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PCRE(3)								       PCRE(3)

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);

       void pcre_free_substring(const char *stringptr);

       void pcre_free_substring_list(const char **stringptr);

       const unsigned char *pcre_maketables(void);

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
	    int what, void *where);

       int pcre_info(const pcre *code, int *optptr, int *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 regular expres‐
       sion 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, with some  additional	 features  from	 later
       versions. This includes some experimental, incomplete support for UTF-8
       encoded strings. Details of exactly what is and what is	not  supported
       are given below.

       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 regular
       expression 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 applica‐
       tion which calls it. The header file defines the macros PCRE_MAJOR  and
       PCRE_MINOR  to  contain	the  major  and	 minor release numbers for the
       library. Applications can use these to include  support	for  different
       releases.

       The  functions  pcre_compile(),	pcre_study(), and pcre_exec() are used
       for compiling and matching regular expressions. A sample	 program  that
       demonstrates  the simplest way of using them is given in the file pcre‐
       demo.c. The last section of this man page describes how to run it.

       The   functions	 pcre_copy_substring(),	  pcre_get_substring(),	   and
       pcre_get_substring_list() are convenience functions for extracting cap‐
       tured substrings from a matched subject	string;	 pcre_free_substring()
       and  pcre_free_substring_list()	are  also provided, to free the memory
       used for extracted strings.

       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_fullinfo()  is used to find out information about a
       compiled pattern; pcre_info() is an obsolete version which returns only
       some  of	 the available information, but is retained for backwards com‐
       patibility.  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  respec‐
       tively. 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 applications, 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 match‐
       ing, 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 the
       returned block; this is a typedef for a structure  whose	 contents  are
       not  externally defined. It is up to the caller to free the memory when
       it is no longer required.

       Although the compiled code of a PCRE regex is relocatable, that is,  it
       does not depend on memory location, the complete pcre data block is not
       fully relocatable, because it contains a copy of the tableptr argument,
       which is an address (see below).

       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
       replicated.

       The options argument contains independent bits that affect the compila‐
       tion.  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 expressions below). For these options, the contents  of  the
       options	argument specifies their initial settings at the start of com‐
       pilation 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 mes‐
       sage. 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.

       This code fragment shows a typical straightforward  call	 to  pcre_com‐
       pile():

	 pcre *re;
	 const char *error;
	 int erroffset;
	 re = pcre_compile(
	   "^A.*Z",	     /* the pattern */
	   0,		     /* default options */
	   &error,	     /* for error message */
	   &erroffset,	     /* for error offset */
	   NULL);	     /* use default character tables */

       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	newlines).  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 char‐
       acters, including newlines. Without it,	newlines  are  excluded.  This
       option is equivalent to Perl's /s option. A negative class such as [^a]
       always matches a newline character, independent of the setting of  this
       option.

	 PCRE_EXTENDED

       If  this	 bit  is  set,	whitespace  data characters in the pattern are
       totally ignored except when escaped or inside a	character  class,  and
       characters  between  an	unescaped  # outside 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 char‐
       acter sequences in a pattern, for example within the sequence (?( which
       introduces a conditional subpattern.

	 PCRE_EXTRA

       This  option  was invented in order to turn on additional functionality
       of PCRE that is incompatible with Perl, but it  is  currently  of  very
       little  use. When set, 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. It can also be set by a (?X) option setting within a pattern.

	 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_DOL‐
       LAR_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  new‐
       line  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"
       characters  in  a subject string, or no occurrences of ^ or $ in a pat‐
       tern, 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.

	 PCRE_UTF8

       This  option  causes PCRE to regard both the pattern and the subject as
       strings of UTF-8 characters instead of just byte strings.  However,  it
       is  available  only if PCRE has been built to include UTF-8 support. If
       not, the use of this option provokes an error.  Support	for  UTF-8  is
       new,  experimental, and incomplete.  Details of exactly what it entails
       are given below.

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 match‐
       ing. 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 typedef for a structure with hidden contents) containing addi‐
       tional	information   about   the  pattern;  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  mes‐
       sage.

