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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 strings in the original library, and a
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  second library that supports 16-bit and UTF-16 character strings. To use these
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  features, PCRE must be built to include appropriate support. When using UTF
27  places below. There is also a summary of UTF-8 features in the  strings you must either call the compiling function with the PCRE_UTF8 or
28  .\" HTML <a href="pcre.html#utf8support">  PCRE_UTF16 option, or the pattern must start with one of these special
29  .\" </a>  sequences:
30  section on UTF-8 support  .sp
31  .\"    (*UTF8)
32  in the main    (*UTF16)
33    .sp
34    Starting a pattern with such a sequence is equivalent to setting the relevant
35    option. This feature is not Perl-compatible. How setting a UTF mode affects
36    pattern matching is mentioned in several places below. There is also a summary
37    of features in the
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43    Another special sequence that may appear at the start of a pattern or in
44    combination with (*UTF8) or (*UTF16) is:
45    .sp
46      (*UCP)
47    .sp
48    This has the same effect as setting the PCRE_UCP option: it causes sequences
49    such as \ed and \ew to use Unicode properties to determine character types,
50    instead of recognizing only characters with codes less than 128 via a lookup
51    table.
52    .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .P
58  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
59  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60  From release 6.0, PCRE offers a second matching function,  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62  Perl-compatible. The advantages and disadvantages of the alternative function,  a different algorithm that is not Perl-compatible. Some of the features
63  and how it differs from the normal function, are discussed in the  discussed below are not available when DFA matching is used. The advantages and
64    disadvantages of the alternative functions, and how they differ from the normal
65    functions, are discussed in the
66  .\" HREF  .\" HREF
67  \fBpcrematching\fP  \fBpcrematching\fP
68  .\"  .\"
69  page.  page.
70  .  .
71  .  .
72    .\" HTML <a name="newlines"></a>
73    .SH "NEWLINE CONVENTIONS"
74    .rs
75    .sp
76    PCRE supports five different conventions for indicating line breaks in
77    strings: a single CR (carriage return) character, a single LF (linefeed)
78    character, the two-character sequence CRLF, any of the three preceding, or any
79    Unicode newline sequence. The
80    .\" HREF
81    \fBpcreapi\fP
82    .\"
83    page has
84    .\" HTML <a href="pcreapi.html#newlines">
85    .\" </a>
86    further discussion
87    .\"
88    about newlines, and shows how to set the newline convention in the
89    \fIoptions\fP arguments for the compiling and matching functions.
90    .P
91    It is also possible to specify a newline convention by starting a pattern
92    string with one of the following five sequences:
93    .sp
94      (*CR)        carriage return
95      (*LF)        linefeed
96      (*CRLF)      carriage return, followed by linefeed
97      (*ANYCRLF)   any of the three above
98      (*ANY)       all Unicode newline sequences
99    .sp
100    These override the default and the options given to the compiling function. For
101    example, on a Unix system where LF is the default newline sequence, the pattern
102    .sp
103      (*CR)a.b
104    .sp
105    changes the convention to CR. That pattern matches "a\enb" because LF is no
106    longer a newline. Note that these special settings, which are not
107    Perl-compatible, are recognized only at the very start of a pattern, and that
108    they must be in upper case. If more than one of them is present, the last one
109    is used.
110    .P
111    The newline convention affects the interpretation of the dot metacharacter when
112    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113    affect what the \eR escape sequence matches. By default, this is any Unicode
114    newline sequence, for Perl compatibility. However, this can be changed; see the
115    description of \eR in the section entitled
116    .\" HTML <a href="#newlineseq">
117    .\" </a>
118    "Newline sequences"
119    .\"
120    below. A change of \eR setting can be combined with a change of newline
121    convention.
122    .
123    .
124  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
125  .rs  .rs
126  .sp  .sp
# Line 49  corresponding characters in the subject. Line 132  corresponding characters in the subject.
132  .sp  .sp
133  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
134  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
135  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
136  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
137  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
138  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
139  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
140  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
141  UTF-8 support.  UTF support.
142  .P  .P
143  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
144  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 101  The following sections describe the use Line 184  The following sections describe the use
184  .rs  .rs
185  .sp  .sp
186  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
187  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
188  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
189  outside character classes.  both inside and outside character classes.
190  .P  .P
191  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.
192  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 111  otherwise be interpreted as a metacharac Line 194  otherwise be interpreted as a metacharac
194  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
195  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
196  .P  .P
197  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
198    backslash. All other characters (in particular, those whose codepoints are
199    greater than 127) are treated as literals.
200    .P
201    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
202  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
203  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
204  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.
205  .P  .P
206  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
207  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 130  Perl, $ and @ cause variable interpolati Line 217  Perl, $ and @ cause variable interpolati
217    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
218  .sp  .sp
219  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
220    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221    by \eE later in the pattern, the literal interpretation continues to the end of
222    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223    a character class, this causes an error, because the character class is not
224    terminated.
225  .  .
226  .  .
227  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 139  The \eQ...\eE sequence is recognized bot Line 231  The \eQ...\eE sequence is recognized bot
231  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
232  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
233  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
234  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
235  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:  
236  .sp  .sp
237    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
238    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
239    \ee        escape (hex 1B)    \ee        escape (hex 1B)
240    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
241    \en        newline (hex 0A)    \en        linefeed (hex 0A)
242    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
243    \et        tab (hex 09)    \et        tab (hex 09)
244    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
245    \exhh      character with hex code hh    \exhh      character with hex code hh
246    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247      \euhhhh    character with hex code hhhh (JavaScript mode only)
248  .sp  .sp
249  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
250  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
251  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
252  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
254  After \ex, from zero to two hexadecimal digits are read (letters can be in  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
255  upper or lower case). Any number of hexadecimal digits may appear between \ex{  lower case letter is converted to upper case, and then the 0xc0 bits are
256  and }, but the value of the character code must be less than 256 in non-UTF-8  flipped.)
257  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  .P
258  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  and }, or if there is no terminating }, this form of escape is not recognized.  can be in upper or lower case). Any number of hexadecimal digits may appear
260  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  between \ex{ and }, but the character code is constrained as follows:
261  with no following digits, giving a character whose value is zero.  .sp
262      8-bit non-UTF mode    less than 0x100
263      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
264      16-bit non-UTF mode   less than 0x10000
265      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
266    .sp
267    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
268    "surrogate" codepoints).
269    .P
270    If characters other than hexadecimal digits appear between \ex{ and }, or if
271    there is no terminating }, this form of escape is not recognized. Instead, the
272    initial \ex will be interpreted as a basic hexadecimal escape, with no
273    following digits, giving a character whose value is zero.
274    .P
275    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
276    as just described only when it is followed by two hexadecimal digits.
277    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
278    code points greater than 256 is provided by \eu, which must be followed by
279    four hexadecimal digits; otherwise it matches a literal "u" character.
280    Character codes specified by \eu in JavaScript mode are constrained in the same
281    was as those specified by \ex in non-JavaScript mode.
282  .P  .P
283  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
284  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
285  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
286    \eu00dc in JavaScript mode).
287  .P  .P
288  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
289  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 196  parenthesized subpatterns. Line 309  parenthesized subpatterns.
309  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
310  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
311  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
312  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
313  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.
314  to \e777 are permitted. For example:  For example:
315  .sp  .sp
316    \e040   is another way of writing a space    \e040   is another way of writing a space
317  .\" JOIN  .\" JOIN
# Line 215  to \e777 are permitted. For example: Line 328  to \e777 are permitted. For example:
328              character with octal code 113              character with octal code 113
329  .\" JOIN  .\" JOIN
330    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
331              the byte consisting entirely of 1 bits              the value 255 (decimal)
332  .\" JOIN  .\" JOIN
333    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
334              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 224  Note that octal values of 100 or greater Line 337  Note that octal values of 100 or greater
337  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
338  .P  .P
339  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
340  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
341  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
342  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
343  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
344  meanings  inside a character class. Like other unrecognized escape sequences, they are
345  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
346  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
347  (see below).  sequences have different meanings.
348  .\"  .
349    .
350    .SS "Unsupported escape sequences"
351    .rs
352    .sp
353    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
354    handler and used to modify the case of following characters. By default, PCRE
355    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
356    option is set, \eU matches a "U" character, and \eu can be used to define a
357    character by code point, as described in the previous section.
358  .  .
359  .  .
