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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 strings in the original library, and a
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  second library that supports 16-bit and UTF-16 character strings. To use these
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  features, PCRE must be built to include appropriate support. When using UTF
27  places below. There is also a summary of UTF-8 features in the  strings you must either call the compiling function with the PCRE_UTF8 or
28  .\" HTML <a href="pcre.html#utf8support">  PCRE_UTF16 option, or the pattern must start with one of these special
29  .\" </a>  sequences:
30  section on UTF-8 support  .sp
31  .\"    (*UTF8)
32  in the main    (*UTF16)
33    .sp
34    Starting a pattern with such a sequence is equivalent to setting the relevant
35    option. This feature is not Perl-compatible. How setting a UTF mode affects
36    pattern matching is mentioned in several places below. There is also a summary
37    of features in the
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43    Another special sequence that may appear at the start of a pattern or in
44    combination with (*UTF8) or (*UTF16) is:
45    .sp
46      (*UCP)
47    .sp
48    This has the same effect as setting the PCRE_UCP option: it causes sequences
49    such as \ed and \ew to use Unicode properties to determine character types,
50    instead of recognizing only characters with codes less than 128 via a lookup
51    table.
52    .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .P
58  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
59  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60  From release 6.0, PCRE offers a second matching function,  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62  Perl-compatible. The advantages and disadvantages of the alternative function,  a different algorithm that is not Perl-compatible. Some of the features
63  and how it differs from the normal function, are discussed in the  discussed below are not available when DFA matching is used. The advantages and
64    disadvantages of the alternative functions, and how they differ from the normal
65    functions, are discussed in the
66  .\" HREF  .\" HREF
67  \fBpcrematching\fP  \fBpcrematching\fP
68  .\"  .\"
69  page.  page.
70    .
71    .
72    .\" HTML <a name="newlines"></a>
73    .SH "NEWLINE CONVENTIONS"
74    .rs
75    .sp
76    PCRE supports five different conventions for indicating line breaks in
77    strings: a single CR (carriage return) character, a single LF (linefeed)
78    character, the two-character sequence CRLF, any of the three preceding, or any
79    Unicode newline sequence. The
80    .\" HREF
81    \fBpcreapi\fP
82    .\"
83    page has
84    .\" HTML <a href="pcreapi.html#newlines">
85    .\" </a>
86    further discussion
87    .\"
88    about newlines, and shows how to set the newline convention in the
89    \fIoptions\fP arguments for the compiling and matching functions.
90  .P  .P
91    It is also possible to specify a newline convention by starting a pattern
92    string with one of the following five sequences:
93    .sp
94      (*CR)        carriage return
95      (*LF)        linefeed
96      (*CRLF)      carriage return, followed by linefeed
97      (*ANYCRLF)   any of the three above
98      (*ANY)       all Unicode newline sequences
99    .sp
100    These override the default and the options given to the compiling function. For
101    example, on a Unix system where LF is the default newline sequence, the pattern
102    .sp
103      (*CR)a.b
104    .sp
105    changes the convention to CR. That pattern matches "a\enb" because LF is no
106    longer a newline. Note that these special settings, which are not
107    Perl-compatible, are recognized only at the very start of a pattern, and that
108    they must be in upper case. If more than one of them is present, the last one
109    is used.
110    .P
111    The newline convention affects the interpretation of the dot metacharacter when
112    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113    affect what the \eR escape sequence matches. By default, this is any Unicode
114    newline sequence, for Perl compatibility. However, this can be changed; see the
115    description of \eR in the section entitled
116    .\" HTML <a href="#newlineseq">
117    .\" </a>
118    "Newline sequences"
119    .\"
120    below. A change of \eR setting can be combined with a change of newline
121    convention.
122    .
123    .
124    .SH "CHARACTERS AND METACHARACTERS"
125    .rs
126    .sp
127  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
128  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
129  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 45  corresponding characters in the subject. Line 132  corresponding characters in the subject.
132  .sp  .sp
133  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
134  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
135  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
136  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
137  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
138  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
139  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
140  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
141  UTF-8 support.  UTF support.
142  .P  .P
143  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
144  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 60  interpreted in some special way. Line 147  interpreted in some special way.
147  .P  .P
148  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
149  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
150  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
151  as follows:  are as follows:
152  .sp  .sp
153    \e      general escape character with several uses    \e      general escape character with several uses
154    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 179  a character class the only metacharacter
179  .sp  .sp
180  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
181  .  .
182    .
183  .SH BACKSLASH  .SH BACKSLASH
184  .rs  .rs
185  .sp  .sp
186  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
187  non-alphanumeric character, it takes away any special meaning that character may  character that is not a number or a letter, it takes away any special meaning
188  have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
189  outside character classes.  both inside and outside character classes.
190  .P  .P
191  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
192  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 106  otherwise be interpreted as a metacharac Line 194  otherwise be interpreted as a metacharac
194  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
195  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
196  .P  .P
197  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  In a UTF mode, only ASCII numbers and letters have any special meaning after a
198    backslash. All other characters (in particular, those whose codepoints are
199    greater than 127) are treated as literals.
200    .P
201    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
202  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
203  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
204  backslash can be used to include a whitespace or # character as part of the  be used to include a white space or # character as part of the pattern.
 pattern.  
205  .P  .P
206  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
207  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 126  Perl, $ and @ cause variable interpolati Line 217  Perl, $ and @ cause variable interpolati
217    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
218  .sp  .sp
219  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
220    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221    by \eE later in the pattern, the literal interpretation continues to the end of
222    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223    a character class, this causes an error, because the character class is not
224    terminated.
225  .  .
226  .  .
227  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 135  The \eQ...\eE sequence is recognized bot Line 231  The \eQ...\eE sequence is recognized bot
231  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
232  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
233  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
234  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
235  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
236  .sp  .sp
237    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
238    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
239    \ee        escape (hex 1B)    \ee        escape (hex 1B)
240    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
241    \en        newline (hex 0A)    \en        linefeed (hex 0A)
242    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
243    \et        tab (hex 09)    \et        tab (hex 09)
244    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
245    \exhh      character with hex code hh    \exhh      character with hex code hh
246    \ex{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247      \euhhhh    character with hex code hhhh (JavaScript mode only)
248  .sp  .sp
249  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
250  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
251  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
252  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
253  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
254  After \ex, from zero to two hexadecimal digits are read (letters can be in  compile-time error occurs. This locks out non-ASCII characters in all modes.
255  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  .P
256  appear between \ex{ and }, but the value of the character code must be less  The \ec facility was designed for use with ASCII characters, but with the
257  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  extension to Unicode it is even less useful than it once was. It is, however,
258  other than hexadecimal digits appear between \ex{ and }, or if there is no  recognized when PCRE is compiled in EBCDIC mode, where data items are always
259  terminating }, this form of escape is not recognized. Instead, the initial  bytes. In this mode, all values are valid after \ec. If the next character is a
260  \ex will be interpreted as a basic hexadecimal escape, with no following  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
261  digits, giving a character whose value is zero.  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
262    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
263    characters also generate different values.
264    .P
265    By default, after \ex, from zero to two hexadecimal digits are read (letters
266    can be in upper or lower case). Any number of hexadecimal digits may appear
267    between \ex{ and }, but the character code is constrained as follows:
268    .sp
269      8-bit non-UTF mode    less than 0x100
270      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
271      16-bit non-UTF mode   less than 0x10000
272      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
273    .sp
274    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
275    "surrogate" codepoints).
276    .P
277    If characters other than hexadecimal digits appear between \ex{ and }, or if
278    there is no terminating }, this form of escape is not recognized. Instead, the
279    initial \ex will be interpreted as a basic hexadecimal escape, with no
280    following digits, giving a character whose value is zero.
281    .P
282    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
283    as just described only when it is followed by two hexadecimal digits.
284    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
285    code points greater than 256 is provided by \eu, which must be followed by
286    four hexadecimal digits; otherwise it matches a literal "u" character.
287    Character codes specified by \eu in JavaScript mode are constrained in the same
288    was as those specified by \ex in non-JavaScript mode.
289  .P  .P
290  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
291  syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
292  way they are handled. For example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
293  .P  \eu00dc in JavaScript mode).
294  After \e0 up to two further octal digits are read. In both cases, if there  .P
295  are fewer than two digits, just those that are present are used. Thus the  After \e0 up to two further octal digits are read. If there are fewer than two
296  sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character  digits, just those that are present are used. Thus the sequence \e0\ex\e07
297  (code value 7). Make sure you supply two digits after the initial zero if the  specifies two binary zeros followed by a BEL character (code value 7). Make
298  pattern character that follows is itself an octal digit.  sure you supply two digits after the initial zero if the pattern character that
299    follows is itself an octal digit.
300  .P  .P
301  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.
302  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
# Line 191  parenthesized subpatterns. Line 315  parenthesized subpatterns.
315  .P  .P
316  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
317  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
318  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
319  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. The value of the character is
320    constrained in the same way as characters specified in hexadecimal.
321  For example:  For example:
322  .sp  .sp
323    \e040   is another way of writing a space    \e040   is another way of writing a space
# Line 210  For example: Line 335  For example:
335              character with octal code 113              character with octal code 113
336  .\" JOIN  .\" JOIN
337    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
338              the byte consisting entirely of 1 bits              the value 255 (decimal)
339  .\" JOIN  .\" JOIN
340    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
341              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 218  For example: Line 343  For example:
343  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
344  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
345  .P  .P
346  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
347  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, \eb is
348  addition, inside a character class, the sequence \eb is interpreted as the  interpreted as the backspace character (hex 08).
349  backspace character (hex 08), and the sequence \eX is interpreted as the  .P
350  character "X". Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
351  meanings  inside a character class. Like other unrecognized escape sequences, they are
352  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
353    error if the PCRE_EXTRA option is set. Outside a character class, these
354    sequences have different meanings.
355    .
356    .
357    .SS "Unsupported escape sequences"
358    .rs
359    .sp
360    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
361    handler and used to modify the case of following characters. By default, PCRE
362    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
363    option is set, \eU matches a "U" character, and \eu can be used to define a
364    character by code point, as described in the previous section.
365    .
366    .
367    .SS "Absolute and relative back references"
368    .rs
369    .sp
370    The sequence \eg followed by an unsigned or a negative number, optionally
371    enclosed in braces, is an absolute or relative back reference. A named back
372    reference can be coded as \eg{name}. Back references are discussed
373    .\" HTML <a href="#backreferences">
374  .\" </a>  .\" </a>
375  (see below).  later,
376    .\"
377    following the discussion of
378    .\" HTML <a href="#subpattern">
379    .\" </a>
380    parenthesized subpatterns.
381  .\"  .\"
382  .  .
