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pcreapi(3)





NAME

       PCRE - Perl-compatible regular expressions


PCRE NATIVE API


       #include <pcre.h>

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

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            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_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       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_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       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_refcount(pcre *code, int adjust);

       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 API OVERVIEW


       PCRE has its own native API, which is described in this document. There
       are also some wrapper functions that correspond to  the  POSIX  regular
       expression  API.  These  are  described in the pcreposix documentation.
       Both of these APIs define a set of C function calls. A C++  wrapper  is
       distributed with PCRE. It is documented in the pcrecpp page.

       The  native  API  C  function prototypes are defined in the header file
       pcre.h, and on Unix systems the library itself is called  libpcre.   It
       can normally be accessed by adding -lpcre to the command for linking an
       application  that  uses  PCRE.  The  header  file  defines  the  macros
       PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
       bers for the library.  Applications can use these  to  include  support
       for different releases of PCRE.

       The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
       pcre_exec() are used for compiling and matching regular expressions  in
       a  Perl-compatible  manner. A sample program that demonstrates the sim-
       plest way of using them is provided in the file  called  pcredemo.c  in
       the  source distribution. The pcresample documentation describes how to
       run it.

       A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble,  is  also provided. This uses a different algorithm for the match-
       ing. The alternative algorithm finds all possible matches (at  a  given
       point  in  the subject), and scans the subject just once. However, this
       algorithm does not return captured substrings. A description of the two
       matching  algorithms and their advantages and disadvantages is given in
       the pcrematching documentation.

       In addition to the main compiling and  matching  functions,  there  are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

         pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

       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  to build a set of character
       tables  in  the  current  locale   for   passing   to   pcre_compile(),
       pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
       provided for specialist use.  Most  commonly,  no  special  tables  are
       passed,  in  which case internal tables that are generated when PCRE is
       built are used.

       The function pcre_fullinfo() is used to find out  information  about  a
       compiled  pattern; pcre_info() is an obsolete version that 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 function pcre_refcount() maintains a  reference  count  in  a  data
       block  containing  a compiled pattern. This is provided for the benefit
       of object-oriented applications.

       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, when running the pcre_exec()
       function. See the pcrebuild documentation for  details  of  how  to  do
       this.  It  is  a non-standard way of building PCRE, for use in environ-
       ments 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.
       There  is  a discussion about PCRE's stack usage in the pcrestack docu-
       mentation.

       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.


NEWLINES


       PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each  of  the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE is built, a  default
       can  be  specified.  The default default is LF, which is the Unix stan-
       dard. When PCRE is run, the default can be overridden,  either  when  a
       pattern is compiled, or when it is matched.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter or pair of characters that indicate a line break". The choice  of
       newline  convention  affects  the  handling of the dot, circumflex, and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF  is a recognized line ending sequence, the match position advance-
       ment for a non-anchored pattern. The choice of newline convention  does
       not affect the interpretation of the \n or \r escape sequences.


MULTITHREADING


       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.


SAVING PRECOMPILED PATTERNS FOR LATER USE


       The compiled form of a regular expression can be saved and re-used at a
       later time, possibly by a different program, and even on a  host  other
       than  the  one  on  which  it  was  compiled.  Details are given in the
       pcreprecompile documentation. However, compiling a  regular  expression
       with  one version of PCRE for use with a different version is not guar-
       anteed to work and may cause crashes.


CHECKING BUILD-TIME OPTIONS


       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-
       tures.

       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
       available:

         PCRE_CONFIG_UTF8

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

         PCRE_CONFIG_UNICODE_PROPERTIES

       The  output  is  an  integer  that is set to one if support for Unicode
       character properties is available; otherwise it is set to zero.

         PCRE_CONFIG_NEWLINE

       The output is an integer whose value specifies  the  default  character
       sequence  that is recognized as meaning "newline". The four values that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and  -1  for  ANY. The default should normally be the standard sequence
       for your operating system.

         PCRE_CONFIG_LINK_SIZE

       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.

         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

       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.

         PCRE_CONFIG_MATCH_LIMIT

       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.

         PCRE_CONFIG_MATCH_LIMIT_RECURSION

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

         PCRE_CONFIG_STACKRECURSE

       The  output is an integer that is set to one if internal recursion when
       running pcre_exec() 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.


