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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "26 April 2013" "PCRE 8.33"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
 regular expressions in great detail. This description of PCRE's regular  
 expressions is intended as reference material.  
 .P  
 The original operation of PCRE was on strings of one-byte characters. However,  
 there is now also support for UTF-8 character strings. To use this, you must  
 build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  
 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  
 places below. There is also a summary of UTF-8 features in the  
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
11  .\"  .\"
12  in the main  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22    .P
23    This document discusses the patterns that are supported by PCRE when one its
24    main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25    (16- or 32-bit), is used. PCRE also has alternative matching functions,
26    \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27    different algorithm that is not Perl-compatible. Some of the features discussed
28    below are not available when DFA matching is used. The advantages and
29    disadvantages of the alternative functions, and how they differ from the normal
30    functions, are discussed in the
31  .\" HREF  .\" HREF
32  \fBpcre\fP  \fBpcrematching\fP
33  .\"  .\"
34  page.  page.
35  .P  .
36  The remainder of this document discusses the patterns that are supported by  .
37  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  .SH "SPECIAL START-OF-PATTERN ITEMS"
38  From release 6.0, PCRE offers a second matching function,  .rs
39  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  .sp
40  Perl-compatible. Some of the features discussed below are not available when  A number of options that can be passed to \fBpcre_compile()\fP can also be set
41  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  by special items at the start of a pattern. These are not Perl-compatible, but
42  alternative function, and how it differs from the normal function, are  are provided to make these options accessible to pattern writers who are not
43  discussed in the  able to change the program that processes the pattern. Any number of these
44    items may appear, but they must all be together right at the start of the
45    pattern string, and the letters must be in upper case.
46    .
47    .
48    .SS "UTF support"
49    .rs
50    .sp
51    The original operation of PCRE was on strings of one-byte characters. However,
52    there is now also support for UTF-8 strings in the original library, an
53    extra library that supports 16-bit and UTF-16 character strings, and a
54    third library that supports 32-bit and UTF-32 character strings. To use these
55    features, PCRE must be built to include appropriate support. When using UTF
56    strings you must either call the compiling function with the PCRE_UTF8,
57    PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58    these special sequences:
59    .sp
60      (*UTF8)
61      (*UTF16)
62      (*UTF32)
63      (*UTF)
64    .sp
65    (*UTF) is a generic sequence that can be used with any of the libraries.
66    Starting a pattern with such a sequence is equivalent to setting the relevant
67    option. How setting a UTF mode affects pattern matching is mentioned in several
68    places below. There is also a summary of features in the
69  .\" HREF  .\" HREF
70  \fBpcrematching\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73    .P
74    Some applications that allow their users to supply patterns may wish to
75    restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76    option is set at compile time, (*UTF) etc. are not allowed, and their
77    appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82    .sp
83    Another special sequence that may appear at the start of a pattern is
84    .sp
85      (*UCP)
86    .sp
87    This has the same effect as setting the PCRE_UCP option: it causes sequences
88    such as \ed and \ew to use Unicode properties to determine character types,
89    instead of recognizing only characters with codes less than 128 via a lookup
90    table.
91    .
92    .
93    .SS "Disabling start-up optimizations"
94    .rs
95    .sp
96    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
97    PCRE_NO_START_OPTIMIZE option either at compile or matching time.
98    .
99    .
100    .\" HTML <a name="newlines"></a>
101    .SS "Newline conventions"
102    .rs
103    .sp
104    PCRE supports five different conventions for indicating line breaks in
105    strings: a single CR (carriage return) character, a single LF (linefeed)
106    character, the two-character sequence CRLF, any of the three preceding, or any
107    Unicode newline sequence. The
108    .\" HREF
109    \fBpcreapi\fP
110    .\"
111    page has
112    .\" HTML <a href="pcreapi.html#newlines">
113    .\" </a>
114    further discussion
115    .\"
116    about newlines, and shows how to set the newline convention in the
117    \fIoptions\fP arguments for the compiling and matching functions.
118    .P
119    It is also possible to specify a newline convention by starting a pattern
120    string with one of the following five sequences:
121    .sp
122      (*CR)        carriage return
123      (*LF)        linefeed
124      (*CRLF)      carriage return, followed by linefeed
125      (*ANYCRLF)   any of the three above
126      (*ANY)       all Unicode newline sequences
127    .sp
128    These override the default and the options given to the compiling function. For
129    example, on a Unix system where LF is the default newline sequence, the pattern
130    .sp
131      (*CR)a.b
132    .sp
133    changes the convention to CR. That pattern matches "a\enb" because LF is no
134    longer a newline. If more than one of these settings is present, the last one
135    is used.
136    .P
137    The newline convention affects where the circumflex and dollar assertions are
138    true. It also affects the interpretation of the dot metacharacter when
139    PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
140    what the \eR escape sequence matches. By default, this is any Unicode newline
141    sequence, for Perl compatibility. However, this can be changed; see the
142    description of \eR in the section entitled
143    .\" HTML <a href="#newlineseq">
144    .\" </a>
145    "Newline sequences"
146    .\"
147    below. A change of \eR setting can be combined with a change of newline
148    convention.
149    .
150    .
151    .SS "Setting match and recursion limits"
152    .rs
153    .sp
154    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
155    internal \fBmatch()\fP function is called and on the maximum depth of
156    recursive calls. These facilities are provided to catch runaway matches that
157    are provoked by patterns with huge matching trees (a typical example is a
158    pattern with nested unlimited repeats) and to avoid running out of system stack
159    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
160    gives an error return. The limits can also be set by items at the start of the
161    pattern of the form
162    .sp
163      (*LIMIT_MATCH=d)
164      (*LIMIT_RECURSION=d)
165    .sp
166    where d is any number of decimal digits. However, the value of the setting must
167    be less than the value set by the caller of \fBpcre_exec()\fP for it to have
168    any effect. In other words, the pattern writer can lower the limit set by the
169    programmer, but not raise it. If there is more than one setting of one of these
170    limits, the lower value is used.
171    .
172    .
173    .SH "EBCDIC CHARACTER CODES"
174    .rs
175    .sp
176    PCRE can be compiled to run in an environment that uses EBCDIC as its character
177    code rather than ASCII or Unicode (typically a mainframe system). In the
178    sections below, character code values are ASCII or Unicode; in an EBCDIC
179    environment these characters may have different code values, and there are no
180    code points greater than 255.
181  .  .
182  .  .
183  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
# Line 51  corresponding characters in the subject. Line 191  corresponding characters in the subject.
191  .sp  .sp
192  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
193  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
194  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
195  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
196  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
197  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
198  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
199  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
200  UTF-8 support.  UTF support.
201  .P  .P
202  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
203  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 103  The following sections describe the use Line 243  The following sections describe the use
243  .rs  .rs
244  .sp  .sp
245  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
246  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
247  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
248  outside character classes.  both inside and outside character classes.
249  .P  .P
250  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.
251  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 113  otherwise be interpreted as a metacharac Line 253  otherwise be interpreted as a metacharac
253  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
254  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
255  .P  .P
256  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
257    backslash. All other characters (in particular, those whose codepoints are
258    greater than 127) are treated as literals.
259    .P
260    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
261  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
262  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
263  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.
264  .P  .P
265  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
266  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 132  Perl, $ and @ cause variable interpolati Line 276  Perl, $ and @ cause variable interpolati
276    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
277  .sp  .sp
278  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
279    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
280    by \eE later in the pattern, the literal interpretation continues to the end of
281    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
282    a character class, this causes an error, because the character class is not
283    terminated.
284  .  .
285  .  .
286  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 141  The \eQ...\eE sequence is recognized bot Line 290  The \eQ...\eE sequence is recognized bot
290  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
291  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
292  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
293  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
294  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:  
295  .sp  .sp
296    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
297    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
298    \ee        escape (hex 1B)    \ee        escape (hex 1B)
299    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
300    \en        newline (hex 0A)    \en        linefeed (hex 0A)
301    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
302    \et        tab (hex 09)    \et        tab (hex 09)
303    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
304    \exhh      character with hex code hh    \exhh      character with hex code hh
305    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
306      \euhhhh    character with hex code hhhh (JavaScript mode only)
307  .sp  .sp
308  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
309  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
310  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
311  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
312  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
313  After \ex, from zero to two hexadecimal digits are read (letters can be in  compile-time error occurs. This locks out non-ASCII characters in all modes.
314  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
315  and }, but the value of the character code must be less than 256 in non-UTF-8  The \ec facility was designed for use with ASCII characters, but with the
316  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  extension to Unicode it is even less useful than it once was. It is, however,
317  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  recognized when PCRE is compiled in EBCDIC mode, where data items are always
318  and }, or if there is no terminating }, this form of escape is not recognized.  bytes. In this mode, all values are valid after \ec. If the next character is a
319  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
320  with no following digits, giving a character whose value is zero.  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
321    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
322    characters also generate different values.
323    .P
324    By default, after \ex, from zero to two hexadecimal digits are read (letters
325    can be in upper or lower case). Any number of hexadecimal digits may appear
326    between \ex{ and }, but the character code is constrained as follows:
327    .sp
328      8-bit non-UTF mode    less than 0x100
329      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
330      16-bit non-UTF mode   less than 0x10000
331      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
332      32-bit non-UTF mode   less than 0x80000000
333      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
334    .sp
335    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
336    "surrogate" codepoints), and 0xffef.