       This is a typical call to pcre_study():

	 pcre_extra *pe;
	 pe = pcre_study(
	   re,		   /* result of pcre_compile() */
	   0,		   /* no options exist */
	   &error);	   /* set to NULL or points to a 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	possi‐
       ble starting characters is created.

LOCALE SUPPORT
       PCRE  handles  caseless matching, and determines whether characters 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 compiled. 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 argu‐
       ments, 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 char‐
       acters with codes greater than 128 are treated as letters), the follow‐
       ing 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 pat‐
       tern, 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 responsibility to
       ensure that the memory containing the tables remains available  for  as
       long as it is needed.

INFORMATION ABOUT A PATTERN
       The  pcre_fullinfo() function returns information about a compiled pat‐
       tern. It replaces the obsolete pcre_info() function, which is neverthe‐
       less retained for backwards compability (and is documented below).

       The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
       pattern. The second argument is the result of pcre_study(), or NULL  if
       the  pattern  was not studied. The third argument specifies which piece
       of information is required, while the fourth argument is a pointer to a
       variable	 to  receive  the  data. The yield of the function is zero for
       success, or one of the following negative numbers:

	 PCRE_ERROR_NULL       the argument code was NULL
			       the argument where was NULL
	 PCRE_ERROR_BADMAGIC   the "magic number" was not found
	 PCRE_ERROR_BADOPTION  the value of what was invalid

       Here is a typical call of pcre_fullinfo(), to obtain the length of  the
       compiled pattern:

	 int rc;
	 unsigned long int length;
	 rc = pcre_fullinfo(
	   re,		     /* result of pcre_compile() */
	   pe,		     /* result of pcre_study(), or NULL */
	   PCRE_INFO_SIZE,   /* what is required */
	   &length);	     /* where to put the data */

       The  possible  values for the third argument are defined in pcre.h, and
       are as follows:

	 PCRE_INFO_OPTIONS

       Return a copy of the options with which the pattern was	compiled.  The
       fourth  argument	 should	 point to an unsigned long int variable. 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 forcibly set if the form of the  pattern  implies
       that it can match only at the start of a subject string.

	 PCRE_INFO_SIZE

       Return  the  size  of the compiled pattern, that is, the value that was
       passed as the argument to pcre_malloc() when PCRE was getting memory in
       which to place the compiled data. The fourth argument should point to a
       size_t variable.

	 PCRE_INFO_CAPTURECOUNT

       Return the number of capturing subpatterns in the pattern.  The	fourth
       argument should point to an int variable.

	 PCRE_INFO_BACKREFMAX

       Return  the  number  of	the highest back reference in the pattern. The
       fourth argument should point to an int variable. Zero  is  returned  if
       there are no back references.

	 PCRE_INFO_FIRSTCHAR

       Return information about the first character of any matched string, for
       a non-anchored pattern. If there is a fixed first character, e.g.  from
       a  pattern  such	 as  (cat|cow|coyote),	it  is returned in the integer
       pointed to by where. 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),

       -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.  For anchored patterns, -2 is returned.

	 PCRE_INFO_FIRSTTABLE

       If the pattern was studied, and this resulted in the construction of  a
       256-bit	table indicating a fixed set of characters for the first char‐
       acter in any matching string, a pointer to the table is returned.  Oth‐
       erwise  NULL  is	 returned.  The	 fourth	 argument  should  point to an
       unsigned char * variable.

	 PCRE_INFO_LASTLITERAL

       For a non-anchored pattern, return the value of the  rightmost  literal
       character  which	 must  exist  in any matched string, other than at its
       start. The fourth argument should point to an int variable. If there is
       no  such	 character, or if the pattern is anchored, -1 is returned. For
       example, for the pattern /a\d+z\d+/ the returned value is 'z'.

       The pcre_info() function is now obsolete because its interface  is  too
       restrictive  to return all the available data about a compiled pattern.
       New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of
       pcre_info()  is the number of capturing subpatterns, or one of the fol‐
       lowing 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 (see
       PCRE_INFO_OPTIONS above).