COMPILING A PATTERN


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

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the  two interfaces is that pcre_compile2() has an additional argument,
       errorcodeptr, via which a numerical error code can be returned.

       The pattern is a C string terminated by a binary zero, and is passed in
       the  pattern  argument.  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 (via pcre_free) 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 may contain a copy of the tableptr  argu-
       ment, which is an address (see below).

       The options argument contains various bit settings that affect the com-
       pilation. It should be zero if no options are required.  The  available
       options  are  described  below. Some of them, in particular, those that
       are compatible with Perl, can also be set and  unset  from  within  the
       pattern  (see  the  detailed  description in the pcrepattern documenta-
       tion). For these options, the contents of the options  argument  speci-
       fies  their initial settings at the start of compilation and execution.
       The PCRE_ANCHORED and PCRE_NEWLINE_xxx options 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. This is a static string that is part of the library. You must not
       try to free it. The offset from the start of the pattern to the charac-
       ter 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 pcre_compile2() is used instead of pcre_compile(),  and  the  error-
       codeptr  argument is not NULL, a non-zero error code number is returned
       via this argument in the event of an error. This is in addition to  the
       textual error message. Error codes and messages are listed below.

       If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
       character tables that are  built  when  PCRE  is  compiled,  using  the
       default  C  locale.  Otherwise, tableptr must be an address that is the
       result of a call to pcre_maketables(). This value is  stored  with  the
       compiled  pattern,  and used again by pcre_exec(), unless another table
       pointer is passed to it. For more discussion, 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 names for option bits are defined in  the  pcre.h  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 first matching point in the  string
       that  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_AUTO_CALLOUT

       If this bit is set, pcre_compile() automatically inserts callout items,
       all with number 255, before each pattern item. For  discussion  of  the
       callout facility, see the pcrecallout documentation.

         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,  and  it  can  be
       changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
       always understands the concept of case for characters whose values  are
       less  than 128, so caseless matching is always possible. For characters
       with higher values, the concept of case is supported if  PCRE  is  com-
       piled  with Unicode property support, but not otherwise. If you want to
       use caseless matching for characters 128 and  above,  you  must  ensure
       that  PCRE  is  compiled  with Unicode property support as well as with
       UTF-8 support.

         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 a newline at the end of the string (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.

         PCRE_DOTALL

       If this bit is set, a dot metacharater in the pattern matches all char-
       acters, including those that indicate newline. Without it, a  dot  does
       not  match  when  the  current position is at a newline. 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
       newline characters, independent of the setting of this option.

         PCRE_DUPNAMES

       If this bit is set, names used to identify capturing  subpatterns  need
       not be unique. This can be helpful for certain types of pattern when it
       is known that only one instance of the named  subpattern  can  ever  be
       matched.  There  are  more details of named subpatterns below; see also
       the pcrepattern documentation.

         PCRE_EXTENDED

       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, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
       option,  and  it  can be changed within a pattern by a (?x) option set-
       ting.

       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.

         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. (Perl can, however, be persuaded to give a warning for  this.)
       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_FIRSTLINE

       If this option is set, an  unanchored  pattern  is  required  to  match
       before  or  at  the  first  newline  in  the subject string, though the
       matched text may continue over the newline.

         PCRE_MULTILINE

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

       When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  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 new-
       lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
       setting PCRE_MULTILINE has no effect.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These  options  override the default newline definition that was chosen
       when PCRE was built. Setting the first or the second specifies  that  a
       newline  is  indicated  by a single character (CR or LF, respectively).
       Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
       two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
       U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
       (paragraph  separator,  U+2029).  The  last  two are recognized only in
       UTF-8 mode.

       The newline setting in the  options  word  uses  three  bits  that  are
       treated as a number, giving eight possibilities. Currently only six are
       used (default plus the five values above). This means that if  you  set
       more  than one newline option, the combination may or may not be sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
       cause an error.

       The only time that a line break is specially recognized when  compiling
       a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
       character class is encountered. This indicates  a  comment  that  lasts
       until  after the next line break sequence. In other circumstances, line
       break  sequences  are  treated  as  literal  data,   except   that   in
       PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
       and are therefore ignored.

       The newline option that is set at compile time becomes the default that
       is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

         PCRE_NO_AUTO_CAPTURE

       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.

