NAME
     pcre - Perl-compatible regular expressions.



SYNOPSIS
     #include <pcre.h>

     pcre *pcre_compile(const char *pattern, int options,
          const char **errptr, int *erroffset,
          const unsigned char *tableptr);

     pcre_extra *pcre_study(const pcre *code, int options,
          const char **errptr);

     int pcre_exec(const pcre *code, const pcre_extra *extra,
          const char *subject, int length, int startoffset,
          int options, int *ovector, int ovecsize);

     int pcre_copy_substring(const char *subject, int *ovector,
          int stringcount, int stringnumber, char *buffer,
          int buffersize);

     int pcre_get_substring(const char *subject, int *ovector,
          int stringcount, int stringnumber,
          const char **stringptr);

     int pcre_get_substring_list(const char *subject,
          int *ovector, int stringcount, const char ***listptr);

     const unsigned char *pcre_maketables(void);

     int pcre_info(const pcre *code, int *optptr, *firstcharptr);

     char *pcre_version(void);

     void *(*pcre_malloc)(size_t);

     void (*pcre_free)(void *);




DESCRIPTION
     The PCRE library is a set of functions that implement  regu-
     lar  expression  pattern  matching using the same syntax and
     semantics as Perl  5,  with  just  a  few  differences  (see
     below).  The  current  implementation  corresponds  to  Perl
     5.005.

     PCRE has its own native API,  which  is  described  in  this
     document.  There  is  also  a  set of wrapper functions that
     correspond to the POSIX API.  These  are  described  in  the
     pcreposix documentation.
     The native API function prototypes are defined in the header
     file  pcre.h,  and  on  Unix  systems  the library itself is
     called libpcre.a, so can be accessed by adding -lpcre to the
     command for linking an application which calls it.

     The functions pcre_compile(), pcre_study(), and  pcre_exec()
     are  used  for  compiling  and matching regular expressions,
     while   pcre_copy_substring(),   pcre_get_substring(),   and
     pcre_get_substring_list()   are  convenience  functions  for
     extracting  captured  substrings  from  a  matched   subject
     string.  The function pcre_maketables() is used (optionally)
     to build a set of character tables in the current locale for
     passing to pcre_compile().

     The function pcre_info() is used  to  find  out  information
     about  a compiled pattern, while the function pcre_version()
     returns a pointer to a string containing the version of PCRE
     and its date of release.

     The global variables  pcre_malloc  and  pcre_free  initially
     contain the entry points of the standard malloc() and free()
     functions respectively. PCRE  calls  the  memory  management
     functions  via  these  variables,  so  a calling program can
     replace them if it  wishes  to  intercept  the  calls.  This
     should be done before calling any PCRE functions.



MULTI-THREADING
     The PCRE functions can be used in  multi-threading  applica-
     tions, with the proviso that the memory management functions
     pointed to by pcre_malloc and pcre_free are  shared  by  all
     threads.

     The compiled form of a regular  expression  is  not  altered
     during  matching, so the same compiled pattern can safely be
     used by several threads at once.



COMPILING A PATTERN
     The function pcre_compile() is called to compile  a  pattern
     into  an internal form. The pattern is a C string terminated
     by a binary zero, and is passed in the argument  pattern.  A
     pointer  to  a  single  block of memory that is obtained via
     pcre_malloc is returned. This contains the compiled code and
     related data. The pcre type is defined for this for conveni-
     ence, but in fact pcre is just a typedef for void, since the
     contents  of  the block are not externally defined. It is up
     to the caller to free  the  memory  when  it  is  no  longer
     required.

     The size of a compiled pattern is  roughly  proportional  to
     the length of the pattern string, except that each character
     class (other than those containing just a single  character,
     negated  or  not)  requires 33 bytes, and repeat quantifiers
     with a minimum greater than one or a bounded  maximum  cause
     the  relevant  portions of the compiled pattern to be repli-
     cated.

     The options argument contains independent bits  that  affect
     the  compilation.  It  should  be  zero  if  no  options are
     required. Some of the options, in particular, those that are
     compatible  with Perl, can also be set and unset from within
     the pattern (see the detailed description of regular expres-
     sions below). For these options, the contents of the options
     argument specifies their initial settings at  the  start  of
     compilation  and  execution. The PCRE_ANCHORED option can be
     set at the time of matching as well as at compile time.

     If errptr is NULL, pcre_compile() returns NULL  immediately.
     Otherwise, if compilation of a pattern fails, pcre_compile()
     returns NULL, and sets the variable pointed to by errptr  to
     point  to a textual error message. The offset from the start
     of  the  pattern  to  the  character  where  the  error  was
     discovered   is   placed  in  the  variable  pointed  to  by
     erroffset, which must not be NULL. If it  is,  an  immediate
     error is given.

     If the final  argument,  tableptr,  is  NULL,  PCRE  uses  a
     default  set  of character tables which are built when it is
     compiled, using the default C  locale.  Otherwise,  tableptr
     must  be  the result of a call to pcre_maketables(). See the
     section on locale support below.

     The following option bits are defined in the header file:

       PCRE_ANCHORED

     If this bit is set, the pattern is forced to be  "anchored",
     that is, it is constrained to match only at the start of the
     string which is being searched (the "subject string").  This
     effect can also be achieved by appropriate constructs in the
     pattern itself, which is the only way to do it in Perl.

       PCRE_CASELESS

     If this bit is set, letters in the pattern match both  upper
     and  lower  case  letters.  It  is  equivalent  to Perl's /i
     option.

       PCRE_DOLLAR_ENDONLY

     If this bit is set, a dollar metacharacter  in  the  pattern
     matches  only at the end of the subject string. Without this
     option, a dollar also matches immediately before  the  final
     character  if it is a newline (but not before any other new-
     lines).  The  PCRE_DOLLAR_ENDONLY  option  is   ignored   if
     PCRE_MULTILINE is set. There is no equivalent to this option
     in Perl.

       PCRE_DOTALL

     If this bit is  set,  a  dot  metacharater  in  the  pattern
     matches all characters, including newlines. Without it, new-
     lines are excluded. This option is equivalent to  Perl's  /s
     option.  A negative class such as [^a] always matches a new-
     line character, independent of the setting of this option.

       PCRE_EXTENDED

     If this bit is set, whitespace data characters in  the  pat-
     tern  are  totally  ignored  except when escaped or inside a
     character class, and characters between an unescaped #  out-
     side  a  character  class  and  the  next newline character,
     inclusive, are also ignored. This is equivalent to Perl's /x
     option,  and  makes  it  possible to include comments inside
     complicated patterns. Note, however, that this applies  only
     to  data  characters. Whitespace characters may never appear
     within special character sequences in a pattern, for example
     within  the sequence (?( which introduces a conditional sub-
     pattern.

       PCRE_EXTRA

     This option turns on additional functionality of  PCRE  that
     is  incompatible  with Perl. Any backslash in a pattern that
     is followed by a letter that has no special  meaning  causes
     an  error,  thus  reserving  these  combinations  for future
     expansion. By default, as in Perl, a backslash followed by a
     letter  with  no  special  meaning  is treated as a literal.
     There are at present no other features  controlled  by  this
     option.

       PCRE_MULTILINE

     By default, PCRE treats the subject string as consisting  of
     a  single "line" of characters (even if it actually contains
     several newlines). The "start  of  line"  metacharacter  (^)
     matches  only  at the start of the string, while the "end of
     line" metacharacter ($) matches  only  at  the  end  of  the
     string,    or   before   a   terminating   newline   (unless
     PCRE_DOLLAR_ENDONLY is set). This is the same as Perl.

     When PCRE_MULTILINE it is set, the "start of line" and  "end
     of   line"   constructs   match   immediately  following  or
     immediately  before  any  newline  in  the  subject  string,
     respectively,  as well as at the very start and end. This is
     equivalent to Perl's /m option. If there are no "\n" charac-
     ters  in  a subject string, or no occurrences of ^ or $ in a
     pattern, setting PCRE_MULTILINE has no effect.

       PCRE_UNGREEDY

     This option inverts the "greediness" of the  quantifiers  so
     that  they  are  not greedy by default, but become greedy if
     followed by "?". It is not compatible with Perl. It can also
     be set by a (?U) option setting within the pattern.



