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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 488 by ph10, Mon Jan 11 15:29:42 2010 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 83  string with one of the following five se Line 83  string with one of the following five se
83    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
84    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
85  .sp  .sp
86  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
87  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88  newline sequence, the pattern  newline sequence, the pattern
89  .sp  .sp
# Line 217  one of the following escape sequences th Line 217  one of the following escape sequences th
217    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
221    \exhh      character with hex code hh    \exhh      character with hex code hh
222    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
223  .sp  .sp
# Line 333  syntax for referencing a subpattern as a Line 333  syntax for referencing a subpattern as a
333  later.  later.
334  .\"  .\"
335  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
337  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
338  .\" </a>  .\" </a>
339  subroutine  subroutine
# Line 468  one of the following sequences: Line 468  one of the following sequences:
468    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
469    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
470  .sp  .sp
471  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
472  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
473  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
# Line 741  different meaning, namely the backspace Line 741  different meaning, namely the backspace
741  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
742  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
743  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
744  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745  separte "start of word" or "end of word" metasequence. However, whatever  separte "start of word" or "end of word" metasequence. However, whatever
746  follows \eb normally determines which it is. For example, the fragment  follows \eb normally determines which it is. For example, the fragment
747  \eba matches "a" at the start of a word.  \eba matches "a" at the start of a word.
748  .P  .P
749  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
# Line 876  the lookbehind. Line 876  the lookbehind.
876  .rs  .rs
877  .sp  .sp
878  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
879  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
880  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
881  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
882  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
883  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
# Line 1163  stored. Line 1163  stored.
1163    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1165  .sp  .sp
1166  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1167  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1168  "defdef":  or "defdef":
1169  .sp  .sp
1170    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1171  .sp  .sp
1172  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173  refers to the first one in the pattern with the given number. The following  refers to the first one in the pattern with the given number. The following
1174  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1175  .sp  .sp
1176    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1177  .sp  .sp
1178    If a
1179    .\" HTML <a href="#conditions">
1180    .\" </a>
1181    condition test
1182    .\"
1183    for a subpattern's having matched refers to a non-unique number, the test is
1184    true if any of the subpatterns of that number have matched.
1185  .P  .P
1186  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1187  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1188  .  .
1189  .  .
# Line 1189  if an expression is modified, the number Line 1196  if an expression is modified, the number
1196  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1197  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1198  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1199  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1200    have different names, but PCRE does not.
1201  .P  .P
1202  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1203  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1204  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1205  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1206  .\" </a>  .\" </a>
1207  backreferences,  back references,
1208  .\"  .\"
1209  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1210  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1224  extracting the name-to-number translatio
1224  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1225  .P  .P
1226  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1227  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1228  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1229  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1230  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1231  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1232    name, and in both cases you want to extract the abbreviation. This pattern
1233    (ignoring the line breaks) does the job:
1234  .sp  .sp
1235    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1236    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1244  subpattern, as described in the previous
1244  .P  .P
1245  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1246  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1247  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1248  make a reference to a non-unique named subpattern from elsewhere in the  .P
1249  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1250  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1251    used. In the absence of duplicate numbers (see the previous section) this is
1252    the one with the lowest number. If you use a named reference in a condition
1253    test (see the
1254    .\"
1255    .\" HTML <a href="#conditions">
1256    .\" </a>
1257    section about conditions
1258    .\"
1259    below), either to check whether a subpattern has matched, or to check for
1260    recursion, all subpatterns with the same name are tested. If the condition is
1261    true for any one of them, the overall condition is true. This is the same
1262    behaviour as testing by number. For further details of the interfaces for
1263    handling named subpatterns, see the
1264  .\" HREF  .\" HREF
1265  \fBpcreapi\fP  \fBpcreapi\fP
1266  .\"  .\"
1267  documentation.  documentation.
1268  .P  .P
1269  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1270  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1271  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1272    are given to subpatterns with the same number. However, you can give the same
1273    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1274  .  .
1275  .  .