360  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
361  .rs  .rs
362  .sp  .sp
363  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
364  in braces, is an absolute or relative back reference. Back references are  enclosed in braces, is an absolute or relative back reference. A named back
365  discussed  reference can be coded as \eg{name}. Back references are discussed
366  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
367  .\" </a>  .\" </a>
368  later,  later,
# Line 252  parenthesized subpatterns. Line 374  parenthesized subpatterns.
374  .\"  .\"
375  .  .
376  .  .
377    .SS "Absolute and relative subroutine calls"
378    .rs
379    .sp
380    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
381    a number enclosed either in angle brackets or single quotes, is an alternative
382    syntax for referencing a subpattern as a "subroutine". Details are discussed
383    .\" HTML <a href="#onigurumasubroutines">
384    .\" </a>
385    later.
386    .\"
387    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
388    synonymous. The former is a back reference; the latter is a
389    .\" HTML <a href="#subpatternsassubroutines">
390    .\" </a>
391    subroutine
392    .\"
393    call.
394    .
395    .
396    .\" HTML <a name="genericchartypes"></a>
397  .SS "Generic character types"  .SS "Generic character types"
398  .rs  .rs
399  .sp  .sp
400  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:  
401  .sp  .sp
402    \ed     any decimal digit    \ed     any decimal digit
403    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
404    \es     any whitespace character    \eh     any horizontal white space character
405    \eS     any character that is not a whitespace character    \eH     any character that is not a horizontal white space character
406      \es     any white space character
407      \eS     any character that is not a white space character
408      \ev     any vertical white space character
409      \eV     any character that is not a vertical white space character
410    \ew     any "word" character    \ew     any "word" character
411    \eW     any "non-word" character    \eW     any "non-word" character
412  .sp  .sp
413  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.
414  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
415    .\" HTML <a href="#fullstopdot">
416    .\" </a>
417    the "." metacharacter
418    .\"
419    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
420    PCRE does not support this.
421  .P  .P
422  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
423    of characters into two disjoint sets. Any given character matches one, and only
424    one, of each pair. The sequences can appear both inside and outside character
425  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
426  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
427  there is no character to match.  there is no character to match.
428  .P  .P
429  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).
430  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
431  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
432  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
433  does.)  does.
434  .P  .P
435  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
436  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
437  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
438  place (see  place (see
439  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 293  in the Line 446  in the
446  .\"  .\"
447  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
448  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
449  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
450    Unicode is discouraged.
451  .P  .P
452  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in a UTF mode, characters with values greater than 128 never match
453  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
454  character property support is available. The use of locales with Unicode is  their original meanings from before UTF support was available, mainly for
455  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
456    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
457    properties are used to determine character types, as follows:
458    .sp
459      \ed  any character that \ep{Nd} matches (decimal digit)
460      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
461      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
462    .sp
463    The upper case escapes match the inverse sets of characters. Note that \ed
464    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
465    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
466    \eB because they are defined in terms of \ew and \eW. Matching these sequences
467    is noticeably slower when PCRE_UCP is set.
468    .P
469    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
470    release 5.10. In contrast to the other sequences, which match only ASCII
471    characters by default, these always match certain high-valued codepoints,
472    whether or not PCRE_UCP is set. The horizontal space characters are:
473    .sp
474      U+0009     Horizontal tab
475      U+0020     Space
476      U+00A0     Non-break space
477      U+1680     Ogham space mark
478      U+180E     Mongolian vowel separator
479      U+2000     En quad
480      U+2001     Em quad
481      U+2002     En space
482      U+2003     Em space
483      U+2004     Three-per-em space
484      U+2005     Four-per-em space
485      U+2006     Six-per-em space
486      U+2007     Figure space
487      U+2008     Punctuation space
488      U+2009     Thin space
489      U+200A     Hair space
490      U+202F     Narrow no-break space
491      U+205F     Medium mathematical space
492      U+3000     Ideographic space
493    .sp
494    The vertical space characters are:
495    .sp
496      U+000A     Linefeed
497      U+000B     Vertical tab
498      U+000C     Form feed
499      U+000D     Carriage return
500      U+0085     Next line
501      U+2028     Line separator
502      U+2029     Paragraph separator
503    .sp
504    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
505    relevant.
506  .  .
507  .  .
508    .\" HTML <a name="newlineseq"></a>
509  .SS "Newline sequences"  .SS "Newline sequences"
510  .rs  .rs
511  .sp  .sp
512  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
513  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
514  the following:  following:
515  .sp  .sp
516    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
517  .sp  .sp
# Line 317  below. Line 522  below.
522  .\"  .\"
523  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
524  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,
525  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
526  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
527  cannot be split.  cannot be split.
528  .P  .P
529  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
530  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).
531  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
532  recognized.  recognized.
533  .P  .P
534  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
535    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
536    either at compile time or when the pattern is matched. (BSR is an abbrevation
537    for "backslash R".) This can be made the default when PCRE is built; if this is
538    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
539    It is also possible to specify these settings by starting a pattern string with
540    one of the following sequences:
541    .sp
542      (*BSR_ANYCRLF)   CR, LF, or CRLF only
543      (*BSR_UNICODE)   any Unicode newline sequence
544    .sp
545    These override the default and the options given to the compiling function, but
546    they can themselves be overridden by options given to a matching function. Note
547    that these special settings, which are not Perl-compatible, are recognized only
548    at the very start of a pattern, and that they must be in upper case. If more
549    than one of them is present, the last one is used. They can be combined with a
550    change of newline convention; for example, a pattern can start with:
551    .sp
552      (*ANY)(*BSR_ANYCRLF)
553    .sp
554    They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
555    sequences. Inside a character class, \eR is treated as an unrecognized escape
556    sequence, and so matches the letter "R" by default, but causes an error if
557    PCRE_EXTRA is set.
558  .  .
559  .  .
560  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 334  Inside a character class, \eR matches th Line 562  Inside a character class, \eR matches th
562  .rs  .rs
563  .sp  .sp
564  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
565  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
566  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
567    characters whose codepoints are less than 256, but they do work in this mode.
568    The extra escape sequences are:
569  .sp  .sp
570    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
571    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
572    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
573  .sp  .sp
574  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
575  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
576  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
577  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
578  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
579    .\" </a>
580    next section).
581    .\"
582    Other Perl properties such as "InMusicalSymbols" are not currently supported by
583    PCRE. Note that \eP{Any} does not match any characters, so always causes a
584    match failure.
585  .P  .P
586  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
587  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 359  Those that are not part of an identified Line 595  Those that are not part of an identified