337    .P
338    If characters other than hexadecimal digits appear between \ex{ and }, or if
339    there is no terminating }, this form of escape is not recognized. Instead, the
340    initial \ex will be interpreted as a basic hexadecimal escape, with no
341    following digits, giving a character whose value is zero.
342    .P
343    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
344    as just described only when it is followed by two hexadecimal digits.
345    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
346    code points greater than 256 is provided by \eu, which must be followed by
347    four hexadecimal digits; otherwise it matches a literal "u" character.
348    Character codes specified by \eu in JavaScript mode are constrained in the same
349    was as those specified by \ex in non-JavaScript mode.
350  .P  .P
351  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
352  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
353  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
354    \eu00dc in JavaScript mode).
355  .P  .P
356  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
357  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 198  parenthesized subpatterns. Line 377  parenthesized subpatterns.
377  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
378  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
379  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
380  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
381  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.
382  to \e777 are permitted. For example:  For example:
383  .sp  .sp
384    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
385  .\" JOIN  .\" JOIN
386    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
387              previous capturing subpatterns              previous capturing subpatterns
# Line 217  to \e777 are permitted. For example: Line 396  to \e777 are permitted. For example:
396              character with octal code 113              character with octal code 113
397  .\" JOIN  .\" JOIN
398    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
399              the byte consisting entirely of 1 bits              the value 255 (decimal)
400  .\" JOIN  .\" JOIN
401    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
402              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 226  Note that octal values of 100 or greater Line 405  Note that octal values of 100 or greater
405  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
406  .P  .P
407  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
408  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
409  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
410  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
411  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
412  meanings  inside a character class. Like other unrecognized escape sequences, they are
413  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
414  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
415  (see below).  sequences have different meanings.
416  .\"  .
417    .
418    .SS "Unsupported escape sequences"
419    .rs
420    .sp
421    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
422    handler and used to modify the case of following characters. By default, PCRE
423    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
424    option is set, \eU matches a "U" character, and \eu can be used to define a
425    character by code point, as described in the previous section.
426  .  .
427  .  .
428  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
429  .rs  .rs
430  .sp  .sp
431  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
432  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
433  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
434  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
435  .\" </a>  .\" </a>
436  later,  later,
# Line 254  parenthesized subpatterns. Line 442  parenthesized subpatterns.
442  .\"  .\"
443  .  .
444  .  .
445    .SS "Absolute and relative subroutine calls"
446    .rs
447    .sp
448    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
449    a number enclosed either in angle brackets or single quotes, is an alternative
450    syntax for referencing a subpattern as a "subroutine". Details are discussed
451    .\" HTML <a href="#onigurumasubroutines">
452    .\" </a>
453    later.
454    .\"
455    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
456    synonymous. The former is a back reference; the latter is a
457    .\" HTML <a href="#subpatternsassubroutines">
458    .\" </a>
459    subroutine
460    .\"
461    call.
462    .
463    .
464    .\" HTML <a name="genericchartypes"></a>
465  .SS "Generic character types"  .SS "Generic character types"
466  .rs  .rs
467  .sp  .sp
468  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:  
469  .sp  .sp
470    \ed     any decimal digit    \ed     any decimal digit
471    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
472    \es     any whitespace character    \eh     any horizontal white space character
473    \eS     any character that is not a whitespace character    \eH     any character that is not a horizontal white space character
474      \es     any white space character
475      \eS     any character that is not a white space character
476      \ev     any vertical white space character
477      \eV     any character that is not a vertical white space character
478    \ew     any "word" character    \ew     any "word" character
479    \eW     any "non-word" character    \eW     any "non-word" character
480  .sp  .sp
481  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.
482  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
483  .P  .\" HTML <a href="#fullstopdot">
484  These character type sequences can appear both inside and outside character  .\" </a>
485    the "." metacharacter
486    .\"
487    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
488    PCRE does not support this.
489    .P
490    Each pair of lower and upper case escape sequences partitions the complete set
491    of characters into two disjoint sets. Any given character matches one, and only
492    one, of each pair. The sequences can appear both inside and outside character
493  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
494  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
495  there is no character to match.  there is no character to match.
496  .P  .P
497  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).
498  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
499  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
500  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
501  does.)  does.
502  .P  .P
503  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
504  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
505  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
506  place (see  place (see
507  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 295  in the Line 514  in the
514  .\"  .\"
515  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
516  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
517  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
518    Unicode is discouraged.
519  .P  .P
520  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in a UTF mode, characters with values greater than 128 never match
521  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
522  character property support is available. The use of locales with Unicode is  their original meanings from before UTF support was available, mainly for
523  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
524    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
525    properties are used to determine character types, as follows:
526    .sp
527      \ed  any character that \ep{Nd} matches (decimal digit)
528      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
529      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
530    .sp
531    The upper case escapes match the inverse sets of characters. Note that \ed
532    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
533    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
534    \eB because they are defined in terms of \ew and \eW. Matching these sequences
535    is noticeably slower when PCRE_UCP is set.
536    .P
537    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
538    release 5.10. In contrast to the other sequences, which match only ASCII
539    characters by default, these always match certain high-valued codepoints,
540    whether or not PCRE_UCP is set. The horizontal space characters are:
541    .sp
542      U+0009     Horizontal tab (HT)
543      U+0020     Space
544      U+00A0     Non-break space
545      U+1680     Ogham space mark
546      U+180E     Mongolian vowel separator
547      U+2000     En quad
548      U+2001     Em quad
549      U+2002     En space
550      U+2003     Em space
551      U+2004     Three-per-em space
552      U+2005     Four-per-em space
553      U+2006     Six-per-em space
554      U+2007     Figure space
555      U+2008     Punctuation space
556      U+2009     Thin space
557      U+200A     Hair space
558      U+202F     Narrow no-break space
559      U+205F     Medium mathematical space
560      U+3000     Ideographic space
561    .sp
562    The vertical space characters are:
563    .sp
564      U+000A     Linefeed (LF)
565      U+000B     Vertical tab (VT)
566      U+000C     Form feed (FF)
567      U+000D     Carriage return (CR)
568      U+0085     Next line (NEL)
569      U+2028     Line separator
570      U+2029     Paragraph separator
571    .sp
572    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
573    relevant.
574  .  .
575  .  .
576    .\" HTML <a name="newlineseq"></a>
577  .SS "Newline sequences"  .SS "Newline sequences"
578  .rs  .rs
579  .sp  .sp
580  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
581  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
582  the following:  following:
583  .sp  .sp
584    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
585  .sp  .sp
# Line 319  below. Line 590  below.
590  .\"  .\"
591  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
592  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,
593  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
594  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
595  cannot be split.  cannot be split.
596  .P  .P
597  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
598  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).
599  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
600  recognized.  recognized.
601  .P  .P
602  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
603    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
604    either at compile time or when the pattern is matched. (BSR is an abbrevation
605    for "backslash R".) This can be made the default when PCRE is built; if this is
606    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
607    It is also possible to specify these settings by starting a pattern string with
608    one of the following sequences:
609    .sp
610      (*BSR_ANYCRLF)   CR, LF, or CRLF only
611      (*BSR_UNICODE)   any Unicode newline sequence
612    .sp
613    These override the default and the options given to the compiling function, but
614    they can themselves be overridden by options given to a matching function. Note
615    that these special settings, which are not Perl-compatible, are recognized only
616    at the very start of a pattern, and that they must be in upper case. If more
617    than one of them is present, the last one is used. They can be combined with a
618    change of newline convention; for example, a pattern can start with:
619    .sp
620      (*ANY)(*BSR_ANYCRLF)
621    .sp
622    They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
623    (*UCP) special sequences. Inside a character class, \eR is treated as an
624    unrecognized escape sequence, and so matches the letter "R" by default, but
625    causes an error if PCRE_EXTRA is set.
626  .  .
627  .  .
628  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 336  Inside a character class, \eR matches th Line 630  Inside a character class, \eR matches th
630  .rs  .rs
631  .sp  .sp
632  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
633  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
634  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
635    characters whose codepoints are less than 256, but they do work in this mode.
636    The extra escape sequences are:
637  .sp  .sp
638    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
639    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
640    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
641  .sp  .sp
642  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
643  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
644  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
645  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
646  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
647    .\" </a>
648    next section).
649    .\"
650    Other Perl properties such as "InMusicalSymbols" are not currently supported by
651    PCRE. Note that \eP{Any} does not match any characters, so always causes a
652    match failure.