1276  .SH REPETITION  .SH REPETITION
# Line 1263  items: Line 1288  items:
1288    a character class    a character class
1289    a back reference (see next section)    a back reference (see next section)
1290    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1291    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1292  .sp  .sp
1293  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1294  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1399  worth setting PCRE_DOTALL in order to ob
1399  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1400  .P  .P
1401  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1402  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1403  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1404  succeeds. Consider, for example:  succeeds. Consider, for example:
1405  .sp  .sp
# Line 1589  references to it always fail by default. Line 1614  references to it always fail by default.
1614  .sp  .sp
1615    (a|(bc))\e2    (a|(bc))\e2
1616  .sp  .sp
1617  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1618  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1619  unset value matches an empty string.  unset value matches an empty string.
1620  .P  .P
1621  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
# Line 1603  whitespace. Otherwise, the \eg{ syntax o Line 1628  whitespace. Otherwise, the \eg{ syntax o
1628  "Comments"  "Comments"
1629  .\"  .\"
1630  below) can be used.  below) can be used.
1631  .P  .
1632    .SS "Recursive back references"
1633    .rs
1634    .sp
1635  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1636  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1637  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1617  to the previous iteration. In order for Line 1645  to the previous iteration. In order for
1645  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1646  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1647  minimum of zero.  minimum of zero.
1648    .P
1649    Back references of this type cause the group that they reference to be treated
1650    as an
1651    .\" HTML <a href="#atomicgroup">
1652    .\" </a>
1653    atomic group.
1654    .\"
1655    Once the whole group has been matched, a subsequent matching failure cannot
1656    cause backtracking into the middle of the group.
1657  .  .
1658  .  .
1659  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1712  In some cases, the Perl 5.10 escape sequ Line 1749  In some cases, the Perl 5.10 escape sequ
1749  .\" </a>  .\" </a>
1750  (see above)  (see above)
1751  .\"  .\"
1752  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
1753  restriction.  restriction.
1754  .P  .P
1755  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1730  different numbers of bytes, are also not Line 1767  different numbers of bytes, are also not
1767  "Subroutine"  "Subroutine"
1768  .\"  .\"
1769  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1770  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1771  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1772  .\" </a>  .\" </a>
1773  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 1840  characters that are not "999".
1840  .sp  .sp
1841  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1842  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1843  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1844  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1845  .sp  .sp
1846    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1821  recursion, a pseudo-condition called DEF Line 1858  recursion, a pseudo-condition called DEF
1858  .sp  .sp
1859  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1860  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1861  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
1862  (see the earlier  (see the earlier
1863  .\"  .\"
1864  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1865  .\" </a>  .\" </a>
# Line 1874  Rewriting the above example to use a nam Line 1911  Rewriting the above example to use a nam
1911  .sp  .sp
1912    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1913  .sp  .sp
1914    If the name used in a condition of this kind is a duplicate, the test is
1915    applied to all subpatterns of the same name, and is true if any one of them has
1916    matched.
1917  .  .
1918  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
1919  .rs  .rs
# Line 1887  letter R, for example: Line 1927  letter R, for example:
1927  .sp  .sp
1928  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
1929  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
1930  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
1931    applied to all subpatterns of the same name, and is true if any one of them is
1932    the most recent recursion.
1933  .P  .P
1934  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
1935  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1936  .\" </a>  .\" </a>
1937  Recursive patterns  The syntax for recursive patterns
1938  .\"  .\"
1939  are described below.  is described below.
1940  .  .
1941  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1942  .rs  .rs
# Line 1903  If the condition is the string (DEFINE), Line 1945  If the condition is the string (DEFINE),
1945  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
1946  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1947  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
1948  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
1949  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
1950  .\" </a>  .\" </a>
1951  "subroutines"  "subroutines"
# Line 1980  this kind of recursion was subsequently Line 2022  this kind of recursion was subsequently
2022  .P  .P
2023  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2024  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2025  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2026  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2027  .\" </a>  .\" </a>
2028  "subroutine"  "subroutine"
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2038  PCRE_EXTENDED option is set so that whit
2038  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2039  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2040  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2041  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2042  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2043  .P  .P
2044  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 2044  the match runs for a very long time inde Line 2086  the match runs for a very long time inde
2086  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2087  before failure can be reported.  before failure can be reported.
