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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "11 November 2012" "PCRE 8.32"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 strings in the original library, an
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  extra library that supports 16-bit and UTF-16 character strings, and a
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  third library that supports 32-bit and UTF-32 character strings. To use these
27  places below. There is also a summary of UTF-8 features in the  features, PCRE must be built to include appropriate support. When using UTF
28  .\" HTML <a href="pcre.html#utf8support">  strings you must either call the compiling function with the PCRE_UTF8,
29  .\" </a>  PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
30  section on UTF-8 support  these special sequences:
31  .\"  .sp
32  in the main    (*UTF8)
33      (*UTF16)
34      (*UTF32)
35      (*UTF)
36    .sp
37    (*UTF) is a generic sequence that can be used with any of the libraries.
38    Starting a pattern with such a sequence is equivalent to setting the relevant
39    option. This feature is not Perl-compatible. How setting a UTF mode affects
40    pattern matching is mentioned in several places below. There is also a summary
41    of features in the
42  .\" HREF  .\" HREF
43  \fBpcre\fP  \fBpcreunicode\fP
44  .\"  .\"
45  page.  page.
46  .P  .P
47    Another special sequence that may appear at the start of a pattern or in
48    combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) is:
49    .sp
50      (*UCP)
51    .sp
52    This has the same effect as setting the PCRE_UCP option: it causes sequences
53    such as \ed and \ew to use Unicode properties to determine character types,
54    instead of recognizing only characters with codes less than 128 via a lookup
55    table.
56    .P
57    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
58    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
59    also some more of these special sequences that are concerned with the handling
60    of newlines; they are described below.
61    .P
62  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
63  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
64  From release 6.0, PCRE offers a second matching function,  \fBpcre[16|32]_exec()\fP (16- or 32-bit), is used. PCRE also has alternative
65  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  matching functions, \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP,
66  Perl-compatible. Some of the features discussed below are not available when  which match using a different algorithm that is not Perl-compatible. Some of
67  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  the features discussed below are not available when DFA matching is used. The
68  alternative function, and how it differs from the normal function, are  advantages and disadvantages of the alternative functions, and how they differ
69  discussed in the  from the normal functions, are discussed in the
70  .\" HREF  .\" HREF
71  \fBpcrematching\fP  \fBpcrematching\fP
72  .\"  .\"
73  page.  page.
74  .  .
75  .  .
76    .SH "EBCDIC CHARACTER CODES"
77    .rs
78    .sp
79    PCRE can be compiled to run in an environment that uses EBCDIC as its character
80    code rather than ASCII or Unicode (typically a mainframe system). In the
81    sections below, character code values are ASCII or Unicode; in an EBCDIC
82    environment these characters may have different code values, and there are no
83    code points greater than 255.
84    .
85    .
86    .\" HTML <a name="newlines"></a>
87  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
88  .rs  .rs
89  .sp  .sp
# Line 76  string with one of the following five se Line 111  string with one of the following five se
111    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
112    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
113  .sp  .sp
114  These override the default and the options given to \fBpcre_compile()\fP. For  These override the default and the options given to the compiling function. For
115  example, on a Unix system where LF is the default newline sequence, the pattern  example, on a Unix system where LF is the default newline sequence, the pattern
116  .sp  .sp
117    (*CR)a.b    (*CR)a.b
# Line 87  Perl-compatible, are recognized only at Line 122  Perl-compatible, are recognized only at
122  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
123  is used.  is used.
124  .P  .P
125  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects where the circumflex and dollar assertions are
126  default, this is any Unicode newline sequence, for Perl compatibility. However,  true. It also affects the interpretation of the dot metacharacter when
127  this can be changed; see the description of \eR in the section entitled  PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
128    what the \eR escape sequence matches. By default, this is any Unicode newline
129    sequence, for Perl compatibility. However, this can be changed; see the
130    description of \eR in the section entitled
131  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
132  .\" </a>  .\" </a>
133  "Newline sequences"  "Newline sequences"
# Line 109  corresponding characters in the subject. Line 147  corresponding characters in the subject.
147  .sp  .sp
148  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
149  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
150  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
151  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
152  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
153  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
154  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
155  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
156  UTF-8 support.  UTF support.
157  .P  .P
158  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
159  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 161  The following sections describe the use Line 199  The following sections describe the use
199  .rs  .rs
200  .sp  .sp
201  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
202  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
203  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
204  outside character classes.  both inside and outside character classes.
205  .P  .P
206  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
207  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 171  otherwise be interpreted as a metacharac Line 209  otherwise be interpreted as a metacharac
209  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
210  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
211  .P  .P
212  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  In a UTF mode, only ASCII numbers and letters have any special meaning after a
213    backslash. All other characters (in particular, those whose codepoints are
214    greater than 127) are treated as literals.
215    .P
216    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
217  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
218  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
219  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
220  .P  .P
221  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
222  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 190  Perl, $ and @ cause variable interpolati Line 232  Perl, $ and @ cause variable interpolati
232    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
233  .sp  .sp
234  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
235    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
236    by \eE later in the pattern, the literal interpretation continues to the end of
237    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
238    a character class, this causes an error, because the character class is not
239    terminated.
240  .  .
241  .  .
242  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 199  The \eQ...\eE sequence is recognized bot Line 246  The \eQ...\eE sequence is recognized bot
246  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
247  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
248  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
249  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
250  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
251  .sp  .sp
252    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
253    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
254    \ee        escape (hex 1B)    \ee        escape (hex 1B)
255    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
256    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
257    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
258    \et        tab (hex 09)    \et        tab (hex 09)
259    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
260    \exhh      character with hex code hh    \exhh      character with hex code hh
261    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
262      \euhhhh    character with hex code hhhh (JavaScript mode only)
263    .sp
264    The precise effect of \ecx on ASCII characters is as follows: if x is a lower
265    case letter, it is converted to upper case. Then bit 6 of the character (hex
266    40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
267    but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
268    data item (byte or 16-bit value) following \ec has a value greater than 127, a
269    compile-time error occurs. This locks out non-ASCII characters in all modes.
270    .P
271    The \ec facility was designed for use with ASCII characters, but with the
272    extension to Unicode it is even less useful than it once was. It is, however,
273    recognized when PCRE is compiled in EBCDIC mode, where data items are always
274    bytes. In this mode, all values are valid after \ec. If the next character is a
275    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
276    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
277    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
278    characters also generate different values.
279    .P
280    By default, after \ex, from zero to two hexadecimal digits are read (letters
281    can be in upper or lower case). Any number of hexadecimal digits may appear
282    between \ex{ and }, but the character code is constrained as follows:
283    .sp
284      8-bit non-UTF mode    less than 0x100
285      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
286      16-bit non-UTF mode   less than 0x10000
287      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
288      32-bit non-UTF mode   less than 0x80000000
289      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
290  .sp  .sp
291  The precise effect of \ecx is as follows: if x is a lower case letter, it  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
292  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  "surrogate" codepoints), and 0xffef.
 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  
 7B.  
 .P  
 After \ex, from zero to two hexadecimal digits are read (letters can be in  
 upper or lower case). Any number of hexadecimal digits may appear between \ex{  
 and }, but the value of the character code must be less than 256 in non-UTF-8  
 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  
 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  
 point, which is 10FFFF.  
293  .P  .P
294  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
295  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
296  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
297  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
298  .P  .P
299    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
300    as just described only when it is followed by two hexadecimal digits.
301    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
302    code points greater than 256 is provided by \eu, which must be followed by
303    four hexadecimal digits; otherwise it matches a literal "u" character.
304    Character codes specified by \eu in JavaScript mode are constrained in the same
305    was as those specified by \ex in non-JavaScript mode.
306    .P
307  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
308  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
309  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
310    \eu00dc in JavaScript mode).
311  .P  .P
312  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
313  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 259  parenthesized subpatterns. Line 333  parenthesized subpatterns.
