized subpatterns. Line 396  parenthesized subpatterns.
396  .\"  .\"
397  .  .
398  .  .
399    .SS "Absolute and relative subroutine calls"
400    .rs
401    .sp
402    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
403    a number enclosed either in angle brackets or single quotes, is an alternative
404    syntax for referencing a subpattern as a "subroutine". Details are discussed
405    .\" HTML <a href="#onigurumasubroutines">
406    .\" </a>
407    later.
408    .\"
409    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
410    synonymous. The former is a back reference; the latter is a
411    .\" HTML <a href="#subpatternsassubroutines">
412    .\" </a>
413    subroutine
414    .\"
415    call.
416    .
417    .
418    .\" HTML <a name="genericchartypes"></a>
419  .SS "Generic character types"  .SS "Generic character types"
420  .rs  .rs
421  .sp  .sp
422  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:  
423  .sp  .sp
424    \ed     any decimal digit    \ed     any decimal digit
425    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
426    \eh     any horizontal whitespace character    \eh     any horizontal white space character
427    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
428    \es     any whitespace character    \es     any white space character
429    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
430    \ev     any vertical whitespace character    \ev     any vertical white space character
431    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
432    \ew     any "word" character    \ew     any "word" character
433    \eW     any "non-word" character    \eW     any "non-word" character
434  .sp  .sp
435  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.
436  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
437  .P  .\" HTML <a href="#fullstopdot">
438  These character type sequences can appear both inside and outside character  .\" </a>
439    the "." metacharacter
440    .\"
441    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
442    PCRE does not support this.
443    .P
444    Each pair of lower and upper case escape sequences partitions the complete set
445    of characters into two disjoint sets. Any given character matches one, and only
446    one, of each pair. The sequences can appear both inside and outside character
447  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
448  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
449  there is no character to match.  there is no character to match.
450  .P  .P
451  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 292  are HT (9), LF (10), FF (12), CR (13), a Line 454  are HT (9), LF (10), FF (12), CR (13), a
454  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
455  does.  does.
456  .P  .P
457  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
458  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
459  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
460  meanings from before UTF-8 support was available, mainly for efficiency  place (see
461  reasons.  .\" HTML <a href="pcreapi.html#localesupport">
462  .P  .\" </a>
463  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  "Locale support"
464  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\"
465  The horizontal space characters are:  in the
466    .\" HREF
467    \fBpcreapi\fP
468    .\"
469    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
470    or "french" in Windows, some character codes greater than 128 are used for
471    accented letters, and these are then matched by \ew. The use of locales with
472    Unicode is discouraged.
473    .P
474    By default, in a UTF mode, characters with values greater than 128 never match
475    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
476    their original meanings from before UTF support was available, mainly for
477    efficiency reasons. However, if PCRE is compiled with Unicode property support,
478    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
479    properties are used to determine character types, as follows:
480    .sp
481      \ed  any character that \ep{Nd} matches (decimal digit)
482      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
483      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
484    .sp
485    The upper case escapes match the inverse sets of characters. Note that \ed
486    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
487    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
488    \eB because they are defined in terms of \ew and \eW. Matching these sequences
489    is noticeably slower when PCRE_UCP is set.
490    .P
491    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
492    release 5.10. In contrast to the other sequences, which match only ASCII
493    characters by default, these always match certain high-valued codepoints,
494    whether or not PCRE_UCP is set. The horizontal space characters are:
495  .sp  .sp
496    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
497    U+0020     Space    U+0020     Space
498    U+00A0     Non-break space    U+00A0     Non-break space
499    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 324  The horizontal space characters are: Line 515  The horizontal space characters are:
515  .sp  .sp
516  The vertical space characters are:  The vertical space characters are:
517  .sp  .sp
518    U+000A     Linefeed    U+000A     Linefeed (LF)
519    U+000B     Vertical tab    U+000B     Vertical tab (VT)
520    U+000C     Formfeed    U+000C     Form feed (FF)
521    U+000D     Carriage return    U+000D     Carriage return (CR)
522    U+0085     Next line    U+0085     Next line (NEL)
523    U+2028     Line separator    U+2028     Line separator
524    U+2029     Paragraph separator    U+2029     Paragraph separator
525  .P  .sp
526  A "word" character is an underscore or any character less than 256 that is a  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
527  letter or digit. The definition of letters and digits is controlled by PCRE's  relevant.
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
528  .  .
529  .  .
530    .\" HTML <a name="newlineseq"></a>
531  .SS "Newline sequences"  .SS "Newline sequences"
532  .rs  .rs
533  .sp  .sp
534  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
535  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
536  the following:  following:
537  .sp  .sp
538    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
539  .sp  .sp
# Line 366  below. Line 544  below.
544  .\"  .\"
545  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
546  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,
547  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
548  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
549  cannot be split.  cannot be split.
550  .P  .P
551  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
552  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).
553  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
554  recognized.  recognized.
555  .P  .P
556  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
557    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
558    either at compile time or when the pattern is matched. (BSR is an abbrevation
559    for "backslash R".) This can be made the default when PCRE is built; if this is
560    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
561    It is also possible to specify these settings by starting a pattern string with
562    one of the following sequences:
563    .sp
564      (*BSR_ANYCRLF)   CR, LF, or CRLF only
565      (*BSR_UNICODE)   any Unicode newline sequence
566    .sp
567    These override the default and the options given to the compiling function, but
568    they can themselves be overridden by options given to a matching function. Note
569    that these special settings, which are not Perl-compatible, are recognized only
570    at the very start of a pattern, and that they must be in upper case. If more
571    than one of them is present, the last one is used. They can be combined with a
572    change of newline convention; for example, a pattern can start with:
573    .sp
574      (*ANY)(*BSR_ANYCRLF)
575    .sp
576    They can also be combined with the (*UTF8), (*UTF16), (*UTF32) or (*UCP) special
577    sequences. Inside a character class, \eR is treated as an unrecognized escape
578    sequence, and so matches the letter "R" by default, but causes an error if
579    PCRE_EXTRA is set.
580  .  .
581  .  .
582  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 384  Inside a character class, \eR matches th Line 585  Inside a character class, \eR matches th
585  .sp  .sp
586  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
587  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
588  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
589  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
590  The extra escape sequences are:  The extra escape sequences are:
591  .sp  .sp
592    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
593    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
594    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
595  .sp  .sp
596  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
597  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
598  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
599  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
600  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
601    .\" </a>
602    next section).
603    .\"
604    Other Perl properties such as "InMusicalSymbols" are not currently supported by
605    PCRE. Note that \eP{Any} does not match any characters, so always causes a
606    match failure.