         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 single-byte  character  strings.
       However,  it is available only when PCRE is built to include UTF-8 sup-
       port. 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.

         PCRE_NO_UTF8_CHECK

       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 this option can  also  be  passed  to
       pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
       ing of subject strings.


COMPILATION ERROR CODES


       The following table lists the error  codes  than  may  be  returned  by
       pcre_compile2(),  along with the error messages that may be returned by
       both compiling functions. As PCRE has developed, some error codes  have
       fallen out of use. To avoid confusion, they have not been re-used.

          0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (?
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  [this code is not in use]
         20  regular expression too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?[+-]digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is not compiled with PCRE_UTF8 support
         33  [this code is not in use]
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error in subpattern name (missing terminator)
         43  two named subpatterns have the same name
         44  invalid UTF-8 string
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10,000)
         50  repeated subpattern is too long
         51  octal value is greater than \377 (not in UTF-8 mode)
         52  internal error: overran compiling workspace
         53   internal  error:  previously-checked  referenced  subpattern not
       found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options"
         57  \g is not followed by a braced name or an optionally braced
               non-zero number
         58  (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number


STUDYING A PATTERN


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

       If a compiled pattern is going to be used several times,  it  is  worth
       spending more time analyzing it in order to speed up the time taken for
       matching. The function pcre_study() takes a pointer to a compiled  pat-
       tern as its first argument. If studying the pattern produces additional
       information 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  pcre_study()  can  be  passed  directly  to
       pcre_exec().  However,  a  pcre_extra  block also contains other fields
       that can be set by the caller before the block  is  passed;  these  are
       described below in the section on matching a pattern.

       If  studying  the  pattern  does not produce any additional information
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants  to  pass  any of the other fields to pcre_exec(), it must set up
       its own pcre_extra block.

       The second argument of pcre_study() contains option bits.  At  present,
       no options are defined, 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 is set to point to a textual
       error message. This is a static string that is part of the library. You
       must  not  try  to  free it. You should test the error pointer for NULL
       after calling 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 bytes is created.


LOCALE SUPPORT


       PCRE handles caseless matching, and determines whether  characters  are
       letters,  digits, or whatever, by reference to a set of tables, indexed
       by character value. When running in UTF-8 mode, this  applies  only  to
       characters  with  codes  less than 128. Higher-valued codes never match
       escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
       with  Unicode  character property support. The use of locales with Uni-
       code is discouraged. If you are handling characters with codes  greater
       than  128, you should either use UTF-8 and Unicode, or use locales, but
       not try to mix the two.

       PCRE contains an internal set of tables that are used  when  the  final
       argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

       The  internal tables can always be overridden by tables supplied by the
       application that calls PCRE. These may be created in a different locale
       from  the  default.  As more and more applications change to using Uni-
       code, the need for this locale support is expected to die away.

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

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

       The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale is "french".

       When pcre_maketables() runs, the tables are built  in  memory  that  is
       obtained  via  pcre_malloc. It is the caller's responsibility to ensure
       that the memory containing the tables remains available for as long  as
       it is needed.

       The pointer that is passed to pcre_compile() is saved with the compiled
       pattern, and the same tables are used via this pointer by  pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

       It  is  possible to pass a table pointer or NULL (indicating the use of
       the internal tables) to pcre_exec(). Although  not  intended  for  this
       purpose,  this facility could be used to match a pattern in a different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.


INFORMATION ABOUT A PATTERN


       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

       The "magic number" is placed at the start of each compiled  pattern  as
       an  simple check against passing an arbitrary memory pointer. Here is a
       typical call of pcre_fullinfo(), to obtain the length of  the  compiled
       pattern:

         int rc;
         size_t 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_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_CAPTURECOUNT

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

         PCRE_INFO_DEFAULT_TABLES

       Return  a pointer to the internal default character tables within PCRE.
       The fourth argument should point to an unsigned char *  variable.  This
       information call is provided for internal use by the pcre_study() func-
       tion. External callers can cause PCRE to use  its  internal  tables  by
       passing a NULL table pointer.

         PCRE_INFO_FIRSTBYTE

       Return  information  about  the first byte of any matched string, for a
       non-anchored pattern. The fourth argument should point to an int  vari-
       able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
       is still recognized for backwards compatibility.)

