/[pcre]/code/trunk/doc/pcre.txt
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revision 226 by ph10, Thu Aug 9 09:52:43 2007 UTC revision 227 by ph10, Tue Aug 21 15:00:15 2007 UTC
# Line 908  NEWLINES Line 908  NEWLINES
908         dard. When PCRE is run, the default can be overridden,  either  when  a         dard. When PCRE is run, the default can be overridden,  either  when  a
909         pattern is compiled, or when it is matched.         pattern is compiled, or when it is matched.
910    
911           At compile time, the newline convention can be specified by the options
912           argument of pcre_compile(), or it can be specified by special  text  at
913           the start of the pattern itself; this overrides any other settings. See
914           the pcrepattern page for details of the special character sequences.
915    
916         In the PCRE documentation the word "newline" is used to mean "the char-         In the PCRE documentation the word "newline" is used to mean "the char-
917         acter or pair of characters that indicate a line break". The choice  of         acter  or pair of characters that indicate a line break". The choice of
918         newline  convention  affects  the  handling of the dot, circumflex, and         newline convention affects the handling of  the  dot,  circumflex,  and
919         dollar metacharacters, the handling of #-comments in /x mode, and, when         dollar metacharacters, the handling of #-comments in /x mode, and, when
920         CRLF  is a recognized line ending sequence, the match position advance-         CRLF is a recognized line ending sequence, the match position  advance-
921         ment for a non-anchored pattern. The choice of newline convention  does         ment for a non-anchored pattern. There is more detail about this in the
922         not affect the interpretation of the \n or \r escape sequences.         section on pcre_exec() options below. The choice of newline  convention
923           does not affect the interpretation of the \n or \r escape sequences.
924    
925    
926  MULTITHREADING  MULTITHREADING
# Line 1522  INFORMATION ABOUT A PATTERN Line 1528  INFORMATION ABOUT A PATTERN
1528         returned.  The fourth argument should point to an unsigned char * vari-         returned.  The fourth argument should point to an unsigned char * vari-
1529         able.         able.
1530    
1531             PCRE_INFO_HASCRORLF
1532    
1533           Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
1534           characters,  otherwise  0.  The  fourth argument should point to an int
1535           variable.
1536    
1537           PCRE_INFO_JCHANGED           PCRE_INFO_JCHANGED
1538    
1539         Return 1 if the (?J) option setting is used in the  pattern,  otherwise         Return 1 if the (?J) option setting is used in the  pattern,  otherwise
# Line 1812  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 1824  MATCHING A PATTERN: THE TRADITIONAL FUNC
1824         tion of pcre_compile()  above.  During  matching,  the  newline  choice         tion of pcre_compile()  above.  During  matching,  the  newline  choice
1825         affects  the  behaviour  of the dot, circumflex, and dollar metacharac-         affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
1826         ters. It may also alter the way the match position is advanced after  a         ters. It may also alter the way the match position is advanced after  a
1827         match  failure  for  an  unanchored  pattern.  When  PCRE_NEWLINE_CRLF,         match failure for an unanchored pattern.
1828         PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a  match  attempt  
1829         fails  when the current position is at a CRLF sequence, the match posi-         When  PCRE_NEWLINE_CRLF,  PCRE_NEWLINE_ANYCRLF,  or PCRE_NEWLINE_ANY is
1830         tion is advanced by two characters instead of one, in other  words,  to         set, and a match attempt for an unanchored pattern fails when the  cur-
1831         after the CRLF.         rent  position  is  at  a  CRLF  sequence,  and the pattern contains no
1832           explicit matches for  CR  or  NL  characters,  the  match  position  is
1833           advanced by two characters instead of one, in other words, to after the
1834           CRLF.
1835    
1836           The above rule is a compromise that makes the most common cases work as
1837           expected.  For  example,  if  the  pattern  is .+A (and the PCRE_DOTALL
1838           option is not set), it does not match the string "\r\nA" because, after
1839           failing  at the start, it skips both the CR and the LF before retrying.
1840           However, the pattern [\r\n]A does match that string,  because  it  con-
1841           tains an explicit CR or LF reference, and so advances only by one char-
1842           acter after the first failure.  Note than an explicit CR or  LF  refer-
1843           ence occurs for negated character classes such as [^X] because they can
1844           match CR or LF characters.
1845    
1846           Notwithstanding the above, anomalous effects may still occur when  CRLF
1847           is a valid newline sequence and explicit \r or \n escapes appear in the
1848           pattern.
1849    
1850           PCRE_NOTBOL           PCRE_NOTBOL
1851    
1852         This option specifies that first character of the subject string is not         This option specifies that first character of the subject string is not
1853         the beginning of a line, so the  circumflex  metacharacter  should  not         the  beginning  of  a  line, so the circumflex metacharacter should not
1854         match  before it. Setting this without PCRE_MULTILINE (at compile time)         match before it. Setting this without PCRE_MULTILINE (at compile  time)
1855         causes circumflex never to match. This option affects only  the  behav-         causes  circumflex  never to match. This option affects only the behav-
1856         iour of the circumflex metacharacter. It does not affect \A.         iour of the circumflex metacharacter. It does not affect \A.
1857    
1858           PCRE_NOTEOL           PCRE_NOTEOL
1859    
1860         This option specifies that the end of the subject string is not the end         This option specifies that the end of the subject string is not the end
1861         of a line, so the dollar metacharacter should not match it nor  (except         of  a line, so the dollar metacharacter should not match it nor (except
1862         in  multiline mode) a newline immediately before it. Setting this with-         in multiline mode) a newline immediately before it. Setting this  with-
1863         out PCRE_MULTILINE (at compile time) causes dollar never to match. This         out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1864         option  affects only the behaviour of the dollar metacharacter. It does         option affects only the behaviour of the dollar metacharacter. It  does
1865         not affect \Z or \z.         not affect \Z or \z.
1866    
1867           PCRE_NOTEMPTY           PCRE_NOTEMPTY
1868    
1869         An empty string is not considered to be a valid match if this option is         An empty string is not considered to be a valid match if this option is
1870         set.  If  there are alternatives in the pattern, they are tried. If all         set. If there are alternatives in the pattern, they are tried.  If  all
1871         the alternatives match the empty string, the entire  match  fails.  For         the  alternatives  match  the empty string, the entire match fails. For
1872         example, if the pattern         example, if the pattern
1873    
1874           a?b?           a?b?
1875    
1876         is  applied  to  a string not beginning with "a" or "b", it matches the         is applied to a string not beginning with "a" or "b",  it  matches  the
1877         empty string at the start of the subject. With PCRE_NOTEMPTY set,  this         empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1878         match is not valid, so PCRE searches further into the string for occur-         match is not valid, so PCRE searches further into the string for occur-
1879         rences of "a" or "b".         rences of "a" or "b".
1880    
1881         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-         Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1882         cial  case  of  a  pattern match of the empty string within its split()         cial case of a pattern match of the empty  string  within  its  split()
1883         function, and when using the /g modifier. It  is  possible  to  emulate         function,  and  when  using  the /g modifier. It is possible to emulate
1884         Perl's behaviour after matching a null string by first trying the match         Perl's behaviour after matching a null string by first trying the match
1885         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then         again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1886         if  that  fails by advancing the starting offset (see below) and trying         if that fails by advancing the starting offset (see below)  and  trying
1887         an ordinary match again. There is some code that demonstrates how to do         an ordinary match again. There is some code that demonstrates how to do
1888         this in the pcredemo.c sample program.         this in the pcredemo.c sample program.
1889    
1890           PCRE_NO_UTF8_CHECK           PCRE_NO_UTF8_CHECK
1891    
1892         When PCRE_UTF8 is set at compile time, the validity of the subject as a         When PCRE_UTF8 is set at compile time, the validity of the subject as a
1893         UTF-8 string is automatically checked when pcre_exec() is  subsequently         UTF-8  string is automatically checked when pcre_exec() is subsequently
1894         called.   The  value  of  startoffset is also checked to ensure that it         called.  The value of startoffset is also checked  to  ensure  that  it
1895         points to the start of a UTF-8 character. There is a  discussion  about         points  to  the start of a UTF-8 character. There is a discussion about
1896         the  validity  of  UTF-8 strings in the section on UTF-8 support in the         the validity of UTF-8 strings in the section on UTF-8  support  in  the
1897         main pcre page. If  an  invalid  UTF-8  sequence  of  bytes  is  found,         main  pcre  page.  If  an  invalid  UTF-8  sequence  of bytes is found,
1898         pcre_exec()  returns  the error PCRE_ERROR_BADUTF8. If startoffset con-         pcre_exec() returns the error PCRE_ERROR_BADUTF8. If  startoffset  con-
1899         tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.         tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1900    
1901         If you already know that your subject is valid, and you  want  to  skip         If  you  already  know that your subject is valid, and you want to skip
1902         these    checks    for   performance   reasons,   you   can   set   the         these   checks   for   performance   reasons,   you   can    set    the
1903         PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to         PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
1904         do  this  for the second and subsequent calls to pcre_exec() if you are         do this for the second and subsequent calls to pcre_exec() if  you  are
1905         making repeated calls to find all  the  matches  in  a  single  subject         making  repeated  calls  to  find  all  the matches in a single subject
1906         string.  However,  you  should  be  sure  that the value of startoffset         string. However, you should be  sure  that  the  value  of  startoffset
1907         points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is         points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1908         set,  the  effect of passing an invalid UTF-8 string as a subject, or a         set, the effect of passing an invalid UTF-8 string as a subject,  or  a
1909         value of startoffset that does not point to the start of a UTF-8  char-         value  of startoffset that does not point to the start of a UTF-8 char-
1910         acter, is undefined. Your program may crash.         acter, is undefined. Your program may crash.
1911    
1912           PCRE_PARTIAL           PCRE_PARTIAL
1913    
1914         This  option  turns  on  the  partial  matching feature. If the subject         This option turns on the  partial  matching  feature.  If  the  subject
1915         string fails to match the pattern, but at some point during the  match-         string  fails to match the pattern, but at some point during the match-
1916         ing  process  the  end of the subject was reached (that is, the subject         ing process the end of the subject was reached (that  is,  the  subject
1917         partially matches the pattern and the failure to  match  occurred  only         partially  matches  the  pattern and the failure to match occurred only
1918         because  there were not enough subject characters), pcre_exec() returns         because there were not enough subject characters), pcre_exec()  returns
1919         PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is         PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1920         used,  there  are restrictions on what may appear in the pattern. These         used, there are restrictions on what may appear in the  pattern.  These
1921         are discussed in the pcrepartial documentation.         are discussed in the pcrepartial documentation.
1922    
1923     The string to be matched by pcre_exec()     The string to be matched by pcre_exec()
1924    
1925         The subject string is passed to pcre_exec() as a pointer in subject,  a         The  subject string is passed to pcre_exec() as a pointer in subject, a
1926         length  in  length, and a starting byte offset in startoffset. In UTF-8         length in length, and a starting byte offset in startoffset.  In  UTF-8
1927         mode, the byte offset must point to the start  of  a  UTF-8  character.         mode,  the  byte  offset  must point to the start of a UTF-8 character.
1928         Unlike  the  pattern string, the subject may contain binary zero bytes.         Unlike the pattern string, the subject may contain binary  zero  bytes.
1929         When the starting offset is zero, the search for a match starts at  the         When  the starting offset is zero, the search for a match starts at the
1930         beginning of the subject, and this is by far the most common case.         beginning of the subject, and this is by far the most common case.
1931    
1932         A  non-zero  starting offset is useful when searching for another match         A non-zero starting offset is useful when searching for  another  match
1933         in the same subject by calling pcre_exec() again after a previous  suc-         in  the same subject by calling pcre_exec() again after a previous suc-
1934         cess.   Setting  startoffset differs from just passing over a shortened         cess.  Setting startoffset differs from just passing over  a  shortened
1935         string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins         string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
1936         with any kind of lookbehind. For example, consider the pattern         with any kind of lookbehind. For example, consider the pattern
1937    
1938           \Biss\B           \Biss\B
1939    
1940         which  finds  occurrences  of "iss" in the middle of words. (\B matches         which finds occurrences of "iss" in the middle of  words.  (\B  matches
1941         only if the current position in the subject is not  a  word  boundary.)         only  if  the  current position in the subject is not a word boundary.)
1942         When  applied  to the string "Mississipi" the first call to pcre_exec()         When applied to the string "Mississipi" the first call  to  pcre_exec()
1943         finds the first occurrence. If pcre_exec() is called  again  with  just         finds  the  first  occurrence. If pcre_exec() is called again with just
1944         the  remainder  of  the  subject,  namely  "issipi", it does not match,         the remainder of the subject,  namely  "issipi",  it  does  not  match,
1945         because \B is always false at the start of the subject, which is deemed         because \B is always false at the start of the subject, which is deemed
1946         to  be  a  word  boundary. However, if pcre_exec() is passed the entire         to be a word boundary. However, if pcre_exec()  is  passed  the  entire
1947         string again, but with startoffset set to 4, it finds the second occur-         string again, but with startoffset set to 4, it finds the second occur-
1948         rence  of "iss" because it is able to look behind the starting point to         rence of "iss" because it is able to look behind the starting point  to
1949         discover that it is preceded by a letter.         discover that it is preceded by a letter.
1950    
1951         If a non-zero starting offset is passed when the pattern  is  anchored,         If  a  non-zero starting offset is passed when the pattern is anchored,
1952         one attempt to match at the given offset is made. This can only succeed         one attempt to match at the given offset is made. This can only succeed
1953         if the pattern does not require the match to be at  the  start  of  the         if  the  pattern  does  not require the match to be at the start of the
1954         subject.         subject.
1955    
1956     How pcre_exec() returns captured substrings     How pcre_exec() returns captured substrings
1957    
1958         In  general, a pattern matches a certain portion of the subject, and in         In general, a pattern matches a certain portion of the subject, and  in
1959         addition, further substrings from the subject  may  be  picked  out  by         addition,  further  substrings  from  the  subject may be picked out by
1960         parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,         parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
1961         this is called "capturing" in what follows, and the  phrase  "capturing         this  is  called "capturing" in what follows, and the phrase "capturing
1962         subpattern"  is  used for a fragment of a pattern that picks out a sub-         subpattern" is used for a fragment of a pattern that picks out  a  sub-
1963         string. PCRE supports several other kinds of  parenthesized  subpattern         string.  PCRE  supports several other kinds of parenthesized subpattern
1964         that do not cause substrings to be captured.         that do not cause substrings to be captured.
1965    
1966         Captured  substrings are returned to the caller via a vector of integer         Captured substrings are returned to the caller via a vector of  integer
1967         offsets whose address is passed in ovector. The number of  elements  in         offsets  whose  address is passed in ovector. The number of elements in
1968         the  vector is passed in ovecsize, which must be a non-negative number.         the vector is passed in ovecsize, which must be a non-negative  number.
1969         Note: this argument is NOT the size of ovector in bytes.         Note: this argument is NOT the size of ovector in bytes.
1970    
1971         The first two-thirds of the vector is used to pass back  captured  sub-         The  first  two-thirds of the vector is used to pass back captured sub-
1972         strings,  each  substring using a pair of integers. The remaining third         strings, each substring using a pair of integers. The  remaining  third
1973         of the vector is used as workspace by pcre_exec() while  matching  cap-         of  the  vector is used as workspace by pcre_exec() while matching cap-
1974         turing  subpatterns, and is not available for passing back information.         turing subpatterns, and is not available for passing back  information.
1975         The length passed in ovecsize should always be a multiple of three.  If         The  length passed in ovecsize should always be a multiple of three. If
1976         it is not, it is rounded down.         it is not, it is rounded down.
1977    
1978         When  a  match  is successful, information about captured substrings is         When a match is successful, information about  captured  substrings  is
1979         returned in pairs of integers, starting at the  beginning  of  ovector,         returned  in  pairs  of integers, starting at the beginning of ovector,
1980         and  continuing  up  to two-thirds of its length at the most. The first         and continuing up to two-thirds of its length at the  most.  The  first
1981         element of a pair is set to the offset of the first character in a sub-         element of a pair is set to the offset of the first character in a sub-
1982         string,  and  the  second  is  set to the offset of the first character         string, and the second is set to the  offset  of  the  first  character
1983         after the end of a substring. The  first  pair,  ovector[0]  and  ovec-         after  the  end  of  a  substring. The first pair, ovector[0] and ovec-
1984         tor[1],  identify  the  portion  of  the  subject string matched by the         tor[1], identify the portion of  the  subject  string  matched  by  the
1985         entire pattern. The next pair is used for the first  capturing  subpat-         entire  pattern.  The next pair is used for the first capturing subpat-
1986         tern, and so on. The value returned by pcre_exec() is one more than the         tern, and so on. The value returned by pcre_exec() is one more than the
1987         highest numbered pair that has been set. For example, if two substrings         highest numbered pair that has been set. For example, if two substrings
1988         have  been captured, the returned value is 3. If there are no capturing         have been captured, the returned value is 3. If there are no  capturing
1989         subpatterns, the return value from a successful match is 1,  indicating         subpatterns,  the return value from a successful match is 1, indicating
1990         that just the first pair of offsets has been set.         that just the first pair of offsets has been set.
1991    
1992         If a capturing subpattern is matched repeatedly, it is the last portion         If a capturing subpattern is matched repeatedly, it is the last portion
1993         of the string that it matched that is returned.         of the string that it matched that is returned.
1994    
1995         If the vector is too small to hold all the captured substring  offsets,         If  the vector is too small to hold all the captured substring offsets,
1996         it is used as far as possible (up to two-thirds of its length), and the         it is used as far as possible (up to two-thirds of its length), and the
1997         function returns a value of zero. In particular, if the substring  off-         function  returns a value of zero. In particular, if the substring off-
1998         sets are not of interest, pcre_exec() may be called with ovector passed         sets are not of interest, pcre_exec() may be called with ovector passed
1999         as NULL and ovecsize as zero. However, if  the  pattern  contains  back         as  NULL  and  ovecsize  as zero. However, if the pattern contains back
2000         references  and  the  ovector is not big enough to remember the related         references and the ovector is not big enough to  remember  the  related
2001         substrings, PCRE has to get additional memory for use during  matching.         substrings,  PCRE has to get additional memory for use during matching.
2002         Thus it is usually advisable to supply an ovector.         Thus it is usually advisable to supply an ovector.
2003    
2004         The  pcre_info()  function  can  be used to find out how many capturing         The pcre_info() function can be used to find  out  how  many  capturing
2005         subpatterns there are in a compiled  pattern.  The  smallest  size  for         subpatterns  there  are  in  a  compiled pattern. The smallest size for
2006         ovector  that  will allow for n captured substrings, in addition to the         ovector that will allow for n captured substrings, in addition  to  the
2007         offsets of the substring matched by the whole pattern, is (n+1)*3.         offsets of the substring matched by the whole pattern, is (n+1)*3.
2008    
2009         It is possible for capturing subpattern number n+1 to match  some  part         It  is  possible for capturing subpattern number n+1 to match some part
2010         of the subject when subpattern n has not been used at all. For example,         of the subject when subpattern n has not been used at all. For example,
2011         if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the         if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
2012         return from the function is 4, and subpatterns 1 and 3 are matched, but         return from the function is 4, and subpatterns 1 and 3 are matched, but
2013         2 is not. When this happens, both values in  the  offset  pairs  corre-         2  is  not.  When  this happens, both values in the offset pairs corre-
2014         sponding to unused subpatterns are set to -1.         sponding to unused subpatterns are set to -1.
2015    
2016         Offset  values  that correspond to unused subpatterns at the end of the         Offset values that correspond to unused subpatterns at the end  of  the
2017         expression are also set to -1. For example,  if  the  string  "abc"  is         expression  are  also  set  to  -1. For example, if the string "abc" is
2018         matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not         matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not
2019         matched. The return from the function is 2, because  the  highest  used         matched.  The  return  from the function is 2, because the highest used
2020         capturing subpattern number is 1. However, you can refer to the offsets         capturing subpattern number is 1. However, you can refer to the offsets
2021         for the second and third capturing subpatterns if  you  wish  (assuming         for  the  second  and third capturing subpatterns if you wish (assuming
2022         the vector is large enough, of course).         the vector is large enough, of course).
2023    
2024         Some  convenience  functions  are  provided for extracting the captured         Some convenience functions are provided  for  extracting  the  captured
2025         substrings as separate strings. These are described below.         substrings as separate strings. These are described below.
2026    
2027     Error return values from pcre_exec()     Error return values from pcre_exec()
2028    
2029         If pcre_exec() fails, it returns a negative number. The  following  are         If  pcre_exec()  fails, it returns a negative number. The following are
2030         defined in the header file:         defined in the header file:
2031    
2032           PCRE_ERROR_NOMATCH        (-1)           PCRE_ERROR_NOMATCH        (-1)
# Line 2006  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2035  MATCHING A PATTERN: THE TRADITIONAL FUNC
2035    
2036           PCRE_ERROR_NULL           (-2)           PCRE_ERROR_NULL           (-2)
2037    
2038         Either  code  or  subject  was  passed as NULL, or ovector was NULL and         Either code or subject was passed as NULL,  or  ovector  was  NULL  and
2039         ovecsize was not zero.         ovecsize was not zero.
2040    
2041           PCRE_ERROR_BADOPTION      (-3)           PCRE_ERROR_BADOPTION      (-3)
# Line 2015  MATCHING A PATTERN: THE TRADITIONAL FUNC Line 2044  MATCHING A PATTERN: THE TRADITIONAL FUNC
2044    
2045           PCRE_ERROR_BADMAGIC       (-4)           PCRE_ERROR_BADMAGIC       (-4)
2046    
2047         PCRE stores a 4-byte "magic number" at the start of the compiled  code,         PCRE  stores a 4-byte "magic number" at the start of the compiled code,
2048         to catch the case when it is passed a junk pointer and to detect when a         to catch the case when it is passed a junk pointer and to detect when a
2049         pattern that was compiled in an environment of one endianness is run in         pattern that was compiled in an environment of one endianness is run in
2050         an  environment  with the other endianness. This is the error that PCRE         an environment with the other endianness. This is the error  that  PCRE
2051         gives when the magic number is not present.         gives when the magic number is not present.
2052    
2053           PCRE_ERROR_UNKNOWN_OPCODE (-5)           PCRE_ERROR_UNKNOWN_OPCODE (-5)
2054    
2055         While running the pattern match, an unknown item was encountered in the         While running the pattern match, an unknown item was encountered in the
2056         compiled  pattern.  This  error  could be caused by a bug in PCRE or by         compiled pattern. This error could be caused by a bug  in  PCRE  or  by
2057         overwriting of the compiled pattern.         overwriting of the compiled pattern.
2058    
2059           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2060    
2061         If a pattern contains back references, but the ovector that  is  passed         If  a  pattern contains back references, but the ovector that is passed
2062         to pcre_exec() is not big enough to remember the referenced substrings,         to pcre_exec() is not big enough to remember the referenced substrings,
2063         PCRE gets a block of memory at the start of matching to  use  for  this         PCRE  gets  a  block of memory at the start of matching to use for this
2064         purpose.  If the call via pcre_malloc() fails, this error is given. The         purpose. If the call via pcre_malloc() fails, this error is given.  The
2065         memory is automatically freed at the end of matching.         memory is automatically freed at the end of matching.
2066    
2067           PCRE_ERROR_NOSUBSTRING    (-7)           PCRE_ERROR_NOSUBSTRING    (-7)
2068    
2069         This error is used by the pcre_copy_substring(),  pcre_get_substring(),         This  error is used by the pcre_copy_substring(), pcre_get_substring(),
2070         and  pcre_get_substring_list()  functions  (see  below).  It  is  never         and  pcre_get_substring_list()  functions  (see  below).  It  is  never
2071         returned by pcre_exec().         returned by pcre_exec().