607  .P  .P
608  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
609  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 410  Those that are not part of an identified Line 617  Those that are not part of an identified
617  .P  .P
618  Arabic,  Arabic,
619  Armenian,  Armenian,
620    Avestan,
621  Balinese,  Balinese,
622    Bamum,
623    Batak,
624  Bengali,  Bengali,
625  Bopomofo,  Bopomofo,
626    Brahmi,
627  Braille,  Braille,
628  Buginese,  Buginese,
629  Buhid,  Buhid,
630  Canadian_Aboriginal,  Canadian_Aboriginal,
631    Carian,
632    Chakma,
633    Cham,
634  Cherokee,  Cherokee,
635  Common,  Common,
636  Coptic,  Coptic,
# Line 425  Cypriot, Line 639  Cypriot,
639  Cyrillic,  Cyrillic,
640  Deseret,  Deseret,
641  Devanagari,  Devanagari,
642    Egyptian_Hieroglyphs,
643  Ethiopic,  Ethiopic,
644  Georgian,  Georgian,
645  Glagolitic,  Glagolitic,
# Line 437  Hangul, Line 652  Hangul,
652  Hanunoo,  Hanunoo,
653  Hebrew,  Hebrew,
654  Hiragana,  Hiragana,
655    Imperial_Aramaic,
656  Inherited,  Inherited,
657    Inscriptional_Pahlavi,
658    Inscriptional_Parthian,
659    Javanese,
660    Kaithi,
661  Kannada,  Kannada,
662  Katakana,  Katakana,
663    Kayah_Li,
664  Kharoshthi,  Kharoshthi,
665  Khmer,  Khmer,
666  Lao,  Lao,
667  Latin,  Latin,
668    Lepcha,
669  Limbu,  Limbu,
670  Linear_B,  Linear_B,
671    Lisu,
672    Lycian,
673    Lydian,
674  Malayalam,  Malayalam,
675    Mandaic,
676    Meetei_Mayek,
677    Meroitic_Cursive,
678    Meroitic_Hieroglyphs,
679    Miao,
680  Mongolian,  Mongolian,
681  Myanmar,  Myanmar,
682  New_Tai_Lue,  New_Tai_Lue,
# Line 454  Nko, Line 684  Nko,
684  Ogham,  Ogham,
685  Old_Italic,  Old_Italic,
686  Old_Persian,  Old_Persian,
687    Old_South_Arabian,
688    Old_Turkic,
689    Ol_Chiki,
690  Oriya,  Oriya,
691  Osmanya,  Osmanya,
692  Phags_Pa,  Phags_Pa,
693  Phoenician,  Phoenician,
694    Rejang,
695  Runic,  Runic,
696    Samaritan,
697    Saurashtra,
698    Sharada,
699  Shavian,  Shavian,
700  Sinhala,  Sinhala,
701    Sora_Sompeng,
702    Sundanese,
703  Syloti_Nagri,  Syloti_Nagri,
704  Syriac,  Syriac,
705  Tagalog,  Tagalog,
706  Tagbanwa,  Tagbanwa,
707  Tai_Le,  Tai_Le,
708    Tai_Tham,
709    Tai_Viet,
710    Takri,
711  Tamil,  Tamil,
712  Telugu,  Telugu,
713  Thaana,  Thaana,
# Line 473  Thai, Line 715  Thai,
715  Tibetan,  Tibetan,
716  Tifinagh,  Tifinagh,
717  Ugaritic,  Ugaritic,
718    Vai,
719  Yi.  Yi.
720  .P  .P
721  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
722  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
723  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
724  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
725  .P  .P
726  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
727  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 539  the Lu, Ll, or Lt property, in other wor Line 782  the Lu, Ll, or Lt property, in other wor
782  a modifier or "other".  a modifier or "other".
783  .P  .P
784  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
785  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
786  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
787  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
788    PCRE_NO_UTF32_CHECK in the
789  .\" HREF  .\" HREF
790  \fBpcreapi\fP  \fBpcreapi\fP
791  .\"  .\"
792  page).  page). Perl does not support the Cs property.
793  .P  .P
794  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
795  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
796  properties with "Is".  properties with "Is".
797  .P  .P
# Line 558  Unicode table. Line 802  Unicode table.
802  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
803  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
804  .P  .P
805  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
806  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
807  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
808    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
809    PCRE_UCP option or by starting the pattern with (*UCP).
810    .
811    .
812    .SS Extended grapheme clusters
813    .rs
814  .sp  .sp
815  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
816  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  
817  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
818  .\" </a>  .\" </a>
819  (see below).  (see below).
820  .\"  .\"
821  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
822  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
823  non-UTF-8 mode \eX matches any one character.  .sp
824  .P    (?>\ePM\epM*)
825  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
826  a structure that contains data for over fifteen thousand characters. That is  That is, it matched a character without the "mark" property, followed by zero
827  why the traditional escape sequences such as \ed and \ew do not use Unicode  or more characters with the "mark" property. Characters with the "mark"
828  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
829    .P
830    This simple definition was extended in Unicode to include more complicated
831    kinds of composite character by giving each character a grapheme breaking
832    property, and creating rules that use these properties to define the boundaries
833    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
834    one of these clusters.
835    .P
836    \eX always matches at least one character. Then it decides whether to add
837    additional characters according to the following rules for ending a cluster:
838    .P
839    1. End at the end of the subject string.
840    .P
841    2. Do not end between CR and LF; otherwise end after any control character.
842    .P
843    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
844    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
845    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
846    character; an LVT or T character may be follwed only by a T character.
847    .P
848    4. Do not end before extending characters or spacing marks. Characters with
849    the "mark" property always have the "extend" grapheme breaking property.
850    .P
851    5. Do not end after prepend characters.
852    .P
853    6. Otherwise, end the cluster.
854    .
855    .
856    .\" HTML <a name="extraprops"></a>
857    .SS PCRE's additional properties
858    .rs
859    .sp
860    As well as the standard Unicode properties described above, PCRE supports four
861    more that make it possible to convert traditional escape sequences such as \ew
862    and \es and POSIX character classes to use Unicode properties. PCRE uses these
863    non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
864    .sp
865      Xan   Any alphanumeric character
866      Xps   Any POSIX space character
867      Xsp   Any Perl space character
868      Xwd   Any Perl "word" character
869    .sp
870    Xan matches characters that have either the L (letter) or the N (number)
871    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
872    carriage return, and any other character that has the Z (separator) property.
873    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
874    same characters as Xan, plus underscore.
875  .  .
876  .  .
877  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
878  .SS "Resetting the match start"  .SS "Resetting the match start"
879  .rs  .rs
880  .sp  .sp
881  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
882  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
883  .sp  .sp
884    foo\eKbar    foo\eKbar
885  .sp  .sp
# Line 608  For example, when the pattern Line 901  For example, when the pattern
901    (foo)\eKbar    (foo)\eKbar
902  .sp  .sp
903  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
904    .P
905    Perl documents that the use of \eK within assertions is "not well defined". In
906    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
907    ignored in negative assertions.
908  .  .
909  .  .
910  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 632  The backslashed assertions are: Line 929  The backslashed assertions are:
929    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
930    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
931  .sp  .sp
932  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
933  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
934    default it matches the corresponding literal character (for example, \eB
935    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
936    escape sequence" error is generated instead.
937  .P  .P
938  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
939  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
940  \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
941  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
942    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
943    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
944    of word" or "end of word" metasequence. However, whatever follows \eb normally
945    determines which it is. For example, the fragment \eba matches "a" at the start
946    of a word.