       If there is a fixed first byte, for example, from  a  pattern  such  as
       (cat|cow|coyote), its value is returned. 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.

         PCRE_INFO_FIRSTTABLE

       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-
       able.

         PCRE_INFO_JCHANGED

       Return  1  if the (?J) option setting is used in the pattern, otherwise
       0. The fourth argument should point to an int variable. The (?J) inter-
       nal option setting changes the local PCRE_DUPNAMES option.

         PCRE_INFO_LASTLITERAL

       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_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

       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 numbers. Several convenience functions such as
       pcre_get_named_substring() are provided for  extracting  captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers in the output vector (described with pcre_exec() below). To do
       the conversion, you need  to  use  the  name-to-number  map,  which  is
       described by these three values.

       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.
       When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
       theses numbers. For example, consider  the  following  pattern  (assume
       PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
       ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<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 hexadecimal, 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 using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE_INFO_OKPARTIAL

       Return  1 if the pattern can be used for partial matching, otherwise 0.
       The fourth argument should point to an int  variable.  The  pcrepartial
       documentation  lists  the restrictions that apply to patterns when par-
       tial matching is used.

         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.

       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().

         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_STUDYSIZE

       Return 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
       variable.


OBSOLETE INFO FUNCTION


       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).


REFERENCE COUNTS


       int pcre_refcount(pcre *code, int adjust);

       The pcre_refcount() function is used to maintain a reference  count  in
       the data block that contains a compiled pattern. It is provided for the
       benefit of applications that  operate  in  an  object-oriented  manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

       When a pattern is compiled, the reference count field is initialized to
       zero.   It is changed only by calling this function, whose action is to
       add the adjust value (which may be positive or  negative)  to  it.  The
       yield of the function is the new value. However, the value of the count
       is constrained to lie between 0 and 65535, inclusive. If the new  value
       is outside these limits, it is forced to the appropriate limit value.

       Except  when it is zero, the reference count is not correctly preserved
       if a pattern is compiled on one host and then  transferred  to  a  host
       whose byte-order is different. (This seems a highly unlikely scenario.)


MATCHING A PATTERN: THE TRADITIONAL FUNCTION


       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
       compiled  pattern, which is passed in the code argument. If the pattern
       has been studied, the result of the study should be passed in the extra
       argument.  This  function is the main matching facility of the library,
       and it operates in a Perl-like manner. For specialist use there is also
       an  alternative matching function, which is described below in the sec-
       tion about the pcre_dfa_exec() function.

       In most applications, the pattern will have been compiled (and  option-
       ally  studied)  in the same process that calls pcre_exec(). However, it
       is possible to save compiled patterns and study data, and then use them
       later  in  different processes, possibly even on different hosts. For a
       discussion about this, see the pcreprecompile documentation.

       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 of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

   Extra data for pcre_exec()

       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 pcre_extra block contains the following
       fields (not necessarily in this order):

         unsigned long int flags;
         void *study_data;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;

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

         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_TABLES

       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 may
       add to the block by setting the other fields  and  their  corresponding
       flag bits.

       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 repeat-
       edly (sometimes recursively). The limit set by match_limit  is  imposed
       on  the  number  of times this function is called during a match, which
       has the effect of limiting the amount of  backtracking  that  can  take
       place. For patterns that are not anchored, the count restarts from zero
       for each position in the subject string.

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

       The  match_limit_recursion field is similar to match_limit, but instead
       of limiting the total number of times that match() is called, it limits
       the  depth  of  recursion. The recursion depth is a smaller number than
       the total number of calls, because not all calls to match() are  recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting the recursion depth limits the amount of  stack  that  can  be
       used, or, when PCRE has been compiled to use memory on the heap instead
       of the stack, the amount of heap memory that can be used.

       The default value for match_limit_recursion can be  set  when  PCRE  is
       built;  the  default  default  is  the  same  value  as the default for
       match_limit. You can override the default by suppling pcre_exec()  with
       a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

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

       The tables field  is  used  to  pass  a  character  tables  pointer  to
       pcre_exec();  this overrides the value that is stored with the compiled
       pattern. A non-NULL value is stored with the compiled pattern  only  if
       custom  tables  were  supplied to pcre_compile() via its tableptr argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's  internal  tables  to be used. This facility is helpful when re-
       using patterns that have been saved after compiling  with  an  external
       set  of  tables,  because  the  external tables might be at a different
       address when pcre_exec() is called. See the  pcreprecompile  documenta-
       tion for a discussion of saving compiled patterns for later use.

