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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "03 March 2013" "PCRE 8.33"
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 a
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  third 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      (*UTF)
36    .sp
37    (*UTF) is a generic sequence that can be used with any of the libraries.
38    Starting a pattern with such a sequence is equivalent to setting the relevant
39    option. This feature is not Perl-compatible. How setting a UTF mode affects
40    pattern matching is mentioned in several places below. There is also a summary
41    of features in the
42  .\" HREF  .\" HREF
43  \fBpcre\fP  \fBpcreunicode\fP
44  .\"  .\"
45  page.  page.
46  .P  .P
47    Another special sequence that may appear at the start of a pattern or in
48    combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) is:
49    .sp
50      (*UCP)
51    .sp
52    This has the same effect as setting the PCRE_UCP option: it causes sequences
53    such as \ed and \ew to use Unicode properties to determine character types,
54    instead of recognizing only characters with codes less than 128 via a lookup
55    table.
56    .P
57    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
58    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
59    also some more of these special sequences that are concerned with the handling
60    of newlines; they are described below.
61    .P
62  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
63  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
64  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
65  \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,
66  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
67  \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
68  alternative function, and how it differs from the normal function, are  advantages and disadvantages of the alternative functions, and how they differ
69  discussed in the  from the normal functions, are discussed in the
70  .\" HREF  .\" HREF
71  \fBpcrematching\fP  \fBpcrematching\fP
72  .\"  .\"
73  page.  page.
74  .  .
75  .  .
76    .SH "EBCDIC CHARACTER CODES"
77    .rs
78    .sp
79    PCRE can be compiled to run in an environment that uses EBCDIC as its character
80    code rather than ASCII or Unicode (typically a mainframe system). In the
81    sections below, character code values are ASCII or Unicode; in an EBCDIC
82    environment these characters may have different code values, and there are no
83    code points greater than 255.
84    .
85    .
86    .\" HTML <a name="newlines"></a>
87    .SH "NEWLINE CONVENTIONS"
88    .rs
89    .sp
90    PCRE supports five different conventions for indicating line breaks in
91    strings: a single CR (carriage return) character, a single LF (linefeed)
92    character, the two-character sequence CRLF, any of the three preceding, or any
93    Unicode newline sequence. The
94    .\" HREF
95    \fBpcreapi\fP
96    .\"
97    page has
98    .\" HTML <a href="pcreapi.html#newlines">
99    .\" </a>
100    further discussion
101    .\"
102    about newlines, and shows how to set the newline convention in the
103    \fIoptions\fP arguments for the compiling and matching functions.
104    .P
105    It is also possible to specify a newline convention by starting a pattern
106    string with one of the following five sequences:
107    .sp
108      (*CR)        carriage return
109      (*LF)        linefeed
110      (*CRLF)      carriage return, followed by linefeed
111      (*ANYCRLF)   any of the three above
112      (*ANY)       all Unicode newline sequences
113    .sp
114    These override the default and the options given to the compiling function. For
115    example, on a Unix system where LF is the default newline sequence, the pattern
116    .sp
117      (*CR)a.b
118    .sp
119    changes the convention to CR. That pattern matches "a\enb" because LF is no
120    longer a newline. Note that these special settings, which are not
121    Perl-compatible, are recognized only at the very start of a pattern, and that
122    they must be in upper case. If more than one of them is present, the last one
123    is used.
124    .P
125    The newline convention affects where the circumflex and dollar assertions are
126    true. It also affects the interpretation of the dot metacharacter when
127    PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
128    what the \eR escape sequence matches. By default, this is any Unicode newline
129    sequence, for Perl compatibility. However, this can be changed; see the
130    description of \eR in the section entitled
131    .\" HTML <a href="#newlineseq">
132    .\" </a>
133    "Newline sequences"
134    .\"
135    below. A change of \eR setting can be combined with a change of newline
136    convention.
137    .
138    .
139  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
140  .rs  .rs
141  .sp  .sp
# Line 55  corresponding characters in the subject. Line 147  corresponding characters in the subject.
147  .sp  .sp
148  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
149  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
150  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
151  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
152  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
153  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
154  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
155  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
156  UTF-8 support.  UTF support.
157  .P  .P
158  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
159  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 199  The following sections describe the use
199  .rs  .rs
200  .sp  .sp
201  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
202  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
203  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
204  outside character classes.  both inside and outside character classes.
205  .P  .P
206  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.
207  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 209  otherwise be interpreted as a metacharac
209  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
210  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
211  .P  .P
212  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
213    backslash. All other characters (in particular, those whose codepoints are
214    greater than 127) are treated as literals.
215    .P
216    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
217  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
218  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
219  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.
220  .P  .P
221  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
222  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 232  Perl, $ and @ cause variable interpolati
232    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
233  .sp  .sp
234  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
235    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
236    by \eE later in the pattern, the literal interpretation continues to the end of
237    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
238    a character class, this causes an error, because the character class is not
239    terminated.
240  .  .
241  .  .
242  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 145  The \eQ...\eE sequence is recognized bot Line 246  The \eQ...\eE sequence is recognized bot
246  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
247  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
248  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
249  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
250  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:  
251  .sp  .sp
252    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
253    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
254    \ee        escape (hex 1B)    \ee        escape (hex 1B)
255    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
256    \en        newline (hex 0A)    \en        linefeed (hex 0A)
257    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
258    \et        tab (hex 09)    \et        tab (hex 09)
259    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
260    \exhh      character with hex code hh    \exhh      character with hex code hh
261    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
262      \euhhhh    character with hex code hhhh (JavaScript mode only)
263  .sp  .sp
264  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
265  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
266  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),
267  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
268  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
269  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.
270  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
271  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
272  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,
273  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
274  point, which is 10FFFF.  bytes. In this mode, all values are valid after \ec. If the next character is a
275    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
276    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
277    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
278    characters also generate different values.
279    .P
280    By default, after \ex, from zero to two hexadecimal digits are read (letters
281    can be in upper or lower case). Any number of hexadecimal digits may appear
282    between \ex{ and }, but the character code is constrained as follows:
283    .sp
284      8-bit non-UTF mode    less than 0x100
285      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
286      16-bit non-UTF mode   less than 0x10000
287      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
288      32-bit non-UTF mode   less than 0x80000000
289      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
290    .sp
291    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
292    "surrogate" codepoints), and 0xffef.
293  .P  .P
294  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
295  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
296  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
297  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
298  .P  .P
299    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
300    as just described only when it is followed by two hexadecimal digits.
301    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
302    code points greater than 256 is provided by \eu, which must be followed by
303    four hexadecimal digits; otherwise it matches a literal "u" character.
304    Character codes specified by \eu in JavaScript mode are constrained in the same
305    was as those specified by \ex in non-JavaScript mode.
306    .P
307  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
308  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
309  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
310    \eu00dc in JavaScript mode).
311  .P  .P
312  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
313  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 333  parenthesized subpatterns.
333  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
334  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
335  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
336  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
337  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.
338  to \e777 are permitted. For example:  For example:
339  .sp  .sp
340    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
341  .\" JOIN  .\" JOIN
342    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
343              previous capturing subpatterns              previous capturing subpatterns
# Line 224  to \e777 are permitted. For example: Line 352  to \e777 are permitted. For example:
352              character with octal code 113              character with octal code 113
353  .\" JOIN  .\" JOIN
354    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
355              the byte consisting entirely of 1 bits              the value 255 (decimal)
356  .\" JOIN  .\" JOIN
357    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
358              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 361  Note that octal values of 100 or greater
361  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
362  .P  .P
363  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
364  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
365  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
366  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
367  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
368  meanings  inside a character class. Like other unrecognized escape sequences, they are
369  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
370  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
371  (see below).  sequences have different meanings.
372  .\"  .
373    .
374    .SS "Unsupported escape sequences"
375    .rs
376    .sp
377    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
378    handler and used to modify the case of following characters. By default, PCRE
379    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
380    option is set, \eU matches a "U" character, and \eu can be used to define a
381    character by code point, as described in the previous section.
382  .  .