653  .P  .P
654  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
655  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 361  Those that are not part of an identified Line 663  Those that are not part of an identified
663  .P  .P
664  Arabic,  Arabic,
665  Armenian,  Armenian,
666    Avestan,
667  Balinese,  Balinese,
668    Bamum,
669    Batak,
670  Bengali,  Bengali,
671  Bopomofo,  Bopomofo,
672    Brahmi,
673  Braille,  Braille,
674  Buginese,  Buginese,
675  Buhid,  Buhid,
676  Canadian_Aboriginal,  Canadian_Aboriginal,
677    Carian,
678    Chakma,
679    Cham,
680  Cherokee,  Cherokee,
681  Common,  Common,
682  Coptic,  Coptic,
# Line 376  Cypriot, Line 685  Cypriot,
685  Cyrillic,  Cyrillic,
686  Deseret,  Deseret,
687  Devanagari,  Devanagari,
688    Egyptian_Hieroglyphs,
689  Ethiopic,  Ethiopic,
690  Georgian,  Georgian,
691  Glagolitic,  Glagolitic,
# Line 388  Hangul, Line 698  Hangul,
698  Hanunoo,  Hanunoo,
699  Hebrew,  Hebrew,
700  Hiragana,  Hiragana,
701    Imperial_Aramaic,
702  Inherited,  Inherited,
703    Inscriptional_Pahlavi,
704    Inscriptional_Parthian,
705    Javanese,
706    Kaithi,
707  Kannada,  Kannada,
708  Katakana,  Katakana,
709    Kayah_Li,
710  Kharoshthi,  Kharoshthi,
711  Khmer,  Khmer,
712  Lao,  Lao,
713  Latin,  Latin,
714    Lepcha,
715  Limbu,  Limbu,
716  Linear_B,  Linear_B,
717    Lisu,
718    Lycian,
719    Lydian,
720  Malayalam,  Malayalam,
721    Mandaic,
722    Meetei_Mayek,
723    Meroitic_Cursive,
724    Meroitic_Hieroglyphs,
725    Miao,
726  Mongolian,  Mongolian,
727  Myanmar,  Myanmar,
728  New_Tai_Lue,  New_Tai_Lue,
# Line 405  Nko, Line 730  Nko,
730  Ogham,  Ogham,
731  Old_Italic,  Old_Italic,
732  Old_Persian,  Old_Persian,
733    Old_South_Arabian,
734    Old_Turkic,
735    Ol_Chiki,
736  Oriya,  Oriya,
737  Osmanya,  Osmanya,
738  Phags_Pa,  Phags_Pa,
739  Phoenician,  Phoenician,
740    Rejang,
741  Runic,  Runic,
742    Samaritan,
743    Saurashtra,
744    Sharada,
745  Shavian,  Shavian,
746  Sinhala,  Sinhala,
747    Sora_Sompeng,
748    Sundanese,
749  Syloti_Nagri,  Syloti_Nagri,
750  Syriac,  Syriac,
751  Tagalog,  Tagalog,
752  Tagbanwa,  Tagbanwa,
753  Tai_Le,  Tai_Le,
754    Tai_Tham,
755    Tai_Viet,
756    Takri,
757  Tamil,  Tamil,
758  Telugu,  Telugu,
759  Thaana,  Thaana,
# Line 424  Thai, Line 761  Thai,
761  Tibetan,  Tibetan,
762  Tifinagh,  Tifinagh,
763  Ugaritic,  Ugaritic,
764    Vai,
765  Yi.  Yi.
766  .P  .P
767  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
768  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
769  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
770  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
771  .P  .P
772  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
773  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 489  The special property L& is also supporte Line 827  The special property L& is also supporte
827  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
828  a modifier or "other".  a modifier or "other".
829  .P  .P
830  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
831    U+DFFF. Such characters are not valid in Unicode strings and so
832    cannot be tested by PCRE, unless UTF validity checking has been turned off
833    (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
834    PCRE_NO_UTF32_CHECK in the
835    .\" HREF
836    \fBpcreapi\fP
837    .\"
838    page). Perl does not support the Cs property.
839    .P
840    The long synonyms for property names that Perl supports (such as \ep{Letter})
841  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
842  properties with "Is".  properties with "Is".
843  .P  .P
# Line 498  Instead, this property is assumed for an Line 846  Instead, this property is assumed for an
846  Unicode table.  Unicode table.
847  .P  .P
848  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
849  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
850    the behaviour of current versions of Perl.
851  .P  .P
852  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
853  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
854  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
855    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
856    PCRE_UCP option or by starting the pattern with (*UCP).
857    .
858    .
859    .SS Extended grapheme clusters
860    .rs
861  .sp  .sp
862  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
863  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  
864  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
865  .\" </a>  .\" </a>
866  (see below).  (see below).
867  .\"  .\"
868  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
869  preceding character.  that was equivalent to
870    .sp
871      (?>\ePM\epM*)
872    .sp
873    That is, it matched a character without the "mark" property, followed by zero
874    or more characters with the "mark" property. Characters with the "mark"
875    property are typically non-spacing accents that affect the preceding character.
876    .P
877    This simple definition was extended in Unicode to include more complicated
878    kinds of composite character by giving each character a grapheme breaking
879    property, and creating rules that use these properties to define the boundaries
880    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
881    one of these clusters.
882    .P
883    \eX always matches at least one character. Then it decides whether to add
884    additional characters according to the following rules for ending a cluster:
885    .P
886    1. End at the end of the subject string.
887    .P
888    2. Do not end between CR and LF; otherwise end after any control character.
889    .P
890    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
891    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
892    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
893    character; an LVT or T character may be follwed only by a T character.
894  .P  .P
895  Matching characters by Unicode property is not fast, because PCRE has to search  4. Do not end before extending characters or spacing marks. Characters with
896  a structure that contains data for over fifteen thousand characters. That is  the "mark" property always have the "extend" grapheme breaking property.
897  why the traditional escape sequences such as \ed and \ew do not use Unicode  .P
898  properties in PCRE.  5. Do not end after prepend characters.
899    .P
900    6. Otherwise, end the cluster.
901    .
902    .
903    .\" HTML <a name="extraprops"></a>
904    .SS PCRE's additional properties
905    .rs
906    .sp
907    As well as the standard Unicode properties described above, PCRE supports four
908    more that make it possible to convert traditional escape sequences such as \ew
909    and \es and POSIX character classes to use Unicode properties. PCRE uses these
910    non-standard, non-Perl properties internally when PCRE_UCP is set. However,
911    they may also be used explicitly. These properties are:
912    .sp
913      Xan   Any alphanumeric character
914      Xps   Any POSIX space character
915      Xsp   Any Perl space character
916      Xwd   Any Perl "word" character
917    .sp
918    Xan matches characters that have either the L (letter) or the N (number)
919    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
920    carriage return, and any other character that has the Z (separator) property.
921    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
922    same characters as Xan, plus underscore.
923    .P
924    There is another non-standard property, Xuc, which matches any character that
925    can be represented by a Universal Character Name in C++ and other programming
926    languages. These are the characters $, @, ` (grave accent), and all characters
927    with Unicode code points greater than or equal to U+00A0, except for the
928    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
929    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
930    where H is a hexadecimal digit. Note that the Xuc property does not match these
931    sequences but the characters that they represent.)
932  .  .
933  .  .
934  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
935  .SS "Resetting the match start"  .SS "Resetting the match start"
936  .rs  .rs
937  .sp  .sp
938  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
939  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:  
940  .sp  .sp
941    foo\eKbar    foo\eKbar
942  .sp  .sp
# Line 549  For example, when the pattern Line 958  For example, when the pattern
958    (foo)\eKbar    (foo)\eKbar
959  .sp  .sp
960  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
961    .P
962    Perl documents that the use of \eK within assertions is "not well defined". In
963    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
964    ignored in negative assertions.
965  .  .
966  .  .
967  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 573  The backslashed assertions are: Line 986  The backslashed assertions are:
986    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
987    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
988  .sp  .sp
989  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
990  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
991    default it matches the corresponding literal character (for example, \eB
992    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
993    escape sequence" error is generated instead.
994  .P  .P
995  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
996  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
997  \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
998  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
999    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1000    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1001    of word" or "end of word" metasequence. However, whatever follows \eb normally
1002    determines which it is. For example, the fragment \eba matches "a" at the start
1003    of a word.
1004  .P  .P
1005  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1006  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 613  regular expression. Line 1034  regular expression.
1034  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1035  .rs  .rs
1036  .sp  .sp
1037    The circumflex and dollar metacharacters are zero-width assertions. That is,
1038    they test for a particular condition being true without consuming any
1039    characters from the subject string.
1040    .P
1041  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1042  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
1043  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
1044  \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
1045  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1046  meaning  meaning
# Line 632  constrained to match only at the start o Line 1057  constrained to match only at the start o
1057  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1058  to be anchored.)  to be anchored.)
1059  .P  .P
1060  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
1061  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
1062  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
1063  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
1064  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
1065  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1066  .P  .P
1067  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
1068  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 663  end of the subject in both modes, and if Line 1088  end of the subject in both modes, and if
1088  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1089  .  .
1090  .  .
1091  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1092    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1093  .rs  .rs
1094  .sp  .sp
1095  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
1096  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
1097  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1098  .P  .P
1099  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
1100  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 685  to match it. Line 1111  to match it.
1111  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
1112  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
1113  special meaning in a character class.  special meaning in a character class.
1114  .  .P
1115  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1116  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1117  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1118  .sp  name; PCRE does not support this.
1119  Outside a character class, the escape sequence \eC matches any one byte, both  .
1120  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1121  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1122  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1123  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1124  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1125    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1126    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1127    a 32-bit unit. Unlike a dot, \eC always
1128    matches line-ending characters. The feature is provided in Perl in order to
1129    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1130    used. Because \eC breaks up characters into individual data units, matching one
1131    unit with \eC in a UTF mode means that the rest of the string may start with a
1132    malformed UTF character. This has undefined results, because PCRE assumes that
1133    it is dealing with valid UTF strings (and by default it checks this at the
1134    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1135    PCRE_NO_UTF32_CHECK option is used).
1136  .P  .P
1137  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1138  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1139  .\" </a>  .\" </a>
1140  (described below),  (described below)
1141  .\"  .\"
1142  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
1143  the lookbehind.  the lookbehind.