383  .  .
384    .SS "Absolute and relative subroutine calls"
385    .rs
386    .sp
387    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
388    a number enclosed either in angle brackets or single quotes, is an alternative
389    syntax for referencing a subpattern as a "subroutine". Details are discussed
390    .\" HTML <a href="#onigurumasubroutines">
391    .\" </a>
392    later.
393    .\"
394    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
395    synonymous. The former is a back reference; the latter is a
396    .\" HTML <a href="#subpatternsassubroutines">
397    .\" </a>
398    subroutine
399    .\"
400    call.
401    .
402    .
403    .\" HTML <a name="genericchartypes"></a>
404  .SS "Generic character types"  .SS "Generic character types"
405  .rs  .rs
406  .sp  .sp
407  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
408  .sp  .sp
409    \ed     any decimal digit    \ed     any decimal digit
410    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
411    \es     any whitespace character    \eh     any horizontal white space character
412    \eS     any character that is not a whitespace character    \eH     any character that is not a horizontal white space character
413      \es     any white space character
414      \eS     any character that is not a white space character
415      \ev     any vertical white space character
416      \eV     any character that is not a vertical white space character
417    \ew     any "word" character    \ew     any "word" character
418    \eW     any "non-word" character    \eW     any "non-word" character
419  .sp  .sp
420  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
421  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
422    .\" HTML <a href="#fullstopdot">
423    .\" </a>
424    the "." metacharacter
425    .\"
426    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
427    PCRE does not support this.
428  .P  .P
429  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
430    of characters into two disjoint sets. Any given character matches one, and only
431    one, of each pair. The sequences can appear both inside and outside character
432  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
433  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
434  there is no character to match.  there is no character to match.
435  .P  .P
436  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
437  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
438  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
439    included in a Perl script, \es may match the VT character. In PCRE, it never
440    does.
441  .P  .P
442  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
443  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
444  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
445  place (see  place (see
446  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 267  in the Line 451  in the
451  .\" HREF  .\" HREF
452  \fBpcreapi\fP  \fBpcreapi\fP
453  .\"  .\"
454  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
455  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
456  .P  accented letters, and these are then matched by \ew. The use of locales with
457  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
458  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
459  character property support is available.  By default, in a UTF mode, characters with values greater than 128 never match
460    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
461    their original meanings from before UTF support was available, mainly for
462    efficiency reasons. However, if PCRE is compiled with Unicode property support,
463    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
464    properties are used to determine character types, as follows:
465    .sp
466      \ed  any character that \ep{Nd} matches (decimal digit)
467      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
468      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
469    .sp
470    The upper case escapes match the inverse sets of characters. Note that \ed
471    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
472    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
473    \eB because they are defined in terms of \ew and \eW. Matching these sequences
474    is noticeably slower when PCRE_UCP is set.
475    .P
476    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
477    release 5.10. In contrast to the other sequences, which match only ASCII
478    characters by default, these always match certain high-valued codepoints,
479    whether or not PCRE_UCP is set. The horizontal space characters are:
480    .sp
481      U+0009     Horizontal tab
482      U+0020     Space
483      U+00A0     Non-break space
484      U+1680     Ogham space mark
485      U+180E     Mongolian vowel separator
486      U+2000     En quad
487      U+2001     Em quad
488      U+2002     En space
489      U+2003     Em space
490      U+2004     Three-per-em space
491      U+2005     Four-per-em space
492      U+2006     Six-per-em space
493      U+2007     Figure space
494      U+2008     Punctuation space
495      U+2009     Thin space
496      U+200A     Hair space
497      U+202F     Narrow no-break space
498      U+205F     Medium mathematical space
499      U+3000     Ideographic space
500    .sp
501    The vertical space characters are:
502    .sp
503      U+000A     Linefeed
504      U+000B     Vertical tab
505      U+000C     Form feed
506      U+000D     Carriage return
507      U+0085     Next line
508      U+2028     Line separator
509      U+2029     Paragraph separator
510    .sp
511    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
512    relevant.
513    .
514    .
515    .\" HTML <a name="newlineseq"></a>
516    .SS "Newline sequences"
517    .rs
518    .sp
519    Outside a character class, by default, the escape sequence \eR matches any
520    Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
521    following:
522    .sp
523      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
524    .sp
525    This is an example of an "atomic group", details of which are given
526    .\" HTML <a href="#atomicgroup">
527    .\" </a>
528    below.
529    .\"
530    This particular group matches either the two-character sequence CR followed by
531    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
532    U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
533    line, U+0085). The two-character sequence is treated as a single unit that
534    cannot be split.
535    .P
536    In other modes, two additional characters whose codepoints are greater than 255
537    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
538    Unicode character property support is not needed for these characters to be
539    recognized.
540    .P
541    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
542    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
543    either at compile time or when the pattern is matched. (BSR is an abbrevation
544    for "backslash R".) This can be made the default when PCRE is built; if this is
545    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
546    It is also possible to specify these settings by starting a pattern string with
547    one of the following sequences:
548    .sp
549      (*BSR_ANYCRLF)   CR, LF, or CRLF only
550      (*BSR_UNICODE)   any Unicode newline sequence
551    .sp
552    These override the default and the options given to the compiling function, but
553    they can themselves be overridden by options given to a matching function. Note
554    that these special settings, which are not Perl-compatible, are recognized only
555    at the very start of a pattern, and that they must be in upper case. If more
556    than one of them is present, the last one is used. They can be combined with a
557    change of newline convention; for example, a pattern can start with:
558    .sp
559      (*ANY)(*BSR_ANYCRLF)
560    .sp
561    They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
562    sequences. Inside a character class, \eR is treated as an unrecognized escape
563    sequence, and so matches the letter "R" by default, but causes an error if
564    PCRE_EXTRA is set.
565  .  .
566  .  .
567  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 280  character property support is available. Line 569  character property support is available.
569  .rs  .rs
570  .sp  .sp
571  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
572  escape sequences to match generic character types are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
573  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
574  .sp  characters whose codepoints are less than 256, but they do work in this mode.
575   \ep{\fIxx\fP}   a character with the \fIxx\fP property  The extra escape sequences are:
576   \eP{\fIxx\fP}   a character without the \fIxx\fP property  .sp
577   \eX       an extended Unicode sequence    \ep{\fIxx\fP}   a character with the \fIxx\fP property
578  .sp    \eP{\fIxx\fP}   a character without the \fIxx\fP property
579  The property names represented by \fIxx\fP above are limited to the    \eX       a Unicode extended grapheme cluster
580  Unicode general category properties. Each character has exactly one such  .sp
581  property, specified by a two-letter abbreviation. For compatibility with Perl,  The property names represented by \fIxx\fP above are limited to the Unicode
582  negation can be specified by including a circumflex between the opening brace  script names, the general category properties, "Any", which matches any
583  and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.  character (including newline), and some special PCRE properties (described
584  .P  in the
585  If only one letter is specified with \ep or \eP, it includes all the properties  .\" HTML <a href="#extraprops">
586  that start with that letter. In this case, in the absence of negation, the  .\" </a>
587  curly brackets in the escape sequence are optional; these two examples have  next section).
588  the same effect:  .\"
589    Other Perl properties such as "InMusicalSymbols" are not currently supported by
590    PCRE. Note that \eP{Any} does not match any characters, so always causes a
591    match failure.
592    .P
593    Sets of Unicode characters are defined as belonging to certain scripts. A
594    character from one of these sets can be matched using a script name. For
595    example:
596    .sp
597      \ep{Greek}
598      \eP{Han}
599    .sp
600    Those that are not part of an identified script are lumped together as
601    "Common". The current list of scripts is:
602    .P
603    Arabic,
604    Armenian,
605    Avestan,
606    Balinese,
607    Bamum,
608    Batak,
609    Bengali,
610    Bopomofo,
611    Brahmi,
612    Braille,
613    Buginese,
614    Buhid,
615    Canadian_Aboriginal,
616    Carian,
617    Chakma,
618    Cham,
619    Cherokee,
620    Common,
621    Coptic,
622    Cuneiform,
623    Cypriot,
624    Cyrillic,
625    Deseret,
626    Devanagari,
627    Egyptian_Hieroglyphs,
628    Ethiopic,
629    Georgian,
630    Glagolitic,
631    Gothic,
632    Greek,
633    Gujarati,
634    Gurmukhi,
635    Han,
636    Hangul,
637    Hanunoo,
638    Hebrew,
639    Hiragana,
640    Imperial_Aramaic,
641    Inherited,
642    Inscriptional_Pahlavi,
643    Inscriptional_Parthian,
644    Javanese,
645    Kaithi,
646    Kannada,
647    Katakana,
648    Kayah_Li,
649    Kharoshthi,
650    Khmer,
651    Lao,
652    Latin,
653    Lepcha,
654    Limbu,
655    Linear_B,
656    Lisu,
657    Lycian,
658    Lydian,
659    Malayalam,
660    Mandaic,
661    Meetei_Mayek,
662    Meroitic_Cursive,
663    Meroitic_Hieroglyphs,
664    Miao,
665    Mongolian,
666    Myanmar,
667    New_Tai_Lue,
668    Nko,
669    Ogham,
670    Old_Italic,
671    Old_Persian,
672    Old_South_Arabian,
673    Old_Turkic,
674    Ol_Chiki,
675    Oriya,
676    Osmanya,
677    Phags_Pa,
678    Phoenician,
679    Rejang,
680    Runic,
681    Samaritan,
682    Saurashtra,
683    Sharada,
684    Shavian,
685    Sinhala,
686    Sora_Sompeng,
687    Sundanese,
688    Syloti_Nagri,
689    Syriac,
690    Tagalog,
691    Tagbanwa,
692    Tai_Le,
693    Tai_Tham,
694    Tai_Viet,
695    Takri,
696    Tamil,
697    Telugu,
698    Thaana,
699    Thai,
700    Tibetan,
701    Tifinagh,
702    Ugaritic,
703    Vai,
704    Yi.
705    .P
706    Each character has exactly one Unicode general category property, specified by
707    a two-letter abbreviation. For compatibility with Perl, negation can be
708    specified by including a circumflex between the opening brace and the property
709    name. For example, \ep{^Lu} is the same as \eP{Lu}.
710    .P
711    If only one letter is specified with \ep or \eP, it includes all the general
712    category properties that start with that letter. In this case, in the absence
713    of negation, the curly brackets in the escape sequence are optional; these two
714    examples have the same effect:
715  .sp  .sp
716    \ep{L}    \ep{L}
717    \epL    \epL
718  .sp  .sp
719  The following property codes are supported:  The following general category property codes are supported:
720  .sp  .sp
721    C     Other    C     Other
722    Cc    Control    Cc    Control
# Line 347  The following property codes are support Line 762  The following property codes are support
762    Zp    Paragraph separator    Zp    Paragraph separator
763    Zs    Space separator    Zs    Space separator
764  .sp  .sp
765  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
766  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
767    a modifier or "other".