   Option bits for pcre_exec()

       The  unused  bits of the options argument for pcre_exec() must be zero.
       The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
       PCRE_PARTIAL.

         PCRE_ANCHORED

       The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
       matching  position.  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.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These  options  override  the  newline  definition  that  was chosen or
       defaulted when the pattern was compiled. For details, see the  descrip-
       tion  of  pcre_compile()  above.  During  matching,  the newline choice
       affects the behaviour of the dot, circumflex,  and  dollar  metacharac-
       ters.  It may also alter the way the match position is advanced after a
       match  failure  for  an  unanchored  pattern.  When  PCRE_NEWLINE_CRLF,
       PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY is set, and a match attempt
       fails when the current position is at a CRLF sequence, the match  posi-
       tion  is  advanced by two characters instead of one, in other words, to
       after the CRLF.

         PCRE_NOTBOL

       This option specifies that first character of the subject 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. This option affects only the behav-
       iour of the circumflex metacharacter. It does not affect \A.

         PCRE_NOTEOL

       This option specifies that the end of the subject string is not the end
       of  a line, so the dollar metacharacter should not match it nor (except
       in multiline mode) a newline immediately before it. Setting this  with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option affects only the behaviour of the dollar metacharacter. It  does
       not affect \Z or \z.

         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 and PCRE_ANCHORED, and then
       if that fails by advancing the starting offset (see below)  and  trying
       an ordinary match again. There is some code that demonstrates how to do
       this in the pcredemo.c sample program.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8  string is automatically checked when pcre_exec() is subsequently
       called.  The value of startoffset is also checked  to  ensure  that  it
       points  to the start of a UTF-8 character. 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.

         PCRE_PARTIAL

       This  option  turns  on  the  partial  matching feature. If the subject
       string fails to match the pattern, but at some point during the  match-
       ing  process  the  end of the subject was reached (that is, the subject
       partially matches the pattern and the failure to  match  occurred  only
       because  there were not enough subject characters), pcre_exec() returns
       PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
       used,  there  are restrictions on what may appear in the pattern. These
       are discussed in the pcrepartial documentation.

   The string to be matched by pcre_exec()

       The subject string is passed to pcre_exec() as a pointer in subject,  a
       length  in  length, and a starting byte offset in startoffset. In UTF-8
       mode, the byte offset must point to the start  of  a  UTF-8  character.
       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.

       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 made. This can only succeed
       if the pattern does not require the match to be at  the  start  of  the
       subject.

   How pcre_exec() returns captured substrings

       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, which must be a non-negative number.
       Note: this argument is NOT the size of ovector in bytes.

       The first two-thirds of the vector is used to pass back  captured  sub-
       strings,  each  substring using a pair of integers. The remaining third
       of the vector is used as workspace by pcre_exec() while  matching  cap-
       turing  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  is 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 one more than the
       highest numbered pair that has been set. For example, if two substrings
       have  been captured, the returned value is 3. 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.

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

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

       The  pcre_info()  function  can  be used to find out how many capturing
       subpatterns there are in a compiled  pattern.  The  smallest  size  for
       ovector  that  will allow for n captured substrings, in addition to the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

       It is possible for 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)  the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2 is not. When this happens, both values in  the  offset  pairs  corre-
       sponding to unused subpatterns are set to -1.

       Offset  values  that correspond to unused subpatterns at the end of the
       expression are also set to -1. For example,  if  the  string  "abc"  is
       matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
       matched. The return from the function is 2, because  the  highest  used
       capturing subpattern number is 1. However, you can refer to the offsets
       for the second and third capturing subpatterns if  you  wish  (assuming
       the vector is large enough, of course).

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

   Error return values from pcre_exec()

       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 and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an  environment  with the other endianness. This is the error that PCRE
       gives when the magic number is not present.

         PCRE_ERROR_UNKNOWN_OPCODE (-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 automatically freed at the end of matching.

         PCRE_ERROR_NOSUBSTRING    (-7)

       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().