1144    .P
1145    In general, the \eC escape sequence is best avoided. However, one
1146    way of using it that avoids the problem of malformed UTF characters is to use a
1147    lookahead to check the length of the next character, as in this pattern, which
1148    could be used with a UTF-8 string (ignore white space and line breaks):
1149    .sp
1150      (?| (?=[\ex00-\ex7f])(\eC) |
1151          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1152          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1153          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1154    .sp
1155    A group that starts with (?| resets the capturing parentheses numbers in each
1156    alternative (see
1157    .\" HTML <a href="#dupsubpatternnumber">
1158    .\" </a>
1159    "Duplicate Subpattern Numbers"
1160    .\"
1161    below). The assertions at the start of each branch check the next UTF-8
1162    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1163    character's individual bytes are then captured by the appropriate number of
1164    groups.
1165  .  .
1166  .  .
1167  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 711  the lookbehind. Line 1169  the lookbehind.
1169  .rs  .rs
1170  .sp  .sp
1171  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1172  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.
1173  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
1174  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
1175  escaped with a backslash.  a member of the class, it should be the first data character in the class
1176  .P  (after an initial circumflex, if present) or escaped with a backslash.
1177  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1178  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
1179  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
1180  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
1181  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
1182  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
1183    member of the class, ensure it is not the first character, or escape it with a
1184  backslash.  backslash.
1185  .P  .P
1186  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1187  [^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
1188  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1189  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
1190  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
1191  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
1192  string.  string.
1193  .P  .P
1194  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)
1195  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
1196    \ex{ escaping mechanism.
1197  .P  .P
1198  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
1199  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
1200  "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
1201  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
1202  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
1203  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1204  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1205  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
1206  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
1207  UTF-8 support.  well as with UTF support.
1208  .P  .P
1209  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
1210  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 766  followed by two other characters. The oc Line 1226  followed by two other characters. The oc
1226  "]" can also be used to end a range.  "]" can also be used to end a range.
1227  .P  .P
1228  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
1229  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1230  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}].  
1231  .P  .P
1232  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
1233  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
1234  [][\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
1235  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
1236  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
1237  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
1238  property support.  property support.
1239  .P  .P
1240  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,
1241  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
1242  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1243  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
1244  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
1245  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1246    .\" HTML <a href="#genericchartypes">
1247    .\" </a>
1248    "Generic character types"
1249    .\"
1250    above. The escape sequence \eb has a different meaning inside a character
1251    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1252    are not special inside a character class. Like any other unrecognized escape
1253    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1254    default, but cause an error if the PCRE_EXTRA option is set.
1255    .P
1256    A circumflex can conveniently be used with the upper case character types to
1257    specify a more restricted set of characters than the matching lower case type.
1258    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1259    whereas [\ew] includes underscore. A positive character class should be read as
1260    "something OR something OR ..." and a negative class as "NOT something AND NOT
1261    something AND NOT ...".
1262  .P  .P
1263  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1264  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 803  this notation. For example, Line 1278  this notation. For example,
1278    [01[:alpha:]%]    [01[:alpha:]%]
1279  .sp  .sp
1280  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1281  are  are:
1282  .sp  .sp
1283    alnum    letters and digits    alnum    letters and digits
1284    alpha    letters    alpha    letters
# Line 814  are Line 1289  are
1289    graph    printing characters, excluding space    graph    printing characters, excluding space
1290    lower    lower case letters    lower    lower case letters
1291    print    printing characters, including space    print    printing characters, including space
1292    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1293    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1294    upper    upper case letters    upper    upper case letters
1295    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 835  matches "1", "2", or any non-digit. PCRE Line 1310  matches "1", "2", or any non-digit. PCRE
1310  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
1311  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1312  .P  .P
1313  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
1314  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1315    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1316    character properties are used. This is achieved by replacing the POSIX classes
1317    by other sequences, as follows:
1318    .sp
1319      [:alnum:]  becomes  \ep{Xan}
1320      [:alpha:]  becomes  \ep{L}
1321      [:blank:]  becomes  \eh
1322      [:digit:]  becomes  \ep{Nd}
1323      [:lower:]  becomes  \ep{Ll}
1324      [:space:]  becomes  \ep{Xps}
1325      [:upper:]  becomes  \ep{Lu}
1326      [:word:]   becomes  \ep{Xwd}
1327    .sp
1328    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1329    classes are unchanged, and match only characters with code points less than
1330    128.
1331  .  .
1332  .  .
1333  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 863  alternative in the subpattern. Line 1354  alternative in the subpattern.
1354  .rs  .rs
1355  .sp  .sp
1356  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1357  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1358  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1359    The option letters are
1360  .sp  .sp
1361    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1362    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 878  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1370  PCRE_MULTILINE while unsetting PCRE_DOTA
1370  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1371  unset.  unset.
1372  .P  .P
1373  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1374  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1375  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1376  the global options (and it will therefore show up in data extracted by the  .P
1377  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1378    subpattern parentheses), the change applies to the remainder of the pattern
1379    that follows. If the change is placed right at the start of a pattern, PCRE
1380    extracts it into the global options (and it will therefore show up in data
1381    extracted by the \fBpcre_fullinfo()\fP function).
1382  .P  .P
1383  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1384  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
1385  .sp  .sp
1386    (a(?i)b)c    (a(?i)b)c
1387  .sp  .sp
# Line 901  branch is abandoned before the option se Line 1397  branch is abandoned before the option se
1397  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1398  behaviour otherwise.  behaviour otherwise.
1399  .P  .P
1400  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1401  changed in the same way as the Perl-compatible options by using the characters  application when the compiling or matching functions are called. In some cases
1402  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1403    what the application has set or what has been defaulted. Details are given in
1404    the section entitled
1405    .\" HTML <a href="#newlineseq">
1406    .\" </a>
1407    "Newline sequences"
1408    .\"
1409    above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1410    sequences that can be used to set UTF and Unicode property modes; they are
1411    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1412    options, respectively. The (*UTF) sequence is a generic version that can be
1413    used with any of the libraries. However, the application can set the
1414    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1415  .  .
1416  .  .
1417  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 917  Turning part of a pattern into a subpatt Line 1425  Turning part of a pattern into a subpatt
1425  .sp  .sp
1426    cat(aract|erpillar|)    cat(aract|erpillar|)
1427  .sp  .sp
1428  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1429  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1430  .sp  .sp
1431  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
1432  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
1433  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
1434  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1435  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1436  .P  .P
1437  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
1438    numbers for the capturing subpatterns. For example, if the string "the red
1439    king" is matched against the pattern
1440  .sp  .sp
1441    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1442  .sp  .sp
# Line 958  is reached, an option setting in one bra Line 1468  is reached, an option setting in one bra
1468  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1469  .  .
1470  .  .
1471    .\" HTML <a name="dupsubpatternnumber"></a>
1472    .SH "DUPLICATE SUBPATTERN NUMBERS"
1473    .rs
1474    .sp
1475    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1476    the same numbers for its capturing parentheses. Such a subpattern starts with
1477    (?| and is itself a non-capturing subpattern. For example, consider this
1478    pattern:
1479    .sp
1480      (?|(Sat)ur|(Sun))day
1481    .sp
1482    Because the two alternatives are inside a (?| group, both sets of capturing
1483    parentheses are numbered one. Thus, when the pattern matches, you can look
1484    at captured substring number one, whichever alternative matched. This construct
1485    is useful when you want to capture part, but not all, of one of a number of
1486    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1487    number is reset at the start of each branch. The numbers of any capturing
1488    parentheses that follow the subpattern start after the highest number used in
1489    any branch. The following example is taken from the Perl documentation. The
1490    numbers underneath show in which buffer the captured content will be stored.
1491    .sp
1492      # before  ---------------branch-reset----------- after
1493      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1494      # 1            2         2  3        2     3     4
1495    .sp
1496    A back reference to a numbered subpattern uses the most recent value that is
1497    set for that number by any subpattern. The following pattern matches "abcabc"
1498    or "defdef":
1499    .sp
1500      /(?|(abc)|(def))\e1/
1501    .sp
1502    In contrast, a subroutine call to a numbered subpattern always refers to the
1503    first one in the pattern with the given number. The following pattern matches
1504    "abcabc" or "defabc":
1505    .sp
1506      /(?|(abc)|(def))(?1)/
1507    .sp
1508    If a
1509    .\" HTML <a href="#conditions">
1510    .\" </a>
1511    condition test
1512    .\"
1513    for a subpattern's having matched refers to a non-unique number, the test is
1514    true if any of the subpatterns of that number have matched.
1515    .P
1516    An alternative approach to using this "branch reset" feature is to use
1517    duplicate named subpatterns, as described in the next section.
1518    .
1519    .
1520  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1521  .rs  .rs
1522  .sp  .sp
# Line 967  if an expression is modified, the number Line 1526  if an expression is modified, the number
1526  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1527  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
1528  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
1529  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1530    have different names, but PCRE does not.