2072    
2073           PCRE_ERROR_MATCHLIMIT     (-8)           PCRE_ERROR_MATCHLIMIT     (-8)
2074    
2075         The backtracking limit, as specified by  the  match_limit  field  in  a         The  backtracking  limit,  as  specified  by the match_limit field in a
2076         pcre_extra  structure  (or  defaulted) was reached. See the description         pcre_extra structure (or defaulted) was reached.  See  the  description
2077         above.         above.
2078    
2079           PCRE_ERROR_CALLOUT        (-9)           PCRE_ERROR_CALLOUT        (-9)
2080    
2081         This error is never generated by pcre_exec() itself. It is provided for         This error is never generated by pcre_exec() itself. It is provided for
2082         use  by  callout functions that want to yield a distinctive error code.         use by callout functions that want to yield a distinctive  error  code.
2083         See the pcrecallout documentation for details.         See the pcrecallout documentation for details.
2084    
2085           PCRE_ERROR_BADUTF8        (-10)           PCRE_ERROR_BADUTF8        (-10)
2086    
2087         A string that contains an invalid UTF-8 byte sequence was passed  as  a         A  string  that contains an invalid UTF-8 byte sequence was passed as a
2088         subject.         subject.
2089    
2090           PCRE_ERROR_BADUTF8_OFFSET (-11)           PCRE_ERROR_BADUTF8_OFFSET (-11)
2091    
2092         The UTF-8 byte sequence that was passed as a subject was valid, but the         The UTF-8 byte sequence that was passed as a subject was valid, but the
2093         value of startoffset did not point to the beginning of a UTF-8  charac-         value  of startoffset did not point to the beginning of a UTF-8 charac-
2094         ter.         ter.
2095    
2096           PCRE_ERROR_PARTIAL        (-12)           PCRE_ERROR_PARTIAL        (-12)
2097    
2098         The  subject  string did not match, but it did match partially. See the         The subject string did not match, but it did match partially.  See  the
2099         pcrepartial documentation for details of partial matching.         pcrepartial documentation for details of partial matching.
2100    
2101           PCRE_ERROR_BADPARTIAL     (-13)           PCRE_ERROR_BADPARTIAL     (-13)
2102    
2103         The PCRE_PARTIAL option was used with  a  compiled  pattern  containing         The  PCRE_PARTIAL  option  was  used with a compiled pattern containing
2104         items  that are not supported for partial matching. See the pcrepartial         items that are not supported for partial matching. See the  pcrepartial
2105         documentation for details of partial matching.         documentation for details of partial matching.
2106    
2107           PCRE_ERROR_INTERNAL       (-14)           PCRE_ERROR_INTERNAL       (-14)
2108    
2109         An unexpected internal error has occurred. This error could  be  caused         An  unexpected  internal error has occurred. This error could be caused
2110         by a bug in PCRE or by overwriting of the compiled pattern.         by a bug in PCRE or by overwriting of the compiled pattern.
2111    
2112           PCRE_ERROR_BADCOUNT       (-15)           PCRE_ERROR_BADCOUNT       (-15)
2113    
2114         This  error is given if the value of the ovecsize argument is negative.         This error is given if the value of the ovecsize argument is  negative.
2115    
2116           PCRE_ERROR_RECURSIONLIMIT (-21)           PCRE_ERROR_RECURSIONLIMIT (-21)
2117    
2118         The internal recursion limit, as specified by the match_limit_recursion         The internal recursion limit, as specified by the match_limit_recursion
2119         field  in  a  pcre_extra  structure (or defaulted) was reached. See the         field in a pcre_extra structure (or defaulted)  was  reached.  See  the
2120         description above.         description above.
2121    
2122           PCRE_ERROR_BADNEWLINE     (-23)           PCRE_ERROR_BADNEWLINE     (-23)
# Line 2110  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER Line 2139  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2139         int pcre_get_substring_list(const char *subject,         int pcre_get_substring_list(const char *subject,
2140              int *ovector, int stringcount, const char ***listptr);              int *ovector, int stringcount, const char ***listptr);
2141    
2142         Captured substrings can be  accessed  directly  by  using  the  offsets         Captured  substrings  can  be  accessed  directly  by using the offsets
2143         returned  by  pcre_exec()  in  ovector.  For convenience, the functions         returned by pcre_exec() in  ovector.  For  convenience,  the  functions
2144         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2145         string_list()  are  provided for extracting captured substrings as new,         string_list() are provided for extracting captured substrings  as  new,
2146         separate, zero-terminated strings. These functions identify  substrings         separate,  zero-terminated strings. These functions identify substrings
2147         by  number.  The  next section describes functions for extracting named         by number. The next section describes functions  for  extracting  named
2148         substrings.         substrings.
2149    
2150         A substring that contains a binary zero is correctly extracted and  has         A  substring that contains a binary zero is correctly extracted and has
2151         a  further zero added on the end, but the result is not, of course, a C         a further zero added on the end, but the result is not, of course, a  C
2152         string.  However, you can process such a string  by  referring  to  the         string.   However,  you  can  process such a string by referring to the
2153         length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-         length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
2154         string().  Unfortunately, the interface to pcre_get_substring_list() is         string().  Unfortunately, the interface to pcre_get_substring_list() is
2155         not  adequate for handling strings containing binary zeros, because the         not adequate for handling strings containing binary zeros, because  the
2156         end of the final string is not independently indicated.         end of the final string is not independently indicated.
2157    
2158         The first three arguments are the same for all  three  of  these  func-         The  first  three  arguments  are the same for all three of these func-
2159         tions:  subject  is  the subject string that has just been successfully         tions: subject is the subject string that has  just  been  successfully
2160         matched, ovector is a pointer to the vector of integer offsets that was         matched, ovector is a pointer to the vector of integer offsets that was
2161         passed to pcre_exec(), and stringcount is the number of substrings that         passed to pcre_exec(), and stringcount is the number of substrings that
2162         were captured by the match, including the substring  that  matched  the         were  captured  by  the match, including the substring that matched the
2163         entire regular expression. This is the value returned by pcre_exec() if         entire regular expression. This is the value returned by pcre_exec() if
2164         it is greater than zero. If pcre_exec() returned zero, indicating  that         it  is greater than zero. If pcre_exec() returned zero, indicating that
2165         it  ran out of space in ovector, the value passed as stringcount should         it ran out of space in ovector, the value passed as stringcount  should
2166         be the number of elements in the vector divided by three.         be the number of elements in the vector divided by three.
2167    
2168         The functions pcre_copy_substring() and pcre_get_substring() extract  a         The  functions pcre_copy_substring() and pcre_get_substring() extract a
2169         single  substring,  whose  number  is given as stringnumber. A value of         single substring, whose number is given as  stringnumber.  A  value  of
2170         zero extracts the substring that matched the  entire  pattern,  whereas         zero  extracts  the  substring that matched the entire pattern, whereas
2171         higher  values  extract  the  captured  substrings.  For pcre_copy_sub-         higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
2172         string(), the string is placed in buffer,  whose  length  is  given  by         string(),  the  string  is  placed  in buffer, whose length is given by
2173         buffersize,  while  for  pcre_get_substring()  a new block of memory is         buffersize, while for pcre_get_substring() a new  block  of  memory  is
2174         obtained via pcre_malloc, and its address is  returned  via  stringptr.         obtained  via  pcre_malloc,  and its address is returned via stringptr.
2175         The  yield  of  the function is the length of the string, not including         The yield of the function is the length of the  string,  not  including
2176         the terminating zero, or one of these error codes:         the terminating zero, or one of these error codes:
2177    
2178           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2179    
2180         The buffer was too small for pcre_copy_substring(), or the  attempt  to         The  buffer  was too small for pcre_copy_substring(), or the attempt to
2181         get memory failed for pcre_get_substring().         get memory failed for pcre_get_substring().
2182    
2183           PCRE_ERROR_NOSUBSTRING    (-7)           PCRE_ERROR_NOSUBSTRING    (-7)
2184    
2185         There is no substring whose number is stringnumber.         There is no substring whose number is stringnumber.
2186    
2187         The  pcre_get_substring_list()  function  extracts  all  available sub-         The pcre_get_substring_list()  function  extracts  all  available  sub-
2188         strings and builds a list of pointers to them. All this is  done  in  a         strings  and  builds  a list of pointers to them. All this is done in a
2189         single block of memory that is obtained via pcre_malloc. The address of         single block of memory that is obtained via pcre_malloc. The address of
2190         the memory block is returned via listptr, which is also  the  start  of         the  memory  block  is returned via listptr, which is also the start of
2191         the  list  of  string pointers. The end of the list is marked by a NULL         the list of string pointers. The end of the list is marked  by  a  NULL
2192         pointer. The yield of the function is zero if all  went  well,  or  the         pointer.  The  yield  of  the function is zero if all went well, or the
2193         error code         error code
2194    
2195           PCRE_ERROR_NOMEMORY       (-6)           PCRE_ERROR_NOMEMORY       (-6)
2196    
2197         if the attempt to get the memory block failed.         if the attempt to get the memory block failed.
2198    
2199         When  any of these functions encounter a substring that is unset, which         When any of these functions encounter a substring that is unset,  which
2200         can happen when capturing subpattern number n+1 matches  some  part  of         can  happen  when  capturing subpattern number n+1 matches some part of
2201         the  subject, but subpattern n has not been used at all, they return an         the subject, but subpattern n has not been used at all, they return  an
2202         empty string. This can be distinguished from a genuine zero-length sub-         empty string. This can be distinguished from a genuine zero-length sub-
2203         string  by inspecting the appropriate offset in ovector, which is nega-         string by inspecting the appropriate offset in ovector, which is  nega-
2204         tive for unset substrings.         tive for unset substrings.
2205    
2206         The two convenience functions pcre_free_substring() and  pcre_free_sub-         The  two convenience functions pcre_free_substring() and pcre_free_sub-
2207         string_list()  can  be  used  to free the memory returned by a previous         string_list() can be used to free the memory  returned  by  a  previous
2208         call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-         call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2209         tively.  They  do  nothing  more  than  call the function pointed to by         tively. They do nothing more than  call  the  function  pointed  to  by
2210         pcre_free, which of course could be called directly from a  C  program.         pcre_free,  which  of course could be called directly from a C program.
2211         However,  PCRE is used in some situations where it is linked via a spe-         However, PCRE is used in some situations where it is linked via a  spe-
2212         cial  interface  to  another  programming  language  that  cannot   use         cial   interface  to  another  programming  language  that  cannot  use
2213         pcre_free  directly;  it is for these cases that the functions are pro-         pcre_free directly; it is for these cases that the functions  are  pro-
2214         vided.         vided.
2215    
2216    
# Line 2200  EXTRACTING CAPTURED SUBSTRINGS BY NAME Line 2229  EXTRACTING CAPTURED SUBSTRINGS BY NAME
2229              int stringcount, const char *stringname,              int stringcount, const char *stringname,
2230              const char **stringptr);              const char **stringptr);
2231    
2232         To extract a substring by name, you first have to find associated  num-         To  extract a substring by name, you first have to find associated num-
2233         ber.  For example, for this pattern         ber.  For example, for this pattern
2234    
2235           (a+)b(?<xxx>\d+)...           (a+)b(?<xxx>\d+)...
# Line 2209  EXTRACTING CAPTURED SUBSTRINGS BY NAME Line 2238  EXTRACTING CAPTURED SUBSTRINGS BY NAME
2238         be unique (PCRE_DUPNAMES was not set), you can find the number from the         be unique (PCRE_DUPNAMES was not set), you can find the number from the
2239         name by calling pcre_get_stringnumber(). The first argument is the com-         name by calling pcre_get_stringnumber(). The first argument is the com-
2240         piled pattern, and the second is the name. The yield of the function is         piled pattern, and the second is the name. The yield of the function is
2241         the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no         the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no
2242         subpattern of that name.         subpattern of that name.
2243    
2244         Given the number, you can extract the substring directly, or use one of         Given the number, you can extract the substring directly, or use one of
2245         the functions described in the previous section. For convenience, there         the functions described in the previous section. For convenience, there
2246         are also two functions that do the whole job.         are also two functions that do the whole job.
2247    
2248         Most   of   the   arguments    of    pcre_copy_named_substring()    and         Most    of    the    arguments   of   pcre_copy_named_substring()   and
2249         pcre_get_named_substring()  are  the  same  as  those for the similarly         pcre_get_named_substring() are the same  as  those  for  the  similarly
2250         named functions that extract by number. As these are described  in  the         named  functions  that extract by number. As these are described in the
2251         previous  section,  they  are not re-described here. There are just two         previous section, they are not re-described here. There  are  just  two
2252         differences:         differences:
2253    
2254         First, instead of a substring number, a substring name is  given.  Sec-         First,  instead  of a substring number, a substring name is given. Sec-
2255         ond, there is an extra argument, given at the start, which is a pointer         ond, there is an extra argument, given at the start, which is a pointer
2256         to the compiled pattern. This is needed in order to gain access to  the         to  the compiled pattern. This is needed in order to gain access to the
2257         name-to-number translation table.         name-to-number translation table.
2258    
2259         These  functions call pcre_get_stringnumber(), and if it succeeds, they         These functions call pcre_get_stringnumber(), and if it succeeds,  they
2260         then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-         then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
2261         ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the         ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
2262         behaviour may not be what you want (see the next section).         behaviour may not be what you want (see the next section).
2263    
2264    
# Line 2238  DUPLICATE SUBPATTERN NAMES Line 2267  DUPLICATE SUBPATTERN NAMES
2267         int pcre_get_stringtable_entries(const pcre *code,         int pcre_get_stringtable_entries(const pcre *code,
2268              const char *name, char **first, char **last);              const char *name, char **first, char **last);
2269    
2270         When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for         When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
2271         subpatterns  are  not  required  to  be unique. Normally, patterns with         subpatterns are not required to  be  unique.  Normally,  patterns  with
2272         duplicate names are such that in any one match, only one of  the  named         duplicate  names  are such that in any one match, only one of the named
2273         subpatterns  participates. An example is shown in the pcrepattern docu-         subpatterns participates. An example is shown in the pcrepattern  docu-
2274         mentation.         mentation.
2275    
2276         When   duplicates   are   present,   pcre_copy_named_substring()    and         When    duplicates   are   present,   pcre_copy_named_substring()   and
2277         pcre_get_named_substring()  return the first substring corresponding to         pcre_get_named_substring() return the first substring corresponding  to
2278         the given name that is set. If  none  are  set,  PCRE_ERROR_NOSUBSTRING         the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING
2279         (-7)  is  returned;  no  data  is returned. The pcre_get_stringnumber()         (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
2280         function returns one of the numbers that are associated with the  name,         function  returns one of the numbers that are associated with the name,
2281         but it is not defined which it is.         but it is not defined which it is.
2282    
2283         If  you want to get full details of all captured substrings for a given         If you want to get full details of all captured substrings for a  given
2284         name, you must use  the  pcre_get_stringtable_entries()  function.  The         name,  you  must  use  the pcre_get_stringtable_entries() function. The
2285         first argument is the compiled pattern, and the second is the name. The         first argument is the compiled pattern, and the second is the name. The
2286         third and fourth are pointers to variables which  are  updated  by  the         third  and  fourth  are  pointers to variables which are updated by the
2287         function. After it has run, they point to the first and last entries in         function. After it has run, they point to the first and last entries in
2288         the name-to-number table  for  the  given  name.  The  function  itself         the  name-to-number  table  for  the  given  name.  The function itself
2289         returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if         returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
2290         there are none. The format of the table is described above in the  sec-         there  are none. The format of the table is described above in the sec-
2291         tion  entitled  Information  about  a  pattern.  Given all the relevant         tion entitled Information about a  pattern.   Given  all  the  relevant
2292         entries for the name, you can extract each of their numbers, and  hence         entries  for the name, you can extract each of their numbers, and hence
2293         the captured data, if any.         the captured data, if any.
2294    
2295    
2296  FINDING ALL POSSIBLE MATCHES  FINDING ALL POSSIBLE MATCHES
2297    
2298         The  traditional  matching  function  uses a similar algorithm to Perl,         The traditional matching function uses a  similar  algorithm  to  Perl,
2299         which stops when it finds the first match, starting at a given point in         which stops when it finds the first match, starting at a given point in
2300         the  subject.  If you want to find all possible matches, or the longest         the subject. If you want to find all possible matches, or  the  longest
2301         possible match, consider using the alternative matching  function  (see         possible  match,  consider using the alternative matching function (see
2302         below)  instead.  If you cannot use the alternative function, but still         below) instead. If you cannot use the alternative function,  but  still
2303         need to find all possible matches, you can kludge it up by  making  use         need  to  find all possible matches, you can kludge it up by making use
2304         of the callout facility, which is described in the pcrecallout documen-         of the callout facility, which is described in the pcrecallout documen-
2305         tation.         tation.
2306    
2307         What you have to do is to insert a callout right at the end of the pat-         What you have to do is to insert a callout right at the end of the pat-
2308         tern.   When your callout function is called, extract and save the cur-         tern.  When your callout function is called, extract and save the  cur-
2309         rent matched substring. Then return  1,  which  forces  pcre_exec()  to         rent  matched  substring.  Then  return  1, which forces pcre_exec() to
2310         backtrack  and  try other alternatives. Ultimately, when it runs out of         backtrack and try other alternatives. Ultimately, when it runs  out  of
2311         matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.         matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2312    
2313    
# Line 2289  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2318  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2318              int options, int *ovector, int ovecsize,              int options, int *ovector, int ovecsize,
2319              int *workspace, int wscount);              int *workspace, int wscount);
2320    
2321         The function pcre_dfa_exec()  is  called  to  match  a  subject  string         The  function  pcre_dfa_exec()  is  called  to  match  a subject string
2322         against  a  compiled pattern, using a matching algorithm that scans the         against a compiled pattern, using a matching algorithm that  scans  the
2323         subject string just once, and does not backtrack.  This  has  different         subject  string  just  once, and does not backtrack. This has different
2324         characteristics  to  the  normal  algorithm, and is not compatible with         characteristics to the normal algorithm, and  is  not  compatible  with
2325         Perl. Some of the features of PCRE patterns are not  supported.  Never-         Perl.  Some  of the features of PCRE patterns are not supported. Never-
2326         theless,  there are times when this kind of matching can be useful. For         theless, there are times when this kind of matching can be useful.  For
2327         a discussion of the two matching algorithms, see the pcrematching docu-         a discussion of the two matching algorithms, see the pcrematching docu-
2328         mentation.         mentation.
2329    
2330         The  arguments  for  the  pcre_dfa_exec()  function are the same as for         The arguments for the pcre_dfa_exec() function  are  the  same  as  for
2331         pcre_exec(), plus two extras. The ovector argument is used in a differ-         pcre_exec(), plus two extras. The ovector argument is used in a differ-
2332         ent  way,  and  this is described below. The other common arguments are         ent way, and this is described below. The other  common  arguments  are
2333         used in the same way as for pcre_exec(), so their  description  is  not         used  in  the  same way as for pcre_exec(), so their description is not
2334         repeated here.         repeated here.
2335    
2336         The  two  additional  arguments provide workspace for the function. The         The two additional arguments provide workspace for  the  function.  The
2337         workspace vector should contain at least 20 elements. It  is  used  for         workspace  vector  should  contain at least 20 elements. It is used for
2338         keeping  track  of  multiple  paths  through  the  pattern  tree.  More         keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2339         workspace will be needed for patterns and subjects where  there  are  a         workspace  will  be  needed for patterns and subjects where there are a
2340         lot of potential matches.         lot of potential matches.
2341    
2342         Here is an example of a simple call to pcre_dfa_exec():         Here is an example of a simple call to pcre_dfa_exec():
# Line 2329  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2358  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2358    
2359     Option bits for pcre_dfa_exec()     Option bits for pcre_dfa_exec()
2360    
2361         The  unused  bits  of  the options argument for pcre_dfa_exec() must be         The unused bits of the options argument  for  pcre_dfa_exec()  must  be
2362         zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-         zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
2363         LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,         LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,
2364         PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last         PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2365         three of these are the same as for pcre_exec(), so their description is         three of these are the same as for pcre_exec(), so their description is
2366         not repeated here.         not repeated here.
2367    
2368           PCRE_PARTIAL           PCRE_PARTIAL
2369    
2370         This has the same general effect as it does for  pcre_exec(),  but  the         This  has  the  same general effect as it does for pcre_exec(), but the
2371         details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for         details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
2372         pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into         pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
2373         PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have         PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
2374         been no complete matches, but there is still at least one matching pos-         been no complete matches, but there is still at least one matching pos-
2375         sibility.  The portion of the string that provided the partial match is         sibility. The portion of the string that provided the partial match  is
2376         set as the first matching string.         set as the first matching string.
2377    
2378           PCRE_DFA_SHORTEST           PCRE_DFA_SHORTEST
2379    
2380         Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to         Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
2381         stop as soon as it has found one match. Because of the way the alterna-         stop as soon as it has found one match. Because of the way the alterna-
2382         tive algorithm works, this is necessarily the shortest  possible  match         tive  algorithm  works, this is necessarily the shortest possible match
2383         at the first possible matching point in the subject string.         at the first possible matching point in the subject string.
2384    
2385           PCRE_DFA_RESTART           PCRE_DFA_RESTART
2386    
2387         When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and         When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
2388         returns a partial match, it is possible to call it  again,  with  addi-         returns  a  partial  match, it is possible to call it again, with addi-
2389         tional  subject  characters,  and have it continue with the same match.         tional subject characters, and have it continue with  the  same  match.
2390         The PCRE_DFA_RESTART option requests this action; when it is  set,  the         The  PCRE_DFA_RESTART  option requests this action; when it is set, the
2391         workspace  and wscount options must reference the same vector as before         workspace and wscount options must reference the same vector as  before
2392         because data about the match so far is left in  them  after  a  partial         because  data  about  the  match so far is left in them after a partial
2393         match.  There  is  more  discussion of this facility in the pcrepartial         match. There is more discussion of this  facility  in  the  pcrepartial
2394         documentation.         documentation.
2395    
2396     Successful returns from pcre_dfa_exec()     Successful returns from pcre_dfa_exec()
2397    
2398         When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-         When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
2399         string in the subject. Note, however, that all the matches from one run         string in the subject. Note, however, that all the matches from one run
2400         of the function start at the same point in  the  subject.  The  shorter         of  the  function  start  at the same point in the subject. The shorter
2401         matches  are all initial substrings of the longer matches. For example,         matches are all initial substrings of the longer matches. For  example,
2402         if the pattern         if the pattern
2403    
2404           <.*>           <.*>
# Line 2384  MATCHING A PATTERN: THE ALTERNATIVE FUNC Line 2413  MATCHING A PATTERN: THE ALTERNATIVE FUNC
2413           <something> <something else>           <something> <something else>
2414           <something> <something else> <something further>           <something> <something else> <something further>
2415    
2416         On success, the yield of the function is a number  greater  than  zero,         On  success,  the  yield of the function is a number greater than zero,
2417         which  is  the  number of matched substrings. The substrings themselves         which is the number of matched substrings.  The  substrings  themselves
2418         are returned in ovector. Each string uses two elements;  the  first  is         are  returned  in  ovector. Each string uses two elements; the first is
2419         the  offset  to  the start, and the second is the offset to the end. In         the offset to the start, and the second is the offset to  the  end.  In
2420         fact, all the strings have the same start  offset.  (Space  could  have         fact,  all  the  strings  have the same start offset. (Space could have
2421         been  saved by giving this only once, but it was decided to retain some         been saved by giving this only once, but it was decided to retain  some
2422         compatibility with the way pcre_exec() returns data,  even  though  the         compatibility  with  the  way pcre_exec() returns data, even though the
2423         meaning of the strings is different.)         meaning of the strings is different.)