       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 (see PCRE_INFO_FIRSTCHAR above).

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 pat‐
       tern has been studied, the result of the study should be passed in  the
       extra argument. Otherwise this must be NULL.

       Here is an example of a simple call to pcre_exec():

	 int rc;
	 int ovector[30];
	 rc = pcre_exec(
	   re,		   /* result of pcre_compile() */
	   NULL,	   /* we didn't study the pattern */
	   "some string",  /* the subject string */
	   11,		   /* the length of the subject string */
	   0,		   /* start at offset 0 in the subject */
	   0,		   /* default options */
	   ovector,	   /* vector for substring information */
	   30);		   /* number of elements in the vector */

       The  PCRE_ANCHORED  option can be passed in the options argument, 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 dollar metachar‐
       acter  should  not  match  it  nor (except in multiline 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 pattern, 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 occur‐
       rences of "a" or "b".

       Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe‐
       cial case of a pattern match of the empty  string  within  its  split()
       function,  and  when  using  the /g modifier. It is possible to emulate
       Perl's behaviour after matching a null string by first trying the match
       again at the same offset with PCRE_NOTEMPTY set, and then if that fails
       by advancing the starting offset (see below)  and  trying  an  ordinary
       match again.

       The  subject  string  is	 passed	 as  a pointer in subject, a length in
       length, and a  starting	offset	in  startoffset.  Unlike  the  pattern
       string, the subject may contain binary zero characters. When the start‐
       ing 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  suc‐
       cess.   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 occur‐
       rence  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 suc‐
       ceed 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 subject, 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 sub‐
       string. PCRE supports several other kinds of  parenthesized  subpattern
       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 sub‐
       string, 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 subpat‐
       tern, 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 substrings,	it  is
       used as far as possible (up to two-thirds of its length), and the func‐
       tion 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 refer‐
       ences  and  the	ovector	 isn't big enough to remember the related sub‐
       strings, 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 capturing sub‐
       patterns 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	following  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 compiled	 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 encountered 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_sub‐
       string_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 functions: 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 cap‐
       tured 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 ovector, 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 stringnumber. A value of
       zero extracts the substring that	 matched  the  entire  pattern,	 while
       higher  values  extract	the  captured  substrings.  For pcre_copy_sub‐
       string(), the string is placed in buffer,  whose	 length	 is  given  by
       buffersize,  while  for	pcre_get_substring()  a new block of memory 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	available sub‐
       strings 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 sub‐
       string by inspecting the appropriate offset in ovector, which is	 nega‐
       tive for unset substrings.

       The  two convenience functions pcre_free_substring() and pcre_free_sub‐
       string_list() can be used to free the memory  returned  by  a  previous
       call  of	 pcre_get_substring()  or  pcre_get_substring_list(),  respec‐
       tively. They do nothing more than  call	the  function  pointed	to  by
       pcre_free,  which  of course could be called directly from a C program.
       However, PCRE is used in some situations where it is linked via a  spe‐
       cial  interface	to  another  programming  language  which  cannot  use
       pcre_free directly; it is for these cases that the functions  are  pro‐
       vided.

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  pat‐
       tern is 65539 (sic) bytes.  All values in repeating quantifiers must be
       less than 65536.	 There maximum	number	of  capturing  subpatterns  is
       65535.	There  is no limit to the number of non-capturing subpatterns,
       but the maximum depth of nesting of all kinds of parenthesized  subpat‐
       tern,  including	 capturing subpatterns, assertions, and other types of
       subpattern, is 200.

       The maximum length of a subject string is the largest  positive	number
       that an integer variable can hold. However, PCRE uses recursion to han‐
       dle subpatterns and indefinite repetition. This means that  the	avail‐
       able  stack  space  may	limit the size of a subject string that can be
       processed by certain 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
       characters. The \v escape that was in the Perl documentation for a long
       time was never in fact recognized. However, the	character  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 lookahead asser‐
       tions are counted, but their entries in the offsets  vector  are	 never
       set.  Perl sets its numerical variables from any such patterns that are
       matched before the assertion fails to match something (thereby succeed‐
       ing),  but  only	 if the negative lookahead assertion contains just one
       branch.