STUDYING A PATTERN
     When a pattern is going to be  used  several  times,  it  is
     worth  spending  more time analyzing it in order to speed up
     the time taken for matching. The function pcre_study() takes
     a  pointer  to a compiled pattern as its first argument, and
     returns a  pointer  to  a  pcre_extra  block  (another  void
     typedef)  containing  additional  information about the pat-
     tern; this can be passed to pcre_exec().  If  no  additional
     information is available, NULL is returned.

     The second argument contains option  bits.  At  present,  no
     options  are  defined  for  pcre_study(),  and this argument
     should always be zero.

     The third argument for pcre_study() is a pointer to an error
     message. If studying succeeds (even if no data is returned),
     the variable it points to  is  set  to  NULL.  Otherwise  it
     points to a textual error message.

     At present, studying a  pattern  is  useful  only  for  non-
     anchored  patterns  that do not have a single fixed starting
     character. A  bitmap  of  possible  starting  characters  is
     created.



LOCALE SUPPORT
     PCRE handles caseless matching, and determines whether char-
     acters  are  letters, digits, or whatever, by reference to a
     set of tables. The library contains a default set of  tables
     which  is  created in the default C locale when PCRE is com-
     piled.  This  is   used   when   the   final   argument   of
     pcre_compile()  is NULL, and is sufficient for many applica-
     tions.

     An alternative set of tables can, however, be supplied. Such
     tables  are built by calling the pcre_maketables() function,
     which has no arguments, in the relevant locale.  The  result
     can  then be passed to pcre_compile() as often as necessary.
     For example, to build and use tables  that  are  appropriate
     for  the French locale (where accented characters with codes
     greater than 128 are treated as letters), the following code
     could be used:

       setlocale(LC_CTYPE, "fr");
       tables = pcre_maketables();
       re = pcre_compile(..., tables);

     The  tables  are  built  in  memory  that  is  obtained  via
     pcre_malloc.  The  pointer that is passed to pcre_compile is
     saved with the compiled pattern, and  the  same  tables  are
     used  via this pointer by pcre_study() and pcre_exec(). Thus
     for any single pattern, compilation, studying  and  matching
     all happen in the same locale, but different patterns can be
     compiled in different locales. It is the caller's  responsi-
     bility  to  ensure  that  the  memory  containing the tables
     remains available for as long as it is needed.



INFORMATION ABOUT A PATTERN
     The pcre_info() function returns information  about  a  com-
     piled pattern.  Its yield is the number of capturing subpat-
     terns, or one of the following negative numbers:

       PCRE_ERROR_NULL       the argument code was NULL
       PCRE_ERROR_BADMAGIC   the "magic number" was not found

     If the optptr argument is not NULL, a copy  of  the  options
     with which the pattern was compiled is placed in the integer
     it points to. These option bits are those specified  in  the
     call  to  pcre_compile(),  modified  by any top-level option
     settings  within  the   pattern   itself,   and   with   the
     PCRE_ANCHORED  bit  set  if  the form of the pattern implies
     that it can match only at the start of a subject string.

     If the pattern is not anchored and the firstcharptr argument
     is  not  NULL, it is used to pass back information about the
     first character of any matched string. If there is  a  fixed
     first    character,    e.g.   from   a   pattern   such   as
     (cat|cow|coyote), then it is returned in the integer pointed
     to by firstcharptr. Otherwise, if either

     (a) the pattern was compiled with the PCRE_MULTILINE option,
     and every branch starts with "^", or

     (b) every  branch  of  the  pattern  starts  with  ".*"  and
     PCRE_DOTALL is not set (if it were set, the pattern would be
     anchored),
     then -1 is returned, indicating  that  the  pattern  matches
     only  at  the  start  of  a subject string or after any "\n"
     within the string. Otherwise -2 is returned.



MATCHING A PATTERN
     The function pcre_exec() is called to match a subject string
     against  a pre-compiled pattern, which is passed in the code
     argument. If the pattern has been studied, the result of the
     study should be passed in the extra argument. Otherwise this
     must be NULL.

     The PCRE_ANCHORED option can be passed in the options  argu-
     ment,  whose unused bits must be zero. However, if a pattern
     was  compiled  with  PCRE_ANCHORED,  or  turned  out  to  be
     anchored  by  virtue  of  its  contents,  it  cannot be made
     unachored at matching time.

     There are also three further options that can be set only at
     matching time:

       PCRE_NOTBOL

     The first character of the string is not the beginning of  a
     line,  so  the  circumflex  metacharacter  should  not match
     before it. Setting this without PCRE_MULTILINE  (at  compile
     time) causes circumflex never to match.

       PCRE_NOTEOL

     The end of the string is not the end of a line, so the  dol-
     lar  metacharacter should not match it nor (except in multi-
     line mode) a newline immediately  before  it.  Setting  this
     without PCRE_MULTILINE (at compile time) causes dollar never
     to match.

       PCRE_NOTEMPTY

     An empty string is not considered to be  a  valid  match  if
     this  option  is  set. If there are alternatives in the pat-
     tern, they are tried. If  all  the  alternatives  match  the
     empty  string,  the  entire match fails. For example, if the
     pattern

       a?b?

     is applied to a string not beginning with  "a"  or  "b",  it
     matches  the  empty string at the start of the subject. With
     PCRE_NOTEMPTY set, this match is not valid, so PCRE searches
     further into the string for occurrences of "a" or "b".  Perl
     has no direct equivalent of this  option,  but  it  makes  a
     special  case  of a pattern match of the empty string within
     its split() function, or when using the /g  modifier.  Using
     PCRE_NOTEMPTY it is possible to emulate this behaviour.

     The subject string is passed as  a  pointer  in  subject,  a
     length  in  length,  and  a  starting offset in startoffset.
     Unlike the pattern string, it may contain binary zero  char-
     acters.  When  the starting offset is zero, the search for a
     match starts at the beginning of the subject, and this is by
     far the most common case.

     A non-zero starting offset  is  useful  when  searching  for
     another  match  in  the  same subject by calling pcre_exec()
     again after a previous success.  Setting startoffset differs
     from  just  passing  over  a  shortened  string  and setting
     PCRE_NOTBOL in the case of a pattern that  begins  with  any
     kind of lookbehind. For example, consider the pattern

       \Biss\B

     which finds occurrences of "iss" in the middle of words. (\B
     matches only if the current position in the subject is not a
     word boundary.) When applied to the string "Mississipi"  the
     first  call  to  pcre_exec()  finds the first occurrence. If
     pcre_exec() is called again with just the remainder  of  the
     subject,  namely  "issipi", it does not match, because \B is
     always false at the start of the subject, which is deemed to
     be  a  word  boundary. However, if pcre_exec() is passed the
     entire string again, but with startoffset set to 4, it finds
     the  second  occurrence  of "iss" because it is able to look
     behind the starting point to discover that it is preceded by
     a letter.

     If a non-zero starting offset is passed when the pattern  is
     anchored, one attempt to match at the given offset is tried.
     This can only succeed if the pattern does  not  require  the
     match to be at the start of the subject.

     In general, a pattern matches a certain portion of the  sub-
     ject,  and  in addition, further substrings from the subject
     may be picked out by parts of  the  pattern.  Following  the
     usage  in  Jeffrey Friedl's book, this is called "capturing"
     in what follows, and the phrase  "capturing  subpattern"  is
     used for a fragment of a pattern that picks out a substring.
     PCRE supports several other kinds of  parenthesized  subpat-
     tern that do not cause substrings to be captured.

     Captured substrings are returned to the caller via a  vector
     of  integer  offsets whose address is passed in ovector. The
     number of elements in the vector is passed in ovecsize.  The
     first two-thirds of the vector is used to pass back captured
     substrings, each substring using a  pair  of  integers.  The
     remaining  third  of  the  vector  is  used  as workspace by
     pcre_exec() while matching capturing subpatterns, and is not
     available for passing back information. The length passed in
     ovecsize should always be a multiple of three. If it is not,
     it is rounded down.