1531  .P  .P
1532  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
1533  (?'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
1534  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1535  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1536  .\" </a>  .\" </a>
1537  backreferences,  back references,
1538  .\"  .\"
1539  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1540  .\" </a>  .\" </a>
# Line 994  extracting the name-to-number translatio Line 1554  extracting the name-to-number translatio
1554  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1555  .P  .P
1556  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
1557  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
1558  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
1559  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
1560  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
1561  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
1562    name, and in both cases you want to extract the abbreviation. This pattern
1563    (ignoring the line breaks) does the job:
1564  .sp  .sp
1565    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1566    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1007  abbreviation. This pattern (ignoring the Line 1569  abbreviation. This pattern (ignoring the
1569    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1570  .sp  .sp
1571  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1572    (An alternative way of solving this problem is to use a "branch reset"
1573    subpattern, as described in the previous section.)
1574    .P
1575  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1576  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
1577  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1578  make a reference to a non-unique named subpattern from elsewhere in the  .P
1579  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
1580  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1581    used. In the absence of duplicate numbers (see the previous section) this is
1582    the one with the lowest number. If you use a named reference in a condition
1583    test (see the
1584    .\"
1585    .\" HTML <a href="#conditions">
1586    .\" </a>
1587    section about conditions
1588    .\"
1589    below), either to check whether a subpattern has matched, or to check for
1590    recursion, all subpatterns with the same name are tested. If the condition is
1591    true for any one of them, the overall condition is true. This is the same
1592    behaviour as testing by number. For further details of the interfaces for
1593    handling named subpatterns, see the
1594  .\" HREF  .\" HREF
1595  \fBpcreapi\fP  \fBpcreapi\fP
1596  .\"  .\"
1597  documentation.  documentation.
1598    .P
1599    \fBWarning:\fP You cannot use different names to distinguish between two
1600    subpatterns with the same number because PCRE uses only the numbers when
1601    matching. For this reason, an error is given at compile time if different names
1602    are given to subpatterns with the same number. However, you can give the same
1603    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1604  .  .
1605  .  .
1606  .SH REPETITION  .SH REPETITION
# Line 1028  items: Line 1612  items:
1612    a literal data character    a literal data character
1613    the dot metacharacter    the dot metacharacter
1614    the \eC escape sequence    the \eC escape sequence
1615    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1616    the \eR escape sequence    the \eR escape sequence
1617    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1618    a character class    a character class
1619    a back reference (see next section)    a back reference (see next section)
1620    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1621      a subroutine call to a subpattern (recursive or otherwise)
1622  .sp  .sp
1623  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1624  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1058  where a quantifier is not allowed, or on Line 1643  where a quantifier is not allowed, or on
1643  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
1644  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1645  .P  .P
1646  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
1647  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
1648  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,
1649  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
1650  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1651  .P  .P
1652  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
1653  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1654    subpatterns that are referenced as
1655    .\" HTML <a href="#subpatternsassubroutines">
1656    .\" </a>
1657    subroutines
1658    .\"
1659    from elsewhere in the pattern (but see also the section entitled
1660    .\" HTML <a href="#subdefine">
1661    .\" </a>
1662    "Defining subpatterns for use by reference only"
1663    .\"
1664    below). Items other than subpatterns that have a {0} quantifier are omitted
1665    from the compiled pattern.
1666  .P  .P
1667  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1668  abbreviations:  abbreviations:
# Line 1136  In cases where it is known that the subj Line 1733  In cases where it is known that the subj
1733  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1734  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1735  .P  .P
1736  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1737  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1738  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
1739  succeeds. Consider, for example:  succeeds. Consider, for example:
1740  .sp  .sp
# Line 1146  succeeds. Consider, for example: Line 1743  succeeds. Consider, for example:
1743  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
1744  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1745  .P  .P
1746    Another case where implicit anchoring is not applied is when the leading .* is
1747    inside an atomic group. Once again, a match at the start may fail where a later
1748    one succeeds. Consider this pattern:
1749    .sp
1750      (?>.*?a)b
1751    .sp
1752    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1753    (*PRUNE) and (*SKIP) also disable this optimization.
1754    .P
1755  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1756  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1757  .sp  .sp
# Line 1212  previous example can be rewritten as Line 1818  previous example can be rewritten as
1818  .sp  .sp
1819    \ed++foo    \ed++foo
1820  .sp  .sp
1821    Note that a possessive quantifier can be used with an entire group, for
1822    example:
1823    .sp
1824      (abc|xyz){2,3}+
1825    .sp
1826  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1827  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1828  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1285  no such problem when named parentheses a Line 1896  no such problem when named parentheses a
1896  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1897  .P  .P
1898  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
1899  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
1900  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1901  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1902  .sp  .sp
1903    (ring), \e1    (ring), \e1
1904    (ring), \eg1    (ring), \eg1
1905    (ring), \eg{1}    (ring), \eg{1}
1906  .sp  .sp
1907  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1908  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1909  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1910    example:
1911  .sp  .sp
1912    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1913  .sp  .sp
1914  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
1915  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.
1916  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
1917  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
1918  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1919  .P  .P
1920  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1921  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1342  after the reference. Line 1954  after the reference.
1954  .P  .P
1955  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
1956  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1957  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1958  .sp  .sp
1959    (a|(bc))\e2    (a|(bc))\e2
1960  .sp  .sp
1961  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
1962  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
1963  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1964  with a digit character, some delimiter must be used to terminate the back  .P
1965  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1966  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1967    If the pattern continues with a digit character, some delimiter must be used to
1968    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1969    white space. Otherwise, the \eg{ syntax or an empty comment (see
1970  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1971  .\" </a>  .\" </a>
1972  "Comments"  "Comments"
1973  .\"  .\"
1974  below) can be used.  below) can be used.
1975  .P  .
1976    .SS "Recursive back references"
1977    .rs
1978    .sp
1979  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
1980  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.
1981  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1371  to the previous iteration. In order for Line 1989  to the previous iteration. In order for
1989  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
1990  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
1991  minimum of zero.  minimum of zero.
1992    .P
1993    Back references of this type cause the group that they reference to be treated
1994    as an
1995    .\" HTML <a href="#atomicgroup">
1996    .\" </a>
1997    atomic group.
1998    .\"
1999    Once the whole group has been matched, a subsequent matching failure cannot
2000    cause backtracking into the middle of the group.
2001  .  .
2002  .  .
2003  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1390  those that look ahead of the current pos Line 2017  those that look ahead of the current pos
2017  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,
2018  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.
2019  .P  .P
2020  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2021  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
2022  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2023  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2024  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2025  because it does not make sense for negative assertions.  .P
2026    For compatibility with Perl, assertion subpatterns may be repeated; though
2027    it makes no sense to assert the same thing several times, the side effect of
2028    capturing parentheses may occasionally be useful. In practice, there only three
2029    cases:
2030    .sp
2031    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2032    However, it may contain internal capturing parenthesized groups that are called
2033    from elsewhere via the
2034    .\" HTML <a href="#subpatternsassubroutines">
2035    .\" </a>
2036    subroutine mechanism.
2037    .\"
2038    .sp
2039    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2040    were {0,1}. At run time, the rest of the pattern match is tried with and
2041    without the assertion, the order depending on the greediness of the quantifier.
2042    .sp
2043    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2044    The assertion is obeyed just once when encountered during matching.
2045  .  .
2046  .  .
2047  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1424  lookbehind assertion is needed to achiev Line 2070  lookbehind assertion is needed to achiev
2070  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
2071  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
2072  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.
2073    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2074  .  .
2075  .  .
2076  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1448  is permitted, but Line 2095  is permitted, but
2095  .sp  .sp
2096  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2097  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
2098  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
2099  match the same length of string. An assertion such as  length of string. An assertion such as
2100  .sp  .sp
2101    (?<=ab(c|de))    (?<=ab(c|de))
2102  .sp  .sp
2103  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
2104  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
2105    branches:
2106  .sp  .sp
2107    (?<=abc|abde)    (?<=abc|abde)
2108  .sp  .sp
2109  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2110  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2111  .\" </a>  .\" </a>
2112  (see above)  (see above)
2113  .\"  .\"
2114  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
2115  fixed-length.  restriction.
2116  .P  .P
2117  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2118  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
2119  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2120  assertion fails.  assertion fails.
2121  .P  .P
2122  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
2123  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
2124  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
2125  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2126    permitted.
2127    .P
2128    .\" HTML <a href="#subpatternsassubroutines">
2129    .\" </a>
2130    "Subroutine"
2131    .\"
2132    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2133    as the subpattern matches a fixed-length string.
2134    .\" HTML <a href="#recursion">
2135    .\" </a>
2136    Recursion,
2137    .\"
2138    however, is not supported.
2139  .P  .P
2140  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2141  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
2142  pattern such as  strings. Consider a simple pattern such as
2143  .sp  .sp
2144    abcd$    abcd$
2145  .sp  .sp
# Line 1542  characters that are not "999". Line 2203  characters that are not "999".
2203  .sp  .sp
2204  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2205  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2206  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2207  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2208  .sp  .sp
2209    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2210    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2211  .sp  .sp
2212  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2213  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
2214  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2215    itself contain nested subpatterns of any form, including conditional
2216    subpatterns; the restriction to two alternatives applies only at the level of
2217    the condition. This pattern fragment is an example where the alternatives are
2218    complex:
2219    .sp
2220      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2221    .sp
2222  .P  .P
2223  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2224  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1559  recursion, a pseudo-condition called DEF Line 2227  recursion, a pseudo-condition called DEF
2227  .rs  .rs
2228  .sp  .sp
2229  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
2230  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2231  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
2232  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2233  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2234  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2235  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2236    section about duplicate subpattern numbers),
2237    .\"
2238    the condition is true if any of them have matched. An alternative notation is
2239    to precede the digits with a plus or minus sign. In this case, the subpattern
2240    number is relative rather than absolute. The most recently opened parentheses
2241    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2242    loops it can also make sense to refer to subsequent groups. The next
2243    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2244    zero in any of these forms is not used; it provokes a compile-time error.)