         PCRE_ERROR_MATCHLIMIT     (-8)

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

         PCRE_ERROR_CALLOUT        (-9)

       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.

         PCRE_ERROR_BADUTF8        (-10)

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

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       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-
       ter.

         PCRE_ERROR_PARTIAL        (-12)

       The  subject  string did not match, but it did match partially. See the
       pcrepartial documentation for details of partial matching.

         PCRE_ERROR_BADPARTIAL     (-13)

       The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
       items  that are not supported for partial matching. See the pcrepartial
       documentation for details of partial matching.

         PCRE_ERROR_INTERNAL       (-14)

       An unexpected internal error has occurred. This error could  be  caused
       by a bug in PCRE or by overwriting of the compiled pattern.

         PCRE_ERROR_BADCOUNT       (-15)

       This  error is given if the value of the ovecsize argument is negative.

         PCRE_ERROR_RECURSIONLIMIT (-21)

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

         PCRE_ERROR_NULLWSLIMIT    (-22)

       When a group that can match an empty  substring  is  repeated  with  an
       unbounded  upper  limit, the subject position at the start of the group
       must be remembered, so that a test for an empty string can be made when
       the  end  of the group is reached. Some workspace is required for this;
       if it runs out, this error is given.

         PCRE_ERROR_BADNEWLINE     (-23)

       An invalid combination of PCRE_NEWLINE_xxx options was given.

       Error numbers -16 to -20 are not used by pcre_exec().


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER


       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.  However, you can process such a string  by  referring  to  the
       length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not  adequate for handling strings containing binary zeros, because the
       end of the final string is not independently indicated.

       The first three arguments are the same for all  three  of  these  func-
       tions:  subject  is  the subject string that 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 number of elements in 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,  whereas
       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 these error codes:

         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 that 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  the
       error code

         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  that  cannot   use
       pcre_free  directly;  it is for these cases that the functions are pro-
       vided.


EXTRACTING CAPTURED SUBSTRINGS BY NAME


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

       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_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.  For example, for this pattern

         (a+)b(?<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
       subpattern of that name.

       Given the number, you can 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 similarly
       named functions that extract by number. As these are described  in  the
       previous  section,  they  are not re-described here. There are just two
       differences:

       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-
       ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
       behaviour may not be what you want (see the next section).


DUPLICATE SUBPATTERN NAMES


       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
       subpatterns  are  not  required  to  be unique. Normally, patterns with
       duplicate names are such that in any one match, only one of  the  named
       subpatterns  participates. An example is shown in the pcrepattern docu-
       mentation. When duplicates are present, pcre_copy_named_substring() and
       pcre_get_named_substring()  return the first substring corresponding to
       the given name that is set.  If  none  are  set,  an  empty  string  is
       returned.  The pcre_get_stringnumber() function returns one of the num-
       bers that are associated with the name, but it is not defined which  it
       is.

       If  you want to get full details of all captured substrings for a given
       name, you must use  the  pcre_get_stringtable_entries()  function.  The
       first argument is the compiled pattern, and the second is the name. The
       third and fourth are pointers to variables which  are  updated  by  the
       function. After it has run, they point to the first and last entries in
       the name-to-number table  for  the  given  name.  The  function  itself
       returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
       there are none. The format of the table is described above in the  sec-
       tion  entitled  Information  about  a  pattern.  Given all the relevant
       entries for the name, you can extract each of their numbers, and  hence
       the captured data, if any.


FINDING ALL POSSIBLE MATCHES


       The  traditional  matching  function  uses a similar algorithm to Perl,
       which stops when it finds the first match, starting at a given point in
       the  subject.  If you want to find all possible matches, or the longest
       possible match, consider using the alternative matching  function  (see
       below)  instead.  If you cannot use the alternative function, but still
       need to find all possible matches, you can kludge it up by  making  use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.   When your callout function is called, extract and save the cur-
       rent matched substring. Then return  1,  which  forces  pcre_exec()  to
       backtrack  and  try other alternatives. Ultimately, when it runs out of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION


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

       The function pcre_dfa_exec()  is  called  to  match  a  subject  string
       against  a  compiled pattern, using a matching algorithm that scans the
       subject string just once, and does not backtrack.  This  has  different
       characteristics  to  the  normal  algorithm, and is not compatible with
       Perl. Some of the features of PCRE patterns are not  supported.  Never-
       theless,  there are times when this kind of matching can be useful. For
       a discussion of the two matching algorithms, see the pcrematching docu-
       mentation.