2424    
2425         The strings are returned in reverse order of length; that is, the long-         The strings are returned in reverse order of length; that is, the long-
2426         est matching string is given first. If there were too many  matches  to         est  matching  string is given first. If there were too many matches to
2427         fit  into ovector, the yield of the function is zero, and the vector is         fit into ovector, the yield of the function is zero, and the vector  is
2428         filled with the longest matches.         filled with the longest matches.
2429    
2430     Error returns from pcre_dfa_exec()     Error returns from pcre_dfa_exec()
2431    
2432         The pcre_dfa_exec() function returns a negative number when  it  fails.         The  pcre_dfa_exec()  function returns a negative number when it fails.
2433         Many  of  the  errors  are  the  same as for pcre_exec(), and these are         Many of the errors are the same  as  for  pcre_exec(),  and  these  are
2434         described above.  There are in addition the following errors  that  are         described  above.   There are in addition the following errors that are
2435         specific to pcre_dfa_exec():         specific to pcre_dfa_exec():
2436    
2437           PCRE_ERROR_DFA_UITEM      (-16)           PCRE_ERROR_DFA_UITEM      (-16)
2438    
2439         This  return is given if pcre_dfa_exec() encounters an item in the pat-         This return is given if pcre_dfa_exec() encounters an item in the  pat-
2440         tern that it does not support, for instance, the use of \C  or  a  back         tern  that  it  does not support, for instance, the use of \C or a back
2441         reference.         reference.
2442    
2443           PCRE_ERROR_DFA_UCOND      (-17)           PCRE_ERROR_DFA_UCOND      (-17)
2444    
2445         This  return  is  given  if pcre_dfa_exec() encounters a condition item         This return is given if pcre_dfa_exec()  encounters  a  condition  item
2446         that uses a back reference for the condition, or a test  for  recursion         that  uses  a back reference for the condition, or a test for recursion
2447         in a specific group. These are not supported.         in a specific group. These are not supported.
2448    
2449           PCRE_ERROR_DFA_UMLIMIT    (-18)           PCRE_ERROR_DFA_UMLIMIT    (-18)
2450    
2451         This  return  is given if pcre_dfa_exec() is called with an extra block         This return is given if pcre_dfa_exec() is called with an  extra  block
2452         that contains a setting of the match_limit field. This is not supported         that contains a setting of the match_limit field. This is not supported
2453         (it is meaningless).         (it is meaningless).
2454    
2455           PCRE_ERROR_DFA_WSSIZE     (-19)           PCRE_ERROR_DFA_WSSIZE     (-19)
2456    
2457         This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the         This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
2458         workspace vector.         workspace vector.
2459    
2460           PCRE_ERROR_DFA_RECURSE    (-20)           PCRE_ERROR_DFA_RECURSE    (-20)
2461    
2462         When a recursive subpattern is processed, the matching  function  calls         When  a  recursive subpattern is processed, the matching function calls
2463         itself  recursively,  using  private vectors for ovector and workspace.         itself recursively, using private vectors for  ovector  and  workspace.
2464         This error is given if the output vector  is  not  large  enough.  This         This  error  is  given  if  the output vector is not large enough. This
2465         should be extremely rare, as a vector of size 1000 is used.         should be extremely rare, as a vector of size 1000 is used.
2466    
2467    
2468  SEE ALSO  SEE ALSO
2469    
2470         pcrebuild(3),  pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-         pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
2471         tial(3), pcreposix(3), pcreprecompile(3), pcresample(3),  pcrestack(3).         tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2472    
2473    
2474  AUTHOR  AUTHOR
# Line 2451  AUTHOR Line 2480  AUTHOR
2480    
2481  REVISION  REVISION
2482    
2483         Last updated: 09 August 2007         Last updated: 21 August 2007
2484         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
2485  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
2486    
# Line 2797  PCRE REGULAR EXPRESSION DETAILS Line 2826  PCRE REGULAR EXPRESSION DETAILS
2826         discussed in the pcrematching page.         discussed in the pcrematching page.
2827    
2828    
2829    NEWLINE CONVENTIONS
2830    
2831           PCRE  supports five different conventions for indicating line breaks in
2832           strings: a single CR (carriage return) character, a  single  LF  (line-
2833           feed) character, the two-character sequence CRLF, any of the three pre-
2834           ceding, or any Unicode newline sequence. The pcreapi page  has  further
2835           discussion  about newlines, and shows how to set the newline convention
2836           in the options arguments for the compiling and matching functions.
2837    
2838           It is also possible to specify a newline convention by starting a  pat-
2839           tern string with one of the following five sequences:
2840    
2841             (*CR)        carriage return
2842             (*LF)        linefeed
2843             (*CRLF)      carriage return, followed by linefeed
2844             (*ANYCRLF)   any of the three above
2845             (*ANY)       all Unicode newline sequences
2846    
2847           These override the default and the options given to pcre_compile(). For
2848           example, on a Unix system where LF is the default newline sequence, the
2849           pattern
2850    
2851             (*CR)a.b
2852    
2853           changes the convention to CR. That pattern matches "a\nb" because LF is
2854           no longer a newline. Note that these special settings,  which  are  not
2855           Perl-compatible,  are  recognized  only at the very start of a pattern,
2856           and that they must be in upper case.
2857    
2858    
2859  CHARACTERS AND METACHARACTERS  CHARACTERS AND METACHARACTERS
2860    
2861         A  regular  expression  is  a pattern that is matched against a subject         A regular expression is a pattern that is  matched  against  a  subject
2862         string from left to right. Most characters stand for  themselves  in  a         string  from  left  to right. Most characters stand for themselves in a
2863         pattern,  and  match  the corresponding characters in the subject. As a         pattern, and match the corresponding characters in the  subject.  As  a
2864         trivial example, the pattern         trivial example, the pattern
2865    
2866           The quick brown fox           The quick brown fox
2867    
2868         matches a portion of a subject string that is identical to itself. When         matches a portion of a subject string that is identical to itself. When
2869         caseless  matching is specified (the PCRE_CASELESS option), letters are         caseless matching is specified (the PCRE_CASELESS option), letters  are
2870         matched independently of case. In UTF-8 mode, PCRE  always  understands         matched  independently  of case. In UTF-8 mode, PCRE always understands
2871         the  concept  of case for characters whose values are less than 128, so         the concept of case for characters whose values are less than  128,  so
2872         caseless matching is always possible. For characters with  higher  val-         caseless  matching  is always possible. For characters with higher val-
2873         ues,  the concept of case is supported if PCRE is compiled with Unicode         ues, the concept of case is supported if PCRE is compiled with  Unicode
2874         property support, but not otherwise.   If  you  want  to  use  caseless         property  support,  but  not  otherwise.   If  you want to use caseless
2875         matching  for  characters  128  and above, you must ensure that PCRE is         matching for characters 128 and above, you must  ensure  that  PCRE  is
2876         compiled with Unicode property support as well as with UTF-8 support.         compiled with Unicode property support as well as with UTF-8 support.
2877    
2878         The power of regular expressions comes  from  the  ability  to  include         The  power  of  regular  expressions  comes from the ability to include
2879         alternatives  and  repetitions in the pattern. These are encoded in the         alternatives and repetitions in the pattern. These are encoded  in  the
2880         pattern by the use of metacharacters, which do not stand for themselves         pattern by the use of metacharacters, which do not stand for themselves
2881         but instead are interpreted in some special way.         but instead are interpreted in some special way.
2882    
2883         There  are  two different sets of metacharacters: those that are recog-         There are two different sets of metacharacters: those that  are  recog-
2884         nized anywhere in the pattern except within square brackets, and  those         nized  anywhere in the pattern except within square brackets, and those
2885         that  are  recognized  within square brackets. Outside square brackets,         that are recognized within square brackets.  Outside  square  brackets,
2886         the metacharacters are as follows:         the metacharacters are as follows:
2887    
2888           \      general escape character with several uses           \      general escape character with several uses
# Line 2842  CHARACTERS AND METACHARACTERS Line 2901  CHARACTERS AND METACHARACTERS
2901                  also "possessive quantifier"                  also "possessive quantifier"
2902           {      start min/max quantifier           {      start min/max quantifier
2903    
2904         Part of a pattern that is in square brackets  is  called  a  "character         Part  of  a  pattern  that is in square brackets is called a "character
2905         class". In a character class the only metacharacters are:         class". In a character class the only metacharacters are:
2906    
2907           \      general escape character           \      general escape character
# Line 2852  CHARACTERS AND METACHARACTERS Line 2911  CHARACTERS AND METACHARACTERS
2911                    syntax)                    syntax)
2912           ]      terminates the character class           ]      terminates the character class
2913    
2914         The  following sections describe the use of each of the metacharacters.         The following sections describe the use of each of the  metacharacters.
2915    
2916    
2917  BACKSLASH  BACKSLASH
2918    
2919         The backslash character has several uses. Firstly, if it is followed by         The backslash character has several uses. Firstly, if it is followed by
2920         a  non-alphanumeric  character,  it takes away any special meaning that         a non-alphanumeric character, it takes away any  special  meaning  that
2921         character may have. This  use  of  backslash  as  an  escape  character         character  may  have.  This  use  of  backslash  as an escape character
2922         applies both inside and outside character classes.         applies both inside and outside character classes.
2923    
2924         For  example,  if  you want to match a * character, you write \* in the         For example, if you want to match a * character, you write  \*  in  the
2925         pattern.  This escaping action applies whether  or  not  the  following         pattern.   This  escaping  action  applies whether or not the following
2926         character  would  otherwise be interpreted as a metacharacter, so it is         character would otherwise be interpreted as a metacharacter, so  it  is
2927         always safe to precede a non-alphanumeric  with  backslash  to  specify         always  safe  to  precede  a non-alphanumeric with backslash to specify
2928         that  it stands for itself. In particular, if you want to match a back-         that it stands for itself. In particular, if you want to match a  back-
2929         slash, you write \\.         slash, you write \\.
2930    
2931         If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in         If  a  pattern is compiled with the PCRE_EXTENDED option, whitespace in
2932         the  pattern (other than in a character class) and characters between a         the pattern (other than in a character class) and characters between  a
2933         # outside a character class and the next newline are ignored. An escap-         # outside a character class and the next newline are ignored. An escap-
2934         ing  backslash  can  be  used to include a whitespace or # character as         ing backslash can be used to include a whitespace  or  #  character  as
2935         part of the pattern.         part of the pattern.
2936    
2937         If you want to remove the special meaning from a  sequence  of  charac-         If  you  want  to remove the special meaning from a sequence of charac-
2938         ters,  you can do so by putting them between \Q and \E. This is differ-         ters, you can do so by putting them between \Q and \E. This is  differ-
2939         ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E         ent  from  Perl  in  that  $  and  @ are handled as literals in \Q...\E
2940         sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-         sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
2941         tion. Note the following examples:         tion. Note the following examples:
2942    
2943           Pattern            PCRE matches   Perl matches           Pattern            PCRE matches   Perl matches
# Line 2888  BACKSLASH Line 2947  BACKSLASH
2947           \Qabc\$xyz\E       abc\$xyz       abc\$xyz           \Qabc\$xyz\E       abc\$xyz       abc\$xyz
2948           \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz           \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
2949    
2950         The \Q...\E sequence is recognized both inside  and  outside  character         The  \Q...\E  sequence  is recognized both inside and outside character
2951         classes.         classes.
2952    
2953     Non-printing characters     Non-printing characters
2954    
2955         A second use of backslash provides a way of encoding non-printing char-         A second use of backslash provides a way of encoding non-printing char-
2956         acters in patterns in a visible manner. There is no restriction on  the         acters  in patterns in a visible manner. There is no restriction on the
2957         appearance  of non-printing characters, apart from the binary zero that         appearance of non-printing characters, apart from the binary zero  that
2958         terminates a pattern, but when a pattern  is  being  prepared  by  text         terminates  a  pattern,  but  when  a pattern is being prepared by text
2959         editing,  it  is  usually  easier  to  use  one of the following escape         editing, it is usually easier  to  use  one  of  the  following  escape
2960         sequences than the binary character it represents:         sequences than the binary character it represents:
2961    
2962           \a        alarm, that is, the BEL character (hex 07)           \a        alarm, that is, the BEL character (hex 07)
2963           \cx       "control-x", where x is any character           \cx       "control-x", where x is any character
2964           \e        escape (hex 1B)           \e        escape (hex 1B)
2965           \f        formfeed (hex 0C)           \f        formfeed (hex 0C)
2966           \n        newline (hex 0A)           \n        linefeed (hex 0A)
2967           \r        carriage return (hex 0D)           \r        carriage return (hex 0D)
2968           \t        tab (hex 09)           \t        tab (hex 09)
2969           \ddd      character with octal code ddd, or backreference           \ddd      character with octal code ddd, or backreference
2970           \xhh      character with hex code hh           \xhh      character with hex code hh
2971           \x{hhh..} character with hex code hhh..           \x{hhh..} character with hex code hhh..
2972    
2973         The precise effect of \cx is as follows: if x is a lower  case  letter,         The  precise  effect of \cx is as follows: if x is a lower case letter,
2974         it  is converted to upper case. Then bit 6 of the character (hex 40) is         it is converted to upper case. Then bit 6 of the character (hex 40)  is
2975         inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;         inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
2976         becomes hex 7B.         becomes hex 7B.
2977    
2978         After  \x, from zero to two hexadecimal digits are read (letters can be         After \x, from zero to two hexadecimal digits are read (letters can  be
2979         in upper or lower case). Any number of hexadecimal  digits  may  appear         in  upper  or  lower case). Any number of hexadecimal digits may appear
2980         between  \x{  and  },  but the value of the character code must be less         between \x{ and }, but the value of the character  code  must  be  less
2981         than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,         than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
2982         the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger         the maximum value in hexadecimal is 7FFFFFFF. Note that this is  bigger
2983         than the largest Unicode code point, which is 10FFFF.         than the largest Unicode code point, which is 10FFFF.
2984    
2985         If characters other than hexadecimal digits appear between \x{  and  },         If  characters  other than hexadecimal digits appear between \x{ and },
2986         or if there is no terminating }, this form of escape is not recognized.         or if there is no terminating }, this form of escape is not recognized.
2987         Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal         Instead,  the  initial  \x  will  be interpreted as a basic hexadecimal
2988         escape,  with  no  following  digits, giving a character whose value is         escape, with no following digits, giving a  character  whose  value  is
2989         zero.         zero.
2990    
2991         Characters whose value is less than 256 can be defined by either of the         Characters whose value is less than 256 can be defined by either of the
2992         two  syntaxes  for  \x. There is no difference in the way they are han-         two syntaxes for \x. There is no difference in the way  they  are  han-
2993         dled. For example, \xdc is exactly the same as \x{dc}.         dled. For example, \xdc is exactly the same as \x{dc}.
2994    
2995         After \0 up to two further octal digits are read. If  there  are  fewer         After  \0  up  to two further octal digits are read. If there are fewer
2996         than  two  digits,  just  those  that  are  present  are used. Thus the         than two digits, just  those  that  are  present  are  used.  Thus  the
2997         sequence \0\x\07 specifies two binary zeros followed by a BEL character         sequence \0\x\07 specifies two binary zeros followed by a BEL character
2998         (code  value 7). Make sure you supply two digits after the initial zero         (code value 7). Make sure you supply two digits after the initial  zero
2999         if the pattern character that follows is itself an octal digit.         if the pattern character that follows is itself an octal digit.
3000    
3001         The handling of a backslash followed by a digit other than 0 is compli-         The handling of a backslash followed by a digit other than 0 is compli-
3002         cated.  Outside a character class, PCRE reads it and any following dig-         cated.  Outside a character class, PCRE reads it and any following dig-
3003         its as a decimal number. If the number is less than  10,  or  if  there         its  as  a  decimal  number. If the number is less than 10, or if there
3004         have been at least that many previous capturing left parentheses in the         have been at least that many previous capturing left parentheses in the
3005         expression, the entire  sequence  is  taken  as  a  back  reference.  A         expression,  the  entire  sequence  is  taken  as  a  back reference. A
3006         description  of how this works is given later, following the discussion         description of how this works is given later, following the  discussion
3007         of parenthesized subpatterns.         of parenthesized subpatterns.
3008    
3009         Inside a character class, or if the decimal number is  greater  than  9         Inside  a  character  class, or if the decimal number is greater than 9
3010         and  there have not been that many capturing subpatterns, PCRE re-reads         and there have not been that many capturing subpatterns, PCRE  re-reads
3011         up to three octal digits following the backslash, and uses them to gen-         up to three octal digits following the backslash, and uses them to gen-
3012         erate  a data character. Any subsequent digits stand for themselves. In         erate a data character. Any subsequent digits stand for themselves.  In
3013         non-UTF-8 mode, the value of a character specified  in  octal  must  be         non-UTF-8  mode,  the  value  of a character specified in octal must be
3014         less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For         less than \400. In UTF-8 mode, values up to  \777  are  permitted.  For
3015         example:         example:
3016    
3017           \040   is another way of writing a space           \040   is another way of writing a space
# Line 2970  BACKSLASH Line 3029  BACKSLASH
3029           \81    is either a back reference, or a binary zero           \81    is either a back reference, or a binary zero
3030                     followed by the two characters "8" and "1"                     followed by the two characters "8" and "1"
3031    
3032         Note that octal values of 100 or greater must not be  introduced  by  a         Note  that  octal  values of 100 or greater must not be introduced by a
3033         leading zero, because no more than three octal digits are ever read.         leading zero, because no more than three octal digits are ever read.
3034    
3035         All the sequences that define a single character value can be used both         All the sequences that define a single character value can be used both
3036         inside and outside character classes. In addition, inside  a  character         inside  and  outside character classes. In addition, inside a character
3037         class,  the  sequence \b is interpreted as the backspace character (hex         class, the sequence \b is interpreted as the backspace  character  (hex
3038         08), and the sequences \R and \X are interpreted as the characters  "R"         08),  and the sequences \R and \X are interpreted as the characters "R"
3039         and  "X", respectively. Outside a character class, these sequences have         and "X", respectively. Outside a character class, these sequences  have
3040         different meanings (see below).         different meanings (see below).
3041    
3042     Absolute and relative back references     Absolute and relative back references
3043    
3044         The sequence \g followed by an unsigned or a negative  number,  option-         The  sequence  \g followed by an unsigned or a negative number, option-
3045         ally  enclosed  in braces, is an absolute or relative back reference. A         ally enclosed in braces, is an absolute or relative back  reference.  A
3046         named back reference can be coded as \g{name}. Back references are dis-         named back reference can be coded as \g{name}. Back references are dis-
3047         cussed later, following the discussion of parenthesized subpatterns.         cussed later, following the discussion of parenthesized subpatterns.
3048    
# Line 3004  BACKSLASH Line 3063  BACKSLASH
3063           \W     any "non-word" character           \W     any "non-word" character
3064    
3065         Each pair of escape sequences partitions the complete set of characters         Each pair of escape sequences partitions the complete set of characters
3066         into  two disjoint sets. Any given character matches one, and only one,         into two disjoint sets. Any given character matches one, and only  one,
3067         of each pair.         of each pair.
3068    
3069         These character type sequences can appear both inside and outside char-         These character type sequences can appear both inside and outside char-
3070         acter  classes.  They each match one character of the appropriate type.         acter classes. They each match one character of the  appropriate  type.
3071         If the current matching point is at the end of the subject string,  all         If  the current matching point is at the end of the subject string, all
3072         of them fail, since there is no character to match.         of them fail, since there is no character to match.
3073    
3074         For  compatibility  with Perl, \s does not match the VT character (code         For compatibility with Perl, \s does not match the VT  character  (code
3075         11).  This makes it different from the the POSIX "space" class. The  \s         11).   This makes it different from the the POSIX "space" class. The \s
3076         characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If         characters are HT (9), LF (10), FF (12), CR (13), and  space  (32).  If
3077         "use locale;" is included in a Perl script, \s may match the VT charac-         "use locale;" is included in a Perl script, \s may match the VT charac-
3078         ter. In PCRE, it never does.         ter. In PCRE, it never does.
3079    
3080         In  UTF-8 mode, characters with values greater than 128 never match \d,         In UTF-8 mode, characters with values greater than 128 never match  \d,
3081         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3082         code  character  property  support is available. These sequences retain         code character property support is available.  These  sequences  retain
3083         their original meanings from before UTF-8 support was available, mainly         their original meanings from before UTF-8 support was available, mainly
3084         for efficiency reasons.         for efficiency reasons.
3085    
3086         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3087         the other sequences, these do match certain high-valued  codepoints  in         the  other  sequences, these do match certain high-valued codepoints in
3088         UTF-8 mode.  The horizontal space characters are:         UTF-8 mode.  The horizontal space characters are:
3089    
3090           U+0009     Horizontal tab           U+0009     Horizontal tab
# Line 3059  BACKSLASH Line 3118  BACKSLASH
3118           U+2029     Paragraph separator           U+2029     Paragraph separator
3119    
3120         A "word" character is an underscore or any character less than 256 that         A "word" character is an underscore or any character less than 256 that
3121         is a letter or digit. The definition of  letters  and  digits  is  con-         is  a  letter  or  digit.  The definition of letters and digits is con-
3122         trolled  by PCRE's low-valued character tables, and may vary if locale-         trolled by PCRE's low-valued character tables, and may vary if  locale-
3123         specific matching is taking place (see "Locale support" in the  pcreapi         specific  matching is taking place (see "Locale support" in the pcreapi
3124         page).  For  example,  in  a French locale such as "fr_FR" in Unix-like         page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
3125         systems, or "french" in Windows, some character codes greater than  128         systems,  or "french" in Windows, some character codes greater than 128
3126         are  used for accented letters, and these are matched by \w. The use of         are used for accented letters, and these are matched by \w. The use  of
3127         locales with Unicode is discouraged.         locales with Unicode is discouraged.
3128    
3129     Newline sequences     Newline sequences
3130    
3131         Outside a character class, the escape sequence \R matches  any  Unicode         Outside  a  character class, the escape sequence \R matches any Unicode
3132         newline  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is         newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R  is
3133         equivalent to the following:         equivalent to the following:
3134    
3135           (?>\r\n|\n|\x0b|\f|\r|\x85)           (?>\r\n|\n|\x0b|\f|\r|\x85)
3136    
3137         This is an example of an "atomic group", details  of  which  are  given         This  is  an  example  of an "atomic group", details of which are given
3138         below.  This particular group matches either the two-character sequence         below.  This particular group matches either the two-character sequence
3139         CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,         CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
3140         U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage         U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3141         return, U+000D), or NEL (next line, U+0085). The two-character sequence         return, U+000D), or NEL (next line, U+0085). The two-character sequence
3142         is treated as a single unit that cannot be split.         is treated as a single unit that cannot be split.
3143    
3144         In  UTF-8  mode, two additional characters whose codepoints are greater         In UTF-8 mode, two additional characters whose codepoints  are  greater
3145         than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-         than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3146         rator,  U+2029).   Unicode character property support is not needed for         rator, U+2029).  Unicode character property support is not  needed  for
3147         these characters to be recognized.         these characters to be recognized.
3148    
3149         Inside a character class, \R matches the letter "R".         Inside a character class, \R matches the letter "R".