2245  .P  .P
2246  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2247  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 1575  three parts for ease of discussion: Line 2252  three parts for ease of discussion:
2252  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2253  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
2254  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
2255  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2256  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,
2257  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
2258  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2259  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 1605  Rewriting the above example to use a nam Line 2282  Rewriting the above example to use a nam
2282  .sp  .sp
2283    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2284  .sp  .sp
2285    If the name used in a condition of this kind is a duplicate, the test is
2286    applied to all subpatterns of the same name, and is true if any one of them has
2287    matched.
2288  .  .
2289  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2290  .rs  .rs
# Line 1616  letter R, for example: Line 2296  letter R, for example:
2296  .sp  .sp
2297    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2298  .sp  .sp
2299  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
2300  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
2301  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2302    applied to all subpatterns of the same name, and is true if any one of them is
2303    the most recent recursion.
2304  .P  .P
2305  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2306  patterns are described below.  .\" HTML <a href="#recursion">
2307    .\" </a>
2308    The syntax for recursive patterns
2309    .\"
2310    is described below.
2311  .  .
2312    .\" HTML <a name="subdefine"></a>
2313  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2314  .rs  .rs
2315  .sp  .sp
# Line 1630  If the condition is the string (DEFINE), Line 2317  If the condition is the string (DEFINE),
2317  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
2318  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2319  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
2320  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2321  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2322  written like this (ignore whitespace and line breaks):  .\" </a>
2323    subroutines
2324    .\"
2325    is described below.) For example, a pattern to match an IPv4 address such as
2326    "192.168.23.245" could be written like this (ignore white space and line
2327    breaks):
2328  .sp  .sp
2329    (?(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) )
2330    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1640  written like this (ignore whitespace and Line 2332  written like this (ignore whitespace and
2332  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
2333  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2334  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
2335  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2336  .P  pattern uses references to the named group to match the four dot-separated
2337  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.  
2338  .  .
2339  .SS "Assertion conditions"  .SS "Assertion conditions"
2340  .rs  .rs
# Line 1669  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2359  dd-aaa-dd or dd-dd-dd, where aaa are let
2359  .SH COMMENTS  .SH COMMENTS
2360  .rs  .rs
2361  .sp  .sp
2362  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
2363  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,
2364  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
2365    subpattern name or number. The characters that make up a comment play no part
2366    in the pattern matching.
2367  .P  .P
2368  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
2369  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2370  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2371    this case continues to immediately after the next newline character or
2372    character sequence in the pattern. Which characters are interpreted as newlines
2373    is controlled by the options passed to a compiling function or by a special
2374    sequence at the start of the pattern, as described in the section entitled
2375    .\" HTML <a href="#newlines">
2376    .\" </a>
2377    "Newline conventions"
2378    .\"
2379    above. Note that the end of this type of comment is a literal newline sequence
2380    in the pattern; escape sequences that happen to represent a newline do not
2381    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2382    default newline convention is in force:
2383    .sp
2384      abc #comment \en still comment
2385    .sp
2386    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2387    a newline in the pattern. The sequence \en is still literal at this stage, so
2388    it does not terminate the comment. Only an actual character with the code value
2389    0x0a (the default newline) does so.
2390  .  .
2391  .  .
2392  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1701  recursively to the pattern in which it a Line 2412  recursively to the pattern in which it a
2412  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2413  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2414  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2415  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.
2416  .P  .P
2417  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
2418  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
2419  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
2420    .\" HTML <a href="#subpatternsassubroutines">
2421    .\" </a>
2422    non-recursive subroutine
2423    .\"
2424  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
2425  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2426  .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  
2427  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2428  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2429  .sp  .sp
2430    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2431  .sp  .sp
2432  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2433  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
2434  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2435  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2436    to avoid backtracking into sequences of non-parentheses.
2437  .P  .P
2438  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
2439  pattern, so instead you could use this:  pattern, so instead you could use this:
2440  .sp  .sp
2441    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2442  .sp  .sp
2443  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
2444  them instead of the whole pattern.  them instead of the whole pattern.
2445  .P  .P
2446  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
2447  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
2448  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
2449  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2450  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.  
2451  .P  .P
2452  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2453  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2454  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2455  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2456    .\" </a>
2457    non-recursive subroutine
2458    .\"
2459    calls, as described in the next section.
2460  .P  .P
2461  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2462  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
2463  could rewrite the above example as follows:  could rewrite the above example as follows:
2464  .sp  .sp
2465    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2466  .sp  .sp
2467  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
2468  used.  used.
2469  .P  .P
2470  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2471  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
2472  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
2473  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2474  .sp  .sp
2475    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2476  .sp  .sp
2477  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,
2478  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
2479  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
2480  before failure can be reported.  before failure can be reported.
2481  .P  .P
2482  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
2483  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
2484  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  
2485  .\" HREF  .\" HREF
2486  \fBpcrecallout\fP  \fBpcrecallout\fP
2487  .\"  .\"
# Line 1776  documentation). If the pattern above is Line 2489  documentation). If the pattern above is
2489  .sp  .sp
2490    (ab(cd)ef)    (ab(cd)ef)
2491  .sp  .sp
2492  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
2493  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
2494  .sp  matched at the top level, its final captured value is unset, even if it was
2495    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2496       ^                        ^  .P
2497       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2498  .sp  obtain extra memory to store data during a recursion, which it does by using
2499  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
2500  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.  
2501  .P  .P
2502  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.
2503  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1801  different alternatives for the recursive Line 2511  different alternatives for the recursive
2511  is the actual recursive call.  is the actual recursive call.
2512  .  .
2513  .  .
2514    .\" HTML <a name="recursiondifference"></a>
2515    .SS "Differences in recursion processing between PCRE and Perl"
2516    .rs
2517    .sp
2518    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2519    (like Python, but unlike Perl), a recursive subpattern call is always treated
2520    as an atomic group. That is, once it has matched some of the subject string, it
2521    is never re-entered, even if it contains untried alternatives and there is a
2522    subsequent matching failure. This can be illustrated by the following pattern,
2523    which purports to match a palindromic string that contains an odd number of
2524    characters (for example, "a", "aba", "abcba", "abcdcba"):
2525    .sp
2526      ^(.|(.)(?1)\e2)$
2527    .sp
2528    The idea is that it either matches a single character, or two identical
2529    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2530    it does not if the pattern is longer than three characters. Consider the
2531    subject string "abcba":
2532    .P
2533    At the top level, the first character is matched, but as it is not at the end
2534    of the string, the first alternative fails; the second alternative is taken
2535    and the recursion kicks in. The recursive call to subpattern 1 successfully
2536    matches the next character ("b"). (Note that the beginning and end of line
2537    tests are not part of the recursion).
2538    .P
2539    Back at the top level, the next character ("c") is compared with what
2540    subpattern 2 matched, which was "a". This fails. Because the recursion is
2541    treated as an atomic group, there are now no backtracking points, and so the
2542    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2543    try the second alternative.) However, if the pattern is written with the
2544    alternatives in the other order, things are different:
2545    .sp
2546      ^((.)(?1)\e2|.)$
2547    .sp
2548    This time, the recursing alternative is tried first, and continues to recurse
2549    until it runs out of characters, at which point the recursion fails. But this
2550    time we do have another alternative to try at the higher level. That is the big
2551    difference: in the previous case the remaining alternative is at a deeper
2552    recursion level, which PCRE cannot use.
2553    .P
2554    To change the pattern so that it matches all palindromic strings, not just
2555    those with an odd number of characters, it is tempting to change the pattern to
2556    this:
2557    .sp
2558      ^((.)(?1)\e2|.?)$
2559    .sp
2560    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2561    deeper recursion has matched a single character, it cannot be entered again in
2562    order to match an empty string. The solution is to separate the two cases, and
2563    write out the odd and even cases as alternatives at the higher level:
2564    .sp
2565      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2566    .sp
2567    If you want to match typical palindromic phrases, the pattern has to ignore all
2568    non-word characters, which can be done like this:
2569    .sp
2570      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2571    .sp
2572    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2573    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2574    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2575    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2576    more) to match typical phrases, and Perl takes so long that you think it has
2577    gone into a loop.
2578    .P
2579    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2580    string does not start with a palindrome that is shorter than the entire string.
2581    For example, although "abcba" is correctly matched, if the subject is "ababa",
2582    PCRE finds the palindrome "aba" at the start, then fails at top level because
2583    the end of the string does not follow. Once again, it cannot jump back into the
2584    recursion to try other alternatives, so the entire match fails.
2585    .P
2586    The second way in which PCRE and Perl differ in their recursion processing is
2587    in the handling of captured values. In Perl, when a subpattern is called
2588    recursively or as a subpattern (see the next section), it has no access to any
2589    values that were captured outside the recursion, whereas in PCRE these values
2590    can be referenced. Consider this pattern:
2591    .sp
2592      ^(.)(\e1|a(?2))
2593    .sp
2594    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2595    then in the second group, when the back reference \e1 fails to match "b", the
2596    second alternative matches "a" and then recurses. In the recursion, \e1 does
2597    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2598    match because inside the recursive call \e1 cannot access the externally set
2599    value.
