php-src/ext/pcre/pcrelib/doc/pcre.txt

-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. There are
separate text files for the pcregrep and pcretest commands.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

INTRODUCTION

       The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a  few  differences.  The current implementation of PCRE (release
       5.x) corresponds approximately with Perl  5.8,  including  support  for
       UTF-8 encoded strings and Unicode general category properties. However,
       this support has to be explicitly enabled; it is not the default.

       PCRE is written in C and released as a C library. A  number  of  people
       have  written  wrappers and interfaces of various kinds. A C++ class is
       included in these contributions, which can  be  found  in  the  Contrib
       directory at the primary FTP site, which is:

       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

       Details  of  exactly which Perl regular expression features are and are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern and pcrecompat pages.

       Some  features  of  PCRE can be included, excluded, or changed when the
       library is built. The pcre_config() function makes it  possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcrebuild page. Documentation about  build-
       ing  PCRE for various operating systems can be found in the README file
       in the source distribution.


USER DOCUMENTATION

       The user documentation for PCRE comprises a number  of  different  sec-
       tions.  In the "man" format, each of these is a separate "man page". In
       the HTML format, each is a separate page, linked from the  index  page.
       In  the  plain text format, all the sections are concatenated, for ease
       of searching. The sections are as follows:

         pcre              this document
         pcreapi           details of PCRE's native API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcregrep          description of the pcregrep command
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible API
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the sample program
         pcretest          description of the pcretest testing command

       In  addition,  in the "man" and HTML formats, there is a short page for
       each library function, listing its arguments and results.


LIMITATIONS

       There are some size limitations in PCRE but it is hoped that they  will
       never in practice be relevant.

       The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
       is compiled with the default internal linkage size of 2. If you want to
       process  regular  expressions  that are truly enormous, you can compile
       PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
       the  source  distribution and the pcrebuild documentation for details).
       In these cases the limit is substantially larger.  However,  the  speed
       of execution will be slower.

       All values in repeating quantifiers must be less than 65536.  The maxi-
       mum number of capturing subpatterns is 65535.

       There is no limit to the number of non-capturing subpatterns,  but  the
       maximum  depth  of  nesting  of  all kinds of parenthesized subpattern,
       including capturing subpatterns, assertions, and other types of subpat-
       tern, is 200.

       The  maximum  length of a subject string is the largest positive number
       that an integer variable can hold. However, PCRE uses recursion to han-
       dle  subpatterns  and indefinite repetition. This means that the avail-
       able stack space may limit the size of a subject  string  that  can  be
       processed by certain patterns.


UTF-8 AND UNICODE PROPERTY SUPPORT

       From  release  3.3,  PCRE  has  had  some support for character strings
       encoded in the UTF-8 format. For release 4.0 this was greatly  extended
       to  cover  most common requirements, and in release 5.0 additional sup-
       port for Unicode general category properties was added.

       In order process UTF-8 strings, you must build PCRE  to  include  UTF-8
       support  in  the  code,  and, in addition, you must call pcre_compile()
       with the PCRE_UTF8 option flag. When you do this, both the pattern  and
       any  subject  strings  that are matched against it are treated as UTF-8
       strings instead of just strings of bytes.

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

       If PCRE is built with Unicode character property support (which implies
       UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
       ported.  The available properties that can be tested are limited to the
       general category properties such as Lu for an upper case letter  or  Nd
       for  a decimal number. A full list is given in the pcrepattern documen-
       tation. The PCRE library is increased in size by about 90K when Unicode
       property support is included.

       The following comments apply when PCRE is running in UTF-8 mode:

       1.  When you set the PCRE_UTF8 flag, the strings passed as patterns and
       subjects are checked for validity on entry to the  relevant  functions.
       If an invalid UTF-8 string is passed, an error return is given. In some
       situations, you may already know  that  your  strings  are  valid,  and
       therefore want to skip these checks in order to improve performance. If
       you set the PCRE_NO_UTF8_CHECK flag at compile time  or  at  run  time,
       PCRE  assumes  that  the  pattern or subject it is given (respectively)
       contains only valid UTF-8 codes. In this case, it does not diagnose  an
       invalid  UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
       PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  may
       crash.