333  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
334  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
335  digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
336  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. The value of the character is
337  character specified in octal must be less than \e400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
338  to \e777 are permitted. For example:  For example:
339  .sp  .sp
340    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
341  .\" JOIN  .\" JOIN
342    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
343              previous capturing subpatterns              previous capturing subpatterns
# Line 278  to \e777 are permitted. For example: Line 352  to \e777 are permitted. For example:
352              character with octal code 113              character with octal code 113
353  .\" JOIN  .\" JOIN
354    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
355              the byte consisting entirely of 1 bits              the value 255 (decimal)
356  .\" JOIN  .\" JOIN
357    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
358              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 287  Note that octal values of 100 or greater Line 361  Note that octal values of 100 or greater
361  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
362  .P  .P
363  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
364  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
365  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
366  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
367  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
368  meanings  inside a character class. Like other unrecognized escape sequences, they are
369  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
370  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
371  (see below).  sequences have different meanings.
372  .\"  .
373    .
374    .SS "Unsupported escape sequences"
375    .rs
376    .sp
377    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
378    handler and used to modify the case of following characters. By default, PCRE
379    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
380    option is set, \eU matches a "U" character, and \eu can be used to define a
381    character by code point, as described in the previous section.
382  .  .
383  .  .
384  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 326  syntax for referencing a subpattern as a Line 409  syntax for referencing a subpattern as a
409  later.  later.
410  .\"  .\"
411  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
412  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
413    .\" HTML <a href="#subpatternsassubroutines">
414    .\" </a>
415    subroutine
416    .\"
417    call.
418  .  .
419  .  .
420    .\" HTML <a name="genericchartypes"></a>
421  .SS "Generic character types"  .SS "Generic character types"
422  .rs  .rs
423  .sp  .sp
424  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
425  .sp  .sp
426    \ed     any decimal digit    \ed     any decimal digit
427    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
428    \eh     any horizontal whitespace character    \eh     any horizontal white space character
429    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
430    \es     any whitespace character    \es     any white space character
431    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
432    \ev     any vertical whitespace character    \ev     any vertical white space character
433    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
434    \ew     any "word" character    \ew     any "word" character
435    \eW     any "non-word" character    \eW     any "non-word" character
436  .sp  .sp
437  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
438  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
439  .P  .\" HTML <a href="#fullstopdot">
440  These character type sequences can appear both inside and outside character  .\" </a>
441    the "." metacharacter
442    .\"
443    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
444    PCRE does not support this.
445    .P
446    Each pair of lower and upper case escape sequences partitions the complete set
447    of characters into two disjoint sets. Any given character matches one, and only
448    one, of each pair. The sequences can appear both inside and outside character
449  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
450  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
451  there is no character to match.  there is no character to match.
452  .P  .P
453  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 360  are HT (9), LF (10), FF (12), CR (13), a Line 456  are HT (9), LF (10), FF (12), CR (13), a
456  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
457  does.  does.
458  .P  .P
459  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
460  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
461  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
462  meanings from before UTF-8 support was available, mainly for efficiency  place (see
463  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
464  and \eW.  .\" </a>
465  .P  "Locale support"
466  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
467  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
468  The horizontal space characters are:  .\" HREF
469    \fBpcreapi\fP
470    .\"
471    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
472    or "french" in Windows, some character codes greater than 128 are used for
473    accented letters, and these are then matched by \ew. The use of locales with
474    Unicode is discouraged.
475    .P
476    By default, in a UTF mode, characters with values greater than 128 never match
477    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
478    their original meanings from before UTF support was available, mainly for
479    efficiency reasons. However, if PCRE is compiled with Unicode property support,
480    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
481    properties are used to determine character types, as follows:
482    .sp
483      \ed  any character that \ep{Nd} matches (decimal digit)
484      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
485      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
486    .sp
487    The upper case escapes match the inverse sets of characters. Note that \ed
488    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
489    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
490    \eB because they are defined in terms of \ew and \eW. Matching these sequences
491    is noticeably slower when PCRE_UCP is set.
492    .P
493    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
494    release 5.10. In contrast to the other sequences, which match only ASCII
495    characters by default, these always match certain high-valued codepoints,
496    whether or not PCRE_UCP is set. The horizontal space characters are:
497  .sp  .sp
498    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
499    U+0020     Space    U+0020     Space
500    U+00A0     Non-break space    U+00A0     Non-break space
501    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 393  The horizontal space characters are: Line 517  The horizontal space characters are:
517  .sp  .sp
518  The vertical space characters are:  The vertical space characters are:
519  .sp  .sp
520    U+000A     Linefeed    U+000A     Linefeed (LF)
521    U+000B     Vertical tab    U+000B     Vertical tab (VT)
522    U+000C     Formfeed    U+000C     Form feed (FF)
523    U+000D     Carriage return    U+000D     Carriage return (CR)
524    U+0085     Next line    U+0085     Next line (NEL)
525    U+2028     Line separator    U+2028     Line separator
526    U+2029     Paragraph separator    U+2029     Paragraph separator
527  .P  .sp
528  A "word" character is an underscore or any character less than 256 that is a  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
529  letter or digit. The definition of letters and digits is controlled by PCRE's  relevant.
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
530  .  .
531  .  .
532  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 424  is discouraged. Line 534  is discouraged.
534  .rs  .rs
535  .sp  .sp
536  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
537  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
538  equivalent to the following:  following:
539  .sp  .sp
540    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
541  .sp  .sp
# Line 436  below. Line 546  below.
546  .\"  .\"
547  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
548  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
549  U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next  U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
550  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
551  cannot be split.  cannot be split.
552  .P  .P
553  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
554  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
555  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
556  recognized.  recognized.
# Line 456  one of the following sequences: Line 566  one of the following sequences:
566    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
567    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
568  .sp  .sp
569  These override the default and the options given to \fBpcre_compile()\fP, but  These override the default and the options given to the compiling function, but
570  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  they can themselves be overridden by options given to a matching function. Note
571  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
572  very start of a pattern, and that they must be in upper case. If more than one  at the very start of a pattern, and that they must be in upper case. If more
573  of them is present, the last one is used. They can be combined with a change of  than one of them is present, the last one is used. They can be combined with a
574  newline convention, for example, a pattern can start with:  change of newline convention; for example, a pattern can start with:
575  .sp  .sp
576    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
577  .sp  .sp
578  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
579    (*UCP) special sequences. Inside a character class, \eR is treated as an
580    unrecognized escape sequence, and so matches the letter "R" by default, but
581    causes an error if PCRE_EXTRA is set.
582  .  .
583  .  .
584  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 474  Inside a character class, \eR matches th Line 587  Inside a character class, \eR matches th
587  .sp  .sp
588  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
589  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
590  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
591  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
592  The extra escape sequences are:  The extra escape sequences are:
593  .sp  .sp
594    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
595    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
596    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
597  .sp  .sp
598  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
599  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
600  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
601  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
602  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
603    .\" </a>
604    next section).
605    .\"
606    Other Perl properties such as "InMusicalSymbols" are not currently supported by
607    PCRE. Note that \eP{Any} does not match any characters, so always causes a
608    match failure.
