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1  .TH PCRE 3  .TH PCREPATTERN 3
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 other books, some of which have copious  .\" HREF
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  \fBpcresyntax\fP
11  O'Reilly, covers them in great detail. The description here is intended as  .\"
12  reference documentation.  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  The basic operation of PCRE is on strings of bytes. However, there is also  conflict with the Perl syntax) in order to provide some compatibility with
15  support for UTF-8 character strings. To use this support you must build PCRE to  regular expressions in Python, .NET, and Oniguruma.
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  .P
17  option. How this affects the pattern matching is mentioned in several places  Perl's regular expressions are described in its own documentation, and
18  below. There is also a summary of UTF-8 features in the  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    The original operation of PCRE was on strings of one-byte characters. However,
24    there is now also support for UTF-8 character strings. To use this,
25    PCRE must be built to include UTF-8 support, and you must call
26    \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27    is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
38  .\"  .\"
39  in the main  in the main
40  .\" HREF  .\" HREF
41  \fBpcre\fR  \fBpcre\fP
42  .\"  .\"
43  page.  page.
44    .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60    The remainder of this document discusses the patterns that are supported by
61    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62    From release 6.0, PCRE offers a second matching function,
63    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
64    Perl-compatible. Some of the features discussed below are not available when
65    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
66    alternative function, and how it differs from the normal function, are
67    discussed in the
68    .\" HREF
69    \fBpcrematching\fP
70    .\"
71    page.
72    .
73    .
74    .\" HTML <a name="newlines"></a>
75    .SH "NEWLINE CONVENTIONS"
76    .rs
77    .sp
78    PCRE supports five different conventions for indicating line breaks in
79    strings: a single CR (carriage return) character, a single LF (linefeed)
80    character, the two-character sequence CRLF, any of the three preceding, or any
81    Unicode newline sequence. The
82    .\" HREF
83    \fBpcreapi\fP
84    .\"
85    page has
86    .\" HTML <a href="pcreapi.html#newlines">
87    .\" </a>
88    further discussion
89    .\"
90    about newlines, and shows how to set the newline convention in the
91    \fIoptions\fP arguments for the compiling and matching functions.
92    .P
93    It is also possible to specify a newline convention by starting a pattern
94    string with one of the following five sequences:
95    .sp
96      (*CR)        carriage return
97      (*LF)        linefeed
98      (*CRLF)      carriage return, followed by linefeed
99      (*ANYCRLF)   any of the three above
100      (*ANY)       all Unicode newline sequences
101    .sp
102    These override the default and the options given to \fBpcre_compile()\fP or
103    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
104    newline sequence, the pattern
105    .sp
106      (*CR)a.b
107    .sp
108    changes the convention to CR. That pattern matches "a\enb" because LF is no
109    longer a newline. Note that these special settings, which are not
110    Perl-compatible, are recognized only at the very start of a pattern, and that
111    they must be in upper case. If more than one of them is present, the last one
112    is used.
113    .P
114    The newline convention affects the interpretation of the dot metacharacter when
115    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
116    affect what the \eR escape sequence matches. By default, this is any Unicode
117    newline sequence, for Perl compatibility. However, this can be changed; see the
118    description of \eR in the section entitled
119    .\" HTML <a href="#newlineseq">
120    .\" </a>
121    "Newline sequences"
122    .\"
123    below. A change of \eR setting can be combined with a change of newline
124    convention.
125    .
126    .
127    .SH "CHARACTERS AND METACHARACTERS"
128    .rs
129    .sp
130  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
131  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
132  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
133    .sp
134    The quick brown fox    The quick brown fox
135    .sp
136  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
137  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
138  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
139  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
140    always possible. For characters with higher values, the concept of case is
141    supported if PCRE is compiled with Unicode property support, but not otherwise.
142    If you want to use caseless matching for characters 128 and above, you must
143    ensure that PCRE is compiled with Unicode property support as well as with
144    UTF-8 support.
145    .P
146    The power of regular expressions comes from the ability to include alternatives
147    and repetitions in the pattern. These are encoded in the pattern by the use of
148    \fImetacharacters\fP, which do not stand for themselves but instead are
149  interpreted in some special way.  interpreted in some special way.
150    .P
151  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
152  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
153  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
154  as follows:  are as follows:
155    .sp
156    \\      general escape character with several uses    \e      general escape character with several uses
157    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
158    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
159    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 168  as follows:
168    +      1 or more quantifier    +      1 or more quantifier
169           also "possessive quantifier"           also "possessive quantifier"
170    {      start min/max quantifier    {      start min/max quantifier
171    .sp
172  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
173  a character class the only meta-characters are:  a character class the only metacharacters are:
174    .sp
175    \\      general escape character    \e      general escape character
176    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
177    -      indicates character range    -      indicates character range
178    .\" JOIN
179    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
180             syntax)             syntax)
181    ]      terminates the character class    ]      terminates the character class
182    .sp
183  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
184    .
185    .
186  .SH BACKSLASH  .SH BACKSLASH
187  .rs  .rs
188  .sp  .sp
189  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
190  non-alphameric character, it takes away any special meaning that character may  character that is not a number or a letter, it takes away any special meaning
191  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
192  outside character classes.  both inside and outside character classes.
193    .P
194  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
195  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
196  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
197  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
198  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
199    .P
200    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201    backslash. All other characters (in particular, those whose codepoints are
202    greater than 127) are treated as literals.
203    .P
204  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
205  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
206  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
207  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
208  pattern.  .P
   
209  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
210  can do so by putting them between \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
211  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
212  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
213    .sp
214    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
215    .sp
216    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
217      \eQabc$xyz\eE        abc$xyz        abc followed by the
218                                        contents of $xyz                                        contents of $xyz
219    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
220    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
221    .sp
222  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
223    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224    by \eE later in the pattern, the literal interpretation continues to the end of
225    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226    a character class, this causes an error, because the character class is not
227    terminated.
228    .
229    .
230    .\" HTML <a name="digitsafterbackslash"></a>
231    .SS "Non-printing characters"
232    .rs
233    .sp
234  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
235  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
236  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
237  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
238  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
239  represents:  .sp
240      \ea        alarm, that is, the BEL character (hex 07)
241    \\a        alarm, that is, the BEL character (hex 07)    \ecx       "control-x", where x is any ASCII character
242    \\cx       "control-x", where x is any character    \ee        escape (hex 1B)
243    \\e        escape (hex 1B)    \ef        formfeed (hex 0C)
244    \\f        formfeed (hex 0C)    \en        linefeed (hex 0A)
245    \\n        newline (hex 0A)    \er        carriage return (hex 0D)
246    \\r        carriage return (hex 0D)    \et        tab (hex 09)
247    \\t        tab (hex 09)    \eddd      character with octal code ddd, or back reference
248    \\ddd      character with octal code ddd, or backreference    \exhh      character with hex code hh
249    \\xhh      character with hex code hh    \ex{hhh..} character with hex code hhh..
250    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)  .sp
251    The precise effect of \ecx is as follows: if x is a lower case letter, it
 The precise effect of \\cx is as follows: if x is a lower case letter, it  
252  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
253  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
254  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255    than 127, a compile-time error occurs. This locks out non-ASCII characters in
256  After \\x, from zero to two hexadecimal digits are read (letters can be in  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  values are valid. A lower case letter is converted to upper case, and then the
258  appear between \\x{ and }, but the value of the character code must be less  0xc0 bits are flipped.)
259  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  .P
260  other than hexadecimal digits appear between \\x{ and }, or if there is no  After \ex, from zero to two hexadecimal digits are read (letters can be in
261  terminating }, this form of escape is not recognized. Instead, the initial  upper or lower case). Any number of hexadecimal digits may appear between \ex{
262  \\x will be interpreted as a basic hexadecimal escape, with no following  and }, but the value of the character code must be less than 256 in non-UTF-8
263  digits, giving a byte whose value is zero.  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
264    hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
265    point, which is 10FFFF.
266    .P
267    If characters other than hexadecimal digits appear between \ex{ and }, or if
268    there is no terminating }, this form of escape is not recognized. Instead, the
269    initial \ex will be interpreted as a basic hexadecimal escape, with no
270    following digits, giving a character whose value is zero.
271    .P
272  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
273  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
274  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
275    .P
276  After \\0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
277  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
278  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
279  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
280  character that follows is itself an octal digit.  follows is itself an octal digit.
281    .P
282  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
283  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
284  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
285  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
286  taken as a \fIback reference\fR. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
287  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
288    .\" </a>
289    later,
290    .\"
291    following the discussion of
292    .\" HTML <a href="#subpattern">
293    .\" </a>
294    parenthesized subpatterns.
295    .\"
296    .P
297  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
298  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
299  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
300  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
301  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
302    to \e777 are permitted. For example:
303    \\040   is another way of writing a space  .sp
304    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
305    .\" JOIN
306      \e40    is the same, provided there are fewer than 40
307              previous capturing subpatterns              previous capturing subpatterns
308    \\7     is always a back reference    \e7     is always a back reference
309    \\11    might be a back reference, or another way of  .\" JOIN
310      \e11    might be a back reference, or another way of
311              writing a tab              writing a tab
312    \\011   is always a tab    \e011   is always a tab
313    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
314    \\113   might be a back reference, otherwise the  .\" JOIN
315      \e113   might be a back reference, otherwise the
316              character with octal code 113              character with octal code 113
317    \\377   might be a back reference, otherwise  .\" JOIN
318      \e377   might be a back reference, otherwise
319              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
320    \\81    is either a back reference, or a binary zero  .\" JOIN
321      \e81    is either a back reference, or a binary zero
322              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
323    .sp
324  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
325  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
326    .P
327  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
328  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
329  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
330  backspace character (hex 08). Outside a character class it has a different  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
331  meaning (see below).  unrecognized escape sequences, they are treated as the literal characters "B",
332    "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
333  The third use of backslash is for specifying generic character types:  set. Outside a character class, these sequences have different meanings.
334    .
335    \\d     any decimal digit  .
336    \\D     any character that is not a decimal digit  .SS "Absolute and relative back references"
337    \\s     any whitespace character  .rs
338    \\S     any character that is not a whitespace character  .sp
339    \\w     any "word" character  The sequence \eg followed by an unsigned or a negative number, optionally
340    \\W     any "non-word" character  enclosed in braces, is an absolute or relative back reference. A named back
341    reference can be coded as \eg{name}. Back references are discussed
342  Each pair of escape sequences partitions the complete set of characters into  .\" HTML <a href="#backreferences">
343  two disjoint sets. Any given character matches one, and only one, of each pair.  .\" </a>
344    later,
345  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  .\"
346  \\w, and always match \\D, \\S, and \\W.  following the discussion of
347    .\" HTML <a href="#subpattern">
348  For compatibility with Perl, \\s does not match the VT character (code 11).  .\" </a>
349  This makes it different from the the POSIX "space" class. The \\s characters  parenthesized subpatterns.
350  are HT (9), LF (10), FF (12), CR (13), and space (32).  .\"
351    .