768    .P
769    The Cs (Surrogate) property applies only to characters in the range U+D800 to
770    U+DFFF. Such characters are not valid in Unicode strings and so
771    cannot be tested by PCRE, unless UTF validity checking has been turned off
772    (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
773    .\" HREF
774    \fBpcreapi\fP
775    .\"
776    page). Perl does not support the Cs property.
777    .P
778    The long synonyms for property names that Perl supports (such as \ep{Letter})
779    are not supported by PCRE, nor is it permitted to prefix any of these
780    properties with "Is".
781    .P
782    No character that is in the Unicode table has the Cn (unassigned) property.
783    Instead, this property is assumed for any code point that is not in the
784    Unicode table.
785  .P  .P
786  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
787  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
788  .P  .P
789  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
790  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
791  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
792    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
793    PCRE_UCP option or by starting the pattern with (*UCP).
794    .
795    .
796    .SS Extended grapheme clusters
797    .rs
798  .sp  .sp
799  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
800  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
801  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
802  .\" </a>  .\" </a>
803  (see below).  (see below).
804  .\"  .\"
805  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
806  preceding character.  that was equivalent to
807    .sp
808      (?>\ePM\epM*)
809    .sp
810    That is, it matched a character without the "mark" property, followed by zero
811    or more characters with the "mark" property. Characters with the "mark"
812    property are typically non-spacing accents that affect the preceding character.
813    .P
814    This simple definition was extended in Unicode to include more complicated
815    kinds of composite character by giving each character a grapheme breaking
816    property, and creating rules that use these properties to define the boundaries
817    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
818    one of these clusters.
819    .P
820    \eX always matches at least one character. Then it decides whether to add
821    additional characters according to the following rules for ending a cluster:
822    .P
823    1. End at the end of the subject string.
824    .P
825    2. Do not end between CR and LF; otherwise end after any control character.
826    .P
827    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
828    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
829    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
830    character; an LVT or T character may be follwed only by a T character.
831    .P
832    4. Do not end before extending characters or spacing marks. Characters with
833    the "mark" property always have the "extend" grapheme breaking property.
834    .P
835    5. Do not end after prepend characters.
836    .P
837    6. Otherwise, end the cluster.
838    .
839    .
840    .\" HTML <a name="extraprops"></a>
841    .SS PCRE's additional properties
842    .rs
843    .sp
844    As well as the standard Unicode properties described above, PCRE supports four
845    more that make it possible to convert traditional escape sequences such as \ew
846    and \es and POSIX character classes to use Unicode properties. PCRE uses these
847    non-standard, non-Perl properties internally when PCRE_UCP is set. They are:
848    .sp
849      Xan   Any alphanumeric character
850      Xps   Any POSIX space character
851      Xsp   Any Perl space character
852      Xwd   Any Perl "word" character
853    .sp
854    Xan matches characters that have either the L (letter) or the N (number)
855    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
856    carriage return, and any other character that has the Z (separator) property.
857    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
858    same characters as Xan, plus underscore.
859    .
860    .
861    .\" HTML <a name="resetmatchstart"></a>
862    .SS "Resetting the match start"
863    .rs
864    .sp
865    The escape sequence \eK causes any previously matched characters not to be
866    included in the final matched sequence. For example, the pattern:
867    .sp
868      foo\eKbar
869    .sp
870    matches "foobar", but reports that it has matched "bar". This feature is
871    similar to a lookbehind assertion
872    .\" HTML <a href="#lookbehind">
873    .\" </a>
874    (described below).
875    .\"
876    However, in this case, the part of the subject before the real match does not
877    have to be of fixed length, as lookbehind assertions do. The use of \eK does
878    not interfere with the setting of
879    .\" HTML <a href="#subpattern">
880    .\" </a>
881    captured substrings.
882    .\"
883    For example, when the pattern
884    .sp
885      (foo)\eKbar
886    .sp
887    matches "foobar", the first substring is still set to "foo".
888  .P  .P
889  Matching characters by Unicode property is not fast, because PCRE has to search  Perl documents that the use of \eK within assertions is "not well defined". In
890  a structure that contains data for over fifteen thousand characters. That is  PCRE, \eK is acted upon when it occurs inside positive assertions, but is
891  why the traditional escape sequences such as \ed and \ew do not use Unicode  ignored in negative assertions.
 properties in PCRE.  
892  .  .
893  .  .
894  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
895  .SS "Simple assertions"  .SS "Simple assertions"
896  .rs  .rs
897  .sp  .sp
898  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
899  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,
900  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
901  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 386  subpatterns for more complicated asserti Line 903  subpatterns for more complicated asserti
903  .\" </a>  .\" </a>
904  below.  below.
905  .\"  .\"
906  The backslashed  The backslashed assertions are:
 assertions are:  
907  .sp  .sp
908    \eb     matches at a word boundary    \eb     matches at a word boundary
909    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
910    \eA     matches at start of subject    \eA     matches at the start of the subject
911    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
912    \ez     matches at end of subject            also matches before a newline at the end of the subject
913    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
914  .sp    \eG     matches at the first matching position in the subject
915  These assertions may not appear in character classes (but note that \eb has a  .sp
916  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
917    character. If any other of these assertions appears in a character class, by
918    default it matches the corresponding literal character (for example, \eB
919    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
920    escape sequence" error is generated instead.
921  .P  .P
922  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
923  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
924  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
925  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
926    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
927    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
928    of word" or "end of word" metasequence. However, whatever follows \eb normally
929    determines which it is. For example, the fragment \eba matches "a" at the start
930    of a word.
931  .P  .P
932  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
933  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 412  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 937  PCRE_NOTBOL or PCRE_NOTEOL options, whic
937  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
938  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
939  at a point other than the beginning of the subject, \eA can never match. The  at a point other than the beginning of the subject, \eA can never match. The
940  difference between \eZ and \ez is that \eZ matches before a newline that is the  difference between \eZ and \ez is that \eZ matches before a newline at the end
941  last character of the string as well as at the end of the string, whereas \ez  of the string as well as at the very end, whereas \ez matches only at the end.
 matches only at the end.  
942  .P  .P
943  The \eG assertion is true only when the current matching position is at the  The \eG assertion is true only when the current matching position is at the
944  start point of the match, as specified by the \fIstartoffset\fP argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
# Line 458  to be anchored.) Line 982  to be anchored.)
982  .P  .P
983  A dollar character is an assertion that is true only if the current matching  A dollar character is an assertion that is true only if the current matching
984  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
985  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
986  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
987  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
988  Dollar has no special meaning in a character class.  character class.
989  .P  .P
990  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
991  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
992  does not affect the \eZ assertion.  does not affect the \eZ assertion.
993  .P  .P
994  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
995  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
996  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
997  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
998  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
999  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
1000  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
1001  branches start with ^ are not anchored in multiline mode, and a match for  .P
1002  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
1003  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is  \en represents a newline) in multiline mode, but not otherwise. Consequently,
1004  set.  patterns that are anchored in single line mode because all branches start with
1005    ^ are not anchored in multiline mode, and a match for circumflex is possible
1006    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
1007    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1008  .P  .P
1009  Note that the sequences \eA, \eZ, and \ez can be used to match the start and  Note that the sequences \eA, \eZ, and \ez can be used to match the start and
1010  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
1011  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1012  .  .
1013  .  .
1014  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1015    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1016  .rs  .rs
1017  .sp  .sp
1018  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
1019  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
1020  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line.
1021  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
1022  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
1023  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
1024  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
1025  .  (including isolated CRs and LFs). When any Unicode line endings are being
1026  .  recognized, dot does not match CR or LF or any of the other line ending
1027  .SH "MATCHING A SINGLE BYTE"  characters.
1028  .rs  .P
1029  .sp  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1030  Outside a character class, the escape sequence \eC matches any one byte, both  option is set, a dot matches any one character, without exception. If the
1031  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is  two-character sequence CRLF is present in the subject string, it takes two dots
1032  provided in Perl in order to match individual bytes in UTF-8 mode. Because it  to match it.
1033  breaks up UTF-8 characters into individual bytes, what remains in the string  .P
1034  may be a malformed UTF-8 string. For this reason, the \eC escape sequence is  The handling of dot is entirely independent of the handling of circumflex and
1035  best avoided.  dollar, the only relationship being that they both involve newlines. Dot has no
1036    special meaning in a character class.
1037    .P
1038    The escape sequence \eN behaves like a dot, except that it is not affected by
1039    the PCRE_DOTALL option. In other words, it matches any character except one
1040    that signifies the end of a line. Perl also uses \eN to match characters by
1041    name; PCRE does not support this.
1042    .
1043    .
1044    .SH "MATCHING A SINGLE DATA UNIT"
1045    .rs
1046    .sp
1047    Outside a character class, the escape sequence \eC matches any one data unit,
1048    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1049    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1050    matches line-ending characters. The feature is provided in Perl in order to
1051    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1052    used. Because \eC breaks up characters into individual data units, matching one
1053    unit with \eC in a UTF mode means that the rest of the string may start with a
1054    malformed UTF character. This has undefined results, because PCRE assumes that
1055    it is dealing with valid UTF strings (and by default it checks this at the
1056    start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1057    is used).
1058  .P  .P
1059  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1060  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1061  .\" </a>  .\" </a>
1062  (described below),  (described below)
1063  .\"  .\"
1064  because in UTF-8 mode this would make it impossible to calculate the length of  in a UTF mode, because this would make it impossible to calculate the length of
1065  the lookbehind.  the lookbehind.
1066    .P
1067    In general, the \eC escape sequence is best avoided. However, one
1068    way of using it that avoids the problem of malformed UTF characters is to use a
1069    lookahead to check the length of the next character, as in this pattern, which
1070    could be used with a UTF-8 string (ignore white space and line breaks):
1071    .sp
1072      (?| (?=[\ex00-\ex7f])(\eC) |
1073          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1074          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1075          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1076    .sp
1077    A group that starts with (?| resets the capturing parentheses numbers in each
1078    alternative (see
1079    .\" HTML <a href="#dupsubpatternnumber">
1080    .\" </a>
1081    "Duplicate Subpattern Numbers"
1082    .\"
1083    below). The assertions at the start of each branch check the next UTF-8
1084    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1085    character's individual bytes are then captured by the appropriate number of
1086    groups.
1087  .  .
1088  .  .
1089  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 520  the lookbehind. Line 1091  the lookbehind.
1091  .rs  .rs
1092  .sp  .sp
1093  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1094  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.
