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revision 556 by ph10, Tue Oct 26 11:06:44 2010 UTC revision 575 by ph10, Sun Nov 21 12:55:42 2010 UTC
# Line 182  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  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
185  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
186  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
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  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.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 192  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  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
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  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 225  but when a pattern is being prepared by Line 229  but when a pattern is being prepared by
229  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
230  .sp  .sp
231    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
232    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
233    \ee        escape (hex 1B)    \ee        escape (hex 1B)
234    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
235    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 237  one of the following escape sequences th Line 241  one of the following escape sequences th
241  .sp  .sp
242  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
243  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.
244  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
245  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
246    than 127, a compile-time error occurs. This locks out non-ASCII characters in
247    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
248    values are valid. A lower case letter is converted to upper case, and then the
249    0xc0 bits are flipped.)
250  .P  .P
251  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
252  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 424  any Unicode letter, and underscore. Note Line 432  any Unicode letter, and underscore. Note
432  \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
433  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
434  .P  .P
435  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
436  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
437  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
438  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
439    are:
440  .sp  .sp
441    U+0009     Horizontal tab    U+0009     Horizontal tab
442    U+0020     Space    U+0020     Space
# Line 465  The vertical space characters are: Line 474  The vertical space characters are:
474  .rs  .rs
475  .sp  .sp
476  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
477  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:  
478  .sp  .sp
479    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
480  .sp  .sp
# Line 774  same characters as Xan, plus underscore. Line 782  same characters as Xan, plus underscore.
782  .SS "Resetting the match start"  .SS "Resetting the match start"
783  .rs  .rs
784  .sp  .sp
785  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
786  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:  
787  .sp  .sp
788    foo\eKbar    foo\eKbar
789  .sp  .sp
# Line 948  The handling of dot is entirely independ Line 955  The handling of dot is entirely independ
955  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
956  special meaning in a character class.  special meaning in a character class.
957  .P  .P
958  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
959  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
960  end of a line.  that signifies the end of a line.
961  .  .
962  .  .
963  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 959  end of a line. Line 966  end of a line.
966  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
967  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
968  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
969  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
970  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,
971  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
972  .P  .P
973  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 1045  characters in both cases. In UTF-8 mode, Line 1052  characters in both cases. In UTF-8 mode,
1052  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
1053  property support.  property support.
1054  .P  .P
1055  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,
1056  \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
1057  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1058  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
1059    and their upper case partners, just as it does when they appear outside a
1060    character class, as described in the section entitled
1061    .\" HTML <a href="#genericchartypes">
1062    .\" </a>
1063    "Generic character types"
1064    .\"
1065    above. The escape sequence \eb has a different meaning inside a character
1066    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1067    are not special inside a character class. Like any other unrecognized escape
1068    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1069    default, but cause an error if the PCRE_EXTRA option is set.
1070    .P
1071    A circumflex can conveniently be used with the upper case character types to
1072  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.
1073  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,
1074    whereas [\ew] includes underscore. A positive character class should be read as
1075    "something OR something OR ..." and a negative class as "NOT something AND NOT
1076    something AND NOT ...".
1077  .P  .P
1078  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1079  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 1173  extracts it into the global options (and Line 1196  extracts it into the global options (and
1196  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1197  .P  .P
1198  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1199  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
1200  .sp  .sp
1201    (a(?i)b)c    (a(?i)b)c
1202  .sp  .sp
# Line 1214  Turning part of a pattern into a subpatt Line 1237  Turning part of a pattern into a subpatt
1237  .sp  .sp
1238    cat(aract|erpillar|)    cat(aract|erpillar|)
1239  .sp  .sp
1240  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1241  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1242  .sp  .sp
1243  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
1244  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
1245  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
1246  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1247  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1248  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1249  .sp  .sp
1250    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1251  .sp  .sp
# Line 1272  at captured substring number one, whiche Line 1294  at captured substring number one, whiche
1294  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
1295  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1296  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
1297  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
1298  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1299  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.  
1300  .sp  .sp
1301    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1302    / ( 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 1402  items: Line 1423  items:
1423    the \eC escape sequence    the \eC escape sequence
1424    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1425    the \eR escape sequence    the \eR escape sequence
1426    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1427    a character class    a character class
1428    a back reference (see next section)    a back reference (see next section)
1429    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
# Line 1444  subpatterns that are referenced as Line 1465  subpatterns that are referenced as
1465  .\" </a>  .\" </a>
1466  subroutines  subroutines
1467  .\"  .\"
1468  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1469  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1470    .\" </a>
1471    "Defining subpatterns for use by reference only"
1472    .\"
1473    below). Items other than subpatterns that have a {0} quantifier are omitted
1474    from the compiled pattern.
1475  .P  .P
1476  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1477  abbreviations:  abbreviations:
# Line 1670  no such problem when named parentheses a Line 1696  no such problem when named parentheses a
1696  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1697  .P  .P
1698  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
1699  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
1700  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
1701  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1702  .sp  .sp
1703    (ring), \e1    (ring), \e1
1704    (ring), \eg1    (ring), \eg1
# Line 1686  example: Line 1712  example:
1712    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1713  .sp  .sp
1714  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
1715  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.