383  .  .
384  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 261  parenthesized subpatterns. Line 398  parenthesized subpatterns.
398  .\"  .\"
399  .  .
400  .  .
401    .SS "Absolute and relative subroutine calls"
402    .rs
403    .sp
404    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
405    a number enclosed either in angle brackets or single quotes, is an alternative
406    syntax for referencing a subpattern as a "subroutine". Details are discussed
407    .\" HTML <a href="#onigurumasubroutines">
408    .\" </a>
409    later.
410    .\"
411    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
412    synonymous. The former is a back reference; the latter is a
413    .\" HTML <a href="#subpatternsassubroutines">
414    .\" </a>
415    subroutine
416    .\"
417    call.
418    .
419    .
420    .\" HTML <a name="genericchartypes"></a>
421  .SS "Generic character types"  .SS "Generic character types"
422  .rs  .rs
423  .sp  .sp
424  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:  
425  .sp  .sp
426    \ed     any decimal digit    \ed     any decimal digit
427    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
428    \eh     any horizontal whitespace character    \eh     any horizontal white space character
429    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
430    \es     any whitespace character    \es     any white space character
431    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
432    \ev     any vertical whitespace character    \ev     any vertical white space character
433    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
434    \ew     any "word" character    \ew     any "word" character
435    \eW     any "non-word" character    \eW     any "non-word" character
436  .sp  .sp
437  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.
438  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
439  .P  .\" HTML <a href="#fullstopdot">
440  These character type sequences can appear both inside and outside character  .\" </a>
441    the "." metacharacter
442    .\"
443    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
444    PCRE does not support this.
445    .P
446    Each pair of lower and upper case escape sequences partitions the complete set
447    of characters into two disjoint sets. Any given character matches one, and only
448    one, of each pair. The sequences can appear both inside and outside character
449  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
450  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
451  there is no character to match.  there is no character to match.
452  .P  .P
453  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 456  are HT (9), LF (10), FF (12), CR (13), a
456  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
457  does.  does.
458  .P  .P
459  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.
460  \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
461  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
462  meanings from before UTF-8 support was available, mainly for efficiency  place (see
463  reasons.  .\" HTML <a href="pcreapi.html#localesupport">
464  .P  .\" </a>
465  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  "Locale support"
466  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\"
467  The horizontal space characters are:  in the
468    .\" HREF
469    \fBpcreapi\fP
470    .\"
471    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
472    or "french" in Windows, some character codes greater than 128 are used for
473    accented letters, and these are then matched by \ew. The use of locales with
474    Unicode is discouraged.
475    .P
476    By default, in a UTF mode, characters with values greater than 128 never match
477    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
478    their original meanings from before UTF support was available, mainly for
479    efficiency reasons. However, if PCRE is compiled with Unicode property support,
480    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
481    properties are used to determine character types, as follows:
482    .sp
483      \ed  any character that \ep{Nd} matches (decimal digit)
484      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
485      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
486    .sp
487    The upper case escapes match the inverse sets of characters. Note that \ed
488    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
489    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
490    \eB because they are defined in terms of \ew and \eW. Matching these sequences
491    is noticeably slower when PCRE_UCP is set.
492    .P
493    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
494    release 5.10. In contrast to the other sequences, which match only ASCII
495    characters by default, these always match certain high-valued codepoints,
496    whether or not PCRE_UCP is set. The horizontal space characters are:
497  .sp  .sp
498    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
499    U+0020     Space    U+0020     Space
500    U+00A0     Non-break space    U+00A0     Non-break space
501    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 324  The horizontal space characters are: Line 517  The horizontal space characters are:
517  .sp  .sp
518  The vertical space characters are:  The vertical space characters are:
519  .sp  .sp
520    U+000A     Linefeed    U+000A     Linefeed (LF)
521    U+000B     Vertical tab    U+000B     Vertical tab (VT)
522    U+000C     Formfeed    U+000C     Form feed (FF)
523    U+000D     Carriage return    U+000D     Carriage return (CR)
524    U+0085     Next line    U+0085     Next line (NEL)
525    U+2028     Line separator    U+2028     Line separator
526    U+2029     Paragraph separator    U+2029     Paragraph separator
527  .P  .sp
528  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
529  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.  
530  .  .
531  .  .
532    .\" HTML <a name="newlineseq"></a>
533  .SS "Newline sequences"  .SS "Newline sequences"
534  .rs  .rs
535  .sp  .sp
536  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
537  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
538  the following:  following:
539  .sp  .sp
540    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
541  .sp  .sp
# Line 366  below. Line 546  below.
546  .\"  .\"
547  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
548  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,
549  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
550  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
551  cannot be split.  cannot be split.
552  .P  .P
553  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
554  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).
555  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
556  recognized.  recognized.
557  .P  .P
558  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
559    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
560    either at compile time or when the pattern is matched. (BSR is an abbrevation
561    for "backslash R".) This can be made the default when PCRE is built; if this is
562    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
563    It is also possible to specify these settings by starting a pattern string with
564    one of the following sequences:
565    .sp
566      (*BSR_ANYCRLF)   CR, LF, or CRLF only
567      (*BSR_UNICODE)   any Unicode newline sequence
568    .sp
569    These override the default and the options given to the compiling function, but
570    they can themselves be overridden by options given to a matching function. Note
571    that these special settings, which are not Perl-compatible, are recognized only
572    at the very start of a pattern, and that they must be in upper case. If more
573    than one of them is present, the last one is used. They can be combined with a
574    change of newline convention; for example, a pattern can start with:
575    .sp
576      (*ANY)(*BSR_ANYCRLF)
577    .sp
578    They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
579    (*UCP) special sequences. Inside a character class, \eR is treated as an
580    unrecognized escape sequence, and so matches the letter "R" by default, but
581    causes an error if PCRE_EXTRA is set.
582  .  .
583  .  .
584  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 384  Inside a character class, \eR matches th Line 587  Inside a character class, \eR matches th
587  .sp  .sp
588  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
589  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
590  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
591  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.
592  The extra escape sequences are:  The extra escape sequences are:
593  .sp  .sp
594    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
595    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
596    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
597  .sp  .sp
598  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
599  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
600  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
601  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
602  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
603    .\" </a>
604    next section).
605    .\"
606    Other Perl properties such as "InMusicalSymbols" are not currently supported by
607    PCRE. Note that \eP{Any} does not match any characters, so always causes a
608    match failure.