       4. Though binary zero characters are supported in the  subject  string,
       they are not allowed in a pattern string because it is passed as a nor‐
       mal 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-han‐
       dling and are not part of its pattern matching engine.

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

       7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
       constructions.  However,	 there is some experimental support for recur‐
       sive patterns using the non-Perl item (?R).

       8. There are at the time of writing some oddities in Perl 5.005_02 con‐
       cerned  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) are 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 facil‐
       ities:

       (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 spe‐
       cial meaning is faulted.

       (d)  If	PCRE_UNGREEDY is set, the greediness of the repetition quanti‐
       fiers is inverted, that is, by default they are not greedy, but if fol‐
       lowed 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.

       (g)  The (?R) construct allows for recursive pattern matching (Perl 5.6
       can do this using the (?p{code}) construct, which PCRE cannot of course
       support.)

REGULAR EXPRESSION DETAILS
       The  syntax  and semantics of the regular expressions supported 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 copi‐
       ous examples. Jeffrey Friedl's "Mastering  Regular  Expressions",  pub‐
       lished by O'Reilly (ISBN 1-56592-257), covers them in great detail.

       The  description here is intended as reference documentation. The basic
       operation of PCRE is on strings of bytes. However, there is the	begin‐
       nings  of some support for UTF-8 character strings. To use this support
       you must configure PCRE to include it,  and  then  call	pcre_compile()
       with  the  PCRE_UTF8  option.  How this affects the pattern matching is
       described in the final section of this document.

       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 characters 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 alterna‐
       tives  and repetitions in the pattern. 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 recog‐
       nized 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 following 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 particular, if you want to match a backslash, you write "\\".

       If a pattern is compiled with the PCRE_EXTENDED option,	whitespace  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 char‐
       acters  in patterns in a visible manner. There is no restriction on the
       appearance of non-printing characters, 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 let‐
       ter, 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 compli‐
       cated.  Outside a character class, PCRE reads it and any following dig‐
       its  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 following the backslash, and generates a sin‐
       gle 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 introduced 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 character types:

	 \d	any decimal digit
	 \D	any character that is not a decimal digit
	 \s	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  defini‐
       tion  of	 letters  and digits is controlled 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 charac‐
       ter 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 char‐
       acter 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 assertions. An asser‐
       tion  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 argument 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 argu‐
       ment 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 alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start	 of  the  sub‐
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs 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 immedi‐
       ately after and immediately before an internal "\n" character,  respec‐
       tively,	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. Consequently, patterns
       that are anchored in single line mode because all branches  start  with
       "^"  are	 not anchored in multiline 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 it is 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 charac‐
       ter  in	the  subject,  including a non-printing character, but not (by
       default) newline.  If the PCRE_DOTALL option is set,  dots  match  new‐
       lines  as well. The handling of dot is entirely independent of the han‐
       dling of circumflex and dollar, the only relationship being  that  they
       both  involve newline characters. Dot has no special meaning in a char‐
       acter class.

SQUARE BRACKETS
       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe‐
       cial. 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
       circumflex, if present) or escaped with a backslash.

       A character class matches a single character in the subject; the	 char‐
       acter must be in the set of characters defined by the class, unless the
       first character in the class is a circumflex, 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 convenient 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 caseful 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 charac‐
       ters 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 interpreted 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 charac‐
       ter  of a range. A pattern such as [W-]46] is interpreted as a class of
       two characters ("W" and "-") followed 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  restricted	 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.