1716  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
1717  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
1718  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1719  .P  .P
1720  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1721  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1825  lookbehind assertion is needed to achiev Line 1851  lookbehind assertion is needed to achiev
1851  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
1852  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
1853  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.
1854  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 (?!).  
1855  .  .
1856  .  .
1857  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1851  is permitted, but Line 1876  is permitted, but
1876  .sp  .sp
1877  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1878  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
1879  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
1880  match the same length of string. An assertion such as  length of string. An assertion such as
1881  .sp  .sp
1882    (?<=ab(c|de))    (?<=ab(c|de))
1883  .sp  .sp
# Line 1862  branches: Line 1887  branches:
1887  .sp  .sp
1888    (?<=abc|abde)    (?<=abc|abde)
1889  .sp  .sp
1890  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1891  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1892  .\" </a>  .\" </a>
1893  (see above)  (see above)
# Line 1966  already been matched. The two possible f Line 1991  already been matched. The two possible f
1991  .sp  .sp
1992  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1993  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
1994  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1995    itself contain nested subpatterns of any form, including conditional
1996    subpatterns; the restriction to two alternatives applies only at the level of
1997    the condition. This pattern fragment is an example where the alternatives are
1998    complex:
1999    .sp
2000      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2001    .sp
2002  .P  .P
2003  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2004  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1983  matched. If there is more than one captu Line 2015  matched. If there is more than one captu
2015  .\" </a>  .\" </a>
2016  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2017  .\"  .\"
2018  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
2019  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
2020  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2021  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
2022  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
2023  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2024    zero in any of these forms is not used; it provokes a compile-time error.)
2025  .P  .P
2026  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2027  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 1999  three parts for ease of discussion: Line 2032  three parts for ease of discussion:
2032  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2033  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
2034  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
2035  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2036  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,
2037  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
2038  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2039  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 2056  The syntax for recursive patterns Line 2089  The syntax for recursive patterns
2089  .\"  .\"
2090  is described below.  is described below.
2091  .  .
2092    .\" HTML <a name="subdefine"></a>
2093  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2094  .rs  .rs
2095  .sp  .sp
# Line 2068  point in the pattern; the idea of DEFINE Line 2102  point in the pattern; the idea of DEFINE
2102  .\" </a>  .\" </a>
2103  "subroutines"  "subroutines"
2104  .\"  .\"
2105  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
2106  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2107    breaks):
2108  .sp  .sp
2109    (?(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) )
2110    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2104  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2139  dd-aaa-dd or dd-dd-dd, where aaa are let
2139  .SH COMMENTS  .SH COMMENTS
2140  .rs  .rs
2141  .sp  .sp
2142  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
2143  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,
2144  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
2145    subpattern name or number. The characters that make up a comment play no part
2146    in the pattern matching.
2147  .P  .P
2148  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
2149  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2150  next newline character or character sequence in the pattern. Which characters  option is set, an unescaped # character also introduces a comment, which in
2151  are interpreted as newlines is controlled by the options passed to  this case continues to immediately after the next newline character or
2152  \fBpcre_compile()\fP or by a special sequence at the start of the pattern, as  character sequence in the pattern. Which characters are interpreted as newlines
2153  described in the section entitled  is controlled by the options passed to \fBpcre_compile()\fP or by a special
2154  .\" HTML <a href="#recursion">  sequence at the start of the pattern, as described in the section entitled
2155    .\" HTML <a href="#newlines">
2156  .\" </a>  .\" </a>
2157  "Newline conventions"  "Newline conventions"
2158  .\"  .\"
2159  above. Note that end of a comment is a literal newline sequence in the pattern;  above. Note that the end of this type of comment is a literal newline sequence
2160  escape sequences that happen to represent a newline do not terminate a comment.  in the pattern; escape sequences that happen to represent a newline do not
2161  For example, consider this pattern when PCRE_EXTENDED is set, and the default  count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2162  newline convention is in force:  default newline convention is in force:
2163  .sp  .sp
2164    abc #comment \en still comment    abc #comment \en still comment
2165  .sp  .sp
2166  On encountering the # character, \fBpcre_compile()\fP skips along, looking for  On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2167  a newline in the pattern. The sequence \en is still literal at this stage, so  a newline in the pattern. The sequence \en is still literal at this stage, so
2168  it does not terminate the comment. Only an actual character with the code value  it does not terminate the comment. Only an actual character with the code value
2169  0x0a does so.  0x0a (the default newline) does so.
2170  .  .
2171  .  .
2172  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2186  We have put the pattern into parentheses Line 2224  We have put the pattern into parentheses
2224  them instead of the whole pattern.  them instead of the whole pattern.
2225  .P  .P
2226  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
2227  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
2228  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
2229  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2230  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.  
2231  .P  .P
2232  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2233  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
# Line 2293  time we do have another alternative to t Line 2330  time we do have another alternative to t
2330  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2331  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2332  .P  .P
2333  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
2334  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
2335    this:
2336  .sp  .sp
2337    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2338  .sp  .sp
# Line 2704  Cambridge CB2 3QH, England. Line 2742  Cambridge CB2 3QH, England.
2742  .rs  .rs
2743  .sp  .sp
2744  .nf  .nf
2745  Last updated: 26 October 2010  Last updated: 21 November 2010
2746  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2747  .fi  .fi

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