609  .P  .P
610  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
611  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 619  Those that are not part of an identified
619  .P  .P
620  Arabic,  Arabic,
621  Armenian,  Armenian,
622    Avestan,
623  Balinese,  Balinese,
624    Bamum,
625    Batak,
626  Bengali,  Bengali,
627  Bopomofo,  Bopomofo,
628    Brahmi,
629  Braille,  Braille,
630  Buginese,  Buginese,
631  Buhid,  Buhid,
632  Canadian_Aboriginal,  Canadian_Aboriginal,
633    Carian,
634    Chakma,
635    Cham,
636  Cherokee,  Cherokee,
637  Common,  Common,
638  Coptic,  Coptic,
# Line 425  Cypriot, Line 641  Cypriot,
641  Cyrillic,  Cyrillic,
642  Deseret,  Deseret,
643  Devanagari,  Devanagari,
644    Egyptian_Hieroglyphs,
645  Ethiopic,  Ethiopic,
646  Georgian,  Georgian,
647  Glagolitic,  Glagolitic,
# Line 437  Hangul, Line 654  Hangul,
654  Hanunoo,  Hanunoo,
655  Hebrew,  Hebrew,
656  Hiragana,  Hiragana,
657    Imperial_Aramaic,
658  Inherited,  Inherited,
659    Inscriptional_Pahlavi,
660    Inscriptional_Parthian,
661    Javanese,
662    Kaithi,
663  Kannada,  Kannada,
664  Katakana,  Katakana,
665    Kayah_Li,
666  Kharoshthi,  Kharoshthi,
667  Khmer,  Khmer,
668  Lao,  Lao,
669  Latin,  Latin,
670    Lepcha,
671  Limbu,  Limbu,
672  Linear_B,  Linear_B,
673    Lisu,
674    Lycian,
675    Lydian,
676  Malayalam,  Malayalam,
677    Mandaic,
678    Meetei_Mayek,
679    Meroitic_Cursive,
680    Meroitic_Hieroglyphs,
681    Miao,
682  Mongolian,  Mongolian,
683  Myanmar,  Myanmar,
684  New_Tai_Lue,  New_Tai_Lue,
# Line 454  Nko, Line 686  Nko,
686  Ogham,  Ogham,
687  Old_Italic,  Old_Italic,
688  Old_Persian,  Old_Persian,
689    Old_South_Arabian,
690    Old_Turkic,
691    Ol_Chiki,
692  Oriya,  Oriya,
693  Osmanya,  Osmanya,
694  Phags_Pa,  Phags_Pa,
695  Phoenician,  Phoenician,
696    Rejang,
697  Runic,  Runic,
698    Samaritan,
699    Saurashtra,
700    Sharada,
701  Shavian,  Shavian,
702  Sinhala,  Sinhala,
703    Sora_Sompeng,
704    Sundanese,
705  Syloti_Nagri,  Syloti_Nagri,
706  Syriac,  Syriac,
707  Tagalog,  Tagalog,
708  Tagbanwa,  Tagbanwa,
709  Tai_Le,  Tai_Le,
710    Tai_Tham,
711    Tai_Viet,
712    Takri,
713  Tamil,  Tamil,
714  Telugu,  Telugu,
715  Thaana,  Thaana,
# Line 473  Thai, Line 717  Thai,
717  Tibetan,  Tibetan,
718  Tifinagh,  Tifinagh,
719  Ugaritic,  Ugaritic,
720    Vai,
721  Yi.  Yi.
722  .P  .P
723  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
724  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
725  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
726  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
727  .P  .P
728  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
729  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 784  the Lu, Ll, or Lt property, in other wor
784  a modifier or "other".  a modifier or "other".
785  .P  .P
786  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
787  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
788  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
789  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
790    PCRE_NO_UTF32_CHECK in the
791  .\" HREF  .\" HREF
792  \fBpcreapi\fP  \fBpcreapi\fP
793  .\"  .\"
794  page).  page). Perl does not support the Cs property.
795  .P  .P
796  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})
797  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
798  properties with "Is".  properties with "Is".
799  .P  .P
# Line 556  Instead, this property is assumed for an Line 802  Instead, this property is assumed for an
802  Unicode table.  Unicode table.
803  .P  .P
804  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
805  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
806    the behaviour of current versions of Perl.
807  .P  .P
808  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
809  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
810  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
811    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
812    PCRE_UCP option or by starting the pattern with (*UCP).
813    .
814    .
815    .SS Extended grapheme clusters
816    .rs
817  .sp  .sp
818  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
819  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  
820  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
821  .\" </a>  .\" </a>
822  (see below).  (see below).
823  .\"  .\"
824  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
825  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
826  non-UTF-8 mode \eX matches any one character.  .sp
827  .P    (?>\ePM\epM*)
828  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
829  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
830  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"
831  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
832    .P
833    This simple definition was extended in Unicode to include more complicated
834    kinds of composite character by giving each character a grapheme breaking
835    property, and creating rules that use these properties to define the boundaries
836    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
837    one of these clusters.
838    .P
839    \eX always matches at least one character. Then it decides whether to add
840    additional characters according to the following rules for ending a cluster:
841    .P
842    1. End at the end of the subject string.
843    .P
844    2. Do not end between CR and LF; otherwise end after any control character.
845    .P
846    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
847    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
848    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
849    character; an LVT or T character may be follwed only by a T character.
850    .P
851    4. Do not end before extending characters or spacing marks. Characters with
852    the "mark" property always have the "extend" grapheme breaking property.
853    .P
854    5. Do not end after prepend characters.
855    .P
856    6. Otherwise, end the cluster.
857    .
858    .
859    .\" HTML <a name="extraprops"></a>
860    .SS PCRE's additional properties
861    .rs
862    .sp
863    As well as the standard Unicode properties described above, PCRE supports four
864    more that make it possible to convert traditional escape sequences such as \ew
865    and \es and POSIX character classes to use Unicode properties. PCRE uses these
866    non-standard, non-Perl properties internally when PCRE_UCP is set. However,
867    they may also be used explicitly. These properties are:
868    .sp
869      Xan   Any alphanumeric character
870      Xps   Any POSIX space character
871      Xsp   Any Perl space character
872      Xwd   Any Perl "word" character
873    .sp
874    Xan matches characters that have either the L (letter) or the N (number)
875    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
876    carriage return, and any other character that has the Z (separator) property.
877    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
878    same characters as Xan, plus underscore.
879    .P
880    There is another non-standard property, Xuc, which matches any character that
881    can be represented by a Universal Character Name in C++ and other programming
882    languages. These are the characters $, @, ` (grave accent), and all characters
883    with Unicode code points greater than or equal to U+00A0, except for the
884    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
885    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
886    where H is a hexadecimal digit. Note that the Xuc property does not match these
887    sequences but the characters that they represent.)
888  .  .
889  .  .
890  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
891  .SS "Resetting the match start"  .SS "Resetting the match start"
892  .rs  .rs
893  .sp  .sp
894  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
895  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:  
896  .sp  .sp
897    foo\eKbar    foo\eKbar
898  .sp  .sp
# Line 608  For example, when the pattern Line 914  For example, when the pattern
914    (foo)\eKbar    (foo)\eKbar
915  .sp  .sp
916  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
917    .P
918    Perl documents that the use of \eK within assertions is "not well defined". In
919    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
920    ignored in negative assertions.
921  .  .
922  .  .
923  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 632  The backslashed assertions are: Line 942  The backslashed assertions are:
942    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
943    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
944  .sp  .sp
945  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
946  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
947    default it matches the corresponding literal character (for example, \eB
948    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
949    escape sequence" error is generated instead.
950  .P  .P
951  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
952  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
953  \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
954  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
955    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
956    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
957    of word" or "end of word" metasequence. However, whatever follows \eb normally
958    determines which it is. For example, the fragment \eba matches "a" at the start
959    of a word.
960  .P  .P
961  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
962  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 990  regular expression.
990  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
991  .rs  .rs
992  .sp  .sp
993    The circumflex and dollar metacharacters are zero-width assertions. That is,
994    they test for a particular condition being true without consuming any
995    characters from the subject string.
996    .P
997  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
998  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
999  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
1000  \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
1001  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1002  meaning  meaning
# Line 691  constrained to match only at the start o Line 1013  constrained to match only at the start o
1013  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1014  to be anchored.)  to be anchored.)
1015  .P  .P
1016  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
1017  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
1018  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
1019  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
1020  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
1021  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1022  .P  .P
1023  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
1024  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 1044  end of the subject in both modes, and if
1044  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1045  .  .
1046  .  .
1047  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1048    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1049  .rs  .rs
1050  .sp  .sp
1051  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
1052  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
1053  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1054  .P  .P
1055  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
1056  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 1067  to match it.
1067  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
1068  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
1069  special meaning in a character class.  special meaning in a character class.
1070  .  .P
1071  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1072  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1073  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1074  .sp  name; PCRE does not support this.
1075  Outside a character class, the escape sequence \eC matches any one byte, both  .
1076  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1077  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1078  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1079  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1080  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1081    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1082    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1083    a 32-bit unit. Unlike a dot, \eC always
1084    matches line-ending characters. The feature is provided in Perl in order to
1085    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1086    used. Because \eC breaks up characters into individual data units, matching one
1087    unit with \eC in a UTF mode means that the rest of the string may start with a
1088    malformed UTF character. This has undefined results, because PCRE assumes that
1089    it is dealing with valid UTF strings (and by default it checks this at the
1090    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1091    PCRE_NO_UTF32_CHECK option is used).
1092  .P  .P
1093  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1094  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1095  .\" </a>  .\" </a>
1096  (described below),  (described below)
1097  .\"  .\"
1098  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
1099  the lookbehind.  the lookbehind.