POSIX CHARACTER CLASSES
       Perl  5.6 (not yet released at the time of writing) is going to support
       the POSIX notation for character classes, which uses names enclosed  by
       [:  and	:]  within  the	 enclosing square brackets. PCRE supports this
       notation. For example,

	 [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are

	 alnum	  letters and digits
	 alpha	  letters
	 ascii	  character codes 0 - 127
	 cntrl	  control characters
	 digit	  decimal digits (same as \d)
	 graph	  printing characters, excluding space
	 lower	  lower case letters
	 print	  printing characters, including space
	 punct	  printing characters, excluding letters and digits
	 space	  white space (same as \s)
	 upper	  upper case letters
	 word	  "word" characters (same as \w)
	 xdigit	  hexadecimal digits

       The  names  "ascii" and "word" are Perl extensions. Another Perl exten‐
       sion is negation, which is indicated by a ^ character after the	colon.
       For example,

	 [12[:^digit:]]

       matches	"1", "2", or any non-digit. PCRE (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

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 alternatives  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 alterna‐
       tives are within a subpattern (defined below), "succeeds" means	match‐
       ing the rest of the main pattern as well as the alternative in the sub‐
       pattern.

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 possi‐
       ble to unset these options by preceding the letter with a hyphen, and a
       combined	 setting and unsetting such as (?im-sx), which sets PCRE_CASE‐
       LESS 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_CASE‐
       LESS set.  In other words, such "top level" settings apply to the whole
       pattern (unless there are other changes inside subpatterns).  If	 there
       is more than one setting of the same option at top level, the rightmost
       setting is used.

       If an option change occurs inside a subpattern, the effect  is  differ‐
       ent.  This  is  a  change  of behaviour in Perl 5.005. An option change
       inside a subpattern affects only that part of the subpattern that  fol‐
       lows 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 otherwise.

       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 subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

	 cat(aract|erpillar|)

       matches	one  of the words "cat", "cataract", or "caterpillar". 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 portion  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 capturing
       subpatterns.

       For example, if the string "the red king" is matched against  the  pat‐
       tern

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

       the captured substrings are "red king", "red", and "king", and are num‐
       bered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil  two  functions  is  not	always
       helpful.	  There are often times when a grouping subpattern 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 pattern

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

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of captured substrings is 99, and the maxi‐
       mum  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 num‐
       ber 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	 exam‐
       ple, {,6} is not a quantifier, 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  bro‐
       ken.

       By  default,  the quantifiers are "greedy", that is, they match as much
       as possible (up to the maximum  number  of  permitted  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, indi‐
       vidual * and / characters may appear. An attempt to match C comments by
       applying the pattern

	 /\*.*\*/

       to the string

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

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

       However, if a quantifier is followed by a question mark, 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 preferred number of
       matches.	 Do not confuse this use of question 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), 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 maximum, 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 (equiv‐
       alent to Perl's /s) is set, thus allowing the . to match newlines,  the
       pattern	is implicitly anchored, because whatever follows will be tried
       against every character 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 pattern begins with .* in  order  to
       obtain  this optimization, or alternatively using ^ to indicate anchor‐
       ing explicitly.

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

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

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are  nested  capturing  subpatterns,
       the  corresponding captured values may have been set in previous itera‐
       tions. For example, 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 sub‐
       pattern 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 pat‐
       tern. 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 handling of  dig‐
       its following a backslash.

       A  back	reference matches whatever actually matched the capturing sub‐
       pattern 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 responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of the back reference, the case of letters is relevant. For exam‐
       ple,

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

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

       There  may be more than one back reference to the same subpattern. If a
       subpattern has not actually been used in a particular match,  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 con‐
       tinues with a digit character, 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  sub‐
       patterns. For example, the pattern

	 (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter‐
       ation of the subpattern,	 the  back  reference  matches	the  character
       string  corresponding  to  the previous iteration. 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 complicated 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	 semi‐
       colon 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 assertions 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 sev‐
       eral  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  different	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 rewritten 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 cur‐
       rent position, the match is deemed to fail. Lookbehinds in  conjunction
       with  once-only	subpatterns can be particularly useful for matching 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, and then there is  a	check  that  the  same
       three characters are not "999".	This pattern does not match "foo" pre‐
       ceded by six characters, the first of which are	digits	and  the  last
       three  of  which	 are not "999". For example, it doesn't match "123abc‐
       foo". 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 sub‐
       patterns 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 dif‐
       ferent number of repeats allows the rest of the pattern to match. Some‐
       times  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, start‐
       ing with (?> as in this example:

	 (?>\d+)bar

       This  kind  of  parenthesis "locks up" the  part of the pattern it con‐
       tains once it has matched, and a failure further into  the  pattern  is
       prevented  from	backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a subpattern of  this	 type  matches
       the  string  of	characters  that an identical standalone 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 maximizing 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 complicated subpat‐
       terns, and it can be nested.

       Once-only subpatterns can be used in conjunction with lookbehind asser‐
       tions 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. 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$

       the initial .* matches the entire string at first, but when this	 fails
       (because there is no following "a"), 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 pattern is written as

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

       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
       processing time.

       When a pattern contains an unlimited repeat inside  a  subpattern  that
       can itself be repeated an unlimited number of times, the use of a once-
       only subpattern is the only way to avoid some failing matches taking  a
       very long time indeed.  The pattern

	 (\D+|<\d+>)*[!?]

       matches	an  unlimited number of substrings that either consist of non-
       digits, or digits enclosed in <>, followed by either ! or  ?.  When  it
       matches, it runs quickly. However, if it is applied to

	 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it  takes  a  long  time	 before reporting failure. This is because the
       string can be divided between the two repeats  in  a  large  number  of
       ways,  and  all	have to be tried. (The example used [!?] rather than a
       single character at the end, because both PCRE and Perl have  an	 opti‐
       mization	 that allows for fast failure when a single character is used.
       They remember the last single character that is required for  a	match,
       and  fail early if it is not present in the string.)  If the pattern is
       changed to

	 ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.

CONDITIONAL SUBPATTERNS
       It is possible to cause the matching process to obey a subpattern  con‐
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a previous capturing  subpat‐
       tern  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;	otherwise  the
       no-pattern  (if	present)  is used. If there are more than two alterna‐
       tives 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, the condition is satisfied if the
       capturing subpattern of that number has previously matched. The	number
       must  be greater than zero.  Consider the following pattern, which con‐
       tains 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 sec‐
       ond 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-pat‐
       tern 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 containing non-significant white space,
       and with the two alternatives 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 continues 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 continues up to the next new‐
       line character in the pattern.

RECURSIVE PATTERNS
       Consider	 the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of	 recursion,  the  best
       that  can  be  done  is	to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to	handle	an  arbitrary  nesting
       depth. Perl 5.6 has provided an experimental facility that allows regu‐
       lar expressions to recurse (amongst other  things).  It	does  this  by
       interpolating Perl code in the expression at run time, and the code can
       refer to the expression itself. A Perl pattern to solve the parentheses
       problem can be created like this:

	 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears. Obviously,  PCRE
       cannot  support	the  interpolation  of Perl code. Instead, the special
       item (?R) is provided for the specific case  of	recursion.  This  PCRE
       pattern solves the parentheses problem (assume the PCRE_EXTENDED option
       is set so that white space is ignored):

	 \( ( (?>[^()]+) | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (i.e. a  correctly	 parenthesized
       substring). Finally there is a closing parenthesis.

       This  particular example pattern contains nested unlimited repeats, and
       so the use of a once-only subpattern for matching strings of non-paren‐
       theses  is  important  when applying the pattern to strings that do not
       match. For example, when it is applied to

	 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a once-only subpattern is not
       used,  the  match runs for a very long time indeed because there are so
       many different ways the + and * repeats can carve up the	 subject,  and
       all have to be tested before failure can be reported.