     When a match has been successful, information about captured
     substrings is returned in pairs of integers, starting at the
     beginning of ovector, and continuing up to two-thirds of its
     length  at  the  most. The first element of a pair is set to
     the offset of the first character in a  substring,  and  the
     second is set to the offset of the first character after the
     end of a substring. The first  pair,  ovector[0]  and  ovec-
     tor[1],  identify  the portion of the subject string matched
     by the entire pattern. The next pair is used for  the  first
     capturing  subpattern,  and  so  on.  The  value returned by
     pcre_exec() is the number of pairs that have  been  set.  If
     there  are no capturing subpatterns, the return value from a
     successful match is 1, indicating that just the  first  pair
     of offsets has been set.

     Some convenience functions are provided for  extracting  the
     captured substrings as separate strings. These are described
     in the following section.

     It is possible for an capturing  subpattern  number  n+1  to
     match  some  part  of  the subject when subpattern n has not
     been used at all.  For  example,  if  the  string  "abc"  is
     matched  against the pattern (a|(z))(bc) subpatterns 1 and 3
     are matched, but 2 is not. When this  happens,  both  offset
     values corresponding to the unused subpattern are set to -1.

     If a capturing subpattern is matched repeatedly, it  is  the
     last  portion  of  the  string  that  it  matched  that gets
     returned.

     If the vector is too small to hold  all  the  captured  sub-
     strings,  it is used as far as possible (up to two-thirds of
     its length), and the function returns a value  of  zero.  In
     particular,  if  the  substring offsets are not of interest,
     pcre_exec() may be called with ovector passed  as  NULL  and
     ovecsize  as  zero.  However,  if  the pattern contains back
     references and the ovector isn't big enough to remember  the
     related  substrings,  PCRE  has to get additional memory for
     use during matching. Thus it is usually advisable to  supply
     an ovector.

     Note that pcre_info() can be used to find out how many  cap-
     turing  subpatterns  there  are  in  a compiled pattern. The
     smallest size for ovector that will  allow  for  n  captured
     substrings  in  addition  to  the  offsets  of the substring
     matched by the whole pattern is (n+1)*3.
     If pcre_exec() fails, it returns a negative number. The fol-
     lowing are defined in the header file:

       PCRE_ERROR_NOMATCH        (-1)

     The subject string did not match the pattern.

       PCRE_ERROR_NULL           (-2)

     Either code or subject was passed as NULL,  or  ovector  was
     NULL and ovecsize was not zero.

       PCRE_ERROR_BADOPTION      (-3)

     An unrecognized bit was set in the options argument.

       PCRE_ERROR_BADMAGIC       (-4)

     PCRE stores a 4-byte "magic number" at the start of the com-
     piled  code,  to  catch  the  case  when it is passed a junk
     pointer. This is the error it gives when  the  magic  number
     isn't present.

       PCRE_ERROR_UNKNOWN_NODE   (-5)

     While running the pattern match, an unknown item was encoun-
     tered in the compiled pattern. This error could be caused by
     a bug in PCRE or by overwriting of the compiled pattern.

       PCRE_ERROR_NOMEMORY       (-6)

     If a pattern contains back references, but the ovector  that
     is  passed  to pcre_exec() is not big enough to remember the
     referenced substrings, PCRE gets a block of  memory  at  the
     start  of  matching to use for this purpose. If the call via
     pcre_malloc() fails, this error  is  given.  The  memory  is
     freed at the end of matching.



EXTRACTING CAPTURED SUBSTRINGS
     Captured substrings can be accessed directly  by  using  the
     offsets returned by pcre_exec() in ovector. For convenience,
     the functions  pcre_copy_substring(),  pcre_get_substring(),
     and  pcre_get_substring_list()  are  provided for extracting
     captured  substrings  as  new,   separate,   zero-terminated
     strings.   A  substring  that  contains  a  binary  zero  is
     correctly extracted and has a further zero added on the end,
     but the result does not, of course, function as a C string.

     The first three arguments are the same for all  three  func-
     tions:  subject  is  the  subject string which has just been
     successfully matched, ovector is a pointer to the vector  of
     integer   offsets   that  was  passed  to  pcre_exec(),  and
     stringcount is the number of substrings that  were  captured
     by  the  match,  including  the  substring  that matched the
     entire regular expression. This is  the  value  returned  by
     pcre_exec  if  it  is  greater  than  zero.  If  pcre_exec()
     returned zero, indicating that it ran out of space in  ovec-
     tor, then the value passed as stringcount should be the size
     of the vector divided by three.

     The functions pcre_copy_substring() and pcre_get_substring()
     extract a single substring, whose number is given as string-
     number. A value of zero extracts the substring that  matched
     the entire pattern, while higher values extract the captured
     substrings. For pcre_copy_substring(), the string is  placed
     in  buffer,  whose  length is given by buffersize, while for
     pcre_get_substring() a new block of store  is  obtained  via
     pcre_malloc,  and its address is returned via stringptr. The
     yield of the function is  the  length  of  the  string,  not
     including the terminating zero, or one of

       PCRE_ERROR_NOMEMORY       (-6)

     The buffer was too small for pcre_copy_substring(),  or  the
     attempt to get memory failed for pcre_get_substring().

       PCRE_ERROR_NOSUBSTRING    (-7)

     There is no substring whose number is stringnumber.

     The pcre_get_substring_list() function extracts  all  avail-
     able  substrings  and builds a list of pointers to them. All
     this is done in a single block of memory which  is  obtained
     via pcre_malloc. The address of the memory block is returned
     via listptr, which is also the start of the list  of  string
     pointers.  The  end of the list is marked by a NULL pointer.
     The yield of the function is zero if all went well, or

       PCRE_ERROR_NOMEMORY       (-6)

     if the attempt to get the memory block failed.

     When any of these functions encounter a  substring  that  is
     unset, which can happen when capturing subpattern number n+1
     matches some part of the subject, but subpattern n  has  not
     been  used  at all, they return an empty string. This can be
     distinguished  from  a  genuine  zero-length  substring   by
     inspecting the appropriate offset in ovector, which is nega-
     tive for unset substrings.



LIMITATIONS
     There are some size limitations in PCRE but it is hoped that
     they will never in practice be relevant.  The maximum length
     of a compiled pattern is 65539 (sic) bytes.  All  values  in
     repeating  quantifiers must be less than 65536.  The maximum
     number of capturing subpatterns is 99.  The  maximum  number
     of  all  parenthesized subpatterns, including capturing sub-
     patterns, assertions, and other types of subpattern, is 200.

     The maximum length of a subject string is the largest  posi-
     tive number that an integer variable can hold. However, PCRE
     uses recursion to handle subpatterns and indefinite  repeti-
     tion.  This  means  that the available stack space may limit
     the size of a subject string that can be processed  by  cer-
     tain patterns.



DIFFERENCES FROM PERL
     The differences described here  are  with  respect  to  Perl
     5.005.

     1. By default, a whitespace character is any character  that
     the  C  library  function isspace() recognizes, though it is
     possible to compile PCRE  with  alternative  character  type
     tables. Normally isspace() matches space, formfeed, newline,
     carriage return, horizontal tab, and vertical tab. Perl 5 no
     longer  includes vertical tab in its set of whitespace char-
     acters. The \v escape that was in the Perl documentation for
     a long time was never in fact recognized. However, the char-
     acter itself was treated as whitespace at least up to 5.002.
     In 5.004 and 5.005 it does not match \s.

     2. PCRE does  not  allow  repeat  quantifiers  on  lookahead
     assertions. Perl permits them, but they do not mean what you
     might think. For example, (?!a){3} does not assert that  the
     next  three characters are not "a". It just asserts that the
     next character is not "a" three times.

     3. Capturing subpatterns that occur inside  negative  looka-
     head  assertions  are  counted,  but  their  entries  in the
     offsets vector are never set. Perl sets its numerical  vari-
     ables  from  any  such  patterns that are matched before the
     assertion fails to match something (thereby succeeding), but
     only  if  the negative lookahead assertion contains just one
     branch.