1095  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
1096  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
1097  escaped with a backslash.  a member of the class, it should be the first data character in the class
1098  .P  (after an initial circumflex, if present) or escaped with a backslash.
1099  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1100  character may occupy more than one byte. A matched character must be in the set  A character class matches a single character in the subject. In a UTF mode, the
1101  of characters defined by the class, unless the first character in the class  character may be more than one data unit long. A matched character must be in
1102  definition is a circumflex, in which case the subject character must not be in  the set of characters defined by the class, unless the first character in the
1103  the set defined by the class. If a circumflex is actually required as a member  class definition is a circumflex, in which case the subject character must not
1104  of the class, ensure it is not the first character, or escape it with a  be in the set defined by the class. If a circumflex is actually required as a
1105    member of the class, ensure it is not the first character, or escape it with a
1106  backslash.  backslash.
1107  .P  .P
1108  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1109  [^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
1110  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1111  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1112  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1113  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1114  string.  string.
1115  .P  .P
1116  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1117  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  included in a class as a literal string of data units, or by using the \ex{
1118    escaping mechanism.
1119  .P  .P
1120  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
1121  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
1122  "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
1123  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1124  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1125  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1126  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1127  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1128  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1129  UTF-8 support.  well as with UTF support.
1130  .P  .P
1131  The newline character is never treated in any special way in character classes,  Characters that might indicate line breaks are never treated in any special way
1132  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  when matching character classes, whatever line-ending sequence is in use, and
1133  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1134    such as [^a] always matches one of these characters.
1135  .P  .P
1136  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
1137  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,
# Line 574  followed by two other characters. The oc Line 1148  followed by two other characters. The oc
1148  "]" can also be used to end a range.  "]" can also be used to end a range.
1149  .P  .P
1150  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
1151  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1152  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1153  .P  .P
1154  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
1155  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
1156  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1157  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1158  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1159  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1160  property support.  property support.
1161  .P  .P
1162  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1163  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1164  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1165  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1166  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1167  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1168    .\" HTML <a href="#genericchartypes">
1169    .\" </a>
1170    "Generic character types"
1171    .\"
1172    above. The escape sequence \eb has a different meaning inside a character
1173    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1174    are not special inside a character class. Like any other unrecognized escape
1175    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1176    default, but cause an error if the PCRE_EXTRA option is set.
1177    .P
1178    A circumflex can conveniently be used with the upper case character types to
1179    specify a more restricted set of characters than the matching lower case type.
1180    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1181    whereas [\ew] includes underscore. A positive character class should be read as
1182    "something OR something OR ..." and a negative class as "NOT something AND NOT
1183    something AND NOT ...".
1184  .P  .P
1185  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1186  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 611  this notation. For example, Line 1200  this notation. For example,
1200    [01[:alpha:]%]    [01[:alpha:]%]
1201  .sp  .sp
1202  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1203  are  are:
1204  .sp  .sp
1205    alnum    letters and digits    alnum    letters and digits
1206    alpha    letters    alpha    letters
# Line 622  are Line 1211  are
1211    graph    printing characters, excluding space    graph    printing characters, excluding space
1212    lower    lower case letters    lower    lower case letters
1213    print    printing characters, including space    print    printing characters, including space
1214    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1215    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1216    upper    upper case letters    upper    upper case letters
1217    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 643  matches "1", "2", or any non-digit. PCRE Line 1232  matches "1", "2", or any non-digit. PCRE
1232  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
1233  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1234  .P  .P
1235  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1236  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1237    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1238    character properties are used. This is achieved by replacing the POSIX classes
1239    by other sequences, as follows:
1240    .sp
1241      [:alnum:]  becomes  \ep{Xan}
1242      [:alpha:]  becomes  \ep{L}
1243      [:blank:]  becomes  \eh
1244      [:digit:]  becomes  \ep{Nd}
1245      [:lower:]  becomes  \ep{Ll}
1246      [:space:]  becomes  \ep{Xps}
1247      [:upper:]  becomes  \ep{Lu}
1248      [:word:]   becomes  \ep{Xwd}
1249    .sp
1250    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1251    classes are unchanged, and match only characters with code points less than
1252    128.
1253  .  .
1254  .  .
1255  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 656  the pattern Line 1261  the pattern
1261    gilbert|sullivan    gilbert|sullivan
1262  .sp  .sp
1263  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1264  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1265  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
1266  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
 subpattern  
1267  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
1268  .\" </a>  .\" </a>
1269  (defined below),  (defined below),
# Line 672  alternative in the subpattern. Line 1276  alternative in the subpattern.
1276  .rs  .rs
1277  .sp  .sp
1278  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1279  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
1280  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1281    The option letters are
1282  .sp  .sp
1283    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1284    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 687  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1292  PCRE_MULTILINE while unsetting PCRE_DOTA
1292  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
1293  unset.  unset.
1294  .P  .P
1295  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
1296  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
1297  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1298  the global options (and it will therefore show up in data extracted by the  .P
1299  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1300    subpattern parentheses), the change applies to the remainder of the pattern
1301    that follows. If the change is placed right at the start of a pattern, PCRE
1302    extracts it into the global options (and it will therefore show up in data
1303    extracted by the \fBpcre_fullinfo()\fP function).
1304  .P  .P
1305  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1306  pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1307  .sp  .sp
1308    (a(?i)b)c    (a(?i)b)c
1309  .sp  .sp
# Line 710  branch is abandoned before the option se Line 1319  branch is abandoned before the option se
1319  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1320  behaviour otherwise.  behaviour otherwise.
1321  .P  .P
1322  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
1323  same way as the Perl-compatible options by using the characters U and X  application when the compiling or matching functions are called. In some cases
1324  respectively. The (?X) flag setting is special in that it must always occur  the pattern can contain special leading sequences such as (*CRLF) to override
1325  earlier in the pattern than any of the additional features it turns on, even  what the application has set or what has been defaulted. Details are given in
1326  when it is at top level. It is best to put it at the start.  the section entitled
1327    .\" HTML <a href="#newlineseq">
1328    .\" </a>
1329    "Newline sequences"
1330    .\"
1331    above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1332    can be used to set UTF and Unicode property modes; they are equivalent to
1333    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1334  .  .
1335  .  .
1336  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 728  Turning part of a pattern into a subpatt Line 1344  Turning part of a pattern into a subpatt
1344  .sp  .sp
1345    cat(aract|erpillar|)    cat(aract|erpillar|)
1346  .sp  .sp
1347  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1348  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1349  .sp  .sp
1350  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1351  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1352  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of the
1353  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1354  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1355  .P  .P
1356  For example, if the string "the red king" is matched against the pattern  Opening parentheses are counted from left to right (starting from 1) to obtain
1357    numbers for the capturing subpatterns. For example, if the string "the red
1358    king" is matched against the pattern
1359  .sp  .sp
1360    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1361  .sp  .sp
# Line 754  the string "the white queen" is matched Line 1372  the string "the white queen" is matched
1372    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1373  .sp  .sp
1374  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
1375  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1376  .P  .P
1377  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
1378  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 770  is reached, an option setting in one bra Line 1387  is reached, an option setting in one bra
1387  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1388  .  .
1389  .  .
1390    .\" HTML <a name="dupsubpatternnumber"></a>
1391    .SH "DUPLICATE SUBPATTERN NUMBERS"
1392    .rs
1393    .sp
1394    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1395    the same numbers for its capturing parentheses. Such a subpattern starts with
1396    (?| and is itself a non-capturing subpattern. For example, consider this
1397    pattern:
1398    .sp
1399      (?|(Sat)ur|(Sun))day
1400    .sp
1401    Because the two alternatives are inside a (?| group, both sets of capturing
1402    parentheses are numbered one. Thus, when the pattern matches, you can look
1403    at captured substring number one, whichever alternative matched. This construct
1404    is useful when you want to capture part, but not all, of one of a number of
1405    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1406    number is reset at the start of each branch. The numbers of any capturing
1407    parentheses that follow the subpattern start after the highest number used in
1408    any branch. The following example is taken from the Perl documentation. The
1409    numbers underneath show in which buffer the captured content will be stored.
1410    .sp
1411      # before  ---------------branch-reset----------- after
1412      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1413      # 1            2         2  3        2     3     4
1414    .sp
1415    A back reference to a numbered subpattern uses the most recent value that is
1416    set for that number by any subpattern. The following pattern matches "abcabc"
1417    or "defdef":
1418    .sp
1419      /(?|(abc)|(def))\e1/
1420    .sp
1421    In contrast, a subroutine call to a numbered subpattern always refers to the
1422    first one in the pattern with the given number. The following pattern matches
1423    "abcabc" or "defabc":
1424    .sp
1425      /(?|(abc)|(def))(?1)/
1426    .sp
1427    If a
1428    .\" HTML <a href="#conditions">
1429    .\" </a>
1430    condition test
1431    .\"
1432    for a subpattern's having matched refers to a non-unique number, the test is
1433    true if any of the subpatterns of that number have matched.
1434    .P
1435    An alternative approach to using this "branch reset" feature is to use
1436    duplicate named subpatterns, as described in the next section.
1437    .
1438    .
1439  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1440  .rs  .rs
1441  .sp  .sp
1442  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
1443  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1444  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1445  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1446  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
1447  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
1448  .P  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1449  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1450  PCRE API provides function calls for extracting the name-to-number translation  .P
1451  table from a compiled pattern. There is also a convenience function for  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1452  extracting a captured substring by name. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1453    parentheses from other parts of the pattern, such as
1454    .\" HTML <a href="#backreferences">
1455    .\" </a>
1456    back references,
1457    .\"
1458    .\" HTML <a href="#recursion">
1459    .\" </a>
1460    recursion,
1461    .\"
1462    and
1463    .\" HTML <a href="#conditions">
1464    .\" </a>
1465    conditions,
1466    .\"
1467    can be made by name as well as by number.
1468    .P
1469    Names consist of up to 32 alphanumeric characters and underscores. Named
1470    capturing parentheses are still allocated numbers as well as names, exactly as
1471    if the names were not present. The PCRE API provides function calls for
1472    extracting the name-to-number translation table from a compiled pattern. There
1473    is also a convenience function for extracting a captured substring by name.
1474    .P
1475    By default, a name must be unique within a pattern, but it is possible to relax
1476    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1477    names are also always permitted for subpatterns with the same number, set up as
1478    described in the previous section.) Duplicate names can be useful for patterns
1479    where only one instance of the named parentheses can match. Suppose you want to
1480    match the name of a weekday, either as a 3-letter abbreviation or as the full
1481    name, and in both cases you want to extract the abbreviation. This pattern
1482    (ignoring the line breaks) does the job:
1483    .sp
1484      (?<DN>Mon|Fri|Sun)(?:day)?|
1485      (?<DN>Tue)(?:sday)?|
1486      (?<DN>Wed)(?:nesday)?|
1487      (?<DN>Thu)(?:rsday)?|
1488      (?<DN>Sat)(?:urday)?