947  .P  .P
948  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
949  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 672  regular expression. Line 977  regular expression.
977  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
978  .rs  .rs
979  .sp  .sp
980    The circumflex and dollar metacharacters are zero-width assertions. That is,
981    they test for a particular condition being true without consuming any
982    characters from the subject string.
983    .P
984  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
985  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
986  at the start of the subject string. If the \fIstartoffset\fP argument of  the start of the subject string. If the \fIstartoffset\fP argument of
987  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
988  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
989  meaning  meaning
# Line 691  constrained to match only at the start o Line 1000  constrained to match only at the start o
1000  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1001  to be anchored.)  to be anchored.)
1002  .P  .P
1003  A dollar character is an assertion that is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1004  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1005  at the end of the string (by default). Dollar need not be the last character of  the end of the string (by default). Note, however, that it does not actually
1006  the pattern if a number of alternatives are involved, but it should be the last  match the newline. Dollar need not be the last character of the pattern if a
1007  item in any branch in which it appears. Dollar has no special meaning in a  number of alternatives are involved, but it should be the last item in any
1008  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1009  .P  .P
1010  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1011  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
# Line 722  end of the subject in both modes, and if Line 1031  end of the subject in both modes, and if
1031  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1032  .  .
1033  .  .
1034  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1035    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1036  .rs  .rs
1037  .sp  .sp
1038  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
1039  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
1040  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1041  .P  .P
1042  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
1043  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 744  to match it. Line 1054  to match it.
1054  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
1055  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
1056  special meaning in a character class.  special meaning in a character class.
1057  .  .P
1058  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1059  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1060  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1061  .sp  name; PCRE does not support this.
1062  Outside a character class, the escape sequence \eC matches any one byte, both  .
1063  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1064  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1065  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1066  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1067  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1068    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1069    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1070    a 32-bit unit. Unlike a dot, \eC always
1071    matches line-ending characters. The feature is provided in Perl in order to
1072    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1073    used. Because \eC breaks up characters into individual data units, matching one
1074    unit with \eC in a UTF mode means that the rest of the string may start with a
1075    malformed UTF character. This has undefined results, because PCRE assumes that
1076    it is dealing with valid UTF strings (and by default it checks this at the
1077    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1078    PCRE_NO_UTF32_CHECK option is used).
1079  .P  .P
1080  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1081  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1082  .\" </a>  .\" </a>
1083  (described below),  (described below)
1084  .\"  .\"
1085  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
1086  the lookbehind.  the lookbehind.
1087    .P
1088    In general, the \eC escape sequence is best avoided. However, one
1089    way of using it that avoids the problem of malformed UTF characters is to use a
1090    lookahead to check the length of the next character, as in this pattern, which
1091    could be used with a UTF-8 string (ignore white space and line breaks):
1092    .sp
1093      (?| (?=[\ex00-\ex7f])(\eC) |
1094          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1095          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1096          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1097    .sp
1098    A group that starts with (?| resets the capturing parentheses numbers in each
1099    alternative (see
1100    .\" HTML <a href="#dupsubpatternnumber">
1101    .\" </a>
1102    "Duplicate Subpattern Numbers"
1103    .\"
1104    below). The assertions at the start of each branch check the next UTF-8
1105    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1106    character's individual bytes are then captured by the appropriate number of
1107    groups.
1108  .  .
1109  .  .
1110  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 770  the lookbehind. Line 1112  the lookbehind.
1112  .rs  .rs
1113  .sp  .sp
1114  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1115  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.
1116  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
1117  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
1118  escaped with a backslash.  a member of the class, it should be the first data character in the class
1119  .P  (after an initial circumflex, if present) or escaped with a backslash.
1120  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1121  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
1122  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
1123  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
1124  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
1125  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
1126    member of the class, ensure it is not the first character, or escape it with a
1127  backslash.  backslash.
1128  .P  .P
1129  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1130  [^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
1131  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1132  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
1133  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
1134  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
1135  string.  string.
1136  .P  .P
1137  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)
1138  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
1139    \ex{ escaping mechanism.
1140  .P  .P
1141  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
1142  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
1143  "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
1144  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
1145  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
1146  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1147  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1148  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
1149  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
1150  UTF-8 support.  well as with UTF support.
1151  .P  .P
1152  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
1153  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 825  followed by two other characters. The oc Line 1169  followed by two other characters. The oc
1169  "]" can also be used to end a range.  "]" can also be used to end a range.
1170  .P  .P
1171  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
1172  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1173  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}].  
1174  .P  .P
1175  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
1176  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
1177  [][\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
1178  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
1179  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
1180  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
1181  property support.  property support.
1182  .P  .P
1183  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,
1184  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
1185  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1186  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
1187  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
1188  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1189    .\" HTML <a href="#genericchartypes">
1190    .\" </a>
1191    "Generic character types"
1192    .\"
1193    above. The escape sequence \eb has a different meaning inside a character
1194    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1195    are not special inside a character class. Like any other unrecognized escape
1196    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1197    default, but cause an error if the PCRE_EXTRA option is set.
1198    .P
1199    A circumflex can conveniently be used with the upper case character types to
1200    specify a more restricted set of characters than the matching lower case type.
1201    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1202    whereas [\ew] includes underscore. A positive character class should be read as
1203    "something OR something OR ..." and a negative class as "NOT something AND NOT
1204    something AND NOT ...".
1205  .P  .P
1206  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1207  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 862  this notation. For example, Line 1221  this notation. For example,
1221    [01[:alpha:]%]    [01[:alpha:]%]
1222  .sp  .sp
1223  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1224  are  are:
1225  .sp  .sp
1226    alnum    letters and digits    alnum    letters and digits
1227    alpha    letters    alpha    letters
# Line 873  are Line 1232  are
1232    graph    printing characters, excluding space    graph    printing characters, excluding space
1233    lower    lower case letters    lower    lower case letters
1234    print    printing characters, including space    print    printing characters, including space
1235    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1236    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1237    upper    upper case letters    upper    upper case letters
1238    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 894  matches "1", "2", or any non-digit. PCRE Line 1253  matches "1", "2", or any non-digit. PCRE
1253  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
1254  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1255  .P  .P
1256  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
1257  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1258    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1259    character properties are used. This is achieved by replacing the POSIX classes
1260    by other sequences, as follows:
1261    .sp
1262      [:alnum:]  becomes  \ep{Xan}
1263      [:alpha:]  becomes  \ep{L}
1264      [:blank:]  becomes  \eh
1265      [:digit:]  becomes  \ep{Nd}
1266      [:lower:]  becomes  \ep{Ll}
1267      [:space:]  becomes  \ep{Xps}
1268      [:upper:]  becomes  \ep{Lu}
1269      [:word:]   becomes  \ep{Xwd}
1270    .sp
1271    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1272    classes are unchanged, and match only characters with code points less than
1273    128.
1274  .  .
1275  .  .
1276  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 922  alternative in the subpattern. Line 1297  alternative in the subpattern.
1297  .rs  .rs
1298  .sp  .sp
1299  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1300  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
1301  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1302    The option letters are
1303  .sp  .sp
1304    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1305    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 937  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1313  PCRE_MULTILINE while unsetting PCRE_DOTA
1313  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
1314  unset.  unset.