     4. Though binary zero characters are supported in  the  sub-
     ject  string,  they  are  not  allowed  in  a pattern string
     because it is passed as a normal  C  string,  terminated  by
     zero. The escape sequence "\0" can be used in the pattern to
     represent a binary zero.
     5. The following Perl escape sequences  are  not  supported:
     \l,  \u,  \L,  \U,  \E, \Q. In fact these are implemented by
     Perl's general string-handling and are not part of its  pat-
     tern matching engine.

     6. The Perl \G assertion is  not  supported  as  it  is  not
     relevant to single pattern matches.

     7. Fairly obviously, PCRE does  not  support  the  (?{code})
     construction.

     8. There are at the time of writing some  oddities  in  Perl
     5.005_02  concerned  with  the  settings of captured strings
     when part of a pattern is repeated.  For  example,  matching
     "aba"  against the pattern /^(a(b)?)+$/ sets $2 to the value
     "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves  $2
     unset.    However,    if   the   pattern   is   changed   to
     /^(aa(b(b))?)+$/ then $2 (and $3) get set.

     In Perl 5.004 $2 is set in both cases, and that is also true
     of PCRE. If in the future Perl changes to a consistent state
     that is different, PCRE may change to follow.

     9. Another as yet unresolved discrepancy  is  that  in  Perl
     5.005_02  the  pattern /^(a)?(?(1)a|b)+$/ matches the string
     "a", whereas in PCRE it does not.  However, in both Perl and
     PCRE /^(a)?a/ matched against "a" leaves $1 unset.

     10. PCRE  provides  some  extensions  to  the  Perl  regular
     expression facilities:

     (a) Although lookbehind assertions must match  fixed  length
     strings,  each  alternative branch of a lookbehind assertion
     can match a different length of string. Perl 5.005  requires
     them all to have the same length.

     (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is  not
     set,  the  $ meta- character matches only at the very end of
     the string.

     (c) If PCRE_EXTRA is set, a backslash followed by  a  letter
     with no special meaning is faulted.

     (d) If PCRE_UNGREEDY is set, the greediness of  the  repeti-
     tion  quantifiers  is inverted, that is, by default they are
     not greedy, but if followed by a question mark they are.

     (e) PCRE_ANCHORED can be used to force a pattern to be tried
     only at the start of the subject.

     (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY  options
     for pcre_exec() have no Perl equivalents.



REGULAR EXPRESSION DETAILS
     The syntax and semantics of  the  regular  expressions  sup-
     ported  by PCRE are described below. Regular expressions are
     also described in the Perl documentation and in a number  of
     other  books,  some  of which have copious examples. Jeffrey
     Friedl's  "Mastering  Regular  Expressions",  published   by
     O'Reilly  (ISBN 1-56592-257-3), covers them in great detail.
     The description here is intended as reference documentation.

     A regular expression is a pattern that is matched against  a
     subject string from left to right. Most characters stand for
     themselves in a pattern, and match the corresponding charac-
     ters in the subject. As a trivial example, the pattern

       The quick brown fox

     matches a portion of a subject string that is  identical  to
     itself.  The  power  of  regular  expressions comes from the
     ability to include alternatives and repetitions in the  pat-
     tern.  These  are encoded in the pattern by the use of meta-
     characters, which do not stand for  themselves  but  instead
     are interpreted in some special way.

     There are two different sets of meta-characters: those  that
     are  recognized anywhere in the pattern except within square
     brackets, and those that are recognized in square  brackets.
     Outside square brackets, the meta-characters are as follows:

       \      general escape character with several uses
       ^      assert start of  subject  (or  line,  in  multiline
     mode)
       $      assert end of subject (or line, in multiline mode)
       .      match any character except newline (by default)
       [      start character class definition
       |      start of alternative branch
       (      start subpattern
       )      end subpattern
       ?      extends the meaning of (
              also 0 or 1 quantifier
              also quantifier minimizer
       *      0 or more quantifier
       +      1 or more quantifier
       {      start min/max quantifier

     Part of a pattern that is in square  brackets  is  called  a
     "character  class".  In  a  character  class  the only meta-
     characters are:

       \      general escape character
       ^      negate the class, but only if the first character
       -      indicates character range
       ]      terminates the character class

     The following sections describe  the  use  of  each  of  the
     meta-characters.



BACKSLASH
     The backslash character has several uses. Firstly, if it  is
     followed  by  a  non-alphameric character, it takes away any
     special  meaning  that  character  may  have.  This  use  of
     backslash  as  an  escape  character applies both inside and
     outside character classes.

     For example, if you want to match a "*" character, you write
     "\*" in the pattern. This applies whether or not the follow-
     ing character would otherwise  be  interpreted  as  a  meta-
     character,  so it is always safe to precede a non-alphameric
     with "\" to specify that it stands for itself.  In  particu-
     lar, if you want to match a backslash, you write "\\".

     If a pattern is compiled with the PCRE_EXTENDED option, whi-
     tespace in the pattern (other than in a character class) and
     characters between a "#" outside a character class  and  the
     next  newline  character  are ignored. An escaping backslash
     can be used to include a whitespace or "#" character as part
     of the pattern.

     A second use of backslash provides a way  of  encoding  non-
     printing  characters  in patterns in a visible manner. There
     is no restriction on the appearance of non-printing  charac-
     ters,  apart from the binary zero that terminates a pattern,
     but when a pattern is being prepared by text editing, it  is
     usually  easier to use one of the following escape sequences
     than the binary character it represents:

       \a     alarm, that is, the BEL character (hex 07)
       \cx    "control-x", where x is any character
       \e     escape (hex 1B)
       \f     formfeed (hex 0C)
       \n     newline (hex 0A)
       \r     carriage return (hex 0D)
       \t     tab (hex 09)
       \xhh   character with hex code hh
       \ddd   character with octal code ddd, or backreference

     The precise effect of "\cx" is as follows: if "x" is a lower
     case  letter,  it  is converted to upper case. Then bit 6 of
     the character (hex 40) is inverted.  Thus "\cz" becomes  hex
     1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.

     After "\x", up to two hexadecimal digits are  read  (letters
     can be in upper or lower case).

     After "\0" up to two further octal digits are read. In  both
     cases,  if  there are fewer than two digits, just those that
     are present are used. Thus the sequence "\0\x\07"  specifies
     two binary zeros followed by a BEL character.  Make sure you
     supply two digits after the initial zero  if  the  character
     that follows is itself an octal digit.

     The handling of a backslash followed by a digit other than 0
     is  complicated.   Outside  a character class, PCRE reads it
     and any following digits as a decimal number. If the  number
     is  less  than  10, or if there have been at least that many
     previous capturing left parentheses in the  expression,  the
     entire  sequence is taken as a back reference. A description
     of how this works is given later, following  the  discussion
     of parenthesized subpatterns.

     Inside a character  class,  or  if  the  decimal  number  is
     greater  than  9 and there have not been that many capturing
     subpatterns, PCRE re-reads up to three octal digits  follow-
     ing  the  backslash,  and  generates  a single byte from the
     least significant 8 bits of the value. Any subsequent digits
     stand for themselves.  For example:

       \040   is another way of writing a space
       \40    is the same, provided there are fewer than 40
                 previous capturing subpatterns
       \7     is always a back reference
       \11    might be a back reference, or another way of
                 writing a tab
       \011   is always a tab
       \0113  is a tab followed by the character "3"
       \113   is the character with octal code 113 (since there
                 can be no more than 99 back references)
       \377   is a byte consisting entirely of 1 bits
       \81    is either a back reference, or a binary zero
                 followed by the two characters "8" and "1"

     Note that octal values of 100 or greater must not be  intro-
     duced  by  a  leading zero, because no more than three octal
     digits are ever read.

     All the sequences that define a single  byte  value  can  be
     used both inside and outside character classes. In addition,
     inside a character class, the sequence "\b"  is  interpreted
     as  the  backspace  character  (hex 08). Outside a character
     class it has a different meaning (see below).

     The third use of backslash is for specifying generic charac-
     ter types:

       \d     any decimal digit
       \D     any character that is not a decimal digit
       any whitespace character
       \S     any character that is not a whitespace character
       \w     any "word" character
       \W     any "non-word" character

     Each pair of escape sequences partitions the complete set of
     characters  into  two  disjoint  sets.  Any  given character
     matches one, and only one, of each pair.