595  .P  .P
596  Arabic,  Arabic,
597  Armenian,  Armenian,
598    Avestan,
599  Balinese,  Balinese,
600    Bamum,
601    Batak,
602  Bengali,  Bengali,
603  Bopomofo,  Bopomofo,
604    Brahmi,
605  Braille,  Braille,
606  Buginese,  Buginese,
607  Buhid,  Buhid,
608  Canadian_Aboriginal,  Canadian_Aboriginal,
609    Carian,
610    Chakma,
611    Cham,
612  Cherokee,  Cherokee,
613  Common,  Common,
614  Coptic,  Coptic,
# Line 374  Cypriot, Line 617  Cypriot,
617  Cyrillic,  Cyrillic,
618  Deseret,  Deseret,
619  Devanagari,  Devanagari,
620    Egyptian_Hieroglyphs,
621  Ethiopic,  Ethiopic,
622  Georgian,  Georgian,
623  Glagolitic,  Glagolitic,
# Line 386  Hangul, Line 630  Hangul,
630  Hanunoo,  Hanunoo,
631  Hebrew,  Hebrew,
632  Hiragana,  Hiragana,
633    Imperial_Aramaic,
634  Inherited,  Inherited,
635    Inscriptional_Pahlavi,
636    Inscriptional_Parthian,
637    Javanese,
638    Kaithi,
639  Kannada,  Kannada,
640  Katakana,  Katakana,
641    Kayah_Li,
642  Kharoshthi,  Kharoshthi,
643  Khmer,  Khmer,
644  Lao,  Lao,
645  Latin,  Latin,
646    Lepcha,
647  Limbu,  Limbu,
648  Linear_B,  Linear_B,
649    Lisu,
650    Lycian,
651    Lydian,
652  Malayalam,  Malayalam,
653    Mandaic,
654    Meetei_Mayek,
655    Meroitic_Cursive,
656    Meroitic_Hieroglyphs,
657    Miao,
658  Mongolian,  Mongolian,
659  Myanmar,  Myanmar,
660  New_Tai_Lue,  New_Tai_Lue,
# Line 403  Nko, Line 662  Nko,
662  Ogham,  Ogham,
663  Old_Italic,  Old_Italic,
664  Old_Persian,  Old_Persian,
665    Old_South_Arabian,
666    Old_Turkic,
667    Ol_Chiki,
668  Oriya,  Oriya,
669  Osmanya,  Osmanya,
670  Phags_Pa,  Phags_Pa,
671  Phoenician,  Phoenician,
672    Rejang,
673  Runic,  Runic,
674    Samaritan,
675    Saurashtra,
676    Sharada,
677  Shavian,  Shavian,
678  Sinhala,  Sinhala,
679    Sora_Sompeng,
680    Sundanese,
681  Syloti_Nagri,  Syloti_Nagri,
682  Syriac,  Syriac,
683  Tagalog,  Tagalog,
684  Tagbanwa,  Tagbanwa,
685  Tai_Le,  Tai_Le,
686    Tai_Tham,
687    Tai_Viet,
688    Takri,
689  Tamil,  Tamil,
690  Telugu,  Telugu,
691  Thaana,  Thaana,
# Line 422  Thai, Line 693  Thai,
693  Tibetan,  Tibetan,
694  Tifinagh,  Tifinagh,
695  Ugaritic,  Ugaritic,
696    Vai,
697  Yi.  Yi.
698  .P  .P
699  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
700  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
701  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
702  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
703  .P  .P
704  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
705  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 487  The special property L& is also supporte Line 759  The special property L& is also supporte
759  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
760  a modifier or "other".  a modifier or "other".
761  .P  .P
762  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
763    U+DFFF. Such characters are not valid in Unicode strings and so
764    cannot be tested by PCRE, unless UTF validity checking has been turned off
765    (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
766    .\" HREF
767    \fBpcreapi\fP
768    .\"
769    page). Perl does not support the Cs property.
770    .P
771    The long synonyms for property names that Perl supports (such as \ep{Letter})
772  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
773  properties with "Is".  properties with "Is".
774  .P  .P
# Line 511  atomic group Line 792  atomic group
792  (see below).  (see below).
793  .\"  .\"
794  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
795  preceding character.  preceding character. None of them have codepoints less than 256, so in
796    8-bit non-UTF-8 mode \eX matches any one character.
797    .P
798    Note that recent versions of Perl have changed \eX to match what Unicode calls
799    an "extended grapheme cluster", which has a more complicated definition.
800  .P  .P
801  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
802  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
803  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
804  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
805    PCRE_UCP option or by starting the pattern with (*UCP).
806    .
807    .
808    .\" HTML <a name="extraprops"></a>
809    .SS PCRE's additional properties
810    .rs
811    .sp
812    As well as the standard Unicode properties described in the previous
813    section, PCRE supports four more that make it possible to convert traditional
814    escape sequences such as \ew and \es and POSIX character classes to use Unicode
815    properties. PCRE uses these non-standard, non-Perl properties internally when
816    PCRE_UCP is set. They are:
817    .sp
818      Xan   Any alphanumeric character
819      Xps   Any POSIX space character
820      Xsp   Any Perl space character
821      Xwd   Any Perl "word" character
822    .sp
823    Xan matches characters that have either the L (letter) or the N (number)
824    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
825    carriage return, and any other character that has the Z (separator) property.
826    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
827    same characters as Xan, plus underscore.
828    .
829    .
830    .\" HTML <a name="resetmatchstart"></a>
831    .SS "Resetting the match start"
832    .rs
833    .sp
834    The escape sequence \eK causes any previously matched characters not to be
835    included in the final matched sequence. For example, the pattern:
836    .sp
837      foo\eKbar
838    .sp
839    matches "foobar", but reports that it has matched "bar". This feature is
840    similar to a lookbehind assertion
841    .\" HTML <a href="#lookbehind">
842    .\" </a>
843    (described below).
844    .\"
845    However, in this case, the part of the subject before the real match does not
846    have to be of fixed length, as lookbehind assertions do. The use of \eK does
847    not interfere with the setting of
848    .\" HTML <a href="#subpattern">
849    .\" </a>
850    captured substrings.
851    .\"
852    For example, when the pattern
853    .sp
854      (foo)\eKbar
855    .sp
856    matches "foobar", the first substring is still set to "foo".
857    .P
858    Perl documents that the use of \eK within assertions is "not well defined". In
859    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
860    ignored in negative assertions.
861  .  .
862  .  .
863  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 541  The backslashed assertions are: Line 882  The backslashed assertions are:
882    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
883    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
884  .sp  .sp
885  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
886  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
887    default it matches the corresponding literal character (for example, \eB
888    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
889    escape sequence" error is generated instead.
890  .P  .P
891  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
892  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
893  \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
894  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
895    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
896    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
897    of word" or "end of word" metasequence. However, whatever follows \eb normally
898    determines which it is. For example, the fragment \eba matches "a" at the start
899    of a word.
900  .P  .P
901  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
902  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 631  end of the subject in both modes, and if Line 980  end of the subject in both modes, and if
980  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
981  .  .
982  .  .
983  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
984    .SH "FULL STOP (PERIOD, DOT) AND \eN"
985  .rs  .rs
986  .sp  .sp
987  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
988  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
989  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
990  .P  .P
991  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
992  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 653  to match it. Line 1003  to match it.
1003  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
1004  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
1005  special meaning in a character class.  special meaning in a character class.
1006  .  .P
1007  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1008  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1009  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1010  .sp  name; PCRE does not support this.
1011  Outside a character class, the escape sequence \eC matches any one byte, both  .
1012  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1013  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1014  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1015  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1016  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1017    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1018    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1019    matches line-ending characters. The feature is provided in Perl in order to
1020    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1021    used. Because \eC breaks up characters into individual data units, matching one
1022    unit with \eC in a UTF mode means that the rest of the string may start with a
1023    malformed UTF character. This has undefined results, because PCRE assumes that
1024    it is dealing with valid UTF strings (and by default it checks this at the
1025    start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1026    is used).
1027  .P  .P
1028  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1029  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1030  .\" </a>  .\" </a>
1031  (described below),  (described below)
1032  .\"  .\"
1033  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
1034  the lookbehind.  the lookbehind.
1035    .P
1036    In general, the \eC escape sequence is best avoided. However, one
1037    way of using it that avoids the problem of malformed UTF characters is to use a
1038    lookahead to check the length of the next character, as in this pattern, which
1039    could be used with a UTF-8 string (ignore white space and line breaks):
1040    .sp
1041      (?| (?=[\ex00-\ex7f])(\eC) |
1042          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1043          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1044          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1045    .sp
1046    A group that starts with (?| resets the capturing parentheses numbers in each
1047    alternative (see
1048    .\" HTML <a href="#dupsubpatternnumber">
1049    .\" </a>
1050    "Duplicate Subpattern Numbers"
1051    .\"
1052    below). The assertions at the start of each branch check the next UTF-8
1053    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1054    character's individual bytes are then captured by the appropriate number of
1055    groups.
1056  .  .
1057  .  .
1058  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 679  the lookbehind. Line 1060  the lookbehind.
1060  .rs  .rs
1061  .sp  .sp
1062  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1063  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.
1064  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
1065  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
1066  escaped with a backslash.  a member of the class, it should be the first data character in the class
1067  .P  (after an initial circumflex, if present) or escaped with a backslash.
1068  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1069  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
1070  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
1071  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
1072  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
1073  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
1074    member of the class, ensure it is not the first character, or escape it with a
1075  backslash.  backslash.
1076  .P  .P
1077  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1078  [^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
1079  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1080  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
1081  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
1082  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
1083  string.  string.
1084  .P  .P
1085  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1086  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  included in a class as a literal string of data units, or by using the \ex{
1087    escaping mechanism.
1088  .P  .P
1089  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
1090  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
1091  "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
1092  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
1093  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
1094  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1095  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1096  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
1097  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
1098  UTF-8 support.  well as with UTF support.
1099  .P  .P
1100  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
1101  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 734  followed by two other characters. The oc Line 1117  followed by two other characters. The oc
1117  "]" can also be used to end a range.  "]" can also be used to end a range.
1118  .P  .P
1119  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
1120  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1121  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}].  
1122  .P  .P
1123  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
1124  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
1125  [][\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
1126  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
1127  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
1128  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
1129  property support.  property support.