609  .P  .P
610  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
611  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 500  Those that are not part of an identified Line 619  Those that are not part of an identified
619  .P  .P
620  Arabic,  Arabic,
621  Armenian,  Armenian,
622    Avestan,
623  Balinese,  Balinese,
624    Bamum,
625    Batak,
626  Bengali,  Bengali,
627  Bopomofo,  Bopomofo,
628    Brahmi,
629  Braille,  Braille,
630  Buginese,  Buginese,
631  Buhid,  Buhid,
632  Canadian_Aboriginal,  Canadian_Aboriginal,
633    Carian,
634    Chakma,
635    Cham,
636  Cherokee,  Cherokee,
637  Common,  Common,
638  Coptic,  Coptic,
# Line 515  Cypriot, Line 641  Cypriot,
641  Cyrillic,  Cyrillic,
642  Deseret,  Deseret,
643  Devanagari,  Devanagari,
644    Egyptian_Hieroglyphs,
645  Ethiopic,  Ethiopic,
646  Georgian,  Georgian,
647  Glagolitic,  Glagolitic,
# Line 527  Hangul, Line 654  Hangul,
654  Hanunoo,  Hanunoo,
655  Hebrew,  Hebrew,
656  Hiragana,  Hiragana,
657    Imperial_Aramaic,
658  Inherited,  Inherited,
659    Inscriptional_Pahlavi,
660    Inscriptional_Parthian,
661    Javanese,
662    Kaithi,
663  Kannada,  Kannada,
664  Katakana,  Katakana,
665    Kayah_Li,
666  Kharoshthi,  Kharoshthi,
667  Khmer,  Khmer,
668  Lao,  Lao,
669  Latin,  Latin,
670    Lepcha,
671  Limbu,  Limbu,
672  Linear_B,  Linear_B,
673    Lisu,
674    Lycian,
675    Lydian,
676  Malayalam,  Malayalam,
677    Mandaic,
678    Meetei_Mayek,
679    Meroitic_Cursive,
680    Meroitic_Hieroglyphs,
681    Miao,
682  Mongolian,  Mongolian,
683  Myanmar,  Myanmar,
684  New_Tai_Lue,  New_Tai_Lue,
# Line 544  Nko, Line 686  Nko,
686  Ogham,  Ogham,
687  Old_Italic,  Old_Italic,
688  Old_Persian,  Old_Persian,
689    Old_South_Arabian,
690    Old_Turkic,
691    Ol_Chiki,
692  Oriya,  Oriya,
693  Osmanya,  Osmanya,
694  Phags_Pa,  Phags_Pa,
695  Phoenician,  Phoenician,
696    Rejang,
697  Runic,  Runic,
698    Samaritan,
699    Saurashtra,
700    Sharada,
701  Shavian,  Shavian,
702  Sinhala,  Sinhala,
703    Sora_Sompeng,
704    Sundanese,
705  Syloti_Nagri,  Syloti_Nagri,
706  Syriac,  Syriac,
707  Tagalog,  Tagalog,
708  Tagbanwa,  Tagbanwa,
709  Tai_Le,  Tai_Le,
710    Tai_Tham,
711    Tai_Viet,
712    Takri,
713  Tamil,  Tamil,
714  Telugu,  Telugu,
715  Thaana,  Thaana,
# Line 563  Thai, Line 717  Thai,
717  Tibetan,  Tibetan,
718  Tifinagh,  Tifinagh,
719  Ugaritic,  Ugaritic,
720    Vai,
721  Yi.  Yi.
722  .P  .P
723  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
724  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
725  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
726  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
727  .P  .P
728  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
729  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 629  the Lu, Ll, or Lt property, in other wor Line 784  the Lu, Ll, or Lt property, in other wor
784  a modifier or "other".  a modifier or "other".
785  .P  .P
786  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
787  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
788  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
789  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
790    PCRE_NO_UTF32_CHECK in the
791  .\" HREF  .\" HREF
792  \fBpcreapi\fP  \fBpcreapi\fP
793  .\"  .\"
794  page).  page). Perl does not support the Cs property.
795  .P  .P
796  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
797  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
798  properties with "Is".  properties with "Is".
799  .P  .P
# Line 648  Unicode table. Line 804  Unicode table.
804  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
805  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
806  .P  .P
807  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
808  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
809  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
810    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
811    PCRE_UCP option or by starting the pattern with (*UCP).
812    .
813    .
814    .SS Extended grapheme clusters
815    .rs
816  .sp  .sp
817  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
818  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
819  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
820  .\" </a>  .\" </a>
821  (see below).  (see below).
822  .\"  .\"
823  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
824  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
825  non-UTF-8 mode \eX matches any one character.  .sp
826  .P    (?>\ePM\epM*)
827  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
828  a structure that contains data for over fifteen thousand characters. That is  That is, it matched a character without the "mark" property, followed by zero
829  why the traditional escape sequences such as \ed and \ew do not use Unicode  or more characters with the "mark" property. Characters with the "mark"
830  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
831    .P
832    This simple definition was extended in Unicode to include more complicated
833    kinds of composite character by giving each character a grapheme breaking
834    property, and creating rules that use these properties to define the boundaries
835    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
836    one of these clusters.
837    .P
838    \eX always matches at least one character. Then it decides whether to add
839    additional characters according to the following rules for ending a cluster:
840    .P
841    1. End at the end of the subject string.
842    .P
843    2. Do not end between CR and LF; otherwise end after any control character.
844    .P
845    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
846    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
847    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
848    character; an LVT or T character may be follwed only by a T character.
849    .P
850    4. Do not end before extending characters or spacing marks. Characters with
851    the "mark" property always have the "extend" grapheme breaking property.
852    .P
853    5. Do not end after prepend characters.
854    .P
855    6. Otherwise, end the cluster.
856    .
857    .
858    .\" HTML <a name="extraprops"></a>
859    .SS PCRE's additional properties
860    .rs
861    .sp
862    As well as the standard Unicode properties described above, PCRE supports four
863    more that make it possible to convert traditional escape sequences such as \ew
864    and \es and POSIX character classes to use Unicode properties. PCRE uses these
865    non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
866    .sp
867      Xan   Any alphanumeric character
868      Xps   Any POSIX space character
869      Xsp   Any Perl space character
870      Xwd   Any Perl "word" character
871    .sp
872    Xan matches characters that have either the L (letter) or the N (number)
873    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
874    carriage return, and any other character that has the Z (separator) property.
875    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
876    same characters as Xan, plus underscore.
877  .  .
878  .  .
879  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
880  .SS "Resetting the match start"  .SS "Resetting the match start"
881  .rs  .rs
882  .sp  .sp
883  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
884  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
885  .sp  .sp
886    foo\eKbar    foo\eKbar
887  .sp  .sp
# Line 698  For example, when the pattern Line 903  For example, when the pattern
903    (foo)\eKbar    (foo)\eKbar
904  .sp  .sp
905  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
906    .P
907    Perl documents that the use of \eK within assertions is "not well defined". In
908    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
909    ignored in negative assertions.
910  .  .
911  .  .
912  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 722  The backslashed assertions are: Line 931  The backslashed assertions are:
931    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
932    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
933  .sp  .sp
934  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
935  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
936    default it matches the corresponding literal character (for example, \eB
937    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
938    escape sequence" error is generated instead.
939  .P  .P
940  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
941  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
942  \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
943  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
944    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
945    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
946    of word" or "end of word" metasequence. However, whatever follows \eb normally
947    determines which it is. For example, the fragment \eba matches "a" at the start
948    of a word.
949  .P  .P
950  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
951  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 762  regular expression. Line 979  regular expression.
979  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
980  .rs  .rs
981  .sp  .sp
982    The circumflex and dollar metacharacters are zero-width assertions. That is,
983    they test for a particular condition being true without consuming any
984    characters from the subject string.
985    .P
986  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
987  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
988  at the start of the subject string. If the \fIstartoffset\fP argument of  the start of the subject string. If the \fIstartoffset\fP argument of
989  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
990  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
991  meaning  meaning
# Line 781  constrained to match only at the start o Line 1002  constrained to match only at the start o
1002  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1003  to be anchored.)  to be anchored.)
1004  .P  .P
1005  A dollar character is an assertion that is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1006  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1007  at the end of the string (by default). Dollar need not be the last character of  the end of the string (by default). Note, however, that it does not actually
1008  the pattern if a number of alternatives are involved, but it should be the last  match the newline. Dollar need not be the last character of the pattern if a
1009  item in any branch in which it appears. Dollar has no special meaning in a  number of alternatives are involved, but it should be the last item in any
1010  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1011  .P  .P
1012  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1013  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
# Line 812  end of the subject in both modes, and if Line 1033  end of the subject in both modes, and if
1033  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1034  .  .
1035  .  .