     A "word" character is any letter or digit or the  underscore
     character,  that  is,  any  character which can be part of a
     Perl "word". The definition of letters and  digits  is  con-
     trolled  by PCRE's character tables, and may vary if locale-
     specific matching is  taking  place  (see  "Locale  support"
     above). For example, in the "fr" (French) locale, some char-
     acter codes greater than 128 are used for accented  letters,
     and these are matched by \w.

     These character type sequences can appear  both  inside  and
     outside  character classes. They each match one character of
     the appropriate type. If the current matching  point  is  at
     the end of the subject string, all of them fail, since there
     is no character to match.

     The fourth use of backslash is  for  certain  simple  asser-
     tions. An assertion specifies a condition that has to be met
     at a particular point in  a  match,  without  consuming  any
     characters  from  the subject string. The use of subpatterns
     for more complicated  assertions  is  described  below.  The
     backslashed assertions are

       \b     word boundary
       \B     not a word boundary
       \A     start of subject (independent of multiline mode)
       \Z     end of subject or newline at  end  (independent  of
     multiline mode)
       \z     end of subject (independent of multiline mode)

     These assertions may not appear in  character  classes  (but
     note that "\b" has a different meaning, namely the backspace
     character, inside a character class).

     A word boundary is a position in the  subject  string  where
     the current character and the previous character do not both
     match \w or \W (i.e. one matches \w and  the  other  matches
     \W),  or the start or end of the string if the first or last
     character matches \w, respectively.

     The \A, \Z, and \z assertions differ  from  the  traditional
     circumflex  and  dollar  (described below) in that they only
     ever match at the very start and end of the subject  string,
     whatever  options  are  set.  They  are  not affected by the
     PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-
     ment  of  pcre_exec()  is  non-zero, \A can never match. The
     difference between \Z and \z is that  \Z  matches  before  a
     newline  that is the last character of the string as well as
     at the end of the string, whereas \z  matches  only  at  the
     end.



CIRCUMFLEX AND DOLLAR
     Outside a character class, in the default matching mode, the
     circumflex  character  is an assertion which is true only if
     the current matching point is at the start  of  the  subject
     string.  If  the startoffset argument of pcre_exec() is non-
     zero, circumflex can never match. Inside a character  class,
     circumflex has an entirely different meaning (see below).

     Circumflex need not be the first character of the pattern if
     a  number of alternatives are involved, but it should be the
     first thing in each alternative in which it appears  if  the
     pattern is ever to match that branch. If all possible alter-
     natives start with a circumflex, that is, if the pattern  is
     constrained to match only at the start of the subject, it is
     said to be an "anchored" pattern. (There are also other con-
     structs that can cause a pattern to be anchored.)

     A dollar character is an assertion which is true only if the
     current  matching point is at the end of the subject string,
     or immediately before a newline character that is  the  last
     character in the string (by default). Dollar need not be the
     last character of the pattern if a  number  of  alternatives
     are  involved,  but it should be the last item in any branch
     in which it appears.  Dollar has no  special  meaning  in  a
     character class.

     The meaning of dollar can be changed so that it matches only
     at   the   very   end   of   the   string,  by  setting  the
     PCRE_DOLLAR_ENDONLY option at compile or matching time. This
     does not affect the \Z assertion.

     The meanings of the circumflex  and  dollar  characters  are
     changed  if  the  PCRE_MULTILINE option is set. When this is
     the case,  they  match  immediately  after  and  immediately
     before an internal "\n" character, respectively, in addition
     to matching at the start and end of the subject string.  For
     example,  the  pattern  /^abc$/  matches  the subject string
     "def\nabc" in multiline  mode,  but  not  otherwise.  Conse-
     quently,  patterns  that  are  anchored  in single line mode
     because all branches start with "^" are not anchored in mul-
     tiline mode, and a match for circumflex is possible when the
     startoffset  argument  of  pcre_exec()  is   non-zero.   The
     PCRE_DOLLAR_ENDONLY  option  is ignored if PCRE_MULTILINE is
     set.

     Note that the sequences \A, \Z, and \z can be used to  match
     the  start  and end of the subject in both modes, and if all
     branches of a pattern start with \A is it  always  anchored,
     whether PCRE_MULTILINE is set or not.



FULL STOP (PERIOD, DOT)
     Outside a character class, a dot in the pattern matches  any
     one character in the subject, including a non-printing char-
     acter, but not (by default)  newline.   If  the  PCRE_DOTALL
     option  is  set,  then dots match newlines as well. The han-
     dling of dot is entirely independent of the handling of cir-
     cumflex  and  dollar,  the only relationship being that they
     both involve newline characters.  Dot has no special meaning
     in a character class.



SQUARE BRACKETS
     An opening square bracket introduces a character class, ter-
     minated  by  a  closing  square  bracket.  A  closing square
     bracket on its own is  not  special.  If  a  closing  square
     bracket  is  required as a member of the class, it should be
     the first data character in the class (after an initial cir-
     cumflex, if present) or escaped with a backslash.

     A character class matches a single character in the subject;
     the  character  must  be in the set of characters defined by
     the class, unless the first character in the class is a cir-
     cumflex,  in which case the subject character must not be in
     the set defined by the class. If a  circumflex  is  actually
     required  as  a  member  of  the class, ensure it is not the
     first character, or escape it with a backslash.

     For example, the character class [aeiou] matches  any  lower
     case vowel, while [^aeiou] matches any character that is not
     a lower case vowel. Note that a circumflex is  just  a  con-
     venient  notation for specifying the characters which are in
     the class by enumerating those that are not. It  is  not  an
     assertion:  it  still  consumes a character from the subject
     string, and fails if the current pointer is at  the  end  of
     the string.

     When caseless matching  is  set,  any  letters  in  a  class
     represent  both their upper case and lower case versions, so
     for example, a caseless [aeiou] matches "A" as well as  "a",
     and  a caseless [^aeiou] does not match "A", whereas a case-
     ful version would.

     The newline character is never treated in any special way in
     character  classes,  whatever the setting of the PCRE_DOTALL
     or PCRE_MULTILINE options is. A  class  such  as  [^a]  will
     always match a newline.

     The minus (hyphen) character can be used to specify a  range
     of  characters  in  a  character  class.  For example, [d-m]
     matches any letter between d and m, inclusive.  If  a  minus
     character  is required in a class, it must be escaped with a
     backslash or appear in a position where it cannot be  inter-
     preted as indicating a range, typically as the first or last
     character in the class.

     It is not possible to have the literal character "]" as  the
     end  character  of  a  range.  A  pattern such as [W-]46] is
     interpreted as a class of two characters ("W" and "-")  fol-
     lowed by a literal string "46]", so it would match "W46]" or
     "-46]". However, if the "]" is escaped with a  backslash  it
     is  interpreted  as  the end of range, so [W-\]46] is inter-
     preted as a single class containing a range followed by  two
     separate characters. The octal or hexadecimal representation
     of "]" can also be used to end a range.

     Ranges operate in ASCII collating sequence. They can also be
     used  for  characters  specified  numerically,  for  example
     [\000-\037]. If a range that includes letters is  used  when
     caseless  matching  is set, it matches the letters in either
     case. For example, [W-c] is equivalent  to  [][\^_`wxyzabc],
     matched  caselessly,  and  if  character tables for the "fr"
     locale are in use, [\xc8-\xcb] matches accented E characters
     in both cases.

     The character types \d, \D, \s, \S,  \w,  and  \W  may  also
     appear  in  a  character  class, and add the characters that
     they match to the class. For example, [\dABCDEF] matches any
     hexadecimal  digit.  A  circumflex  can conveniently be used
     with the upper case character types to specify a  more  res-
     tricted set of characters than the matching lower case type.
     For example, the class [^\W_] matches any letter  or  digit,
     but not underscore.

     All non-alphameric characters other than \,  -,  ^  (at  the
     start)  and  the  terminating ] are non-special in character
     classes, but it does no harm if they are escaped.