1489    .sp
1490    There are five capturing substrings, but only one is ever set after a match.
1491    (An alternative way of solving this problem is to use a "branch reset"
1492    subpattern, as described in the previous section.)
1493    .P
1494    The convenience function for extracting the data by name returns the substring
1495    for the first (and in this example, the only) subpattern of that name that
1496    matched. This saves searching to find which numbered subpattern it was.
1497    .P
1498    If you make a back reference to a non-unique named subpattern from elsewhere in
1499    the pattern, the one that corresponds to the first occurrence of the name is
1500    used. In the absence of duplicate numbers (see the previous section) this is
1501    the one with the lowest number. If you use a named reference in a condition
1502    test (see the
1503    .\"
1504    .\" HTML <a href="#conditions">
1505    .\" </a>
1506    section about conditions
1507    .\"
1508    below), either to check whether a subpattern has matched, or to check for
1509    recursion, all subpatterns with the same name are tested. If the condition is
1510    true for any one of them, the overall condition is true. This is the same
1511    behaviour as testing by number. For further details of the interfaces for
1512    handling named subpatterns, see the
1513  .\" HREF  .\" HREF
1514  \fBpcreapi\fP  \fBpcreapi\fP
1515  .\"  .\"
1516  documentation.  documentation.
1517    .P
1518    \fBWarning:\fP You cannot use different names to distinguish between two
1519    subpatterns with the same number because PCRE uses only the numbers when
1520    matching. For this reason, an error is given at compile time if different names
1521    are given to subpatterns with the same number. However, you can give the same
1522    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1523  .  .
1524  .  .
1525  .SH REPETITION  .SH REPETITION
# Line 797  Repetition is specified by quantifiers, Line 1529  Repetition is specified by quantifiers,
1529  items:  items:
1530  .sp  .sp
1531    a literal data character    a literal data character
1532    the . metacharacter    the dot metacharacter
1533    the \eC escape sequence    the \eC escape sequence
1534    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1535    an escape such as \ed that matches a single character    the \eR escape sequence
1536      an escape such as \ed or \epL that matches a single character
1537    a character class    a character class
1538    a back reference (see next section)    a back reference (see next section)
1539    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1540      a subroutine call to a subpattern (recursive or otherwise)
1541  .sp  .sp
1542  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1543  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 828  where a quantifier is not allowed, or on Line 1562  where a quantifier is not allowed, or on
1562  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
1563  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1564  .P  .P
1565  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1566  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1567  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1568  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1569  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1570  .P  .P
1571  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
1572  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1573    subpatterns that are referenced as
1574    .\" HTML <a href="#subpatternsassubroutines">
1575    .\" </a>
1576    subroutines
1577    .\"
1578    from elsewhere in the pattern (but see also the section entitled
1579    .\" HTML <a href="#subdefine">
1580    .\" </a>
1581    "Defining subpatterns for use by reference only"
1582    .\"
1583    below). Items other than subpatterns that have a {0} quantifier are omitted
1584    from the compiled pattern.
1585  .P  .P
1586  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1587  quantifiers have single-character abbreviations:  abbreviations:
1588  .sp  .sp
1589    *    is equivalent to {0,}    *    is equivalent to {0,}
1590    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 886  own right. Because it has two uses, it c Line 1632  own right. Because it has two uses, it c
1632  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
1633  way the rest of the pattern matches.  way the rest of the pattern matches.
1634  .P  .P
1635  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),
1636  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
1637  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
1638  default behaviour.  default behaviour.
# Line 896  is greater than 1 or with a limited maxi Line 1642  is greater than 1 or with a limited maxi
1642  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1643  .P  .P
1644  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
1645  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
1646  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1647  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
1648  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
# Line 906  In cases where it is known that the subj Line 1652  In cases where it is known that the subj
1652  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1653  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1654  .P  .P
1655  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1656  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1657  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
1658  succeed. Consider, for example:  succeeds. Consider, for example:
1659  .sp  .sp
1660    (.*)abc\e1    (.*)abc\e1
1661  .sp  .sp
1662  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
1663  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1664  .P  .P
1665    Another case where implicit anchoring is not applied is when the leading .* is
1666    inside an atomic group. Once again, a match at the start may fail where a later
1667    one succeeds. Consider this pattern:
1668    .sp
1669      (?>.*?a)b
1670    .sp
1671    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1672    (*PRUNE) and (*SKIP) also disable this optimization.
1673    .P
1674  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1675  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1676  .sp  .sp
# Line 935  matches "aba" the value of the second ca Line 1690  matches "aba" the value of the second ca
1690  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1691  .rs  .rs
1692  .sp  .sp
1693  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1694  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
1695  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
1696  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
1697  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
1698  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1699  .P  .P
1700  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1701  .sp  .sp
# Line 952  item, and then with 4, and so on, before Line 1707  item, and then with 4, and so on, before
1707  (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
1708  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.
1709  .P  .P
1710  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
1711  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
1712  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1713  .sp  .sp
# Line 982  previous example can be rewritten as Line 1737  previous example can be rewritten as
1737  .sp  .sp
1738    \ed++foo    \ed++foo
1739  .sp  .sp
1740    Note that a possessive quantifier can be used with an entire group, for
1741    example:
1742    .sp
1743      (abc|xyz){2,3}+
1744    .sp
1745  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1746  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
1747  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
1748  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1749  .P  difference; possessive quantifiers should be slightly faster.
1750  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1751  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1752    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1753    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1754    package, and PCRE copied it from there. It ultimately found its way into Perl
1755    at release 5.10.
1756    .P
1757    PCRE has an optimization that automatically "possessifies" certain simple
1758    pattern constructs. For example, the sequence A+B is treated as A++B because
1759    there is no point in backtracking into a sequence of A's when B must follow.
1760  .P  .P
1761  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
1762  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
# Line 1030  However, if the decimal number following Line 1798  However, if the decimal number following
1798  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
1799  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
1800  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
1801  numbers less than 10. See the subsection entitled "Non-printing characters"  numbers less than 10. A "forward back reference" of this type can make sense
1802    when a repetition is involved and the subpattern to the right has participated
1803    in an earlier iteration.
1804    .P
1805    It is not possible to have a numerical "forward back reference" to a subpattern
1806    whose number is 10 or more using this syntax because a sequence such as \e50 is
1807    interpreted as a character defined in octal. See the subsection entitled
1808    "Non-printing characters"
1809  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1810  .\" </a>  .\" </a>
1811  above  above
1812  .\"  .\"
1813  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1814    no such problem when named parentheses are used. A back reference to any
1815    subpattern is possible using named parentheses (see below).
1816    .P
1817    Another way of avoiding the ambiguity inherent in the use of digits following a
1818    backslash is to use the \eg escape sequence. This escape must be followed by an
1819    unsigned number or a negative number, optionally enclosed in braces. These
1820    examples are all identical:
1821    .sp
1822      (ring), \e1
1823      (ring), \eg1
1824      (ring), \eg{1}
1825    .sp
1826    An unsigned number specifies an absolute reference without the ambiguity that
1827    is present in the older syntax. It is also useful when literal digits follow
1828    the reference. A negative number is a relative reference. Consider this
1829    example:
1830    .sp
1831      (abc(def)ghi)\eg{-1}
1832    .sp
1833    The sequence \eg{-1} is a reference to the most recently started capturing
1834    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1835    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1836    can be helpful in long patterns, and also in patterns that are created by
1837    joining together fragments that contain references within themselves.
1838  .P  .P
1839  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1840  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1057  back reference, the case of letters is r Line 1856  back reference, the case of letters is r
1856  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
1857  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1858  .P  .P
1859  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1860  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1861  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1862    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1863    references, is also supported. We could rewrite the above example in any of
1864    the following ways:
1865    .sp
1866      (?<p1>(?i)rah)\es+\ek<p1>
1867      (?'p1'(?i)rah)\es+\ek{p1}
1868      (?P<p1>(?i)rah)\es+(?P=p1)
1869      (?<p1>(?i)rah)\es+\eg{p1}
1870  .sp  .sp
1871    A subpattern that is referenced by name may appear in the pattern before or
1872    after the reference.
1873    .P
1874  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
1875  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1876  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1877  .sp  .sp
1878    (a|(bc))\e2    (a|(bc))\e2
1879  .sp  .sp
1880  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
1881  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
1882  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1883  with a digit character, some delimiter must be used to terminate the back  .P
1884  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1885  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1886    If the pattern continues with a digit character, some delimiter must be used to
1887    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1888    white space. Otherwise, the \eg{ syntax or an empty comment (see
1889  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1890  .\" </a>  .\" </a>
1891  "Comments"  "Comments"
1892  .\"  .\"
1893  below) can be used.  below) can be used.
1894  .P  .
1895    .SS "Recursive back references"
1896    .rs
1897    .sp
1898  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
1899  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1900  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1093  to the previous iteration. In order for Line 1908  to the previous iteration. In order for
1908  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
1909  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
1910  minimum of zero.  minimum of zero.
1911    .P
1912    Back references of this type cause the group that they reference to be treated
1913    as an
1914    .\" HTML <a href="#atomicgroup">
1915    .\" </a>
1916    atomic group.
1917    .\"
1918    Once the whole group has been matched, a subsequent matching failure cannot
1919    cause backtracking into the middle of the group.
1920  .  .
1921  .  .
1922  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1112  those that look ahead of the current pos Line 1936  those that look ahead of the current pos
1936  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1937  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1938  .P  .P
1939  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1940  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1941  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1942  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1943  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1944  because it does not make sense for negative assertions.  .P
1945    For compatibility with Perl, assertion subpatterns may be repeated; though
1946    it makes no sense to assert the same thing several times, the side effect of
1947    capturing parentheses may occasionally be useful. In practice, there only three
1948    cases:
1949    .sp
1950    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1951    However, it may contain internal capturing parenthesized groups that are called
1952    from elsewhere via the
1953    .\" HTML <a href="#subpatternsassubroutines">
1954    .\" </a>
1955    subroutine mechanism.
1956    .\"
1957    .sp
1958    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1959    were {0,1}. At run time, the rest of the pattern match is tried with and
1960    without the assertion, the order depending on the greediness of the quantifier.
1961    .sp
1962    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1963    The assertion is obeyed just once when encountered during matching.
1964  .  .
1965  .  .