352  A "word" character is any letter or digit or the underscore character, that is,  .
353  any character which can be part of a Perl "word". The definition of letters and  .SS "Absolute and relative subroutine calls"
354  digits is controlled by PCRE's character tables, and may vary if locale-  .rs
355  specific matching is taking place (see  .sp
356    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
357    a number enclosed either in angle brackets or single quotes, is an alternative
358    syntax for referencing a subpattern as a "subroutine". Details are discussed
359    .\" HTML <a href="#onigurumasubroutines">
360    .\" </a>
361    later.
362    .\"
363    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
364    synonymous. The former is a back reference; the latter is a
365    .\" HTML <a href="#subpatternsassubroutines">
366    .\" </a>
367    subroutine
368    .\"
369    call.
370    .
371    .
372    .\" HTML <a name="genericchartypes"></a>
373    .SS "Generic character types"
374    .rs
375    .sp
376    Another use of backslash is for specifying generic character types:
377    .sp
378      \ed     any decimal digit
379      \eD     any character that is not a decimal digit
380      \eh     any horizontal whitespace character
381      \eH     any character that is not a horizontal whitespace character
382      \es     any whitespace character
383      \eS     any character that is not a whitespace character
384      \ev     any vertical whitespace character
385      \eV     any character that is not a vertical whitespace character
386      \ew     any "word" character
387      \eW     any "non-word" character
388    .sp
389    There is also the single sequence \eN, which matches a non-newline character.
390    This is the same as
391    .\" HTML <a href="#fullstopdot">
392    .\" </a>
393    the "." metacharacter
394    .\"
395    when PCRE_DOTALL is not set.
396    .P
397    Each pair of lower and upper case escape sequences partitions the complete set
398    of characters into two disjoint sets. Any given character matches one, and only
399    one, of each pair. The sequences can appear both inside and outside character
400    classes. They each match one character of the appropriate type. If the current
401    matching point is at the end of the subject string, all of them fail, because
402    there is no character to match.
403    .P
404    For compatibility with Perl, \es does not match the VT character (code 11).
405    This makes it different from the the POSIX "space" class. The \es characters
406    are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
407    included in a Perl script, \es may match the VT character. In PCRE, it never
408    does.
409    .P
410    A "word" character is an underscore or any character that is a letter or digit.
411    By default, the definition of letters and digits is controlled by PCRE's
412    low-valued character tables, and may vary if locale-specific matching is taking
413    place (see
414  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
415  .\" </a>  .\" </a>
416  "Locale support"  "Locale support"
417  .\"  .\"
418  in the  in the
419  .\" HREF  .\" HREF
420  \fBpcreapi\fR  \fBpcreapi\fP
421  .\"  .\"
422  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
423  than 128 are used for accented letters, and these are matched by \\w.  or "french" in Windows, some character codes greater than 128 are used for
424    accented letters, and these are then matched by \ew. The use of locales with
425  These character type sequences can appear both inside and outside character  Unicode is discouraged.
426  classes. They each match one character of the appropriate type. If the current  .P
427  matching point is at the end of the subject string, all of them fail, since  By default, in UTF-8 mode, characters with values greater than 128 never match
428  there is no character to match.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
429    their original meanings from before UTF-8 support was available, mainly for
430  The fourth use of backslash is for certain simple assertions. An assertion  efficiency reasons. However, if PCRE is compiled with Unicode property support,
431    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
432    properties are used to determine character types, as follows:
433    .sp
434      \ed  any character that \ep{Nd} matches (decimal digit)
435      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
436      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
437    .sp
438    The upper case escapes match the inverse sets of characters. Note that \ed
439    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
440    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
441    \eB because they are defined in terms of \ew and \eW. Matching these sequences
442    is noticeably slower when PCRE_UCP is set.
443    .P
444    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
445    release 5.10. In contrast to the other sequences, which match only ASCII
446    characters by default, these always match certain high-valued codepoints in
447    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448    are:
449    .sp
450      U+0009     Horizontal tab
451      U+0020     Space
452      U+00A0     Non-break space
453      U+1680     Ogham space mark
454      U+180E     Mongolian vowel separator
455      U+2000     En quad
456      U+2001     Em quad
457      U+2002     En space
458      U+2003     Em space
459      U+2004     Three-per-em space
460      U+2005     Four-per-em space
461      U+2006     Six-per-em space
462      U+2007     Figure space
463      U+2008     Punctuation space
464      U+2009     Thin space
465      U+200A     Hair space
466      U+202F     Narrow no-break space
467      U+205F     Medium mathematical space
468      U+3000     Ideographic space
469    .sp
470    The vertical space characters are:
471    .sp
472      U+000A     Linefeed
473      U+000B     Vertical tab
474      U+000C     Formfeed
475      U+000D     Carriage return
476      U+0085     Next line
477      U+2028     Line separator
478      U+2029     Paragraph separator
479    .
480    .
481    .\" HTML <a name="newlineseq"></a>
482    .SS "Newline sequences"
483    .rs
484    .sp
485    Outside a character class, by default, the escape sequence \eR matches any
486    Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
487    .sp
488      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489    .sp
490    This is an example of an "atomic group", details of which are given
491    .\" HTML <a href="#atomicgroup">
492    .\" </a>
493    below.
494    .\"
495    This particular group matches either the two-character sequence CR followed by
496    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
497    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
498    line, U+0085). The two-character sequence is treated as a single unit that
499    cannot be split.
500    .P
501    In UTF-8 mode, two additional characters whose codepoints are greater than 255
502    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
503    Unicode character property support is not needed for these characters to be
504    recognized.
505    .P
506    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
507    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
508    either at compile time or when the pattern is matched. (BSR is an abbrevation
509    for "backslash R".) This can be made the default when PCRE is built; if this is
510    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
511    It is also possible to specify these settings by starting a pattern string with
512    one of the following sequences:
513    .sp
514      (*BSR_ANYCRLF)   CR, LF, or CRLF only
515      (*BSR_UNICODE)   any Unicode newline sequence
516    .sp
517    These override the default and the options given to \fBpcre_compile()\fP or
518    \fBpcre_compile2()\fP, but they can be overridden by options given to
519    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
520    which are not Perl-compatible, are recognized only at the very start of a
521    pattern, and that they must be in upper case. If more than one of them is
522    present, the last one is used. They can be combined with a change of newline
523    convention; for example, a pattern can start with:
524    .sp
525      (*ANY)(*BSR_ANYCRLF)
526    .sp
527    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
528    a character class, \eR is treated as an unrecognized escape sequence, and so
529    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
530    .
531    .
532    .\" HTML <a name="uniextseq"></a>
533    .SS Unicode character properties
534    .rs
535    .sp
536    When PCRE is built with Unicode character property support, three additional
537    escape sequences that match characters with specific properties are available.
538    When not in UTF-8 mode, these sequences are of course limited to testing
539    characters whose codepoints are less than 256, but they do work in this mode.
540    The extra escape sequences are:
541    .sp
542      \ep{\fIxx\fP}   a character with the \fIxx\fP property
543      \eP{\fIxx\fP}   a character without the \fIxx\fP property
544      \eX       an extended Unicode sequence
545    .sp
546    The property names represented by \fIxx\fP above are limited to the Unicode
547    script names, the general category properties, "Any", which matches any
548    character (including newline), and some special PCRE properties (described
549    in the
550    .\" HTML <a href="#extraprops">
551    .\" </a>
552    next section).
553    .\"
554    Other Perl properties such as "InMusicalSymbols" are not currently supported by
555    PCRE. Note that \eP{Any} does not match any characters, so always causes a
556    match failure.
557    .P
558    Sets of Unicode characters are defined as belonging to certain scripts. A
559    character from one of these sets can be matched using a script name. For
560    example:
561    .sp
562      \ep{Greek}
563      \eP{Han}
564    .sp
565    Those that are not part of an identified script are lumped together as
566    "Common". The current list of scripts is:
567    .P
568    Arabic,
569    Armenian,
570    Avestan,
571    Balinese,
572    Bamum,
573    Bengali,
574    Bopomofo,
575    Braille,
576    Buginese,
577    Buhid,
578    Canadian_Aboriginal,
579    Carian,
580    Cham,
581    Cherokee,
582    Common,
583    Coptic,
584    Cuneiform,
585    Cypriot,
586    Cyrillic,
587    Deseret,
588    Devanagari,
589    Egyptian_Hieroglyphs,
590    Ethiopic,
591    Georgian,
592    Glagolitic,
593    Gothic,
594    Greek,
595    Gujarati,
596    Gurmukhi,
597    Han,
598    Hangul,
599    Hanunoo,
600    Hebrew,
601    Hiragana,
602    Imperial_Aramaic,
603    Inherited,
604    Inscriptional_Pahlavi,
605    Inscriptional_Parthian,
606    Javanese,
607    Kaithi,
608    Kannada,
609    Katakana,
610    Kayah_Li,
611    Kharoshthi,
612    Khmer,
613    Lao,
614    Latin,
615    Lepcha,
616    Limbu,
617    Linear_B,
618    Lisu,
619    Lycian,
620    Lydian,
621    Malayalam,
622    Meetei_Mayek,
623    Mongolian,
624    Myanmar,
625    New_Tai_Lue,
626    Nko,
627    Ogham,
628    Old_Italic,
629    Old_Persian,
630    Old_South_Arabian,
631    Old_Turkic,
632    Ol_Chiki,
633    Oriya,
634    Osmanya,
635    Phags_Pa,
636    Phoenician,
637    Rejang,
638    Runic,
639    Samaritan,
640    Saurashtra,
641    Shavian,
642    Sinhala,
643    Sundanese,
644    Syloti_Nagri,
645    Syriac,
646    Tagalog,
647    Tagbanwa,
648    Tai_Le,
649    Tai_Tham,
650    Tai_Viet,
651    Tamil,
652    Telugu,
653    Thaana,
654    Thai,
655    Tibetan,
656    Tifinagh,
657    Ugaritic,
658    Vai,
659    Yi.
660    .P
661    Each character has exactly one Unicode general category property, specified by
662    a two-letter abbreviation. For compatibility with Perl, negation can be
663    specified by including a circumflex between the opening brace and the property
664    name. For example, \ep{^Lu} is the same as \eP{Lu}.
665    .P
666    If only one letter is specified with \ep or \eP, it includes all the general
667    category properties that start with that letter. In this case, in the absence
668    of negation, the curly brackets in the escape sequence are optional; these two
669    examples have the same effect:
670    .sp
671      \ep{L}
672      \epL
673    .sp
674    The following general category property codes are supported:
675    .sp
676      C     Other
677      Cc    Control
678      Cf    Format
679      Cn    Unassigned
680      Co    Private use
681      Cs    Surrogate
682    .sp
683      L     Letter
684      Ll    Lower case letter
685      Lm    Modifier letter
686      Lo    Other letter
687      Lt    Title case letter
688      Lu    Upper case letter
689    .sp
690      M     Mark
691      Mc    Spacing mark
692      Me    Enclosing mark
693      Mn    Non-spacing mark
694    .sp
695      N     Number
696      Nd    Decimal number
697      Nl    Letter number
698      No    Other number
699    .sp
700      P     Punctuation
701      Pc    Connector punctuation
702      Pd    Dash punctuation
703      Pe    Close punctuation
704      Pf    Final punctuation
705      Pi    Initial punctuation
706      Po    Other punctuation
707      Ps    Open punctuation
708    .sp
709      S     Symbol
710      Sc    Currency symbol
711      Sk    Modifier symbol
712      Sm    Mathematical symbol
713      So    Other symbol
714    .sp
715      Z     Separator
716      Zl    Line separator
717      Zp    Paragraph separator
718      Zs    Space separator
719    .sp
720    The special property L& is also supported: it matches a character that has
721    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
722    a modifier or "other".
