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PCRE(3)								       PCRE(3)

       PCRE - Perl-compatible regular expressions


       #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_named_substring(const pcre *code,
	    const char *subject, int *ovector,
	    int stringcount, const char *stringname,
	    char *buffer, int buffersize);

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

       int pcre_get_named_substring(const pcre *code,
	    const char *subject, int *ovector,
	    int stringcount, const char *stringname,
	    const char **stringptr);

       int pcre_get_stringnumber(const pcre *code,
	    const char *name);

       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);

       int pcre_config(int what, void *where);

       char *pcre_version(void);

       void *(*pcre_malloc)(size_t);

       void (*pcre_free)(void *);

       void *(*pcre_stack_malloc)(size_t);

       void (*pcre_stack_free)(void *);

       int (*pcre_callout)(pcre_callout_block *);


       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

       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 pcresample documentation describes how to run it.

       There are convenience functions for extracting captured substrings from
       a matched subject string. They are:


       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.

       The  global  variables  pcre_stack_malloc  and pcre_stack_free are also
       indirections to memory management functions.  These  special  functions
       are  used  only	when  PCRE is compiled to use the heap for remembering
       data, instead of recursive function calls. This is a  non-standard  way
       of  building  PCRE,  for	 use in environments that have limited stacks.
       Because of the greater use of memory management, it runs	 more  slowly.
       Separate	 functions  are provided so that special-purpose external code
       can be used for this case. When used, these functions are always called
       in  a  stack-like  manner  (last obtained, first freed), and always for
       memory blocks of the same size.

       The global variable pcre_callout initially contains NULL. It can be set
       by  the	caller	to  a "callout" function, which PCRE will then call at
       specified points during a matching operation. Details are given in  the
       pcrecallout documentation.


       The  PCRE  functions  can be used in multi-threading applications, with
       the  proviso  that  the	memory	management  functions  pointed	to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, 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.


       int pcre_config(int what, void *where);

       The function pcre_config() makes it possible for a PCRE client to  dis‐
       cover which optional features have been compiled into the PCRE library.
       The pcrebuild documentation has more details about these optional  fea‐

       The  first  argument  for pcre_config() is an integer, specifying which
       information is required; the second argument is a pointer to a variable
       into  which  the	 information  is  placed. The following information is


       The output is an integer that is set to one if UTF-8 support is	avail‐
       able; otherwise it is set to zero.


       The  output  is an integer that is set to the value of the code that is
       used for the newline character. It is either linefeed (10) or  carriage
       return  (13),  and  should  normally be the standard character for your
       operating system.


       The output is an integer that contains the number  of  bytes  used  for
       internal linkage in compiled regular expressions. The value is 2, 3, or
       4. Larger values allow larger regular expressions to  be	 compiled,  at
       the  expense  of	 slower matching. The default value of 2 is sufficient
       for all but the most massive patterns, since  it	 allows	 the  compiled
       pattern to be up to 64K in size.


       The  output  is	an integer that contains the threshold above which the
       POSIX interface uses malloc() for output vectors. Further  details  are
       given in the pcreposix documentation.


       The output is an integer that gives the default limit for the number of
       internal matching function calls in a  pcre_exec()  execution.  Further
       details are given with pcre_exec() below.


       The  output  is	an integer that is set to one if internal recursion is
       implemented by recursive function calls that use the stack to  remember
       their state. This is the usual way that PCRE is compiled. The output is
       zero if PCRE was compiled to use blocks of data on the heap instead  of
       recursive   function   calls.   In  this	 case,	pcre_stack_malloc  and
       pcre_stack_free are called to manage memory blocks on  the  heap,  thus
       avoiding the use of the stack.


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

       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 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	 in  the pcrepattern documentation). For these
       options, the contents of the options argument specifies	their  initial
       settings	 at  the start of compilation and execution. The PCRE_ANCHORED
       option can be set at the time of matching as well as at compile time.

       If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if  compilation	of  a  pattern fails, pcre_compile() returns NULL, and
       sets the variable pointed to by errptr to point to a textual error 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

       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‐

	 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:


       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in 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.