2088  .P  .P
2089  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2090  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2091  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2092  .\" HREF  .\" HREF
2093  \fBpcrecallout\fP  \fBpcrecallout\fP
2094  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2096  documentation). If the pattern above is
2096  .sp  .sp
2097    (ab(cd)ef)    (ab(cd)ef)
2098  .sp  .sp
2099  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2100  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2101  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2102    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2103       ^                        ^  .P
2104       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2105  .sp  obtain extra memory to store data during a recursion, which it does by using
2106  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2107  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2108  .P  .P
2109  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2110  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2087  is the actual recursive call. Line 2125  is the actual recursive call.
2125  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2126  treated as an atomic group. That is, once it has matched some of the subject  treated as an atomic group. That is, once it has matched some of the subject
2127  string, it is never re-entered, even if it contains untried alternatives and  string, it is never re-entered, even if it contains untried alternatives and
2128  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2129  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2130  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2131  .sp  .sp
2132    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2133  .sp  .sp
2134  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2135  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2136  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2137  subject string "abcba":  subject string "abcba":
2138  .P  .P
2139  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2140  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2141  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2142  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2143  tests are not part of the recursion).  tests are not part of the recursion).
2144  .P  .P
2145  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2146  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2147  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2148  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2149  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2113  alternatives in the other order, things Line 2151  alternatives in the other order, things
2151  .sp  .sp
2152    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2153  .sp  .sp
2154  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2155  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2156  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2157  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2158  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2159  .P  .P
2160  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that matches all palindromic strings, not just those
2161  with an odd number of characters, it is tempting to change the pattern to this:  with an odd number of characters, it is tempting to change the pattern to this:
2162  .sp  .sp
2163    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2164  .sp  .sp
2165  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2166  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2167  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2168  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2169  .sp  .sp
2170    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2171  .sp  .sp
2172  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2173  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2174  .sp  .sp
2175    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2176  .sp  .sp
2177  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2178  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2179  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2180  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2181  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2182  gone into a loop.  gone into a loop.
# Line 2177  matches "sense and sensibility" and "res Line 2215  matches "sense and sensibility" and "res
2215  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2216  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2217  .P  .P
2218  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2219  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2220  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2221  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2222    call revert to their previous values afterwards.
2223  .P  .P
2224  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2225  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 2264  a backtracking algorithm. With the excep Line 2303  a backtracking algorithm. With the excep
2303  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2304  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2305  .P  .P
2306  If any of these verbs are used in an assertion subpattern, their effect is  If any of these verbs are used in an assertion or subroutine subpattern
2307  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2308  Note that assertion subpatterns are processed as anchored at the point where  it does not extend to the surrounding pattern. Note that such subpatterns are
2309  they are tested.  processed as anchored at the point where they are tested.
2310  .P  .P
2311  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
2312  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. In Perl, they are generally of the form
# Line 2289  captured. (This feature was added to PCR Line 2328  captured. (This feature was added to PCR
2328  .sp  .sp
2329    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2330  .sp  .sp
2331  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2332  the outer parentheses.  the outer parentheses.
2333  .sp  .sp
2334    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2370  is used outside of any alternation, it a Line 2409  is used outside of any alternation, it a
2409  .SH "SEE ALSO"  .SH "SEE ALSO"
2410  .rs  .rs
2411  .sp  .sp
2412  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2413  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2414  .  .
2415  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2427  Cambridge CB2 3QH, England.
2427  .rs  .rs
2428  .sp  .sp
2429  .nf  .nf
2430  Last updated: 30 September 2009  Last updated: 11 January 2010
2431  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2432  .fi  .fi

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