1130  .P  .P
1131  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,
1132  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
1133  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1134  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
1135  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
1136  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1137    .\" HTML <a href="#genericchartypes">
1138    .\" </a>
1139    "Generic character types"
1140    .\"
1141    above. The escape sequence \eb has a different meaning inside a character
1142    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1143    are not special inside a character class. Like any other unrecognized escape
1144    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1145    default, but cause an error if the PCRE_EXTRA option is set.
1146    .P
1147    A circumflex can conveniently be used with the upper case character types to
1148    specify a more restricted set of characters than the matching lower case type.
1149    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1150    whereas [\ew] includes underscore. A positive character class should be read as
1151    "something OR something OR ..." and a negative class as "NOT something AND NOT
1152    something AND NOT ...".
1153  .P  .P
1154  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1155  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 771  this notation. For example, Line 1169  this notation. For example,
1169    [01[:alpha:]%]    [01[:alpha:]%]
1170  .sp  .sp
1171  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1172  are  are:
1173  .sp  .sp
1174    alnum    letters and digits    alnum    letters and digits
1175    alpha    letters    alpha    letters
# Line 782  are Line 1180  are
1180    graph    printing characters, excluding space    graph    printing characters, excluding space
1181    lower    lower case letters    lower    lower case letters
1182    print    printing characters, including space    print    printing characters, including space
1183    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1184    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1185    upper    upper case letters    upper    upper case letters
1186    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 803  matches "1", "2", or any non-digit. PCRE Line 1201  matches "1", "2", or any non-digit. PCRE
1201  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
1202  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1203  .P  .P
1204  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
1205  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1206    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1207    character properties are used. This is achieved by replacing the POSIX classes
1208    by other sequences, as follows:
1209    .sp
1210      [:alnum:]  becomes  \ep{Xan}
1211      [:alpha:]  becomes  \ep{L}
1212      [:blank:]  becomes  \eh
1213      [:digit:]  becomes  \ep{Nd}
1214      [:lower:]  becomes  \ep{Ll}
1215      [:space:]  becomes  \ep{Xps}
1216      [:upper:]  becomes  \ep{Lu}
1217      [:word:]   becomes  \ep{Xwd}
1218    .sp
1219    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1220    classes are unchanged, and match only characters with code points less than
1221    128.
1222  .  .
1223  .  .
1224  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 831  alternative in the subpattern. Line 1245  alternative in the subpattern.
1245  .rs  .rs
1246  .sp  .sp
1247  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1248  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
1249  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1250    The option letters are
1251  .sp  .sp
1252    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1253    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1261  PCRE_MULTILINE while unsetting PCRE_DOTA
1261  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
1262  unset.  unset.
1263  .P  .P
1264  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
1265  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
1266  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1267  the global options (and it will therefore show up in data extracted by the  .P
1268  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1269    subpattern parentheses), the change applies to the remainder of the pattern
1270    that follows. If the change is placed right at the start of a pattern, PCRE
1271    extracts it into the global options (and it will therefore show up in data
1272    extracted by the \fBpcre_fullinfo()\fP function).
1273  .P  .P
1274  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1275  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
1276  .sp  .sp
1277    (a(?i)b)c    (a(?i)b)c
1278  .sp  .sp
# Line 869  branch is abandoned before the option se Line 1288  branch is abandoned before the option se
1288  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1289  behaviour otherwise.  behaviour otherwise.
1290  .P  .P
1291  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
1292  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
1293  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1294    what the application has set or what has been defaulted. Details are given in
1295    the section entitled
1296    .\" HTML <a href="#newlineseq">
1297    .\" </a>
1298    "Newline sequences"
1299    .\"
1300    above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1301    can be used to set UTF and Unicode property modes; they are equivalent to
1302    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1303  .  .
1304  .  .
1305  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 885  Turning part of a pattern into a subpatt Line 1313  Turning part of a pattern into a subpatt
1313  .sp  .sp
1314    cat(aract|erpillar|)    cat(aract|erpillar|)
1315  .sp  .sp
1316  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1317  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1318  .sp  .sp
1319  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
1320  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
1321  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
1322  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1323  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1324  .P  .P
1325  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
1326    numbers for the capturing subpatterns. For example, if the string "the red
1327    king" is matched against the pattern
1328  .sp  .sp
1329    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1330  .sp  .sp
# Line 926  is reached, an option setting in one bra Line 1356  is reached, an option setting in one bra
1356  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1357  .  .
1358  .  .
1359    .\" HTML <a name="dupsubpatternnumber"></a>
1360    .SH "DUPLICATE SUBPATTERN NUMBERS"
1361    .rs
1362    .sp
1363    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1364    the same numbers for its capturing parentheses. Such a subpattern starts with
1365    (?| and is itself a non-capturing subpattern. For example, consider this
1366    pattern:
1367    .sp
1368      (?|(Sat)ur|(Sun))day
1369    .sp
1370    Because the two alternatives are inside a (?| group, both sets of capturing
1371    parentheses are numbered one. Thus, when the pattern matches, you can look
1372    at captured substring number one, whichever alternative matched. This construct
1373    is useful when you want to capture part, but not all, of one of a number of
1374    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1375    number is reset at the start of each branch. The numbers of any capturing
1376    parentheses that follow the subpattern start after the highest number used in
1377    any branch. The following example is taken from the Perl documentation. The
1378    numbers underneath show in which buffer the captured content will be stored.
1379    .sp
1380      # before  ---------------branch-reset----------- after
1381      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1382      # 1            2         2  3        2     3     4
1383    .sp
1384    A back reference to a numbered subpattern uses the most recent value that is
1385    set for that number by any subpattern. The following pattern matches "abcabc"
1386    or "defdef":
1387    .sp
1388      /(?|(abc)|(def))\e1/
1389    .sp
1390    In contrast, a subroutine call to a numbered subpattern always refers to the
1391    first one in the pattern with the given number. The following pattern matches
1392    "abcabc" or "defabc":
1393    .sp
1394      /(?|(abc)|(def))(?1)/
1395    .sp
1396    If a
1397    .\" HTML <a href="#conditions">
1398    .\" </a>
1399    condition test
1400    .\"
1401    for a subpattern's having matched refers to a non-unique number, the test is
1402    true if any of the subpatterns of that number have matched.
1403    .P
1404    An alternative approach to using this "branch reset" feature is to use
1405    duplicate named subpatterns, as described in the next section.
1406    .
1407    .
1408  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1409  .rs  .rs
1410  .sp  .sp
# Line 935  if an expression is modified, the number Line 1414  if an expression is modified, the number
1414  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1415  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
1416  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
1417  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1418    have different names, but PCRE does not.
1419  .P  .P
1420  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
1421  (?'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
1422  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1423  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1424  .\" </a>  .\" </a>
1425  backreferences,  back references,
1426  .\"  .\"
1427  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1428  .\" </a>  .\" </a>
# Line 962  extracting the name-to-number translatio Line 1442  extracting the name-to-number translatio
1442  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1443  .P  .P
1444  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
1445  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
1446  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
1447  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
1448  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
1449  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
1450    name, and in both cases you want to extract the abbreviation. This pattern
1451    (ignoring the line breaks) does the job:
1452  .sp  .sp
1453    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1454    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 975  abbreviation. This pattern (ignoring the Line 1457  abbreviation. This pattern (ignoring the
1457    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1458  .sp  .sp
1459  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1460    (An alternative way of solving this problem is to use a "branch reset"
1461    subpattern, as described in the previous section.)
1462    .P
1463  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1464  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
1465  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1466  make a reference to a non-unique named subpattern from elsewhere in the  .P
1467  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
1468  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1469    used. In the absence of duplicate numbers (see the previous section) this is
1470    the one with the lowest number. If you use a named reference in a condition
1471    test (see the
1472    .\"
1473    .\" HTML <a href="#conditions">
1474    .\" </a>
1475    section about conditions
1476    .\"
1477    below), either to check whether a subpattern has matched, or to check for
1478    recursion, all subpatterns with the same name are tested. If the condition is
1479    true for any one of them, the overall condition is true. This is the same
1480    behaviour as testing by number. For further details of the interfaces for
1481    handling named subpatterns, see the
1482  .\" HREF  .\" HREF
1483  \fBpcreapi\fP  \fBpcreapi\fP
1484  .\"  .\"
1485  documentation.  documentation.