723    .P
724    The Cs (Surrogate) property applies only to characters in the range U+D800 to
725    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
726    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
727    (see the discussion of PCRE_NO_UTF8_CHECK in the
728    .\" HREF
729    \fBpcreapi\fP
730    .\"
731    page). Perl does not support the Cs property.
732    .P
733    The long synonyms for property names that Perl supports (such as \ep{Letter})
734    are not supported by PCRE, nor is it permitted to prefix any of these
735    properties with "Is".
736    .P
737    No character that is in the Unicode table has the Cn (unassigned) property.
738    Instead, this property is assumed for any code point that is not in the
739    Unicode table.
740    .P
741    Specifying caseless matching does not affect these escape sequences. For
742    example, \ep{Lu} always matches only upper case letters.
743    .P
744    The \eX escape matches any number of Unicode characters that form an extended
745    Unicode sequence. \eX is equivalent to
746    .sp
747      (?>\ePM\epM*)
748    .sp
749    That is, it matches a character without the "mark" property, followed by zero
750    or more characters with the "mark" property, and treats the sequence as an
751    atomic group
752    .\" HTML <a href="#atomicgroup">
753    .\" </a>
754    (see below).
755    .\"
756    Characters with the "mark" property are typically accents that affect the
757    preceding character. None of them have codepoints less than 256, so in
758    non-UTF-8 mode \eX matches any one character.
759    .P
760    Note that recent versions of Perl have changed \eX to match what Unicode calls
761    an "extended grapheme cluster", which has a more complicated definition.
762    .P
763    Matching characters by Unicode property is not fast, because PCRE has to search
764    a structure that contains data for over fifteen thousand characters. That is
765    why the traditional escape sequences such as \ed and \ew do not use Unicode
766    properties in PCRE by default, though you can make them do so by setting the
767    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
768    (*UCP).
769    .
770    .
771    .\" HTML <a name="extraprops"></a>
772    .SS PCRE's additional properties
773    .rs
774    .sp
775    As well as the standard Unicode properties described in the previous
776    section, PCRE supports four more that make it possible to convert traditional
777    escape sequences such as \ew and \es and POSIX character classes to use Unicode
778    properties. PCRE uses these non-standard, non-Perl properties internally when
779    PCRE_UCP is set. They are:
780    .sp
781      Xan   Any alphanumeric character
782      Xps   Any POSIX space character
783      Xsp   Any Perl space character
784      Xwd   Any Perl "word" character
785    .sp
786    Xan matches characters that have either the L (letter) or the N (number)
787    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
788    carriage return, and any other character that has the Z (separator) property.
789    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
790    same characters as Xan, plus underscore.
791    .
792    .
793    .\" HTML <a name="resetmatchstart"></a>
794    .SS "Resetting the match start"
795    .rs
796    .sp
797    The escape sequence \eK causes any previously matched characters not to be
798    included in the final matched sequence. For example, the pattern:
799    .sp
800      foo\eKbar
801    .sp
802    matches "foobar", but reports that it has matched "bar". This feature is
803    similar to a lookbehind assertion
804    .\" HTML <a href="#lookbehind">
805    .\" </a>
806    (described below).
807    .\"
808    However, in this case, the part of the subject before the real match does not
809    have to be of fixed length, as lookbehind assertions do. The use of \eK does
810    not interfere with the setting of
811    .\" HTML <a href="#subpattern">
812    .\" </a>
813    captured substrings.
814    .\"
815    For example, when the pattern
816    .sp
817      (foo)\eKbar
818    .sp
819    matches "foobar", the first substring is still set to "foo".
820    .P
821    Perl documents that the use of \eK within assertions is "not well defined". In
822    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
823    ignored in negative assertions.
824    .
825    .
826    .\" HTML <a name="smallassertions"></a>
827    .SS "Simple assertions"
828    .rs
829    .sp
830    The final use of backslash is for certain simple assertions. An assertion
831  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
832  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
833  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
834  assertions are  .\" HTML <a href="#bigassertions">
835    .\" </a>
836    \\b     matches at a word boundary  below.
837    \\B     matches when not at a word boundary  .\"
838    \\A     matches at start of subject  The backslashed assertions are:
839    \\Z     matches at end of subject or before newline at end  .sp
840    \\z     matches at end of subject    \eb     matches at a word boundary
841    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
842      \eA     matches at the start of the subject
843  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
844  different meaning, namely the backspace character, inside a character class).            also matches before a newline at the end of the subject
845      \ez     matches only at the end of the subject
846      \eG     matches at the first matching position in the subject
847    .sp
848    Inside a character class, \eb has a different meaning; it matches the backspace
849    character. If any other of these assertions appears in a character class, by
850    default it matches the corresponding literal character (for example, \eB
851    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
852    escape sequence" error is generated instead.
853    .P
854  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
855  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
856  \\w and the other matches \\W), 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
857  first or last character matches \\w, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
858    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
859  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
860  dollar (described below) in that they only ever match at the very start and end  of word" or "end of word" metasequence. However, whatever follows \eb normally
861  of the subject string, whatever options are set. Thus, they are independent of  determines which it is. For example, the fragment \eba matches "a" at the start
862  multiline mode.  of a word.
863    .P
864  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
865  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  dollar (described in the next section) in that they only ever match at the very
866  matching is to start at a point other than the beginning of the subject, \\A  start and end of the subject string, whatever options are set. Thus, they are
867  can never match. The difference between \\Z and \\z is that \\Z matches before  independent of multiline mode. These three assertions are not affected by the
868  a newline that is the last character of the string as well as at the end of the  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
869  string, whereas \\z matches only at the end.  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
870    argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
871  The \\G assertion is true only when the current matching position is at the  at a point other than the beginning of the subject, \eA can never match. The
872  start point of the match, as specified by the \fIstartoffset\fR argument of  difference between \eZ and \ez is that \eZ matches before a newline at the end
873  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  of the string as well as at the very end, whereas \ez matches only at the end.
874  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  .P
875    The \eG assertion is true only when the current matching position is at the
876    start point of the match, as specified by the \fIstartoffset\fP argument of
877    \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
878    non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
879  arguments, you can mimic Perl's /g option, and it is in this kind of  arguments, you can mimic Perl's /g option, and it is in this kind of
880  implementation where \\G can be useful.  implementation where \eG can be useful.
881    .P
882  Note, however, that PCRE's interpretation of \\G, as the start of the current  Note, however, that PCRE's interpretation of \eG, as the start of the current
883  match, is subtly different from Perl's, which defines it as the end of the  match, is subtly different from Perl's, which defines it as the end of the
884  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
885  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
886  reproduce this behaviour.  reproduce this behaviour.
887    .P
888  If all the alternatives of a pattern begin with \\G, the expression is anchored  If all the alternatives of a pattern begin with \eG, the expression is anchored
889  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
890  regular expression.  regular expression.
891    .
892  .SH CIRCUMFLEX AND DOLLAR  .
893    .SH "CIRCUMFLEX AND DOLLAR"
894  .rs  .rs
895  .sp  .sp
896  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
897  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
898  at the start of the subject string. If the \fIstartoffset\fR argument of  at the start of the subject string. If the \fIstartoffset\fP argument of
899  \fBpcre_exec()\fR 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
900  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
901  meaning (see below).  meaning
902    .\" HTML <a href="#characterclass">
903    .\" </a>
904    (see below).
905    .\"
906    .P
907  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
908  alternatives are involved, but it should be the first thing in each alternative  alternatives are involved, but it should be the first thing in each alternative
909  in which it appears if the pattern is ever to match that branch. If all  in which it appears if the pattern is ever to match that branch. If all
# Line 290  possible alternatives start with a circu Line 911  possible alternatives start with a circu
911  constrained to match only at the start of the subject, it is said to be an  constrained to match only at the start of the subject, it is said to be an
912  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
913  to be anchored.)  to be anchored.)
914    .P
915  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
916  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
917  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
918  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
919  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
920  Dollar has no special meaning in a character class.  character class.
921    .P
922  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
923  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
924  does not affect the \\Z assertion.  does not affect the \eZ assertion.
925    .P
926  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
927  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
928  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
929  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
930  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
931  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
932  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
933  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
934  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
935  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
936    patterns that are anchored in single line mode because all branches start with
937  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  ^ are not anchored in multiline mode, and a match for circumflex is possible
938    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
939    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
940    .P
941    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
942  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
943  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
944    .
945  .SH FULL STOP (PERIOD, DOT)  .
946    .\" HTML <a name="fullstopdot"></a>
947    .SH "FULL STOP (PERIOD, DOT) AND \eN"
948  .rs  .rs
949  .sp  .sp
950  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
951  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
952  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
953  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
954  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
955  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
956  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
957    (including isolated CRs and LFs). When any Unicode line endings are being
958  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
959  .rs  characters.
960  .sp  .P
961  Outside a character class, the escape sequence \\C matches any one byte, both  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
962  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  option is set, a dot matches any one character, without exception. If the
963  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  two-character sequence CRLF is present in the subject string, it takes two dots
964  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
965  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
966    The handling of dot is entirely independent of the handling of circumflex and
967  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  dollar, the only relationship being that they both involve newlines. Dot has no
968  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
969    .P
970  .SH SQUARE BRACKETS  The escape sequence \eN behaves like a dot, except that it is not affected by
971    the PCRE_DOTALL option. In other words, it matches any character except one
972    that signifies the end of a line.
973    .
974    .
975    .SH "MATCHING A SINGLE BYTE"
976    .rs
977    .sp
978    Outside a character class, the escape sequence \eC matches any one byte, both
979    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
980    characters. The feature is provided in Perl in order to match individual bytes
981    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
982    rest of the string may start with a malformed UTF-8 character. For this reason,
983    the \eC escape sequence is best avoided.
984    .P
985    PCRE does not allow \eC to appear in lookbehind assertions
986    .\" HTML <a href="#lookbehind">
987    .\" </a>
988    (described below),
989    .\"
990    because in UTF-8 mode this would make it impossible to calculate the length of
991    the lookbehind.
992    .
993    .
994    .\" HTML <a name="characterclass"></a>
995    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
996  .rs  .rs
997  .sp  .sp
998  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
999  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.
1000  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
1001  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
1002  escaped with a backslash.  a member of the class, it should be the first data character in the class
1003    (after an initial circumflex, if present) or escaped with a backslash.
1004    .P
1005  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
1006  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
1007  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
1008  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
1009  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
1010  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
1011  backslash.  backslash.
1012    .P
1013  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1014  [^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
1015  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
1016  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
1017  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion; it still consumes a character from the subject
1018  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
1019    string.