       If this bit is set, letters in the pattern match both upper  and	 lower
       case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
       changed within a pattern by a (?i) option setting.


       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, and no way to set it within a pattern.


       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, and it can be changed within
       a pattern by a (?s) option setting.  A  negative	 class	such  as  [^a]
       always  matches a newline character, independent of the setting of this


       If this bit is set, whitespace  data  characters	 in  the  pattern  are
       totally ignored except when escaped or inside a character class. White‐
       space does not include the VT character (code 11). In addition, charac‐
       ters between an unescaped # outside a character class and the next new‐
       line character, inclusive, are also  ignored.  This  is	equivalent  to
       Perl's  /x  option,  and	 it  can be changed within a pattern by a (?x)
       option setting.

       This option makes it possible to include	 comments  inside  complicated
       patterns.   Note,  however,  that this applies only to data characters.
       Whitespace  characters  may  never  appear  within  special   character
       sequences  in  a	 pattern,  for	example	 within the sequence (?( which
       introduces a conditional subpattern.


       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.


       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, and it can be changed
       within a pattern by a (?m) option setting. If there are no "\n" charac‐
       ters  in	 a  subject  string, or no occurrences of ^ or $ in a pattern,
       setting PCRE_MULTILINE has no effect.


       If this option is set, it disables the use of numbered capturing paren‐
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can	 still
       be  used	 for  capturing	 (and  they acquire numbers in the usual way).
       There is no equivalent of this option in Perl.


       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.


       This  option  causes PCRE to regard both the pattern and the subject as
       strings of UTF-8 characters instead of single-byte  character  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.  Details  of
       how  this option changes the behaviour of PCRE are given in the section
       on UTF-8 support in the main pcre page.


       When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically  checked. If an invalid UTF-8 sequence of bytes is found,
       pcre_compile() returns an error. If you already know that your  pattern
       is  valid, and you want to skip this check for performance reasons, you
       can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
       passing an invalid UTF-8 string as a pattern is undefined. It may cause
       your program to crash.  Note that there is a similar  option  for  sup‐
       pressing the checking of subject strings passed to pcre_exec().


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

       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. If studing the pattern produces additional informa‐
       tion  that  will help speed up matching, pcre_study() returns a pointer
       to a pcre_extra block, in which the  study_data	field  points  to  the
       results of the study.

       The  returned  value  from  a  pcre_study()  can	 be passed directly to
       pcre_exec(). However, the pcre_extra block also contains	 other	fields
       that  can  be  set  by the caller before the block is passed; these are
       described below. If studying the pattern does  not  produce  any	 addi‐
       tional information, pcre_study() returns NULL. In that circumstance, if
       the calling  program  wants  to	pass  some  of	the  other  fields  to
       pcre_exec(), it must set up its own pcre_extra block.

       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 for 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.  You should therefore test the error pointer for NULL after call‐
       ing pcre_study(), to be sure that it has run successfully.

       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.


       PCRE handles caseless matching, and determines whether  characters  are
       letters,	 digits,  or  whatever,	 by reference to a set of tables. When
       running in UTF-8 mode, this applies only to characters with codes  less
       than  256. The library contains a default set of tables that 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

       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.


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

       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, and 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:


       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.


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


       Return  information  about  the first byte of any matched string, for a
       non-anchored   pattern.	 (This	  option    used    to	  be	called
       PCRE_INFO_FIRSTCHAR;  the  old  name  is still recognized for backwards

       If  there  is  a	 fixed	first  byte,  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 newline within the string. Otherwise
       -2 is returned. For anchored patterns, -2 is returned.


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


       Return the value of the rightmost literal byte that must exist  in  any
       matched	string,	 other	than  at  its  start,  if such a byte has been
       recorded. The fourth argument should point to an int variable. If there
       is  no such byte, -1 is returned. For anchored patterns, a last literal
       byte is recorded only if it follows something of variable  length.  For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.


       PCRE supports the use of named as well as numbered capturing  parenthe‐
       ses.  The names are just an additional way of identifying the parenthe‐
       ses, which still acquire a number. A caller that wants to extract  data
       from  a	named subpattern must convert the name to a number in order to
       access the correct  pointers  in	 the  output  vector  (described  with
       pcre_exec()  below). In order to do this, it must first use these three
       values to obtain the name-to-number mapping table for the pattern.