1486    .P
1487    \fBWarning:\fP You cannot use different names to distinguish between two
1488    subpatterns with the same number because PCRE uses only the numbers when
1489    matching. For this reason, an error is given at compile time if different names
1490    are given to subpatterns with the same number. However, you can give the same
1491    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1492  .  .
1493  .  .
1494  .SH REPETITION  .SH REPETITION
# Line 996  items: Line 1500  items:
1500    a literal data character    a literal data character
1501    the dot metacharacter    the dot metacharacter
1502    the \eC escape sequence    the \eC escape sequence
1503    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1504    the \eR escape sequence    the \eR escape sequence
1505    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1506    a character class    a character class
1507    a back reference (see next section)    a back reference (see next section)
1508    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1509      a subroutine call to a subpattern (recursive or otherwise)
1510  .sp  .sp
1511  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1512  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1026  where a quantifier is not allowed, or on Line 1531  where a quantifier is not allowed, or on
1531  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
1532  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1533  .P  .P
1534  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
1535  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
1536  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,
1537  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended sequences, each of which may be several
1538  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1539  .P  .P
1540  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
1541  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1542    subpatterns that are referenced as
1543    .\" HTML <a href="#subpatternsassubroutines">
1544    .\" </a>
1545    subroutines
1546    .\"
1547    from elsewhere in the pattern (but see also the section entitled
1548    .\" HTML <a href="#subdefine">
1549    .\" </a>
1550    "Defining subpatterns for use by reference only"
1551    .\"
1552    below). Items other than subpatterns that have a {0} quantifier are omitted
1553    from the compiled pattern.
1554  .P  .P
1555  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1556  abbreviations:  abbreviations:
# Line 1104  In cases where it is known that the subj Line 1621  In cases where it is known that the subj
1621  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1622  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1623  .P  .P
1624  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1625  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1626  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
1627  succeeds. Consider, for example:  succeeds. Consider, for example:
1628  .sp  .sp
# Line 1114  succeeds. Consider, for example: Line 1631  succeeds. Consider, for example:
1631  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
1632  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1633  .P  .P
1634    Another case where implicit anchoring is not applied is when the leading .* is
1635    inside an atomic group. Once again, a match at the start may fail where a later
1636    one succeeds. Consider this pattern:
1637    .sp
1638      (?>.*?a)b
1639    .sp
1640    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1641    (*PRUNE) and (*SKIP) also disable this optimization.
1642    .P
1643  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1644  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1645  .sp  .sp
# Line 1180  previous example can be rewritten as Line 1706  previous example can be rewritten as
1706  .sp  .sp
1707    \ed++foo    \ed++foo
1708  .sp  .sp
1709    Note that a possessive quantifier can be used with an entire group, for
1710    example:
1711    .sp
1712      (abc|xyz){2,3}+
1713    .sp
1714  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1715  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1716  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1253  no such problem when named parentheses a Line 1784  no such problem when named parentheses a
1784  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1785  .P  .P
1786  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
1787  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
1788  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1789  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1790  .sp  .sp
1791    (ring), \e1    (ring), \e1
1792    (ring), \eg1    (ring), \eg1
1793    (ring), \eg{1}    (ring), \eg{1}
1794  .sp  .sp
1795  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1796  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1797  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1798    example:
1799  .sp  .sp
1800    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1801  .sp  .sp
1802  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
1803  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.
1804  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
1805  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
1806  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1807  .P  .P
1808  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1809  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1293  back reference, the case of letters is r Line 1825  back reference, the case of letters is r
1825  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1826  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1827  .P  .P
1828  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1829  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1830    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1831    back reference syntax, in which \eg can be used for both numeric and named
1832    references, is also supported. We could rewrite the above example in any of
1833  the following ways:  the following ways:
1834  .sp  .sp
1835    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1836      (?'p1'(?i)rah)\es+\ek{p1}
1837    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1838      (?<p1>(?i)rah)\es+\eg{p1}
1839  .sp  .sp
1840  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1841  after the reference.  after the reference.
1842  .P  .P
1843  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
1844  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1845  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1846  .sp  .sp
1847    (a|(bc))\e2    (a|(bc))\e2
1848  .sp  .sp
1849  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
1850  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
1851  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1852  with a digit character, some delimiter must be used to terminate the back  .P
1853  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1854  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1855    If the pattern continues with a digit character, some delimiter must be used to
1856    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1857    white space. Otherwise, the \eg{ syntax or an empty comment (see
1858  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1859  .\" </a>  .\" </a>
1860  "Comments"  "Comments"
1861  .\"  .\"
1862  below) can be used.  below) can be used.
1863  .P  .
1864    .SS "Recursive back references"
1865    .rs
1866    .sp
1867  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
1868  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.
1869  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1334  to the previous iteration. In order for Line 1877  to the previous iteration. In order for
1877  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
1878  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
1879  minimum of zero.  minimum of zero.
1880    .P
1881    Back references of this type cause the group that they reference to be treated
1882    as an
1883    .\" HTML <a href="#atomicgroup">
1884    .\" </a>
1885    atomic group.
1886    .\"
1887    Once the whole group has been matched, a subsequent matching failure cannot
1888    cause backtracking into the middle of the group.
1889  .  .
1890  .  .
1891  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1353  those that look ahead of the current pos Line 1905  those that look ahead of the current pos
1905  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,
1906  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.
1907  .P  .P
1908  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1909  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
1910  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1911  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1912  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1913  because it does not make sense for negative assertions.  .P
1914    For compatibility with Perl, assertion subpatterns may be repeated; though
1915    it makes no sense to assert the same thing several times, the side effect of
1916    capturing parentheses may occasionally be useful. In practice, there only three
1917    cases:
1918    .sp
1919    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1920    However, it may contain internal capturing parenthesized groups that are called
1921    from elsewhere via the
1922    .\" HTML <a href="#subpatternsassubroutines">
1923    .\" </a>
1924    subroutine mechanism.
1925    .\"
1926    .sp
1927    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1928    were {0,1}. At run time, the rest of the pattern match is tried with and
1929    without the assertion, the order depending on the greediness of the quantifier.
1930    .sp
1931    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1932    The assertion is obeyed just once when encountered during matching.
1933  .  .
1934  .  .
1935  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1387  lookbehind assertion is needed to achiev Line 1958  lookbehind assertion is needed to achiev
1958  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
1959  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
1960  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.
1961    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1962  .  .
1963  .  .
1964  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1411  is permitted, but Line 1983  is permitted, but
1983  .sp  .sp
1984  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1985  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
1986  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
1987  match the same length of string. An assertion such as  length of string. An assertion such as
1988  .sp  .sp
1989    (?<=ab(c|de))    (?<=ab(c|de))
1990  .sp  .sp
1991  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
1992  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
1993    branches:
1994  .sp  .sp
1995    (?<=abc|abde)    (?<=abc|abde)
1996  .sp  .sp
1997    In some cases, the escape sequence \eK
1998    .\" HTML <a href="#resetmatchstart">
1999    .\" </a>
2000    (see above)
2001    .\"
2002    can be used instead of a lookbehind assertion to get round the fixed-length
2003    restriction.
2004    .P
2005  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2006  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
2007  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2008  assertion fails.  assertion fails.
2009  .P  .P
2010  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
2011  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
2012  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
2013  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2014    permitted.
2015    .P
2016    .\" HTML <a href="#subpatternsassubroutines">
2017    .\" </a>
2018    "Subroutine"
2019    .\"
2020    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2021    as the subpattern matches a fixed-length string.
2022    .\" HTML <a href="#recursion">
2023    .\" </a>
2024    Recursion,
2025    .\"
2026    however, is not supported.
2027  .P  .P
2028  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2029  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
2030  pattern such as  strings. Consider a simple pattern such as
2031  .sp  .sp
2032    abcd$    abcd$
2033  .sp  .sp
# Line 1497  characters that are not "999". Line 2091  characters that are not "999".
2091  .sp  .sp
2092  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2093  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2094  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2095  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2096  .sp  .sp
2097    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2098    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2099  .sp  .sp
2100  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2101  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
2102  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2103    itself contain nested subpatterns of any form, including conditional
2104    subpatterns; the restriction to two alternatives applies only at the level of
2105    the condition. This pattern fragment is an example where the alternatives are
2106    complex:
2107    .sp
2108      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2109    .sp
2110  .P  .P
2111  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2112  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1514  recursion, a pseudo-condition called DEF Line 2115  recursion, a pseudo-condition called DEF
2115  .rs  .rs
2116  .sp  .sp
2117  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
2118  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2119  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
2120  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2121  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2122  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2123  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2124    section about duplicate subpattern numbers),
2125    .\"
2126    the condition is true if any of them have matched. An alternative notation is
2127    to precede the digits with a plus or minus sign. In this case, the subpattern
2128    number is relative rather than absolute. The most recently opened parentheses
2129    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2130    loops it can also make sense to refer to subsequent groups. The next
2131    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2132    zero in any of these forms is not used; it provokes a compile-time error.)