       2. In a pattern, the escape sequence \x{...}, where the contents of the
       braces is a string of hexadecimal digits, is  interpreted  as  a  UTF-8
       character  whose code number is the given hexadecimal number, for exam-
       ple: \x{1234}. If a non-hexadecimal digit appears between  the  braces,
       the item is not recognized.  This escape sequence can be used either as
       a literal, or within a character class.

       3. The original hexadecimal escape sequence, \xhh, matches  a  two-byte
       UTF-8 character if the value is greater than 127.

       4.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
       vidual bytes, for example: \x{100}{3}.

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

       6.  The  escape sequence \C can be used to match a single byte in UTF-8
       mode, but its use can lead to some strange effects.

       7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
       test  characters of any code value, but the characters that PCRE recog-
       nizes as digits, spaces, or word characters  remain  the  same  set  as
       before, all with values less than 256. This remains true even when PCRE
       includes Unicode property support, because to do otherwise  would  slow
       down  PCRE in many common cases. If you really want to test for a wider
       sense of, say, "digit", you must use Unicode  property  tests  such  as
       \p{Nd}.

       8.  Similarly,  characters that match the POSIX named character classes
       are all low-valued characters.

       9. Case-insensitive matching applies only to  characters  whose  values
       are  less than 128, unless PCRE is built with Unicode property support.
       Even when Unicode property support is available, PCRE  still  uses  its
       own  character  tables when checking the case of low-valued characters,
       so as not to degrade performance.  The Unicode property information  is
       used only for characters with higher values.


AUTHOR

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

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE BUILD-TIME OPTIONS

       This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. They are all selected, or  dese-
       lected, by providing options to the configure script that is run before
       the make command. The complete list of  options  for  configure  (which
       includes  the  standard  ones such as the selection of the installation
       directory) can be obtained by running

         ./configure --help

       The following sections describe certain options whose names begin  with
       --enable  or  --disable. These settings specify changes to the defaults
       for the configure command. Because of the  way  that  configure  works,
       --enable  and  --disable  always  come  in  pairs, so the complementary
       option always exists as well, but as it specifies the  default,  it  is
       not described.


UTF-8 SUPPORT

       To build PCRE with support for UTF-8 character strings, add

         --enable-utf8

       to  the  configure  command.  Of  itself, this does not make PCRE treat
       strings as UTF-8. As well as compiling PCRE with this option, you  also
       have  have to set the PCRE_UTF8 option when you call the pcre_compile()
       function.


UNICODE CHARACTER PROPERTY SUPPORT

       UTF-8 support allows PCRE to process character values greater than  255
       in  the  strings that it handles. On its own, however, it does not pro-
       vide any facilities for accessing the properties of such characters. If
       you  want  to  be able to use the pattern escapes \P, \p, and \X, which
       refer to Unicode character properties, you must add

         --enable-unicode-properties

       to the configure command. This implies UTF-8 support, even if you  have
       not explicitly requested it.

       Including  Unicode  property  support  adds around 90K of tables to the
       PCRE library, approximately doubling its size. Only the  general  cate-
       gory  properties  such as Lu and Nd are supported. Details are given in
       the pcrepattern documentation.


CODE VALUE OF NEWLINE

       By default, PCRE treats character 10 (linefeed) as the newline  charac-
       ter. This is the normal newline character on Unix-like systems. You can
       compile PCRE to use character 13 (carriage return) instead by adding

         --enable-newline-is-cr

       to the configure command. For completeness there is  also  a  --enable-
       newline-is-lf  option,  which explicitly specifies linefeed as the new-
       line character.