1020    .P
1021  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
1022  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1023    .P
1024  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
1025  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
1026  "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
1027  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
1028  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
1029    always possible. For characters with higher values, the concept of case is
1030  The newline character is never treated in any special way in character classes,  supported if PCRE is compiled with Unicode property support, but not otherwise.
1031  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1032  such as [^a] will always match a newline.  you must ensure that PCRE is compiled with Unicode property support as well as
1033    with UTF-8 support.
1034    .P
1035    Characters that might indicate line breaks are never treated in any special way
1036    when matching character classes, whatever line-ending sequence is in use, and
1037    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1038    such as [^a] always matches one of these characters.
1039    .P
1040  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
1041  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
1042  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
1043  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
1044  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class.
1045    .P
1046  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
1047  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
1048  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1049  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1050  the end of range, so [W-\\]46] is interpreted as a single class containing a  the end of range, so [W-\e]46] is interpreted as a class containing a range
1051  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
1052  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
1053    .P
1054  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
1055  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1056  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
1057  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
1058    .P
1059  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
1060  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
1061  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1062  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\exc8-\excb] matches accented E
1063    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1064  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a  characters with values greater than 128 only when it is compiled with Unicode
1065  character class, and add the characters that they match to the class. For  property support.
1066  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
1067  conveniently be used with the upper case character types to specify a more  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1068  restricted set of characters than the matching lower case type. For example,  \eV, \ew, and \eW may appear in a character class, and add the characters that
1069  the class [^\\W_] matches any letter or digit, but not underscore.  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1070    digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1071  All non-alphameric characters other than \\, -, ^ (at the start) and the  and their upper case partners, just as it does when they appear outside a
1072  terminating ] are non-special in character classes, but it does no harm if they  character class, as described in the section entitled
1073  are escaped.  .\" HTML <a href="#genericchartypes">
1074    .\" </a>
1075  .SH POSIX CHARACTER CLASSES  "Generic character types"
1076    .\"
1077    above. The escape sequence \eb has a different meaning inside a character
1078    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1079    are not special inside a character class. Like any other unrecognized escape
1080    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1081    default, but cause an error if the PCRE_EXTRA option is set.
1082    .P
1083    A circumflex can conveniently be used with the upper case character types to
1084    specify a more restricted set of characters than the matching lower case type.
1085    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1086    whereas [\ew] includes underscore. A positive character class should be read as
1087    "something OR something OR ..." and a negative class as "NOT something AND NOT
1088    something AND NOT ...".
1089    .P
1090    The only metacharacters that are recognized in character classes are backslash,
1091    hyphen (only where it can be interpreted as specifying a range), circumflex
1092    (only at the start), opening square bracket (only when it can be interpreted as
1093    introducing a POSIX class name - see the next section), and the terminating
1094    closing square bracket. However, escaping other non-alphanumeric characters
1095    does no harm.
1096    .
1097    .
1098    .SH "POSIX CHARACTER CLASSES"
1099  .rs  .rs
1100  .sp  .sp
1101  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
1102  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1103  this notation. For example,  this notation. For example,
1104    .sp
1105    [01[:alpha:]%]    [01[:alpha:]%]
1106    .sp
1107  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1108  are  are:
1109    .sp
1110    alnum    letters and digits    alnum    letters and digits
1111    alpha    letters    alpha    letters
1112    ascii    character codes 0 - 127    ascii    character codes 0 - 127
1113    blank    space or tab only    blank    space or tab only
1114    cntrl    control characters    cntrl    control characters
1115    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
1116    graph    printing characters, excluding space    graph    printing characters, excluding space
1117    lower    lower case letters    lower    lower case letters
1118    print    printing characters, including space    print    printing characters, including space
1119    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1120    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
1121    upper    upper case letters    upper    upper case letters
1122    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
1123    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1124    .sp
1125  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1126  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
1127  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
1128  compatibility).  compatibility).
1129    .P
1130  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1131  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
1132  after the colon. For example,  after the colon. For example,
1133    .sp
1134    [12[:^digit:]]    [12[:^digit:]]
1135    .sp
1136  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1137  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
1138  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1139    .P
1140  In UTF-8 mode, characters with values greater than 255 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1141  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1142    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1143  .SH VERTICAL BAR  character properties are used. This is achieved by replacing the POSIX classes
1144    by other sequences, as follows:
1145    .sp
1146      [:alnum:]  becomes  \ep{Xan}
1147      [:alpha:]  becomes  \ep{L}
1148      [:blank:]  becomes  \eh
1149      [:digit:]  becomes  \ep{Nd}
1150      [:lower:]  becomes  \ep{Ll}
1151      [:space:]  becomes  \ep{Xps}
1152      [:upper:]  becomes  \ep{Lu}
1153      [:word:]   becomes  \ep{Xwd}
1154    .sp
1155    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1156    classes are unchanged, and match only characters with code points less than
1157    128.
1158    .
1159    .
1160    .SH "VERTICAL BAR"
1161  .rs  .rs
1162  .sp  .sp
1163  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1164  the pattern  the pattern
1165    .sp
1166    gilbert|sullivan    gilbert|sullivan
1167    .sp
1168  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1169  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1170  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1171  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
1172  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
1173  pattern as well as the alternative in the subpattern.  .\" </a>
1174    (defined below),
1175  .SH INTERNAL OPTION SETTING  .\"
1176    "succeeds" means matching the rest of the main pattern as well as the
1177    alternative in the subpattern.
1178    .
1179    .
1180    .SH "INTERNAL OPTION SETTING"
1181  .rs  .rs
1182  .sp  .sp
1183  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1184  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
1185  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1186    The option letters are
1187    .sp
1188    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1189    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
1190    s  for PCRE_DOTALL    s  for PCRE_DOTALL
1191    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
1192    .sp
1193  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
1194  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
1195  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1196  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1197  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
1198  unset.  unset.
1199    .P
1200  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
1201  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
1202  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1203  the global options (and it will therefore show up in data extracted by the  .P
1204  \fBpcre_fullinfo()\fR function).  When one of these option changes occurs at top level (that is, not inside
1205    subpattern parentheses), the change applies to the remainder of the pattern
1206  An option change within a subpattern affects only that part of the current  that follows. If the change is placed right at the start of a pattern, PCRE
1207  pattern that follows it, so  extracts it into the global options (and it will therefore show up in data
1208    extracted by the \fBpcre_fullinfo()\fP function).
1209    .P
1210    An option change within a subpattern (see below for a description of
1211    subpatterns) affects only that part of the subpattern that follows it, so
1212    .sp
1213    (a(?i)b)c    (a(?i)b)c
1214    .sp
1215  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1216  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
1217  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
1218  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
1219    .sp
1220    (a(?i)b|c)    (a(?i)b|c)
1221    .sp
1222  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
1223  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1224  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1225  behaviour otherwise.  behaviour otherwise.
1226    .P
1227  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1228  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1229  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1230  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1231  when it is at top level. It is best put at the start.  section entitled
1232    .\" HTML <a href="#newlineseq">
1233    .\" </a>
1234    "Newline sequences"
1235    .\"
1236    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1237    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1238    PCRE_UTF8 and the PCRE_UCP options, respectively.
1239    .
1240    .
1241    .\" HTML <a name="subpattern"></a>
1242  .SH SUBPATTERNS  .SH SUBPATTERNS
1243  .rs  .rs
1244  .sp  .sp
1245  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1246  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1247    .sp
1248  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
1249    .sp
1250    cat(aract|erpillar|)    cat(aract|erpillar|)
1251    .sp
1252  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1253  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1254    .sp
1255  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
1256  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
1257  the subpattern is passed back to the caller via the \fIovector\fR argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1258  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1259  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1260    string "the red king" is matched against the pattern
1261  For example, if the string "the red king" is matched against the pattern  .sp
   
1262    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1263    .sp
1264  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
1265  2, and 3, respectively.  2, and 3, respectively.
1266    .P
1267  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
1268  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
1269  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
1270  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
1271  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1272  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1273    .sp
1274    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1275    .sp
1276  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1277  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1278  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1279  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1280  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1281  the ":". Thus the two patterns  the ":". Thus the two patterns
1282    .sp
1283    (?i:saturday|sunday)    (?i:saturday|sunday)
1284    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1285    .sp
1286  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1287  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1288  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1289  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1290    .
1291  .SH NAMED SUBPATTERNS  .
1292    .\" HTML <a name="dupsubpatternnumber"></a>
1293    .SH "DUPLICATE SUBPATTERN NUMBERS"
1294    .rs
1295    .sp
1296    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1297    the same numbers for its capturing parentheses. Such a subpattern starts with
1298    (?| and is itself a non-capturing subpattern. For example, consider this
1299    pattern:
1300    .sp
1301      (?|(Sat)ur|(Sun))day
1302    .sp
1303    Because the two alternatives are inside a (?| group, both sets of capturing
1304    parentheses are numbered one. Thus, when the pattern matches, you can look
1305    at captured substring number one, whichever alternative matched. This construct
1306    is useful when you want to capture part, but not all, of one of a number of
1307    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1308    number is reset at the start of each branch. The numbers of any capturing
1309    parentheses that follow the subpattern start after the highest number used in
1310    any branch. The following example is taken from the Perl documentation. The
1311    numbers underneath show in which buffer the captured content will be stored.
1312    .sp
1313      # before  ---------------branch-reset----------- after
1314      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1315      # 1            2         2  3        2     3     4
1316    .sp
1317    A back reference to a numbered subpattern uses the most recent value that is
1318    set for that number by any subpattern. The following pattern matches "abcabc"
1319    or "defdef":
1320    .sp
1321      /(?|(abc)|(def))\e1/
1322    .sp
1323    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1324    refers to the first one in the pattern with the given number. The following
1325    pattern matches "abcabc" or "defabc":
1326    .sp
1327      /(?|(abc)|(def))(?1)/
1328    .sp
1329    If a
1330    .\" HTML <a href="#conditions">
1331    .\" </a>
1332    condition test
1333    .\"
1334    for a subpattern's having matched refers to a non-unique number, the test is
1335    true if any of the subpatterns of that number have matched.
1336    .P
1337    An alternative approach to using this "branch reset" feature is to use
1338    duplicate named subpatterns, as described in the next section.
1339    .
1340    .
1341    .SH "NAMED SUBPATTERNS"
1342  .rs  .rs
1343  .sp  .sp
1344  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1345  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1346  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
1347  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1348  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1349  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1350    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1351  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1352  PCRE API provides function calls for extracting the name-to-number translation  .P
1353  table from a compiled pattern. For further details see the  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1354    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1355    parentheses from other parts of the pattern, such as
1356    .\" HTML <a href="#backreferences">
1357    .\" </a>
1358    back references,
1359    .\"
1360    .\" HTML <a href="#recursion">
1361    .\" </a>
1362    recursion,
1363    .\"
1364    and
1365    .\" HTML <a href="#conditions">
1366    .\" </a>
1367    conditions,
1368    .\"
1369    can be made by name as well as by number.