1315  .P  .P
1316  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
1317  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
1318  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1319  the global options (and it will therefore show up in data extracted by the  .P
1320  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1321    subpattern parentheses), the change applies to the remainder of the pattern
1322    that follows. If the change is placed right at the start of a pattern, PCRE
1323    extracts it into the global options (and it will therefore show up in data
1324    extracted by the \fBpcre_fullinfo()\fP function).
1325  .P  .P
1326  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1327  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
1328  .sp  .sp
1329    (a(?i)b)c    (a(?i)b)c
1330  .sp  .sp
# Line 960  branch is abandoned before the option se Line 1340  branch is abandoned before the option se
1340  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1341  behaviour otherwise.  behaviour otherwise.
1342  .P  .P
1343  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
1344  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
1345  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1346    what the application has set or what has been defaulted. Details are given in
1347    the section entitled
1348    .\" HTML <a href="#newlineseq">
1349    .\" </a>
1350    "Newline sequences"
1351    .\"
1352    above. There are also the (*UTF8), (*UTF16),(*UTF32) and (*UCP) leading
1353    sequences that can be used to set UTF and Unicode property modes; they are
1354    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1355    options, respectively.
1356  .  .
1357  .  .
1358  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 976  Turning part of a pattern into a subpatt Line 1366  Turning part of a pattern into a subpatt
1366  .sp  .sp
1367    cat(aract|erpillar|)    cat(aract|erpillar|)
1368  .sp  .sp
1369  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1370  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1371  .sp  .sp
1372  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
1373  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
1374  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
1375  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1376  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1377  .P  .P
1378  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
1379    numbers for the capturing subpatterns. For example, if the string "the red
1380    king" is matched against the pattern
1381  .sp  .sp
1382    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1383  .sp  .sp
# Line 1017  is reached, an option setting in one bra Line 1409  is reached, an option setting in one bra
1409  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1410  .  .
1411  .  .
1412    .\" HTML <a name="dupsubpatternnumber"></a>
1413  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1414  .rs  .rs
1415  .sp  .sp
# Line 1033  at captured substring number one, whiche Line 1426  at captured substring number one, whiche
1426  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1427  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1428  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1429  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1430  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1431  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1432  .sp  .sp
1433    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1434    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1435    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1436  .sp  .sp
1437  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1438  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1439    or "defdef":
1440    .sp
1441      /(?|(abc)|(def))\e1/
1442    .sp
1443    In contrast, a subroutine call to a numbered subpattern always refers to the
1444    first one in the pattern with the given number. The following pattern matches
1445    "abcabc" or "defabc":
1446    .sp
1447      /(?|(abc)|(def))(?1)/
1448    .sp
1449    If a
1450    .\" HTML <a href="#conditions">
1451    .\" </a>
1452    condition test
1453    .\"
1454    for a subpattern's having matched refers to a non-unique number, the test is
1455    true if any of the subpatterns of that number have matched.
1456  .P  .P
1457  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1458  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1058  if an expression is modified, the number Line 1467  if an expression is modified, the number
1467  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1468  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
1469  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
1470  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1471    have different names, but PCRE does not.
1472  .P  .P
1473  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
1474  (?'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
1475  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1476  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1477  .\" </a>  .\" </a>
1478  backreferences,  back references,
1479  .\"  .\"
1480  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1481  .\" </a>  .\" </a>
# Line 1085  extracting the name-to-number translatio Line 1495  extracting the name-to-number translatio
1495  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1496  .P  .P
1497  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
1498  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
1499  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
1500  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
1501  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
1502  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
1503    name, and in both cases you want to extract the abbreviation. This pattern
1504    (ignoring the line breaks) does the job:
1505  .sp  .sp
1506    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1507    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1103  subpattern, as described in the previous Line 1515  subpattern, as described in the previous
1515  .P  .P
1516  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1517  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
1518  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1519  make a reference to a non-unique named subpattern from elsewhere in the  .P
1520  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
1521  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1522    used. In the absence of duplicate numbers (see the previous section) this is
1523    the one with the lowest number. If you use a named reference in a condition
1524    test (see the
1525    .\"
1526    .\" HTML <a href="#conditions">
1527    .\" </a>
1528    section about conditions
1529    .\"
1530    below), either to check whether a subpattern has matched, or to check for
1531    recursion, all subpatterns with the same name are tested. If the condition is
1532    true for any one of them, the overall condition is true. This is the same
1533    behaviour as testing by number. For further details of the interfaces for
1534    handling named subpatterns, see the
1535  .\" HREF  .\" HREF
1536  \fBpcreapi\fP  \fBpcreapi\fP
1537  .\"  .\"
1538  documentation.  documentation.
1539    .P
1540    \fBWarning:\fP You cannot use different names to distinguish between two
1541    subpatterns with the same number because PCRE uses only the numbers when
1542    matching. For this reason, an error is given at compile time if different names
1543    are given to subpatterns with the same number. However, you can give the same
1544    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1545  .  .
1546  .  .
1547  .SH REPETITION  .SH REPETITION
# Line 1122  items: Line 1553  items:
1553    a literal data character    a literal data character
1554    the dot metacharacter    the dot metacharacter
1555    the \eC escape sequence    the \eC escape sequence
1556    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1557    the \eR escape sequence    the \eR escape sequence
1558    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1559    a character class    a character class
1560    a back reference (see next section)    a back reference (see next section)
1561    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1562      a subroutine call to a subpattern (recursive or otherwise)
1563  .sp  .sp
1564  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1565  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1152  where a quantifier is not allowed, or on Line 1584  where a quantifier is not allowed, or on
1584  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
1585  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1586  .P  .P
1587  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
1588  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
1589  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,
1590  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
1591  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1592  .P  .P
1593  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
1594  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1595    subpatterns that are referenced as
1596    .\" HTML <a href="#subpatternsassubroutines">
1597    .\" </a>
1598    subroutines
1599    .\"
1600    from elsewhere in the pattern (but see also the section entitled
1601    .\" HTML <a href="#subdefine">
1602    .\" </a>
1603    "Defining subpatterns for use by reference only"
1604    .\"
1605    below). Items other than subpatterns that have a {0} quantifier are omitted
1606    from the compiled pattern.
1607  .P  .P
1608  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1609  abbreviations:  abbreviations:
# Line 1230  In cases where it is known that the subj Line 1674  In cases where it is known that the subj
1674  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1675  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1676  .P  .P
1677  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1678  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1679  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
1680  succeeds. Consider, for example:  succeeds. Consider, for example:
1681  .sp  .sp
# Line 1240  succeeds. Consider, for example: Line 1684  succeeds. Consider, for example:
1684  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
1685  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1686  .P  .P
1687    Another case where implicit anchoring is not applied is when the leading .* is
1688    inside an atomic group. Once again, a match at the start may fail where a later
1689    one succeeds. Consider this pattern:
1690    .sp
1691      (?>.*?a)b
1692    .sp
1693    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1694    (*PRUNE) and (*SKIP) also disable this optimization.
