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revision 555 by ph10, Tue Oct 26 08:26:20 2010 UTC revision 637 by ph10, Sun Jul 24 17:44:12 2011 UTC
# Line 52  such as \ed and \ew to use Unicode prope Line 52  such as \ed and \ew to use Unicode prope
52  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
53  table.  table.
54  .P  .P
55    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
61  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 66  discussed in the Line 71  discussed in the
71  page.  page.
72  .  .
73  .  .
74    .\" HTML <a name="newlines"></a>
75  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
76  .rs  .rs
77  .sp  .sp
# Line 181  The following sections describe the use Line 187  The following sections describe the use
187  .rs  .rs
188  .sp  .sp
189  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
190  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
191  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
192  outside character classes.  both inside and outside character classes.
193  .P  .P
194  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.
195  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 191  otherwise be interpreted as a metacharac Line 197  otherwise be interpreted as a metacharac
197  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
198  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
199  .P  .P
200    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201    backslash. All other characters (in particular, those whose codepoints are
202    greater than 127) are treated as literals.
203    .P
204  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
205  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
206  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 210  Perl, $ and @ cause variable interpolati Line 220  Perl, $ and @ cause variable interpolati
220    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
221  .sp  .sp
222  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
223  An isolated \eE that is not preceded by \eQ is ignored.  An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224    by \eE later in the pattern, the literal interpretation continues to the end of
225    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226    a character class, this causes an error, because the character class is not
227    terminated.
228  .  .
229  .  .
230  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 224  but when a pattern is being prepared by Line 238  but when a pattern is being prepared by
238  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
239  .sp  .sp
240    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
241    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
242    \ee        escape (hex 1B)    \ee        escape (hex 1B)
243    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
244    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 236  one of the following escape sequences th Line 250  one of the following escape sequences th
250  .sp  .sp
251  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
252  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
253  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
254  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255    than 127, a compile-time error occurs. This locks out non-ASCII characters in
256    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257    values are valid. A lower case letter is converted to upper case, and then the
258    0xc0 bits are flipped.)
259  .P  .P
260  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
261  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
# Line 423  any Unicode letter, and underscore. Note Line 441  any Unicode letter, and underscore. Note
441  \eB because they are defined in terms of \ew and \eW. Matching these sequences  \eB because they are defined in terms of \ew and \eW. Matching these sequences
442  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
443  .P  .P
444  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
445  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
446  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is  characters by default, these always match certain high-valued codepoints in
447  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448    are:
449  .sp  .sp
450    U+0009     Horizontal tab    U+0009     Horizontal tab
451    U+0020     Space    U+0020     Space
# Line 464  The vertical space characters are: Line 483  The vertical space characters are:
483  .rs  .rs
484  .sp  .sp
485  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
486  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
487  .sp  .sp
488    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489  .sp  .sp
# Line 739  Characters with the "mark" property are Line 757  Characters with the "mark" property are
757  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
758  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
759  .P  .P
760    Note that recent versions of Perl have changed \eX to match what Unicode calls
761    an "extended grapheme cluster", which has a more complicated definition.
762    .P
763  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
764  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
765  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
# Line 773  same characters as Xan, plus underscore. Line 794  same characters as Xan, plus underscore.
794  .SS "Resetting the match start"  .SS "Resetting the match start"
795  .rs  .rs
796  .sp  .sp
797  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
798  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
799  .sp  .sp
800    foo\eKbar    foo\eKbar
801  .sp  .sp
# Line 947  The handling of dot is entirely independ Line 967  The handling of dot is entirely independ
967  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
968  special meaning in a character class.  special meaning in a character class.
969  .P  .P
970  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not  The escape sequence \eN behaves like a dot, except that it is not affected by
971  set. In other words, it matches any one character except one that signifies the  the PCRE_DOTALL option. In other words, it matches any character except one
972  end of a line.  that signifies the end of a line.
973  .  .