# Line 3092  BACKSLASH Line 3151  BACKSLASH
3151     Unicode character properties     Unicode character properties
3152    
3153         When PCRE is built with Unicode character property support, three addi-         When PCRE is built with Unicode character property support, three addi-
3154         tional  escape sequences that match characters with specific properties         tional escape sequences that match characters with specific  properties
3155         are available.  When not in UTF-8 mode, these sequences are  of  course         are  available.   When not in UTF-8 mode, these sequences are of course
3156         limited  to  testing characters whose codepoints are less than 256, but         limited to testing characters whose codepoints are less than  256,  but
3157         they do work in this mode.  The extra escape sequences are:         they do work in this mode.  The extra escape sequences are:
3158    
3159           \p{xx}   a character with the xx property           \p{xx}   a character with the xx property
3160           \P{xx}   a character without the xx property           \P{xx}   a character without the xx property
3161           \X       an extended Unicode sequence           \X       an extended Unicode sequence
3162    
3163         The property names represented by xx above are limited to  the  Unicode         The  property  names represented by xx above are limited to the Unicode
3164         script names, the general category properties, and "Any", which matches         script names, the general category properties, and "Any", which matches
3165         any character (including newline). Other properties such as "InMusical-         any character (including newline). Other properties such as "InMusical-
3166         Symbols"  are  not  currently supported by PCRE. Note that \P{Any} does         Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does
3167         not match any characters, so always causes a match failure.         not match any characters, so always causes a match failure.
3168    
3169         Sets of Unicode characters are defined as belonging to certain scripts.         Sets of Unicode characters are defined as belonging to certain scripts.
3170         A  character from one of these sets can be matched using a script name.         A character from one of these sets can be matched using a script  name.
3171         For example:         For example:
3172    
3173           \p{Greek}           \p{Greek}
3174           \P{Han}           \P{Han}
3175    
3176         Those that are not part of an identified script are lumped together  as         Those  that are not part of an identified script are lumped together as
3177         "Common". The current list of scripts is:         "Common". The current list of scripts is:
3178    
3179         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
3180         Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,         Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,
3181         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3182         Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-         Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3183         gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,         gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,
3184         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
3185         Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,         Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,
3186         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
3187         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3188    
3189         Each  character has exactly one general category property, specified by         Each character has exactly one general category property, specified  by
3190         a two-letter abbreviation. For compatibility with Perl, negation can be         a two-letter abbreviation. For compatibility with Perl, negation can be
3191         specified  by  including a circumflex between the opening brace and the         specified by including a circumflex between the opening brace  and  the
3192         property name. For example, \p{^Lu} is the same as \P{Lu}.         property name. For example, \p{^Lu} is the same as \P{Lu}.
3193    
3194         If only one letter is specified with \p or \P, it includes all the gen-         If only one letter is specified with \p or \P, it includes all the gen-
3195         eral  category properties that start with that letter. In this case, in         eral category properties that start with that letter. In this case,  in
3196         the absence of negation, the curly brackets in the escape sequence  are         the  absence of negation, the curly brackets in the escape sequence are
3197         optional; these two examples have the same effect:         optional; these two examples have the same effect:
3198    
3199           \p{L}           \p{L}
# Line 3186  BACKSLASH Line 3245  BACKSLASH
3245           Zp    Paragraph separator           Zp    Paragraph separator
3246           Zs    Space separator           Zs    Space separator
3247    
3248         The  special property L& is also supported: it matches a character that         The special property L& is also supported: it matches a character  that
3249         has the Lu, Ll, or Lt property, in other words, a letter  that  is  not         has  the  Lu,  Ll, or Lt property, in other words, a letter that is not
3250         classified as a modifier or "other".         classified as a modifier or "other".
3251    
3252         The  Cs  (Surrogate)  property  applies only to characters in the range         The Cs (Surrogate) property applies only to  characters  in  the  range
3253         U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see         U+D800  to  U+DFFF. Such characters are not valid in UTF-8 strings (see
3254         RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-         RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3255         ing has been turned off (see the discussion  of  PCRE_NO_UTF8_CHECK  in         ing  has  been  turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3256         the pcreapi page).         the pcreapi page).
3257    
3258         The  long  synonyms  for  these  properties that Perl supports (such as         The long synonyms for these properties  that  Perl  supports  (such  as
3259         \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix         \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
3260         any of these properties with "Is".         any of these properties with "Is".
3261    
3262         No character that is in the Unicode table has the Cn (unassigned) prop-         No character that is in the Unicode table has the Cn (unassigned) prop-
3263         erty.  Instead, this property is assumed for any code point that is not         erty.  Instead, this property is assumed for any code point that is not
3264         in the Unicode table.         in the Unicode table.
3265    
3266         Specifying  caseless  matching  does not affect these escape sequences.         Specifying caseless matching does not affect  these  escape  sequences.
3267         For example, \p{Lu} always matches only upper case letters.         For example, \p{Lu} always matches only upper case letters.
3268    
3269         The \X escape matches any number of Unicode  characters  that  form  an         The  \X  escape  matches  any number of Unicode characters that form an
3270         extended Unicode sequence. \X is equivalent to         extended Unicode sequence. \X is equivalent to
3271    
3272           (?>\PM\pM*)           (?>\PM\pM*)
3273    
3274         That  is,  it matches a character without the "mark" property, followed         That is, it matches a character without the "mark"  property,  followed
3275         by zero or more characters with the "mark"  property,  and  treats  the         by  zero  or  more  characters with the "mark" property, and treats the
3276         sequence  as  an  atomic group (see below).  Characters with the "mark"         sequence as an atomic group (see below).  Characters  with  the  "mark"
3277         property are typically accents that  affect  the  preceding  character.         property  are  typically  accents  that affect the preceding character.
3278         None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X         None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
3279         matches any one character.         matches any one character.
3280    
3281         Matching characters by Unicode property is not fast, because  PCRE  has         Matching  characters  by Unicode property is not fast, because PCRE has
3282         to  search  a  structure  that  contains data for over fifteen thousand         to search a structure that contains  data  for  over  fifteen  thousand
3283         characters. That is why the traditional escape sequences such as \d and         characters. That is why the traditional escape sequences such as \d and
3284         \w do not use Unicode properties in PCRE.         \w do not use Unicode properties in PCRE.
3285    
3286     Resetting the match start     Resetting the match start
3287    
3288         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3289         ously matched characters not  to  be  included  in  the  final  matched         ously  matched  characters  not  to  be  included  in the final matched
3290         sequence. For example, the pattern:         sequence. For example, the pattern:
3291    
3292           foo\Kbar           foo\Kbar
3293    
3294         matches  "foobar",  but reports that it has matched "bar". This feature         matches "foobar", but reports that it has matched "bar".  This  feature
3295         is similar to a lookbehind assertion (described  below).   However,  in         is  similar  to  a lookbehind assertion (described below).  However, in
3296         this  case, the part of the subject before the real match does not have         this case, the part of the subject before the real match does not  have
3297         to be of fixed length, as lookbehind assertions do. The use of \K  does         to  be of fixed length, as lookbehind assertions do. The use of \K does
3298         not  interfere  with  the setting of captured substrings.  For example,         not interfere with the setting of captured  substrings.   For  example,
3299         when the pattern         when the pattern
3300    
3301           (foo)\Kbar           (foo)\Kbar
# Line 3245  BACKSLASH Line 3304  BACKSLASH
3304    
3305     Simple assertions     Simple assertions
3306    
3307         The final use of backslash is for certain simple assertions. An  asser-         The  final use of backslash is for certain simple assertions. An asser-
3308         tion  specifies a condition that has to be met at a particular point in         tion specifies a condition that has to be met at a particular point  in
3309         a match, without consuming any characters from the subject string.  The         a  match, without consuming any characters from the subject string. The
3310         use  of subpatterns for more complicated assertions is described below.         use of subpatterns for more complicated assertions is described  below.
3311         The backslashed assertions are:         The backslashed assertions are:
3312    
3313           \b     matches at a word boundary           \b     matches at a word boundary
# Line 3259  BACKSLASH Line 3318  BACKSLASH
3318           \z     matches only at the end of the subject           \z     matches only at the end of the subject
3319           \G     matches at the first matching position in the subject           \G     matches at the first matching position in the subject
3320    
3321         These assertions may not appear in character classes (but note that  \b         These  assertions may not appear in character classes (but note that \b
3322         has a different meaning, namely the backspace character, inside a char-         has a different meaning, namely the backspace character, inside a char-
3323         acter class).         acter class).
3324    
3325         A word boundary is a position in the subject string where  the  current         A  word  boundary is a position in the subject string where the current
3326         character  and  the previous character do not both match \w or \W (i.e.         character and the previous character do not both match \w or  \W  (i.e.
3327         one matches \w and the other matches \W), or the start or  end  of  the         one  matches  \w  and the other matches \W), or the start or end of the
3328         string if the first or last character matches \w, respectively.         string if the first or last character matches \w, respectively.
3329    
3330         The  \A,  \Z,  and \z assertions differ from the traditional circumflex         The \A, \Z, and \z assertions differ from  the  traditional  circumflex
3331         and dollar (described in the next section) in that they only ever match         and dollar (described in the next section) in that they only ever match
3332         at  the  very start and end of the subject string, whatever options are         at the very start and end of the subject string, whatever  options  are
3333         set. Thus, they are independent of multiline mode. These  three  asser-         set.  Thus,  they are independent of multiline mode. These three asser-
3334         tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which         tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3335         affect only the behaviour of the circumflex and dollar  metacharacters.         affect  only the behaviour of the circumflex and dollar metacharacters.
3336         However,  if the startoffset argument of pcre_exec() is non-zero, indi-         However, if the startoffset argument of pcre_exec() is non-zero,  indi-
3337         cating that matching is to start at a point other than the beginning of         cating that matching is to start at a point other than the beginning of
3338         the  subject,  \A  can never match. The difference between \Z and \z is         the subject, \A can never match. The difference between \Z  and  \z  is
3339         that \Z matches before a newline at the end of the string as well as at         that \Z matches before a newline at the end of the string as well as at
3340         the very end, whereas \z matches only at the end.         the very end, whereas \z matches only at the end.
3341    
3342         The  \G assertion is true only when the current matching position is at         The \G assertion is true only when the current matching position is  at
3343         the start point of the match, as specified by the startoffset  argument         the  start point of the match, as specified by the startoffset argument
3344         of  pcre_exec().  It  differs  from \A when the value of startoffset is         of pcre_exec(). It differs from \A when the  value  of  startoffset  is
3345         non-zero. By calling pcre_exec() multiple times with appropriate  argu-         non-zero.  By calling pcre_exec() multiple times with appropriate argu-
3346         ments, you can mimic Perl's /g option, and it is in this kind of imple-         ments, you can mimic Perl's /g option, and it is in this kind of imple-
3347         mentation where \G can be useful.         mentation where \G can be useful.
3348    
3349         Note, however, that PCRE's interpretation of \G, as the  start  of  the         Note,  however,  that  PCRE's interpretation of \G, as the start of the
3350         current match, is subtly different from Perl's, which defines it as the         current match, is subtly different from Perl's, which defines it as the
3351         end of the previous match. In Perl, these can  be  different  when  the         end  of  the  previous  match. In Perl, these can be different when the
3352         previously  matched  string was empty. Because PCRE does just one match         previously matched string was empty. Because PCRE does just  one  match
3353         at a time, it cannot reproduce this behaviour.         at a time, it cannot reproduce this behaviour.
3354    
3355         If all the alternatives of a pattern begin with \G, the  expression  is         If  all  the alternatives of a pattern begin with \G, the expression is
3356         anchored to the starting match position, and the "anchored" flag is set         anchored to the starting match position, and the "anchored" flag is set
3357         in the compiled regular expression.         in the compiled regular expression.
3358    
# Line 3301  BACKSLASH Line 3360  BACKSLASH
3360  CIRCUMFLEX AND DOLLAR  CIRCUMFLEX AND DOLLAR
3361    
3362         Outside a character class, in the default matching mode, the circumflex         Outside a character class, in the default matching mode, the circumflex
3363         character  is  an  assertion  that is true only if the current matching         character is an assertion that is true only  if  the  current  matching
3364         point is at the start of the subject string. If the  startoffset  argu-         point  is  at the start of the subject string. If the startoffset argu-
3365         ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the         ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
3366         PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex         PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
3367         has an entirely different meaning (see below).         has an entirely different meaning (see below).
3368    
3369         Circumflex  need  not be the first character of the pattern if a number         Circumflex need not be the first character of the pattern if  a  number
3370         of alternatives are involved, but it should be the first thing in  each         of  alternatives are involved, but it should be the first thing in each
3371         alternative  in  which  it appears if the pattern is ever to match that         alternative in which it appears if the pattern is ever  to  match  that
3372         branch. If all possible alternatives start with a circumflex, that  is,         branch.  If all possible alternatives start with a circumflex, that is,
3373         if  the  pattern  is constrained to match only at the start of the sub-         if the pattern is constrained to match only at the start  of  the  sub-
3374         ject, it is said to be an "anchored" pattern.  (There  are  also  other         ject,  it  is  said  to be an "anchored" pattern. (There are also other
3375         constructs that can cause a pattern to be anchored.)         constructs that can cause a pattern to be anchored.)
3376    
3377         A  dollar  character  is  an assertion that is true only if the current         A dollar character is an assertion that is true  only  if  the  current
3378         matching point is at the end of  the  subject  string,  or  immediately         matching  point  is  at  the  end of the subject string, or immediately
3379         before a newline at the end of the string (by default). Dollar need not         before a newline at the end of the string (by default). Dollar need not
3380         be the last character of the pattern if a number  of  alternatives  are         be  the  last  character of the pattern if a number of alternatives are
3381         involved,  but  it  should  be  the last item in any branch in which it         involved, but it should be the last item in  any  branch  in  which  it
3382         appears. Dollar has no special meaning in a character class.         appears. Dollar has no special meaning in a character class.
3383    
3384         The meaning of dollar can be changed so that it  matches  only  at  the         The  meaning  of  dollar  can be changed so that it matches only at the
3385         very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at         very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
3386         compile time. This does not affect the \Z assertion.         compile time. This does not affect the \Z assertion.
3387    
3388         The meanings of the circumflex and dollar characters are changed if the         The meanings of the circumflex and dollar characters are changed if the
3389         PCRE_MULTILINE  option  is  set.  When  this  is the case, a circumflex         PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex
3390         matches immediately after internal newlines as well as at the start  of         matches  immediately after internal newlines as well as at the start of
3391         the  subject  string.  It  does not match after a newline that ends the         the subject string. It does not match after a  newline  that  ends  the
3392         string. A dollar matches before any newlines in the string, as well  as         string.  A dollar matches before any newlines in the string, as well as
3393         at  the very end, when PCRE_MULTILINE is set. When newline is specified         at the very end, when PCRE_MULTILINE is set. When newline is  specified
3394         as the two-character sequence CRLF, isolated CR and  LF  characters  do         as  the  two-character  sequence CRLF, isolated CR and LF characters do
3395         not indicate newlines.         not indicate newlines.
3396    
3397         For  example, the pattern /^abc$/ matches the subject string "def\nabc"         For example, the pattern /^abc$/ matches the subject string  "def\nabc"
3398         (where \n represents a newline) in multiline mode, but  not  otherwise.         (where  \n  represents a newline) in multiline mode, but not otherwise.
3399         Consequently,  patterns  that  are anchored in single line mode because         Consequently, patterns that are anchored in single  line  mode  because
3400         all branches start with ^ are not anchored in  multiline  mode,  and  a         all  branches  start  with  ^ are not anchored in multiline mode, and a
3401         match  for  circumflex  is  possible  when  the startoffset argument of         match for circumflex is  possible  when  the  startoffset  argument  of
3402         pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is  ignored  if         pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3403         PCRE_MULTILINE is set.         PCRE_MULTILINE is set.
3404    
3405         Note  that  the sequences \A, \Z, and \z can be used to match the start         Note that the sequences \A, \Z, and \z can be used to match  the  start
3406         and end of the subject in both modes, and if all branches of a  pattern         and  end of the subject in both modes, and if all branches of a pattern
3407         start  with  \A it is always anchored, whether or not PCRE_MULTILINE is         start with \A it is always anchored, whether or not  PCRE_MULTILINE  is
3408         set.         set.
3409    
3410    
3411  FULL STOP (PERIOD, DOT)  FULL STOP (PERIOD, DOT)
3412    
3413         Outside a character class, a dot in the pattern matches any one charac-         Outside a character class, a dot in the pattern matches any one charac-
3414         ter  in  the subject string except (by default) a character that signi-         ter in the subject string except (by default) a character  that  signi-
3415         fies the end of a line. In UTF-8 mode, the  matched  character  may  be         fies  the  end  of  a line. In UTF-8 mode, the matched character may be
3416         more than one byte long.         more than one byte long.
3417    
3418         When  a line ending is defined as a single character, dot never matches         When a line ending is defined as a single character, dot never  matches
3419         that character; when the two-character sequence CRLF is used, dot  does         that  character; when the two-character sequence CRLF is used, dot does
3420         not  match  CR  if  it  is immediately followed by LF, but otherwise it         not match CR if it is immediately followed  by  LF,  but  otherwise  it
3421         matches all characters (including isolated CRs and LFs). When any  Uni-         matches  all characters (including isolated CRs and LFs). When any Uni-
3422         code  line endings are being recognized, dot does not match CR or LF or         code line endings are being recognized, dot does not match CR or LF  or
3423         any of the other line ending characters.         any of the other line ending characters.
3424    
3425         The behaviour of dot with regard to newlines can  be  changed.  If  the         The  behaviour  of  dot  with regard to newlines can be changed. If the
3426         PCRE_DOTALL  option  is  set,  a dot matches any one character, without         PCRE_DOTALL option is set, a dot matches  any  one  character,  without
3427         exception. If the two-character sequence CRLF is present in the subject         exception. If the two-character sequence CRLF is present in the subject
3428         string, it takes two dots to match it.         string, it takes two dots to match it.
3429    
3430         The  handling of dot is entirely independent of the handling of circum-         The handling of dot is entirely independent of the handling of  circum-
3431         flex and dollar, the only relationship being  that  they  both  involve         flex  and  dollar,  the  only relationship being that they both involve
3432         newlines. Dot has no special meaning in a character class.         newlines. Dot has no special meaning in a character class.
3433    
3434    
3435  MATCHING A SINGLE BYTE  MATCHING A SINGLE BYTE
3436    
3437         Outside a character class, the escape sequence \C matches any one byte,         Outside a character class, the escape sequence \C matches any one byte,
3438         both in and out of UTF-8 mode. Unlike a  dot,  it  always  matches  any         both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any
3439         line-ending  characters.  The  feature  is provided in Perl in order to         line-ending characters. The feature is provided in  Perl  in  order  to
3440         match individual bytes in UTF-8 mode. Because it breaks up UTF-8  char-         match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3441         acters  into individual bytes, what remains in the string may be a mal-         acters into individual bytes, what remains in the string may be a  mal-
3442         formed UTF-8 string. For this reason, the \C escape  sequence  is  best         formed  UTF-8  string.  For this reason, the \C escape sequence is best
3443         avoided.         avoided.
3444    
3445         PCRE  does  not  allow \C to appear in lookbehind assertions (described         PCRE does not allow \C to appear in  lookbehind  assertions  (described
3446         below), because in UTF-8 mode this would make it impossible  to  calcu-         below),  because  in UTF-8 mode this would make it impossible to calcu-
3447         late the length of the lookbehind.         late the length of the lookbehind.
3448    
3449    
# Line 3393  SQUARE BRACKETS AND CHARACTER CLASSES Line 3452  SQUARE BRACKETS AND CHARACTER CLASSES
3452         An opening square bracket introduces a character class, terminated by a         An opening square bracket introduces a character class, terminated by a
3453         closing square bracket. A closing square bracket on its own is not spe-         closing square bracket. A closing square bracket on its own is not spe-
3454         cial. If a closing square bracket is required as a member of the class,         cial. If a closing square bracket is required as a member of the class,
3455         it should be the first data character in the class  (after  an  initial         it  should  be  the first data character in the class (after an initial
3456         circumflex, if present) or escaped with a backslash.         circumflex, if present) or escaped with a backslash.
3457    
3458         A  character  class matches a single character in the subject. In UTF-8         A character class matches a single character in the subject.  In  UTF-8
3459         mode, the character may occupy more than one byte. A matched  character         mode,  the character may occupy more than one byte. A matched character
3460         must be in the set of characters defined by the class, unless the first         must be in the set of characters defined by the class, unless the first
3461         character in the class definition is a circumflex, in  which  case  the         character  in  the  class definition is a circumflex, in which case the
3462         subject  character  must  not  be in the set defined by the class. If a         subject character must not be in the set defined by  the  class.  If  a
3463         circumflex is actually required as a member of the class, ensure it  is         circumflex  is actually required as a member of the class, ensure it is
3464         not the first character, or escape it with a backslash.         not the first character, or escape it with a backslash.
3465    
3466         For  example, the character class [aeiou] matches any lower case vowel,         For example, the character class [aeiou] matches any lower case  vowel,
3467         while [^aeiou] matches any character that is not a  lower  case  vowel.         while  [^aeiou]  matches  any character that is not a lower case vowel.
3468         Note that a circumflex is just a convenient notation for specifying the         Note that a circumflex is just a convenient notation for specifying the
3469         characters that are in the class by enumerating those that are  not.  A         characters  that  are in the class by enumerating those that are not. A
3470         class  that starts with a circumflex is not an assertion: it still con-         class that starts with a circumflex is not an assertion: it still  con-
3471         sumes a character from the subject string, and therefore  it  fails  if         sumes  a  character  from the subject string, and therefore it fails if
3472         the current pointer is at the end of the string.         the current pointer is at the end of the string.
3473    
3474         In  UTF-8 mode, characters with values greater than 255 can be included         In UTF-8 mode, characters with values greater than 255 can be  included
3475         in a class as a literal string of bytes, or by using the  \x{  escaping         in  a  class as a literal string of bytes, or by using the \x{ escaping
3476         mechanism.         mechanism.
3477    
3478         When  caseless  matching  is set, any letters in a class represent both         When caseless matching is set, any letters in a  class  represent  both
3479         their upper case and lower case versions, so for  example,  a  caseless         their  upper  case  and lower case versions, so for example, a caseless
3480         [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not         [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
3481         match "A", whereas a caseful version would. In UTF-8 mode, PCRE  always         match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3482         understands  the  concept  of case for characters whose values are less         understands the concept of case for characters whose  values  are  less
3483         than 128, so caseless matching is always possible. For characters  with         than  128, so caseless matching is always possible. For characters with
3484         higher  values,  the  concept  of case is supported if PCRE is compiled         higher values, the concept of case is supported  if  PCRE  is  compiled
3485         with Unicode property support, but not otherwise.  If you want  to  use         with  Unicode  property support, but not otherwise.  If you want to use
3486         caseless  matching  for  characters 128 and above, you must ensure that         caseless matching for characters 128 and above, you  must  ensure  that
3487         PCRE is compiled with Unicode property support as well  as  with  UTF-8         PCRE  is  compiled  with Unicode property support as well as with UTF-8
3488         support.         support.
3489    
3490         Characters  that  might  indicate  line breaks are never treated in any         Characters that might indicate line breaks are  never  treated  in  any
3491         special way  when  matching  character  classes,  whatever  line-ending         special  way  when  matching  character  classes,  whatever line-ending
3492         sequence  is  in  use,  and  whatever  setting  of  the PCRE_DOTALL and         sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
3493         PCRE_MULTILINE options is used. A class such as [^a] always matches one         PCRE_MULTILINE options is used. A class such as [^a] always matches one
3494         of these characters.         of these characters.
3495    
3496         The  minus (hyphen) character can be used to specify a range of charac-         The minus (hyphen) character can be used to specify a range of  charac-
3497         ters in a character  class.  For  example,  [d-m]  matches  any  letter         ters  in  a  character  class.  For  example,  [d-m] matches any letter
3498         between  d  and  m,  inclusive.  If  a minus character is required in a         between d and m, inclusive. If a  minus  character  is  required  in  a
3499         class, it must be escaped with a backslash  or  appear  in  a  position         class,  it  must  be  escaped  with a backslash or appear in a position
3500         where  it cannot be interpreted as indicating a range, typically as the         where it cannot be interpreted as indicating a range, typically as  the
3501         first or last character in the class.         first or last character in the class.