       The  values set for any capturing subpatterns are those from the outer‐
       most level of the recursion at which the subpattern value  is  set.  If
       the pattern above is matched against

	 (ab(cd)ef)

       the  value  for	the  capturing	parentheses is "ef", which is the last
       value taken on at the top level. If additional parentheses  are	added,
       giving

	 \( ( ( (?>[^()]+) | (?R) )* ) \)
	    ^			     ^
	    ^			      ^ the string they capture is "ab(cd)ef",
       the contents of the top level parentheses. If there are	more  than  15
       capturing  parentheses in a pattern, PCRE has to obtain extra memory to
       store data during a recursion, which  it	 does  by  using  pcre_malloc,
       freeing	it  via pcre_free afterwards. If no memory can be obtained, it
       saves data for the first 15 capturing parentheses only, as there is  no
       way to give an out-of-memory error from within a recursion.

PERFORMANCE
       Certain	items that may appear in patterns are more efficient than oth‐
       ers. 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 con‐
       struction 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 con‐
       tain 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
       principle 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  sub‐
       ject  string, and if there is not, it fails the match immediately. How‐
       ever, 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.

UTF-8 SUPPORT
       Starting	 at  release  3.3, PCRE has some support for character strings
       encoded in the UTF-8 format. This is incomplete,	 and  is  regarded  as
       experimental.  In  order	 to use it, you must configure PCRE to include
       UTF-8 support in the code, and, in addition, you	 must  call  pcre_com‐
       pile()  with the PCRE_UTF8 option flag. When you do this, both the pat‐
       tern and any subject strings that are matched against it are treated as
       UTF-8  strings  instead of just strings of bytes, but only in the cases
       that are mentioned below.

       If you compile PCRE with UTF-8 support, but do not use it at run	 time,
       the  library will be a bit bigger, but the additional run time overhead
       is limited to testing the PCRE_UTF8 flag in several places,  so	should
       not be very large.

       PCRE assumes that the strings it is given contain valid UTF-8 codes. It
       does not diagnose invalid UTF-8 strings.	 If  you  pass	invalid	 UTF-8
       strings to PCRE, the results are undefined.

       Running with PCRE_UTF8 set causes these changes in the way PCRE works:

       1. In a pattern, the escape sequence \x{...}, where the contents of the
       braces is a string of hexadecimal digits, is  interpreted  as  a	 UTF-8
       character  whose code number is the given hexadecimal number, for exam‐
       ple: \x{1234}. This inserts from one to six literal bytes into the pat‐
       tern,  using  the  UTF-8	 encoding.  If a non-hexadecimal digit appears
       between the braces, the item is not recognized.

       2. The original hexadecimal escape sequence, \xhh, generates a two-byte
       UTF-8 character if its value is greater than 127.

       3. Repeat quantifiers are NOT correctly handled if they follow a multi‐
       byte character. For example, \x{100}* and \xc3+ do  not	work.  If  you
       want  to repeat such characters, you must enclose them in non-capturing
       parentheses, for example (?:\x{100}), at present.

       4. The dot metacharacter matches one UTF-8 character instead of a  sin‐
       gle byte.

       5. Unlike literal UTF-8 characters, the dot metacharacter followed by a
       repeat quantifier does operate correctly on UTF-8 characters instead of
       single bytes.

       4. Although the \x{...} escape is permitted in a character class, char‐
       acters whose values are greater than 255 cannot be included in a class.

       5. A class is matched against a UTF-8 character instead of just a  sin‐
       gle  byte,  but it can match only characters whose values are less than
       256. Characters with greater values always fail to match a class.

       6. Repeated classes work correctly on multiple characters.

       7. Classes containing just a single character whose  value  is  greater
       than  127 (but less than 256), for example, [\x80] or [^\x{93}], do not
       work because these are optimized into single byte matches. In the first
       case, of course, the class brackets are just redundant.

       8.  Lookbehind assertions move backwards in the subject by a fixed num‐
       ber of characters instead of a fixed number of bytes. Simple cases have
       been tested to work correctly, but there may be hidden gotchas herein.

       9.  The	character  types  such as \d and \w do not work correctly with
       UTF-8 characters. They continue to test a single byte.