1100    .P
1101    In general, the \eC escape sequence is best avoided. However, one
1102    way of using it that avoids the problem of malformed UTF characters is to use a
1103    lookahead to check the length of the next character, as in this pattern, which
1104    could be used with a UTF-8 string (ignore white space and line breaks):
1105    .sp
1106      (?| (?=[\ex00-\ex7f])(\eC) |
1107          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1108          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1109          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1110    .sp
1111    A group that starts with (?| resets the capturing parentheses numbers in each
1112    alternative (see
1113    .\" HTML <a href="#dupsubpatternnumber">
1114    .\" </a>
1115    "Duplicate Subpattern Numbers"
1116    .\"
1117    below). The assertions at the start of each branch check the next UTF-8
1118    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1119    character's individual bytes are then captured by the appropriate number of
1120    groups.
1121  .  .
1122  .  .
1123  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 770  the lookbehind. Line 1125  the lookbehind.
1125  .rs  .rs
1126  .sp  .sp
1127  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1128  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.
1129  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
1130  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
1131  escaped with a backslash.  a member of the class, it should be the first data character in the class
1132  .P  (after an initial circumflex, if present) or escaped with a backslash.
1133  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1134  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
1135  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
1136  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
1137  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
1138  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
1139    member of the class, ensure it is not the first character, or escape it with a
1140  backslash.  backslash.
1141  .P  .P
1142  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1143  [^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
1144  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1145  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
1146  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
1147  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
1148  string.  string.
1149  .P  .P
1150  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)
1151  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
1152    \ex{ escaping mechanism.
1153  .P  .P
1154  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
1155  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
1156  "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
1157  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
1158  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
1159  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1160  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1161  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
1162  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
1163  UTF-8 support.  well as with UTF support.
1164  .P  .P
1165  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
1166  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 1182  followed by two other characters. The oc
1182  "]" can also be used to end a range.  "]" can also be used to end a range.
1183  .P  .P
1184  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
1185  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1186  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}].  
1187  .P  .P
1188  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
1189  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
1190  [][\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
1191  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
1192  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
1193  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
1194  property support.  property support.
1195  .P  .P
1196  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,
1197  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
1198  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1199  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
1200  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
1201  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1202    .\" HTML <a href="#genericchartypes">
1203    .\" </a>
1204    "Generic character types"
1205    .\"
1206    above. The escape sequence \eb has a different meaning inside a character
1207    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1208    are not special inside a character class. Like any other unrecognized escape
1209    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1210    default, but cause an error if the PCRE_EXTRA option is set.
1211    .P
1212    A circumflex can conveniently be used with the upper case character types to
1213    specify a more restricted set of characters than the matching lower case type.
1214    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1215    whereas [\ew] includes underscore. A positive character class should be read as
1216    "something OR something OR ..." and a negative class as "NOT something AND NOT
1217    something AND NOT ...".
1218  .P  .P
1219  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1220  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 1234  this notation. For example,
1234    [01[:alpha:]%]    [01[:alpha:]%]
1235  .sp  .sp
1236  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1237  are  are:
1238  .sp  .sp
1239    alnum    letters and digits    alnum    letters and digits
1240    alpha    letters    alpha    letters
# Line 873  are Line 1245  are
1245    graph    printing characters, excluding space    graph    printing characters, excluding space
1246    lower    lower case letters    lower    lower case letters
1247    print    printing characters, including space    print    printing characters, including space
1248    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1249    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1250    upper    upper case letters    upper    upper case letters
1251    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 894  matches "1", "2", or any non-digit. PCRE Line 1266  matches "1", "2", or any non-digit. PCRE
1266  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
1267  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1268  .P  .P
1269  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
1270  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1271    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1272    character properties are used. This is achieved by replacing the POSIX classes
1273    by other sequences, as follows:
1274    .sp
1275      [:alnum:]  becomes  \ep{Xan}
1276      [:alpha:]  becomes  \ep{L}
1277      [:blank:]  becomes  \eh
1278      [:digit:]  becomes  \ep{Nd}
1279      [:lower:]  becomes  \ep{Ll}
1280      [:space:]  becomes  \ep{Xps}
1281      [:upper:]  becomes  \ep{Lu}
1282      [:word:]   becomes  \ep{Xwd}
1283    .sp
1284    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1285    classes are unchanged, and match only characters with code points less than
1286    128.
1287  .  .
1288  .  .
1289  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 922  alternative in the subpattern. Line 1310  alternative in the subpattern.
1310  .rs  .rs
1311  .sp  .sp
1312  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1313  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
1314  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1315    The option letters are
1316  .sp  .sp
1317    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1318    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 937  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1326  PCRE_MULTILINE while unsetting PCRE_DOTA
1326  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
1327  unset.  unset.
1328  .P  .P
1329  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
1330  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
1331  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1332  the global options (and it will therefore show up in data extracted by the  .P
1333  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1334    subpattern parentheses), the change applies to the remainder of the pattern
1335    that follows. If the change is placed right at the start of a pattern, PCRE
1336    extracts it into the global options (and it will therefore show up in data
1337    extracted by the \fBpcre_fullinfo()\fP function).
1338  .P  .P
1339  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1340  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
1341  .sp  .sp
1342    (a(?i)b)c    (a(?i)b)c
1343  .sp  .sp
# Line 960  branch is abandoned before the option se Line 1353  branch is abandoned before the option se
1353  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1354  behaviour otherwise.  behaviour otherwise.
1355  .P  .P
1356  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
1357  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
1358  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1359    what the application has set or what has been defaulted. Details are given in
1360    the section entitled
1361    .\" HTML <a href="#newlineseq">
1362    .\" </a>
1363    "Newline sequences"
1364    .\"
1365    above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1366    sequences that can be used to set UTF and Unicode property modes; they are
1367    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1368    options, respectively. The (*UTF) sequence is a generic version that can be
1369    used with any of the libraries.
1370  .  .
1371  .  .
1372  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 976  Turning part of a pattern into a subpatt Line 1380  Turning part of a pattern into a subpatt
1380  .sp  .sp
1381    cat(aract|erpillar|)    cat(aract|erpillar|)
1382  .sp  .sp
1383  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1384  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1385  .sp  .sp
1386  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
1387  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
1388  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
1389  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1390  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1391  .P  .P
1392  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
1393    numbers for the capturing subpatterns. For example, if the string "the red
1394    king" is matched against the pattern
1395  .sp  .sp
1396    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1397  .sp  .sp
# Line 1017  is reached, an option setting in one bra Line 1423  is reached, an option setting in one bra
1423  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1424  .  .
1425  .  .
1426    .\" HTML <a name="dupsubpatternnumber"></a>
1427  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1428  .rs  .rs
1429  .sp  .sp
# Line 1033  at captured substring number one, whiche Line 1440  at captured substring number one, whiche
1440  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
1441  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1442  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
1443  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
1444  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1445  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.  
1446  .sp  .sp
1447    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1448    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1449    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1450  .sp  .sp
1451  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
1452  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1453    or "defdef":
1454    .sp
1455      /(?|(abc)|(def))\e1/
1456    .sp
1457    In contrast, a subroutine call to a numbered subpattern always refers to the
1458    first one in the pattern with the given number. The following pattern matches
1459    "abcabc" or "defabc":
1460    .sp
1461      /(?|(abc)|(def))(?1)/
1462    .sp
1463    If a
1464    .\" HTML <a href="#conditions">
1465    .\" </a>
1466    condition test
1467    .\"
1468    for a subpattern's having matched refers to a non-unique number, the test is
1469    true if any of the subpatterns of that number have matched.
1470  .P  .P
1471  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1472  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 1481  if an expression is modified, the number
1481  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1482  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
1483  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
1484  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1485    have different names, but PCRE does not.