974  .  .
975  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 958  end of a line. Line 978  end of a line.
978  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
979  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
980  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
981  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
982  what remains in the string may be a malformed UTF-8 string. For this reason,  rest of the string may start with a malformed UTF-8 character. For this reason,
983  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
984  .P  .P
985  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 1044  characters in both cases. In UTF-8 mode, Line 1064  characters in both cases. In UTF-8 mode,
1064  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
1065  property support.  property support.
1066  .P  .P
1067  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1068  \eW may also appear in a character class, and add the characters that they  \eV, \ew, and \eW may appear in a character class, and add the characters that
1069  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1070  circumflex can conveniently be used with the upper case character types to  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1071    and their upper case partners, just as it does when they appear outside a
1072    character class, as described in the section entitled
1073    .\" HTML <a href="#genericchartypes">
1074    .\" </a>
1075    "Generic character types"
1076    .\"
1077    above. The escape sequence \eb has a different meaning inside a character
1078    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1079    are not special inside a character class. Like any other unrecognized escape
1080    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1081    default, but cause an error if the PCRE_EXTRA option is set.
1082    .P
1083    A circumflex can conveniently be used with the upper case character types to
1084  specify a more restricted set of characters than the matching lower case type.  specify a more restricted set of characters than the matching lower case type.
1085  For example, the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore,
1086    whereas [\ew] includes underscore. A positive character class should be read as
1087    "something OR something OR ..." and a negative class as "NOT something AND NOT
1088    something AND NOT ...".
1089  .P  .P
1090  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1091  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 1172  extracts it into the global options (and Line 1208  extracts it into the global options (and
1208  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1209  .P  .P
1210  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1211  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1212  .sp  .sp
1213    (a(?i)b)c    (a(?i)b)c
1214  .sp  .sp
# Line 1213  Turning part of a pattern into a subpatt Line 1249  Turning part of a pattern into a subpatt
1249  .sp  .sp
1250    cat(aract|erpillar|)    cat(aract|erpillar|)
1251  .sp  .sp
1252  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1253  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1254  .sp  .sp
1255  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
1256  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
1257  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
1258  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1259  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1260  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1261  .sp  .sp
1262    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1263  .sp  .sp
# Line 1271  at captured substring number one, whiche Line 1306  at captured substring number one, whiche
1306  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1307  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1308  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1309  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1310  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1311  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1312  .sp  .sp
1313    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1314    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# Line 1401  items: Line 1435  items:
1435    the \eC escape sequence    the \eC escape sequence
1436    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1437    the \eR escape sequence    the \eR escape sequence
1438    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1439    a character class    a character class
1440    a back reference (see next section)    a back reference (see next section)
1441    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1442    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1443  .sp  .sp
1444  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
# Line 1443  subpatterns that are referenced as Line 1477  subpatterns that are referenced as
1477  .\" </a>  .\" </a>
1478  subroutines  subroutines
1479  .\"  .\"
1480  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1481  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1482    .\" </a>
1483    "Defining subpatterns for use by reference only"
1484    .\"
1485    below). Items other than subpatterns that have a {0} quantifier are omitted
1486    from the compiled pattern.
1487  .P  .P
1488  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1489  abbreviations:  abbreviations:
# Line 1669  no such problem when named parentheses a Line 1708  no such problem when named parentheses a
1708  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1709  .P  .P
1710  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1711  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1712  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1713  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1714  .sp  .sp
1715    (ring), \e1    (ring), \e1
1716    (ring), \eg1    (ring), \eg1
# Line 1685  example: Line 1724  example:
1724    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1725  .sp  .sp
1726  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1727  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1728  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1729  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1730  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1731  .P  .P
1732  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1733  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1790  those that look ahead of the current pos Line 1829  those that look ahead of the current pos
1829  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,
1830  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.