BUILDING SHARED AND STATIC LIBRARIES

       The PCRE building process uses libtool to build both shared and  static
       Unix  libraries by default. You can suppress one of these by adding one
       of

         --disable-shared
         --disable-static

       to the configure command, as required.


POSIX MALLOC USAGE

       When PCRE is called through the POSIX interface (see the pcreposix doc-
       umentation),  additional  working  storage  is required for holding the
       pointers to capturing substrings, because PCRE requires three  integers
       per  substring,  whereas  the POSIX interface provides only two. If the
       number of expected substrings is small, the wrapper function uses space
       on the stack, because this is faster than using malloc() for each call.
       The default threshold above which the stack is no longer used is 10; it
       can be changed by adding a setting such as

         --with-posix-malloc-threshold=20

       to the configure command.


LIMITING PCRE RESOURCE USAGE

       Internally,  PCRE has a function called match(), which it calls repeat-
       edly (possibly recursively) when matching a pattern. By controlling the
       maximum  number  of  times  this function may be called during a single
       matching operation, a limit can be placed on the resources  used  by  a
       single  call  to  pcre_exec(). The limit can be changed at run time, as
       described in the pcreapi documentation. The default is 10 million,  but
       this can be changed by adding a setting such as

         --with-match-limit=500000

       to the configure command.


HANDLING VERY LARGE PATTERNS

       Within  a  compiled  pattern,  offset values are used to point from one
       part to another (for example, from an opening parenthesis to an  alter-
       nation  metacharacter).  By default, two-byte values are used for these
       offsets, leading to a maximum size for a  compiled  pattern  of  around
       64K.  This  is sufficient to handle all but the most gigantic patterns.
       Nevertheless, some people do want to process enormous patterns,  so  it
       is  possible  to compile PCRE to use three-byte or four-byte offsets by
       adding a setting such as

         --with-link-size=3

       to the configure command. The value given must be 2,  3,  or  4.  Using
       longer  offsets slows down the operation of PCRE because it has to load
       additional bytes when handling them.

       If you build PCRE with an increased link size, test 2 (and  test  5  if
       you  are using UTF-8) will fail. Part of the output of these tests is a
       representation of the compiled pattern, and this changes with the  link
       size.


AVOIDING EXCESSIVE STACK USAGE

       PCRE  implements  backtracking while matching by making recursive calls
       to an internal function called match(). In environments where the  size
       of the stack is limited, this can severely limit PCRE's operation. (The
       Unix environment does not usually suffer from this problem.) An  alter-
       native  approach  that  uses  memory  from  the  heap to remember data,
       instead of using recursive function calls, has been implemented to work
       round  this  problem. If you want to build a version of PCRE that works
       this way, add

         --disable-stack-for-recursion

       to the configure command. With this configuration, PCRE  will  use  the
       pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
       ment functions. Separate functions are provided because  the  usage  is
       very  predictable:  the  block sizes requested are always the same, and
       the blocks are always freed in reverse order. A calling  program  might
       be  able  to implement optimized functions that perform better than the
       standard malloc() and  free()  functions.  PCRE  runs  noticeably  more
       slowly when built in this way.


USING EBCDIC CODE

       PCRE  assumes  by  default that it will run in an environment where the
       character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
       PCRE  can,  however,  be  compiled  to  run in an EBCDIC environment by
       adding

         --enable-ebcdic

       to the configure command.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(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_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 API OVERVIEW

       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.  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_study(), and pcre_exec() are used
       for compiling and matching regular expressions. A sample  program  that
       demonstrates  the  simplest  way  of using them is provided in the file
       called pcredemo.c in the source distribution. The pcresample documenta-
       tion describes how to run it.

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

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

       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()  or
       pcre_exec().  This is an optional facility that is  provided  for  spe-
       cialist 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 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.


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.


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 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.

         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_STACKRECURSE

       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.


COMPILING A PATTERN

       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 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 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 independent bits that affect the compila-
       tion. 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 option can be set at the time of matching as well  as
       at compile time.