1036  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1037    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1038  .rs  .rs
1039  .sp  .sp
1040  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
1041  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
1042  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1043  .P  .P
1044  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
1045  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 834  to match it. Line 1056  to match it.
1056  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
1057  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
1058  special meaning in a character class.  special meaning in a character class.
1059  .  .P
1060  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1061  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1062  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1063  .sp  name; PCRE does not support this.
1064  Outside a character class, the escape sequence \eC matches any one byte, both  .
1065  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1066  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1067  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1068  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1069  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1070    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1071    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1072    a 32-bit unit. Unlike a dot, \eC always
1073    matches line-ending characters. The feature is provided in Perl in order to
1074    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1075    used. Because \eC breaks up characters into individual data units, matching one
1076    unit with \eC in a UTF mode means that the rest of the string may start with a
1077    malformed UTF character. This has undefined results, because PCRE assumes that
1078    it is dealing with valid UTF strings (and by default it checks this at the
1079    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1080    PCRE_NO_UTF32_CHECK option is used).
1081  .P  .P
1082  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1083  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1084  .\" </a>  .\" </a>
1085  (described below),  (described below)
1086  .\"  .\"
1087  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
1088  the lookbehind.  the lookbehind.
1089    .P
1090    In general, the \eC escape sequence is best avoided. However, one
1091    way of using it that avoids the problem of malformed UTF characters is to use a
1092    lookahead to check the length of the next character, as in this pattern, which
1093    could be used with a UTF-8 string (ignore white space and line breaks):
1094    .sp
1095      (?| (?=[\ex00-\ex7f])(\eC) |
1096          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1097          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1098          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1099    .sp
1100    A group that starts with (?| resets the capturing parentheses numbers in each
1101    alternative (see
1102    .\" HTML <a href="#dupsubpatternnumber">
1103    .\" </a>
1104    "Duplicate Subpattern Numbers"
1105    .\"
1106    below). The assertions at the start of each branch check the next UTF-8
1107    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1108    character's individual bytes are then captured by the appropriate number of
1109    groups.
1110  .  .
1111  .  .
1112  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 860  the lookbehind. Line 1114  the lookbehind.
1114  .rs  .rs
1115  .sp  .sp
1116  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1117  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.
1118  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
1119  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
1120  escaped with a backslash.  a member of the class, it should be the first data character in the class
1121  .P  (after an initial circumflex, if present) or escaped with a backslash.
1122  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1123  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
1124  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
1125  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
1126  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
1127  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
1128    member of the class, ensure it is not the first character, or escape it with a
1129  backslash.  backslash.
1130  .P  .P
1131  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1132  [^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
1133  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1134  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
1135  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
1136  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
1137  string.  string.
1138  .P  .P
1139  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1140  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  can be included in a class as a literal string of data units, or by using the
1141    \ex{ escaping mechanism.
1142  .P  .P
1143  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
1144  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
1145  "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
1146  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
1147  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
1148  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1149  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1150  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
1151  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
1152  UTF-8 support.  well as with UTF support.
1153  .P  .P
1154  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
1155  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 915  followed by two other characters. The oc Line 1171  followed by two other characters. The oc
1171  "]" can also be used to end a range.  "]" can also be used to end a range.
1172  .P  .P
1173  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
1174  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1175  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}].  
1176  .P  .P
1177  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
1178  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
1179  [][\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
1180  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
1181  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
1182  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
1183  property support.  property support.
1184  .P  .P
1185  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,
1186  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
1187  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1188  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
1189  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
1190  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1191    .\" HTML <a href="#genericchartypes">
1192    .\" </a>
1193    "Generic character types"
1194    .\"
1195    above. The escape sequence \eb has a different meaning inside a character
1196    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1197    are not special inside a character class. Like any other unrecognized escape
1198    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1199    default, but cause an error if the PCRE_EXTRA option is set.
1200    .P
1201    A circumflex can conveniently be used with the upper case character types to
1202    specify a more restricted set of characters than the matching lower case type.
1203    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1204    whereas [\ew] includes underscore. A positive character class should be read as
1205    "something OR something OR ..." and a negative class as "NOT something AND NOT
1206    something AND NOT ...".
1207  .P  .P
1208  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1209  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 952  this notation. For example, Line 1223  this notation. For example,
1223    [01[:alpha:]%]    [01[:alpha:]%]
1224  .sp  .sp
1225  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1226  are  are:
1227  .sp  .sp
1228    alnum    letters and digits    alnum    letters and digits
1229    alpha    letters    alpha    letters
# Line 963  are Line 1234  are
1234    graph    printing characters, excluding space    graph    printing characters, excluding space
1235    lower    lower case letters    lower    lower case letters
1236    print    printing characters, including space    print    printing characters, including space
1237    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1238    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1239    upper    upper case letters    upper    upper case letters
1240    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 984  matches "1", "2", or any non-digit. PCRE Line 1255  matches "1", "2", or any non-digit. PCRE
1255  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
1256  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1257  .P  .P
1258  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
1259  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1260    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1261    character properties are used. This is achieved by replacing the POSIX classes
1262    by other sequences, as follows:
1263    .sp
1264      [:alnum:]  becomes  \ep{Xan}
1265      [:alpha:]  becomes  \ep{L}
1266      [:blank:]  becomes  \eh
1267      [:digit:]  becomes  \ep{Nd}
1268      [:lower:]  becomes  \ep{Ll}
1269      [:space:]  becomes  \ep{Xps}
1270      [:upper:]  becomes  \ep{Lu}
1271      [:word:]   becomes  \ep{Xwd}
1272    .sp
1273    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1274    classes are unchanged, and match only characters with code points less than
1275    128.
1276  .  .
1277  .  .
1278  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1032  The PCRE-specific options PCRE_DUPNAMES, Line 1319  The PCRE-specific options PCRE_DUPNAMES,
1319  changed in the same way as the Perl-compatible options by using the characters  changed in the same way as the Perl-compatible options by using the characters
1320  J, U and X respectively.  J, U and X respectively.
1321  .P  .P
1322  When an option change occurs at top level (that is, not inside subpattern  When one of these option changes occurs at top level (that is, not inside
1323  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1324  If the change is placed right at the start of a pattern, PCRE extracts it into  that follows. If the change is placed right at the start of a pattern, PCRE
1325  the global options (and it will therefore show up in data extracted by the  extracts it into the global options (and it will therefore show up in data
1326  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1327  .P  .P
1328  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1329  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
1330  .sp  .sp
1331    (a(?i)b)c    (a(?i)b)c
1332  .sp  .sp
# Line 1056  option settings happen at compile time. Line 1343  option settings happen at compile time.
1343  behaviour otherwise.  behaviour otherwise.
1344  .P  .P
1345  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1346  application when the compile or match functions are called. In some cases the  application when the compiling or matching functions are called. In some cases
1347  pattern can contain special leading sequences to override what the application  the pattern can contain special leading sequences such as (*CRLF) to override
1348  has set or what has been defaulted. Details are given in the section entitled  what the application has set or what has been defaulted. Details are given in
1349    the section entitled
1350  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1351  .\" </a>  .\" </a>
1352  "Newline sequences"  "Newline sequences"
1353  .\"  .\"
1354  above.  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1355    sequences that can be used to set UTF and Unicode property modes; they are
1356    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1357    options, respectively. The (*UTF) sequence is a generic version that can be
1358    used with any of the libraries.
1359  .  .
1360  .  .
1361  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1077  Turning part of a pattern into a subpatt Line 1369  Turning part of a pattern into a subpatt
1369  .sp  .sp
1370    cat(aract|erpillar|)    cat(aract|erpillar|)
1371  .sp  .sp
1372  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1373  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1374  .sp  .sp
1375  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
1376  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
1377  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
1378  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1379  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1380  .P  .P
1381  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
1382    numbers for the capturing subpatterns. For example, if the string "the red
1383    king" is matched against the pattern
1384  .sp  .sp
1385    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1386  .sp  .sp
# Line 1118  is reached, an option setting in one bra Line 1412  is reached, an option setting in one bra
1412  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1413  .  .