1966  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1967  .rs  .rs
1968  .sp  .sp
1969  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1970  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1971  .sp  .sp
1972    \ew+(?=;)    \ew+(?=;)
1973  .sp  .sp
# Line 1146  lookbehind assertion is needed to achiev Line 1989  lookbehind assertion is needed to achiev
1989  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
1990  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
1991  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.
1992    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1993  .  .
1994  .  .
1995  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1159  negative assertions. For example, Line 2003  negative assertions. For example,
2003  .sp  .sp
2004  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
2005  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
2006  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
2007  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
2008  .sp  .sp
2009    (?<=bullock|donkey)    (?<=bullock|donkey)
2010  .sp  .sp
# Line 1170  is permitted, but Line 2014  is permitted, but
2014  .sp  .sp
2015  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2016  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
2017  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
2018  match the same length of string. An assertion such as  length of string. An assertion such as
2019  .sp  .sp
2020    (?<=ab(c|de))    (?<=ab(c|de))
2021  .sp  .sp
2022  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
2023  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
2024    branches:
2025  .sp  .sp
2026    (?<=abc|abde)    (?<=abc|abde)
2027  .sp  .sp
2028    In some cases, the escape sequence \eK
2029    .\" HTML <a href="#resetmatchstart">
2030    .\" </a>
2031    (see above)
2032    .\"
2033    can be used instead of a lookbehind assertion to get round the fixed-length
2034    restriction.
2035    .P
2036  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2037  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
2038  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2039  match is deemed to fail.  assertion fails.
2040  .P  .P
2041  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2042  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2043  the length of the lookbehind. The \eX escape, which can match different numbers  it impossible to calculate the length of the lookbehind. The \eX and \eR
2044  of bytes, is also not permitted.  escapes, which can match different numbers of data units, are also not
2045  .P  permitted.
2046  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
2047  efficient matching at the end of the subject string. Consider a simple pattern  .\" HTML <a href="#subpatternsassubroutines">
2048  such as  .\" </a>
2049    "Subroutine"
2050    .\"
2051    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2052    as the subpattern matches a fixed-length string.
2053    .\" HTML <a href="#recursion">
2054    .\" </a>
2055    Recursion,
2056    .\"
2057    however, is not supported.
2058    .P
2059    Possessive quantifiers can be used in conjunction with lookbehind assertions to
2060    specify efficient matching of fixed-length strings at the end of subject
2061    strings. Consider a simple pattern such as
2062  .sp  .sp
2063    abcd$    abcd$
2064  .sp  .sp
# Line 1208  then all but the last two characters, an Line 2074  then all but the last two characters, an
2074  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,
2075  if the pattern is written as  if the pattern is written as
2076  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
2077    ^.*+(?<=abcd)    ^.*+(?<=abcd)
2078  .sp  .sp
2079  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
2080  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
2081  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2082  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1254  is another pattern that matches "foo" pr Line 2116  is another pattern that matches "foo" pr
2116  characters that are not "999".  characters that are not "999".
2117  .  .
2118  .  .
2119    .\" HTML <a name="conditions"></a>
2120  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
2121  .rs  .rs
2122  .sp  .sp
2123  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2124  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2125  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2126  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2127  .sp  .sp
2128    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2129    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2130  .sp  .sp
2131  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2132  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
2133  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2134    itself contain nested subpatterns of any form, including conditional
2135    subpatterns; the restriction to two alternatives applies only at the level of
2136    the condition. This pattern fragment is an example where the alternatives are
2137    complex:
2138    .sp
2139      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2140    .sp
2141  .P  .P
2142  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
2143  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
2144  subpattern of that number has previously matched. The number must be greater  .
2145  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
2146  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
2147  it into three parts for ease of discussion:  .sp
2148    If the text between the parentheses consists of a sequence of digits, the
2149    condition is true if a capturing subpattern of that number has previously
2150    matched. If there is more than one capturing subpattern with the same number
2151    (see the earlier
2152    .\"
2153    .\" HTML <a href="#recursion">
2154    .\" </a>
2155    section about duplicate subpattern numbers),
2156    .\"
2157    the condition is true if any of them have matched. An alternative notation is
2158    to precede the digits with a plus or minus sign. In this case, the subpattern
2159    number is relative rather than absolute. The most recently opened parentheses
2160    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2161    loops it can also make sense to refer to subsequent groups. The next
2162    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2163    zero in any of these forms is not used; it provokes a compile-time error.)
2164    .P
2165    Consider the following pattern, which contains non-significant white space to
2166    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2167    three parts for ease of discussion:
2168  .sp  .sp
2169    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
2170  .sp  .sp
2171  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2172  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
2173  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
2174  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2175  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,
2176  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
2177  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2178  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2179  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2180  .P  .P
2181  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
2182  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2183  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2184      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2185    .sp
2186    This makes the fragment independent of the parentheses in the larger pattern.
2187    .
2188    .SS "Checking for a used subpattern by name"
2189    .rs
2190    .sp
2191    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2192    subpattern by name. For compatibility with earlier versions of PCRE, which had
2193    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2194    there is a possible ambiguity with this syntax, because subpattern names may
2195    consist entirely of digits. PCRE looks first for a named subpattern; if it
2196    cannot find one and the name consists entirely of digits, PCRE looks for a
2197    subpattern of that number, which must be greater than zero. Using subpattern
2198    names that consist entirely of digits is not recommended.
2199    .P
2200    Rewriting the above example to use a named subpattern gives this:
2201    .sp
2202      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2203    .sp
2204    If the name used in a condition of this kind is a duplicate, the test is
2205    applied to all subpatterns of the same name, and is true if any one of them has
2206    matched.
2207    .
2208    .SS "Checking for pattern recursion"
2209    .rs
2210    .sp
2211    If the condition is the string (R), and there is no subpattern with the name R,
2212    the condition is true if a recursive call to the whole pattern or any
2213    subpattern has been made. If digits or a name preceded by ampersand follow the
2214    letter R, for example:
2215    .sp
2216      (?(R3)...) or (?(R&name)...)
2217    .sp
2218    the condition is true if the most recent recursion is into a subpattern whose
2219    number or name is given. This condition does not check the entire recursion
2220    stack. If the name used in a condition of this kind is a duplicate, the test is
2221    applied to all subpatterns of the same name, and is true if any one of them is
2222    the most recent recursion.
2223  .P  .P
2224  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
2225    .\" HTML <a href="#recursion">
2226    .\" </a>
2227    The syntax for recursive patterns
2228    .\"
2229    is described below.
2230    .
2231    .\" HTML <a name="subdefine"></a>
2232    .SS "Defining subpatterns for use by reference only"
2233    .rs
2234    .sp
2235    If the condition is the string (DEFINE), and there is no subpattern with the
2236    name DEFINE, the condition is always false. In this case, there may be only one
2237    alternative in the subpattern. It is always skipped if control reaches this
2238    point in the pattern; the idea of DEFINE is that it can be used to define
2239    subroutines that can be referenced from elsewhere. (The use of
2240    .\" HTML <a href="#subpatternsassubroutines">
2241    .\" </a>
2242    subroutines
2243    .\"
2244    is described below.) For example, a pattern to match an IPv4 address such as
2245    "192.168.23.245" could be written like this (ignore white space and line
2246    breaks):
2247    .sp
2248      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2249      \eb (?&byte) (\e.(?&byte)){3} \eb
2250    .sp
2251    The first part of the pattern is a DEFINE group inside which a another group
2252    named "byte" is defined. This matches an individual component of an IPv4
2253    address (a number less than 256). When matching takes place, this part of the
2254    pattern is skipped because DEFINE acts like a false condition. The rest of the
2255    pattern uses references to the named group to match the four dot-separated
2256    components of an IPv4 address, insisting on a word boundary at each end.
2257    .
2258    .SS "Assertion conditions"
2259    .rs
2260    .sp
2261    If the condition is not in any of the above formats, it must be an assertion.
2262  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2263  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2264  alternatives on the second line:  alternatives on the second line:
# Line 1312  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2278  dd-aaa-dd or dd-dd-dd, where aaa are let
2278  .SH COMMENTS  .SH COMMENTS
2279  .rs  .rs
2280  .sp  .sp
2281  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2282  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,
2283  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
2284    subpattern name or number. The characters that make up a comment play no part
2285    in the pattern matching.
2286  .P  .P
2287  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2288  character class introduces a comment that continues up to the next newline  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2289  character in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2290    this case continues to immediately after the next newline character or
2291    character sequence in the pattern. Which characters are interpreted as newlines
2292    is controlled by the options passed to a compiling function or by a special
2293    sequence at the start of the pattern, as described in the section entitled
2294    .\" HTML <a href="#newlines">
2295    .\" </a>
2296    "Newline conventions"
2297    .\"
2298    above. Note that the end of this type of comment is a literal newline sequence
2299    in the pattern; escape sequences that happen to represent a newline do not
2300    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2301    default newline convention is in force:
2302    .sp
2303      abc #comment \en still comment
2304    .sp
2305    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2306    a newline in the pattern. The sequence \en is still literal at this stage, so
2307    it does not terminate the comment. Only an actual character with the code value
2308    0x0a (the default newline) does so.
2309  .  .
2310  .  .
2311    .\" HTML <a name="recursion"></a>
2312  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
2313  .rs  .rs
2314  .sp  .sp
2315  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2316  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2317  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
2318  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2319  that allows regular expressions to recurse (amongst other things). It does this  .P
2320  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2321  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2322  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2323    pattern using code interpolation to solve the parentheses problem can be
2324    created like this:
2325  .sp  .sp
2326    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2327  .sp  .sp
2328  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
2329  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2330  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2331  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2332  .P  supports special syntax for recursion of the entire pattern, and also for
2333  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2334  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.
2335  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2336  "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
2337  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive subroutine call of the subpattern of the
2338    given number, provided that it occurs inside that subpattern. (If not, it is a
2339    .\" HTML <a href="#subpatternsassubroutines">
2340    .\" </a>
2341    non-recursive subroutine
2342    .\"
2343    call, which is described in the next section.) The special item (?R) or (?0) is
2344    a recursive call of the entire regular expression.
2345  .P  .P
2346  For example, this PCRE pattern solves the nested parentheses problem (assume  This PCRE pattern solves the nested parentheses problem (assume the
2347  the PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2348  .sp  .sp
2349    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2350  .sp  .sp
2351  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2352  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
2353  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2354  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2355    to avoid backtracking into sequences of non-parentheses.
2356  .P  .P
2357  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
2358  pattern, so instead you could use this:  pattern, so instead you could use this:
2359  .sp  .sp
2360    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2361  .sp  .sp
2362  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
2363  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2364  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2365  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
2366  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
2367  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2368  .sp  parentheses preceding the recursion. In other words, a negative number counts
2369    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  capturing parentheses leftwards from the point at which it is encountered.