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

       If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
       character tables 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. When running in
       UTF-8 mode, case support for high-valued characters is  available  only
       when PCRE is built with Unicode character property 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 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.

         PCRE_DOTALL

       If this bit is set, a dot metacharater in the pattern matches all char-
       acters,  including  newlines.  Without  it, newlines are excluded. This
       option is equivalent to Perl's /s option, 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
       option.

         PCRE_EXTENDED

       If  this  bit  is  set,  whitespace  data characters in the pattern are
       totally ignored except  when  escaped  or  inside  a  character  class.
       Whitespace  does  not  include the VT character (code 11). In addition,
       characters between an unescaped # outside a  character  class  and  the
       next newline character, inclusive, are also ignored. This is equivalent
       to Perl's /x option, and 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.

         PCRE_EXTRA

       This  option  was invented in order to turn on additional functionality
       of PCRE that is incompatible with Perl, but it  is  currently  of  very
       little  use. When set, any backslash in a pattern that is followed by a
       letter that has no special meaning  causes  an  error,  thus  reserving
       these  combinations  for  future  expansion.  By default, as in Perl, a
       backslash followed by a letter with no special meaning is treated as  a
       literal.  There  are  at  present  no other features controlled by this
       option. It can also be set by a (?X) option setting within a pattern.

         PCRE_MULTILINE

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

       When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
       constructs match immediately following or immediately before  any  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.

         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(),  to  suppress  the  UTF-8  validity  checking  of  subject
       strings.


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 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 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.)

       An  internal set of tables is created in the default C locale when PCRE
       is built. This is used when the final  argument  of  pcre_compile()  is
       NULL,  and  is  sufficient for many applications. An alternative set of
       tables can, however, be supplied. These may be created in  a  different
       locale  from the default. As more and more applications change to using
       Unicode, 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);

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

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

         PCRE_INFO_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_DEFAULTTABLES

       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.    (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), 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.

         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_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.  A  convenience  function  called
       pcre_get_named_substring() is provided  for  extracting  an  individual
       captured  substring  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.
       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 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 each entry is likely to
       be different for each compiled pattern.

         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).


MATCHING A PATTERN

       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.

       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 in the vector  (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 fields in a pcre_extra block are as  fol-
       lows:

         unsigned long int flags;
         void *study_data;
         unsigned long int match_limit;
         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_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 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  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_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_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
       behaviour 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  the  number  of
       pairs  that  have  been set. If there are no capturing subpatterns, the
       return value from a successful match is 1,  indicating  that  just  the
       first pair of offsets has been set.

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

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

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

       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.

   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_NODE   (-5)

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

         PCRE_ERROR_NOMEMORY       (-6)

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


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.

       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

         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

         PCRE_ERROR_NOMEMORY       (-6)

       if the attempt to get the memory block failed.

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

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


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. You can find the number
       from the name by calling pcre_get_stringnumber(). The first argument is
       the compiled 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.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout. By  default,  this
       variable contains NULL, which disables all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called.  Different  callout  points  can  be
       identified  by  putting  a number less than 256 after the letter C. The
       default value is zero.  For  example,  this  pattern  has  two  callout
       points:

         (?C1)eabc(?C2)def

       If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
       called, PCRE automatically  inserts  callouts,  all  with  number  255,
       before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
       used with the pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice that there is a callout before and after  each  parenthesis  and
       alternation  bar.  Automatic  callouts  can  be  used  for tracking the
       progress of pattern matching. The pcretest command has an  option  that
       sets  automatic callouts; when it is used, the output indicates how the
       pattern is matched. This is useful information when you are  trying  to
       optimize the performance of a particular pattern.


MISSING CALLOUTS

       You  should  be  aware  that,  because of optimizations in the way PCRE
       matches patterns, callouts sometimes do not happen. For example, if the
       pattern is

         ab(?C4)cd

       PCRE knows that any matching string must contain the letter "d". If the
       subject string is "abyz", the lack of "d" means that  matching  doesn't
       ever  start,  and  the  callout is never reached. However, with "abyd",
       though the result is still no match, the callout is obeyed.