1370    .P
1371    Names consist of up to 32 alphanumeric characters and underscores. Named
1372    capturing parentheses are still allocated numbers as well as names, exactly as
1373    if the names were not present. The PCRE API provides function calls for
1374    extracting the name-to-number translation table from a compiled pattern. There
1375    is also a convenience function for extracting a captured substring by name.
1376    .P
1377    By default, a name must be unique within a pattern, but it is possible to relax
1378    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1379    names are also always permitted for subpatterns with the same number, set up as
1380    described in the previous section.) Duplicate names can be useful for patterns
1381    where only one instance of the named parentheses can match. Suppose you want to
1382    match the name of a weekday, either as a 3-letter abbreviation or as the full
1383    name, and in both cases you want to extract the abbreviation. This pattern
1384    (ignoring the line breaks) does the job:
1385    .sp
1386      (?<DN>Mon|Fri|Sun)(?:day)?|
1387      (?<DN>Tue)(?:sday)?|
1388      (?<DN>Wed)(?:nesday)?|
1389      (?<DN>Thu)(?:rsday)?|
1390      (?<DN>Sat)(?:urday)?
1391    .sp
1392    There are five capturing substrings, but only one is ever set after a match.
1393    (An alternative way of solving this problem is to use a "branch reset"
1394    subpattern, as described in the previous section.)
1395    .P
1396    The convenience function for extracting the data by name returns the substring
1397    for the first (and in this example, the only) subpattern of that name that
1398    matched. This saves searching to find which numbered subpattern it was.
1399    .P
1400    If you make a back reference to a non-unique named subpattern from elsewhere in
1401    the pattern, the one that corresponds to the first occurrence of the name is
1402    used. In the absence of duplicate numbers (see the previous section) this is
1403    the one with the lowest number. If you use a named reference in a condition
1404    test (see the
1405    .\"
1406    .\" HTML <a href="#conditions">
1407    .\" </a>
1408    section about conditions
1409    .\"
1410    below), either to check whether a subpattern has matched, or to check for
1411    recursion, all subpatterns with the same name are tested. If the condition is
1412    true for any one of them, the overall condition is true. This is the same
1413    behaviour as testing by number. For further details of the interfaces for
1414    handling named subpatterns, see the
1415  .\" HREF  .\" HREF
1416  \fBpcreapi\fR  \fBpcreapi\fP
1417  .\"  .\"
1418  documentation.  documentation.
1419    .P
1420    \fBWarning:\fP You cannot use different names to distinguish between two
1421    subpatterns with the same number because PCRE uses only the numbers when
1422    matching. For this reason, an error is given at compile time if different names
1423    are given to subpatterns with the same number. However, you can give the same
1424    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1425    .
1426    .
1427  .SH REPETITION  .SH REPETITION
1428  .rs  .rs
1429  .sp  .sp
1430  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1431  items:  items:
1432    .sp
1433    a literal data character    a literal data character
1434    the . metacharacter    the dot metacharacter
1435    the \\C escape sequence    the \eC escape sequence
1436    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1437      the \eR escape sequence
1438      an escape such as \ed or \epL that matches a single character
1439    a character class    a character class
1440    a back reference (see next section)    a back reference (see next section)
1441    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1442      a recursive or "subroutine" call to a subpattern
1443    .sp
1444  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1445  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1446  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1447  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1448    .sp
1449    z{2,4}    z{2,4}
1450    .sp
1451  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1452  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
1453  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
1454  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1455    .sp
1456    [aeiou]{3,}    [aeiou]{3,}
1457    .sp
1458  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1459    .sp
1460    \\d{8}    \ed{8}
1461    .sp
1462  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
1463  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
1464  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
1465  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1466    .P
1467  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1468  bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1469  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1470    support is available, \eX{3} matches three Unicode extended sequences, each of
1471    which may be several bytes long (and they may be of different lengths).
1472    .P
1473  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
1474  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1475    subpatterns that are referenced as
1476  For convenience (and historical compatibility) the three most common  .\" HTML <a href="#subpatternsassubroutines">
1477  quantifiers have single-character abbreviations:  .\" </a>
1478    subroutines
1479    .\"
1480    from elsewhere in the pattern (but see also the section entitled
1481    .\" HTML <a href="#subdefine">
1482    .\" </a>
1483    "Defining subpatterns for use by reference only"
1484    .\"
1485    below). Items other than subpatterns that have a {0} quantifier are omitted
1486    from the compiled pattern.
1487    .P
1488    For convenience, the three most common quantifiers have single-character
1489    abbreviations:
1490    .sp
1491    *    is equivalent to {0,}    *    is equivalent to {0,}
1492    +    is equivalent to {1,}    +    is equivalent to {1,}
1493    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1494    .sp
1495  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
1496  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
1497    .sp
1498    (a?)*    (a?)*
1499    .sp
1500  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
1501  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
1502  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
1503  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1504    .P
1505  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
1506  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1507  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1508  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
1509  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1510  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1511    .sp
1512    /\\*.*\\*/    /\e*.*\e*/
1513    .sp
1514  to the string  to the string
1515    .sp
1516    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1517    .sp
1518  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
1519  item.  item.
1520    .P
1521  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
1522  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1523  pattern  pattern
1524    .sp
1525    /\\*.*?\\*/    /\e*.*?\e*/
1526    .sp
1527  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
1528  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1529  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1530  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1531    .sp
1532    \\d??\\d    \ed??\ed
1533    .sp
1534  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1535  way the rest of the pattern matches.  way the rest of the pattern matches.
1536    .P
1537  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1538  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1539  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1540  default behaviour.  default behaviour.
1541    .P
1542  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1543  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
1544  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1545    .P
1546  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1547  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1548  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1549  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1550  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
1551  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1552    .P
1553  In cases where it is known that the subject string contains no newlines, it is  In cases where it is known that the subject string contains no newlines, it is
1554  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1555  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1556    .P
1557  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1558  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1559  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1560  succeed. Consider, for example:  succeeds. Consider, for example:
1561    .sp
1562    (.*)abc\\1    (.*)abc\e1
1563    .sp
1564  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
1565  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1566    .P
1567  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1568  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1569    .sp
1570    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1571    .sp
1572  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1573  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1574  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1575  example, after  example, after
1576    .sp
1577    /(a|(b))+/    /(a|(b))+/
1578    .sp
1579  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1580    .
1581  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1582    .\" HTML <a name="atomicgroup"></a>
1583    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1584  .rs  .rs
1585  .sp  .sp
1586  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1587  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1588  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1589  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1590  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1591  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1592    .P
1593  Consider, for example, the pattern \\d+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1594    .sp
1595    123456bar    123456bar
1596    .sp
1597  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1598  action of the matcher is to try again with only 5 digits matching the \\d+  action of the matcher is to try again with only 5 digits matching the \ed+
1599  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1600  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1601  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1602    .P
1603  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1604  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1605  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1606    .sp
1607    (?>\\d+)foo    (?>\ed+)foo
1608    .sp
1609  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1610  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1611  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1612  normal.  normal.
1613    .P
1614  An alternative description is that a subpattern of this type matches the string  An alternative description is that a subpattern of this type matches the string
1615  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1616  the current point in the subject string.  the current point in the subject string.
1617    .P
1618  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1619  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1620  everything it can. So, while both \\d+ and \\d+? are prepared to adjust the  everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1621  number of digits they match in order to make the rest of the pattern match,  number of digits they match in order to make the rest of the pattern match,
1622  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1623    .P
1624  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1625  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1626  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1627  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1628  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1629  previous example can be rewritten as  previous example can be rewritten as
1630    .sp
1631    \\d++bar    \ed++foo
1632    .sp
1633    Note that a possessive quantifier can be used with an entire group, for
1634    example:
1635    .sp
1636      (abc|xyz){2,3}+
1637    .sp
1638  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1639  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
1640  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1641  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1642    difference; possessive quantifiers should be slightly faster.
1643  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1644  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1645    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1646    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1647    package, and PCRE copied it from there. It ultimately found its way into Perl
1648    at release 5.10.
1649    .P
1650    PCRE has an optimization that automatically "possessifies" certain simple
1651    pattern constructs. For example, the sequence A+B is treated as A++B because
1652    there is no point in backtracking into a sequence of A's when B must follow.
1653    .P
1654  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1655  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
1656  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1657  pattern  pattern
1658    .sp
1659    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1660    .sp
1661  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1662  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1663  quickly. However, if it is applied to  quickly. However, if it is applied to
1664    .sp
1665    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1666    .sp
1667  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1668  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \eD+ repeat and the external * repeat in a
1669  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1670  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1671  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1672  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1673  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1674    an atomic group, like this:
1675    ((?>\\D+)|<\\d+>)*[!?]  .sp
1676      ((?>\eD+)|<\ed+>)*[!?]
1677    .sp
1678  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1679    .
1680  .SH BACK REFERENCES  .
1681    .\" HTML <a name="backreferences"></a>
1682    .SH "BACK REFERENCES"
1683  .rs  .rs
1684  .sp  .sp
1685  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1686  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1687  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1688  previous capturing left parentheses.  previous capturing left parentheses.
1689    .P
1690  However, if the decimal number following the backslash is less than 10, it is  However, if the decimal number following the backslash is less than 10, it is
1691  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1692  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1693  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1694  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1695  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1696    in an earlier iteration.
1697    .P
1698    It is not possible to have a numerical "forward back reference" to a subpattern
1699    whose number is 10 or more using this syntax because a sequence such as \e50 is
1700    interpreted as a character defined in octal. See the subsection entitled
1701    "Non-printing characters"
1702    .\" HTML <a href="#digitsafterbackslash">
1703    .\" </a>
1704    above
1705    .\"
1706    for further details of the handling of digits following a backslash. There is
1707    no such problem when named parentheses are used. A back reference to any
1708    subpattern is possible using named parentheses (see below).
1709    .P
1710    Another way of avoiding the ambiguity inherent in the use of digits following a
1711    backslash is to use the \eg escape sequence. This escape must be followed by an
1712    unsigned number or a negative number, optionally enclosed in braces. These
1713    examples are all identical:
1714    .sp
1715      (ring), \e1
1716      (ring), \eg1
1717      (ring), \eg{1}
1718    .sp
1719    An unsigned number specifies an absolute reference without the ambiguity that
1720    is present in the older syntax. It is also useful when literal digits follow
1721    the reference. A negative number is a relative reference. Consider this
1722    example:
1723    .sp
1724      (abc(def)ghi)\eg{-1}
1725    .sp
1726    The sequence \eg{-1} is a reference to the most recently started capturing
1727    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1728    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1729    can be helpful in long patterns, and also in patterns that are created by
1730    joining together fragments that contain references within themselves.
1731    .P
1732  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1733  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1734  itself (see  itself (see
# Line 832  itself (see Line 1737  itself (see
1737  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1738  .\"  .\"
1739  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1740    .sp
1741    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1742    .sp
1743  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1744  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1745  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1746    .sp
1747    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1748    .sp
1749  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1750  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1751    .P
1752  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1753  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1754    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1755    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1756    references, is also supported. We could rewrite the above example in any of
1757    the following ways:
1758    .sp
1759      (?<p1>(?i)rah)\es+\ek<p1>
1760      (?'p1'(?i)rah)\es+\ek{p1}
1761      (?P<p1>(?i)rah)\es+(?P=p1)
1762      (?<p1>(?i)rah)\es+\eg{p1}
1763    .sp
1764    A subpattern that is referenced by name may appear in the pattern before or
1765    after the reference.
