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revision 550 by ph10, Sun Oct 10 16:24:11 2010 UTC revision 628 by ph10, Wed Jul 20 18:03:20 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. 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 223  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 235  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 422  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 463  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 738  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 772  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 946  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 957  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 1043  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 1171  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 1212  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 1270  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 1400  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 (unless it is an assertion)
# Line 1442  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 1668  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 1684  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 1823  lookbehind assertion is needed to achiev Line 1863  lookbehind assertion is needed to achiev
1863  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1864  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1865  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1866  The 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 (?!).  
1867  .  .
1868  .  .
1869  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1849  is permitted, but Line 1888  is permitted, but
1888  .sp  .sp
1889  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1890  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1891  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1892  match the same length of string. An assertion such as  length of string. An assertion such as
1893  .sp  .sp
1894    (?<=ab(c|de))    (?<=ab(c|de))
1895  .sp  .sp
# Line 1860  branches: Line 1899  branches:
1899  .sp  .sp
1900    (?<=abc|abde)    (?<=abc|abde)
1901  .sp  .sp
1902  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1903  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1904  .\" </a>  .\" </a>
1905  (see above)  (see above)
# Line 1964  already been matched. The two possible f Line 2003  already been matched. The two possible f
2003  .sp  .sp
2004  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2005  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2006  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2007    itself contain nested subpatterns of any form, including conditional
2008    subpatterns; the restriction to two alternatives applies only at the level of
2009    the condition. This pattern fragment is an example where the alternatives are
2010    complex:
2011    .sp
2012      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2013    .sp
2014  .P  .P
2015  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2016  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1981  matched. If there is more than one captu Line 2027  matched. If there is more than one captu
2027  .\" </a>  .\" </a>
2028  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2029  .\"  .\"
2030  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
2031  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
2032  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2033  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2034  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
2035  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2036    zero in any of these forms is not used; it provokes a compile-time error.)
2037  .P  .P
2038  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2039  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1997  three parts for ease of discussion: Line 2044  three parts for ease of discussion:
2044  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2045  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2046  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2047  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2048  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2049  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2050  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2051  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 2054  The syntax for recursive patterns Line 2101  The syntax for recursive patterns
2101  .\"  .\"
2102  is described below.  is described below.
2103  .  .
2104    .\" HTML <a name="subdefine"></a>
2105  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2106  .rs  .rs
2107  .sp  .sp
# Line 2066  point in the pattern; the idea of DEFINE Line 2114  point in the pattern; the idea of DEFINE
2114  .\" </a>  .\" </a>
2115  "subroutines"  "subroutines"
2116  .\"  .\"
2117  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
2118  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2119    breaks):
2120  .sp  .sp
2121    (?(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) )
2122    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2102  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2151  dd-aaa-dd or dd-dd-dd, where aaa are let
2151  .SH COMMENTS  .SH COMMENTS
2152  .rs  .rs
2153  .sp  .sp
2154  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
2155  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2156  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2157    subpattern name or number. The characters that make up a comment play no part
2158    in the pattern matching.
2159  .P  .P
2160  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
2161  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2162  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2163    this case continues to immediately after the next newline character or
2164    character sequence in the pattern. Which characters are interpreted as newlines
2165    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2166    sequence at the start of the pattern, as described in the section entitled
2167    .\" HTML <a href="#newlines">
2168    .\" </a>
2169    "Newline conventions"
2170    .\"
2171    above. Note that the end of this type of comment is a literal newline sequence
2172    in the pattern; escape sequences that happen to represent a newline do not
2173    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2174    default newline convention is in force:
2175    .sp
2176      abc #comment \en still comment
2177    .sp
2178    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2179    a newline in the pattern. The sequence \en is still literal at this stage, so
2180    it does not terminate the comment. Only an actual character with the code value
2181    0x0a (the default newline) does so.
2182  .  .
2183  .  .
2184  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2166  We have put the pattern into parentheses Line 2236  We have put the pattern into parentheses
2236  them instead of the whole pattern.  them instead of the whole pattern.
2237  .P  .P
2238  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
2239  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
2240  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
2241  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2242  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.  
2243  .P  .P
2244  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2245  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
# Line 2273  time we do have another alternative to t Line 2342  time we do have another alternative to t
2342  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2343  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2344  .P  .P
2345  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
2346  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
2347    this:
2348  .sp  .sp
2349    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2350  .sp  .sp
# Line 2439  minimum length of matching subject, or t Line 2509  minimum length of matching subject, or t
2509  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
2510  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2511  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2512  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2513    pattern with (*NO_START_OPT).
2514  .  .
2515  .  .
2516  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2630  matching name is found, normal "bumpalon Line 2701  matching name is found, normal "bumpalon
2701  .sp  .sp
2702    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2703  .sp  .sp
2704  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
2705  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
2706  backtracking, but only within the current alternation. Its name comes from the  backtracking, but only within the current alternation. Its name comes from the
2707  observation that it can be used for a pattern-based if-then-else block:  observation that it can be used for a pattern-based if-then-else block:
# Line 2644  behaviour of (*THEN:NAME) is exactly the Line 2715  behaviour of (*THEN:NAME) is exactly the
2715  overall match fails. If (*THEN) is not directly inside an alternation, it acts  overall match fails. If (*THEN) is not directly inside an alternation, it acts
2716  like (*PRUNE).  like (*PRUNE).
2717  .  .
2718    .P
2719    The above verbs provide four different "strengths" of control when subsequent
2720    matching fails. (*THEN) is the weakest, carrying on the match at the next
2721    alternation. (*PRUNE) comes next, failing the match at the current starting
2722    position, but allowing an advance to the next character (for an unanchored
2723    pattern). (*SKIP) is similar, except that the advance may be more than one
2724    character. (*COMMIT) is the strongest, causing the entire match to fail.
2725    .P
2726    If more than one is present in a pattern, the "stongest" one wins. For example,
2727    consider this pattern, where A, B, etc. are complex pattern fragments:
2728    .sp
2729      (A(*COMMIT)B(*THEN)C|D)
2730    .sp
2731    Once A has matched, PCRE is committed to this match, at the current starting
2732    position. If subsequently B matches, but C does not, the normal (*THEN) action
2733    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2734    overrides.
2735    .
2736  .  .
2737  .SH "SEE ALSO"  .SH "SEE ALSO"
2738  .rs  .rs
# Line 2666  Cambridge CB2 3QH, England. Line 2755  Cambridge CB2 3QH, England.
2755  .rs  .rs
2756  .sp  .sp
2757  .nf  .nf
2758  Last updated: 10 October 2010  Last updated: 20 July 2011
2759  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2760  .fi  .fi

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