THE CALLOUT INTERFACE

       During matching, when PCRE reaches a callout point, the external  func-
       tion  defined  by pcre_callout is called (if it is set). The only argu-
       ment is a pointer to a pcre_callout block. This structure contains  the
       following fields:

         int          version;
         int          callout_number;
         int         *offset_vector;
         const char  *subject;
         int          subject_length;
         int          start_match;
         int          current_position;
         int          capture_top;
         int          capture_last;
         void        *callout_data;
         int          pattern_position;
         int          next_item_length;

       The  version  field  is an integer containing the version number of the
       block format. The initial version was 0; the current version is 1.  The
       version  number  will  change  again in future if additional fields are
       added, but the intention is never to remove any of the existing fields.

       The  callout_number  field  contains the number of the callout, as com-
       piled into the pattern (that is, the number after ?C for  manual  call-
       outs, and 255 for automatically generated callouts).

       The  offset_vector field is a pointer to the vector of offsets that was
       passed by the caller to pcre_exec(). The contents can be  inspected  in
       order  to extract substrings that have been matched so far, in the same
       way as for extracting substrings after a match has completed.

       The subject and subject_length fields contain copies of the values that
       were passed to pcre_exec().

       The  start_match  field contains the offset within the subject at which
       the current match attempt started. If the pattern is not anchored,  the
       callout function may be called several times from the same point in the
       pattern for different starting points in the subject.

       The current_position field contains the offset within  the  subject  of
       the current match pointer.

       The  capture_top field contains one more than the number of the highest
       numbered captured substring so far. If no  substrings  have  been  cap-
       tured, the value of capture_top is one.

       The  capture_last  field  contains the number of the most recently cap-
       tured substring. If no substrings have been captured, its value is  -1.

       The  callout_data  field contains a value that is passed to pcre_exec()
       by the caller specifically so that it can be passed back  in  callouts.
       It  is  passed  in the pcre_callout field of the pcre_extra data struc-
       ture. If no such data was  passed,  the  value  of  callout_data  in  a
       pcre_callout  block  is  NULL. There is a description of the pcre_extra
       structure in the pcreapi documentation.

       The pattern_position field is present from version 1 of the  pcre_call-
       out structure. It contains the offset to the next item to be matched in
       the pattern string.

       The next_item_length field is present from version 1 of the  pcre_call-
       out structure. It contains the length of the next item to be matched in
       the pattern string. When the callout immediately precedes  an  alterna-
       tion  bar, a closing parenthesis, or the end of the pattern, the length
       is zero. When the callout precedes an opening parenthesis,  the  length
       is that of the entire subpattern.

       The  pattern_position  and next_item_length fields are intended to help
       in distinguishing between different automatic callouts, which all  have
       the same callout number. However, they are set for all callouts.


RETURN VALUES

       The  external callout function returns an integer to PCRE. If the value
       is zero, matching proceeds as normal. If  the  value  is  greater  than
       zero,  matching  fails  at  the current point, but backtracking to test
       other matching possibilities goes ahead, just as if a lookahead  asser-
       tion  had  failed.  If  the value is less than zero, the match is aban-
       doned, and pcre_exec() returns the negative value.

       Negative  values  should  normally  be   chosen   from   the   set   of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT  is
       reserved  for  use  by callout functions; it will never be used by PCRE
       itself.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl 5.8.

       1.  PCRE does not have full UTF-8 support. Details of what it does have
       are given in the section on UTF-8 support in the main pcre page.

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

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

       4. Though binary zero characters are supported in the  subject  string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5.  The  following Perl escape sequences are not supported: \l, \u, \L,
       \U, and \N. In fact these are implemented by Perl's general string-han-
       dling  and are not part of its pattern matching engine. If any of these
       are encountered by PCRE, an error is generated.