1695    .P
1696  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1697  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1698  .sp  .sp
# Line 1384  no such problem when named parentheses a Line 1837  no such problem when named parentheses a
1837  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1838  .P  .P
1839  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
1840  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
1841  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1842  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1843  .sp  .sp
1844    (ring), \e1    (ring), \e1
1845    (ring), \eg1    (ring), \eg1
# Line 1400  example: Line 1853  example:
1853    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1854  .sp  .sp
1855  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
1856  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.
1857  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
1858  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
1859  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1860  .P  .P
1861  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1862  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1442  after the reference. Line 1895  after the reference.
1895  .P  .P
1896  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
1897  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1898  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1899  .sp  .sp
1900    (a|(bc))\e2    (a|(bc))\e2
1901  .sp  .sp
1902  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
1903  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
1904  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1905  with a digit character, some delimiter must be used to terminate the back  .P
1906  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1907  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1908    If the pattern continues with a digit character, some delimiter must be used to
1909    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1910    white space. Otherwise, the \eg{ syntax or an empty comment (see
1911  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1912  .\" </a>  .\" </a>
1913  "Comments"  "Comments"
1914  .\"  .\"
1915  below) can be used.  below) can be used.
1916  .P  .
1917    .SS "Recursive back references"
1918    .rs
1919    .sp
1920  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
1921  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.
1922  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1471  to the previous iteration. In order for Line 1930  to the previous iteration. In order for
1930  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
1931  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
1932  minimum of zero.  minimum of zero.
1933    .P
1934    Back references of this type cause the group that they reference to be treated
1935    as an
1936    .\" HTML <a href="#atomicgroup">
1937    .\" </a>
1938    atomic group.
1939    .\"
1940    Once the whole group has been matched, a subsequent matching failure cannot
1941    cause backtracking into the middle of the group.
1942  .  .
1943  .  .
1944  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1490  those that look ahead of the current pos Line 1958  those that look ahead of the current pos
1958  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,
1959  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.
1960  .P  .P
1961  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1962  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
1963  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1964  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1965  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1966  because it does not make sense for negative assertions.  .P
1967    For compatibility with Perl, assertion subpatterns may be repeated; though
1968    it makes no sense to assert the same thing several times, the side effect of
1969    capturing parentheses may occasionally be useful. In practice, there only three
1970    cases:
1971    .sp
1972    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1973    However, it may contain internal capturing parenthesized groups that are called
1974    from elsewhere via the
1975    .\" HTML <a href="#subpatternsassubroutines">
1976    .\" </a>
1977    subroutine mechanism.
1978    .\"
1979    .sp
1980    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1981    were {0,1}. At run time, the rest of the pattern match is tried with and
1982    without the assertion, the order depending on the greediness of the quantifier.
1983    .sp
1984    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1985    The assertion is obeyed just once when encountered during matching.
1986  .  .
1987  .  .
1988  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1524  lookbehind assertion is needed to achiev Line 2011  lookbehind assertion is needed to achiev
2011  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
2012  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
2013  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.
2014    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2015  .  .
2016  .  .
2017  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1548  is permitted, but Line 2036  is permitted, but
2036  .sp  .sp
2037  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2038  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
2039  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
2040  match the same length of string. An assertion such as  length of string. An assertion such as
2041  .sp  .sp
2042    (?<=ab(c|de))    (?<=ab(c|de))
2043  .sp  .sp
2044  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
2045  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
2046    branches:
2047  .sp  .sp
2048    (?<=abc|abde)    (?<=abc|abde)
2049  .sp  .sp
2050  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2051  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2052  .\" </a>  .\" </a>
2053  (see above)  (see above)
2054  .\"  .\"
2055  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
2056  fixed-length.  restriction.
2057  .P  .P
2058  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2059  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
2060  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2061  assertion fails.  assertion fails.
2062  .P  .P
2063  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
2064  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
2065  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
2066  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2067    permitted.
2068    .P
2069    .\" HTML <a href="#subpatternsassubroutines">
2070    .\" </a>
2071    "Subroutine"
2072    .\"
2073    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2074    as the subpattern matches a fixed-length string.
2075    .\" HTML <a href="#recursion">
2076    .\" </a>
2077    Recursion,
2078    .\"
2079    however, is not supported.
2080  .P  .P
2081  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2082  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
2083  pattern such as  strings. Consider a simple pattern such as
2084  .sp  .sp
2085    abcd$    abcd$
2086  .sp  .sp
# Line 1642  characters that are not "999". Line 2144  characters that are not "999".
2144  .sp  .sp
2145  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2146  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2147  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2148  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2149  .sp  .sp
2150    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2151    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2152  .sp  .sp
2153  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2154  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
2155  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2156    itself contain nested subpatterns of any form, including conditional
2157    subpatterns; the restriction to two alternatives applies only at the level of
2158    the condition. This pattern fragment is an example where the alternatives are
2159    complex:
2160    .sp
2161      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2162    .sp
2163  .P  .P
2164  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2165  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1659  recursion, a pseudo-condition called DEF Line 2168  recursion, a pseudo-condition called DEF
2168  .rs  .rs
2169  .sp  .sp
2170  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
2171  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2172  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
2173  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2174  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2175  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2176  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2177    section about duplicate subpattern numbers),
2178    .\"
2179    the condition is true if any of them have matched. An alternative notation is
2180    to precede the digits with a plus or minus sign. In this case, the subpattern
2181    number is relative rather than absolute. The most recently opened parentheses
2182    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2183    loops it can also make sense to refer to subsequent groups. The next
2184    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2185    zero in any of these forms is not used; it provokes a compile-time error.)
2186  .P  .P
2187  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2188  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1675  three parts for ease of discussion: Line 2193  three parts for ease of discussion:
2193  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2194  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
2195  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
2196  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2197  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,
2198  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
2199  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2200  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1705  Rewriting the above example to use a nam Line 2223  Rewriting the above example to use a nam
2223  .sp  .sp
2224    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2225  .sp  .sp
2226    If the name used in a condition of this kind is a duplicate, the test is
2227    applied to all subpatterns of the same name, and is true if any one of them has
2228    matched.
2229  .  .
2230  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2231  .rs  .rs
# Line 1716  letter R, for example: Line 2237  letter R, for example:
2237  .sp  .sp
2238    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2239  .sp  .sp
2240  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
2241  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
2242  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2243    applied to all subpatterns of the same name, and is true if any one of them is
2244    the most recent recursion.
2245  .P  .P
2246  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2247  patterns are described below.  .\" HTML <a href="#recursion">
2248    .\" </a>
2249    The syntax for recursive patterns
2250    .\"
2251    is described below.
2252  .  .
2253    .\" HTML <a name="subdefine"></a>
2254  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2255  .rs  .rs
2256  .sp  .sp
# Line 1730  If the condition is the string (DEFINE), Line 2258  If the condition is the string (DEFINE),
2258  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
2259  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2260  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
2261  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2262  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2263  written like this (ignore whitespace and line breaks):  .\" </a>
2264    subroutines
2265    .\"
2266    is described below.) For example, a pattern to match an IPv4 address such as
2267    "192.168.23.245" could be written like this (ignore white space and line
2268    breaks):
2269  .sp  .sp
2270    (?(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) )
2271    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1740  written like this (ignore whitespace and Line 2273  written like this (ignore whitespace and
2273  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
2274  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2275  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
2276  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2277  .P  pattern uses references to the named group to match the four dot-separated
2278  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.  