3502    
3503         It is not possible to have the literal character "]" as the end charac-         It is not possible to have the literal character "]" as the end charac-
3504         ter  of a range. A pattern such as [W-]46] is interpreted as a class of         ter of a range. A pattern such as [W-]46] is interpreted as a class  of
3505         two characters ("W" and "-") followed by a literal string "46]", so  it         two  characters ("W" and "-") followed by a literal string "46]", so it
3506         would  match  "W46]"  or  "-46]". However, if the "]" is escaped with a         would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
3507         backslash it is interpreted as the end of range, so [W-\]46] is  inter-         backslash  it is interpreted as the end of range, so [W-\]46] is inter-
3508         preted  as a class containing a range followed by two other characters.         preted as a class containing a range followed by two other  characters.
3509         The octal or hexadecimal representation of "]" can also be used to  end         The  octal or hexadecimal representation of "]" can also be used to end
3510         a range.         a range.
3511    
3512         Ranges  operate in the collating sequence of character values. They can         Ranges operate in the collating sequence of character values. They  can
3513         also  be  used  for  characters  specified  numerically,  for   example         also   be  used  for  characters  specified  numerically,  for  example
3514         [\000-\037].  In UTF-8 mode, ranges can include characters whose values         [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
3515         are greater than 255, for example [\x{100}-\x{2ff}].         are greater than 255, for example [\x{100}-\x{2ff}].
3516    
3517         If a range that includes letters is used when caseless matching is set,         If a range that includes letters is used when caseless matching is set,
3518         it matches the letters in either case. For example, [W-c] is equivalent         it matches the letters in either case. For example, [W-c] is equivalent
3519         to [][\\^_`wxyzabc], matched caselessly,  and  in  non-UTF-8  mode,  if         to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
3520         character  tables  for  a French locale are in use, [\xc8-\xcb] matches         character tables for a French locale are in  use,  [\xc8-\xcb]  matches
3521         accented E characters in both cases. In UTF-8 mode, PCRE  supports  the         accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
3522         concept  of  case for characters with values greater than 128 only when         concept of case for characters with values greater than 128  only  when
3523         it is compiled with Unicode property support.         it is compiled with Unicode property support.
3524    
3525         The character types \d, \D, \p, \P, \s, \S, \w, and \W may also  appear         The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3526         in  a  character  class,  and add the characters that they match to the         in a character class, and add the characters that  they  match  to  the
3527         class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-         class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3528         flex  can  conveniently  be used with the upper case character types to         flex can conveniently be used with the upper case  character  types  to
3529         specify a more restricted set of characters  than  the  matching  lower         specify  a  more  restricted  set of characters than the matching lower
3530         case  type.  For example, the class [^\W_] matches any letter or digit,         case type. For example, the class [^\W_] matches any letter  or  digit,
3531         but not underscore.         but not underscore.
3532    
3533         The only metacharacters that are recognized in  character  classes  are         The  only  metacharacters  that are recognized in character classes are
3534         backslash,  hyphen  (only  where  it can be interpreted as specifying a         backslash, hyphen (only where it can be  interpreted  as  specifying  a
3535         range), circumflex (only at the start), opening  square  bracket  (only         range),  circumflex  (only  at the start), opening square bracket (only
3536         when  it can be interpreted as introducing a POSIX class name - see the         when it can be interpreted as introducing a POSIX class name - see  the
3537         next section), and the terminating  closing  square  bracket.  However,         next  section),  and  the  terminating closing square bracket. However,
3538         escaping other non-alphanumeric characters does no harm.         escaping other non-alphanumeric characters does no harm.
3539    
3540    
3541  POSIX CHARACTER CLASSES  POSIX CHARACTER CLASSES
3542    
3543         Perl supports the POSIX notation for character classes. This uses names         Perl supports the POSIX notation for character classes. This uses names
3544         enclosed by [: and :] within the enclosing square brackets.  PCRE  also         enclosed  by  [: and :] within the enclosing square brackets. PCRE also
3545         supports this notation. For example,         supports this notation. For example,
3546    
3547           [01[:alpha:]%]           [01[:alpha:]%]
# Line 3505  POSIX CHARACTER CLASSES Line 3564  POSIX CHARACTER CLASSES
3564           word     "word" characters (same as \w)           word     "word" characters (same as \w)
3565           xdigit   hexadecimal digits           xdigit   hexadecimal digits
3566    
3567         The "space" characters are HT (9), LF (10), VT (11), FF (12), CR  (13),         The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3568         and  space  (32). Notice that this list includes the VT character (code         and space (32). Notice that this list includes the VT  character  (code
3569         11). This makes "space" different to \s, which does not include VT (for         11). This makes "space" different to \s, which does not include VT (for
3570         Perl compatibility).         Perl compatibility).
3571    
3572         The  name  "word"  is  a Perl extension, and "blank" is a GNU extension         The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
3573         from Perl 5.8. Another Perl extension is negation, which  is  indicated         from  Perl  5.8. Another Perl extension is negation, which is indicated
3574         by a ^ character after the colon. For example,         by a ^ character after the colon. For example,
3575    
3576           [12[:^digit:]]           [12[:^digit:]]
3577    
3578         matches  "1", "2", or any non-digit. PCRE (and Perl) also recognize the         matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
3579         POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but         POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3580         these are not supported, and an error is given if they are encountered.         these are not supported, and an error is given if they are encountered.
3581    
# Line 3526  POSIX CHARACTER CLASSES Line 3585  POSIX CHARACTER CLASSES
3585    
3586  VERTICAL BAR  VERTICAL BAR
3587    
3588         Vertical  bar characters are used to separate alternative patterns. For         Vertical bar characters are used to separate alternative patterns.  For
3589         example, the pattern         example, the pattern
3590    
3591           gilbert|sullivan           gilbert|sullivan
3592    
3593         matches either "gilbert" or "sullivan". Any number of alternatives  may         matches  either "gilbert" or "sullivan". Any number of alternatives may
3594         appear,  and  an  empty  alternative  is  permitted (matching the empty         appear, and an empty  alternative  is  permitted  (matching  the  empty
3595         string). The matching process tries each alternative in turn, from left         string). The matching process tries each alternative in turn, from left
3596         to  right, and the first one that succeeds is used. If the alternatives         to right, and the first one that succeeds is used. If the  alternatives
3597         are within a subpattern (defined below), "succeeds" means matching  the         are  within a subpattern (defined below), "succeeds" means matching the
3598         rest  of the main pattern as well as the alternative in the subpattern.         rest of the main pattern as well as the alternative in the  subpattern.
3599    
3600    
3601  INTERNAL OPTION SETTING  INTERNAL OPTION SETTING
3602    
3603         The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and         The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3604         PCRE_EXTENDED  options  can  be  changed  from  within the pattern by a         PCRE_EXTENDED options can be changed  from  within  the  pattern  by  a
3605         sequence of Perl option letters enclosed  between  "(?"  and  ")".  The         sequence  of  Perl  option  letters  enclosed between "(?" and ")". The
3606         option letters are         option letters are
3607    
3608           i  for PCRE_CASELESS           i  for PCRE_CASELESS
# Line 3553  INTERNAL OPTION SETTING Line 3612  INTERNAL OPTION SETTING
3612    
3613         For example, (?im) sets caseless, multiline matching. It is also possi-         For example, (?im) sets caseless, multiline matching. It is also possi-
3614         ble to unset these options by preceding the letter with a hyphen, and a         ble to unset these options by preceding the letter with a hyphen, and a
3615         combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-         combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
3616         LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,         LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3617         is  also  permitted.  If  a  letter  appears  both before and after the         is also permitted. If a  letter  appears  both  before  and  after  the
3618         hyphen, the option is unset.         hyphen, the option is unset.
3619    
3620         When an option change occurs at top level (that is, not inside  subpat-         When  an option change occurs at top level (that is, not inside subpat-
3621         tern  parentheses),  the change applies to the remainder of the pattern         tern parentheses), the change applies to the remainder of  the  pattern
3622         that follows.  If the change is placed right at the start of a pattern,         that follows.  If the change is placed right at the start of a pattern,
3623         PCRE extracts it into the global options (and it will therefore show up         PCRE extracts it into the global options (and it will therefore show up
3624         in data extracted by the pcre_fullinfo() function).         in data extracted by the pcre_fullinfo() function).
3625    
3626         An option change within a subpattern (see below for  a  description  of         An  option  change  within a subpattern (see below for a description of
3627         subpatterns) affects only that part of the current pattern that follows         subpatterns) affects only that part of the current pattern that follows
3628         it, so         it, so
3629    
3630           (a(?i)b)c           (a(?i)b)c
3631    
3632         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3633         used).   By  this means, options can be made to have different settings         used).  By this means, options can be made to have  different  settings
3634         in different parts of the pattern. Any changes made in one  alternative         in  different parts of the pattern. Any changes made in one alternative
3635         do  carry  on  into subsequent branches within the same subpattern. For         do carry on into subsequent branches within the  same  subpattern.  For
3636         example,         example,
3637    
3638           (a(?i)b|c)           (a(?i)b|c)
3639    
3640         matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the         matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
3641         first  branch  is  abandoned before the option setting. This is because         first branch is abandoned before the option setting.  This  is  because
3642         the effects of option settings happen at compile time. There  would  be         the  effects  of option settings happen at compile time. There would be
3643         some very weird behaviour otherwise.         some very weird behaviour otherwise.
3644    
3645         The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA         The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
3646         can be changed in the same way as the Perl-compatible options by  using         can  be changed in the same way as the Perl-compatible options by using
3647         the characters J, U and X respectively.         the characters J, U and X respectively.
3648    
3649    
# Line 3597  SUBPATTERNS Line 3656  SUBPATTERNS
3656    
3657           cat(aract|erpillar|)           cat(aract|erpillar|)
3658    
3659         matches one of the words "cat", "cataract", or  "caterpillar".  Without         matches  one  of the words "cat", "cataract", or "caterpillar". Without
3660         the  parentheses,  it  would  match  "cataract", "erpillar" or an empty         the parentheses, it would match  "cataract",  "erpillar"  or  an  empty
3661         string.         string.
3662    
3663         2. It sets up the subpattern as  a  capturing  subpattern.  This  means         2.  It  sets  up  the  subpattern as a capturing subpattern. This means
3664         that,  when  the  whole  pattern  matches,  that portion of the subject         that, when the whole pattern  matches,  that  portion  of  the  subject
3665         string that matched the subpattern is passed back to the caller via the         string that matched the subpattern is passed back to the caller via the
3666         ovector  argument  of pcre_exec(). Opening parentheses are counted from         ovector argument of pcre_exec(). Opening parentheses are  counted  from
3667         left to right (starting from 1) to obtain  numbers  for  the  capturing         left  to  right  (starting  from 1) to obtain numbers for the capturing
3668         subpatterns.         subpatterns.
3669    
3670         For  example,  if the string "the red king" is matched against the pat-         For example, if the string "the red king" is matched against  the  pat-
3671         tern         tern
3672    
3673           the ((red|white) (king|queen))           the ((red|white) (king|queen))
# Line 3616  SUBPATTERNS Line 3675  SUBPATTERNS
3675         the captured substrings are "red king", "red", and "king", and are num-         the captured substrings are "red king", "red", and "king", and are num-
3676         bered 1, 2, and 3, respectively.         bered 1, 2, and 3, respectively.
3677    
3678         The  fact  that  plain  parentheses  fulfil two functions is not always         The fact that plain parentheses fulfil  two  functions  is  not  always
3679         helpful.  There are often times when a grouping subpattern is  required         helpful.   There are often times when a grouping subpattern is required
3680         without  a capturing requirement. If an opening parenthesis is followed         without a capturing requirement. If an opening parenthesis is  followed
3681         by a question mark and a colon, the subpattern does not do any  captur-         by  a question mark and a colon, the subpattern does not do any captur-
3682         ing,  and  is  not  counted when computing the number of any subsequent         ing, and is not counted when computing the  number  of  any  subsequent
3683         capturing subpatterns. For example, if the string "the white queen"  is         capturing  subpatterns. For example, if the string "the white queen" is
3684         matched against the pattern         matched against the pattern
3685    
3686           the ((?:red|white) (king|queen))           the ((?:red|white) (king|queen))
# Line 3629  SUBPATTERNS Line 3688  SUBPATTERNS
3688         the captured substrings are "white queen" and "queen", and are numbered         the captured substrings are "white queen" and "queen", and are numbered
3689         1 and 2. The maximum number of capturing subpatterns is 65535.         1 and 2. The maximum number of capturing subpatterns is 65535.
3690    
3691         As a convenient shorthand, if any option settings are required  at  the         As  a  convenient shorthand, if any option settings are required at the
3692         start  of  a  non-capturing  subpattern,  the option letters may appear         start of a non-capturing subpattern,  the  option  letters  may  appear
3693         between the "?" and the ":". Thus the two patterns         between the "?" and the ":". Thus the two patterns
3694    
3695           (?i:saturday|sunday)           (?i:saturday|sunday)
3696           (?:(?i)saturday|sunday)           (?:(?i)saturday|sunday)
3697    
3698         match exactly the same set of strings. Because alternative branches are         match exactly the same set of strings. Because alternative branches are
3699         tried  from  left  to right, and options are not reset until the end of         tried from left to right, and options are not reset until  the  end  of
3700         the subpattern is reached, an option setting in one branch does  affect         the  subpattern is reached, an option setting in one branch does affect
3701         subsequent  branches,  so  the above patterns match "SUNDAY" as well as         subsequent branches, so the above patterns match "SUNDAY"  as  well  as
3702         "Saturday".         "Saturday".
3703    
3704    
3705  DUPLICATE SUBPATTERN NUMBERS  DUPLICATE SUBPATTERN NUMBERS
3706    
3707         Perl 5.10 introduced a feature whereby each alternative in a subpattern         Perl 5.10 introduced a feature whereby each alternative in a subpattern
3708         uses  the same numbers for its capturing parentheses. Such a subpattern         uses the same numbers for its capturing parentheses. Such a  subpattern
3709         starts with (?| and is itself a non-capturing subpattern. For  example,         starts  with (?| and is itself a non-capturing subpattern. For example,
3710         consider this pattern:         consider this pattern:
3711    
3712           (?|(Sat)ur|(Sun))day           (?|(Sat)ur|(Sun))day
3713    
3714         Because  the two alternatives are inside a (?| group, both sets of cap-         Because the two alternatives are inside a (?| group, both sets of  cap-
3715         turing parentheses are numbered one. Thus, when  the  pattern  matches,         turing  parentheses  are  numbered one. Thus, when the pattern matches,
3716         you  can  look  at captured substring number one, whichever alternative         you can look at captured substring number  one,  whichever  alternative
3717         matched. This construct is useful when you want to  capture  part,  but         matched.  This  construct  is useful when you want to capture part, but
3718         not all, of one of a number of alternatives. Inside a (?| group, paren-         not all, of one of a number of alternatives. Inside a (?| group, paren-
3719         theses are numbered as usual, but the number is reset at the  start  of         theses  are  numbered as usual, but the number is reset at the start of
3720         each  branch. The numbers of any capturing buffers that follow the sub-         each branch. The numbers of any capturing buffers that follow the  sub-
3721         pattern start after the highest number used in any branch. The  follow-         pattern  start after the highest number used in any branch. The follow-
3722         ing  example  is taken from the Perl documentation.  The numbers under-         ing example is taken from the Perl documentation.  The  numbers  under-
3723         neath show in which buffer the captured content will be stored.         neath show in which buffer the captured content will be stored.
3724    
3725           # before  ---------------branch-reset----------- after           # before  ---------------branch-reset----------- after
3726           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3727           # 1            2         2  3        2     3     4           # 1            2         2  3        2     3     4
3728    
3729         A backreference or a recursive call to  a  numbered  subpattern  always         A  backreference  or  a  recursive call to a numbered subpattern always
3730         refers to the first one in the pattern with the given number.         refers to the first one in the pattern with the given number.
3731    
3732         An  alternative approach to using this "branch reset" feature is to use         An alternative approach to using this "branch reset" feature is to  use
3733         duplicate named subpatterns, as described in the next section.         duplicate named subpatterns, as described in the next section.
3734    
3735    
3736  NAMED SUBPATTERNS  NAMED SUBPATTERNS
3737    
3738         Identifying capturing parentheses by number is simple, but  it  can  be         Identifying  capturing  parentheses  by number is simple, but it can be
3739         very  hard  to keep track of the numbers in complicated regular expres-         very hard to keep track of the numbers in complicated  regular  expres-
3740         sions. Furthermore, if an  expression  is  modified,  the  numbers  may         sions.  Furthermore,  if  an  expression  is  modified, the numbers may
3741         change.  To help with this difficulty, PCRE supports the naming of sub-         change. To help with this difficulty, PCRE supports the naming of  sub-
3742         patterns. This feature was not added to Perl until release 5.10. Python         patterns. This feature was not added to Perl until release 5.10. Python
3743         had  the  feature earlier, and PCRE introduced it at release 4.0, using         had the feature earlier, and PCRE introduced it at release  4.0,  using
3744         the Python syntax. PCRE now supports both the Perl and the Python  syn-         the  Python syntax. PCRE now supports both the Perl and the Python syn-
3745         tax.         tax.
3746    
3747         In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)         In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)
3748         or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References         or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References
3749         to capturing parentheses from other parts of the pattern, such as back-         to capturing parentheses from other parts of the pattern, such as back-
3750         references, recursion, and conditions, can be made by name as  well  as         references,  recursion,  and conditions, can be made by name as well as
3751         by number.         by number.
3752    
3753         Names  consist  of  up  to  32 alphanumeric characters and underscores.         Names consist of up to  32  alphanumeric  characters  and  underscores.
3754         Named capturing parentheses are still  allocated  numbers  as  well  as         Named  capturing  parentheses  are  still  allocated numbers as well as
3755         names,  exactly as if the names were not present. The PCRE API provides         names, exactly as if the names were not present. The PCRE API  provides
3756         function calls for extracting the name-to-number translation table from         function calls for extracting the name-to-number translation table from
3757         a compiled pattern. There is also a convenience function for extracting         a compiled pattern. There is also a convenience function for extracting
3758         a captured substring by name.         a captured substring by name.
3759    
3760         By default, a name must be unique within a pattern, but it is  possible         By  default, a name must be unique within a pattern, but it is possible
3761         to relax this constraint by setting the PCRE_DUPNAMES option at compile         to relax this constraint by setting the PCRE_DUPNAMES option at compile
3762         time. This can be useful for patterns where only one  instance  of  the         time.  This  can  be useful for patterns where only one instance of the
3763         named  parentheses  can  match. Suppose you want to match the name of a         named parentheses can match. Suppose you want to match the  name  of  a
3764         weekday, either as a 3-letter abbreviation or as the full name, and  in         weekday,  either as a 3-letter abbreviation or as the full name, and in
3765         both cases you want to extract the abbreviation. This pattern (ignoring         both cases you want to extract the abbreviation. This pattern (ignoring
3766         the line breaks) does the job:         the line breaks) does the job:
3767    
# Line 3712  NAMED SUBPATTERNS Line 3771  NAMED SUBPATTERNS
3771           (?<DN>Thu)(?:rsday)?|           (?<DN>Thu)(?:rsday)?|
3772           (?<DN>Sat)(?:urday)?           (?<DN>Sat)(?:urday)?
3773    
3774         There are five capturing substrings, but only one is ever set  after  a         There  are  five capturing substrings, but only one is ever set after a
3775         match.  (An alternative way of solving this problem is to use a "branch         match.  (An alternative way of solving this problem is to use a "branch
3776         reset" subpattern, as described in the previous section.)         reset" subpattern, as described in the previous section.)
3777    
3778         The convenience function for extracting the data by  name  returns  the         The  convenience  function  for extracting the data by name returns the
3779         substring  for  the first (and in this example, the only) subpattern of         substring for the first (and in this example, the only)  subpattern  of
3780         that name that matched. This saves searching  to  find  which  numbered         that  name  that  matched.  This saves searching to find which numbered
3781         subpattern  it  was. If you make a reference to a non-unique named sub-         subpattern it was. If you make a reference to a non-unique  named  sub-
3782         pattern from elsewhere in the pattern, the one that corresponds to  the         pattern  from elsewhere in the pattern, the one that corresponds to the
3783         lowest  number  is used. For further details of the interfaces for han-         lowest number is used. For further details of the interfaces  for  han-
3784         dling named subpatterns, see the pcreapi documentation.         dling named subpatterns, see the pcreapi documentation.
3785    
3786    
3787  REPETITION  REPETITION
3788    
3789         Repetition is specified by quantifiers, which can  follow  any  of  the         Repetition  is  specified  by  quantifiers, which can follow any of the
3790         following items:         following items:
3791    
3792           a literal data character           a literal data character
# Line 3740  REPETITION Line 3799  REPETITION
3799           a back reference (see next section)           a back reference (see next section)
3800           a parenthesized subpattern (unless it is an assertion)           a parenthesized subpattern (unless it is an assertion)
3801    
3802         The  general repetition quantifier specifies a minimum and maximum num-         The general repetition quantifier specifies a minimum and maximum  num-
3803         ber of permitted matches, by giving the two numbers in  curly  brackets         ber  of  permitted matches, by giving the two numbers in curly brackets
3804         (braces),  separated  by  a comma. The numbers must be less than 65536,         (braces), separated by a comma. The numbers must be  less  than  65536,
3805         and the first must be less than or equal to the second. For example:         and the first must be less than or equal to the second. For example:
3806    
3807           z{2,4}           z{2,4}
3808    
3809         matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a         matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
3810         special  character.  If  the second number is omitted, but the comma is         special character. If the second number is omitted, but  the  comma  is
3811         present, there is no upper limit; if the second number  and  the  comma         present,  there  is  no upper limit; if the second number and the comma
3812         are  both omitted, the quantifier specifies an exact number of required         are both omitted, the quantifier specifies an exact number of  required
3813         matches. Thus         matches. Thus
3814    
3815           [aeiou]{3,}           [aeiou]{3,}
# Line 3759  REPETITION Line 3818  REPETITION
3818    
3819           \d{8}           \d{8}
3820    
3821         matches exactly 8 digits. An opening curly bracket that  appears  in  a         matches  exactly  8  digits. An opening curly bracket that appears in a
3822         position  where a quantifier is not allowed, or one that does not match         position where a quantifier is not allowed, or one that does not  match
3823         the syntax of a quantifier, is taken as a literal character. For  exam-         the  syntax of a quantifier, is taken as a literal character. For exam-
3824         ple, {,6} is not a quantifier, but a literal string of four characters.         ple, {,6} is not a quantifier, but a literal string of four characters.
3825    
3826         In UTF-8 mode, quantifiers apply to UTF-8  characters  rather  than  to         In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
3827         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
3828         acters, each of which is represented by a two-byte sequence. Similarly,         acters, each of which is represented by a two-byte sequence. Similarly,
3829         when Unicode property support is available, \X{3} matches three Unicode         when Unicode property support is available, \X{3} matches three Unicode
3830         extended sequences, each of which may be several bytes long  (and  they         extended  sequences,  each of which may be several bytes long (and they
3831         may be of different lengths).         may be of different lengths).
3832    
3833         The quantifier {0} is permitted, causing the expression to behave as if         The quantifier {0} is permitted, causing the expression to behave as if
3834         the previous item and the quantifier were not present.         the previous item and the quantifier were not present.