VERTICAL BAR
     Vertical bar characters are  used  to  separate  alternative
     patterns. For example, the pattern

       gilbert|sullivan

     matches either "gilbert" or "sullivan". Any number of alter-
     natives  may  appear,  and an empty alternative is permitted
     (matching the empty string).   The  matching  process  tries
     each  alternative in turn, from left to right, and the first
     one that succeeds is used. If the alternatives are within  a
     subpattern  (defined  below),  "succeeds" means matching the
     rest of the main pattern as well as the alternative  in  the
     subpattern.



INTERNAL OPTION SETTING
     The settings of PCRE_CASELESS, PCRE_MULTILINE,  PCRE_DOTALL,
     and  PCRE_EXTENDED can be changed from within the pattern by
     a sequence of Perl option letters enclosed between "(?"  and
     ")". The option letters are

       i  for PCRE_CASELESS
       m  for PCRE_MULTILINE
       s  for PCRE_DOTALL
       x  for PCRE_EXTENDED

     For example, (?im) sets caseless, multiline matching. It  is
     also possible to unset these options by preceding the letter
     with a hyphen, and a combined setting and unsetting such  as
     (?im-sx),  which sets PCRE_CASELESS and PCRE_MULTILINE while
     unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  permitted.
     If  a  letter  appears both before and after the hyphen, the
     option is unset.

     The scope of these option changes depends on  where  in  the
     pattern  the  setting  occurs. For settings that are outside
     any subpattern (defined below), the effect is the same as if
     the  options were set or unset at the start of matching. The
     following patterns all behave in exactly the same way:

       (?i)abc
       a(?i)bc
       ab(?i)c
       abc(?i)

     which in turn is the same as compiling the pattern abc  with
     PCRE_CASELESS  set.   In  other words, such "top level" set-
     tings apply to the whole pattern  (unless  there  are  other
     changes  inside subpatterns). If there is more than one set-
     ting of the same option at top level, the rightmost  setting
     is used.

     If an option change occurs inside a subpattern,  the  effect
     is  different.  This is a change of behaviour in Perl 5.005.
     An option change inside a subpattern affects only that  part
     of the subpattern that follows it, so

       (a(?i)b)c

     matches  abc  and  aBc  and  no  other   strings   (assuming
     PCRE_CASELESS  is  not used).  By this means, options can be
     made to have different settings in different  parts  of  the
     pattern.  Any  changes  made  in one alternative do carry on
     into subsequent branches within  the  same  subpattern.  For
     example,

       (a(?i)b|c)

     matches "ab", "aB", "c", and "C", even though when  matching
     "C" the first branch is abandoned before the option setting.
     This is because the effects of  option  settings  happen  at
     compile  time. There would be some very weird behaviour oth-
     erwise.

     The PCRE-specific options PCRE_UNGREEDY and  PCRE_EXTRA  can
     be changed in the same way as the Perl-compatible options by
     using the characters U and X  respectively.  The  (?X)  flag
     setting  is  special in that it must always occur earlier in
     the pattern than any of the additional features it turns on,
     even when it is at top level. It is best put at the start.



SUBPATTERNS
     Subpatterns are delimited by parentheses  (round  brackets),
     which can be nested.  Marking part of a pattern as a subpat-
     tern does two things:

     1. It localizes a set of alternatives. For example, the pat-
     tern

       cat(aract|erpillar|)

     matches one of the words "cat",  "cataract",  or  "caterpil-
     lar".  Without  the  parentheses, it would match "cataract",
     "erpillar" or the empty string.

     2. It sets up the subpattern as a capturing  subpattern  (as
     defined  above).   When the whole pattern matches, that por-
     tion of the subject string that matched  the  subpattern  is
     passed  back  to  the  caller  via  the  ovector argument of
     pcre_exec(). Opening parentheses are counted  from  left  to
     right (starting from 1) to obtain the numbers of the captur-
     ing subpatterns.

     For example, if the string "the red king" is matched against
     the pattern

       the ((red|white) (king|queen))

     the captured substrings are "red king", "red",  and  "king",
     and are numbered 1, 2, and 3.

     The fact that plain parentheses fulfil two functions is  not
     always  helpful.  There are often times when a grouping sub-
     pattern is required without a capturing requirement.  If  an
     opening parenthesis is followed by "?:", the subpattern does
     not do any capturing, and is not counted when computing  the
     number of any subsequent capturing subpatterns. For example,
     if the string "the white queen" is matched against the  pat-
     tern

       the ((?:red|white) (king|queen))

     the captured substrings are "white queen" and  "queen",  and
     are  numbered  1  and 2. The maximum number of captured sub-
     strings is 99, and the maximum number  of  all  subpatterns,
     both capturing and non-capturing, is 200.

     As a  convenient  shorthand,  if  any  option  settings  are
     required  at  the  start  of a non-capturing subpattern, the
     option letters may appear between the "?" and the ":".  Thus
     the two patterns

       (?i:saturday|sunday)
       (?:(?i)saturday|sunday)

     match exactly the same set of strings.  Because  alternative
     branches  are  tried from left to right, and options are not
     reset until the end of the subpattern is reached, an  option
     setting  in  one  branch does affect subsequent branches, so
     the above patterns match "SUNDAY" as well as "Saturday".



REPETITION
     Repetition is specified by quantifiers, which can follow any
     of the following items:

       a single character, possibly escaped
       the . metacharacter
       a character class
       a back reference (see next section)
       a parenthesized subpattern (unless it is  an  assertion  -
     see below)

     The general repetition quantifier specifies  a  minimum  and
     maximum  number  of  permitted  matches,  by  giving the two
     numbers in curly brackets (braces), separated  by  a  comma.
     The  numbers  must be less than 65536, and the first must be
     less than or equal to the second. For example:

       z{2,4}

     matches "zz", "zzz", or "zzzz". A closing brace on  its  own
     is not a special character. If the second number is omitted,
     but the comma is present, there is no upper  limit;  if  the
     second number and the comma are both omitted, the quantifier
     specifies an exact number of required matches. Thus

       [aeiou]{3,}

     matches at least 3 successive vowels,  but  may  match  many
     more, while

       \d{8}

     matches exactly 8 digits.  An  opening  curly  bracket  that
     appears  in a position where a quantifier is not allowed, or
     one that does not match the syntax of a quantifier, is taken
     as  a literal character. For example, {,6} is not a quantif-
     ier, but a literal string of four characters.

     The quantifier {0} is permitted, causing the  expression  to
     behave  as  if the previous item and the quantifier were not
     present.

     For convenience (and  historical  compatibility)  the  three
     most common quantifiers have single-character abbreviations:

       *    is equivalent to {0,}
       +    is equivalent to {1,}
       ?    is equivalent to {0,1}

     It is possible to construct infinite loops  by  following  a
     subpattern  that  can  match no characters with a quantifier
     that has no upper limit, for example:

       (a?)*

     Earlier versions of Perl and PCRE used to give an  error  at
     compile  time  for such patterns. However, because there are
     cases where this  can  be  useful,  such  patterns  are  now
     accepted,  but  if  any repetition of the subpattern does in
     fact match no characters, the loop is forcibly broken.

     By default, the quantifiers  are  "greedy",  that  is,  they
     match  as much as possible (up to the maximum number of per-
     mitted times), without causing the rest of  the  pattern  to
     fail. The classic example of where this gives problems is in
     trying to match comments in C programs. These appear between
     the  sequences /* and */ and within the sequence, individual
     * and / characters may appear. An attempt to  match  C  com-
     ments by applying the pattern

       /\*.*\*/

     to the string

       /* first command */  not comment  /* second comment */

     fails, because it matches  the  entire  string  due  to  the
     greediness of the .*  item.

     However, if a quantifier is followed  by  a  question  mark,
     then it ceases to be greedy, and instead matches the minimum
     number of times possible, so the pattern

       /\*.*?\*/

     does the right thing with the C comments. The meaning of the
     various  quantifiers is not otherwise changed, just the pre-
     ferred number of matches.  Do not confuse this use of  ques-
     tion  mark  with  its  use as a quantifier in its own right.
     Because it has two uses, it can sometimes appear doubled, as
     in

       \d??\d

     which matches one digit by preference, but can match two  if
     that is the only way the rest of the pattern matches.