1766    .P
1767  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
1768  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1769  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1770    .sp
1771    (a|(bc))\\2    (a|(bc))\e2
1772    .sp
1773  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
1774  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
1775  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1776  with a digit character, some delimiter must be used to terminate the back  .P
1777  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1778  Otherwise an empty comment can be used.  following a backslash are taken as part of a potential back reference number.
1779    If the pattern continues with a digit character, some delimiter must be used to
1780    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1781    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1782    .\" HTML <a href="#comments">
1783    .\" </a>
1784    "Comments"
1785    .\"
1786    below) can be used.
1787    .
1788    .SS "Recursive back references"
1789    .rs
1790    .sp
1791  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
1792  when the subpattern is first used, so, for example, (a\\1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1793  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1794  example, the pattern  example, the pattern
1795    .sp
1796    (a|b\\1)+    (a|b\e1)+
1797    .sp
1798  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1799  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1800  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1801  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
1802  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
1803  minimum of zero.  minimum of zero.
1804    .P
1805    Back references of this type cause the group that they reference to be treated
1806    as an
1807    .\" HTML <a href="#atomicgroup">
1808    .\" </a>
1809    atomic group.
1810    .\"
1811    Once the whole group has been matched, a subsequent matching failure cannot
1812    cause backtracking into the middle of the group.
1813    .
1814    .
1815    .\" HTML <a name="bigassertions"></a>
1816  .SH ASSERTIONS  .SH ASSERTIONS
1817  .rs  .rs
1818  .sp  .sp
1819  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1820  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1821  assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.  assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1822    .\" HTML <a href="#smallassertions">
1823    .\" </a>
1824    above.
1825    .\"
1826    .P
1827  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1828  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1829  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1830    except that it does not cause the current matching position to be changed.
1831  An assertion subpattern is matched in the normal way, except that it does not  .P
1832  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1833  with (?= for positive assertions and (?! for negative assertions. For example,  because it makes no sense to assert the same thing several times. If any kind
1834    of assertion contains capturing subpatterns within it, these are counted for
1835    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1836    However, substring capturing is carried out only for positive assertions,
1837    because it does not make sense for negative assertions.
1838    .
1839    .
1840    .SS "Lookahead assertions"
1841    .rs
1842    .sp
1843    Lookahead assertions start with (?= for positive assertions and (?! for
1844    negative assertions. For example,
1845    .sp
1846      \ew+(?=;)
1847    .sp
1848  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1849  the match, and  the match, and
1850    .sp
1851    foo(?!bar)    foo(?!bar)
1852    .sp
1853  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1854  apparently similar pattern  apparently similar pattern
1855    .sp
1856    (?!foo)bar    (?!foo)bar
1857    .sp
1858  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1859  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1860  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1861  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1862    .P
1863  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
1864  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
1865  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.
1866    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1867    .
1868    .
1869    .\" HTML <a name="lookbehind"></a>
1870    .SS "Lookbehind assertions"
1871    .rs
1872    .sp
1873  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1874  negative assertions. For example,  negative assertions. For example,
1875    .sp
1876    (?<!foo)bar    (?<!foo)bar
1877    .sp
1878  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1879  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1880  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1881  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1882    .sp
1883    (?<=bullock|donkey)    (?<=bullock|donkey)
1884    .sp
1885  is permitted, but  is permitted, but
1886    .sp
1887    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1888    .sp
1889  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1890  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
1891  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
1892  match the same length of string. An assertion such as  length of string. An assertion such as
1893    .sp
1894    (?<=ab(c|de))    (?<=ab(c|de))
1895    .sp
1896  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
1897  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
1898    branches:
1899    .sp
1900    (?<=abc|abde)    (?<=abc|abde)
1901    .sp
1902    In some cases, the escape sequence \eK
1903    .\" HTML <a href="#resetmatchstart">
1904    .\" </a>
1905    (see above)
1906    .\"
1907    can be used instead of a lookbehind assertion to get round the fixed-length
1908    restriction.
1909    .P
1910  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1911  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1912  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1913  match is deemed to fail.  assertion fails.
1914    .P
1915  PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1916  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1917  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1918    different numbers of bytes, are also not permitted.
1919  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1920  efficient matching at the end of the subject string. Consider a simple pattern  .\" HTML <a href="#subpatternsassubroutines">
1921  such as  .\" </a>
1922    "Subroutine"
1923    .\"
1924    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1925    as the subpattern matches a fixed-length string.
1926    .\" HTML <a href="#recursion">
1927    .\" </a>
1928    Recursion,
1929    .\"
1930    however, is not supported.
1931    .P
1932    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1933    specify efficient matching of fixed-length strings at the end of subject
1934    strings. Consider a simple pattern such as
1935    .sp
1936    abcd$    abcd$
1937    .sp
1938  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1939  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1940  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
1941    .sp
1942    ^.*abcd$    ^.*abcd$
1943    .sp
1944  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1945  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1946  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1947  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1948  if the pattern is written as  if the pattern is written as
1949    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1950    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1951    .sp
1952  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1953  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1954  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1955  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1956    .
1957    .
1958    .SS "Using multiple assertions"
1959    .rs
1960    .sp
1961  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1962    .sp
1963    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1964    .sp
1965  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1966  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1967  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1968  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1969  This pattern does \fInot\fR match "foo" preceded by six characters, the first  This pattern does \fInot\fP match "foo" preceded by six characters, the first
1970  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1971  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1972    .sp
1973    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1974    .sp
1975  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1976  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1977  preceding three characters are not "999".  preceding three characters are not "999".
1978    .P
1979  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1980    .sp
1981    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1982    .sp
1983  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1984  preceded by "foo", while  preceded by "foo", while
1985    .sp
1986    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1987    .sp
1988  is another pattern which matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
1989  characters that are not "999".  characters that are not "999".
1990    .
1991  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1992  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1993  of assertion contains capturing subpatterns within it, these are counted for  .SH "CONDITIONAL SUBPATTERNS"
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
   
 .SH CONDITIONAL SUBPATTERNS  
1994  .rs  .rs
1995  .sp  .sp
1996  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1997  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1998  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1999  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2000    .sp
2001    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2002    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2003    .sp
2004  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2005  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
2006  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2007    itself contain nested subpatterns of any form, including conditional
2008  There are three kinds of condition. If the text between the parentheses  subpatterns; the restriction to two alternatives applies only at the level of
2009  consists of a sequence of digits, the condition is satisfied if the capturing  the condition. This pattern fragment is an example where the alternatives are
2010  subpattern of that number has previously matched. The number must be greater  complex:
2011  than zero. Consider the following pattern, which contains non-significant white  .sp
2012  space to make it more readable (assume the PCRE_EXTENDED option) and to divide    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2013  it into three parts for ease of discussion:  .sp
2014    .P
2015    ( \\( )?    [^()]+    (?(1) \\) )  There are four kinds of condition: references to subpatterns, references to
2016    recursion, a pseudo-condition called DEFINE, and assertions.
2017    .
2018    .SS "Checking for a used subpattern by number"
2019    .rs
2020    .sp
2021    If the text between the parentheses consists of a sequence of digits, the
2022    condition is true if a capturing subpattern of that number has previously
2023    matched. If there is more than one capturing subpattern with the same number
2024    (see the earlier
2025    .\"
2026    .\" HTML <a href="#recursion">
2027    .\" </a>
2028    section about duplicate subpattern numbers),
2029    .\"
2030    the condition is true if any of them have matched. An alternative notation is
2031    to precede the digits with a plus or minus sign. In this case, the subpattern
2032    number is relative rather than absolute. The most recently opened parentheses
2033    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2034    loops it can also make sense to refer to subsequent groups. The next
2035    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2036    zero in any of these forms is not used; it provokes a compile-time error.)
2037    .P
2038    Consider the following pattern, which contains non-significant white space to
2039    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2040    three parts for ease of discussion:
2041    .sp
2042      ( \e( )?    [^()]+    (?(1) \e) )
2043    .sp
2044  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2045  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
2046  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
2047  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2048  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,
2049  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
2050  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2051  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2052  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2053    .P
2054  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2055  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2056  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2057      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2058  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
2059    This makes the fragment independent of the parentheses in the larger pattern.
2060    .
2061    .SS "Checking for a used subpattern by name"
2062    .rs
2063    .sp
2064    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2065    subpattern by name. For compatibility with earlier versions of PCRE, which had
2066    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2067    there is a possible ambiguity with this syntax, because subpattern names may
2068    consist entirely of digits. PCRE looks first for a named subpattern; if it
2069    cannot find one and the name consists entirely of digits, PCRE looks for a
2070    subpattern of that number, which must be greater than zero. Using subpattern
2071    names that consist entirely of digits is not recommended.
2072    .P
2073    Rewriting the above example to use a named subpattern gives this:
2074    .sp
2075      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2076    .sp
2077    If the name used in a condition of this kind is a duplicate, the test is
2078    applied to all subpatterns of the same name, and is true if any one of them has
2079    matched.
2080    .
2081    .SS "Checking for pattern recursion"
2082    .rs
2083    .sp
2084    If the condition is the string (R), and there is no subpattern with the name R,
2085    the condition is true if a recursive call to the whole pattern or any
2086    subpattern has been made. If digits or a name preceded by ampersand follow the
2087    letter R, for example:
2088    .sp
2089      (?(R3)...) or (?(R&name)...)
2090    .sp
2091    the condition is true if the most recent recursion is into a subpattern whose
2092    number or name is given. This condition does not check the entire recursion
2093    stack. If the name used in a condition of this kind is a duplicate, the test is
2094    applied to all subpatterns of the same name, and is true if any one of them is
2095    the most recent recursion.
2096    .P
2097    At "top level", all these recursion test conditions are false.
2098    .\" HTML <a href="#recursion">
2099    .\" </a>
2100    The syntax for recursive patterns
2101    .\"
2102    is described below.
2103    .
2104    .\" HTML <a name="subdefine"></a>
2105    .SS "Defining subpatterns for use by reference only"
2106    .rs
2107    .sp
2108    If the condition is the string (DEFINE), and there is no subpattern with the
2109    name DEFINE, the condition is always false. In this case, there may be only one
2110    alternative in the subpattern. It is always skipped if control reaches this
2111    point in the pattern; the idea of DEFINE is that it can be used to define
2112    "subroutines" that can be referenced from elsewhere. (The use of
2113    .\" HTML <a href="#subpatternsassubroutines">
2114    .\" </a>
2115    "subroutines"
2116    .\"
2117    is described below.) For example, a pattern to match an IPv4 address such as
2118    "192.168.23.245" could be written like this (ignore whitespace and line
2119    breaks):
2120    .sp
2121      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2122      \eb (?&byte) (\e.(?&byte)){3} \eb
2123    .sp
2124    The first part of the pattern is a DEFINE group inside which a another group
2125    named "byte" is defined. This matches an individual component of an IPv4
2126    address (a number less than 256). When matching takes place, this part of the
2127    pattern is skipped because DEFINE acts like a false condition. The rest of the
2128    pattern uses references to the named group to match the four dot-separated
2129    components of an IPv4 address, insisting on a word boundary at each end.