3835    
3836         For convenience, the three most common quantifiers have  single-charac-         For  convenience, the three most common quantifiers have single-charac-
3837         ter abbreviations:         ter abbreviations:
3838    
3839           *    is equivalent to {0,}           *    is equivalent to {0,}
3840           +    is equivalent to {1,}           +    is equivalent to {1,}
3841           ?    is equivalent to {0,1}           ?    is equivalent to {0,1}
3842    
3843         It  is  possible  to construct infinite loops by following a subpattern         It is possible to construct infinite loops by  following  a  subpattern
3844         that can match no characters with a quantifier that has no upper limit,         that can match no characters with a quantifier that has no upper limit,
3845         for example:         for example:
3846    
3847           (a?)*           (a?)*
3848    
3849         Earlier versions of Perl and PCRE used to give an error at compile time         Earlier versions of Perl and PCRE used to give an error at compile time
3850         for such patterns. However, because there are cases where this  can  be         for  such  patterns. However, because there are cases where this can be
3851         useful,  such  patterns  are now accepted, but if any repetition of the         useful, such patterns are now accepted, but if any  repetition  of  the
3852         subpattern does in fact match no characters, the loop is forcibly  bro-         subpattern  does in fact match no characters, the loop is forcibly bro-
3853         ken.         ken.
3854    
3855         By  default,  the quantifiers are "greedy", that is, they match as much         By default, the quantifiers are "greedy", that is, they match  as  much
3856         as possible (up to the maximum  number  of  permitted  times),  without         as  possible  (up  to  the  maximum number of permitted times), without
3857         causing  the  rest of the pattern to fail. The classic example of where         causing the rest of the pattern to fail. The classic example  of  where
3858         this gives problems is in trying to match comments in C programs. These         this gives problems is in trying to match comments in C programs. These
3859         appear  between  /*  and  */ and within the comment, individual * and /         appear between /* and */ and within the comment,  individual  *  and  /
3860         characters may appear. An attempt to match C comments by  applying  the         characters  may  appear. An attempt to match C comments by applying the
3861         pattern         pattern
3862    
3863           /\*.*\*/           /\*.*\*/
# Line 3807  REPETITION Line 3866  REPETITION
3866    
3867           /* first comment */  not comment  /* second comment */           /* first comment */  not comment  /* second comment */
3868    
3869         fails,  because it matches the entire string owing to the greediness of         fails, because it matches the entire string owing to the greediness  of
3870         the .*  item.         the .*  item.
3871    
3872         However, if a quantifier is followed by a question mark, it  ceases  to         However,  if  a quantifier is followed by a question mark, it ceases to
3873         be greedy, and instead matches the minimum number of times possible, so         be greedy, and instead matches the minimum number of times possible, so
3874         the pattern         the pattern
3875    
3876           /\*.*?\*/           /\*.*?\*/
3877    
3878         does the right thing with the C comments. The meaning  of  the  various         does  the  right  thing with the C comments. The meaning of the various
3879         quantifiers  is  not  otherwise  changed,  just the preferred number of         quantifiers is not otherwise changed,  just  the  preferred  number  of
3880         matches.  Do not confuse this use of question mark with its  use  as  a         matches.   Do  not  confuse this use of question mark with its use as a
3881         quantifier  in its own right. Because it has two uses, it can sometimes         quantifier in its own right. Because it has two uses, it can  sometimes
3882         appear doubled, as in         appear doubled, as in
3883    
3884           \d??\d           \d??\d
# Line 3827  REPETITION Line 3886  REPETITION
3886         which matches one digit by preference, but can match two if that is the         which matches one digit by preference, but can match two if that is the
3887         only way the rest of the pattern matches.         only way the rest of the pattern matches.
3888    
3889         If  the PCRE_UNGREEDY option is set (an option that is not available in         If the PCRE_UNGREEDY option is set (an option that is not available  in
3890         Perl), the quantifiers are not greedy by default, but  individual  ones         Perl),  the  quantifiers are not greedy by default, but individual ones
3891         can  be  made  greedy  by following them with a question mark. In other         can be made greedy by following them with a  question  mark.  In  other
3892         words, it inverts the default behaviour.         words, it inverts the default behaviour.
3893    
3894         When a parenthesized subpattern is quantified  with  a  minimum  repeat         When  a  parenthesized  subpattern  is quantified with a minimum repeat
3895         count  that is greater than 1 or with a limited maximum, more memory is         count that is greater than 1 or with a limited maximum, more memory  is
3896         required for the compiled pattern, in proportion to  the  size  of  the         required  for  the  compiled  pattern, in proportion to the size of the
3897         minimum or maximum.         minimum or maximum.
3898    
3899         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
3900         alent to Perl's /s) is set, thus allowing the dot  to  match  newlines,         alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
3901         the  pattern  is  implicitly anchored, because whatever follows will be         the pattern is implicitly anchored, because whatever  follows  will  be
3902         tried against every character position in the subject string, so  there         tried  against every character position in the subject string, so there
3903         is  no  point  in  retrying the overall match at any position after the         is no point in retrying the overall match at  any  position  after  the
3904         first. PCRE normally treats such a pattern as though it  were  preceded         first.  PCRE  normally treats such a pattern as though it were preceded
3905         by \A.         by \A.
3906    
3907         In  cases  where  it  is known that the subject string contains no new-         In cases where it is known that the subject  string  contains  no  new-
3908         lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-         lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
3909         mization, or alternatively using ^ to indicate anchoring explicitly.         mization, or alternatively using ^ to indicate anchoring explicitly.
3910    
3911         However,  there is one situation where the optimization cannot be used.         However, there is one situation where the optimization cannot be  used.
3912         When .*  is inside capturing parentheses that  are  the  subject  of  a         When  .*   is  inside  capturing  parentheses that are the subject of a
3913         backreference  elsewhere  in the pattern, a match at the start may fail         backreference elsewhere in the pattern, a match at the start  may  fail
3914         where a later one succeeds. Consider, for example:         where a later one succeeds. Consider, for example:
3915    
3916           (.*)abc\1           (.*)abc\1
3917    
3918         If the subject is "xyz123abc123" the match point is the fourth  charac-         If  the subject is "xyz123abc123" the match point is the fourth charac-
3919         ter. For this reason, such a pattern is not implicitly anchored.         ter. For this reason, such a pattern is not implicitly anchored.
3920    
3921         When a capturing subpattern is repeated, the value captured is the sub-         When a capturing subpattern is repeated, the value captured is the sub-
# Line 3865  REPETITION Line 3924  REPETITION
3924           (tweedle[dume]{3}\s*)+           (tweedle[dume]{3}\s*)+
3925    
3926         has matched "tweedledum tweedledee" the value of the captured substring         has matched "tweedledum tweedledee" the value of the captured substring
3927         is  "tweedledee".  However,  if there are nested capturing subpatterns,         is "tweedledee". However, if there are  nested  capturing  subpatterns,
3928         the corresponding captured values may have been set in previous  itera-         the  corresponding captured values may have been set in previous itera-
3929         tions. For example, after         tions. For example, after
3930    
3931           /(a|(b))+/           /(a|(b))+/
# Line 3876  REPETITION Line 3935  REPETITION
3935    
3936  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
3937    
3938         With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")         With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
3939         repetition, failure of what follows normally causes the  repeated  item         repetition,  failure  of what follows normally causes the repeated item
3940         to  be  re-evaluated to see if a different number of repeats allows the         to be re-evaluated to see if a different number of repeats  allows  the
3941         rest of the pattern to match. Sometimes it is useful to  prevent  this,         rest  of  the pattern to match. Sometimes it is useful to prevent this,
3942         either  to  change the nature of the match, or to cause it fail earlier         either to change the nature of the match, or to cause it  fail  earlier
3943         than it otherwise might, when the author of the pattern knows there  is         than  it otherwise might, when the author of the pattern knows there is
3944         no point in carrying on.         no point in carrying on.
3945    
3946         Consider,  for  example, the pattern \d+foo when applied to the subject         Consider, for example, the pattern \d+foo when applied to  the  subject
3947         line         line
3948    
3949           123456bar           123456bar
3950    
3951         After matching all 6 digits and then failing to match "foo", the normal         After matching all 6 digits and then failing to match "foo", the normal
3952         action  of  the matcher is to try again with only 5 digits matching the         action of the matcher is to try again with only 5 digits  matching  the
3953         \d+ item, and then with  4,  and  so  on,  before  ultimately  failing.         \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
3954         "Atomic  grouping"  (a  term taken from Jeffrey Friedl's book) provides         "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
3955         the means for specifying that once a subpattern has matched, it is  not         the  means for specifying that once a subpattern has matched, it is not
3956         to be re-evaluated in this way.         to be re-evaluated in this way.
3957    
3958         If  we  use atomic grouping for the previous example, the matcher gives         If we use atomic grouping for the previous example, the  matcher  gives
3959         up immediately on failing to match "foo" the first time.  The  notation         up  immediately  on failing to match "foo" the first time. The notation
3960         is a kind of special parenthesis, starting with (?> as in this example:         is a kind of special parenthesis, starting with (?> as in this example:
3961    
3962           (?>\d+)foo           (?>\d+)foo
3963    
3964         This kind of parenthesis "locks up" the  part of the  pattern  it  con-         This  kind  of  parenthesis "locks up" the  part of the pattern it con-
3965         tains  once  it  has matched, and a failure further into the pattern is         tains once it has matched, and a failure further into  the  pattern  is
3966         prevented from backtracking into it. Backtracking past it  to  previous         prevented  from  backtracking into it. Backtracking past it to previous
3967         items, however, works as normal.         items, however, works as normal.
3968    
3969         An  alternative  description  is that a subpattern of this type matches         An alternative description is that a subpattern of  this  type  matches
3970         the string of characters that an  identical  standalone  pattern  would         the  string  of  characters  that an identical standalone pattern would
3971         match, if anchored at the current point in the subject string.         match, if anchored at the current point in the subject string.
3972    
3973         Atomic grouping subpatterns are not capturing subpatterns. Simple cases         Atomic grouping subpatterns are not capturing subpatterns. Simple cases
3974         such as the above example can be thought of as a maximizing repeat that         such as the above example can be thought of as a maximizing repeat that
3975         must  swallow  everything  it can. So, while both \d+ and \d+? are pre-         must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
3976         pared to adjust the number of digits they match in order  to  make  the         pared  to  adjust  the number of digits they match in order to make the
3977         rest of the pattern match, (?>\d+) can only match an entire sequence of         rest of the pattern match, (?>\d+) can only match an entire sequence of
3978         digits.         digits.
3979    
3980         Atomic groups in general can of course contain arbitrarily  complicated         Atomic  groups in general can of course contain arbitrarily complicated
3981         subpatterns,  and  can  be  nested. However, when the subpattern for an         subpatterns, and can be nested. However, when  the  subpattern  for  an
3982         atomic group is just a single repeated item, as in the example above, a         atomic group is just a single repeated item, as in the example above, a
3983         simpler  notation,  called  a "possessive quantifier" can be used. This         simpler notation, called a "possessive quantifier" can  be  used.  This
3984         consists of an additional + character  following  a  quantifier.  Using         consists  of  an  additional  + character following a quantifier. Using
3985         this notation, the previous example can be rewritten as         this notation, the previous example can be rewritten as
3986    
3987           \d++foo           \d++foo
# Line 3932  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE Line 3991  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE
3991    
3992           (abc|xyz){2,3}+           (abc|xyz){2,3}+
3993    
3994         Possessive  quantifiers  are  always  greedy;  the   setting   of   the         Possessive   quantifiers   are   always  greedy;  the  setting  of  the
3995         PCRE_UNGREEDY option is ignored. They are a convenient notation for the         PCRE_UNGREEDY option is ignored. They are a convenient notation for the
3996         simpler forms of atomic group. However, there is no difference  in  the         simpler  forms  of atomic group. However, there is no difference in the
3997         meaning  of  a  possessive  quantifier and the equivalent atomic group,         meaning of a possessive quantifier and  the  equivalent  atomic  group,
3998         though there may be a performance  difference;  possessive  quantifiers         though  there  may  be a performance difference; possessive quantifiers
3999         should be slightly faster.         should be slightly faster.
4000    
4001         The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-         The possessive quantifier syntax is an extension to the Perl  5.8  syn-
4002         tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first         tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
4003         edition of his book. Mike McCloskey liked it, so implemented it when he         edition of his book. Mike McCloskey liked it, so implemented it when he
4004         built Sun's Java package, and PCRE copied it from there. It  ultimately         built  Sun's Java package, and PCRE copied it from there. It ultimately
4005         found its way into Perl at release 5.10.         found its way into Perl at release 5.10.
4006    
4007         PCRE has an optimization that automatically "possessifies" certain sim-         PCRE has an optimization that automatically "possessifies" certain sim-
4008         ple pattern constructs. For example, the sequence  A+B  is  treated  as         ple  pattern  constructs.  For  example, the sequence A+B is treated as
4009         A++B  because  there is no point in backtracking into a sequence of A's         A++B because there is no point in backtracking into a sequence  of  A's
4010         when B must follow.         when B must follow.
4011    
4012         When a pattern contains an unlimited repeat inside  a  subpattern  that         When  a  pattern  contains an unlimited repeat inside a subpattern that
4013         can  itself  be  repeated  an  unlimited number of times, the use of an         can itself be repeated an unlimited number of  times,  the  use  of  an
4014         atomic group is the only way to avoid some  failing  matches  taking  a         atomic  group  is  the  only way to avoid some failing matches taking a
4015         very long time indeed. The pattern         very long time indeed. The pattern
4016    
4017           (\D+|<\d+>)*[!?]           (\D+|<\d+>)*[!?]
4018    
4019         matches  an  unlimited number of substrings that either consist of non-         matches an unlimited number of substrings that either consist  of  non-
4020         digits, or digits enclosed in <>, followed by either ! or  ?.  When  it         digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
4021         matches, it runs quickly. However, if it is applied to         matches, it runs quickly. However, if it is applied to
4022    
4023           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4024    
4025         it  takes  a  long  time  before reporting failure. This is because the         it takes a long time before reporting  failure.  This  is  because  the
4026         string can be divided between the internal \D+ repeat and the  external         string  can be divided between the internal \D+ repeat and the external
4027         *  repeat  in  a  large  number of ways, and all have to be tried. (The         * repeat in a large number of ways, and all  have  to  be  tried.  (The
4028         example uses [!?] rather than a single character at  the  end,  because         example  uses  [!?]  rather than a single character at the end, because
4029         both  PCRE  and  Perl have an optimization that allows for fast failure         both PCRE and Perl have an optimization that allows  for  fast  failure
4030         when a single character is used. They remember the last single  charac-         when  a single character is used. They remember the last single charac-
4031         ter  that  is required for a match, and fail early if it is not present         ter that is required for a match, and fail early if it is  not  present
4032         in the string.) If the pattern is changed so that  it  uses  an  atomic         in  the  string.)  If  the pattern is changed so that it uses an atomic
4033         group, like this:         group, like this:
4034    
4035           ((?>\D+)|<\d+>)*[!?]           ((?>\D+)|<\d+>)*[!?]
4036    
4037         sequences  of non-digits cannot be broken, and failure happens quickly.         sequences of non-digits cannot be broken, and failure happens  quickly.
4038    
4039    
4040  BACK REFERENCES  BACK REFERENCES
4041    
4042         Outside a character class, a backslash followed by a digit greater than         Outside a character class, a backslash followed by a digit greater than
4043         0 (and possibly further digits) is a back reference to a capturing sub-         0 (and possibly further digits) is a back reference to a capturing sub-
4044         pattern earlier (that is, to its left) in the pattern,  provided  there         pattern  earlier  (that is, to its left) in the pattern, provided there
4045         have been that many previous capturing left parentheses.         have been that many previous capturing left parentheses.
4046    
4047         However, if the decimal number following the backslash is less than 10,         However, if the decimal number following the backslash is less than 10,
4048         it is always taken as a back reference, and causes  an  error  only  if         it  is  always  taken  as a back reference, and causes an error only if
4049         there  are  not that many capturing left parentheses in the entire pat-         there are not that many capturing left parentheses in the  entire  pat-
4050         tern. In other words, the parentheses that are referenced need  not  be         tern.  In  other words, the parentheses that are referenced need not be
4051         to  the left of the reference for numbers less than 10. A "forward back         to the left of the reference for numbers less than 10. A "forward  back
4052         reference" of this type can make sense when a  repetition  is  involved         reference"  of  this  type can make sense when a repetition is involved
4053         and  the  subpattern to the right has participated in an earlier itera-         and the subpattern to the right has participated in an  earlier  itera-
4054         tion.         tion.
4055    
4056         It is not possible to have a numerical "forward back  reference"  to  a         It  is  not  possible to have a numerical "forward back reference" to a
4057         subpattern  whose  number  is  10  or  more using this syntax because a         subpattern whose number is 10 or  more  using  this  syntax  because  a
4058         sequence such as \50 is interpreted as a character  defined  in  octal.         sequence  such  as  \50 is interpreted as a character defined in octal.
4059         See the subsection entitled "Non-printing characters" above for further         See the subsection entitled "Non-printing characters" above for further
4060         details of the handling of digits following a backslash.  There  is  no         details  of  the  handling of digits following a backslash. There is no
4061         such  problem  when named parentheses are used. A back reference to any         such problem when named parentheses are used. A back reference  to  any
4062         subpattern is possible using named parentheses (see below).         subpattern is possible using named parentheses (see below).
4063    
4064         Another way of avoiding the ambiguity inherent in  the  use  of  digits         Another  way  of  avoiding  the ambiguity inherent in the use of digits
4065         following a backslash is to use the \g escape sequence, which is a fea-         following a backslash is to use the \g escape sequence, which is a fea-
4066         ture introduced in Perl 5.10.  This  escape  must  be  followed  by  an         ture  introduced  in  Perl  5.10.  This  escape  must be followed by an
4067         unsigned  number  or  a negative number, optionally enclosed in braces.         unsigned number or a negative number, optionally  enclosed  in  braces.
4068         These examples are all identical:         These examples are all identical:
4069    
4070           (ring), \1           (ring), \1
4071           (ring), \g1           (ring), \g1
4072           (ring), \g{1}           (ring), \g{1}
4073    
4074         An unsigned number specifies an absolute reference without the  ambigu-         An  unsigned number specifies an absolute reference without the ambigu-
4075         ity that is present in the older syntax. It is also useful when literal         ity that is present in the older syntax. It is also useful when literal
4076         digits follow the reference. A negative number is a relative reference.         digits follow the reference. A negative number is a relative reference.
4077         Consider this example:         Consider this example:
# Line 4020  BACK REFERENCES Line 4079  BACK REFERENCES
4079           (abc(def)ghi)\g{-1}           (abc(def)ghi)\g{-1}
4080    
4081         The sequence \g{-1} is a reference to the most recently started captur-         The sequence \g{-1} is a reference to the most recently started captur-
4082         ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,         ing  subpattern  before \g, that is, is it equivalent to \2. Similarly,
4083         \g{-2} would be equivalent to \1. The use of relative references can be         \g{-2} would be equivalent to \1. The use of relative references can be
4084         helpful in long patterns, and also in  patterns  that  are  created  by         helpful  in  long  patterns,  and  also in patterns that are created by
4085         joining together fragments that contain references within themselves.         joining together fragments that contain references within themselves.
4086    
4087         A  back  reference matches whatever actually matched the capturing sub-         A back reference matches whatever actually matched the  capturing  sub-
4088         pattern in the current subject string, rather  than  anything  matching         pattern  in  the  current subject string, rather than anything matching
4089         the subpattern itself (see "Subpatterns as subroutines" below for a way         the subpattern itself (see "Subpatterns as subroutines" below for a way
4090         of doing that). So the pattern         of doing that). So the pattern
4091    
4092           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4093    
4094         matches "sense and sensibility" and "response and responsibility",  but         matches  "sense and sensibility" and "response and responsibility", but
4095         not  "sense and responsibility". If caseful matching is in force at the         not "sense and responsibility". If caseful matching is in force at  the
4096         time of the back reference, the case of letters is relevant. For  exam-         time  of the back reference, the case of letters is relevant. For exam-
4097         ple,         ple,
4098    
4099           ((?i)rah)\s+\1           ((?i)rah)\s+\1
4100    
4101         matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the         matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
4102         original capturing subpattern is matched caselessly.         original capturing subpattern is matched caselessly.
4103    
4104         There are several different ways of writing back  references  to  named         There  are  several  different ways of writing back references to named
4105         subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or         subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or
4106         \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's         \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's
4107         unified back reference syntax, in which \g can be used for both numeric         unified back reference syntax, in which \g can be used for both numeric
4108         and named references, is also supported. We  could  rewrite  the  above         and  named  references,  is  also supported. We could rewrite the above
4109         example in any of the following ways:         example in any of the following ways:
4110    
4111           (?<p1>(?i)rah)\s+\k<p1>           (?<p1>(?i)rah)\s+\k<p1>
# Line 4054  BACK REFERENCES Line 4113  BACK REFERENCES
4113           (?P<p1>(?i)rah)\s+(?P=p1)           (?P<p1>(?i)rah)\s+(?P=p1)
4114           (?<p1>(?i)rah)\s+\g{p1}           (?<p1>(?i)rah)\s+\g{p1}
4115    
4116         A  subpattern  that  is  referenced  by  name may appear in the pattern         A subpattern that is referenced by  name  may  appear  in  the  pattern
4117         before or after the reference.         before or after the reference.
4118    
4119         There may be more than one back reference to the same subpattern. If  a         There  may be more than one back reference to the same subpattern. If a
4120         subpattern  has  not actually been used in a particular match, any back         subpattern has not actually been used in a particular match,  any  back
4121         references to it always fail. For example, the pattern         references to it always fail. For example, the pattern
4122    
4123           (a|(bc))\2           (a|(bc))\2
4124    
4125         always fails if it starts to match "a" rather than "bc". Because  there         always  fails if it starts to match "a" rather than "bc". Because there
4126         may  be  many  capturing parentheses in a pattern, all digits following         may be many capturing parentheses in a pattern,  all  digits  following
4127         the backslash are taken as part of a potential back  reference  number.         the  backslash  are taken as part of a potential back reference number.
4128         If the pattern continues with a digit character, some delimiter must be         If the pattern continues with a digit character, some delimiter must be
4129         used to terminate the back reference. If the  PCRE_EXTENDED  option  is         used  to  terminate  the back reference. If the PCRE_EXTENDED option is
4130         set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-         set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-
4131         ments" below) can be used.         ments" below) can be used.
4132    
4133         A back reference that occurs inside the parentheses to which it  refers         A  back reference that occurs inside the parentheses to which it refers
4134         fails  when  the subpattern is first used, so, for example, (a\1) never         fails when the subpattern is first used, so, for example,  (a\1)  never
4135         matches.  However, such references can be useful inside  repeated  sub-         matches.   However,  such references can be useful inside repeated sub-
4136         patterns. For example, the pattern         patterns. For example, the pattern
4137    
4138           (a|b\1)+           (a|b\1)+
4139    
4140         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4141         ation of the subpattern,  the  back  reference  matches  the  character         ation  of  the  subpattern,  the  back  reference matches the character
4142         string  corresponding  to  the previous iteration. In order for this to         string corresponding to the previous iteration. In order  for  this  to
4143         work, the pattern must be such that the first iteration does  not  need         work,  the  pattern must be such that the first iteration does not need
4144         to  match the back reference. This can be done using alternation, as in         to match the back reference. This can be done using alternation, as  in
4145         the example above, or by a quantifier with a minimum of zero.         the example above, or by a quantifier with a minimum of zero.
4146    
4147    
4148  ASSERTIONS  ASSERTIONS
4149    
4150         An assertion is a test on the characters  following  or  preceding  the         An  assertion  is  a  test on the characters following or preceding the
4151         current  matching  point that does not actually consume any characters.         current matching point that does not actually consume  any  characters.
4152         The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are         The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are
4153         described above.         described above.