2279  .  .
2280  .SS "Assertion conditions"  .SS "Assertion conditions"
2281  .rs  .rs
# Line 1769  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2300  dd-aaa-dd or dd-dd-dd, where aaa are let
2300  .SH COMMENTS  .SH COMMENTS
2301  .rs  .rs
2302  .sp  .sp
2303  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
2304  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,
2305  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
2306    subpattern name or number. The characters that make up a comment play no part
2307    in the pattern matching.
2308  .P  .P
2309  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
2310  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2311  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2312    this case continues to immediately after the next newline character or
2313    character sequence in the pattern. Which characters are interpreted as newlines
2314    is controlled by the options passed to a compiling function or by a special
2315    sequence at the start of the pattern, as described in the section entitled
2316    .\" HTML <a href="#newlines">
2317    .\" </a>
2318    "Newline conventions"
2319    .\"
2320    above. Note that the end of this type of comment is a literal newline sequence
2321    in the pattern; escape sequences that happen to represent a newline do not
2322    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2323    default newline convention is in force:
2324    .sp
2325      abc #comment \en still comment
2326    .sp
2327    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2328    a newline in the pattern. The sequence \en is still literal at this stage, so
2329    it does not terminate the comment. Only an actual character with the code value
2330    0x0a (the default newline) does so.
2331  .  .
2332  .  .
2333  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1801  recursively to the pattern in which it a Line 2353  recursively to the pattern in which it a
2353  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2354  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2355  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2356  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.
2357  .P  .P
2358  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
2359  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
2360  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
2361    .\" HTML <a href="#subpatternsassubroutines">
2362    .\" </a>
2363    non-recursive subroutine
2364    .\"
2365  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
2366  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2367  .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  
2368  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2369  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2370  .sp  .sp
2371    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2372  .sp  .sp
2373  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2374  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
2375  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2376  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2377    to avoid backtracking into sequences of non-parentheses.
2378  .P  .P
2379  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
2380  pattern, so instead you could use this:  pattern, so instead you could use this:
2381  .sp  .sp
2382    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2383  .sp  .sp
2384  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
2385  them instead of the whole pattern.  them instead of the whole pattern.
2386  .P  .P
2387  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
2388  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
2389  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
2390  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2391  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.  
2392  .P  .P
2393  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2394  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2395  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2396  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2397    .\" </a>
2398    non-recursive subroutine
2399    .\"
2400    calls, as described in the next section.
2401  .P  .P
2402  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2403  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
2404  could rewrite the above example as follows:  could rewrite the above example as follows:
2405  .sp  .sp
2406    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2407  .sp  .sp
2408  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
2409  used.  used.
2410  .P  .P
2411  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2412  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
2413  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
2414  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2415  .sp  .sp
2416    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2417  .sp  .sp
2418  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,
2419  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
2420  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
2421  before failure can be reported.  before failure can be reported.
2422  .P  .P
2423  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
2424  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
2425  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  
2426  .\" HREF  .\" HREF
2427  \fBpcrecallout\fP  \fBpcrecallout\fP
2428  .\"  .\"
# Line 1876  documentation). If the pattern above is Line 2430  documentation). If the pattern above is
2430  .sp  .sp
2431    (ab(cd)ef)    (ab(cd)ef)
2432  .sp  .sp
2433  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
2434  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
2435  .sp  matched at the top level, its final captured value is unset, even if it was
2436    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2437       ^                        ^  .P
2438       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2439  .sp  obtain extra memory to store data during a recursion, which it does by using
2440  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
2441  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.  
2442  .P  .P
2443  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.
2444  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1901  different alternatives for the recursive Line 2452  different alternatives for the recursive
2452  is the actual recursive call.  is the actual recursive call.
2453  .  .
2454  .  .
2455    .\" HTML <a name="recursiondifference"></a>
2456    .SS "Differences in recursion processing between PCRE and Perl"
2457    .rs
2458    .sp
2459    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2460    (like Python, but unlike Perl), a recursive subpattern call is always treated
2461    as an atomic group. That is, once it has matched some of the subject string, it
2462    is never re-entered, even if it contains untried alternatives and there is a
2463    subsequent matching failure. This can be illustrated by the following pattern,
2464    which purports to match a palindromic string that contains an odd number of
2465    characters (for example, "a", "aba", "abcba", "abcdcba"):
2466    .sp
2467      ^(.|(.)(?1)\e2)$
2468    .sp
2469    The idea is that it either matches a single character, or two identical
2470    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2471    it does not if the pattern is longer than three characters. Consider the
2472    subject string "abcba":
2473    .P
2474    At the top level, the first character is matched, but as it is not at the end
2475    of the string, the first alternative fails; the second alternative is taken
2476    and the recursion kicks in. The recursive call to subpattern 1 successfully
2477    matches the next character ("b"). (Note that the beginning and end of line
2478    tests are not part of the recursion).
2479    .P
2480    Back at the top level, the next character ("c") is compared with what
2481    subpattern 2 matched, which was "a". This fails. Because the recursion is
2482    treated as an atomic group, there are now no backtracking points, and so the
2483    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2484    try the second alternative.) However, if the pattern is written with the
2485    alternatives in the other order, things are different:
2486    .sp
2487      ^((.)(?1)\e2|.)$
2488    .sp
2489    This time, the recursing alternative is tried first, and continues to recurse
2490    until it runs out of characters, at which point the recursion fails. But this
2491    time we do have another alternative to try at the higher level. That is the big
2492    difference: in the previous case the remaining alternative is at a deeper
2493    recursion level, which PCRE cannot use.
2494    .P
2495    To change the pattern so that it matches all palindromic strings, not just
2496    those with an odd number of characters, it is tempting to change the pattern to
2497    this:
2498    .sp
2499      ^((.)(?1)\e2|.?)$
2500    .sp
2501    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2502    deeper recursion has matched a single character, it cannot be entered again in
2503    order to match an empty string. The solution is to separate the two cases, and
2504    write out the odd and even cases as alternatives at the higher level:
2505    .sp
2506      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2507    .sp
2508    If you want to match typical palindromic phrases, the pattern has to ignore all
2509    non-word characters, which can be done like this:
2510    .sp
2511      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2512    .sp
2513    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2514    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2515    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2516    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2517    more) to match typical phrases, and Perl takes so long that you think it has
2518    gone into a loop.
2519    .P
2520    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2521    string does not start with a palindrome that is shorter than the entire string.
2522    For example, although "abcba" is correctly matched, if the subject is "ababa",
2523    PCRE finds the palindrome "aba" at the start, then fails at top level because
2524    the end of the string does not follow. Once again, it cannot jump back into the
2525    recursion to try other alternatives, so the entire match fails.