1414  .  .
1415    .\" HTML <a name="dupsubpatternnumber"></a>
1416  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1417  .rs  .rs
1418  .sp  .sp
# Line 1134  at captured substring number one, whiche Line 1429  at captured substring number one, whiche
1429  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1430  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1431  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1432  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1433  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1434  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1435  .sp  .sp
1436    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1437    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1438    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1439  .sp  .sp
1440  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1441  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1442    or "defdef":
1443    .sp
1444      /(?|(abc)|(def))\e1/
1445    .sp
1446    In contrast, a subroutine call to a numbered subpattern always refers to the
1447    first one in the pattern with the given number. The following pattern matches
1448    "abcabc" or "defabc":
1449    .sp
1450      /(?|(abc)|(def))(?1)/
1451    .sp
1452    If a
1453    .\" HTML <a href="#conditions">
1454    .\" </a>
1455    condition test
1456    .\"
1457    for a subpattern's having matched refers to a non-unique number, the test is
1458    true if any of the subpatterns of that number have matched.
1459  .P  .P
1460  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1461  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1159  if an expression is modified, the number Line 1470  if an expression is modified, the number
1470  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1471  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
1472  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
1473  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1474    have different names, but PCRE does not.
1475  .P  .P
1476  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
1477  (?'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
1478  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1479  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1480  .\" </a>  .\" </a>
1481  backreferences,  back references,
1482  .\"  .\"
1483  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1484  .\" </a>  .\" </a>
# Line 1186  extracting the name-to-number translatio Line 1498  extracting the name-to-number translatio
1498  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1499  .P  .P
1500  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
1501  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
1502  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
1503  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
1504  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
1505  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
1506    name, and in both cases you want to extract the abbreviation. This pattern
1507    (ignoring the line breaks) does the job:
1508  .sp  .sp
1509    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1510    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1204  subpattern, as described in the previous Line 1518  subpattern, as described in the previous
1518  .P  .P
1519  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1520  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
1521  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1522  make a reference to a non-unique named subpattern from elsewhere in the  .P
1523  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
1524  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1525    used. In the absence of duplicate numbers (see the previous section) this is
1526    the one with the lowest number. If you use a named reference in a condition
1527    test (see the
1528    .\"
1529    .\" HTML <a href="#conditions">
1530    .\" </a>
1531    section about conditions
1532    .\"
1533    below), either to check whether a subpattern has matched, or to check for
1534    recursion, all subpatterns with the same name are tested. If the condition is
1535    true for any one of them, the overall condition is true. This is the same
1536    behaviour as testing by number. For further details of the interfaces for
1537    handling named subpatterns, see the
1538  .\" HREF  .\" HREF
1539  \fBpcreapi\fP  \fBpcreapi\fP
1540  .\"  .\"
1541  documentation.  documentation.
1542  .P  .P
1543  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1544  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1545  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1546    are given to subpatterns with the same number. However, you can give the same
1547    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1548  .  .
1549  .  .
1550  .SH REPETITION  .SH REPETITION
# Line 1227  items: Line 1556  items:
1556    a literal data character    a literal data character
1557    the dot metacharacter    the dot metacharacter
1558    the \eC escape sequence    the \eC escape sequence
1559    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1560    the \eR escape sequence    the \eR escape sequence
1561    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1562    a character class    a character class
1563    a back reference (see next section)    a back reference (see next section)
1564    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1565      a subroutine call to a subpattern (recursive or otherwise)
1566  .sp  .sp
1567  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1568  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1257  where a quantifier is not allowed, or on Line 1587  where a quantifier is not allowed, or on
1587  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
1588  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1589  .P  .P
1590  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
1591  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
1592  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,
1593  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1594  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1595  .P  .P
1596  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
1597  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
# Line 1270  subpatterns that are referenced as Line 1600  subpatterns that are referenced as
1600  .\" </a>  .\" </a>
1601  subroutines  subroutines
1602  .\"  .\"
1603  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1604  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1605    .\" </a>
1606    "Defining subpatterns for use by reference only"
1607    .\"
1608    below). Items other than subpatterns that have a {0} quantifier are omitted
1609    from the compiled pattern.
1610  .P  .P
1611  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1612  abbreviations:  abbreviations:
# Line 1342  In cases where it is known that the subj Line 1677  In cases where it is known that the subj
1677  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1678  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1679  .P  .P
1680  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1681  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1682  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
1683  succeeds. Consider, for example:  succeeds. Consider, for example:
1684  .sp  .sp
# Line 1352  succeeds. Consider, for example: Line 1687  succeeds. Consider, for example:
1687  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
1688  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1689  .P  .P
1690    Another case where implicit anchoring is not applied is when the leading .* is
1691    inside an atomic group. Once again, a match at the start may fail where a later
1692    one succeeds. Consider this pattern:
1693    .sp
1694      (?>.*?a)b
1695    .sp
1696    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1697    (*PRUNE) and (*SKIP) also disable this optimization.
1698    .P
1699  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1700  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1701  .sp  .sp
# Line 1496  no such problem when named parentheses a Line 1840  no such problem when named parentheses a
1840  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1841  .P  .P
1842  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
1843  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
1844  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1845  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1846  .sp  .sp
1847    (ring), \e1    (ring), \e1
1848    (ring), \eg1    (ring), \eg1
# Line 1512  example: Line 1856  example:
1856    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1857  .sp  .sp
1858  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
1859  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.
1860  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
1861  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
1862  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1863  .P  .P
1864  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1865  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1554  after the reference. Line 1898  after the reference.
1898  .P  .P
1899  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
1900  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1901  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1902  .sp  .sp
1903    (a|(bc))\e2    (a|(bc))\e2
1904  .sp  .sp
1905  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
1906  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
1907  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1908  with a digit character, some delimiter must be used to terminate the back  .P
1909  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1910  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1911    If the pattern continues with a digit character, some delimiter must be used to
1912    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1913    white space. Otherwise, the \eg{ syntax or an empty comment (see
1914  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1915  .\" </a>  .\" </a>
1916  "Comments"  "Comments"
1917  .\"  .\"
1918  below) can be used.  below) can be used.
1919  .P  .
1920    .SS "Recursive back references"
1921    .rs
1922    .sp
1923  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
1924  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.
1925  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1583  to the previous iteration. In order for Line 1933  to the previous iteration. In order for
1933  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
1934  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
1935  minimum of zero.  minimum of zero.
1936    .P
1937    Back references of this type cause the group that they reference to be treated
1938    as an
1939    .\" HTML <a href="#atomicgroup">
1940    .\" </a>
1941    atomic group.
1942    .\"
1943    Once the whole group has been matched, a subsequent matching failure cannot
1944    cause backtracking into the middle of the group.
1945  .  .
1946  .  .
1947  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1602  those that look ahead of the current pos Line 1961  those that look ahead of the current pos
1961  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,
1962  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.
1963  .P  .P
1964  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1965  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
1966  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1967  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1968  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1969  because it does not make sense for negative assertions.  .P
1970    For compatibility with Perl, assertion subpatterns may be repeated; though
1971    it makes no sense to assert the same thing several times, the side effect of
1972    capturing parentheses may occasionally be useful. In practice, there only three
1973    cases:
1974    .sp
1975    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1976    However, it may contain internal capturing parenthesized groups that are called
1977    from elsewhere via the
1978    .\" HTML <a href="#subpatternsassubroutines">
1979    .\" </a>
1980    subroutine mechanism.
1981    .\"
1982    .sp
1983    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1984    were {0,1}. At run time, the rest of the pattern match is tried with and
1985    without the assertion, the order depending on the greediness of the quantifier.
1986    .sp
1987    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1988    The assertion is obeyed just once when encountered during matching.
1989  .  .
1990  .  .
1991  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1636  lookbehind assertion is needed to achiev Line 2014  lookbehind assertion is needed to achiev
2014  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
2015  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
2016  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.
2017    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2018  .  .
2019  .  .