4154    
4155         More  complicated  assertions  are  coded as subpatterns. There are two         More complicated assertions are coded as  subpatterns.  There  are  two
4156         kinds: those that look ahead of the current  position  in  the  subject         kinds:  those  that  look  ahead of the current position in the subject
4157         string,  and  those  that  look  behind  it. An assertion subpattern is         string, and those that look  behind  it.  An  assertion  subpattern  is
4158         matched in the normal way, except that it does not  cause  the  current         matched  in  the  normal way, except that it does not cause the current
4159         matching position to be changed.         matching position to be changed.
4160    
4161         Assertion  subpatterns  are  not  capturing subpatterns, and may not be         Assertion subpatterns are not capturing subpatterns,  and  may  not  be
4162         repeated, because it makes no sense to assert the  same  thing  several         repeated,  because  it  makes no sense to assert the same thing several
4163         times.  If  any kind of assertion contains capturing subpatterns within         times. If any kind of assertion contains capturing  subpatterns  within
4164         it, these are counted for the purposes of numbering the capturing  sub-         it,  these are counted for the purposes of numbering the capturing sub-
4165         patterns in the whole pattern.  However, substring capturing is carried         patterns in the whole pattern.  However, substring capturing is carried
4166         out only for positive assertions, because it does not  make  sense  for         out  only  for  positive assertions, because it does not make sense for
4167         negative assertions.         negative assertions.
4168    
4169     Lookahead assertions     Lookahead assertions
# Line 4114  ASSERTIONS Line 4173  ASSERTIONS
4173    
4174           \w+(?=;)           \w+(?=;)
4175    
4176         matches a word followed by a semicolon, but does not include the  semi-         matches  a word followed by a semicolon, but does not include the semi-
4177         colon in the match, and         colon in the match, and
4178    
4179           foo(?!bar)           foo(?!bar)
4180    
4181         matches  any  occurrence  of  "foo" that is not followed by "bar". Note         matches any occurrence of "foo" that is not  followed  by  "bar".  Note
4182         that the apparently similar pattern         that the apparently similar pattern
4183    
4184           (?!foo)bar           (?!foo)bar
4185    
4186         does not find an occurrence of "bar"  that  is  preceded  by  something         does  not  find  an  occurrence  of "bar" that is preceded by something
4187         other  than "foo"; it finds any occurrence of "bar" whatsoever, because         other than "foo"; it finds any occurrence of "bar" whatsoever,  because
4188         the assertion (?!foo) is always true when the next three characters are         the assertion (?!foo) is always true when the next three characters are
4189         "bar". A lookbehind assertion is needed to achieve the other effect.         "bar". A lookbehind assertion is needed to achieve the other effect.
4190    
4191         If you want to force a matching failure at some point in a pattern, the         If you want to force a matching failure at some point in a pattern, the
4192         most convenient way to do it is  with  (?!)  because  an  empty  string         most  convenient  way  to  do  it  is with (?!) because an empty string
4193         always  matches, so an assertion that requires there not to be an empty         always matches, so an assertion that requires there not to be an  empty
4194         string must always fail.         string must always fail.
4195    
4196     Lookbehind assertions     Lookbehind assertions
4197    
4198         Lookbehind assertions start with (?<= for positive assertions and  (?<!         Lookbehind  assertions start with (?<= for positive assertions and (?<!
4199         for negative assertions. For example,         for negative assertions. For example,
4200    
4201           (?<!foo)bar           (?<!foo)bar
4202    
4203         does  find  an  occurrence  of "bar" that is not preceded by "foo". The         does find an occurrence of "bar" that is not  preceded  by  "foo".  The
4204         contents of a lookbehind assertion are restricted  such  that  all  the         contents  of  a  lookbehind  assertion are restricted such that all the
4205         strings it matches must have a fixed length. However, if there are sev-         strings it matches must have a fixed length. However, if there are sev-
4206         eral top-level alternatives, they do not all  have  to  have  the  same         eral  top-level  alternatives,  they  do  not all have to have the same
4207         fixed length. Thus         fixed length. Thus
4208    
4209           (?<=bullock|donkey)           (?<=bullock|donkey)
# Line 4153  ASSERTIONS Line 4212  ASSERTIONS
4212    
4213           (?<!dogs?|cats?)           (?<!dogs?|cats?)
4214    
4215         causes  an  error at compile time. Branches that match different length         causes an error at compile time. Branches that match  different  length
4216         strings are permitted only at the top level of a lookbehind  assertion.         strings  are permitted only at the top level of a lookbehind assertion.
4217         This  is  an  extension  compared  with  Perl (at least for 5.8), which         This is an extension compared with  Perl  (at  least  for  5.8),  which
4218         requires all branches to match the same length of string. An  assertion         requires  all branches to match the same length of string. An assertion
4219         such as         such as
4220    
4221           (?<=ab(c|de))           (?<=ab(c|de))
4222    
4223         is  not  permitted,  because  its single top-level branch can match two         is not permitted, because its single top-level  branch  can  match  two
4224         different lengths, but it is acceptable if rewritten to  use  two  top-         different  lengths,  but  it is acceptable if rewritten to use two top-
4225         level branches:         level branches:
4226    
4227           (?<=abc|abde)           (?<=abc|abde)
4228    
4229         In some cases, the Perl 5.10 escape sequence \K (see above) can be used         In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4230         instead of a lookbehind assertion; this is not restricted to  a  fixed-         instead  of  a lookbehind assertion; this is not restricted to a fixed-
4231         length.         length.
4232    
4233         The  implementation  of lookbehind assertions is, for each alternative,         The implementation of lookbehind assertions is, for  each  alternative,
4234         to temporarily move the current position back by the fixed  length  and         to  temporarily  move the current position back by the fixed length and
4235         then try to match. If there are insufficient characters before the cur-         then try to match. If there are insufficient characters before the cur-
4236         rent position, the assertion fails.         rent position, the assertion fails.
4237    
4238         PCRE does not allow the \C escape (which matches a single byte in UTF-8         PCRE does not allow the \C escape (which matches a single byte in UTF-8
4239         mode)  to appear in lookbehind assertions, because it makes it impossi-         mode) to appear in lookbehind assertions, because it makes it  impossi-
4240         ble to calculate the length of the lookbehind. The \X and  \R  escapes,         ble  to  calculate the length of the lookbehind. The \X and \R escapes,
4241         which can match different numbers of bytes, are also not permitted.         which can match different numbers of bytes, are also not permitted.
4242    
4243         Possessive  quantifiers  can  be  used  in  conjunction with lookbehind         Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
4244         assertions to specify efficient matching at  the  end  of  the  subject         assertions  to  specify  efficient  matching  at the end of the subject
4245         string. Consider a simple pattern such as         string. Consider a simple pattern such as
4246    
4247           abcd$           abcd$
4248    
4249         when  applied  to  a  long string that does not match. Because matching         when applied to a long string that does  not  match.  Because  matching
4250         proceeds from left to right, PCRE will look for each "a" in the subject         proceeds from left to right, PCRE will look for each "a" in the subject
4251         and  then  see  if what follows matches the rest of the pattern. If the         and then see if what follows matches the rest of the  pattern.  If  the
4252         pattern is specified as         pattern is specified as
4253    
4254           ^.*abcd$           ^.*abcd$
4255    
4256         the initial .* matches the entire string at first, but when this  fails         the  initial .* matches the entire string at first, but when this fails
4257         (because there is no following "a"), it backtracks to match all but the         (because there is no following "a"), it backtracks to match all but the
4258         last character, then all but the last two characters, and so  on.  Once         last  character,  then all but the last two characters, and so on. Once
4259         again  the search for "a" covers the entire string, from right to left,         again the search for "a" covers the entire string, from right to  left,
4260         so we are no better off. However, if the pattern is written as         so we are no better off. However, if the pattern is written as
4261    
4262           ^.*+(?<=abcd)           ^.*+(?<=abcd)
4263    
4264         there can be no backtracking for the .*+ item; it can  match  only  the         there  can  be  no backtracking for the .*+ item; it can match only the
4265         entire  string.  The subsequent lookbehind assertion does a single test         entire string. The subsequent lookbehind assertion does a  single  test
4266         on the last four characters. If it fails, the match fails  immediately.         on  the last four characters. If it fails, the match fails immediately.
4267         For  long  strings, this approach makes a significant difference to the         For long strings, this approach makes a significant difference  to  the
4268         processing time.         processing time.
4269    
4270     Using multiple assertions     Using multiple assertions
# Line 4214  ASSERTIONS Line 4273  ASSERTIONS
4273    
4274           (?<=\d{3})(?<!999)foo           (?<=\d{3})(?<!999)foo
4275    
4276         matches "foo" preceded by three digits that are not "999". Notice  that         matches  "foo" preceded by three digits that are not "999". Notice that
4277         each  of  the  assertions is applied independently at the same point in         each of the assertions is applied independently at the  same  point  in
4278         the subject string. First there is a  check  that  the  previous  three         the  subject  string.  First  there  is a check that the previous three
4279         characters  are  all  digits,  and  then there is a check that the same         characters are all digits, and then there is  a  check  that  the  same
4280         three characters are not "999".  This pattern does not match "foo" pre-         three characters are not "999".  This pattern does not match "foo" pre-
4281         ceded  by  six  characters,  the first of which are digits and the last         ceded by six characters, the first of which are  digits  and  the  last
4282         three of which are not "999". For example, it  doesn't  match  "123abc-         three  of  which  are not "999". For example, it doesn't match "123abc-
4283         foo". A pattern to do that is         foo". A pattern to do that is
4284    
4285           (?<=\d{3}...)(?<!999)foo           (?<=\d{3}...)(?<!999)foo
4286    
4287         This  time  the  first assertion looks at the preceding six characters,         This time the first assertion looks at the  preceding  six  characters,
4288         checking that the first three are digits, and then the second assertion         checking that the first three are digits, and then the second assertion
4289         checks that the preceding three characters are not "999".         checks that the preceding three characters are not "999".
4290    
# Line 4233  ASSERTIONS Line 4292  ASSERTIONS
4292    
4293           (?<=(?<!foo)bar)baz           (?<=(?<!foo)bar)baz
4294    
4295         matches  an occurrence of "baz" that is preceded by "bar" which in turn         matches an occurrence of "baz" that is preceded by "bar" which in  turn
4296         is not preceded by "foo", while         is not preceded by "foo", while
4297    
4298           (?<=\d{3}(?!999)...)foo           (?<=\d{3}(?!999)...)foo
4299    
4300         is another pattern that matches "foo" preceded by three digits and  any         is  another pattern that matches "foo" preceded by three digits and any
4301         three characters that are not "999".         three characters that are not "999".
4302    
4303    
4304  CONDITIONAL SUBPATTERNS  CONDITIONAL SUBPATTERNS
4305    
4306         It  is possible to cause the matching process to obey a subpattern con-         It is possible to cause the matching process to obey a subpattern  con-
4307         ditionally or to choose between two alternative subpatterns,  depending         ditionally  or to choose between two alternative subpatterns, depending
4308         on  the result of an assertion, or whether a previous capturing subpat-         on the result of an assertion, or whether a previous capturing  subpat-
4309         tern matched or not. The two possible forms of  conditional  subpattern         tern  matched  or not. The two possible forms of conditional subpattern
4310         are         are
4311    
4312           (?(condition)yes-pattern)           (?(condition)yes-pattern)
4313           (?(condition)yes-pattern|no-pattern)           (?(condition)yes-pattern|no-pattern)
4314    
4315         If  the  condition is satisfied, the yes-pattern is used; otherwise the         If the condition is satisfied, the yes-pattern is used;  otherwise  the
4316         no-pattern (if present) is used. If there are more  than  two  alterna-         no-pattern  (if  present)  is used. If there are more than two alterna-
4317         tives in the subpattern, a compile-time error occurs.         tives in the subpattern, a compile-time error occurs.
4318    
4319         There  are  four  kinds of condition: references to subpatterns, refer-         There are four kinds of condition: references  to  subpatterns,  refer-
4320         ences to recursion, a pseudo-condition called DEFINE, and assertions.         ences to recursion, a pseudo-condition called DEFINE, and assertions.
4321    
4322     Checking for a used subpattern by number     Checking for a used subpattern by number
4323    
4324         If the text between the parentheses consists of a sequence  of  digits,         If  the  text between the parentheses consists of a sequence of digits,
4325         the  condition  is  true if the capturing subpattern of that number has         the condition is true if the capturing subpattern of  that  number  has
4326         previously matched. An alternative notation is to  precede  the  digits         previously  matched.  An  alternative notation is to precede the digits
4327         with a plus or minus sign. In this case, the subpattern number is rela-         with a plus or minus sign. In this case, the subpattern number is rela-
4328         tive rather than absolute.  The most recently opened parentheses can be         tive rather than absolute.  The most recently opened parentheses can be
4329         referenced  by  (?(-1),  the  next most recent by (?(-2), and so on. In         referenced by (?(-1), the next most recent by (?(-2),  and  so  on.  In
4330         looping constructs it can also make sense to refer to subsequent groups         looping constructs it can also make sense to refer to subsequent groups
4331         with constructs such as (?(+2).         with constructs such as (?(+2).
4332    
4333         Consider  the  following  pattern, which contains non-significant white         Consider the following pattern, which  contains  non-significant  white
4334         space to make it more readable (assume the PCRE_EXTENDED option) and to         space to make it more readable (assume the PCRE_EXTENDED option) and to
4335         divide it into three parts for ease of discussion:         divide it into three parts for ease of discussion:
4336    
4337           ( \( )?    [^()]+    (?(1) \) )           ( \( )?    [^()]+    (?(1) \) )
4338    
4339         The  first  part  matches  an optional opening parenthesis, and if that         The first part matches an optional opening  parenthesis,  and  if  that
4340         character is present, sets it as the first captured substring. The sec-         character is present, sets it as the first captured substring. The sec-
4341         ond  part  matches one or more characters that are not parentheses. The         ond part matches one or more characters that are not  parentheses.  The
4342         third part is a conditional subpattern that tests whether the first set         third part is a conditional subpattern that tests whether the first set
4343         of parentheses matched or not. If they did, that is, if subject started         of parentheses matched or not. If they did, that is, if subject started
4344         with an opening parenthesis, the condition is true, and so the yes-pat-         with an opening parenthesis, the condition is true, and so the yes-pat-
4345         tern  is  executed  and  a  closing parenthesis is required. Otherwise,         tern is executed and a  closing  parenthesis  is  required.  Otherwise,
4346         since no-pattern is not present, the  subpattern  matches  nothing.  In         since  no-pattern  is  not  present, the subpattern matches nothing. In
4347         other  words,  this  pattern  matches  a  sequence  of non-parentheses,         other words,  this  pattern  matches  a  sequence  of  non-parentheses,
4348         optionally enclosed in parentheses.         optionally enclosed in parentheses.
4349    
4350         If you were embedding this pattern in a larger one,  you  could  use  a         If  you  were  embedding  this pattern in a larger one, you could use a
4351         relative reference:         relative reference:
4352    
4353           ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...           ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
4354    
4355         This  makes  the  fragment independent of the parentheses in the larger         This makes the fragment independent of the parentheses  in  the  larger
4356         pattern.         pattern.
4357    
4358     Checking for a used subpattern by name     Checking for a used subpattern by name
4359    
4360         Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a         Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
4361         used  subpattern  by  name.  For compatibility with earlier versions of         used subpattern by name. For compatibility  with  earlier  versions  of
4362         PCRE, which had this facility before Perl, the syntax  (?(name)...)  is         PCRE,  which  had this facility before Perl, the syntax (?(name)...) is
4363         also  recognized. However, there is a possible ambiguity with this syn-         also recognized. However, there is a possible ambiguity with this  syn-
4364         tax, because subpattern names may  consist  entirely  of  digits.  PCRE         tax,  because  subpattern  names  may  consist entirely of digits. PCRE
4365         looks  first for a named subpattern; if it cannot find one and the name         looks first for a named subpattern; if it cannot find one and the  name
4366         consists entirely of digits, PCRE looks for a subpattern of  that  num-         consists  entirely  of digits, PCRE looks for a subpattern of that num-
4367         ber,  which must be greater than zero. Using subpattern names that con-         ber, which must be greater than zero. Using subpattern names that  con-
4368         sist entirely of digits is not recommended.         sist entirely of digits is not recommended.
4369    
4370         Rewriting the above example to use a named subpattern gives this:         Rewriting the above example to use a named subpattern gives this:
# Line 4316  CONDITIONAL SUBPATTERNS Line 4375  CONDITIONAL SUBPATTERNS
4375     Checking for pattern recursion     Checking for pattern recursion
4376    
4377         If the condition is the string (R), and there is no subpattern with the         If the condition is the string (R), and there is no subpattern with the
4378         name  R, the condition is true if a recursive call to the whole pattern         name R, the condition is true if a recursive call to the whole  pattern
4379         or any subpattern has been made. If digits or a name preceded by amper-         or any subpattern has been made. If digits or a name preceded by amper-
4380         sand follow the letter R, for example:         sand follow the letter R, for example:
4381    
4382           (?(R3)...) or (?(R&name)...)           (?(R3)...) or (?(R&name)...)
4383    
4384         the  condition is true if the most recent recursion is into the subpat-         the condition is true if the most recent recursion is into the  subpat-
4385         tern whose number or name is given. This condition does not  check  the         tern  whose  number or name is given. This condition does not check the
4386         entire recursion stack.         entire recursion stack.
4387    
4388         At  "top  level", all these recursion test conditions are false. Recur-         At "top level", all these recursion test conditions are  false.  Recur-
4389         sive patterns are described below.         sive patterns are described below.
4390    
4391     Defining subpatterns for use by reference only     Defining subpatterns for use by reference only
4392    
4393         If the condition is the string (DEFINE), and  there  is  no  subpattern         If  the  condition  is  the string (DEFINE), and there is no subpattern
4394         with  the  name  DEFINE,  the  condition is always false. In this case,         with the name DEFINE, the condition is  always  false.  In  this  case,
4395         there may be only one alternative  in  the  subpattern.  It  is  always         there  may  be  only  one  alternative  in the subpattern. It is always
4396         skipped  if  control  reaches  this  point  in the pattern; the idea of         skipped if control reaches this point  in  the  pattern;  the  idea  of
4397         DEFINE is that it can be used to define "subroutines" that can be  ref-         DEFINE  is that it can be used to define "subroutines" that can be ref-
4398         erenced  from elsewhere. (The use of "subroutines" is described below.)         erenced from elsewhere. (The use of "subroutines" is described  below.)
4399         For example, a pattern to match an IPv4 address could be  written  like         For  example,  a pattern to match an IPv4 address could be written like
4400         this (ignore whitespace and line breaks):         this (ignore whitespace and line breaks):
4401    
4402           (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )           (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4403           \b (?&byte) (\.(?&byte)){3} \b           \b (?&byte) (\.(?&byte)){3} \b
4404    
4405         The  first part of the pattern is a DEFINE group inside which a another         The first part of the pattern is a DEFINE group inside which a  another
4406         group named "byte" is defined. This matches an individual component  of         group  named "byte" is defined. This matches an individual component of
4407         an  IPv4  address  (a number less than 256). When matching takes place,         an IPv4 address (a number less than 256). When  matching  takes  place,
4408         this part of the pattern is skipped because DEFINE acts  like  a  false         this  part  of  the pattern is skipped because DEFINE acts like a false
4409         condition.         condition.
4410    
4411         The rest of the pattern uses references to the named group to match the         The rest of the pattern uses references to the named group to match the
4412         four dot-separated components of an IPv4 address, insisting on  a  word         four  dot-separated  components of an IPv4 address, insisting on a word
4413         boundary at each end.         boundary at each end.
4414    
4415     Assertion conditions     Assertion conditions
4416    
4417         If  the  condition  is  not  in any of the above formats, it must be an         If the condition is not in any of the above  formats,  it  must  be  an
4418         assertion.  This may be a positive or negative lookahead or  lookbehind         assertion.   This may be a positive or negative lookahead or lookbehind
4419         assertion.  Consider  this  pattern,  again  containing non-significant         assertion. Consider  this  pattern,  again  containing  non-significant
4420         white space, and with the two alternatives on the second line:         white space, and with the two alternatives on the second line:
4421    
4422           (?(?=[^a-z]*[a-z])           (?(?=[^a-z]*[a-z])
4423           \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )           \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
4424    
4425         The condition  is  a  positive  lookahead  assertion  that  matches  an         The  condition  is  a  positive  lookahead  assertion  that  matches an
4426         optional  sequence of non-letters followed by a letter. In other words,         optional sequence of non-letters followed by a letter. In other  words,
4427         it tests for the presence of at least one letter in the subject.  If  a         it  tests  for the presence of at least one letter in the subject. If a
4428         letter  is found, the subject is matched against the first alternative;         letter is found, the subject is matched against the first  alternative;
4429         otherwise it is  matched  against  the  second.  This  pattern  matches         otherwise  it  is  matched  against  the  second.  This pattern matches
4430         strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are         strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
4431         letters and dd are digits.         letters and dd are digits.
4432    
4433    
4434  COMMENTS  COMMENTS
4435    
4436         The sequence (?# marks the start of a comment that continues up to  the         The  sequence (?# marks the start of a comment that continues up to the
4437         next  closing  parenthesis.  Nested  parentheses are not permitted. The         next closing parenthesis. Nested parentheses  are  not  permitted.  The
4438         characters that make up a comment play no part in the pattern  matching         characters  that make up a comment play no part in the pattern matching
4439         at all.         at all.
4440    
4441         If  the PCRE_EXTENDED option is set, an unescaped # character outside a         If the PCRE_EXTENDED option is set, an unescaped # character outside  a
4442         character class introduces a  comment  that  continues  to  immediately         character  class  introduces  a  comment  that continues to immediately
4443         after the next newline in the pattern.         after the next newline in the pattern.
4444    
4445    
4446  RECURSIVE PATTERNS  RECURSIVE PATTERNS
4447    
4448         Consider  the problem of matching a string in parentheses, allowing for         Consider the problem of matching a string in parentheses, allowing  for
4449         unlimited nested parentheses. Without the use of  recursion,  the  best         unlimited  nested  parentheses.  Without the use of recursion, the best
4450         that  can  be  done  is  to use a pattern that matches up to some fixed         that can be done is to use a pattern that  matches  up  to  some  fixed
4451         depth of nesting. It is not possible to  handle  an  arbitrary  nesting         depth  of  nesting.  It  is not possible to handle an arbitrary nesting
4452         depth.         depth.
4453    
4454         For some time, Perl has provided a facility that allows regular expres-         For some time, Perl has provided a facility that allows regular expres-
4455         sions to recurse (amongst other things). It does this by  interpolating         sions  to recurse (amongst other things). It does this by interpolating
4456         Perl  code in the expression at run time, and the code can refer to the         Perl code in the expression at run time, and the code can refer to  the
4457         expression itself. A Perl pattern using code interpolation to solve the         expression itself. A Perl pattern using code interpolation to solve the
4458         parentheses problem can be created like this:         parentheses problem can be created like this:
4459    
# Line 4404  RECURSIVE PATTERNS Line 4463  RECURSIVE PATTERNS
4463         refers recursively to the pattern in which it appears.         refers recursively to the pattern in which it appears.
4464    
4465         Obviously, PCRE cannot support the interpolation of Perl code. Instead,         Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4466         it  supports  special  syntax  for recursion of the entire pattern, and         it supports special syntax for recursion of  the  entire  pattern,  and
4467         also for individual subpattern recursion.  After  its  introduction  in         also  for  individual  subpattern  recursion. After its introduction in
4468         PCRE  and  Python,  this  kind of recursion was introduced into Perl at         PCRE and Python, this kind of recursion was  introduced  into  Perl  at
4469         release 5.10.         release 5.10.