     If the PCRE_UNGREEDY option is set (an option which  is  not
     available  in  Perl)  then the quantifiers are not greedy by
     default, but individual ones can be made greedy by following
     them  with  a  question mark. In other words, it inverts the
     default behaviour.

     When a parenthesized subpattern is quantified with a minimum
     repeat  count  that is greater than 1 or with a limited max-
     imum, more store is required for the  compiled  pattern,  in
     proportion to the size of the minimum or maximum.

     If a pattern starts with .* or  .{0,}  and  the  PCRE_DOTALL
     option (equivalent to Perl's /s) is set, thus allowing the .
     to match newlines, then the pattern is implicitly  anchored,
     because whatever follows will be tried against every charac-
     ter position in the subject string, so there is no point  in
     retrying  the overall match at any position after the first.
     PCRE treats such a pattern as though it were preceded by \A.
     In  cases where it is known that the subject string contains
     no newlines, it is worth setting PCRE_DOTALL when  the  pat-
     tern begins with .* in order to obtain this optimization, or
     alternatively using ^ to indicate anchoring explicitly.

     When a capturing subpattern is repeated, the value  captured
     is  the  substring  that  matched  the  final iteration. For
     example, after

       (tweedle[dume]{3}\s*)+

     has matched "tweedledum tweedledee" the value  of  the  cap-
     tured  substring  is  "tweedledee".  However,  if  there are
     nested capturing  subpatterns,  the  corresponding  captured
     values  may  have been set in previous iterations. For exam-
     ple, after

       /(a|(b))+/

     matches "aba" the value of the second captured substring  is
     "b".



BACK REFERENCES
     Outside a character class, a backslash followed by  a  digit
     greater  than  0  (and  possibly  further  digits) is a back
     reference to a capturing subpattern  earlier  (i.e.  to  its
     left)  in  the  pattern,  provided there have been that many
     previous capturing left parentheses.

     However, if the decimal number following  the  backslash  is
     less  than  10,  it is always taken as a back reference, and
     causes an error only if there are not  that  many  capturing
     left  parentheses in the entire pattern. In other words, the
     parentheses that are referenced need not be to the  left  of
     the  reference  for  numbers  less  than 10. See the section
     entitled "Backslash" above for further details of  the  han-
     dling of digits following a backslash.

     A back reference matches whatever actually matched the  cap-
     turing subpattern in the current subject string, rather than
     anything matching the subpattern itself. So the pattern

       (sens|respons)e and \1ibility

     matches "sense and sensibility" and "response and  responsi-
     bility",  but  not  "sense  and  responsibility". If caseful
     matching is in force at the time of the back reference, then
     the case of letters is relevant. For example,

       ((?i)rah)\s+\1

     matches "rah rah" and "RAH RAH", but  not  "RAH  rah",  even
     though  the  original  capturing subpattern is matched case-
     lessly.

     There may be more than one back reference to the  same  sub-
     pattern.  If  a  subpattern  has not actually been used in a
     particular match, then any  back  references  to  it  always
     fail. For example, the pattern

       (a|(bc))\2

     always fails if it starts to match  "a"  rather  than  "bc".
     Because  there  may  be up to 99 back references, all digits
     following the backslash are taken as  part  of  a  potential
     back reference number. If the pattern continues with a digit
     character, then some delimiter must be used to terminate the
     back reference. If the PCRE_EXTENDED option is set, this can
     be whitespace.  Otherwise an empty comment can be used.

     A back reference that occurs inside the parentheses to which
     it  refers  fails when the subpattern is first used, so, for
     example, (a\1) never matches.  However, such references  can
     be useful inside repeated subpatterns. For example, the pat-
     tern

       (a|b\1)+

     matches any number of "a"s and also "aba", "ababaa" etc.  At
     each iteration of the subpattern, the back reference matches
     the character string corresponding to  the  previous  itera-
     tion.  In  order  for this to work, the pattern must be such
     that the first iteration does not need  to  match  the  back
     reference.  This  can  be  done using alternation, as in the
     example above, or by a quantifier with a minimum of zero.



ASSERTIONS
     An assertion is  a  test  on  the  characters  following  or
     preceding  the current matching point that does not actually
     consume any characters. The simple assertions coded  as  \b,
     \B,  \A,  \Z,  \z, ^ and $ are described above. More compli-
     cated assertions are coded as  subpatterns.  There  are  two
     kinds:  those that look ahead of the current position in the
     subject string, and those that look behind it.

     An assertion subpattern is matched in the normal way, except
     that  it  does not cause the current matching position to be
     changed. Lookahead assertions start with  (?=  for  positive
     assertions and (?! for negative assertions. For example,

       \w+(?=;)

     matches a word followed by a semicolon, but does not include
     the semicolon in the match, and

       foo(?!bar)

     matches any occurrence of "foo"  that  is  not  followed  by
     "bar". Note that the apparently similar pattern

       (?!foo)bar

     does not find an occurrence of "bar"  that  is  preceded  by
     something other than "foo"; it finds any occurrence of "bar"
     whatsoever, because the assertion  (?!foo)  is  always  true
     when  the  next  three  characters  are  "bar". A lookbehind
     assertion is needed to achieve this effect.

     Lookbehind assertions start with (?<=  for  positive  asser-
     tions and (?<! for negative assertions. For example,

       (?<!foo)bar

     does find an occurrence of "bar" that  is  not  preceded  by
     "foo". The contents of a lookbehind assertion are restricted
     such that all the strings  it  matches  must  have  a  fixed
     length.  However, if there are several alternatives, they do
     not all have to have the same fixed length. Thus

       (?<=bullock|donkey)

     is permitted, but

       (?<!dogs?|cats?)

     causes an error at compile time. Branches  that  match  dif-
     ferent length strings are permitted only at the top level of
     a lookbehind assertion. This is an extension  compared  with
     Perl  5.005,  which  requires all branches to match the same
     length of string. An assertion such as

       (?<=ab(c|de))

     is not permitted, because its single  top-level  branch  can
     match two different lengths, but it is acceptable if rewrit-
     ten to use two top-level branches:

       (?<=abc|abde)

     The implementation of lookbehind  assertions  is,  for  each
     alternative,  to  temporarily move the current position back
     by the fixed width and then  try  to  match.  If  there  are
     insufficient  characters  before  the  current position, the
     match is deemed to fail.  Lookbehinds  in  conjunction  with
     once-only  subpatterns can be particularly useful for match-
     ing at the ends of strings; an example is given at  the  end
     of the section on once-only subpatterns.

     Several assertions (of any sort) may  occur  in  succession.
     For example,

       (?<=\d{3})(?<!999)foo

     matches "foo" preceded by three digits that are  not  "999".
     Notice  that each of the assertions is applied independently
     at the same point in the subject string. First  there  is  a
     check  that  the  previous  three characters are all digits,
     then there is a check that the same three characters are not
     "999".   This  pattern  does not match "foo" preceded by six
     characters, the first of which are digits and the last three
     of  which  are  not  "999".  For  example,  it doesn't match
     "123abcfoo". A pattern to do that is

       (?<=\d{3}...)(?<!999)foo

     This time the first assertion looks  at  the  preceding  six
     characters,  checking  that  the first three are digits, and
     then the second assertion checks that  the  preceding  three
     characters are not "999".

     Assertions can be nested in any combination. For example,

       (?<=(?<!foo)bar)baz

     matches an occurrence of "baz" that  is  preceded  by  "bar"
     which in turn is not preceded by "foo", while

       (?<=\d{3}(?!999)...)foo

     is another pattern which matches  "foo"  preceded  by  three
     digits and any three characters that are not "999".

     Assertion subpatterns are not capturing subpatterns, and may
     not  be  repeated,  because  it makes no sense to assert the
     same thing several times. If any kind of assertion  contains
     capturing  subpatterns  within it, these are counted for the
     purposes of numbering the capturing subpatterns in the whole
     pattern.   However,  substring capturing is carried out only
     for positive assertions, because it does not make sense  for
     negative assertions.

     Assertions count towards the maximum  of  200  parenthesized
     subpatterns.