1486  .P  .P
1487  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
1488  (?'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
1489  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1490  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1491  .\" </a>  .\" </a>
1492  backreferences,  back references,
1493  .\"  .\"
1494  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1495  .\" </a>  .\" </a>
# Line 1085  extracting the name-to-number translatio Line 1509  extracting the name-to-number translatio
1509  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1510  .P  .P
1511  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
1512  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
1513  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
1514  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
1515  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
1516  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
1517    name, and in both cases you want to extract the abbreviation. This pattern
1518    (ignoring the line breaks) does the job:
1519  .sp  .sp
1520    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1521    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1103  subpattern, as described in the previous Line 1529  subpattern, as described in the previous
1529  .P  .P
1530  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1531  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
1532  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1533  make a reference to a non-unique named subpattern from elsewhere in the  .P
1534  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
1535  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1536    used. In the absence of duplicate numbers (see the previous section) this is
1537    the one with the lowest number. If you use a named reference in a condition
1538    test (see the
1539    .\"
1540    .\" HTML <a href="#conditions">
1541    .\" </a>
1542    section about conditions
1543    .\"
1544    below), either to check whether a subpattern has matched, or to check for
1545    recursion, all subpatterns with the same name are tested. If the condition is
1546    true for any one of them, the overall condition is true. This is the same
1547    behaviour as testing by number. For further details of the interfaces for
1548    handling named subpatterns, see the
1549  .\" HREF  .\" HREF
1550  \fBpcreapi\fP  \fBpcreapi\fP
1551  .\"  .\"
1552  documentation.  documentation.
1553    .P
1554    \fBWarning:\fP You cannot use different names to distinguish between two
1555    subpatterns with the same number because PCRE uses only the numbers when
1556    matching. For this reason, an error is given at compile time if different names
1557    are given to subpatterns with the same number. However, you can give the same
1558    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1559  .  .
1560  .  .
1561  .SH REPETITION  .SH REPETITION
# Line 1122  items: Line 1567  items:
1567    a literal data character    a literal data character
1568    the dot metacharacter    the dot metacharacter
1569    the \eC escape sequence    the \eC escape sequence
1570    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1571    the \eR escape sequence    the \eR escape sequence
1572    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1573    a character class    a character class
1574    a back reference (see next section)    a back reference (see next section)
1575    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1576      a subroutine call to a subpattern (recursive or otherwise)
1577  .sp  .sp
1578  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1579  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 1598  where a quantifier is not allowed, or on
1598  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
1599  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1600  .P  .P
1601  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
1602  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
1603  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,
1604  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
1605  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1606  .P  .P
1607  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
1608  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1609    subpatterns that are referenced as
1610    .\" HTML <a href="#subpatternsassubroutines">
1611    .\" </a>
1612    subroutines
1613    .\"
1614    from elsewhere in the pattern (but see also the section entitled
1615    .\" HTML <a href="#subdefine">
1616    .\" </a>
1617    "Defining subpatterns for use by reference only"
1618    .\"
1619    below). Items other than subpatterns that have a {0} quantifier are omitted
1620    from the compiled pattern.
1621  .P  .P
1622  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1623  abbreviations:  abbreviations:
# Line 1230  In cases where it is known that the subj Line 1688  In cases where it is known that the subj
1688  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1689  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1690  .P  .P
1691  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1692  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1693  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
1694  succeeds. Consider, for example:  succeeds. Consider, for example:
1695  .sp  .sp
# Line 1240  succeeds. Consider, for example: Line 1698  succeeds. Consider, for example:
1698  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
1699  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1700  .P  .P
1701    Another case where implicit anchoring is not applied is when the leading .* is
1702    inside an atomic group. Once again, a match at the start may fail where a later
1703    one succeeds. Consider this pattern:
1704    .sp
1705      (?>.*?a)b
1706    .sp
1707    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1708    (*PRUNE) and (*SKIP) also disable this optimization.
1709    .P
1710  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1711  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1712  .sp  .sp
# Line 1384  no such problem when named parentheses a Line 1851  no such problem when named parentheses a
1851  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1852  .P  .P
1853  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
1854  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
1855  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
1856  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1857  .sp  .sp
1858    (ring), \e1    (ring), \e1
1859    (ring), \eg1    (ring), \eg1
# Line 1400  example: Line 1867  example:
1867    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1868  .sp  .sp
1869  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
1870  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.
1871  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
1872  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
1873  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1874  .P  .P
1875  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1876  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 1909  after the reference.
1909  .P  .P
1910  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
1911  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1912  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1913  .sp  .sp
1914    (a|(bc))\e2    (a|(bc))\e2
1915  .sp  .sp
1916  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
1917  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
1918  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1919  with a digit character, some delimiter must be used to terminate the back  .P
1920  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1921  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1922    If the pattern continues with a digit character, some delimiter must be used to
1923    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1924    white space. Otherwise, the \eg{ syntax or an empty comment (see
1925  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1926  .\" </a>  .\" </a>
1927  "Comments"  "Comments"
1928  .\"  .\"
1929  below) can be used.  below) can be used.
1930  .P  .
1931    .SS "Recursive back references"
1932    .rs
1933    .sp
1934  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
1935  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.
1936  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 1944  to the previous iteration. In order for
1944  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
1945  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
1946  minimum of zero.  minimum of zero.
1947    .P
1948    Back references of this type cause the group that they reference to be treated
1949    as an
1950    .\" HTML <a href="#atomicgroup">
1951    .\" </a>
1952    atomic group.
1953    .\"
1954    Once the whole group has been matched, a subsequent matching failure cannot
1955    cause backtracking into the middle of the group.
1956  .  .
1957  .  .
1958  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1490  those that look ahead of the current pos Line 1972  those that look ahead of the current pos
1972  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,
1973  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.
1974  .P  .P
1975  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1976  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
1977  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1978  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1979  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1980  because it does not make sense for negative assertions.  .P
1981    For compatibility with Perl, assertion subpatterns may be repeated; though
1982    it makes no sense to assert the same thing several times, the side effect of
1983    capturing parentheses may occasionally be useful. In practice, there only three
1984    cases:
1985    .sp
1986    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1987    However, it may contain internal capturing parenthesized groups that are called
1988    from elsewhere via the
1989    .\" HTML <a href="#subpatternsassubroutines">
1990    .\" </a>
1991    subroutine mechanism.
1992    .\"
1993    .sp
1994    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1995    were {0,1}. At run time, the rest of the pattern match is tried with and
1996    without the assertion, the order depending on the greediness of the quantifier.
1997    .sp
1998    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1999    The assertion is obeyed just once when encountered during matching.
2000  .  .
2001  .  .
2002  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1524  lookbehind assertion is needed to achiev Line 2025  lookbehind assertion is needed to achiev
2025  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
2026  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
2027  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.
2028    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2029  .  .
2030  .  .
2031  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1548  is permitted, but Line 2050  is permitted, but
2050  .sp  .sp
2051  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2052  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
2053  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
2054  match the same length of string. An assertion such as  length of string. An assertion such as
2055  .sp  .sp
2056    (?<=ab(c|de))    (?<=ab(c|de))
2057  .sp  .sp
2058  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
2059  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
2060    branches:
2061  .sp  .sp
2062    (?<=abc|abde)    (?<=abc|abde)
2063  .sp  .sp
2064  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2065  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2066  .\" </a>  .\" </a>
2067  (see above)  (see above)
2068  .\"  .\"
2069  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
2070  fixed-length.  restriction.
2071  .P  .P
2072  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2073  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
2074  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2075  assertion fails.  assertion fails.
2076  .P  .P
2077  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
2078  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
2079  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
2080  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2081    permitted.
2082    .P
2083    .\" HTML <a href="#subpatternsassubroutines">
2084    .\" </a>
2085    "Subroutine"
2086    .\"
2087    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2088    as the subpattern matches a fixed-length string.
2089    .\" HTML <a href="#recursion">
2090    .\" </a>
2091    Recursion,
2092    .\"
2093    however, is not supported.