2526    .P
2527    The second way in which PCRE and Perl differ in their recursion processing is
2528    in the handling of captured values. In Perl, when a subpattern is called
2529    recursively or as a subpattern (see the next section), it has no access to any
2530    values that were captured outside the recursion, whereas in PCRE these values
2531    can be referenced. Consider this pattern:
2532    .sp
2533      ^(.)(\e1|a(?2))
2534    .sp
2535    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2536    then in the second group, when the back reference \e1 fails to match "b", the
2537    second alternative matches "a" and then recurses. In the recursion, \e1 does
2538    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2539    match because inside the recursive call \e1 cannot access the externally set
2540    value.
2541    .
2542    .
2543  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2544  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2545  .rs  .rs
2546  .sp  .sp
2547  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
2548  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
2549  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2550  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2551  relative, as in these examples:  relative, as in these examples:
2552  .sp  .sp
# Line 1927  matches "sense and sensibility" and "res Line 2566  matches "sense and sensibility" and "res
2566  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
2567  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2568  .P  .P
2569  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2570  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
2571  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
2572  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2573  .P  subroutine call revert to their previous values afterwards.
2574  When a subpattern is used as a subroutine, processing options such as  .P
2575  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2576  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2577    different calls. For example, consider this pattern:
2578  .sp  .sp
2579    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2580  .sp  .sp
# Line 1942  It matches "abcabc". It does not match " Line 2582  It matches "abcabc". It does not match "
2582  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2583  .  .
2584  .  .
2585    .\" HTML <a name="onigurumasubroutines"></a>
2586    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2587    .rs
2588    .sp
2589    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2590    a number enclosed either in angle brackets or single quotes, is an alternative
2591    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2592    are two of the examples used above, rewritten using this syntax:
2593    .sp
2594      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2595      (sens|respons)e and \eg'1'ibility
2596    .sp
2597    PCRE supports an extension to Oniguruma: if a number is preceded by a
2598    plus or a minus sign it is taken as a relative reference. For example:
2599    .sp
2600      (abc)(?i:\eg<-1>)
2601    .sp
2602    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2603    synonymous. The former is a back reference; the latter is a subroutine call.
2604    .
2605    .
2606  .SH CALLOUTS  .SH CALLOUTS
2607  .rs  .rs
2608  .sp  .sp
# Line 1952  same pair of parentheses when there is a Line 2613  same pair of parentheses when there is a
2613  .P  .P
2614  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
2615  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
2616  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
2617    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2618  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2619  .P  .P
2620  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 1962  For example, this pattern has two callou Line 2624  For example, this pattern has two callou
2624  .sp  .sp
2625    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2626  .sp  .sp
2627  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
2628  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2629  255.  255.
2630  .P  .P
2631  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
2632  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
2633  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
2634  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2635  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
2636  description of the interface to the callout function is given in the  the callout function is given in the
2637  .\" HREF  .\" HREF
2638  \fBpcrecallout\fP  \fBpcrecallout\fP
2639  .\"  .\"
2640  documentation.  documentation.
2641  .  .
2642  .  .
2643  .SH "BACTRACKING CONTROL"  .\" HTML <a name="backtrackcontrol"></a>
2644    .SH "BACKTRACKING CONTROL"
2645  .rs  .rs
2646  .sp  .sp
2647  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
# Line 1987  or removal in a future version of Perl". Line 2650  or removal in a future version of Perl".
2650  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2651  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2652  .P  .P
2653  Since these verbs are specifically related to backtracking, they can be used  Since these verbs are specifically related to backtracking, most of them can be
2654  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  used only when the pattern is to be matched using one of the traditional
2655  backtracking algorithm. They cause an error if encountered by  matching functions, which use a backtracking algorithm. With the exception of
2656  \fBpcre_dfa_exec()\fP.  (*FAIL), which behaves like a failing negative assertion, they cause an error
2657    if encountered by a DFA matching function.
2658    .P
2659    If any of these verbs are used in an assertion or in a subpattern that is
2660    called as a subroutine (whether or not recursively), their effect is confined
2661    to that subpattern; it does not extend to the surrounding pattern, with one
2662    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2663    a successful positive assertion \fIis\fP passed back when a match succeeds
2664    (compare capturing parentheses in assertions). Note that such subpatterns are
2665    processed as anchored at the point where they are tested. Note also that Perl's
2666    treatment of subroutines and assertions is different in some cases.
2667  .P  .P
2668  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2669  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2670  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2671  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2672  are two kinds:  characters that does not include a closing parenthesis. The maximum length of
2673    name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit library.
2674    If the name is empty, that is, if the closing parenthesis immediately follows
2675    the colon, the effect is as if the colon were not there. Any number of these
2676    verbs may occur in a pattern.
2677    .
2678    .
2679    .\" HTML <a name="nooptimize"></a>
2680    .SS "Optimizations that affect backtracking verbs"
2681    .rs
2682    .sp
2683    PCRE contains some optimizations that are used to speed up matching by running
2684    some checks at the start of each match attempt. For example, it may know the
2685    minimum length of matching subject, or that a particular character must be
2686    present. When one of these optimizations suppresses the running of a match, any
2687    included backtracking verbs will not, of course, be processed. You can suppress
2688    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2689    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2690    pattern with (*NO_START_OPT). There is more discussion of this option in the
2691    section entitled
2692    .\" HTML <a href="pcreapi.html#execoptions">
2693    .\" </a>
2694    "Option bits for \fBpcre_exec()\fP"
2695    .\"
2696    in the
2697    .\" HREF
2698    \fBpcreapi\fP
2699    .\"
2700    documentation.
2701    .P
2702    Experiments with Perl suggest that it too has similar optimizations, sometimes
2703    leading to anomalous results.
2704    .
2705  .  .
2706  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2707  .rs  .rs
2708  .sp  .sp
2709  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2710    followed by a name.
2711  .sp  .sp
2712     (*ACCEPT)     (*ACCEPT)
2713  .sp  .sp
2714  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2715  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2716  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2717  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2718  captured. For example:  far is captured. For example:
2719  .sp  .sp
2720    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2721  .sp  .sp
2722  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2723  captured.  the outer parentheses.
2724  .sp  .sp
2725    (*FAIL) or (*F)    (*FAIL) or (*F)
2726  .sp  .sp
2727  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2728  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2729  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2730  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2029  callout feature, as for example in this Line 2735  callout feature, as for example in this
2735  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2736  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2737  .  .
2738    .
2739    .SS "Recording which path was taken"
2740    .rs
2741    .sp
2742    There is one verb whose main purpose is to track how a match was arrived at,
2743    though it also has a secondary use in conjunction with advancing the match
2744    starting point (see (*SKIP) below).
2745    .sp
2746      (*MARK:NAME) or (*:NAME)
2747    .sp
2748    A name is always required with this verb. There may be as many instances of
2749    (*MARK) as you like in a pattern, and their names do not have to be unique.