2020  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1660  is permitted, but Line 2039  is permitted, but
2039  .sp  .sp
2040  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2041  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
2042  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
2043  match the same length of string. An assertion such as  length of string. An assertion such as
2044  .sp  .sp
2045    (?<=ab(c|de))    (?<=ab(c|de))
2046  .sp  .sp
2047  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
2048  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
2049    branches:
2050  .sp  .sp
2051    (?<=abc|abde)    (?<=abc|abde)
2052  .sp  .sp
2053  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2054  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2055  .\" </a>  .\" </a>
2056  (see above)  (see above)
2057  .\"  .\"
2058  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
2059  fixed-length.  restriction.
2060  .P  .P
2061  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2062  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
2063  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2064  assertion fails.  assertion fails.
2065  .P  .P
2066  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
2067  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
2068  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
2069  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2070    permitted.
2071    .P
2072    .\" HTML <a href="#subpatternsassubroutines">
2073    .\" </a>
2074    "Subroutine"
2075    .\"
2076    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2077    as the subpattern matches a fixed-length string.
2078    .\" HTML <a href="#recursion">
2079    .\" </a>
2080    Recursion,
2081    .\"
2082    however, is not supported.
2083  .P  .P
2084  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2085  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
2086  pattern such as  strings. Consider a simple pattern such as
2087  .sp  .sp
2088    abcd$    abcd$
2089  .sp  .sp
# Line 1754  characters that are not "999". Line 2147  characters that are not "999".
2147  .sp  .sp
2148  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2149  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2150  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2151  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2152  .sp  .sp
2153    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2154    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2155  .sp  .sp
2156  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2157  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
2158  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2159    itself contain nested subpatterns of any form, including conditional
2160    subpatterns; the restriction to two alternatives applies only at the level of
2161    the condition. This pattern fragment is an example where the alternatives are
2162    complex:
2163    .sp
2164      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2165    .sp
2166  .P  .P
2167  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2168  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1771  recursion, a pseudo-condition called DEF Line 2171  recursion, a pseudo-condition called DEF
2171  .rs  .rs
2172  .sp  .sp
2173  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
2174  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2175  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
2176  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2177  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2178  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2179  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2180    section about duplicate subpattern numbers),
2181    .\"
2182    the condition is true if any of them have matched. An alternative notation is
2183    to precede the digits with a plus or minus sign. In this case, the subpattern
2184    number is relative rather than absolute. The most recently opened parentheses
2185    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2186    loops it can also make sense to refer to subsequent groups. The next
2187    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2188    zero in any of these forms is not used; it provokes a compile-time error.)
2189  .P  .P
2190  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2191  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 1787  three parts for ease of discussion: Line 2196  three parts for ease of discussion:
2196  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2197  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
2198  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
2199  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2200  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,
2201  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
2202  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2203  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1817  Rewriting the above example to use a nam Line 2226  Rewriting the above example to use a nam
2226  .sp  .sp
2227    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2228  .sp  .sp
2229    If the name used in a condition of this kind is a duplicate, the test is
2230    applied to all subpatterns of the same name, and is true if any one of them has
2231    matched.
2232  .  .
2233  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2234  .rs  .rs
# Line 1828  letter R, for example: Line 2240  letter R, for example:
2240  .sp  .sp
2241    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2242  .sp  .sp
2243  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
2244  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
2245  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2246    applied to all subpatterns of the same name, and is true if any one of them is
2247    the most recent recursion.
2248  .P  .P
2249  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2250  patterns are described below.  .\" HTML <a href="#recursion">
2251    .\" </a>
2252    The syntax for recursive patterns
2253    .\"
2254    is described below.
2255  .  .
2256    .\" HTML <a name="subdefine"></a>
2257  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2258  .rs  .rs
2259  .sp  .sp
# Line 1842  If the condition is the string (DEFINE), Line 2261  If the condition is the string (DEFINE),
2261  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
2262  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2263  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
2264  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2265  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2266  written like this (ignore whitespace and line breaks):  .\" </a>
2267    subroutines
2268    .\"
2269    is described below.) For example, a pattern to match an IPv4 address such as
2270    "192.168.23.245" could be written like this (ignore white space and line
2271    breaks):
2272  .sp  .sp
2273    (?(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) )
2274    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1852  written like this (ignore whitespace and Line 2276  written like this (ignore whitespace and
2276  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
2277  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2278  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
2279  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2280  .P  pattern uses references to the named group to match the four dot-separated
2281  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.  
2282  .  .
2283  .SS "Assertion conditions"  .SS "Assertion conditions"
2284  .rs  .rs
# Line 1881  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2303  dd-aaa-dd or dd-dd-dd, where aaa are let
2303  .SH COMMENTS  .SH COMMENTS
2304  .rs  .rs
2305  .sp  .sp
2306  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
2307  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,
2308  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
2309    subpattern name or number. The characters that make up a comment play no part
2310    in the pattern matching.
2311  .P  .P
2312  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
2313  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2314  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2315    this case continues to immediately after the next newline character or
2316    character sequence in the pattern. Which characters are interpreted as newlines
2317    is controlled by the options passed to a compiling function or by a special
2318    sequence at the start of the pattern, as described in the section entitled
2319    .\" HTML <a href="#newlines">
2320    .\" </a>
2321    "Newline conventions"
2322    .\"
2323    above. Note that the end of this type of comment is a literal newline sequence
2324    in the pattern; escape sequences that happen to represent a newline do not
2325    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2326    default newline convention is in force:
2327    .sp
2328      abc #comment \en still comment
2329    .sp
2330    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2331    a newline in the pattern. The sequence \en is still literal at this stage, so
2332    it does not terminate the comment. Only an actual character with the code value
2333    0x0a (the default newline) does so.
2334  .  .
2335  .  .
2336  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1913  recursively to the pattern in which it a Line 2356  recursively to the pattern in which it a
2356  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2357  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2358  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2359  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.
2360  .P  .P
2361  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
2362  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
2363  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
2364    .\" HTML <a href="#subpatternsassubroutines">
2365    .\" </a>
2366    non-recursive subroutine
2367    .\"
2368  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
2369  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2370  .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  
2371  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2372  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2373  .sp  .sp
2374    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2375  .sp  .sp
2376  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2377  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
2378  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2379  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2380    to avoid backtracking into sequences of non-parentheses.
2381  .P  .P
2382  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
2383  pattern, so instead you could use this:  pattern, so instead you could use this:
2384  .sp  .sp
2385    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2386  .sp  .sp
2387  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
2388  them instead of the whole pattern.  them instead of the whole pattern.
2389  .P  .P
2390  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
2391  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
2392  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
2393  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2394  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.  
2395  .P  .P
2396  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2397  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2398  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2399  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2400    .\" </a>
2401    non-recursive subroutine
2402    .\"
2403    calls, as described in the next section.
2404  .P  .P
2405  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2406  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
2407  could rewrite the above example as follows:  could rewrite the above example as follows:
2408  .sp  .sp
2409    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2410  .sp  .sp
2411  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
2412  used.  used.
2413  .P  .P
2414  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2415  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
2416  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
2417  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2418  .sp  .sp
2419    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2420  .sp  .sp
2421  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,
2422  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
2423  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
2424  before failure can be reported.  before failure can be reported.
2425  .P  .P
2426  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
2427  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
2428  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  
2429  .\" HREF  .\" HREF
2430  \fBpcrecallout\fP  \fBpcrecallout\fP
2431  .\"  .\"
# Line 1988  documentation). If the pattern above is Line 2433  documentation). If the pattern above is
2433  .sp  .sp
2434    (ab(cd)ef)    (ab(cd)ef)
2435  .sp  .sp
2436  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
2437  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
2438  .sp  matched at the top level, its final captured value is unset, even if it was
2439    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2440       ^                        ^  .P
2441       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2442  .sp  obtain extra memory to store data during a recursion, which it does by using
2443  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
2444  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.  
2445  .P  .P
2446  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.