4470    
4471         A special item that consists of (? followed by a  number  greater  than         A  special  item  that consists of (? followed by a number greater than
4472         zero and a closing parenthesis is a recursive call of the subpattern of         zero and a closing parenthesis is a recursive call of the subpattern of
4473         the given number, provided that it occurs inside that  subpattern.  (If         the  given  number, provided that it occurs inside that subpattern. (If
4474         not,  it  is  a  "subroutine" call, which is described in the next sec-         not, it is a "subroutine" call, which is described  in  the  next  sec-
4475         tion.) The special item (?R) or (?0) is a recursive call of the  entire         tion.)  The special item (?R) or (?0) is a recursive call of the entire
4476         regular expression.         regular expression.
4477    
4478         In  PCRE (like Python, but unlike Perl), a recursive subpattern call is         In PCRE (like Python, but unlike Perl), a recursive subpattern call  is
4479         always treated as an atomic group. That is, once it has matched some of         always treated as an atomic group. That is, once it has matched some of
4480         the subject string, it is never re-entered, even if it contains untried         the subject string, it is never re-entered, even if it contains untried
4481         alternatives and there is a subsequent matching failure.         alternatives and there is a subsequent matching failure.
4482    
4483         This PCRE pattern solves the nested  parentheses  problem  (assume  the         This  PCRE  pattern  solves  the nested parentheses problem (assume the
4484         PCRE_EXTENDED option is set so that white space is ignored):         PCRE_EXTENDED option is set so that white space is ignored):
4485    
4486           \( ( (?>[^()]+) | (?R) )* \)           \( ( (?>[^()]+) | (?R) )* \)
4487    
4488         First  it matches an opening parenthesis. Then it matches any number of         First it matches an opening parenthesis. Then it matches any number  of
4489         substrings which can either be a  sequence  of  non-parentheses,  or  a         substrings  which  can  either  be  a sequence of non-parentheses, or a
4490         recursive  match  of the pattern itself (that is, a correctly parenthe-         recursive match of the pattern itself (that is, a  correctly  parenthe-
4491         sized substring).  Finally there is a closing parenthesis.         sized substring).  Finally there is a closing parenthesis.
4492    
4493         If this were part of a larger pattern, you would not  want  to  recurse         If  this  were  part of a larger pattern, you would not want to recurse
4494         the entire pattern, so instead you could use this:         the entire pattern, so instead you could use this:
4495    
4496           ( \( ( (?>[^()]+) | (?1) )* \) )           ( \( ( (?>[^()]+) | (?1) )* \) )
4497    
4498         We  have  put the pattern into parentheses, and caused the recursion to         We have put the pattern into parentheses, and caused the  recursion  to
4499         refer to them instead of the whole pattern.         refer to them instead of the whole pattern.
4500    
4501         In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be         In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be
4502         tricky.  This is made easier by the use of relative references. (A Perl         tricky. This is made easier by the use of relative references. (A  Perl
4503         5.10 feature.)  Instead of (?1) in the  pattern  above  you  can  write         5.10  feature.)   Instead  of  (?1)  in the pattern above you can write
4504         (?-2) to refer to the second most recently opened parentheses preceding         (?-2) to refer to the second most recently opened parentheses preceding
4505         the recursion. In other  words,  a  negative  number  counts  capturing         the  recursion.  In  other  words,  a  negative number counts capturing
4506         parentheses leftwards from the point at which it is encountered.         parentheses leftwards from the point at which it is encountered.
4507    
4508         It  is  also  possible  to refer to subsequently opened parentheses, by         It is also possible to refer to  subsequently  opened  parentheses,  by
4509         writing references such as (?+2). However, these  cannot  be  recursive         writing  references  such  as (?+2). However, these cannot be recursive
4510         because  the  reference  is  not inside the parentheses that are refer-         because the reference is not inside the  parentheses  that  are  refer-
4511         enced. They are always "subroutine" calls, as  described  in  the  next         enced.  They  are  always  "subroutine" calls, as described in the next
4512         section.         section.
4513    
4514         An  alternative  approach is to use named parentheses instead. The Perl         An alternative approach is to use named parentheses instead.  The  Perl
4515         syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also         syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also
4516         supported. We could rewrite the above example as follows:         supported. We could rewrite the above example as follows:
4517    
4518           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4519    
4520         If  there  is more than one subpattern with the same name, the earliest         If there is more than one subpattern with the same name,  the  earliest
4521         one is used.         one is used.
4522    
4523         This particular example pattern that we have been looking  at  contains         This  particular  example pattern that we have been looking at contains
4524         nested  unlimited repeats, and so the use of atomic grouping for match-         nested unlimited repeats, and so the use of atomic grouping for  match-
4525         ing strings of non-parentheses is important when applying  the  pattern         ing  strings  of non-parentheses is important when applying the pattern
4526         to strings that do not match. For example, when this pattern is applied         to strings that do not match. For example, when this pattern is applied
4527         to         to
4528    
4529           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4530    
4531         it yields "no match" quickly. However, if atomic grouping is not  used,         it  yields "no match" quickly. However, if atomic grouping is not used,
4532         the  match  runs  for a very long time indeed because there are so many         the match runs for a very long time indeed because there  are  so  many
4533         different ways the + and * repeats can carve up the  subject,  and  all         different  ways  the  + and * repeats can carve up the subject, and all
4534         have to be tested before failure can be reported.         have to be tested before failure can be reported.
4535    
4536         At the end of a match, the values set for any capturing subpatterns are         At the end of a match, the values set for any capturing subpatterns are
4537         those from the outermost level of the recursion at which the subpattern         those from the outermost level of the recursion at which the subpattern
4538         value  is  set.   If  you want to obtain intermediate values, a callout         value is set.  If you want to obtain  intermediate  values,  a  callout
4539         function can be used (see below and the pcrecallout documentation).  If         function  can be used (see below and the pcrecallout documentation). If
4540         the pattern above is matched against         the pattern above is matched against
4541    
4542           (ab(cd)ef)           (ab(cd)ef)
4543    
4544         the  value  for  the  capturing  parentheses is "ef", which is the last         the value for the capturing parentheses is  "ef",  which  is  the  last
4545         value taken on at the top level. If additional parentheses  are  added,         value  taken  on at the top level. If additional parentheses are added,
4546         giving         giving
4547    
4548           \( ( ( (?>[^()]+) | (?R) )* ) \)           \( ( ( (?>[^()]+) | (?R) )* ) \)
4549              ^                        ^              ^                        ^
4550              ^                        ^              ^                        ^
4551    
4552         the  string  they  capture is "ab(cd)ef", the contents of the top level         the string they capture is "ab(cd)ef", the contents of  the  top  level
4553         parentheses. If there are more than 15 capturing parentheses in a  pat-         parentheses.  If there are more than 15 capturing parentheses in a pat-
4554         tern, PCRE has to obtain extra memory to store data during a recursion,         tern, PCRE has to obtain extra memory to store data during a recursion,
4555         which it does by using pcre_malloc, freeing  it  via  pcre_free  after-         which  it  does  by  using pcre_malloc, freeing it via pcre_free after-
4556         wards.  If  no  memory  can  be  obtained,  the  match  fails  with the         wards. If  no  memory  can  be  obtained,  the  match  fails  with  the
4557         PCRE_ERROR_NOMEMORY error.         PCRE_ERROR_NOMEMORY error.
4558    
4559         Do not confuse the (?R) item with the condition (R),  which  tests  for         Do  not  confuse  the (?R) item with the condition (R), which tests for
4560         recursion.   Consider  this pattern, which matches text in angle brack-         recursion.  Consider this pattern, which matches text in  angle  brack-
4561         ets, allowing for arbitrary nesting. Only digits are allowed in  nested         ets,  allowing for arbitrary nesting. Only digits are allowed in nested
4562         brackets  (that is, when recursing), whereas any characters are permit-         brackets (that is, when recursing), whereas any characters are  permit-
4563         ted at the outer level.         ted at the outer level.
4564    
4565           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
4566    
4567         In this pattern, (?(R) is the start of a conditional  subpattern,  with         In  this  pattern, (?(R) is the start of a conditional subpattern, with
4568         two  different  alternatives for the recursive and non-recursive cases.         two different alternatives for the recursive and  non-recursive  cases.
4569         The (?R) item is the actual recursive call.         The (?R) item is the actual recursive call.
4570    
4571    
4572  SUBPATTERNS AS SUBROUTINES  SUBPATTERNS AS SUBROUTINES
4573    
4574         If the syntax for a recursive subpattern reference (either by number or         If the syntax for a recursive subpattern reference (either by number or
4575         by  name)  is used outside the parentheses to which it refers, it oper-         by name) is used outside the parentheses to which it refers,  it  oper-
4576         ates like a subroutine in a programming language. The "called"  subpat-         ates  like a subroutine in a programming language. The "called" subpat-
4577         tern may be defined before or after the reference. A numbered reference         tern may be defined before or after the reference. A numbered reference
4578         can be absolute or relative, as in these examples:         can be absolute or relative, as in these examples:
4579    
# Line 4526  SUBPATTERNS AS SUBROUTINES Line 4585  SUBPATTERNS AS SUBROUTINES
4585    
4586           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4587    
4588         matches "sense and sensibility" and "response and responsibility",  but         matches  "sense and sensibility" and "response and responsibility", but
4589         not "sense and responsibility". If instead the pattern         not "sense and responsibility". If instead the pattern
4590    
4591           (sens|respons)e and (?1)ibility           (sens|respons)e and (?1)ibility
4592    
4593         is  used, it does match "sense and responsibility" as well as the other         is used, it does match "sense and responsibility" as well as the  other
4594         two strings. Another example is  given  in  the  discussion  of  DEFINE         two  strings.  Another  example  is  given  in the discussion of DEFINE
4595         above.         above.
4596    
4597         Like recursive subpatterns, a "subroutine" call is always treated as an         Like recursive subpatterns, a "subroutine" call is always treated as an
4598         atomic group. That is, once it has matched some of the subject  string,         atomic  group. That is, once it has matched some of the subject string,
4599         it  is  never  re-entered, even if it contains untried alternatives and         it is never re-entered, even if it contains  untried  alternatives  and
4600         there is a subsequent matching failure.         there is a subsequent matching failure.
4601    
4602         When a subpattern is used as a subroutine, processing options  such  as         When  a  subpattern is used as a subroutine, processing options such as
4603         case-independence are fixed when the subpattern is defined. They cannot         case-independence are fixed when the subpattern is defined. They cannot
4604         be changed for different calls. For example, consider this pattern:         be changed for different calls. For example, consider this pattern:
4605    
4606           (abc)(?i:(?-1))           (abc)(?i:(?-1))
4607    
4608         It matches "abcabc". It does not match "abcABC" because the  change  of         It  matches  "abcabc". It does not match "abcABC" because the change of
4609         processing option does not affect the called subpattern.         processing option does not affect the called subpattern.
4610    
4611    
4612  CALLOUTS  CALLOUTS
4613    
4614         Perl has a feature whereby using the sequence (?{...}) causes arbitrary         Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4615         Perl code to be obeyed in the middle of matching a regular  expression.         Perl  code to be obeyed in the middle of matching a regular expression.
4616         This makes it possible, amongst other things, to extract different sub-         This makes it possible, amongst other things, to extract different sub-
4617         strings that match the same pair of parentheses when there is a repeti-         strings that match the same pair of parentheses when there is a repeti-
4618         tion.         tion.
4619    
4620         PCRE provides a similar feature, but of course it cannot obey arbitrary         PCRE provides a similar feature, but of course it cannot obey arbitrary
4621         Perl code. The feature is called "callout". The caller of PCRE provides         Perl code. The feature is called "callout". The caller of PCRE provides
4622         an  external function by putting its entry point in the global variable         an external function by putting its entry point in the global  variable
4623         pcre_callout.  By default, this variable contains NULL, which  disables         pcre_callout.   By default, this variable contains NULL, which disables
4624         all calling out.         all calling out.
4625    
4626         Within  a  regular  expression,  (?C) indicates the points at which the         Within a regular expression, (?C) indicates the  points  at  which  the
4627         external function is to be called. If you want  to  identify  different         external  function  is  to be called. If you want to identify different
4628         callout  points, you can put a number less than 256 after the letter C.         callout points, you can put a number less than 256 after the letter  C.
4629         The default value is zero.  For example, this pattern has  two  callout         The  default  value is zero.  For example, this pattern has two callout
4630         points:         points:
4631    
4632           (?C1)abc(?C2)def           (?C1)abc(?C2)def
4633    
4634         If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are         If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4635         automatically installed before each item in the pattern. They  are  all         automatically  installed  before each item in the pattern. They are all
4636         numbered 255.         numbered 255.
4637    
4638         During matching, when PCRE reaches a callout point (and pcre_callout is         During matching, when PCRE reaches a callout point (and pcre_callout is
4639         set), the external function is called. It is provided with  the  number         set),  the  external function is called. It is provided with the number
4640         of  the callout, the position in the pattern, and, optionally, one item         of the callout, the position in the pattern, and, optionally, one  item
4641         of data originally supplied by the caller of pcre_exec().  The  callout         of  data  originally supplied by the caller of pcre_exec(). The callout
4642         function  may cause matching to proceed, to backtrack, or to fail alto-         function may cause matching to proceed, to backtrack, or to fail  alto-
4643         gether. A complete description of the interface to the callout function         gether. A complete description of the interface to the callout function
4644         is given in the pcrecallout documentation.         is given in the pcrecallout documentation.
4645    
4646    
4647  BACTRACKING CONTROL  BACTRACKING CONTROL
4648    
4649         Perl  5.10 introduced a number of "Special Backtracking Control Verbs",         Perl 5.10 introduced a number of "Special Backtracking Control  Verbs",
4650         which are described in the Perl documentation as "experimental and sub-         which are described in the Perl documentation as "experimental and sub-
4651         ject  to  change or removal in a future version of Perl". It goes on to         ject to change or removal in a future version of Perl". It goes  on  to
4652         say: "Their usage in production code should be noted to avoid  problems         say:  "Their usage in production code should be noted to avoid problems
4653         during upgrades." The same remarks apply to the PCRE features described         during upgrades." The same remarks apply to the PCRE features described
4654         in this section.         in this section.
4655    
4656         Since these verbs are specifically related to backtracking, they can be         Since these verbs are specifically related to backtracking, they can be
4657         used  only  when  the pattern is to be matched using pcre_exec(), which         used only when the pattern is to be matched  using  pcre_exec(),  which
4658         uses a backtracking algorithm. They cause an error  if  encountered  by         uses  a  backtracking  algorithm. They cause an error if encountered by
4659         pcre_dfa_exec().         pcre_dfa_exec().
4660    
4661         The  new verbs make use of what was previously invalid syntax: an open-         The new verbs make use of what was previously invalid syntax: an  open-
4662         ing parenthesis followed by an asterisk. In Perl, they are generally of         ing parenthesis followed by an asterisk. In Perl, they are generally of
4663         the form (*VERB:ARG) but PCRE does not support the use of arguments, so         the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4664         its general form is just (*VERB). Any number of these verbs  may  occur         its  general  form is just (*VERB). Any number of these verbs may occur
4665         in a pattern. There are two kinds:         in a pattern. There are two kinds:
4666    
4667     Verbs that act immediately     Verbs that act immediately
# Line 4611  BACTRACKING CONTROL Line 4670  BACTRACKING CONTROL
4670    
4671            (*ACCEPT)            (*ACCEPT)
4672    
4673         This  verb causes the match to end successfully, skipping the remainder         This verb causes the match to end successfully, skipping the  remainder
4674         of the pattern. When inside a recursion, only the innermost pattern  is         of  the pattern. When inside a recursion, only the innermost pattern is
4675         ended  immediately.  PCRE  differs  from  Perl  in  what happens if the         ended immediately. PCRE differs  from  Perl  in  what  happens  if  the
4676         (*ACCEPT) is inside capturing parentheses. In Perl, the data so far  is         (*ACCEPT)  is inside capturing parentheses. In Perl, the data so far is
4677         captured: in PCRE no data is captured. For example:         captured: in PCRE no data is captured. For example:
4678    
4679           A(A|B(*ACCEPT)|C)D           A(A|B(*ACCEPT)|C)D
4680    
4681         This  matches  "AB", "AAD", or "ACD", but when it matches "AB", no data         This matches "AB", "AAD", or "ACD", but when it matches "AB",  no  data
4682         is captured.         is captured.
4683    
4684           (*FAIL) or (*F)           (*FAIL) or (*F)
4685    
4686         This verb causes the match to fail, forcing backtracking to  occur.  It         This  verb  causes the match to fail, forcing backtracking to occur. It
4687         is  equivalent to (?!) but easier to read. The Perl documentation notes         is equivalent to (?!) but easier to read. The Perl documentation  notes
4688         that it is probably useful only when combined  with  (?{})  or  (??{}).         that  it  is  probably  useful only when combined with (?{}) or (??{}).
4689         Those  are,  of course, Perl features that are not present in PCRE. The         Those are, of course, Perl features that are not present in  PCRE.  The
4690         nearest equivalent is the callout feature, as for example in this  pat-         nearest  equivalent is the callout feature, as for example in this pat-
4691         tern:         tern:
4692    
4693           a+(?C)(*FAIL)           a+(?C)(*FAIL)
4694    
4695         A  match  with the string "aaaa" always fails, but the callout is taken         A match with the string "aaaa" always fails, but the callout  is  taken
4696         before each backtrack happens (in this example, 10 times).         before each backtrack happens (in this example, 10 times).
4697    
4698     Verbs that act after backtracking     Verbs that act after backtracking
4699    
4700         The following verbs do nothing when they are encountered. Matching con-         The following verbs do nothing when they are encountered. Matching con-
4701         tinues  with what follows, but if there is no subsequent match, a fail-         tinues with what follows, but if there is no subsequent match, a  fail-
4702         ure is forced.  The verbs  differ  in  exactly  what  kind  of  failure         ure  is  forced.   The  verbs  differ  in  exactly what kind of failure
4703         occurs.         occurs.
4704    
4705           (*COMMIT)           (*COMMIT)
4706    
4707         This  verb  causes  the whole match to fail outright if the rest of the         This verb causes the whole match to fail outright if the  rest  of  the
4708         pattern does not match. Even if the pattern is unanchored,  no  further         pattern  does  not match. Even if the pattern is unanchored, no further
4709         attempts  to find a match by advancing the start point take place. Once         attempts to find a match by advancing the start point take place.  Once
4710         (*COMMIT) has been passed, pcre_exec() is committed to finding a  match         (*COMMIT)  has been passed, pcre_exec() is committed to finding a match
4711         at the current starting point, or not at all. For example:         at the current starting point, or not at all. For example:
4712    
4713           a+(*COMMIT)b           a+(*COMMIT)b
4714    
4715         This  matches  "xxaab" but not "aacaab". It can be thought of as a kind         This matches "xxaab" but not "aacaab". It can be thought of as  a  kind
4716         of dynamic anchor, or "I've started, so I must finish."         of dynamic anchor, or "I've started, so I must finish."
4717    
4718           (*PRUNE)           (*PRUNE)
4719    
4720         This verb causes the match to fail at the current position if the  rest         This  verb causes the match to fail at the current position if the rest
4721         of the pattern does not match. If the pattern is unanchored, the normal         of the pattern does not match. If the pattern is unanchored, the normal
4722         "bumpalong" advance to the next starting character then happens.  Back-         "bumpalong"  advance to the next starting character then happens. Back-
4723         tracking  can  occur as usual to the left of (*PRUNE), or when matching         tracking can occur as usual to the left of (*PRUNE), or  when  matching
4724         to the right of (*PRUNE), but if there is no match to the right,  back-         to  the right of (*PRUNE), but if there is no match to the right, back-
4725         tracking  cannot  cross (*PRUNE).  In simple cases, the use of (*PRUNE)         tracking cannot cross (*PRUNE).  In simple cases, the use  of  (*PRUNE)
4726         is just an alternative to an atomic group or possessive quantifier, but         is just an alternative to an atomic group or possessive quantifier, but
4727         there  are  some uses of (*PRUNE) that cannot be expressed in any other         there are some uses of (*PRUNE) that cannot be expressed in  any  other
4728         way.         way.
4729    
4730           (*SKIP)           (*SKIP)
4731    
4732         This verb is like (*PRUNE), except that if the pattern  is  unanchored,         This  verb  is like (*PRUNE), except that if the pattern is unanchored,
4733         the  "bumpalong" advance is not to the next character, but to the posi-         the "bumpalong" advance is not to the next character, but to the  posi-
4734         tion in the subject where (*SKIP) was  encountered.  (*SKIP)  signifies         tion  in  the  subject where (*SKIP) was encountered. (*SKIP) signifies
4735         that  whatever  text  was  matched leading up to it cannot be part of a         that whatever text was matched leading up to it cannot  be  part  of  a
4736         successful match. Consider:         successful match. Consider:
4737    
4738           a+(*SKIP)b           a+(*SKIP)b
4739    
4740         If the subject is "aaaac...",  after  the  first  match  attempt  fails         If  the  subject  is  "aaaac...",  after  the first match attempt fails
4741         (starting  at  the  first  character in the string), the starting point         (starting at the first character in the  string),  the  starting  point
4742         skips on to start the next attempt at "c". Note that a possessive quan-         skips on to start the next attempt at "c". Note that a possessive quan-
4743         tifer  does not have the same effect in this example; although it would         tifer does not have the same effect in this example; although it  would
4744         suppress backtracking  during  the  first  match  attempt,  the  second         suppress  backtracking  during  the  first  match  attempt,  the second
4745         attempt  would  start at the second character instead of skipping on to         attempt would start at the second character instead of skipping  on  to
4746         "c".         "c".
4747    
4748           (*THEN)           (*THEN)
4749    
4750         This verb causes a skip to the next alternation if the rest of the pat-         This verb causes a skip to the next alternation if the rest of the pat-
4751         tern does not match. That is, it cancels pending backtracking, but only         tern does not match. That is, it cancels pending backtracking, but only
4752         within the current alternation. Its name  comes  from  the  observation         within  the  current  alternation.  Its name comes from the observation
4753         that it can be used for a pattern-based if-then-else block:         that it can be used for a pattern-based if-then-else block:
4754    
4755           ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...           ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4756    
4757         If  the COND1 pattern matches, FOO is tried (and possibly further items         If the COND1 pattern matches, FOO is tried (and possibly further  items
4758         after the end of the group if FOO succeeds);  on  failure  the  matcher         after  the  end  of  the group if FOO succeeds); on failure the matcher
4759         skips  to  the second alternative and tries COND2, without backtracking         skips to the second alternative and tries COND2,  without  backtracking
4760         into COND1. If (*THEN) is used outside  of  any  alternation,  it  acts         into  COND1.  If  (*THEN)  is  used outside of any alternation, it acts
4761         exactly like (*PRUNE).         exactly like (*PRUNE).
4762    
4763    
# Line 4716  AUTHOR Line 4775  AUTHOR
4775    
4776  REVISION  REVISION
4777    
4778         Last updated: 09 August 2007         Last updated: 21 August 2007
4779         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
4780  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
4781    
# Line 5013  BACKTRACKING CONTROL Line 5072  BACKTRACKING CONTROL
5072           (*THEN)        local failure, backtrack to next alternation           (*THEN)        local failure, backtrack to next alternation
5073    
5074    
5075    NEWLINE CONVENTIONS
5076    
5077           These are recognized only at the very start of a pattern.
5078    
5079             (*CR)
5080             (*LF)
5081             (*CRLF)
5082             (*ANYCRLF)
5083             (*ANY)
5084    
5085    
5086  CALLOUTS  CALLOUTS
5087    
5088           (?C)      callout           (?C)      callout
# Line 5033  AUTHOR Line 5103  AUTHOR
5103    
5104  REVISION  REVISION
5105    
5106         Last updated: 08 August 2007         Last updated: 21 August 2007
5107         Copyright (c) 1997-2007 University of Cambridge.         Copyright (c) 1997-2007 University of Cambridge.
5108  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5109    

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