       The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of each entry; both of these  return  an	 int  value.  The  entry  size
       depends	on the length of the longest name. PCRE_INFO_NAMETABLE returns
       a pointer to the first entry of the table  (a  pointer  to  char).  The
       first two bytes of each entry are the number of the capturing parenthe‐
       sis, most significant byte first. The rest of the entry is  the	corre‐
       sponding	 name,	zero  terminated. The names are in alphabetical order.
       For example, consider the following pattern  (assume  PCRE_EXTENDED  is
       set, so white space - including newlines - is ignored):

	 (?P<date> (?P<year>(\d\d)?\d\d) -
	 (?P<month>\d\d) - (?P<day>\d\d) )

       There  are  four	 named subpatterns, so the table has four entries, and
       each entry in the table is eight bytes long. The table is  as  follows,
       with non-printing bytes shows in hex, and undefined bytes shown as ??:

	 00 01 d  a  t	e  00 ??
	 00 05 d  a  y	00 ?? ??
	 00 04 m  o  n	t  h  00
	 00 02 y  e  a	r  00 ??

       When writing code to extract data from named subpatterns, remember that
       the length of each entry may be different for each compiled pattern.


       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.

       A  pattern  is  automatically  anchored by PCRE if all of its top-level
       alternatives begin with one of the following:

	 ^     unless PCRE_MULTILINE is set
	 \A    always
	 \G    always
	 .*    if PCRE_DOTALL is set and there are no back
		 references to the subpattern in which .* appears

       For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().


       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.


       Returns the size of the data block pointed to by the  study_data	 field
       in  a  pcre_extra  block.  That	is, it is the value that was passed to
       pcre_malloc() when PCRE was getting memory into which to place the data
       created	by  pcre_study(). The fourth argument should point to a size_t


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

       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_FIRSTBYTE above).


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

       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.

       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 */

       If  the	extra argument is not NULL, it must point to a pcre_extra data
       block. The pcre_study() function returns such a block (when it  doesn't
       return  NULL), but you can also create one for yourself, and pass addi‐
       tional information in it. The fields in the block are as follows:

	 unsigned long int flags;
	 void *study_data;
	 unsigned long int match_limit;
	 void *callout_data;

       The flags field is a bitmap that specifies which of  the	 other	fields
       are set. The flag bits are:


       Other  flag  bits should be set to zero. The study_data field is set in
       the pcre_extra block that is returned by	 pcre_study(),	together  with
       the appropriate flag bit. You should not set this yourself, but you can
       add to the block by setting the other fields.

       The match_limit field provides a means of preventing PCRE from using up
       a  vast amount of resources when running patterns that are not going to
       match, but which have a very large number  of  possibilities  in	 their
       search  trees.  The  classic  example  is  the  use of nested unlimited
       repeats. Internally, PCRE uses a function called match() which it calls
       repeatedly  (sometimes recursively). The limit is imposed on the number
       of times this function is called during a match, which has  the	effect
       of  limiting  the  amount  of  recursion and backtracking that can take
       place. For patterns that are not anchored, the count starts  from  zero
       for each position in the subject string.

       The  default  limit  for the library can be set when PCRE is built; the
       default default is 10 million, which handles all but the	 most  extreme
       cases.  You  can	 reduce	 the  default  by  suppling pcre_exec() with a
       pcre_extra block in which match_limit is set to a  smaller  value,  and
       PCRE_EXTRA_MATCH_LIMIT  is  set	in  the	 flags	field. If the limit is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

       The pcre_callout field is used in conjunction with the  "callout"  fea‐
       ture, which is described in the pcrecallout documentation.

       The  PCRE_ANCHORED  option can be passed in the options argument, whose
       unused bits must be zero. This limits pcre_exec() to  matching  at  the
       first  matching	position.  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.