2447  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2013  different alternatives for the recursive Line 2455  different alternatives for the recursive
2455  is the actual recursive call.  is the actual recursive call.
2456  .  .
2457  .  .
2458    .\" HTML <a name="recursiondifference"></a>
2459    .SS "Differences in recursion processing between PCRE and Perl"
2460    .rs
2461    .sp
2462    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2463    (like Python, but unlike Perl), a recursive subpattern call is always treated
2464    as an atomic group. That is, once it has matched some of the subject string, it
2465    is never re-entered, even if it contains untried alternatives and there is a
2466    subsequent matching failure. This can be illustrated by the following pattern,
2467    which purports to match a palindromic string that contains an odd number of
2468    characters (for example, "a", "aba", "abcba", "abcdcba"):
2469    .sp
2470      ^(.|(.)(?1)\e2)$
2471    .sp
2472    The idea is that it either matches a single character, or two identical
2473    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2474    it does not if the pattern is longer than three characters. Consider the
2475    subject string "abcba":
2476    .P
2477    At the top level, the first character is matched, but as it is not at the end
2478    of the string, the first alternative fails; the second alternative is taken
2479    and the recursion kicks in. The recursive call to subpattern 1 successfully
2480    matches the next character ("b"). (Note that the beginning and end of line
2481    tests are not part of the recursion).
2482    .P
2483    Back at the top level, the next character ("c") is compared with what
2484    subpattern 2 matched, which was "a". This fails. Because the recursion is
2485    treated as an atomic group, there are now no backtracking points, and so the
2486    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2487    try the second alternative.) However, if the pattern is written with the
2488    alternatives in the other order, things are different:
2489    .sp
2490      ^((.)(?1)\e2|.)$
2491    .sp
2492    This time, the recursing alternative is tried first, and continues to recurse
2493    until it runs out of characters, at which point the recursion fails. But this
2494    time we do have another alternative to try at the higher level. That is the big
2495    difference: in the previous case the remaining alternative is at a deeper
2496    recursion level, which PCRE cannot use.
2497    .P
2498    To change the pattern so that it matches all palindromic strings, not just
2499    those with an odd number of characters, it is tempting to change the pattern to
2500    this:
2501    .sp
2502      ^((.)(?1)\e2|.?)$
2503    .sp
2504    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2505    deeper recursion has matched a single character, it cannot be entered again in
2506    order to match an empty string. The solution is to separate the two cases, and
2507    write out the odd and even cases as alternatives at the higher level:
2508    .sp
2509      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2510    .sp
2511    If you want to match typical palindromic phrases, the pattern has to ignore all
2512    non-word characters, which can be done like this:
2513    .sp
2514      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2515    .sp
2516    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2517    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2518    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2519    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2520    more) to match typical phrases, and Perl takes so long that you think it has
2521    gone into a loop.
2522    .P
2523    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2524    string does not start with a palindrome that is shorter than the entire string.
2525    For example, although "abcba" is correctly matched, if the subject is "ababa",
2526    PCRE finds the palindrome "aba" at the start, then fails at top level because
2527    the end of the string does not follow. Once again, it cannot jump back into the
2528    recursion to try other alternatives, so the entire match fails.
2529    .P
2530    The second way in which PCRE and Perl differ in their recursion processing is
2531    in the handling of captured values. In Perl, when a subpattern is called
2532    recursively or as a subpattern (see the next section), it has no access to any
2533    values that were captured outside the recursion, whereas in PCRE these values
2534    can be referenced. Consider this pattern:
2535    .sp
2536      ^(.)(\e1|a(?2))
2537    .sp
2538    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2539    then in the second group, when the back reference \e1 fails to match "b", the
2540    second alternative matches "a" and then recurses. In the recursion, \e1 does
2541    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2542    match because inside the recursive call \e1 cannot access the externally set
2543    value.
2544    .
2545    .
2546  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2547  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2548  .rs  .rs
2549  .sp  .sp
2550  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
2551  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
2552  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2553  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2554  relative, as in these examples:  relative, as in these examples:
2555  .sp  .sp
# Line 2039  matches "sense and sensibility" and "res Line 2569  matches "sense and sensibility" and "res
2569  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
2570  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2571  .P  .P
2572  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2573  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
2574  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
2575  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2576  .P  subroutine call revert to their previous values afterwards.
2577  When a subpattern is used as a subroutine, processing options such as  .P
2578  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2579  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2580    different calls. For example, consider this pattern:
2581  .sp  .sp
2582    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2583  .sp  .sp
# Line 2085  same pair of parentheses when there is a Line 2616  same pair of parentheses when there is a
2616  .P  .P
2617  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
2618  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
2619  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
2620    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2621  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2622  .P  .P
2623  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
# Line 2095  For example, this pattern has two callou Line 2627  For example, this pattern has two callou
2627  .sp  .sp
2628    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2629  .sp  .sp
2630  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
2631  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2632  255.  255.
2633  .P  .P
2634  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
2635  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
2636  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
2637  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2638  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
2639  description of the interface to the callout function is given in the  the callout function is given in the
2640  .\" HREF  .\" HREF
2641  \fBpcrecallout\fP  \fBpcrecallout\fP
2642  .\"  .\"
2643  documentation.  documentation.
2644  .  .
2645  .  .
2646    .\" HTML <a name="backtrackcontrol"></a>
2647  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2648  .rs  .rs
2649  .sp  .sp
# Line 2121  production code should be noted to avoid Line 2654  production code should be noted to avoid
2654  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2655  .P  .P
2656  Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
2657  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  used only when the pattern is to be matched using one of the traditional
2658  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2659  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2660  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2661    .P
2662    If any of these verbs are used in an assertion or in a subpattern that is
2663    called as a subroutine (whether or not recursively), their effect is confined
2664    to that subpattern; it does not extend to the surrounding pattern, with one
2665    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2666    a successful positive assertion \fIis\fP passed back when a match succeeds
2667    (compare capturing parentheses in assertions). Note that such subpatterns are
2668    processed as anchored at the point where they are tested. Note also that Perl's
2669    treatment of subroutines and assertions is different in some cases.
2670  .P  .P
2671  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2672  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2673  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2674  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2675  are two kinds:  characters that does not include a closing parenthesis. The maximum length of
2676    name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit library.
2677    If the name is empty, that is, if the closing parenthesis immediately follows
2678    the colon, the effect is as if the colon were not there. Any number of these
2679    verbs may occur in a pattern.
2680    .
2681    .
2682    .\" HTML <a name="nooptimize"></a>
2683    .SS "Optimizations that affect backtracking verbs"
2684    .rs
2685    .sp
2686    PCRE contains some optimizations that are used to speed up matching by running
2687    some checks at the start of each match attempt. For example, it may know the
2688    minimum length of matching subject, or that a particular character must be
2689    present. When one of these optimizations suppresses the running of a match, any
2690    included backtracking verbs will not, of course, be processed. You can suppress
2691    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2692    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2693    pattern with (*NO_START_OPT). There is more discussion of this option in the
2694    section entitled
2695    .\" HTML <a href="pcreapi.html#execoptions">
2696    .\" </a>
2697    "Option bits for \fBpcre_exec()\fP"
2698    .\"
2699    in the
2700    .\" HREF
2701    \fBpcreapi\fP
2702    .\"
2703    documentation.
2704    .P
2705    Experiments with Perl suggest that it too has similar optimizations, sometimes
2706    leading to anomalous results.
2707    .
2708  .  .
2709  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2710  .rs  .rs
2711  .sp  .sp
2712  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2713    followed by a name.
2714  .sp  .sp
2715     (*ACCEPT)     (*ACCEPT)
2716  .sp  .sp
2717  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2718  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2719  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2720  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2721  captured. For example:  far is captured. For example:
2722  .sp  .sp
2723    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2724  .sp  .sp
2725  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2726  captured.  the outer parentheses.