       6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
       is  built  with Unicode character property support. The properties that
       can be tested with \p and \P are limited to the general category  prop-
       erties such as Lu and Nd.

       7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters in between are treated as literals.  This  is  slightly  different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course  PCRE
       does not have variables). Note the following examples:

           Pattern            PCRE matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
       constructions.  However,  there is support for recursive patterns using
       the non-Perl items (?R),  (?number),  and  (?P>name).  Also,  the  PCRE
       "callout"  feature allows an external function to be called during pat-
       tern matching. See the pcrecallout documentation for details.

       9. There are some differences that are concerned with the  settings  of
       captured  strings  when  part  of  a  pattern is repeated. For example,
       matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
       unset, but in PCRE it is set to "b".

       10. PCRE provides some extensions to the Perl regular expression facil-
       ities:

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

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

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

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

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

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

       (g) The (?R), (?number), and (?P>name) constructs allows for  recursive
       pattern  matching  (Perl  can  do  this using the (?p{code}) construct,
       which PCRE cannot support.)

       (h) PCRE supports named capturing substrings, using the Python  syntax.

       (i)  PCRE  supports  the  possessive quantifier "++" syntax, taken from
       Sun's Java package.

       (j) The (R) condition, for testing recursion, is a PCRE extension.

       (k) The callout facility is PCRE-specific.

       (l) The partial matching facility is PCRE-specific.

       (m) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS

       The  syntax  and semantics of the regular expressions supported by PCRE
       are described below. Regular expressions are also described in the Perl
       documentation  and  in  a  number  of books, some of which have copious
       examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
       by  O'Reilly, covers regular expressions in great detail. This descrip-
       tion of PCRE's regular expressions is intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, you must build PCRE to  include  UTF-8  support,  and  then  call
       pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
       matching is mentioned in several places below. There is also a  summary
       of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
       page.

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

         The quick brown fox

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

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

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

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

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The  following sections describe the use of each of the metacharacters.


BACKSLASH

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

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

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

       If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

   Non-printing characters

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

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        newline (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or backreference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh... (UTF-8 mode only)

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

       After  \x, from zero to two hexadecimal digits are read (letters can be
       in upper or lower case). In UTF-8 mode, any number of hexadecimal  dig-
       its  may  appear between \x{ and }, but the value of the character code
       must be less than 2**31 (that is,  the  maximum  hexadecimal  value  is
       7FFFFFFF).  If  characters other than hexadecimal digits appear between
       \x{ and }, or if there is no terminating }, this form of escape is  not
       recognized. Instead, the initial \x will be interpreted as a basic hex-
       adecimal escape, with no following digits,  giving  a  character  whose
       value is zero.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x when PCRE is in UTF-8 mode. There is no  difference
       in  the  way they are handled. For example, \xdc is exactly the same as
       \x{dc}.

       After \0 up to two further octal digits are read.  In  both  cases,  if
       there  are fewer than two digits, just those that are present are used.
       Thus the sequence \0\x\07 specifies two binary zeros followed by a  BEL
       character  (code  value  7).  Make sure you supply two digits after the
       initial zero if the pattern character that follows is itself  an  octal
       digit.

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

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

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

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

       All the sequences that define a single byte value  or  a  single  UTF-8
       character (in UTF-8 mode) can be used both inside and outside character
       classes. In addition, inside a character  class,  the  sequence  \b  is
       interpreted as the backspace character (hex 08), and the sequence \X is
       interpreted as the character "X".  Outside  a  character  class,  these
       sequences have different meanings (see below).

   Generic character types

       The  third  use of backslash is for specifying generic character types.
       The following are always recognized:

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

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

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

       For  compatibility  with Perl, \s does not match the VT character (code
       11).  This makes it different from the the POSIX "space" class. The  \s
       characters are HT (9), LF (10), FF (12), CR (13), and space (32).