2370  .sp  .P
2371  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2372  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2373  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
2374  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2375    .\" </a>
2376    non-recursive subroutine
2377    .\"
2378    calls, as described in the next section.
2379    .P
2380    An alternative approach is to use named parentheses instead. The Perl syntax
2381    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2382    could rewrite the above example as follows:
2383    .sp
2384      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2385    .sp
2386    If there is more than one subpattern with the same name, the earliest one is
2387    used.
2388    .P
2389    This particular example pattern that we have been looking at contains nested
2390    unlimited repeats, and so the use of a possessive quantifier for matching
2391    strings of non-parentheses is important when applying the pattern to strings
2392    that do not match. For example, when this pattern is applied to
2393  .sp  .sp
2394    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2395  .sp  .sp
2396  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,
2397  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
2398  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
2399  before failure can be reported.  before failure can be reported.
2400  .P  .P
2401  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
2402  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
2403  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2404  .\" HREF  .\" HREF
2405  \fBpcrecallout\fP  \fBpcrecallout\fP
2406  .\"  .\"
# Line 1393  documentation). If the pattern above is Line 2408  documentation). If the pattern above is
2408  .sp  .sp
2409    (ab(cd)ef)    (ab(cd)ef)
2410  .sp  .sp
2411  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
2412  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
2413  .sp  matched at the top level, its final captured value is unset, even if it was
2414    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2415       ^                        ^  .P
2416       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2417  .sp  obtain extra memory to store data during a recursion, which it does by using
2418  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
2419  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2420  .P  .P
2421  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.
2422  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1418  different alternatives for the recursive Line 2430  different alternatives for the recursive
2430  is the actual recursive call.  is the actual recursive call.
2431  .  .
2432  .  .
2433    .\" HTML <a name="recursiondifference"></a>
2434    .SS "Differences in recursion processing between PCRE and Perl"
2435    .rs
2436    .sp
2437    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2438    (like Python, but unlike Perl), a recursive subpattern call is always treated
2439    as an atomic group. That is, once it has matched some of the subject string, it
2440    is never re-entered, even if it contains untried alternatives and there is a
2441    subsequent matching failure. This can be illustrated by the following pattern,
2442    which purports to match a palindromic string that contains an odd number of
2443    characters (for example, "a", "aba", "abcba", "abcdcba"):
2444    .sp
2445      ^(.|(.)(?1)\e2)$
2446    .sp
2447    The idea is that it either matches a single character, or two identical
2448    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2449    it does not if the pattern is longer than three characters. Consider the
2450    subject string "abcba":
2451    .P
2452    At the top level, the first character is matched, but as it is not at the end
2453    of the string, the first alternative fails; the second alternative is taken
2454    and the recursion kicks in. The recursive call to subpattern 1 successfully
2455    matches the next character ("b"). (Note that the beginning and end of line
2456    tests are not part of the recursion).
2457    .P
2458    Back at the top level, the next character ("c") is compared with what
2459    subpattern 2 matched, which was "a". This fails. Because the recursion is
2460    treated as an atomic group, there are now no backtracking points, and so the
2461    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2462    try the second alternative.) However, if the pattern is written with the
2463    alternatives in the other order, things are different:
2464    .sp
2465      ^((.)(?1)\e2|.)$
2466    .sp
2467    This time, the recursing alternative is tried first, and continues to recurse
2468    until it runs out of characters, at which point the recursion fails. But this
2469    time we do have another alternative to try at the higher level. That is the big
2470    difference: in the previous case the remaining alternative is at a deeper
2471    recursion level, which PCRE cannot use.
2472    .P
2473    To change the pattern so that it matches all palindromic strings, not just
2474    those with an odd number of characters, it is tempting to change the pattern to
2475    this:
2476    .sp
2477      ^((.)(?1)\e2|.?)$
2478    .sp
2479    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2480    deeper recursion has matched a single character, it cannot be entered again in
2481    order to match an empty string. The solution is to separate the two cases, and
2482    write out the odd and even cases as alternatives at the higher level:
2483    .sp
2484      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2485    .sp
2486    If you want to match typical palindromic phrases, the pattern has to ignore all
2487    non-word characters, which can be done like this:
2488    .sp
2489      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2490    .sp
2491    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2492    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2493    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2494    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2495    more) to match typical phrases, and Perl takes so long that you think it has
2496    gone into a loop.
2497    .P
2498    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2499    string does not start with a palindrome that is shorter than the entire string.
2500    For example, although "abcba" is correctly matched, if the subject is "ababa",
2501    PCRE finds the palindrome "aba" at the start, then fails at top level because
2502    the end of the string does not follow. Once again, it cannot jump back into the
2503    recursion to try other alternatives, so the entire match fails.
2504    .P
2505    The second way in which PCRE and Perl differ in their recursion processing is
2506    in the handling of captured values. In Perl, when a subpattern is called
2507    recursively or as a subpattern (see the next section), it has no access to any
2508    values that were captured outside the recursion, whereas in PCRE these values
2509    can be referenced. Consider this pattern:
2510    .sp
2511      ^(.)(\e1|a(?2))
2512    .sp
2513    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2514    then in the second group, when the back reference \e1 fails to match "b", the
2515    second alternative matches "a" and then recurses. In the recursion, \e1 does
2516    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2517    match because inside the recursive call \e1 cannot access the externally set
2518    value.
2519    .
2520    .
2521  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2522  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2523  .rs  .rs
2524  .sp  .sp
2525  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2526  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
2527  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2528  pattern  before or after the reference. A numbered reference can be absolute or
2529    relative, as in these examples:
2530    .sp
2531      (...(absolute)...)...(?2)...
2532      (...(relative)...)...(?-1)...
2533      (...(?+1)...(relative)...
2534    .sp
2535    An earlier example pointed out that the pattern
2536  .sp  .sp
2537    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2538  .sp  .sp
# Line 1435  matches "sense and sensibility" and "res Line 2542  matches "sense and sensibility" and "res
2542    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2543  .sp  .sp
2544  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
2545  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2546  refer.  .P
2547    All subroutine calls, whether recursive or not, are always treated as atomic
2548    groups. That is, once a subroutine has matched some of the subject string, it
2549    is never re-entered, even if it contains untried alternatives and there is a
2550    subsequent matching failure. Any capturing parentheses that are set during the
2551    subroutine call revert to their previous values afterwards.
2552    .P
2553    Processing options such as case-independence are fixed when a subpattern is
2554    defined, so if it is used as a subroutine, such options cannot be changed for
2555    different calls. For example, consider this pattern:
2556    .sp
2557      (abc)(?i:(?-1))
2558    .sp
2559    It matches "abcabc". It does not match "abcABC" because the change of
2560    processing option does not affect the called subpattern.
2561    .
2562    .
2563    .\" HTML <a name="onigurumasubroutines"></a>
2564    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2565    .rs
2566    .sp
2567    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2568    a number enclosed either in angle brackets or single quotes, is an alternative
2569    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2570    are two of the examples used above, rewritten using this syntax:
2571    .sp
2572      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2573      (sens|respons)e and \eg'1'ibility
2574    .sp
2575    PCRE supports an extension to Oniguruma: if a number is preceded by a
2576    plus or a minus sign it is taken as a relative reference. For example:
2577    .sp
2578      (abc)(?i:\eg<-1>)
2579    .sp
2580    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2581    synonymous. The former is a back reference; the latter is a subroutine call.
2582  .  .
2583  .  .
2584  .SH CALLOUTS  .SH CALLOUTS
# Line 1449  same pair of parentheses when there is a Line 2591  same pair of parentheses when there is a
2591  .P  .P
2592  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
2593  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
2594  function by putting its entry point in the global variable \fIpcre_callout\fP.  function by putting its entry point in the global variable \fIpcre_callout\fP
2595  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2596    variable contains NULL, which disables all calling out.
2597  .P  .P
2598  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2599  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
2600  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.
2601  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2602  .sp  .sp
2603    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2604  .sp  .sp
2605  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2606  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2607  255.  255.
2608  .P  .P
2609  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  During matching, when PCRE reaches a callout point, the external function is
2610  set), the external function is called. It is provided with the number of the  called. It is provided with the number of the callout, the position in the
2611  callout, the position in the pattern, and, optionally, one item of data  pattern, and, optionally, one item of data originally supplied by the caller of
2612  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2613  may cause matching to proceed, to backtrack, or to fail altogether. A complete  backtrack, or to fail altogether. A complete description of the interface to
2614  description of the interface to the callout function is given in the  the callout function is given in the
2615  .\" HREF  .\" HREF
2616  \fBpcrecallout\fP  \fBpcrecallout\fP
2617  .\"  .\"
2618  documentation.  documentation.
2619    .
2620    .
2621    .\" HTML <a name="backtrackcontrol"></a>
2622    .SH "BACKTRACKING CONTROL"
2623    .rs
2624    .sp
2625    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2626    are described in the Perl documentation as "experimental and subject to change
2627    or removal in a future version of Perl". It goes on to say: "Their usage in
2628    production code should be noted to avoid problems during upgrades." The same
2629    remarks apply to the PCRE features described in this section.
2630    .P
2631    Since these verbs are specifically related to backtracking, most of them can be
2632    used only when the pattern is to be matched using one of the traditional
2633    matching functions, which use a backtracking algorithm. With the exception of
2634    (*FAIL), which behaves like a failing negative assertion, they cause an error
2635    if encountered by a DFA matching function.
2636    .P
2637    If any of these verbs are used in an assertion or in a subpattern that is
2638    called as a subroutine (whether or not recursively), their effect is confined
2639    to that subpattern; it does not extend to the surrounding pattern, with one
2640    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2641    a successful positive assertion \fIis\fP passed back when a match succeeds
2642    (compare capturing parentheses in assertions). Note that such subpatterns are
2643    processed as anchored at the point where they are tested. Note also that Perl's
2644    treatment of subroutines and assertions is different in some cases.
2645    .P
2646    The new verbs make use of what was previously invalid syntax: an opening
2647    parenthesis followed by an asterisk. They are generally of the form
2648    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2649    depending on whether or not an argument is present. A name is any sequence of
2650    characters that does not include a closing parenthesis. The maximum length of
2651    name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2652    is empty, that is, if the closing parenthesis immediately follows the colon,
2653    the effect is as if the colon were not there. Any number of these verbs may
2654    occur in a pattern.
2655    .
2656    .