       The  arguments  for  the  pcre_dfa_exec()  function are the same as for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent  way,  and  this is described below. The other common arguments are
       used in the same way as for pcre_exec(), so their  description  is  not
       repeated here.

       The  two  additional  arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It  is  used  for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace will be needed for patterns and subjects where  there  are  a
       lot of potential matches.

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

         int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_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 of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_exec()

       The  unused  bits  of  the options argument for pcre_dfa_exec() must be
       zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
       LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
       PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
       three of these are the same as for pcre_exec(), so their description is
       not repeated here.

         PCRE_PARTIAL

       This has the same general effect as it does for  pcre_exec(),  but  the
       details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
       pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
       PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
       been no complete matches, but there is still at least one matching pos-
       sibility.  The portion of the string that provided the partial match is
       set as the first matching string.

         PCRE_DFA_SHORTEST

       Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
       stop as soon as it has found one match. Because of the way the alterna-
       tive algorithm works, this is necessarily the shortest  possible  match
       at the first possible matching point in the subject string.

         PCRE_DFA_RESTART

       When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
       returns a partial match, it is possible to call it  again,  with  addi-
       tional  subject  characters,  and have it continue with the same match.
       The PCRE_DFA_RESTART option requests this action; when it is  set,  the
       workspace  and wscount options must reference the same vector as before
       because data about the match so far is left in  them  after  a  partial
       match.  There  is  more  discussion of this facility in the pcrepartial
       documentation.

   Successful returns from pcre_dfa_exec()

       When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
       string in the subject. Note, however, that all the matches from one run
       of the function start at the same point in  the  subject.  The  shorter
       matches  are all initial substrings of the longer matches. For example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something>
         <something> <something else>
         <something> <something else> <something further>

       On success, the yield of the function is a number  greater  than  zero,
       which  is  the  number of matched substrings. The substrings themselves
       are returned in ovector. Each string uses two elements;  the  first  is
       the  offset  to  the start, and the second is the offset to the end. In
       fact, all the strings have the same start  offset.  (Space  could  have
       been  saved by giving this only once, but it was decided to retain some
       compatibility with the way pcre_exec() returns data,  even  though  the
       meaning of the strings is different.)

       The strings are returned in reverse order of length; that is, the long-
       est matching string is given first. If there were too many  matches  to
       fit  into ovector, the yield of the function is zero, and the vector is
       filled with the longest matches.

   Error returns from pcre_dfa_exec()

       The pcre_dfa_exec() function returns a negative number when  it  fails.
       Many  of  the  errors  are  the  same as for pcre_exec(), and these are
       described above.  There are in addition the following errors  that  are
       specific to pcre_dfa_exec():

         PCRE_ERROR_DFA_UITEM      (-16)

       This  return is given if pcre_dfa_exec() encounters an item in the pat-
       tern that it does not support, for instance, the use of \C  or  a  back
       reference.

         PCRE_ERROR_DFA_UCOND      (-17)

       This  return  is  given  if pcre_dfa_exec() encounters a condition item
       that uses a back reference for the condition, or a test  for  recursion
       in a specific group. These are not supported.

         PCRE_ERROR_DFA_UMLIMIT    (-18)

       This  return  is given if pcre_dfa_exec() is called with an extra block
       that contains a setting of the match_limit field. This is not supported
       (it is meaningless).

         PCRE_ERROR_DFA_WSSIZE     (-19)

       This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
       workspace vector.

         PCRE_ERROR_DFA_RECURSE    (-20)

       When a recursive subpattern is processed, the matching  function  calls
       itself  recursively,  using  private vectors for ovector and workspace.
       This error is given if the output vector  is  not  large  enough.  This
       should be extremely rare, as a vector of size 1000 is used.


SEE ALSO


       pcrebuild(3),  pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
       tial(3), pcreposix(3), pcreprecompile(3), pcresample(3),  pcrestack(3).


AUTHOR


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION


       Last updated: 13 June 2007
       Copyright (c) 1997-2007 University of Cambridge.

                                                                    PCREAPI(3)

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