       10. Anything not explicitly mentioned here continues to work  in	 bytes
       rather than in characters.

       The following UTF-8 features of Perl 5.6 are not implemented:

       1. The escape sequence \C to match a single byte.

       2. The use of Unicode tables and properties and escapes \p, \P, and \X.

SAMPLE PROGRAM
       The  code below is a simple, complete demonstration program, to get you
       started with using PCRE. This code is also supplied in the  file	 pcre‐
       demo.c in the PCRE distribution.

       The program compiles the regular expression that is its first argument,
       and matches it against the subject string in its	 second	 argument.  No
       options	are  set,  and	default character tables are used. If matching
       succeeds, the program outputs the portion of the subject that  matched,
       together with the contents of any captured substrings.

       On a Unix system that has PCRE installed in /usr/local, you can compile
       the demonstration program using a command like this:

	 gcc  -o  pcredemo  pcredemo.c	-I/usr/local/include  -L/usr/local/lib
       -lpcre

       Then you can run simple tests like this:

	 ./pcredemo 'cat|dog' 'the cat sat on the mat'

       Note  that  there  is  a	 much  more comprehensive test program, called
       pcretest, which supports	 many  more  facilities	 for  testing  regular
       expressions.  The pcredemo program is provided as a simple coding exam‐
       ple.

       On some operating systems (e.g. Solaris) you may get an error like this
       when you try to run pcredemo:

	 ld.so.1:  a.out:  fatal:  libpcre.so.0:  open failed: No such file or
       directory

       This is caused by the way shared library support works  on  those  sys‐
       tems. You need to add

	 -R/usr/local/lib

       to the compile command to get round this problem. Here's the code:

	 #include <stdio.h>
	 #include <string.h>
	 #include <pcre.h>

	 #define OVECCOUNT 30	 /* should be a multiple of 3 */

	 int main(int argc, char **argv)
	 {
	 pcre *re;
	 const char *error;
	 int erroffset;
	 int ovector[OVECCOUNT];
	 int rc, i;

	 if (argc != 3)
	   {
	   printf("Two arguments required: a regex and a "
	     "subject string\n");
	   return 1;
	   }

	 /* Compile the regular expression in the first argument */

	 re = pcre_compile(
	   argv[1],	/* the pattern */
	   0,		/* default options */
	   &error,	/* for error message */
	   &erroffset,	/* for error offset */
	   NULL);	/* use default character tables */

	 /* Compilation failed: print the error message and exit */

	 if (re == NULL)
	   {
	   printf("PCRE compilation failed at offset %d: %s\n",
	     erroffset, error);
	   return 1;
	   }

	 /* Compilation succeeded: match the subject in the second
	    argument */

	 rc = pcre_exec(
	   re,		/* the compiled pattern */
	   NULL,	/* we didn't study the pattern */
	   argv[2],	/* the subject string */
	   (int)strlen(argv[2]), /* the length of the subject */
	   0,		/* start at offset 0 in the subject */
	   0,		/* default options */
	   ovector,	/* vector for substring information */
	   OVECCOUNT);	/* number of elements in the vector */

	 /* Matching failed: handle error cases */

	 if (rc < 0)
	   {
	   switch(rc)
	     {
	     case PCRE_ERROR_NOMATCH: printf("No match\n"); break;
	     /*
	     Handle other special cases if you like
	     */
	     default: printf("Matching error %d\n", rc); break;
	     }
	   return 1;
	   }

	 /* Match succeded */

	 printf("Match succeeded\n");

	 /* The output vector wasn't big enough */

	 if (rc == 0)
	   {
	   rc = OVECCOUNT/3;
	   printf("ovector only has room for %d captured "
	     substrings\n", rc - 1);
	   }

	 /* Show substrings stored in the output vector */

	 for (i = 0; i < rc; i++)
	   {
	   char *substring_start = argv[2] + ovector[2*i];
	   int substring_length = ovector[2*i+1] - ovector[2*i];
	   printf("%2d: %.*s\n", i, substring_length,
	     substring_start);
	   }

	 return 0;
	 }

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

       Last updated: 15 August 2001
       Copyright (c) 1997-2001 University of Cambridge.

								       PCRE(3)
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