2133  .P  .P
2134  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2135  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1530  three parts for ease of discussion: Line 2140  three parts for ease of discussion:
2140  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2141  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
2142  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
2143  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2144  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,
2145  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
2146  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2147  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2148  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2149  .P  .P
2150  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
2151  reference:  reference:
2152  .sp  .sp
2153    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1560  Rewriting the above example to use a nam Line 2170  Rewriting the above example to use a nam
2170  .sp  .sp
2171    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2172  .sp  .sp
2173    If the name used in a condition of this kind is a duplicate, the test is
2174    applied to all subpatterns of the same name, and is true if any one of them has
2175    matched.
2176  .  .
2177  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2178  .rs  .rs
# Line 1571  letter R, for example: Line 2184  letter R, for example:
2184  .sp  .sp
2185    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2186  .sp  .sp
2187  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
2188  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
2189  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2190    applied to all subpatterns of the same name, and is true if any one of them is
2191    the most recent recursion.
2192  .P  .P
2193  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2194  patterns are described below.  .\" HTML <a href="#recursion">
2195    .\" </a>
2196    The syntax for recursive patterns
2197    .\"
2198    is described below.
2199  .  .
2200    .\" HTML <a name="subdefine"></a>
2201  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2202  .rs  .rs
2203  .sp  .sp
# Line 1585  If the condition is the string (DEFINE), Line 2205  If the condition is the string (DEFINE),
2205  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
2206  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2207  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
2208  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2209  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2210  written like this (ignore whitespace and line breaks):  .\" </a>
2211    subroutines
2212    .\"
2213    is described below.) For example, a pattern to match an IPv4 address such as
2214    "192.168.23.245" could be written like this (ignore white space and line
2215    breaks):
2216  .sp  .sp
2217    (?(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) )
2218    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1595  written like this (ignore whitespace and Line 2220  written like this (ignore whitespace and
2220  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
2221  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2222  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
2223  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2224  .P  pattern uses references to the named group to match the four dot-separated
2225  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.  
2226  .  .
2227  .SS "Assertion conditions"  .SS "Assertion conditions"
2228  .rs  .rs
# Line 1624  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2247  dd-aaa-dd or dd-dd-dd, where aaa are let
2247  .SH COMMENTS  .SH COMMENTS
2248  .rs  .rs
2249  .sp  .sp
2250  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
2251  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,
2252  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
2253    subpattern name or number. The characters that make up a comment play no part
2254    in the pattern matching.
2255  .P  .P
2256  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
2257  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2258  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2259    this case continues to immediately after the next newline character or
2260    character sequence in the pattern. Which characters are interpreted as newlines
2261    is controlled by the options passed to a compiling function or by a special
2262    sequence at the start of the pattern, as described in the section entitled
2263    .\" HTML <a href="#newlines">
2264    .\" </a>
2265    "Newline conventions"
2266    .\"
2267    above. Note that the end of this type of comment is a literal newline sequence
2268    in the pattern; escape sequences that happen to represent a newline do not
2269    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2270    default newline convention is in force:
2271    .sp
2272      abc #comment \en still comment
2273    .sp
2274    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2275    a newline in the pattern. The sequence \en is still literal at this stage, so
2276    it does not terminate the comment. Only an actual character with the code value
2277    0x0a (the default newline) does so.
2278  .  .
2279  .  .
2280  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1656  recursively to the pattern in which it a Line 2300  recursively to the pattern in which it a
2300  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2301  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2302  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2303  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.
2304  .P  .P
2305  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
2306  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
2307  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
2308    .\" HTML <a href="#subpatternsassubroutines">
2309    .\" </a>
2310    non-recursive subroutine
2311    .\"
2312  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
2313  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2314  .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  
2315  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2316  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2317  .sp  .sp
2318    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2319  .sp  .sp
2320  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2321  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
2322  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2323  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2324    to avoid backtracking into sequences of non-parentheses.
2325  .P  .P
2326  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
2327  pattern, so instead you could use this:  pattern, so instead you could use this:
2328  .sp  .sp
2329    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2330  .sp  .sp
2331  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
2332  them instead of the whole pattern.  them instead of the whole pattern.
2333  .P  .P
2334  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
2335  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
2336  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
2337  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2338  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.  
2339  .P  .P
2340  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2341  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2342  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2343  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2344    .\" </a>
2345    non-recursive subroutine
2346    .\"
2347    calls, as described in the next section.
2348  .P  .P
2349  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2350  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
2351  could rewrite the above example as follows:  could rewrite the above example as follows:
2352  .sp  .sp
2353    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2354  .sp  .sp
2355  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
2356  used.  used.
2357  .P  .P
2358  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2359  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
2360  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
2361  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2362  .sp  .sp
2363    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2364  .sp  .sp
2365  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,
2366  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
2367  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
2368  before failure can be reported.  before failure can be reported.
2369  .P  .P
2370  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
2371  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
2372  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  
2373  .\" HREF  .\" HREF
2374  \fBpcrecallout\fP  \fBpcrecallout\fP
2375  .\"  .\"
# Line 1731  documentation). If the pattern above is Line 2377  documentation). If the pattern above is
2377  .sp  .sp
2378    (ab(cd)ef)    (ab(cd)ef)
2379  .sp  .sp
2380  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
2381  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
2382  .sp  matched at the top level, its final captured value is unset, even if it was
2383    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2384       ^                        ^  .P
2385       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2386  .sp  obtain extra memory to store data during a recursion, which it does by using
2387  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
2388  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.  
2389  .P  .P
2390  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.
2391  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1756  different alternatives for the recursive Line 2399  different alternatives for the recursive
2399  is the actual recursive call.  is the actual recursive call.
2400  .  .
2401  .  .
2402    .\" HTML <a name="recursiondifference"></a>
2403    .SS "Differences in recursion processing between PCRE and Perl"
2404    .rs
2405    .sp
2406    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2407    (like Python, but unlike Perl), a recursive subpattern call is always treated
2408    as an atomic group. That is, once it has matched some of the subject string, it
2409    is never re-entered, even if it contains untried alternatives and there is a
2410    subsequent matching failure. This can be illustrated by the following pattern,
2411    which purports to match a palindromic string that contains an odd number of
2412    characters (for example, "a", "aba", "abcba", "abcdcba"):
2413    .sp
2414      ^(.|(.)(?1)\e2)$
2415    .sp
2416    The idea is that it either matches a single character, or two identical
2417    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2418    it does not if the pattern is longer than three characters. Consider the
2419    subject string "abcba":
2420    .P
2421    At the top level, the first character is matched, but as it is not at the end
2422    of the string, the first alternative fails; the second alternative is taken
2423    and the recursion kicks in. The recursive call to subpattern 1 successfully
2424    matches the next character ("b"). (Note that the beginning and end of line
2425    tests are not part of the recursion).
2426    .P
2427    Back at the top level, the next character ("c") is compared with what
2428    subpattern 2 matched, which was "a". This fails. Because the recursion is
2429    treated as an atomic group, there are now no backtracking points, and so the
2430    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2431    try the second alternative.) However, if the pattern is written with the
2432    alternatives in the other order, things are different:
2433    .sp
2434      ^((.)(?1)\e2|.)$
2435    .sp
2436    This time, the recursing alternative is tried first, and continues to recurse
2437    until it runs out of characters, at which point the recursion fails. But this
2438    time we do have another alternative to try at the higher level. That is the big
2439    difference: in the previous case the remaining alternative is at a deeper
2440    recursion level, which PCRE cannot use.
2441    .P
2442    To change the pattern so that it matches all palindromic strings, not just
2443    those with an odd number of characters, it is tempting to change the pattern to
2444    this:
2445    .sp
2446      ^((.)(?1)\e2|.?)$
2447    .sp
2448    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2449    deeper recursion has matched a single character, it cannot be entered again in
2450    order to match an empty string. The solution is to separate the two cases, and
2451    write out the odd and even cases as alternatives at the higher level:
2452    .sp
2453      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2454    .sp
2455    If you want to match typical palindromic phrases, the pattern has to ignore all
2456    non-word characters, which can be done like this:
2457    .sp
2458      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2459    .sp
2460    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2461    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2462    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2463    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2464    more) to match typical phrases, and Perl takes so long that you think it has
2465    gone into a loop.