1831  .P  .P
1832  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1833  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
1834  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1835  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1836  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1837  because it does not make sense for negative assertions.  .P
1838    For compatibility with Perl, assertion subpatterns may be repeated, even though
1839    it makes no sense to assert the same thing several times. In practice, there
1840    only three cases:
1841    .sp
1842    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1843    However, it may contain internal capturing parenthesized groups that are called
1844    from elsewhere via the
1845    .\" HTML <a href="#subpatternsassubroutines">
1846    .\" </a>
1847    subroutine mechanism.
1848    .\"
1849    .sp
1850    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1851    were {0,1}. At run time, the rest of the pattern match is tried with and
1852    without the assertion, the order depending on the greediness of the quantifier.
1853    .sp
1854    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1855    The assertion is obeyed just once when encountered during matching.
1856  .  .
1857  .  .
1858  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1824  lookbehind assertion is needed to achiev Line 1881  lookbehind assertion is needed to achiev
1881  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
1882  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
1883  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.
1884  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1885  .  .
1886  .  .
1887  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1850  is permitted, but Line 1906  is permitted, but
1906  .sp  .sp
1907  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1908  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
1909  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1910  match the same length of string. An assertion such as  length of string. An assertion such as
1911  .sp  .sp
1912    (?<=ab(c|de))    (?<=ab(c|de))
1913  .sp  .sp
# Line 1861  branches: Line 1917  branches:
1917  .sp  .sp
1918    (?<=abc|abde)    (?<=abc|abde)
1919  .sp  .sp
1920  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1921  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1922  .\" </a>  .\" </a>
1923  (see above)  (see above)
# Line 1965  already been matched. The two possible f Line 2021  already been matched. The two possible f
2021  .sp  .sp
2022  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2023  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
2024  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2025    itself contain nested subpatterns of any form, including conditional
2026    subpatterns; the restriction to two alternatives applies only at the level of
2027    the condition. This pattern fragment is an example where the alternatives are
2028    complex:
2029    .sp
2030      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2031    .sp
2032  .P  .P
2033  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2034  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1982  matched. If there is more than one captu Line 2045  matched. If there is more than one captu
2045  .\" </a>  .\" </a>
2046  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2047  .\"  .\"
2048  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2049  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2050  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2051  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2052  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2053  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2054    zero in any of these forms is not used; it provokes a compile-time error.)
2055  .P  .P
2056  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2057  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1998  three parts for ease of discussion: Line 2062  three parts for ease of discussion:
2062  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2063  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
2064  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
2065  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2066  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,
2067  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
2068  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2069  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 2055  The syntax for recursive patterns Line 2119  The syntax for recursive patterns
2119  .\"  .\"
2120  is described below.  is described below.
2121  .  .
2122    .\" HTML <a name="subdefine"></a>
2123  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2124  .rs  .rs
2125  .sp  .sp
# Line 2067  point in the pattern; the idea of DEFINE Line 2132  point in the pattern; the idea of DEFINE
2132  .\" </a>  .\" </a>
2133  "subroutines"  "subroutines"
2134  .\"  .\"
2135  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2136  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2137    breaks):
2138  .sp  .sp
2139    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2140    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2103  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2169  dd-aaa-dd or dd-dd-dd, where aaa are let
2169  .SH COMMENTS  .SH COMMENTS
2170  .rs  .rs
2171  .sp  .sp
2172  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
2173  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,
2174  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
2175    subpattern name or number. The characters that make up a comment play no part
2176    in the pattern matching.
2177  .P  .P
2178  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
2179  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2180  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2181    this case continues to immediately after the next newline character or
2182    character sequence in the pattern. Which characters are interpreted as newlines
2183    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2184    sequence at the start of the pattern, as described in the section entitled
2185    .\" HTML <a href="#newlines">
2186    .\" </a>
2187    "Newline conventions"
2188    .\"
2189    above. Note that the end of this type of comment is a literal newline sequence
2190    in the pattern; escape sequences that happen to represent a newline do not
2191    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2192    default newline convention is in force:
2193    .sp
2194      abc #comment \en still comment
2195    .sp
2196    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2197    a newline in the pattern. The sequence \en is still literal at this stage, so
2198    it does not terminate the comment. Only an actual character with the code value
2199    0x0a (the default newline) does so.