2750    .P
2751    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2752    path is passed back to the caller as described in the section entitled
2753    .\" HTML <a href="pcreapi.html#extradata">
2754    .\" </a>
2755    "Extra data for \fBpcre_exec()\fP"
2756    .\"
2757    in the
2758    .\" HREF
2759    \fBpcreapi\fP
2760    .\"
2761    documentation. Here is an example of \fBpcretest\fP output, where the /K
2762    modifier requests the retrieval and outputting of (*MARK) data:
2763    .sp
2764        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2765      data> XY
2766       0: XY
2767      MK: A
2768      XZ
2769       0: XZ
2770      MK: B
2771    .sp
2772    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2773    indicates which of the two alternatives matched. This is a more efficient way
2774    of obtaining this information than putting each alternative in its own
2775    capturing parentheses.
2776    .P
2777    If (*MARK) is encountered in a positive assertion, its name is recorded and
2778    passed back if it is the last-encountered. This does not happen for negative
2779    assertions.
2780    .P
2781    After a partial match or a failed match, the name of the last encountered
2782    (*MARK) in the entire match process is returned. For example:
2783    .sp
2784        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2785      data> XP
2786      No match, mark = B
2787    .sp
2788    Note that in this unanchored example the mark is retained from the match
2789    attempt that started at the letter "X" in the subject. Subsequent match
2790    attempts starting at "P" and then with an empty string do not get as far as the
2791    (*MARK) item, but nevertheless do not reset it.
2792    .P
2793    If you are interested in (*MARK) values after failed matches, you should
2794    probably set the PCRE_NO_START_OPTIMIZE option
2795    .\" HTML <a href="#nooptimize">
2796    .\" </a>
2797    (see above)
2798    .\"
2799    to ensure that the match is always attempted.
2800    .
2801    .
2802  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2803  .rs  .rs
2804  .sp  .sp
2805  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2806  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2807  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2808    the verb. However, when one of these verbs appears inside an atomic group, its
2809    effect is confined to that group, because once the group has been matched,
2810    there is never any backtracking into it. In this situation, backtracking can
2811    "jump back" to the left of the entire atomic group. (Remember also, as stated
2812    above, that this localization also applies in subroutine calls and assertions.)
2813    .P
2814    These verbs differ in exactly what kind of failure occurs when backtracking
2815    reaches them.
2816  .sp  .sp
2817    (*COMMIT)    (*COMMIT)
2818  .sp  .sp
2819  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2820  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2821  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2822  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2823  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2824  .sp  .sp
2825    a+(*COMMIT)b    a+(*COMMIT)b
2826  .sp  .sp
2827  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2828  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2829  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2830    (*PRUNE)  match failure.
2831  .sp  .P
2832  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2833  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2834  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2835  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2836  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2837  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2838  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2839  be expressed in any other way.    xyzabc\eY
2840      No match
2841    .sp
2842    PCRE knows that any match must start with "a", so the optimization skips along
2843    the subject to "a" before running the first match attempt, which succeeds. When
2844    the optimization is disabled by the \eY escape in the second subject, the match
2845    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2846    starting points.
2847    .sp
2848      (*PRUNE) or (*PRUNE:NAME)
2849    .sp
2850    This verb causes the match to fail at the current starting position in the
2851    subject if the rest of the pattern does not match. If the pattern is
2852    unanchored, the normal "bumpalong" advance to the next starting character then
2853    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2854    reached, or when matching to the right of (*PRUNE), but if there is no match to
2855    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2856    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2857    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2858    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2859    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2860  .sp  .sp
2861    (*SKIP)    (*SKIP)
2862  .sp  .sp
2863  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2864  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2865  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2866  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2867    successful match. Consider:
2868  .sp  .sp
2869    a+(*SKIP)b    a+(*SKIP)b
2870  .sp  .sp
2871  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2872  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2873  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2874  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2875  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2876  instead of skipping on to "c".  instead of skipping on to "c".
2877  .sp  .sp
2878    (*THEN)    (*SKIP:NAME)
2879  .sp  .sp
2880  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2881  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2882  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2883  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2884    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2885    matching name is found, the (*SKIP) is ignored.
2886    .sp
2887      (*THEN) or (*THEN:NAME)
2888    .sp
2889    This verb causes a skip to the next innermost alternative if the rest of the
2890    pattern does not match. That is, it cancels pending backtracking, but only
2891    within the current alternative. Its name comes from the observation that it can
2892    be used for a pattern-based if-then-else block:
2893  .sp  .sp
2894    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2895  .sp  .sp
2896  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2897  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2898  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2899  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2900    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2901    .P
2902    Note that a subpattern that does not contain a | character is just a part of
2903    the enclosing alternative; it is not a nested alternation with only one
2904    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2905    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2906    pattern fragments that do not contain any | characters at this level:
2907    .sp
2908      A (B(*THEN)C) | D
2909    .sp
2910    If A and B are matched, but there is a failure in C, matching does not
2911    backtrack into A; instead it moves to the next alternative, that is, D.
2912    However, if the subpattern containing (*THEN) is given an alternative, it
2913    behaves differently:
2914    .sp
2915      A (B(*THEN)C | (*FAIL)) | D
2916    .sp
2917    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2918    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2919    because there are no more alternatives to try. In this case, matching does now
2920    backtrack into A.
2921    .P
2922    Note also that a conditional subpattern is not considered as having two
2923    alternatives, because only one is ever used. In other words, the | character in
2924    a conditional subpattern has a different meaning. Ignoring white space,
2925    consider:
2926    .sp
2927      ^.*? (?(?=a) a | b(*THEN)c )
2928    .sp
2929    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2930    it initially matches zero characters. The condition (?=a) then fails, the
2931    character "b" is matched, but "c" is not. At this point, matching does not
2932    backtrack to .*? as might perhaps be expected from the presence of the |
2933    character. The conditional subpattern is part of the single alternative that
2934    comprises the whole pattern, and so the match fails. (If there was a backtrack
2935    into .*?, allowing it to match "b", the match would succeed.)
2936    .P
2937    The verbs just described provide four different "strengths" of control when
2938    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2939    next alternative. (*PRUNE) comes next, failing the match at the current
2940    starting position, but allowing an advance to the next character (for an
2941    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2942    than one character. (*COMMIT) is the strongest, causing the entire match to
2943    fail.
2944    .P
2945    If more than one such verb is present in a pattern, the "strongest" one wins.
2946    For example, consider this pattern, where A, B, etc. are complex pattern
2947    fragments:
2948    .sp
2949      (A(*COMMIT)B(*THEN)C|D)
2950    .sp
2951    Once A has matched, PCRE is committed to this match, at the current starting
2952    position. If subsequently B matches, but C does not, the normal (*THEN) action
2953    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2954    overrides.
2955  .  .
2956  .  .
2957  .SH "SEE ALSO"  .SH "SEE ALSO"
2958  .rs  .rs
2959  .sp  .sp
2960  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2961    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
2962  .  .
2963  .  .
2964  .SH AUTHOR  .SH AUTHOR
# Line 2111  Cambridge CB2 3QH, England. Line 2975  Cambridge CB2 3QH, England.
2975  .rs  .rs
2976  .sp  .sp
2977  .nf  .nf
2978  Last updated: 09 August 2007  Last updated: 07 November 2012
2979  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2980  .fi  .fi

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