2130    .
2131    .SS "Assertion conditions"
2132    .rs
2133    .sp
2134    If the condition is not in any of the above formats, it must be an assertion.
2135  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2136  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2137  alternatives on the second line:  alternatives on the second line:
2138    .sp
2139    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
2140    \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )    \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
2141    .sp
2142  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
2143  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
2144  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
2145  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
2146  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
2147  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2148    .
2149    .
2150    .\" HTML <a name="comments"></a>
2151  .SH COMMENTS  .SH COMMENTS
2152  .rs  .rs
2153  .sp  .sp
2154  The sequence (?# marks the start of a comment which continues up to the next  There are two ways of including comments in patterns that are processed by
2155  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,
2156  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
2157    subpattern name or number. The characters that make up a comment play no part
2158  If the PCRE_EXTENDED option is set, an unescaped # character outside a  in the pattern matching.
2159  character class introduces a comment that continues up to the next newline  .P
2160  character in the pattern.  The sequence (?# marks the start of a comment that continues up to the next
2161    closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2162  .SH RECURSIVE PATTERNS  option is set, an unescaped # character also introduces a comment, which in
2163    this case continues to immediately after the next newline character or
2164    character sequence in the pattern. Which characters are interpreted as newlines
2165    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2166    sequence at the start of the pattern, as described in the section entitled
2167    .\" HTML <a href="#newlines">
2168    .\" </a>
2169    "Newline conventions"
2170    .\"
2171    above. Note that the end of this type of comment is a literal newline sequence
2172    in the pattern; escape sequences that happen to represent a newline do not
2173    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2174    default newline convention is in force:
2175    .sp
2176      abc #comment \en still comment
2177    .sp
2178    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2179    a newline in the pattern. The sequence \en is still literal at this stage, so
2180    it does not terminate the comment. Only an actual character with the code value
2181    0x0a (the default newline) does so.
2182    .
2183    .
2184    .\" HTML <a name="recursion"></a>
2185    .SH "RECURSIVE PATTERNS"
2186  .rs  .rs
2187  .sp  .sp
2188  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2189  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2190  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2191  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
2192  experimental facility that allows regular expressions to recurse (amongst other  .P
2193  things). It does this by interpolating Perl code in the expression at run time,  For some time, Perl has provided a facility that allows regular expressions to
2194  and the code can refer to the expression itself. A Perl pattern to solve the  recurse (amongst other things). It does this by interpolating Perl code in the
2195  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2196    pattern using code interpolation to solve the parentheses problem can be
2197    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
2198    .sp
2199      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2200    .sp
2201  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2202  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2203  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2204  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2205    supports special syntax for recursion of the entire pattern, and also for
2206  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2207  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2208  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2209  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2210  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2211    provided that it occurs inside that subpattern. (If not, it is a
2212  For example, this PCRE pattern solves the nested parentheses problem (assume  .\" HTML <a href="#subpatternsassubroutines">
2213  the PCRE_EXTENDED option is set so that white space is ignored):  .\" </a>
2214    "subroutine"
2215    \\( ( (?>[^()]+) | (?R) )* \\)  .\"
2216    call, which is described in the next section.) The special item (?R) or (?0) is
2217    a recursive call of the entire regular expression.
2218    .P
2219    This PCRE pattern solves the nested parentheses problem (assume the
2220    PCRE_EXTENDED option is set so that white space is ignored):
2221    .sp
2222      \e( ( [^()]++ | (?R) )* \e)
2223    .sp
2224  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2225  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
2226  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2227  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2228    to avoid backtracking into sequences of non-parentheses.
2229    .P
2230  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
2231  pattern, so instead you could use this:  pattern, so instead you could use this:
2232    .sp
2233    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( [^()]++ | (?1) )* \e) )
2234    .sp
2235  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
2236  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2237  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2238  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2239  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. Instead of (?1) in the
2240  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2241    parentheses preceding the recursion. In other words, a negative number counts
2242    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  capturing parentheses leftwards from the point at which it is encountered.
2243    .P
2244  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2245  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2246  when applying the pattern to strings that do not match. For example, when this  reference is not inside the parentheses that are referenced. They are always
2247  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2248    .\" </a>
2249    "subroutine"
2250    .\"
2251    calls, as described in the next section.
2252    .P
2253    An alternative approach is to use named parentheses instead. The Perl syntax
2254    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2255    could rewrite the above example as follows:
2256    .sp
2257      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2258    .sp
2259    If there is more than one subpattern with the same name, the earliest one is
2260    used.
2261    .P
2262    This particular example pattern that we have been looking at contains nested
2263    unlimited repeats, and so the use of a possessive quantifier for matching
2264    strings of non-parentheses is important when applying the pattern to strings
2265    that do not match. For example, when this pattern is applied to
2266    .sp
2267    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2268    .sp
2269  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,
2270  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
2271  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
2272  before failure can be reported.  before failure can be reported.
2273    .P
2274  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
2275  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
2276  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  
2277  .\" HREF  .\" HREF
2278  \fBpcrecallout\fR  \fBpcrecallout\fP
2279  .\"  .\"
2280  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2281    .sp
2282    (ab(cd)ef)    (ab(cd)ef)
2283    .sp
2284  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
2285  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
2286    matched at the top level, its final value is unset, even if it is (temporarily)
2287    \\( ( ( (?>[^()]+) | (?R) )* ) \\)  set at a deeper level.
2288       ^                        ^  .P
2289       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2290    obtain extra memory to store data during a recursion, which it does by using
2291  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
2292  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2293  has to obtain extra memory to store data during a recursion, which it does by  .P
 using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
   
2294  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.
2295  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
2296  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2297  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
2298    .sp
2299    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
2300    .sp
2301  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
2302  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2303  is the actual recursive call.  is the actual recursive call.
2304    .
2305    .
2306    .\" HTML <a name="recursiondifference"></a>
2307    .SS "Recursion difference from Perl"
2308    .rs
2309    .sp
2310    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2311    treated as an atomic group. That is, once it has matched some of the subject
2312    string, it is never re-entered, even if it contains untried alternatives and
2313    there is a subsequent matching failure. This can be illustrated by the
2314    following pattern, which purports to match a palindromic string that contains
2315    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2316    .sp
2317      ^(.|(.)(?1)\e2)$
2318    .sp
2319    The idea is that it either matches a single character, or two identical
2320    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2321    it does not if the pattern is longer than three characters. Consider the
2322    subject string "abcba":
2323    .P
2324    At the top level, the first character is matched, but as it is not at the end
2325    of the string, the first alternative fails; the second alternative is taken
2326    and the recursion kicks in. The recursive call to subpattern 1 successfully
2327    matches the next character ("b"). (Note that the beginning and end of line
2328    tests are not part of the recursion).
2329    .P
2330    Back at the top level, the next character ("c") is compared with what
2331    subpattern 2 matched, which was "a". This fails. Because the recursion is
2332    treated as an atomic group, there are now no backtracking points, and so the
2333    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2334    try the second alternative.) However, if the pattern is written with the
2335    alternatives in the other order, things are different:
2336    .sp
2337      ^((.)(?1)\e2|.)$
2338    .sp
2339    This time, the recursing alternative is tried first, and continues to recurse
2340    until it runs out of characters, at which point the recursion fails. But this
2341    time we do have another alternative to try at the higher level. That is the big
2342    difference: in the previous case the remaining alternative is at a deeper
2343    recursion level, which PCRE cannot use.
2344    .P
2345    To change the pattern so that it matches all palindromic strings, not just
2346    those with an odd number of characters, it is tempting to change the pattern to
2347    this:
2348    .sp
2349      ^((.)(?1)\e2|.?)$
2350    .sp
2351    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2352    deeper recursion has matched a single character, it cannot be entered again in
2353    order to match an empty string. The solution is to separate the two cases, and
2354    write out the odd and even cases as alternatives at the higher level:
2355    .sp
2356      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2357    .sp
2358    If you want to match typical palindromic phrases, the pattern has to ignore all
2359    non-word characters, which can be done like this:
2360    .sp
2361      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2362    .sp
2363    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2364    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2365    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2366    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2367    more) to match typical phrases, and Perl takes so long that you think it has
2368    gone into a loop.
2369    .P
2370    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2371    string does not start with a palindrome that is shorter than the entire string.
2372    For example, although "abcba" is correctly matched, if the subject is "ababa",
2373    PCRE finds the palindrome "aba" at the start, then fails at top level because
2374    the end of the string does not follow. Once again, it cannot jump back into the
2375    recursion to try other alternatives, so the entire match fails.
2376    .
2377    .
2378  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2379  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
2380  .rs  .rs
2381  .sp  .sp
2382  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
2383  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
2384  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2385  pattern  before or after the reference. A numbered reference can be absolute or
2386    relative, as in these examples:
2387    (sens|respons)e and \\1ibility  .sp
2388      (...(absolute)...)...(?2)...
2389      (...(relative)...)...(?-1)...
2390      (...(?+1)...(relative)...
2391    .sp
2392    An earlier example pointed out that the pattern
2393    .sp
2394      (sens|respons)e and \e1ibility
2395    .sp
2396  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
2397  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
2398    .sp
2399    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2400    .sp
2401  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
2402  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2403  refer.  .P
2404    Like recursive subpatterns, a subroutine call is always treated as an atomic
2405    group. That is, once it has matched some of the subject string, it is never
2406    re-entered, even if it contains untried alternatives and there is a subsequent
2407    matching failure. Any capturing parentheses that are set during the subroutine
2408    call revert to their previous values afterwards.
2409    .P
2410    When a subpattern is used as a subroutine, processing options such as
2411    case-independence are fixed when the subpattern is defined. They cannot be
2412    changed for different calls. For example, consider this pattern:
2413    .sp
2414      (abc)(?i:(?-1))
2415    .sp
2416    It matches "abcabc". It does not match "abcABC" because the change of
2417    processing option does not affect the called subpattern.
2418    .
2419    .
2420    .\" HTML <a name="onigurumasubroutines"></a>
2421    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2422    .rs
2423    .sp
2424    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2425    a number enclosed either in angle brackets or single quotes, is an alternative
2426    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2427    are two of the examples used above, rewritten using this syntax:
2428    .sp
2429      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2430      (sens|respons)e and \eg'1'ibility
2431    .sp
2432    PCRE supports an extension to Oniguruma: if a number is preceded by a
2433    plus or a minus sign it is taken as a relative reference. For example:
2434    .sp
2435      (abc)(?i:\eg<-1>)
2436    .sp
2437    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2438    synonymous. The former is a back reference; the latter is a subroutine call.
2439    .
2440    .
2441  .SH CALLOUTS  .SH CALLOUTS
2442  .rs  .rs
2443  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 2445  Perl has a feature whereby using the seq
2445  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
2446  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
2447  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
2448    .P
2449  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
2450  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
2451  function by putting its entry point in the global variable \fIpcre_callout\fR.  function by putting its entry point in the global variable \fIpcre_callout\fP.