2094  .P  .P
2095  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2096  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
2097  pattern such as  strings. Consider a simple pattern such as
2098  .sp  .sp
2099    abcd$    abcd$
2100  .sp  .sp
# Line 1642  characters that are not "999". Line 2158  characters that are not "999".
2158  .sp  .sp
2159  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2160  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2161  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2162  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2163  .sp  .sp
2164    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2165    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2166  .sp  .sp
2167  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2168  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
2169  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2170    itself contain nested subpatterns of any form, including conditional
2171    subpatterns; the restriction to two alternatives applies only at the level of
2172    the condition. This pattern fragment is an example where the alternatives are
2173    complex:
2174    .sp
2175      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2176    .sp
2177  .P  .P
2178  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2179  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 2182  recursion, a pseudo-condition called DEF
2182  .rs  .rs
2183  .sp  .sp
2184  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
2185  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2186  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
2187  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2188  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2189  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2190  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2191    section about duplicate subpattern numbers),
2192    .\"
2193    the condition is true if any of them have matched. An alternative notation is
2194    to precede the digits with a plus or minus sign. In this case, the subpattern
2195    number is relative rather than absolute. The most recently opened parentheses
2196    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2197    loops it can also make sense to refer to subsequent groups. The next
2198    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2199    zero in any of these forms is not used; it provokes a compile-time error.)
2200  .P  .P
2201  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2202  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 2207  three parts for ease of discussion:
2207  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2208  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
2209  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
2210  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2211  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,
2212  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
2213  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2214  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 2237  Rewriting the above example to use a nam
2237  .sp  .sp
2238    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2239  .sp  .sp
2240    If the name used in a condition of this kind is a duplicate, the test is
2241    applied to all subpatterns of the same name, and is true if any one of them has
2242    matched.
2243  .  .
2244  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2245  .rs  .rs
# Line 1716  letter R, for example: Line 2251  letter R, for example:
2251  .sp  .sp
2252    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2253  .sp  .sp
2254  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
2255  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
2256  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2257    applied to all subpatterns of the same name, and is true if any one of them is
2258    the most recent recursion.
2259  .P  .P
2260  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2261  patterns are described below.  .\" HTML <a href="#recursion">
2262    .\" </a>
2263    The syntax for recursive patterns
2264    .\"
2265    is described below.
2266  .  .
2267    .\" HTML <a name="subdefine"></a>
2268  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2269  .rs  .rs
2270  .sp  .sp
# Line 1730  If the condition is the string (DEFINE), Line 2272  If the condition is the string (DEFINE),
2272  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
2273  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2274  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
2275  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2276  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2277  written like this (ignore whitespace and line breaks):  .\" </a>
2278    subroutines
2279    .\"
2280    is described below.) For example, a pattern to match an IPv4 address such as
2281    "192.168.23.245" could be written like this (ignore white space and line
2282    breaks):
2283  .sp  .sp
2284    (?(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) )
2285    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1740  written like this (ignore whitespace and Line 2287  written like this (ignore whitespace and
2287  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
2288  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2289  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
2290  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2291  .P  pattern uses references to the named group to match the four dot-separated
2292  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.  
2293  .  .
2294  .SS "Assertion conditions"  .SS "Assertion conditions"
2295  .rs  .rs
# Line 1769  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2314  dd-aaa-dd or dd-dd-dd, where aaa are let
2314  .SH COMMENTS  .SH COMMENTS
2315  .rs  .rs
2316  .sp  .sp
2317  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
2318  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,
2319  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
2320    subpattern name or number. The characters that make up a comment play no part
2321    in the pattern matching.
2322  .P  .P
2323  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
2324  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2325  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2326    this case continues to immediately after the next newline character or
2327    character sequence in the pattern. Which characters are interpreted as newlines
2328    is controlled by the options passed to a compiling function or by a special
2329    sequence at the start of the pattern, as described in the section entitled
2330    .\" HTML <a href="#newlines">
2331    .\" </a>
2332    "Newline conventions"
2333    .\"
2334    above. Note that the end of this type of comment is a literal newline sequence
2335    in the pattern; escape sequences that happen to represent a newline do not
2336    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2337    default newline convention is in force:
2338    .sp
2339      abc #comment \en still comment
2340    .sp
2341    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2342    a newline in the pattern. The sequence \en is still literal at this stage, so
2343    it does not terminate the comment. Only an actual character with the code value
2344    0x0a (the default newline) does so.
2345  .  .
2346  .  .
2347  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1801  recursively to the pattern in which it a Line 2367  recursively to the pattern in which it a
2367  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2368  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2369  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2370  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.
2371  .P  .P
2372  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
2373  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
2374  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
2375    .\" HTML <a href="#subpatternsassubroutines">
2376    .\" </a>
2377    non-recursive subroutine
2378    .\"
2379  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
2380  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2381  .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  
2382  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2383  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2384  .sp  .sp
2385    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2386  .sp  .sp
2387  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2388  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
2389  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2390  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2391    to avoid backtracking into sequences of non-parentheses.
2392  .P  .P
2393  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
2394  pattern, so instead you could use this:  pattern, so instead you could use this:
2395  .sp  .sp
2396    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2397  .sp  .sp
2398  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
2399  them instead of the whole pattern.  them instead of the whole pattern.
2400  .P  .P
2401  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
2402  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
2403  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
2404  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2405  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.  
2406  .P  .P
2407  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2408  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2409  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2410  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2411    .\" </a>
2412    non-recursive subroutine
2413    .\"
2414    calls, as described in the next section.
2415  .P  .P
2416  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2417  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
2418  could rewrite the above example as follows:  could rewrite the above example as follows:
2419  .sp  .sp
2420    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2421  .sp  .sp
2422  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
2423  used.  used.
2424  .P  .P
2425  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2426  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
2427  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
2428  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2429  .sp  .sp
2430    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2431  .sp  .sp
2432  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,
2433  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
2434  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
2435  before failure can be reported.  before failure can be reported.
2436  .P  .P
2437  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
2438  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
2439  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  
2440  .\" HREF  .\" HREF
2441  \fBpcrecallout\fP  \fBpcrecallout\fP
2442  .\"  .\"
# Line 1876  documentation). If the pattern above is Line 2444  documentation). If the pattern above is
2444  .sp  .sp
2445    (ab(cd)ef)    (ab(cd)ef)
2446  .sp  .sp
2447  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
2448  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
2449  .sp  matched at the top level, its final captured value is unset, even if it was
2450    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2451       ^                        ^  .P
2452       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2453  .sp  obtain extra memory to store data during a recursion, which it does by using
2454  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
2455  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.  
2456  .P  .P
2457  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.
2458  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 2466  different alternatives for the recursive
2466  is the actual recursive call.  is the actual recursive call.
2467  .  .
2468  .  .
2469    .\" HTML <a name="recursiondifference"></a>
2470    .SS "Differences in recursion processing between PCRE and Perl"
2471    .rs
2472    .sp
2473    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2474    (like Python, but unlike Perl), a recursive subpattern call is always treated
2475    as an atomic group. That is, once it has matched some of the subject string, it
2476    is never re-entered, even if it contains untried alternatives and there is a
2477    subsequent matching failure. This can be illustrated by the following pattern,
2478    which purports to match a palindromic string that contains an odd number of
2479    characters (for example, "a", "aba", "abcba", "abcdcba"):
2480    .sp
2481      ^(.|(.)(?1)\e2)$
2482    .sp
2483    The idea is that it either matches a single character, or two identical
2484    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2485    it does not if the pattern is longer than three characters. Consider the
2486    subject string "abcba":
2487    .P
2488    At the top level, the first character is matched, but as it is not at the end
2489    of the string, the first alternative fails; the second alternative is taken
2490    and the recursion kicks in. The recursive call to subpattern 1 successfully
2491    matches the next character ("b"). (Note that the beginning and end of line
2492    tests are not part of the recursion).
2493    .P
2494    Back at the top level, the next character ("c") is compared with what
2495    subpattern 2 matched, which was "a". This fails. Because the recursion is
2496    treated as an atomic group, there are now no backtracking points, and so the
2497    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2498    try the second alternative.) However, if the pattern is written with the
2499    alternatives in the other order, things are different:
2500    .sp
2501      ^((.)(?1)\e2|.)$
2502    .sp
2503    This time, the recursing alternative is tried first, and continues to recurse
2504    until it runs out of characters, at which point the recursion fails. But this
2505    time we do have another alternative to try at the higher level. That is the big
2506    difference: in the previous case the remaining alternative is at a deeper
2507    recursion level, which PCRE cannot use.