2200  .  .
2201  .  .
2202  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2167  We have put the pattern into parentheses Line 2254  We have put the pattern into parentheses
2254  them instead of the whole pattern.  them instead of the whole pattern.
2255  .P  .P
2256  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2257  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2258  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2259  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2260  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2261  .P  .P
2262  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2263  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
# Line 2274  time we do have another alternative to t Line 2360  time we do have another alternative to t
2360  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2361  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2362  .P  .P
2363  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2364  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2365    this:
2366  .sp  .sp
2367    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2368  .sp  .sp
# Line 2423  failing negative assertion, they cause a Line 2510  failing negative assertion, they cause a
2510  .P  .P
2511  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or subroutine subpattern
2512  (including recursive subpatterns), their effect is confined to that subpattern;  (including recursive subpatterns), their effect is confined to that subpattern;
2513  it does not extend to the surrounding pattern. Note that such subpatterns are  it does not extend to the surrounding pattern, with one exception: a *MARK that
2514  processed as anchored at the point where they are tested.  is encountered in a positive assertion \fIis\fP passed back (compare capturing
2515    parentheses in assertions). Note that such subpatterns are processed as
2516    anchored at the point where they are tested.
2517  .P  .P
2518  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2519  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
# Line 2440  minimum length of matching subject, or t Line 2529  minimum length of matching subject, or t
2529  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations suppresses the running of a match, any
2530  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2531  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2532  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2533    pattern with (*NO_START_OPT).
2534  .  .
2535  .  .
2536  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2514  indicates which of the two alternatives Line 2604  indicates which of the two alternatives
2604  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2605  capturing parentheses.  capturing parentheses.
2606  .P  .P
2607    If (*MARK) is encountered in a positive assertion, its name is recorded and
2608    passed back if it is the last-encountered. This does not happen for negative
2609    assetions.
2610    .P
2611  A name may also be returned after a failed match if the final path through the  A name may also be returned after a failed match if the final path through the
2612  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2613  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
# Line 2631  matching name is found, normal "bumpalon Line 2725  matching name is found, normal "bumpalon
2725  .sp  .sp
2726    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2727  .sp  .sp
2728  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next alternation in the innermost enclosing
2729  group if the rest of the pattern does not match. That is, it cancels pending  group if the rest of the pattern does not match. That is, it cancels pending
2730  backtracking, but only within the current alternation. Its name comes from the  backtracking, but only within the current alternation. Its name comes from the
2731  observation that it can be used for a pattern-based if-then-else block:  observation that it can be used for a pattern-based if-then-else block:
# Line 2646  overall match fails. If (*THEN) is not d Line 2740  overall match fails. If (*THEN) is not d
2740  like (*PRUNE).  like (*PRUNE).
2741  .  .
2742  .P  .P
2743  The above verbs provide four different "strengths" of control when subsequent  The above verbs provide four different "strengths" of control when subsequent
2744  matching fails. (*THEN) is the weakest, carrying on the match at the next  matching fails. (*THEN) is the weakest, carrying on the match at the next
2745  alternation. (*PRUNE) comes next, failing the match at the current starting  alternation. (*PRUNE) comes next, failing the match at the current starting
2746  position, but allowing an advance to the next character (for an unanchored  position, but allowing an advance to the next character (for an unanchored
# Line 2685  Cambridge CB2 3QH, England. Line 2779  Cambridge CB2 3QH, England.
2779  .rs  .rs
2780  .sp  .sp
2781  .nf  .nf
2782  Last updated: 26 October 2010  Last updated: 24 July 2011
2783  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2784  .fi  .fi

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