2452  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2453    .P
2454  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2455  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
2456  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2457  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2458    .sp
2459    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2460    .sp
2461  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2462    automatically installed before each item in the pattern. They are all numbered
2463    255.
2464    .P
2465    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2466  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
2467  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
2468  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2469  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
2470  function is given in the  description of the interface to the callout function is given in the
2471  .\" HREF  .\" HREF
2472  \fBpcrecallout\fR  \fBpcrecallout\fP
2473  .\"  .\"
2474  documentation.  documentation.
2475    .
2476  .in 0  .
2477  Last updated: 03 February 2003  .\" HTML <a name="backtrackcontrol"></a>
2478  .br  .SH "BACKTRACKING CONTROL"
2479  Copyright (c) 1997-2003 University of Cambridge.  .rs
2480    .sp
2481    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2482    are described in the Perl documentation as "experimental and subject to change
2483    or removal in a future version of Perl". It goes on to say: "Their usage in
2484    production code should be noted to avoid problems during upgrades." The same
2485    remarks apply to the PCRE features described in this section.
2486    .P
2487    Since these verbs are specifically related to backtracking, most of them can be
2488    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2489    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2490    failing negative assertion, they cause an error if encountered by
2491    \fBpcre_dfa_exec()\fP.
2492    .P
2493    If any of these verbs are used in an assertion or subroutine subpattern
2494    (including recursive subpatterns), their effect is confined to that subpattern;
2495    it does not extend to the surrounding pattern. Note that such subpatterns are
2496    processed as anchored at the point where they are tested.
2497    .P
2498    The new verbs make use of what was previously invalid syntax: an opening
2499    parenthesis followed by an asterisk. They are generally of the form
2500    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2501    depending on whether or not an argument is present. An name is a sequence of
2502    letters, digits, and underscores. If the name is empty, that is, if the closing
2503    parenthesis immediately follows the colon, the effect is as if the colon were
2504    not there. Any number of these verbs may occur in a pattern.
2505    .P
2506    PCRE contains some optimizations that are used to speed up matching by running
2507    some checks at the start of each match attempt. For example, it may know the
2508    minimum length of matching subject, or that a particular character must be
2509    present. When one of these optimizations suppresses the running of a match, any
2510    included backtracking verbs will not, of course, be processed. You can suppress
2511    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2512    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2513    pattern with (*NO_START_OPT).
2514    .
2515    .
2516    .SS "Verbs that act immediately"
2517    .rs
2518    .sp
2519    The following verbs act as soon as they are encountered. They may not be
2520    followed by a name.
2521    .sp
2522       (*ACCEPT)
2523    .sp
2524    This verb causes the match to end successfully, skipping the remainder of the
2525    pattern. When inside a recursion, only the innermost pattern is ended
2526    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2527    captured. (This feature was added to PCRE at release 8.00.) For example:
2528    .sp
2529      A((?:A|B(*ACCEPT)|C)D)
2530    .sp
2531    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2532    the outer parentheses.
2533    .sp
2534      (*FAIL) or (*F)
2535    .sp
2536    This verb causes the match to fail, forcing backtracking to occur. It is
2537    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2538    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2539    Perl features that are not present in PCRE. The nearest equivalent is the
2540    callout feature, as for example in this pattern:
2541    .sp
2542      a+(?C)(*FAIL)
2543    .sp
2544    A match with the string "aaaa" always fails, but the callout is taken before
2545    each backtrack happens (in this example, 10 times).
2546    .
2547    .
2548    .SS "Recording which path was taken"
2549    .rs
2550    .sp
2551    There is one verb whose main purpose is to track how a match was arrived at,
2552    though it also has a secondary use in conjunction with advancing the match
2553    starting point (see (*SKIP) below).
2554    .sp
2555      (*MARK:NAME) or (*:NAME)
2556    .sp
2557    A name is always required with this verb. There may be as many instances of
2558    (*MARK) as you like in a pattern, and their names do not have to be unique.
2559    .P
2560    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2561    to the caller via the \fIpcre_extra\fP data structure, as described in the
2562    .\" HTML <a href="pcreapi.html#extradata">
2563    .\" </a>
2564    section on \fIpcre_extra\fP
2565    .\"
2566    in the
2567    .\" HREF
2568    \fBpcreapi\fP
2569    .\"
2570    documentation. No data is returned for a partial match. Here is an example of
2571    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2572    outputting of (*MARK) data:
2573    .sp
2574      /X(*MARK:A)Y|X(*MARK:B)Z/K
2575      XY
2576       0: XY
2577      MK: A
2578      XZ
2579       0: XZ
2580      MK: B
2581    .sp
2582    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2583    indicates which of the two alternatives matched. This is a more efficient way
2584    of obtaining this information than putting each alternative in its own
2585    capturing parentheses.
2586    .P
2587    A name may also be returned after a failed match if the final path through the
2588    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2589    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2590    starting point for matching is advanced, the final check is often with an empty
2591    string, causing a failure before (*MARK) is reached. For example:
2592    .sp
2593      /X(*MARK:A)Y|X(*MARK:B)Z/K
2594      XP
2595      No match
2596    .sp
2597    There are three potential starting points for this match (starting with X,
2598    starting with P, and with an empty string). If the pattern is anchored, the
2599    result is different:
2600    .sp
2601      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2602      XP
2603      No match, mark = B
2604    .sp
2605    PCRE's start-of-match optimizations can also interfere with this. For example,
2606    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2607    subject length for a match, a shorter subject will not be scanned at all.
2608    .P
2609    Note that similar anomalies (though different in detail) exist in Perl, no
2610    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2611    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2612    .
2613    .
2614    .SS "Verbs that act after backtracking"
2615    .rs
2616    .sp
2617    The following verbs do nothing when they are encountered. Matching continues
2618    with what follows, but if there is no subsequent match, causing a backtrack to
2619    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2620    the verb. However, when one of these verbs appears inside an atomic group, its
2621    effect is confined to that group, because once the group has been matched,
2622    there is never any backtracking into it. In this situation, backtracking can
2623    "jump back" to the left of the entire atomic group. (Remember also, as stated
2624    above, that this localization also applies in subroutine calls and assertions.)
2625    .P
2626    These verbs differ in exactly what kind of failure occurs when backtracking
2627    reaches them.
2628    .sp
2629      (*COMMIT)
2630    .sp
2631    This verb, which may not be followed by a name, causes the whole match to fail
2632    outright if the rest of the pattern does not match. Even if the pattern is
2633    unanchored, no further attempts to find a match by advancing the starting point
2634    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2635    finding a match at the current starting point, or not at all. For example:
2636    .sp
2637      a+(*COMMIT)b
2638    .sp
2639    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2640    dynamic anchor, or "I've started, so I must finish." The name of the most
2641    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2642    match failure.
2643    .P
2644    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2645    unless PCRE's start-of-match optimizations are turned off, as shown in this
2646    \fBpcretest\fP example:
2647    .sp
2648      /(*COMMIT)abc/
2649      xyzabc
2650       0: abc
2651      xyzabc\eY
2652      No match
2653    .sp
2654    PCRE knows that any match must start with "a", so the optimization skips along
2655    the subject to "a" before running the first match attempt, which succeeds. When
2656    the optimization is disabled by the \eY escape in the second subject, the match
2657    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2658    starting points.
2659    .sp
2660      (*PRUNE) or (*PRUNE:NAME)
2661    .sp
2662    This verb causes the match to fail at the current starting position in the
2663    subject if the rest of the pattern does not match. If the pattern is
2664    unanchored, the normal "bumpalong" advance to the next starting character then
2665    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2666    reached, or when matching to the right of (*PRUNE), but if there is no match to
2667    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2668    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2669    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2670    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2671    match fails completely; the name is passed back if this is the final attempt.
2672    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2673    pattern (*PRUNE) has the same effect as (*COMMIT).
2674    .sp
2675      (*SKIP)
2676    .sp
2677    This verb, when given without a name, is like (*PRUNE), except that if the
2678    pattern is unanchored, the "bumpalong" advance is not to the next character,
2679    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2680    signifies that whatever text was matched leading up to it cannot be part of a
2681    successful match. Consider:
2682    .sp
2683      a+(*SKIP)b
2684    .sp
2685    If the subject is "aaaac...", after the first match attempt fails (starting at
2686    the first character in the string), the starting point skips on to start the
2687    next attempt at "c". Note that a possessive quantifer does not have the same
2688    effect as this example; although it would suppress backtracking during the
2689    first match attempt, the second attempt would start at the second character
2690    instead of skipping on to "c".
2691    .sp
2692      (*SKIP:NAME)
2693    .sp
2694    When (*SKIP) has an associated name, its behaviour is modified. If the
2695    following pattern fails to match, the previous path through the pattern is
2696    searched for the most recent (*MARK) that has the same name. If one is found,
2697    the "bumpalong" advance is to the subject position that corresponds to that
2698    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2699    matching name is found, normal "bumpalong" of one character happens (the
2700    (*SKIP) is ignored).
2701    .sp
2702      (*THEN) or (*THEN:NAME)
2703    .sp
2704    This verb causes a skip to the next alternation in the innermost enclosing
2705    group if the rest of the pattern does not match. That is, it cancels pending
2706    backtracking, but only within the current alternation. Its name comes from the
2707    observation that it can be used for a pattern-based if-then-else block:
2708    .sp
2709      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2710    .sp
2711    If the COND1 pattern matches, FOO is tried (and possibly further items after
2712    the end of the group if FOO succeeds); on failure the matcher skips to the
2713    second alternative and tries COND2, without backtracking into COND1. The
2714    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2715    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2716    like (*PRUNE).
2717    .
2718    .P
2719    The above verbs provide four different "strengths" of control when subsequent
2720    matching fails. (*THEN) is the weakest, carrying on the match at the next
2721    alternation. (*PRUNE) comes next, failing the match at the current starting
2722    position, but allowing an advance to the next character (for an unanchored
2723    pattern). (*SKIP) is similar, except that the advance may be more than one
2724    character. (*COMMIT) is the strongest, causing the entire match to fail.
2725    .P
2726    If more than one is present in a pattern, the "stongest" one wins. For example,
2727    consider this pattern, where A, B, etc. are complex pattern fragments:
2728    .sp
2729      (A(*COMMIT)B(*THEN)C|D)
2730    .sp
2731    Once A has matched, PCRE is committed to this match, at the current starting
2732    position. If subsequently B matches, but C does not, the normal (*THEN) action
2733    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2734    overrides.
2735    .
2736    .
2737    .SH "SEE ALSO"
2738    .rs
2739    .sp
2740    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2741    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2742    .
2743    .
2744    .SH AUTHOR
2745    .rs
2746    .sp
2747    .nf
2748    Philip Hazel
2749    University Computing Service
2750    Cambridge CB2 3QH, England.
2751    .fi
2752    .
2753    .
2754    .SH REVISION
2755    .rs
2756    .sp
2757    .nf
2758    Last updated: 20 July 2011
2759    Copyright (c) 1997-2011 University of Cambridge.
2760    .fi

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