       When  PCRE_UTF8 was set at compile time, the validity of the subject as
       a UTF-8 string is automatically checked, and the value  of  startoffset
       is  also checked to ensure that it points to the start of a UTF-8 char‐
       acter. If an invalid UTF-8 sequence  of	bytes  is  found,  pcre_exec()
       returns	the  error  PCRE_ERROR_BADUTF8.	 If  startoffset  contains  an
       invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.

       If you already know that your subject is valid, and you	want  to  skip
       these	checks	  for	performance   reasons,	 you   can   set   the
       PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
       do  this	 for the second and subsequent calls to pcre_exec() if you are
       making repeated calls to find all  the  matches	in  a  single  subject
       string.	However,  you  should  be  sure	 that the value of startoffset
       points to the start of a UTF-8 character.  When	PCRE_NO_UTF8_CHECK  is
       set,  the  effect of passing an invalid UTF-8 string as a subject, or a
       value of startoffset that does not point to the start of a UTF-8	 char‐
       acter, is undefined. Your program may crash.

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


       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


       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.


       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


       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 to pcre_exec() as a pointer in subject,  a
       length in length, and a starting byte offset in startoffset. Unlike the
       pattern string, the subject may contain binary  zero  bytes.  When  the
       starting offset is zero, the search for a match starts at the beginning
       of the subject, and this is by far the most common case.

       If the pattern was compiled with the PCRE_UTF8 option, the subject must
       be  a  sequence of bytes that is a valid UTF-8 string, and the starting
       offset must point to the beginning of a UTF-8 character. If an  invalid
       UTF-8  string  or offset is passed, an error (either PCRE_ERROR_BADUTF8
       or  PCRE_ERROR_BADUTF8_OFFSET)	is   returned,	 unless	  the	option
       PCRE_NO_UTF8_CHECK  is  set,  in	 which	case  PCRE's  behaviour is not

       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


       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

       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:


       The subject string did not match the pattern.


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


       An unrecognized bit was set in the options argument.


       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.


       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.


       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.


       This  error is used by the pcre_copy_substring(), pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see	below).	 It  is	 never
       returned by pcre_exec().


       The  recursion  and backtracking limit, as specified by the match_limit
       field in a pcre_extra structure (or defaulted)  was  reached.  See  the
       description above.


       This error is never generated by pcre_exec() itself. It is provided for
       use by callout functions that want to yield a distinctive  error	 code.
       See the pcrecallout documentation for details.


       A  string  that contains an invalid UTF-8 byte sequence was passed as a


       The UTF-8 byte sequence that was passed as a subject was valid, but the
       value  of startoffset did not point to the beginning of a UTF-8 charac‐


       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);

       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. These functions identify  substrings
       by  number.  The	 next section describes functions for extracting named
       substrings. A substring	that  contains	a  binary  zero	 is  correctly
       extracted  and  has  a further zero added on the end, but the result is
       not, of course, a C string.

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


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


       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


       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‐


       int pcre_copy_named_substring(const pcre *code,
	    const char *subject, int *ovector,
	    int stringcount, const char *stringname,
	    char *buffer, int buffersize);

       int pcre_get_stringnumber(const pcre *code,
	    const char *name);

       int pcre_get_named_substring(const pcre *code,
	    const char *subject, int *ovector,
	    int stringcount, const char *stringname,
	    const char **stringptr);

       To  extract a substring by name, you first have to find associated num‐
       ber. This can be done by	 calling  pcre_get_stringnumber().  The	 first
       argument is the compiled pattern, and the second is the name. For exam‐
       ple, for this pattern


       the number of the subpattern called "xxx" is 1. Given the  number,  you
       can  then  extract  the substring directly, or use one of the functions
       described in the previous section. For convenience, there are also  two
       functions that do the whole job.

       Most    of    the    arguments	of   pcre_copy_named_substring()   and
       pcre_get_named_substring() are the same as those for the functions that
       extract by number, and so are not re-described here. There are just two

       First, instead of a substring number, a substring name is  given.  Sec‐
       ond, there is an extra argument, given at the start, which is a pointer
       to the compiled pattern. This is needed in order to gain access to  the
       name-to-number translation table.

       These  functions call pcre_get_stringnumber(), and if it succeeds, they
       then call pcre_copy_substring() or pcre_get_substring(),	 as  appropri‐

Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.


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