2727  .sp  .sp
2728    (*FAIL) or (*F)    (*FAIL) or (*F)
2729  .sp  .sp
2730  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2731  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2732  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2733  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2163  callout feature, as for example in this Line 2738  callout feature, as for example in this
2738  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2739  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2740  .  .
2741    .
2742    .SS "Recording which path was taken"
2743    .rs
2744    .sp
2745    There is one verb whose main purpose is to track how a match was arrived at,
2746    though it also has a secondary use in conjunction with advancing the match
2747    starting point (see (*SKIP) below).
2748    .sp
2749      (*MARK:NAME) or (*:NAME)
2750    .sp
2751    A name is always required with this verb. There may be as many instances of
2752    (*MARK) as you like in a pattern, and their names do not have to be unique.
2753    .P
2754    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2755    path is passed back to the caller as described in the section entitled
2756    .\" HTML <a href="pcreapi.html#extradata">
2757    .\" </a>
2758    "Extra data for \fBpcre_exec()\fP"
2759    .\"
2760    in the
2761    .\" HREF
2762    \fBpcreapi\fP
2763    .\"
2764    documentation. Here is an example of \fBpcretest\fP output, where the /K
2765    modifier requests the retrieval and outputting of (*MARK) data:
2766    .sp
2767        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2768      data> XY
2769       0: XY
2770      MK: A
2771      XZ
2772       0: XZ
2773      MK: B
2774    .sp
2775    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2776    indicates which of the two alternatives matched. This is a more efficient way
2777    of obtaining this information than putting each alternative in its own
2778    capturing parentheses.
2779    .P
2780    If (*MARK) is encountered in a positive assertion, its name is recorded and
2781    passed back if it is the last-encountered. This does not happen for negative
2782    assertions.
2783    .P
2784    After a partial match or a failed match, the name of the last encountered
2785    (*MARK) in the entire match process is returned. For example:
2786    .sp
2787        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2788      data> XP
2789      No match, mark = B
2790    .sp
2791    Note that in this unanchored example the mark is retained from the match
2792    attempt that started at the letter "X" in the subject. Subsequent match
2793    attempts starting at "P" and then with an empty string do not get as far as the
2794    (*MARK) item, but nevertheless do not reset it.
2795    .P
2796    If you are interested in (*MARK) values after failed matches, you should
2797    probably set the PCRE_NO_START_OPTIMIZE option
2798    .\" HTML <a href="#nooptimize">
2799    .\" </a>
2800    (see above)
2801    .\"
2802    to ensure that the match is always attempted.
2803    .
2804    .
2805  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2806  .rs  .rs
2807  .sp  .sp
2808  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2809  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2810  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2811    the verb. However, when one of these verbs appears inside an atomic group, its
2812    effect is confined to that group, because once the group has been matched,
2813    there is never any backtracking into it. In this situation, backtracking can
2814    "jump back" to the left of the entire atomic group. (Remember also, as stated
2815    above, that this localization also applies in subroutine calls and assertions.)
2816    .P
2817    These verbs differ in exactly what kind of failure occurs when backtracking
2818    reaches them.
2819  .sp  .sp
2820    (*COMMIT)    (*COMMIT)
2821  .sp  .sp
2822  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2823  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2824  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2825  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2826  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2827  .sp  .sp
2828    a+(*COMMIT)b    a+(*COMMIT)b
2829  .sp  .sp
2830  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2831  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2832  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2833    (*PRUNE)  match failure.
2834  .sp  .P
2835  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2836  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2837  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2838  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2839  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2840  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2841  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2842  be expressed in any other way.    xyzabc\eY
2843      No match
2844    .sp
2845    PCRE knows that any match must start with "a", so the optimization skips along
2846    the subject to "a" before running the first match attempt, which succeeds. When
2847    the optimization is disabled by the \eY escape in the second subject, the match
2848    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2849    starting points.
2850    .sp
2851      (*PRUNE) or (*PRUNE:NAME)
2852    .sp
2853    This verb causes the match to fail at the current starting position in the
2854    subject if the rest of the pattern does not match. If the pattern is
2855    unanchored, the normal "bumpalong" advance to the next starting character then
2856    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2857    reached, or when matching to the right of (*PRUNE), but if there is no match to
2858    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2859    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2860    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2861    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2862    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2863  .sp  .sp
2864    (*SKIP)    (*SKIP)
2865  .sp  .sp
2866  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2867  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2868  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2869  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2870    successful match. Consider:
2871  .sp  .sp
2872    a+(*SKIP)b    a+(*SKIP)b
2873  .sp  .sp
2874  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2875  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2876  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2877  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2878  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2879  instead of skipping on to "c".  instead of skipping on to "c".
2880  .sp  .sp
2881    (*THEN)    (*SKIP:NAME)
2882  .sp  .sp
2883  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2884  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2885  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2886  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2887    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2888    matching name is found, the (*SKIP) is ignored.
2889    .sp
2890      (*THEN) or (*THEN:NAME)
2891    .sp
2892    This verb causes a skip to the next innermost alternative if the rest of the
2893    pattern does not match. That is, it cancels pending backtracking, but only
2894    within the current alternative. Its name comes from the observation that it can
2895    be used for a pattern-based if-then-else block:
2896  .sp  .sp
2897    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2898  .sp  .sp
2899  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2900  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2901  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2902  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2903    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2904    .P
2905    Note that a subpattern that does not contain a | character is just a part of
2906    the enclosing alternative; it is not a nested alternation with only one
2907    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2908    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2909    pattern fragments that do not contain any | characters at this level:
2910    .sp
2911      A (B(*THEN)C) | D
2912    .sp
2913    If A and B are matched, but there is a failure in C, matching does not
2914    backtrack into A; instead it moves to the next alternative, that is, D.
2915    However, if the subpattern containing (*THEN) is given an alternative, it
2916    behaves differently:
2917    .sp
2918      A (B(*THEN)C | (*FAIL)) | D
2919    .sp
2920    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2921    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2922    because there are no more alternatives to try. In this case, matching does now
2923    backtrack into A.
2924    .P
2925    Note also that a conditional subpattern is not considered as having two
2926    alternatives, because only one is ever used. In other words, the | character in
2927    a conditional subpattern has a different meaning. Ignoring white space,
2928    consider:
2929    .sp
2930      ^.*? (?(?=a) a | b(*THEN)c )
2931    .sp
2932    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2933    it initially matches zero characters. The condition (?=a) then fails, the
2934    character "b" is matched, but "c" is not. At this point, matching does not
2935    backtrack to .*? as might perhaps be expected from the presence of the |
2936    character. The conditional subpattern is part of the single alternative that
2937    comprises the whole pattern, and so the match fails. (If there was a backtrack
2938    into .*?, allowing it to match "b", the match would succeed.)
2939    .P
2940    The verbs just described provide four different "strengths" of control when
2941    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2942    next alternative. (*PRUNE) comes next, failing the match at the current
2943    starting position, but allowing an advance to the next character (for an
2944    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2945    than one character. (*COMMIT) is the strongest, causing the entire match to
2946    fail.
2947    .P
2948    If more than one such verb is present in a pattern, the "strongest" one wins.
2949    For example, consider this pattern, where A, B, etc. are complex pattern
2950    fragments:
2951    .sp
2952      (A(*COMMIT)B(*THEN)C|D)
2953    .sp
2954    Once A has matched, PCRE is committed to this match, at the current starting
2955    position. If subsequently B matches, but C does not, the normal (*THEN) action
2956    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2957    overrides.
2958  .  .
2959  .  .
2960  .SH "SEE ALSO"  .SH "SEE ALSO"
2961  .rs  .rs
2962  .sp  .sp
2963  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2964    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
2965  .  .
2966  .  .
2967  .SH AUTHOR  .SH AUTHOR
# Line 2245  Cambridge CB2 3QH, England. Line 2978  Cambridge CB2 3QH, England.
2978  .rs  .rs
2979  .sp  .sp
2980  .nf  .nf
2981  Last updated: 18 March 2009  Last updated: 11 November 2012
2982  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2983  .fi  .fi

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