2600    .
2601    .
2602  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2603  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2604  .rs  .rs
2605  .sp  .sp
2606  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
2607  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
2608  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2609  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2610  relative, as in these examples:  relative, as in these examples:
2611  .sp  .sp
# Line 1827  matches "sense and sensibility" and "res Line 2625  matches "sense and sensibility" and "res
2625  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
2626  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2627  .P  .P
2628  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2629  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
2630  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
2631  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2632  .P  subroutine call revert to their previous values afterwards.
2633  When a subpattern is used as a subroutine, processing options such as  .P
2634  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2635  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2636    different calls. For example, consider this pattern:
2637  .sp  .sp
2638    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2639  .sp  .sp
# Line 1842  It matches "abcabc". It does not match " Line 2641  It matches "abcabc". It does not match "
2641  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2642  .  .
2643  .  .
2644    .\" HTML <a name="onigurumasubroutines"></a>
2645    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2646    .rs
2647    .sp
2648    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2649    a number enclosed either in angle brackets or single quotes, is an alternative
2650    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2651    are two of the examples used above, rewritten using this syntax:
2652    .sp
2653      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2654      (sens|respons)e and \eg'1'ibility
2655    .sp
2656    PCRE supports an extension to Oniguruma: if a number is preceded by a
2657    plus or a minus sign it is taken as a relative reference. For example:
2658    .sp
2659      (abc)(?i:\eg<-1>)
2660    .sp
2661    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2662    synonymous. The former is a back reference; the latter is a subroutine call.
2663    .
2664    .
2665  .SH CALLOUTS  .SH CALLOUTS
2666  .rs  .rs
2667  .sp  .sp
# Line 1852  same pair of parentheses when there is a Line 2672  same pair of parentheses when there is a
2672  .P  .P
2673  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
2674  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
2675  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
2676    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2677  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2678  .P  .P
2679  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 1862  For example, this pattern has two callou Line 2683  For example, this pattern has two callou
2683  .sp  .sp
2684    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2685  .sp  .sp
2686  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
2687  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2688  255.  255. If there is a conditional group in the pattern whose condition is an
2689  .P  assertion, an additional callout is inserted just before the condition. An
2690  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  explicit callout may also be set at this position, as in this example:
2691  set), the external function is called. It is provided with the number of the  .sp
2692  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2693  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2694  may cause matching to proceed, to backtrack, or to fail altogether. A complete  Note that this applies only to assertion conditions, not to other types of
2695  description of the interface to the callout function is given in the  condition.
2696    .P
2697    During matching, when PCRE reaches a callout point, the external function is
2698    called. It is provided with the number of the callout, the position in the
2699    pattern, and, optionally, one item of data originally supplied by the caller of
2700    the matching function. The callout function may cause matching to proceed, to
2701    backtrack, or to fail altogether. A complete description of the interface to
2702    the callout function is given in the
2703  .\" HREF  .\" HREF
2704  \fBpcrecallout\fP  \fBpcrecallout\fP
2705  .\"  .\"
2706  documentation.  documentation.
2707  .  .
2708  .  .
2709    .\" HTML <a name="backtrackcontrol"></a>
2710    .SH "BACKTRACKING CONTROL"
2711    .rs
2712    .sp
2713    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2714    are still described in the Perl documentation as "experimental and subject to
2715    change or removal in a future version of Perl". It goes on to say: "Their usage
2716    in production code should be noted to avoid problems during upgrades." The same
2717    remarks apply to the PCRE features described in this section.
2718    .P
2719    The new verbs make use of what was previously invalid syntax: an opening
2720    parenthesis followed by an asterisk. They are generally of the form
2721    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2722    differently depending on whether or not a name is present. A name is any
2723    sequence of characters that does not include a closing parenthesis. The maximum
2724    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2725    libraries. If the name is empty, that is, if the closing parenthesis
2726    immediately follows the colon, the effect is as if the colon were not there.
2727    Any number of these verbs may occur in a pattern.
2728    .P
2729    Since these verbs are specifically related to backtracking, most of them can be
2730    used only when the pattern is to be matched using one of the traditional
2731    matching functions, because these use a backtracking algorithm. With the
2732    exception of (*FAIL), which behaves like a failing negative assertion, the
2733    backtracking control verbs cause an error if encountered by a DFA matching
2734    function.
2735    .P
2736    The behaviour of these verbs in
2737    .\" HTML <a href="#btrepeat">
2738    .\" </a>
2739    repeated groups,
2740    .\"
2741    .\" HTML <a href="#btassert">
2742    .\" </a>
2743    assertions,
2744    .\"
2745    and in
2746    .\" HTML <a href="#btsub">
2747    .\" </a>
2748    subpatterns called as subroutines
2749    .\"
2750    (whether or not recursively) is documented below.
2751    .
2752    .
2753    .\" HTML <a name="nooptimize"></a>
2754    .SS "Optimizations that affect backtracking verbs"
2755    .rs
2756    .sp
2757    PCRE contains some optimizations that are used to speed up matching by running
2758    some checks at the start of each match attempt. For example, it may know the
2759    minimum length of matching subject, or that a particular character must be
2760    present. When one of these optimizations bypasses the running of a match, any
2761    included backtracking verbs will not, of course, be processed. You can suppress
2762    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2763    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2764    pattern with (*NO_START_OPT). There is more discussion of this option in the
2765    section entitled
2766    .\" HTML <a href="pcreapi.html#execoptions">
2767    .\" </a>
2768    "Option bits for \fBpcre_exec()\fP"
2769    .\"
2770    in the
2771    .\" HREF
2772    \fBpcreapi\fP
2773    .\"
2774    documentation.
2775    .P
2776    Experiments with Perl suggest that it too has similar optimizations, sometimes
2777    leading to anomalous results.
2778    .
2779    .
2780    .SS "Verbs that act immediately"
2781    .rs
2782    .sp
2783    The following verbs act as soon as they are encountered. They may not be
2784    followed by a name.
2785    .sp
2786       (*ACCEPT)
2787    .sp
2788    This verb causes the match to end successfully, skipping the remainder of the
2789    pattern. However, when it is inside a subpattern that is called as a
2790    subroutine, only that subpattern is ended successfully. Matching then continues
2791    at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2792    assertion succeeds; in a negative assertion, the assertion fails.
2793    .P
2794    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2795    example:
2796    .sp
2797      A((?:A|B(*ACCEPT)|C)D)
2798    .sp
2799    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2800    the outer parentheses.
2801    .sp
2802      (*FAIL) or (*F)
2803    .sp
2804    This verb causes a matching failure, forcing backtracking to occur. It is
2805    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2806    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2807    Perl features that are not present in PCRE. The nearest equivalent is the
2808    callout feature, as for example in this pattern:
2809    .sp
2810      a+(?C)(*FAIL)
2811    .sp
2812    A match with the string "aaaa" always fails, but the callout is taken before
2813    each backtrack happens (in this example, 10 times).
2814    .
2815    .
2816    .SS "Recording which path was taken"
2817    .rs
2818    .sp
2819    There is one verb whose main purpose is to track how a match was arrived at,
2820    though it also has a secondary use in conjunction with advancing the match
2821    starting point (see (*SKIP) below).
2822    .sp
2823      (*MARK:NAME) or (*:NAME)
2824    .sp
2825    A name is always required with this verb. There may be as many instances of
2826    (*MARK) as you like in a pattern, and their names do not have to be unique.
2827    .P
2828    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2829    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2830    caller as described in the section entitled
2831    .\" HTML <a href="pcreapi.html#extradata">
2832    .\" </a>
2833    "Extra data for \fBpcre_exec()\fP"
2834    .\"
2835    in the
2836    .\" HREF
2837    \fBpcreapi\fP
2838    .\"
2839    documentation. Here is an example of \fBpcretest\fP output, where the /K
2840    modifier requests the retrieval and outputting of (*MARK) data:
2841    .sp
2842        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2843      data> XY
2844       0: XY
2845      MK: A
2846      XZ
2847       0: XZ
2848      MK: B
2849    .sp
2850    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2851    indicates which of the two alternatives matched. This is a more efficient way
2852    of obtaining this information than putting each alternative in its own
2853    capturing parentheses.
2854    .P
2855    If a verb with a name is encountered in a positive assertion that is true, the
2856    name is recorded and passed back if it is the last-encountered. This does not
2857    happen for negative assertions or failing positive assertions.
2858    .P
2859    After a partial match or a failed match, the last encountered name in the
2860    entire match process is returned. For example:
2861    .sp
2862        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2863      data> XP
2864      No match, mark = B
2865    .sp
2866    Note that in this unanchored example the mark is retained from the match
2867    attempt that started at the letter "X" in the subject. Subsequent match
2868    attempts starting at "P" and then with an empty string do not get as far as the
2869    (*MARK) item, but nevertheless do not reset it.
2870    .P
2871    If you are interested in (*MARK) values after failed matches, you should
2872    probably set the PCRE_NO_START_OPTIMIZE option
2873    .\" HTML <a href="#nooptimize">
2874    .\" </a>
2875    (see above)
2876    .\"
2877    to ensure that the match is always attempted.
2878    .
2879    .