ONCE-ONLY SUBPATTERNS
     With both maximizing and minimizing repetition,  failure  of
     what  follows  normally  causes  the repeated item to be re-
     evaluated to see if a different number of repeats allows the
     rest  of  the  pattern  to  match. Sometimes it is useful to
     prevent this, either to change the nature of the  match,  or
     to  cause  it fail earlier than it otherwise might, when the
     author of the pattern knows there is no  point  in  carrying
     on.

     Consider, for example, the pattern \d+foo  when  applied  to
     the subject line

       123456bar

     After matching all 6 digits and then failing to match "foo",
     the normal action of the matcher is to try again with only 5
     digits matching the \d+ item, and then with 4,  and  so  on,
     before ultimately failing. Once-only subpatterns provide the
     means for specifying that once a portion of the pattern  has
     matched,  it  is  not to be re-evaluated in this way, so the
     matcher would give up immediately on failing to match  "foo"
     the  first  time.  The  notation  is another kind of special
     parenthesis, starting with (?> as in this example:

       (?>\d+)bar

     This kind of parenthesis "locks up" the  part of the pattern
     it  contains once it has matched, and a failure further into
     the pattern is prevented from backtracking  into  it.  Back-
     tracking  past  it to previous items, however, works as nor-
     mal.

     An alternative description is that a subpattern of this type
     matches  the  string  of  characters that an identical stan-
     dalone pattern would match, if anchored at the current point
     in the subject string.

     Once-only subpatterns are not capturing subpatterns.  Simple
     cases  such as the above example can be thought of as a max-
     imizing repeat that must  swallow  everything  it  can.  So,
     while both \d+ and \d+? are prepared to adjust the number of
     digits they match in order to make the rest of  the  pattern
     match, (?>\d+) can only match an entire sequence of digits.

     This construction can of course contain arbitrarily  compli-
     cated subpatterns, and it can be nested.

     Once-only subpatterns can be used in conjunction with  look-
     behind  assertions  to specify efficient matching at the end
     of the subject string. Consider a simple pattern such as

       abcd$

     when applied to a long  string  which  does  not  match  it.
     Because matching proceeds from left to right, PCRE will look
     for each "a" in the subject and then  see  if  what  follows
     matches the rest of the pattern. If the pattern is specified
     as

       ^.*abcd$

     then the initial .* matches the entire string at first,  but
     when  this  fails,  it  backtracks to match all but the last
     character, then all but the last two characters, and so  on.
     Once again the search for "a" covers the entire string, from
     right to left, so we are no better off. However, if the pat-
     tern is written as

       ^(?>.*)(?<=abcd)

     then there can be no backtracking for the .*  item;  it  can
     match  only  the  entire  string.  The subsequent lookbehind
     assertion does a single test on the last four characters. If
     it  fails,  the  match  fails immediately. For long strings,
     this approach makes a significant difference to the process-
     ing time.



CONDITIONAL SUBPATTERNS
     It is possible to cause the matching process to obey a  sub-
     pattern  conditionally  or to choose between two alternative
     subpatterns, depending on the result  of  an  assertion,  or
     whether  a previous capturing subpattern matched or not. The
     two possible forms of conditional subpattern are

       (?(condition)yes-pattern)
       (?(condition)yes-pattern|no-pattern)

     If the condition is satisfied, the yes-pattern is used; oth-
     erwise  the  no-pattern  (if  present) is used. If there are
     more than two alternatives in the subpattern, a compile-time
     error occurs.

     There are two kinds of condition. If the  text  between  the
     parentheses  consists of a sequence of digits, then the con-
     dition is satisfied if  the  capturing  subpattern  of  that
     number  has  previously matched. Consider the following pat-
     tern, which contains non-significant white space to make  it
     more  readable  (assume  the  PCRE_EXTENDED  option)  and to
     divide it into three parts for ease of discussion:

       ( \( )?    [^()]+    (?(1) \) )

     The first part matches an optional opening parenthesis,  and
     if  that character is present, sets it as the first captured
     substring. The second part matches one  or  more  characters
     that  are  not  parentheses. The third part is a conditional
     subpattern that tests whether the first set  of  parentheses
     matched  or  not.  If  they did, that is, if subject started
     with an opening parenthesis, the condition is true,  and  so
     the  yes-pattern  is  executed  and a closing parenthesis is
     required. Otherwise, since no-pattern is  not  present,  the
     subpattern  matches  nothing.  In  other words, this pattern
     matches a sequence of non-parentheses,  optionally  enclosed
     in parentheses.

     If the condition is not a sequence of digits, it must be  an
     assertion.  This  may be a positive or negative lookahead or
     lookbehind assertion. Consider this pattern, again  contain-
     ing  non-significant  white space, and with the two alterna-
     tives on the second line:

       (?(?=[^a-z]*[a-z])
       \d{2}[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

     The condition is a positive lookahead assertion that matches
     an optional sequence of non-letters followed by a letter. In
     other words, it tests for  the  presence  of  at  least  one
     letter  in the subject. If a letter is found, the subject is
     matched against  the  first  alternative;  otherwise  it  is
     matched  against the second. This pattern matches strings in
     one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
     letters and dd are digits.



COMMENTS
     The sequence (?# marks the start of a comment which  contin-
     ues  up  to the next closing parenthesis. Nested parentheses
     are not permitted. The characters that  make  up  a  comment
     play no part in the pattern matching at all.

     If the PCRE_EXTENDED option is set, an unescaped # character
     outside  a character class introduces a comment that contin-
     ues up to the next newline character in the pattern.



PERFORMANCE
     Certain items that may appear in patterns are more efficient
     than  others.  It is more efficient to use a character class
     like [aeiou] than a set of alternatives such as (a|e|i|o|u).
     In  general,  the  simplest  construction  that provides the
     required behaviour is usually the  most  efficient.  Jeffrey
     Friedl's  book contains a lot of discussion about optimizing
     regular expressions for efficient performance.

     When a pattern begins with .* and the PCRE_DOTALL option  is
     set,  the  pattern  is implicitly anchored by PCRE, since it
     can match only at the start of a subject string. However, if
     PCRE_DOTALL  is not set, PCRE cannot make this optimization,
     because the . metacharacter does not then match  a  newline,
     and if the subject string contains newlines, the pattern may
     match from the character immediately following one  of  them
     instead of from the very start. For example, the pattern

       (.*) second

     matches the subject "first\nand second" (where \n stands for
     a newline character) with the first captured substring being
     "and". In order to do this, PCRE  has  to  retry  the  match
     starting after every newline in the subject.

     If you are using such a pattern with subject strings that do
     not  contain  newlines,  the best performance is obtained by
     setting PCRE_DOTALL, or starting the  pattern  with  ^.*  to
     indicate  explicit anchoring. That saves PCRE from having to
     scan along the subject looking for a newline to restart at.

     Beware of patterns that contain nested  indefinite  repeats.
     These  can  take a long time to run when applied to a string
     that does not match. Consider the pattern fragment

       (a+)*

     This can match "aaaa" in 33 different ways, and this  number
     increases  very  rapidly  as  the string gets longer. (The *
     repeat can match 0, 1, 2, 3, or 4 times,  and  for  each  of
     those  cases other than 0, the + repeats can match different
     numbers of times.) When the remainder of the pattern is such
     that  the entire match is going to fail, PCRE has in princi-
     ple to try every possible variation, and this  can  take  an
     extremely long time.

     An optimization catches some of the more simple  cases  such
     as

       (a+)*b

     where a literal character follows. Before embarking  on  the
     standard matching procedure, PCRE checks that there is a "b"
     later in the subject string, and if there is not,  it  fails
     the  match  immediately. However, when there is no following
     literal this optimization cannot be used. You  can  see  the
     difference by comparing the behaviour of

       (a+)*\d

     with the pattern above. The former gives  a  failure  almost
     instantly  when  applied  to a whole line of "a" characters,
     whereas the latter takes an appreciable  time  with  strings
     longer than about 20 characters.



AUTHOR
     Philip Hazel <ph10@cam.ac.uk>
     University Computing Service,
     New Museums Site,
     Cambridge CB2 3QG, England.
     Phone: +44 1223 334714

     Last updated: 29 July 1999
     Copyright (c) 1997-1999 University of Cambridge.