2657    .\" HTML <a name="nooptimize"></a>
2658    .SS "Optimizations that affect backtracking verbs"
2659    .rs
2660    .sp
2661    PCRE contains some optimizations that are used to speed up matching by running
2662    some checks at the start of each match attempt. For example, it may know the
2663    minimum length of matching subject, or that a particular character must be
2664    present. When one of these optimizations suppresses the running of a match, any
2665    included backtracking verbs will not, of course, be processed. You can suppress
2666    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2667    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2668    pattern with (*NO_START_OPT). There is more discussion of this option in the
2669    section entitled
2670    .\" HTML <a href="pcreapi.html#execoptions">
2671    .\" </a>
2672    "Option bits for \fBpcre_exec()\fP"
2673    .\"
2674    in the
2675    .\" HREF
2676    \fBpcreapi\fP
2677    .\"
2678    documentation.
2679  .P  .P
2680  .in 0  Experiments with Perl suggest that it too has similar optimizations, sometimes
2681  Last updated: 28 February 2005  leading to anomalous results.
2682  .br  .
2683  Copyright (c) 1997-2005 University of Cambridge.  .
2684    .SS "Verbs that act immediately"
2685    .rs
2686    .sp
2687    The following verbs act as soon as they are encountered. They may not be
2688    followed by a name.
2689    .sp
2690       (*ACCEPT)
2691    .sp
2692    This verb causes the match to end successfully, skipping the remainder of the
2693    pattern. However, when it is inside a subpattern that is called as a
2694    subroutine, only that subpattern is ended successfully. Matching then continues
2695    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2696    far is captured. For example:
2697    .sp
2698      A((?:A|B(*ACCEPT)|C)D)
2699    .sp
2700    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2701    the outer parentheses.
2702    .sp
2703      (*FAIL) or (*F)
2704    .sp
2705    This verb causes a matching failure, forcing backtracking to occur. It is
2706    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2707    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2708    Perl features that are not present in PCRE. The nearest equivalent is the
2709    callout feature, as for example in this pattern:
2710    .sp
2711      a+(?C)(*FAIL)
2712    .sp
2713    A match with the string "aaaa" always fails, but the callout is taken before
2714    each backtrack happens (in this example, 10 times).
2715    .
2716    .
2717    .SS "Recording which path was taken"
2718    .rs
2719    .sp
2720    There is one verb whose main purpose is to track how a match was arrived at,
2721    though it also has a secondary use in conjunction with advancing the match
2722    starting point (see (*SKIP) below).
2723    .sp
2724      (*MARK:NAME) or (*:NAME)
2725    .sp
2726    A name is always required with this verb. There may be as many instances of
2727    (*MARK) as you like in a pattern, and their names do not have to be unique.
2728    .P
2729    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2730    path is passed back to the caller as described in the section entitled
2731    .\" HTML <a href="pcreapi.html#extradata">
2732    .\" </a>
2733    "Extra data for \fBpcre_exec()\fP"
2734    .\"
2735    in the
2736    .\" HREF
2737    \fBpcreapi\fP
2738    .\"
2739    documentation. Here is an example of \fBpcretest\fP output, where the /K
2740    modifier requests the retrieval and outputting of (*MARK) data:
2741    .sp
2742        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2743      data> XY
2744       0: XY
2745      MK: A
2746      XZ
2747       0: XZ
2748      MK: B
2749    .sp
2750    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2751    indicates which of the two alternatives matched. This is a more efficient way
2752    of obtaining this information than putting each alternative in its own
2753    capturing parentheses.
2754    .P
2755    If (*MARK) is encountered in a positive assertion, its name is recorded and
2756    passed back if it is the last-encountered. This does not happen for negative
2757    assertions.
2758    .P
2759    After a partial match or a failed match, the name of the last encountered
2760    (*MARK) in the entire match process is returned. For example:
2761    .sp
2762        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2763      data> XP
2764      No match, mark = B
2765    .sp
2766    Note that in this unanchored example the mark is retained from the match
2767    attempt that started at the letter "X" in the subject. Subsequent match
2768    attempts starting at "P" and then with an empty string do not get as far as the
2769    (*MARK) item, but nevertheless do not reset it.
2770    .P
2771    If you are interested in (*MARK) values after failed matches, you should
2772    probably set the PCRE_NO_START_OPTIMIZE option
2773    .\" HTML <a href="#nooptimize">
2774    .\" </a>
2775    (see above)
2776    .\"
2777    to ensure that the match is always attempted.
2778    .
2779    .
2780    .SS "Verbs that act after backtracking"
2781    .rs
2782    .sp
2783    The following verbs do nothing when they are encountered. Matching continues
2784    with what follows, but if there is no subsequent match, causing a backtrack to
2785    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2786    the verb. However, when one of these verbs appears inside an atomic group, its
2787    effect is confined to that group, because once the group has been matched,
2788    there is never any backtracking into it. In this situation, backtracking can
2789    "jump back" to the left of the entire atomic group. (Remember also, as stated
2790    above, that this localization also applies in subroutine calls and assertions.)
2791    .P
2792    These verbs differ in exactly what kind of failure occurs when backtracking
2793    reaches them.
2794    .sp
2795      (*COMMIT)
2796    .sp
2797    This verb, which may not be followed by a name, causes the whole match to fail
2798    outright if the rest of the pattern does not match. Even if the pattern is
2799    unanchored, no further attempts to find a match by advancing the starting point
2800    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2801    finding a match at the current starting point, or not at all. For example:
2802    .sp
2803      a+(*COMMIT)b
2804    .sp
2805    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2806    dynamic anchor, or "I've started, so I must finish." The name of the most
2807    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2808    match failure.
2809    .P
2810    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2811    unless PCRE's start-of-match optimizations are turned off, as shown in this
2812    \fBpcretest\fP example:
2813    .sp
2814        re> /(*COMMIT)abc/
2815      data> xyzabc
2816       0: abc
2817      xyzabc\eY
2818      No match
2819    .sp
2820    PCRE knows that any match must start with "a", so the optimization skips along
2821    the subject to "a" before running the first match attempt, which succeeds. When
2822    the optimization is disabled by the \eY escape in the second subject, the match
2823    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2824    starting points.
2825    .sp
2826      (*PRUNE) or (*PRUNE:NAME)
2827    .sp
2828    This verb causes the match to fail at the current starting position in the
2829    subject if the rest of the pattern does not match. If the pattern is
2830    unanchored, the normal "bumpalong" advance to the next starting character then
2831    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2832    reached, or when matching to the right of (*PRUNE), but if there is no match to
2833    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2834    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2835    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2836    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2837    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2838    .sp
2839      (*SKIP)
2840    .sp
2841    This verb, when given without a name, is like (*PRUNE), except that if the
2842    pattern is unanchored, the "bumpalong" advance is not to the next character,
2843    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2844    signifies that whatever text was matched leading up to it cannot be part of a
2845    successful match. Consider:
2846    .sp
2847      a+(*SKIP)b
2848    .sp
2849    If the subject is "aaaac...", after the first match attempt fails (starting at
2850    the first character in the string), the starting point skips on to start the
2851    next attempt at "c". Note that a possessive quantifer does not have the same
2852    effect as this example; although it would suppress backtracking during the
2853    first match attempt, the second attempt would start at the second character
2854    instead of skipping on to "c".
2855    .sp
2856      (*SKIP:NAME)
2857    .sp
2858    When (*SKIP) has an associated name, its behaviour is modified. If the
2859    following pattern fails to match, the previous path through the pattern is
2860    searched for the most recent (*MARK) that has the same name. If one is found,
2861    the "bumpalong" advance is to the subject position that corresponds to that
2862    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2863    matching name is found, the (*SKIP) is ignored.
2864    .sp
2865      (*THEN) or (*THEN:NAME)
2866    .sp
2867    This verb causes a skip to the next innermost alternative if the rest of the
2868    pattern does not match. That is, it cancels pending backtracking, but only
2869    within the current alternative. Its name comes from the observation that it can
2870    be used for a pattern-based if-then-else block:
2871    .sp
2872      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2873    .sp
2874    If the COND1 pattern matches, FOO is tried (and possibly further items after
2875    the end of the group if FOO succeeds); on failure, the matcher skips to the
2876    second alternative and tries COND2, without backtracking into COND1. The
2877    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2878    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2879    .P
2880    Note that a subpattern that does not contain a | character is just a part of
2881    the enclosing alternative; it is not a nested alternation with only one
2882    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2883    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2884    pattern fragments that do not contain any | characters at this level:
2885    .sp
2886      A (B(*THEN)C) | D
2887    .sp
2888    If A and B are matched, but there is a failure in C, matching does not
2889    backtrack into A; instead it moves to the next alternative, that is, D.
2890    However, if the subpattern containing (*THEN) is given an alternative, it
2891    behaves differently:
2892    .sp
2893      A (B(*THEN)C | (*FAIL)) | D
2894    .sp
2895    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2896    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2897    because there are no more alternatives to try. In this case, matching does now
2898    backtrack into A.
2899    .P
2900    Note also that a conditional subpattern is not considered as having two
2901    alternatives, because only one is ever used. In other words, the | character in
2902    a conditional subpattern has a different meaning. Ignoring white space,
2903    consider:
2904    .sp
2905      ^.*? (?(?=a) a | b(*THEN)c )
2906    .sp
2907    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2908    it initially matches zero characters. The condition (?=a) then fails, the
2909    character "b" is matched, but "c" is not. At this point, matching does not
2910    backtrack to .*? as might perhaps be expected from the presence of the |
2911    character. The conditional subpattern is part of the single alternative that
2912    comprises the whole pattern, and so the match fails. (If there was a backtrack
2913    into .*?, allowing it to match "b", the match would succeed.)
2914    .P
2915    The verbs just described provide four different "strengths" of control when
2916    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2917    next alternative. (*PRUNE) comes next, failing the match at the current
2918    starting position, but allowing an advance to the next character (for an
2919    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2920    than one character. (*COMMIT) is the strongest, causing the entire match to
2921    fail.
2922    .P
2923    If more than one such verb is present in a pattern, the "strongest" one wins.
2924    For example, consider this pattern, where A, B, etc. are complex pattern
2925    fragments:
2926    .sp
2927      (A(*COMMIT)B(*THEN)C|D)
2928    .sp
2929    Once A has matched, PCRE is committed to this match, at the current starting
2930    position. If subsequently B matches, but C does not, the normal (*THEN) action
2931    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2932    overrides.
2933    .
2934    .
2935    .SH "SEE ALSO"
2936    .rs
2937    .sp
2938    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2939    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2940    .
2941    .
2942    .SH AUTHOR
2943    .rs
2944    .sp
2945    .nf
2946    Philip Hazel
2947    University Computing Service
2948    Cambridge CB2 3QH, England.
2949    .fi
2950    .
2951    .
2952    .SH REVISION
2953    .rs
2954    .sp
2955    .nf
2956    Last updated: 25 August 2012
2957    Copyright (c) 1997-2012 University of Cambridge.
2958    .fi

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