2466    .P
2467    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2468    string does not start with a palindrome that is shorter than the entire string.
2469    For example, although "abcba" is correctly matched, if the subject is "ababa",
2470    PCRE finds the palindrome "aba" at the start, then fails at top level because
2471    the end of the string does not follow. Once again, it cannot jump back into the
2472    recursion to try other alternatives, so the entire match fails.
2473    .P
2474    The second way in which PCRE and Perl differ in their recursion processing is
2475    in the handling of captured values. In Perl, when a subpattern is called
2476    recursively or as a subpattern (see the next section), it has no access to any
2477    values that were captured outside the recursion, whereas in PCRE these values
2478    can be referenced. Consider this pattern:
2479    .sp
2480      ^(.)(\e1|a(?2))
2481    .sp
2482    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2483    then in the second group, when the back reference \e1 fails to match "b", the
2484    second alternative matches "a" and then recurses. In the recursion, \e1 does
2485    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2486    match because inside the recursive call \e1 cannot access the externally set
2487    value.
2488    .
2489    .
2490  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2491  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2492  .rs  .rs
2493  .sp  .sp
2494  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
2495  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
2496  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2497  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2498  relative, as in these examples:  relative, as in these examples:
2499  .sp  .sp
2500    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2501    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2502    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2503  .sp  .sp
2504  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2505  .sp  .sp
# Line 1782  matches "sense and sensibility" and "res Line 2513  matches "sense and sensibility" and "res
2513  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
2514  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2515  .P  .P
2516  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2517  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
2518  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
2519  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2520  .P  subroutine call revert to their previous values afterwards.
2521  When a subpattern is used as a subroutine, processing options such as  .P
2522  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2523  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2524    different calls. For example, consider this pattern:
2525  .sp  .sp
2526    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2527  .sp  .sp
# Line 1797  It matches "abcabc". It does not match " Line 2529  It matches "abcabc". It does not match "
2529  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2530  .  .
2531  .  .
2532    .\" HTML <a name="onigurumasubroutines"></a>
2533    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2534    .rs
2535    .sp
2536    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2537    a number enclosed either in angle brackets or single quotes, is an alternative
2538    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2539    are two of the examples used above, rewritten using this syntax:
2540    .sp
2541      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2542      (sens|respons)e and \eg'1'ibility
2543    .sp
2544    PCRE supports an extension to Oniguruma: if a number is preceded by a
2545    plus or a minus sign it is taken as a relative reference. For example:
2546    .sp
2547      (abc)(?i:\eg<-1>)
2548    .sp
2549    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2550    synonymous. The former is a back reference; the latter is a subroutine call.
2551    .
2552    .
2553  .SH CALLOUTS  .SH CALLOUTS
2554  .rs  .rs
2555  .sp  .sp
# Line 1807  same pair of parentheses when there is a Line 2560  same pair of parentheses when there is a
2560  .P  .P
2561  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
2562  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
2563  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
2564  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2565    variable contains NULL, which disables all calling out.
2566  .P  .P
2567  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2568  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
# Line 1817  For example, this pattern has two callou Line 2571  For example, this pattern has two callou
2571  .sp  .sp
2572    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2573  .sp  .sp
2574  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
2575  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2576  255.  255.
2577  .P  .P
2578  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  During matching, when PCRE reaches a callout point, the external function is
2579  set), the external function is called. It is provided with the number of the  called. It is provided with the number of the callout, the position in the
2580  callout, the position in the pattern, and, optionally, one item of data  pattern, and, optionally, one item of data originally supplied by the caller of
2581  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2582  may cause matching to proceed, to backtrack, or to fail altogether. A complete  backtrack, or to fail altogether. A complete description of the interface to
2583  description of the interface to the callout function is given in the  the callout function is given in the
2584  .\" HREF  .\" HREF
2585  \fBpcrecallout\fP  \fBpcrecallout\fP
2586  .\"  .\"
2587  documentation.  documentation.
2588  .  .
2589  .  .
2590    .\" HTML <a name="backtrackcontrol"></a>
2591    .SH "BACKTRACKING CONTROL"
2592    .rs
2593    .sp
2594    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2595    are described in the Perl documentation as "experimental and subject to change
2596    or removal in a future version of Perl". It goes on to say: "Their usage in
2597    production code should be noted to avoid problems during upgrades." The same
2598    remarks apply to the PCRE features described in this section.
2599    .P
2600    Since these verbs are specifically related to backtracking, most of them can be
2601    used only when the pattern is to be matched using one of the traditional
2602    matching functions, which use a backtracking algorithm. With the exception of
2603    (*FAIL), which behaves like a failing negative assertion, they cause an error
2604    if encountered by a DFA matching function.
2605    .P
2606    If any of these verbs are used in an assertion or in a subpattern that is
2607    called as a subroutine (whether or not recursively), their effect is confined
2608    to that subpattern; it does not extend to the surrounding pattern, with one
2609    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2610    a successful positive assertion \fIis\fP passed back when a match succeeds
2611    (compare capturing parentheses in assertions). Note that such subpatterns are
2612    processed as anchored at the point where they are tested. Note also that Perl's
2613    treatment of subroutines and assertions is different in some cases.
2614    .P
2615    The new verbs make use of what was previously invalid syntax: an opening
2616    parenthesis followed by an asterisk. They are generally of the form
2617    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2618    depending on whether or not an argument is present. A name is any sequence of
2619    characters that does not include a closing parenthesis. The maximum length of
2620    name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2621    is empty, that is, if the closing parenthesis immediately follows the colon,
2622    the effect is as if the colon were not there. Any number of these verbs may
2623    occur in a pattern.
2624    .
2625    .
2626    .\" HTML <a name="nooptimize"></a>
2627    .SS "Optimizations that affect backtracking verbs"
2628    .rs
2629    .sp
2630    PCRE contains some optimizations that are used to speed up matching by running
2631    some checks at the start of each match attempt. For example, it may know the
2632    minimum length of matching subject, or that a particular character must be
2633    present. When one of these optimizations suppresses the running of a match, any
2634    included backtracking verbs will not, of course, be processed. You can suppress
2635    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2636    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2637    pattern with (*NO_START_OPT). There is more discussion of this option in the
2638    section entitled
2639    .\" HTML <a href="pcreapi.html#execoptions">
2640    .\" </a>
2641    "Option bits for \fBpcre_exec()\fP"
2642    .\"
2643    in the
2644    .\" HREF
2645    \fBpcreapi\fP
2646    .\"
2647    documentation.
2648    .P
2649    Experiments with Perl suggest that it too has similar optimizations, sometimes
2650    leading to anomalous results.
2651    .
2652    .
2653    .SS "Verbs that act immediately"
2654    .rs
2655    .sp
2656    The following verbs act as soon as they are encountered. They may not be
2657    followed by a name.
2658    .sp
2659       (*ACCEPT)
2660    .sp
2661    This verb causes the match to end successfully, skipping the remainder of the
2662    pattern. However, when it is inside a subpattern that is called as a
2663    subroutine, only that subpattern is ended successfully. Matching then continues
2664    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2665    far is captured. For example:
2666    .sp
2667      A((?:A|B(*ACCEPT)|C)D)
2668    .sp
2669    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2670    the outer parentheses.
2671    .sp
2672      (*FAIL) or (*F)
2673    .sp
2674    This verb causes a matching failure, forcing backtracking to occur. It is
2675    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2676    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2677    Perl features that are not present in PCRE. The nearest equivalent is the
2678    callout feature, as for example in this pattern:
2679    .sp
2680      a+(?C)(*FAIL)
2681    .sp
2682    A match with the string "aaaa" always fails, but the callout is taken before
2683    each backtrack happens (in this example, 10 times).
2684    .
2685    .
2686    .SS "Recording which path was taken"
2687    .rs
2688    .sp
2689    There is one verb whose main purpose is to track how a match was arrived at,
2690    though it also has a secondary use in conjunction with advancing the match
2691    starting point (see (*SKIP) below).
2692    .sp
2693      (*MARK:NAME) or (*:NAME)
2694    .sp
2695    A name is always required with this verb. There may be as many instances of
2696    (*MARK) as you like in a pattern, and their names do not have to be unique.