2508    .P
2509    To change the pattern so that it matches all palindromic strings, not just
2510    those with an odd number of characters, it is tempting to change the pattern to
2511    this:
2512    .sp
2513      ^((.)(?1)\e2|.?)$
2514    .sp
2515    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2516    deeper recursion has matched a single character, it cannot be entered again in
2517    order to match an empty string. The solution is to separate the two cases, and
2518    write out the odd and even cases as alternatives at the higher level:
2519    .sp
2520      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2521    .sp
2522    If you want to match typical palindromic phrases, the pattern has to ignore all
2523    non-word characters, which can be done like this:
2524    .sp
2525      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2526    .sp
2527    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2528    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2529    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2530    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2531    more) to match typical phrases, and Perl takes so long that you think it has
2532    gone into a loop.
2533    .P
2534    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2535    string does not start with a palindrome that is shorter than the entire string.
2536    For example, although "abcba" is correctly matched, if the subject is "ababa",
2537    PCRE finds the palindrome "aba" at the start, then fails at top level because
2538    the end of the string does not follow. Once again, it cannot jump back into the
2539    recursion to try other alternatives, so the entire match fails.
2540    .P
2541    The second way in which PCRE and Perl differ in their recursion processing is
2542    in the handling of captured values. In Perl, when a subpattern is called
2543    recursively or as a subpattern (see the next section), it has no access to any
2544    values that were captured outside the recursion, whereas in PCRE these values
2545    can be referenced. Consider this pattern:
2546    .sp
2547      ^(.)(\e1|a(?2))
2548    .sp
2549    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2550    then in the second group, when the back reference \e1 fails to match "b", the
2551    second alternative matches "a" and then recurses. In the recursion, \e1 does
2552    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2553    match because inside the recursive call \e1 cannot access the externally set
2554    value.
2555    .
2556    .
2557  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2558  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2559  .rs  .rs
2560  .sp  .sp
2561  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
2562  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
2563  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2564  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2565  relative, as in these examples:  relative, as in these examples:
2566  .sp  .sp
# Line 1927  matches "sense and sensibility" and "res Line 2580  matches "sense and sensibility" and "res
2580  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
2581  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2582  .P  .P
2583  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2584  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
2585  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
2586  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2587  .P  subroutine call revert to their previous values afterwards.
2588  When a subpattern is used as a subroutine, processing options such as  .P
2589  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2590  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2591    different calls. For example, consider this pattern:
2592  .sp  .sp
2593    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2594  .sp  .sp
# Line 1942  It matches "abcabc". It does not match " Line 2596  It matches "abcabc". It does not match "
2596  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2597  .  .
2598  .  .
2599    .\" HTML <a name="onigurumasubroutines"></a>
2600    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2601    .rs
2602    .sp
2603    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2604    a number enclosed either in angle brackets or single quotes, is an alternative
2605    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2606    are two of the examples used above, rewritten using this syntax:
2607    .sp
2608      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2609      (sens|respons)e and \eg'1'ibility
2610    .sp
2611    PCRE supports an extension to Oniguruma: if a number is preceded by a
2612    plus or a minus sign it is taken as a relative reference. For example:
2613    .sp
2614      (abc)(?i:\eg<-1>)
2615    .sp
2616    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2617    synonymous. The former is a back reference; the latter is a subroutine call.
2618    .
2619    .
2620  .SH CALLOUTS  .SH CALLOUTS
2621  .rs  .rs
2622  .sp  .sp
# Line 1952  same pair of parentheses when there is a Line 2627  same pair of parentheses when there is a
2627  .P  .P
2628  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
2629  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
2630  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
2631    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2632  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2633  .P  .P
2634  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 2638  For example, this pattern has two callou
2638  .sp  .sp
2639    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2640  .sp  .sp
2641  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
2642  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2643  255.  255. If there is a conditional group in the pattern whose condition is an
2644  .P  assertion, an additional callout is inserted just before the condition. An
2645  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  explicit callout may also be set at this position, as in this example:
2646  set), the external function is called. It is provided with the number of the  .sp
2647  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2648  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2649  may cause matching to proceed, to backtrack, or to fail altogether. A complete  Note that this applies only to assertion conditions, not to other types of
2650  description of the interface to the callout function is given in the  condition.
2651    .P
2652    During matching, when PCRE reaches a callout point, the external function is
2653    called. It is provided with the number of the callout, the position in the
2654    pattern, and, optionally, one item of data originally supplied by the caller of
2655    the matching function. The callout function may cause matching to proceed, to
2656    backtrack, or to fail altogether. A complete description of the interface to
2657    the callout function is given in the
2658  .\" HREF  .\" HREF
2659  \fBpcrecallout\fP  \fBpcrecallout\fP
2660  .\"  .\"
2661  documentation.  documentation.
2662  .  .
2663  .  .
2664  .SH "BACTRACKING CONTROL"  .\" HTML <a name="backtrackcontrol"></a>
2665    .SH "BACKTRACKING CONTROL"
2666  .rs  .rs
2667  .sp  .sp
2668  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 2671  or removal in a future version of Perl".
2671  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2672  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2673  .P  .P
2674  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
2675  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
2676  backtracking algorithm. They cause an error if encountered by  matching functions, which use a backtracking algorithm. With the exception of
2677  \fBpcre_dfa_exec()\fP.  (*FAIL), which behaves like a failing negative assertion, they cause an error
2678    if encountered by a DFA matching function.
2679    .P
2680    If any of these verbs are used in an assertion or in a subpattern that is
2681    called as a subroutine (whether or not recursively), their effect is confined
2682    to that subpattern; it does not extend to the surrounding pattern, with one
2683    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2684    a successful positive assertion \fIis\fP passed back when a match succeeds
2685    (compare capturing parentheses in assertions). Note that such subpatterns are
2686    processed as anchored at the point where they are tested. Note also that Perl's
2687    treatment of subroutines and assertions is different in some cases.
2688  .P  .P
2689  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
2690  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2691  (*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,
2692  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
2693  are two kinds:  characters that does not include a closing parenthesis. The maximum length of
2694    name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit library.
2695    If the name is empty, that is, if the closing parenthesis immediately follows
2696    the colon, the effect is as if the colon were not there. Any number of these
2697    verbs may occur in a pattern.
2698    .
2699    .
2700    .\" HTML <a name="nooptimize"></a>
2701    .SS "Optimizations that affect backtracking verbs"
2702    .rs
2703    .sp
2704    PCRE contains some optimizations that are used to speed up matching by running
2705    some checks at the start of each match attempt. For example, it may know the
2706    minimum length of matching subject, or that a particular character must be
2707    present. When one of these optimizations suppresses the running of a match, any
2708    included backtracking verbs will not, of course, be processed. You can suppress
2709    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2710    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2711    pattern with (*NO_START_OPT). There is more discussion of this option in the
2712    section entitled
2713    .\" HTML <a href="pcreapi.html#execoptions">
2714    .\" </a>
2715    "Option bits for \fBpcre_exec()\fP"
2716    .\"
2717    in the
2718    .\" HREF
2719    \fBpcreapi\fP
2720    .\"
2721    documentation.
2722    .P
2723    Experiments with Perl suggest that it too has similar optimizations, sometimes
2724    leading to anomalous results.
2725    .
2726  .  .
2727  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2728  .rs  .rs
2729  .sp  .sp
2730  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2731    followed by a name.
2732  .sp  .sp
2733     (*ACCEPT)     (*ACCEPT)
2734  .sp  .sp
2735  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
2736  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2737  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2738  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
2739  captured. For example:  far is captured. For example:
2740  .sp  .sp
2741    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2742  .sp  .sp
2743  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
2744  captured.  the outer parentheses.