2880    .SS "Verbs that act after backtracking"
2881    .rs
2882    .sp
2883    The following verbs do nothing when they are encountered. Matching continues
2884    with what follows, but if there is no subsequent match, causing a backtrack to
2885    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2886    the verb. However, when one of these verbs appears inside an atomic group or an
2887    assertion that is true, its effect is confined to that group, because once the
2888    group has been matched, there is never any backtracking into it. In this
2889    situation, backtracking can "jump back" to the left of the entire atomic group
2890    or assertion. (Remember also, as stated above, that this localization also
2891    applies in subroutine calls.)
2892    .P
2893    These verbs differ in exactly what kind of failure occurs when backtracking
2894    reaches them. The behaviour described below is what happens when the verb is
2895    not in a subroutine or an assertion. Subsequent sections cover these special
2896    cases.
2897    .sp
2898      (*COMMIT)
2899    .sp
2900    This verb, which may not be followed by a name, causes the whole match to fail
2901    outright if there is a later matching failure that causes backtracking to reach
2902    it. Even if the pattern is unanchored, no further attempts to find a match by
2903    advancing the starting point take place. If (*COMMIT) is the only backtracking
2904    verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2905    committed to finding a match at the current starting point, or not at all. For
2906    example:
2907    .sp
2908      a+(*COMMIT)b
2909    .sp
2910    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2911    dynamic anchor, or "I've started, so I must finish." The name of the most
2912    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2913    match failure.
2914    .P
2915    If there is more than one backtracking verb in a pattern, a different one that
2916    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2917    match does not always guarantee that a match must be at this starting point.
2918    .P
2919    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2920    unless PCRE's start-of-match optimizations are turned off, as shown in this
2921    \fBpcretest\fP example:
2922    .sp
2923        re> /(*COMMIT)abc/
2924      data> xyzabc
2925       0: abc
2926      xyzabc\eY
2927      No match
2928    .sp
2929    PCRE knows that any match must start with "a", so the optimization skips along
2930    the subject to "a" before running the first match attempt, which succeeds. When
2931    the optimization is disabled by the \eY escape in the second subject, the match
2932    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2933    starting points.
2934    .sp
2935      (*PRUNE) or (*PRUNE:NAME)
2936    .sp
2937    This verb causes the match to fail at the current starting position in the
2938    subject if there is a later matching failure that causes backtracking to reach
2939    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2940    starting character then happens. Backtracking can occur as usual to the left of
2941    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2942    if there is no match to the right, backtracking cannot cross (*PRUNE). In
2943    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2944    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2945    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2946    as (*COMMIT).
2947    .P
2948    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2949    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2950    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2951    .sp
2952      (*SKIP)
2953    .sp
2954    This verb, when given without a name, is like (*PRUNE), except that if the
2955    pattern is unanchored, the "bumpalong" advance is not to the next character,
2956    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2957    signifies that whatever text was matched leading up to it cannot be part of a
2958    successful match. Consider:
2959    .sp
2960      a+(*SKIP)b
2961    .sp
2962    If the subject is "aaaac...", after the first match attempt fails (starting at
2963    the first character in the string), the starting point skips on to start the
2964    next attempt at "c". Note that a possessive quantifer does not have the same
2965    effect as this example; although it would suppress backtracking during the
2966    first match attempt, the second attempt would start at the second character
2967    instead of skipping on to "c".
2968    .sp
2969      (*SKIP:NAME)
2970    .sp
2971    When (*SKIP) has an associated name, its behaviour is modified. When it is
2972    triggered, the previous path through the pattern is searched for the most
2973    recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
2974    is to the subject position that corresponds to that (*MARK) instead of to where
2975    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
2976    (*SKIP) is ignored.
2977    .P
2978    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2979    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2980    .sp
2981      (*THEN) or (*THEN:NAME)
2982    .sp
2983    This verb causes a skip to the next innermost alternative when backtracking
2984    reaches it. That is, it cancels any further backtracking within the current
2985    alternative. Its name comes from the observation that it can be used for a
2986    pattern-based if-then-else block:
2987    .sp
2988      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2989    .sp
2990    If the COND1 pattern matches, FOO is tried (and possibly further items after
2991    the end of the group if FOO succeeds); on failure, the matcher skips to the
2992    second alternative and tries COND2, without backtracking into COND1. If that
2993    succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
2994    more alternatives, so there is a backtrack to whatever came before the entire
2995    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2996    .P
2997    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
2998    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2999    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3000    .P
3001    A subpattern that does not contain a | character is just a part of the
3002    enclosing alternative; it is not a nested alternation with only one
3003    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3004    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3005    pattern fragments that do not contain any | characters at this level:
3006    .sp
3007      A (B(*THEN)C) | D
3008    .sp
3009    If A and B are matched, but there is a failure in C, matching does not
3010    backtrack into A; instead it moves to the next alternative, that is, D.
3011    However, if the subpattern containing (*THEN) is given an alternative, it
3012    behaves differently:
3013    .sp
3014      A (B(*THEN)C | (*FAIL)) | D
3015    .sp
3016    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3017    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3018    because there are no more alternatives to try. In this case, matching does now
3019    backtrack into A.
3020    .P
3021    Note that a conditional subpattern is not considered as having two
3022    alternatives, because only one is ever used. In other words, the | character in
3023    a conditional subpattern has a different meaning. Ignoring white space,
3024    consider:
3025    .sp
3026      ^.*? (?(?=a) a | b(*THEN)c )
3027    .sp
3028    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3029    it initially matches zero characters. The condition (?=a) then fails, the
3030    character "b" is matched, but "c" is not. At this point, matching does not
3031    backtrack to .*? as might perhaps be expected from the presence of the |
3032    character. The conditional subpattern is part of the single alternative that
3033    comprises the whole pattern, and so the match fails. (If there was a backtrack
3034    into .*?, allowing it to match "b", the match would succeed.)
3035    .P
3036    The verbs just described provide four different "strengths" of control when
3037    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3038    next alternative. (*PRUNE) comes next, failing the match at the current
3039    starting position, but allowing an advance to the next character (for an
3040    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3041    than one character. (*COMMIT) is the strongest, causing the entire match to
3042    fail.
3043    .
3044    .
3045    .SS "More than one backtracking verb"
3046    .rs
3047    .sp
3048    If more than one backtracking verb is present in a pattern, the one that is
3049    backtracked onto first acts. For example, consider this pattern, where A, B,
3050    etc. are complex pattern fragments:
3051    .sp
3052      (A(*COMMIT)B(*THEN)C|ABD)
3053    .sp
3054    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3055    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3056    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3057    not always the same as Perl's. It means that if two or more backtracking verbs
3058    appear in succession, all the the last of them has no effect. Consider this
3059    example:
3060    .sp
3061      ...(*COMMIT)(*PRUNE)...
3062    .sp
3063    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3064    it to be triggered, and its action is taken. There can never be a backtrack
3065    onto (*COMMIT).
3066    .
3067    .
3068    .\" HTML <a name="btrepeat"></a>
3069    .SS "Backtracking verbs in repeated groups"
3070    .rs
3071    .sp
3072    PCRE differs from Perl in its handling of backtracking verbs in repeated
3073    groups. For example, consider:
3074    .sp
3075      /(a(*COMMIT)b)+ac/
3076    .sp
3077    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3078    the second repeat of the group acts.
3079    .
3080    .
3081    .\" HTML <a name="btassert"></a>
3082    .SS "Backtracking verbs in assertions"
3083    .rs
3084    .sp
3085    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3086    .P
3087    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3088    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3089    fail without any further processing.
3090    .P
3091    The other backtracking verbs are not treated specially if they appear in a
3092    positive assertion. In particular, (*THEN) skips to the next alternative in the
3093    innermost enclosing group that has alternations, whether or not this is within
3094    the assertion.
3095    .P
3096    Negative assertions are, however, different, in order to ensure that changing a
3097    positive assertion into a negative assertion changes its result. Backtracking
3098    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3099    without considering any further alternative branches in the assertion.
3100    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3101    within the assertion (the normal behaviour), but if the assertion does not have
3102    such an alternative, (*THEN) behaves like (*PRUNE).
3103    .
3104    .
3105    .\" HTML <a name="btsub"></a>
3106    .SS "Backtracking verbs in subroutines"
3107    .rs
3108    .sp
3109    These behaviours occur whether or not the subpattern is called recursively.
3110    Perl's treatment of subroutines is different in some cases.
3111    .P
3112    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3113    an immediate backtrack.
3114    .P
3115    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3116    succeed without any further processing. Matching then continues after the
3117    subroutine call.
3118    .P
3119    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3120    the subroutine match to fail.
3121    .P
3122    (*THEN) skips to the next alternative in the innermost enclosing group within
3123    the subpattern that has alternatives. If there is no such group within the
3124    subpattern, (*THEN) causes the subroutine match to fail.
3125    .
3126    .
3127  .SH "SEE ALSO"  .SH "SEE ALSO"
3128  .rs  .rs
3129  .sp  .sp
3130  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3131    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3132  .  .
3133  .  .
3134  .SH AUTHOR  .SH AUTHOR
# Line 1898  Cambridge CB2 3QH, England. Line 3145  Cambridge CB2 3QH, England.
3145  .rs  .rs
3146  .sp  .sp
3147  .nf  .nf
3148  Last updated: 29 May 2007  Last updated: 26 April 2013
3149  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3150  .fi  .fi

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