2697    .P
2698    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2699    path is passed back to the caller as described in the section entitled
2700    .\" HTML <a href="pcreapi.html#extradata">
2701    .\" </a>
2702    "Extra data for \fBpcre_exec()\fP"
2703    .\"
2704    in the
2705    .\" HREF
2706    \fBpcreapi\fP
2707    .\"
2708    documentation. Here is an example of \fBpcretest\fP output, where the /K
2709    modifier requests the retrieval and outputting of (*MARK) data:
2710    .sp
2711        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2712      data> XY
2713       0: XY
2714      MK: A
2715      XZ
2716       0: XZ
2717      MK: B
2718    .sp
2719    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2720    indicates which of the two alternatives matched. This is a more efficient way
2721    of obtaining this information than putting each alternative in its own
2722    capturing parentheses.
2723    .P
2724    If (*MARK) is encountered in a positive assertion, its name is recorded and
2725    passed back if it is the last-encountered. This does not happen for negative
2726    assertions.
2727    .P
2728    After a partial match or a failed match, the name of the last encountered
2729    (*MARK) in the entire match process is returned. For example:
2730    .sp
2731        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2732      data> XP
2733      No match, mark = B
2734    .sp
2735    Note that in this unanchored example the mark is retained from the match
2736    attempt that started at the letter "X" in the subject. Subsequent match
2737    attempts starting at "P" and then with an empty string do not get as far as the
2738    (*MARK) item, but nevertheless do not reset it.
2739    .P
2740    If you are interested in (*MARK) values after failed matches, you should
2741    probably set the PCRE_NO_START_OPTIMIZE option
2742    .\" HTML <a href="#nooptimize">
2743    .\" </a>
2744    (see above)
2745    .\"
2746    to ensure that the match is always attempted.
2747    .
2748    .
2749    .SS "Verbs that act after backtracking"
2750    .rs
2751    .sp
2752    The following verbs do nothing when they are encountered. Matching continues
2753    with what follows, but if there is no subsequent match, causing a backtrack to
2754    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2755    the verb. However, when one of these verbs appears inside an atomic group, its
2756    effect is confined to that group, because once the group has been matched,
2757    there is never any backtracking into it. In this situation, backtracking can
2758    "jump back" to the left of the entire atomic group. (Remember also, as stated
2759    above, that this localization also applies in subroutine calls and assertions.)
2760    .P
2761    These verbs differ in exactly what kind of failure occurs when backtracking
2762    reaches them.
2763    .sp
2764      (*COMMIT)
2765    .sp
2766    This verb, which may not be followed by a name, causes the whole match to fail
2767    outright if the rest of the pattern does not match. Even if the pattern is
2768    unanchored, no further attempts to find a match by advancing the starting point
2769    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2770    finding a match at the current starting point, or not at all. For example:
2771    .sp
2772      a+(*COMMIT)b
2773    .sp
2774    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2775    dynamic anchor, or "I've started, so I must finish." The name of the most
2776    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2777    match failure.
2778    .P
2779    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2780    unless PCRE's start-of-match optimizations are turned off, as shown in this
2781    \fBpcretest\fP example:
2782    .sp
2783        re> /(*COMMIT)abc/
2784      data> xyzabc
2785       0: abc
2786      xyzabc\eY
2787      No match
2788    .sp
2789    PCRE knows that any match must start with "a", so the optimization skips along
2790    the subject to "a" before running the first match attempt, which succeeds. When
2791    the optimization is disabled by the \eY escape in the second subject, the match
2792    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2793    starting points.
2794    .sp
2795      (*PRUNE) or (*PRUNE:NAME)
2796    .sp
2797    This verb causes the match to fail at the current starting position in the
2798    subject if the rest of the pattern does not match. If the pattern is
2799    unanchored, the normal "bumpalong" advance to the next starting character then
2800    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2801    reached, or when matching to the right of (*PRUNE), but if there is no match to
2802    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2803    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2804    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2805    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2806    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2807    .sp
2808      (*SKIP)
2809    .sp
2810    This verb, when given without a name, is like (*PRUNE), except that if the
2811    pattern is unanchored, the "bumpalong" advance is not to the next character,
2812    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2813    signifies that whatever text was matched leading up to it cannot be part of a
2814    successful match. Consider:
2815    .sp
2816      a+(*SKIP)b
2817    .sp
2818    If the subject is "aaaac...", after the first match attempt fails (starting at
2819    the first character in the string), the starting point skips on to start the
2820    next attempt at "c". Note that a possessive quantifer does not have the same
2821    effect as this example; although it would suppress backtracking during the
2822    first match attempt, the second attempt would start at the second character
2823    instead of skipping on to "c".
2824    .sp
2825      (*SKIP:NAME)
2826    .sp
2827    When (*SKIP) has an associated name, its behaviour is modified. If the
2828    following pattern fails to match, the previous path through the pattern is
2829    searched for the most recent (*MARK) that has the same name. If one is found,
2830    the "bumpalong" advance is to the subject position that corresponds to that
2831    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2832    matching name is found, the (*SKIP) is ignored.
2833    .sp
2834      (*THEN) or (*THEN:NAME)
2835    .sp
2836    This verb causes a skip to the next innermost alternative if the rest of the
2837    pattern does not match. That is, it cancels pending backtracking, but only
2838    within the current alternative. Its name comes from the observation that it can
2839    be used for a pattern-based if-then-else block:
2840    .sp
2841      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2842    .sp
2843    If the COND1 pattern matches, FOO is tried (and possibly further items after
2844    the end of the group if FOO succeeds); on failure, the matcher skips to the
2845    second alternative and tries COND2, without backtracking into COND1. The
2846    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2847    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2848    .P
2849    Note that a subpattern that does not contain a | character is just a part of
2850    the enclosing alternative; it is not a nested alternation with only one
2851    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2852    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2853    pattern fragments that do not contain any | characters at this level:
2854    .sp
2855      A (B(*THEN)C) | D
2856    .sp
2857    If A and B are matched, but there is a failure in C, matching does not
2858    backtrack into A; instead it moves to the next alternative, that is, D.
2859    However, if the subpattern containing (*THEN) is given an alternative, it
2860    behaves differently:
2861    .sp
2862      A (B(*THEN)C | (*FAIL)) | D
2863    .sp
2864    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2865    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2866    because there are no more alternatives to try. In this case, matching does now
2867    backtrack into A.
2868    .P
2869    Note also that a conditional subpattern is not considered as having two
2870    alternatives, because only one is ever used. In other words, the | character in
2871    a conditional subpattern has a different meaning. Ignoring white space,
2872    consider:
2873    .sp
2874      ^.*? (?(?=a) a | b(*THEN)c )
2875    .sp
2876    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2877    it initially matches zero characters. The condition (?=a) then fails, the
2878    character "b" is matched, but "c" is not. At this point, matching does not
2879    backtrack to .*? as might perhaps be expected from the presence of the |
2880    character. The conditional subpattern is part of the single alternative that
2881    comprises the whole pattern, and so the match fails. (If there was a backtrack
2882    into .*?, allowing it to match "b", the match would succeed.)
2883    .P
2884    The verbs just described provide four different "strengths" of control when
2885    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2886    next alternative. (*PRUNE) comes next, failing the match at the current
2887    starting position, but allowing an advance to the next character (for an
2888    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2889    than one character. (*COMMIT) is the strongest, causing the entire match to
2890    fail.
2891    .P
2892    If more than one such verb is present in a pattern, the "strongest" one wins.
2893    For example, consider this pattern, where A, B, etc. are complex pattern
2894    fragments:
2895    .sp
2896      (A(*COMMIT)B(*THEN)C|D)
2897    .sp
2898    Once A has matched, PCRE is committed to this match, at the current starting
2899    position. If subsequently B matches, but C does not, the normal (*THEN) action
2900    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2901    overrides.
2902    .
2903    .
2904  .SH "SEE ALSO"  .SH "SEE ALSO"
2905  .rs  .rs
2906  .sp  .sp
2907  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2908    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2909  .  .
2910  .  .
2911  .SH AUTHOR  .SH AUTHOR
# Line 1853  Cambridge CB2 3QH, England. Line 2922  Cambridge CB2 3QH, England.
2922  .rs  .rs
2923  .sp  .sp
2924  .nf  .nf
2925  Last updated: 09 May 2007  Last updated: 10 July 2012
2926  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2927  .fi  .fi

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