2745  .sp  .sp
2746    (*FAIL) or (*F)    (*FAIL) or (*F)
2747  .sp  .sp
2748  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
2749  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
2750  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2751  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 2756  callout feature, as for example in this
2756  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
2757  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2758  .  .
2759    .
2760    .SS "Recording which path was taken"
2761    .rs
2762    .sp
2763    There is one verb whose main purpose is to track how a match was arrived at,
2764    though it also has a secondary use in conjunction with advancing the match
2765    starting point (see (*SKIP) below).
2766    .sp
2767      (*MARK:NAME) or (*:NAME)
2768    .sp
2769    A name is always required with this verb. There may be as many instances of
2770    (*MARK) as you like in a pattern, and their names do not have to be unique.
2771    .P
2772    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2773    path is passed back to the caller as described in the section entitled
2774    .\" HTML <a href="pcreapi.html#extradata">
2775    .\" </a>
2776    "Extra data for \fBpcre_exec()\fP"
2777    .\"
2778    in the
2779    .\" HREF
2780    \fBpcreapi\fP
2781    .\"
2782    documentation. Here is an example of \fBpcretest\fP output, where the /K
2783    modifier requests the retrieval and outputting of (*MARK) data:
2784    .sp
2785        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2786      data> XY
2787       0: XY
2788      MK: A
2789      XZ
2790       0: XZ
2791      MK: B
2792    .sp
2793    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2794    indicates which of the two alternatives matched. This is a more efficient way
2795    of obtaining this information than putting each alternative in its own
2796    capturing parentheses.
2797    .P
2798    If (*MARK) is encountered in a positive assertion, its name is recorded and
2799    passed back if it is the last-encountered. This does not happen for negative
2800    assertions.
2801    .P
2802    After a partial match or a failed match, the name of the last encountered
2803    (*MARK) in the entire match process is returned. For example:
2804    .sp
2805        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2806      data> XP
2807      No match, mark = B
2808    .sp
2809    Note that in this unanchored example the mark is retained from the match
2810    attempt that started at the letter "X" in the subject. Subsequent match
2811    attempts starting at "P" and then with an empty string do not get as far as the
2812    (*MARK) item, but nevertheless do not reset it.
2813    .P
2814    If you are interested in (*MARK) values after failed matches, you should
2815    probably set the PCRE_NO_START_OPTIMIZE option
2816    .\" HTML <a href="#nooptimize">
2817    .\" </a>
2818    (see above)
2819    .\"
2820    to ensure that the match is always attempted.
2821    .
2822    .
2823  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2824  .rs  .rs
2825  .sp  .sp
2826  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2827  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
2828  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
2829    the verb. However, when one of these verbs appears inside an atomic group, its
2830    effect is confined to that group, because once the group has been matched,
2831    there is never any backtracking into it. In this situation, backtracking can
2832    "jump back" to the left of the entire atomic group. (Remember also, as stated
2833    above, that this localization also applies in subroutine calls and assertions.)
2834    .P
2835    These verbs differ in exactly what kind of failure occurs when backtracking
2836    reaches them.
2837  .sp  .sp
2838    (*COMMIT)    (*COMMIT)
2839  .sp  .sp
2840  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
2841  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
2842  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
2843  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
2844  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2845  .sp  .sp
2846    a+(*COMMIT)b    a+(*COMMIT)b
2847  .sp  .sp
2848  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
2849  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
2850  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2851    (*PRUNE)  match failure.
2852  .sp  .P
2853  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,
2854  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
2855  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2856  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2857  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2858  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2859  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2860  be expressed in any other way.    xyzabc\eY
2861      No match
2862    .sp
2863    PCRE knows that any match must start with "a", so the optimization skips along
2864    the subject to "a" before running the first match attempt, which succeeds. When
2865    the optimization is disabled by the \eY escape in the second subject, the match
2866    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2867    starting points.
2868    .sp
2869      (*PRUNE) or (*PRUNE:NAME)
2870    .sp
2871    This verb causes the match to fail at the current starting position in the
2872    subject if the rest of the pattern does not match. If the pattern is
2873    unanchored, the normal "bumpalong" advance to the next starting character then
2874    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2875    reached, or when matching to the right of (*PRUNE), but if there is no match to
2876    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2877    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2878    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2879    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2880    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2881  .sp  .sp
2882    (*SKIP)    (*SKIP)
2883  .sp  .sp
2884  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
2885  "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,
2886  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2887  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
2888    successful match. Consider:
2889  .sp  .sp
2890    a+(*SKIP)b    a+(*SKIP)b
2891  .sp  .sp
2892  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
2893  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
2894  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
2895  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2896  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2897  instead of skipping on to "c".  instead of skipping on to "c".
2898  .sp  .sp
2899    (*THEN)    (*SKIP:NAME)
2900  .sp  .sp
2901  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
2902  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2903  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,
2904  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2905    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2906    matching name is found, the (*SKIP) is ignored.
2907    .sp
2908      (*THEN) or (*THEN:NAME)
2909    .sp
2910    This verb causes a skip to the next innermost alternative if the rest of the
2911    pattern does not match. That is, it cancels pending backtracking, but only
2912    within the current alternative. Its name comes from the observation that it can
2913    be used for a pattern-based if-then-else block:
2914  .sp  .sp
2915    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2916  .sp  .sp
2917  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
2918  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
2919  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2920  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2921    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2922    .P
2923    Note that a subpattern that does not contain a | character is just a part of
2924    the enclosing alternative; it is not a nested alternation with only one
2925    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2926    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2927    pattern fragments that do not contain any | characters at this level:
2928    .sp
2929      A (B(*THEN)C) | D
2930    .sp
2931    If A and B are matched, but there is a failure in C, matching does not
2932    backtrack into A; instead it moves to the next alternative, that is, D.
2933    However, if the subpattern containing (*THEN) is given an alternative, it
2934    behaves differently:
2935    .sp
2936      A (B(*THEN)C | (*FAIL)) | D
2937    .sp
2938    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2939    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2940    because there are no more alternatives to try. In this case, matching does now
2941    backtrack into A.
2942    .P
2943    Note also that a conditional subpattern is not considered as having two
2944    alternatives, because only one is ever used. In other words, the | character in
2945    a conditional subpattern has a different meaning. Ignoring white space,
2946    consider:
2947    .sp
2948      ^.*? (?(?=a) a | b(*THEN)c )
2949    .sp
2950    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2951    it initially matches zero characters. The condition (?=a) then fails, the
2952    character "b" is matched, but "c" is not. At this point, matching does not
2953    backtrack to .*? as might perhaps be expected from the presence of the |
2954    character. The conditional subpattern is part of the single alternative that
2955    comprises the whole pattern, and so the match fails. (If there was a backtrack
2956    into .*?, allowing it to match "b", the match would succeed.)
2957    .P
2958    The verbs just described provide four different "strengths" of control when
2959    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2960    next alternative. (*PRUNE) comes next, failing the match at the current
2961    starting position, but allowing an advance to the next character (for an
2962    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2963    than one character. (*COMMIT) is the strongest, causing the entire match to
2964    fail.
2965    .P
2966    If more than one such verb is present in a pattern, the "strongest" one wins.
2967    For example, consider this pattern, where A, B, etc. are complex pattern
2968    fragments:
2969    .sp
2970      (A(*COMMIT)B(*THEN)C|D)
2971    .sp
2972    Once A has matched, PCRE is committed to this match, at the current starting
2973    position. If subsequently B matches, but C does not, the normal (*THEN) action
2974    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2975    overrides.
2976  .  .
2977  .  .
2978  .SH "SEE ALSO"  .SH "SEE ALSO"
2979  .rs  .rs
2980  .sp  .sp
2981  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2982    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
2983  .  .
2984  .  .
2985  .SH AUTHOR  .SH AUTHOR
# Line 2111  Cambridge CB2 3QH, England. Line 2996  Cambridge CB2 3QH, England.
2996  .rs  .rs
2997  .sp  .sp
2998  .nf  .nf
2999  Last updated: 09 August 2007  Last updated: 03 March 2013
3000  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3001  .fi  .fi

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