       A "word" character is an underscore or any character less than 256 that
       is a letter or digit. The definition of  letters  and  digits  is  con-
       trolled  by PCRE's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the  pcreapi
       page).  For  example,  in  the  "fr_FR" (French) locale, some character
       codes greater than 128 are used for accented  letters,  and  these  are
       matched by \w.

       In  UTF-8 mode, characters with values greater than 128 never match \d,
       \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
       code character property support is available.

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional escape sequences to match generic character types are  available
       when UTF-8 mode is selected. They are:

        \p{xx}   a character with the xx property
        \P{xx}   a character without the xx property
        \X       an extended Unicode sequence

       The  property  names represented by xx above are limited to the Unicode
       general category properties. Each character has exactly one such  prop-
       erty,  specified  by  a two-letter abbreviation. For compatibility with
       Perl, negation can be specified by including a circumflex  between  the
       opening  brace  and the property name. For example, \p{^Lu} is the same
       as \P{Lu}.

       If only one letter is specified with \p or  \P,  it  includes  all  the
       properties that start with that letter. In this case, in the absence of
       negation, the curly brackets in the escape sequence are optional; these
       two examples have the same effect:

         \p{L}
         \pL

       The following property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       Extended  properties such as "Greek" or "InMusicalSymbols" are not sup-
       ported by PCRE.

       Specifying caseless matching does not affect  these  escape  sequences.
       For example, \p{Lu} always matches only upper case letters.

       The  \X  escape  matches  any number of Unicode characters that form an
       extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

       That is, it matches a character without the "mark"  property,  followed
       by  zero  or  more  characters with the "mark" property, and treats the
       sequence as an atomic group (see below).  Characters  with  the  "mark"
       property are typically accents that affect the preceding character.

       Matching  characters  by Unicode property is not fast, because PCRE has
       to search a structure that contains  data  for  over  fifteen  thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE.

   Simple assertions

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

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at start of subject
         \Z     matches at end of subject or before newline at end
         \z     matches at end of subject
         \G     matches at first matching position in subject

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

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

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect only the behaviour of the circumflex and dollar  metacharacters.
       However,  if the startoffset argument of pcre_exec() is non-zero, indi-
       cating that matching is to start at a point other than the beginning of
       the  subject,  \A  can never match. The difference between \Z and \z is
       that \Z matches before a newline that is  the  last  character  of  the
       string  as well as at the end of the string, whereas \z matches only at
       the end.

       The \G assertion is true only when the current matching position is  at
       the  start point of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times with appropriate argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note,  however,  that  PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end  of  the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one  match
       at a time, it cannot reproduce this behaviour.

       If  all  the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
       PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
       has an entirely different meaning (see below).

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

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

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

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

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


FULL STOP (PERIOD, DOT)

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject, including a non-printing  character,  but  not  (by
       default)  newline.   In  UTF-8 mode, a dot matches any UTF-8 character,
       which might be more than one byte long, except (by default) newline. If
       the  PCRE_DOTALL  option  is set, dots match newlines as well. The han-
       dling of dot is entirely independent of the handling of circumflex  and
       dollar,  the  only  relationship  being  that they both involve newline
       characters. Dot has no special meaning in a character class.


MATCHING A SINGLE BYTE

       Outside a character class, the escape sequence \C matches any one byte,
       both  in  and  out of UTF-8 mode. Unlike a dot, it can match a newline.
       The feature is provided in Perl in order to match individual  bytes  in
       UTF-8  mode.  Because  it  breaks  up  UTF-8 characters into individual
       bytes, what remains in the string may be a malformed UTF-8 string.  For
       this reason, the \C escape sequence is best avoided.

       PCRE  does  not  allow \C to appear in lookbehind assertions (described
       below), because in UTF-8 mode this would make it impossible  to  calcu-
       late the length of the lookbehind.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial. If a closing square bracket is required as a member of the class,
       it should be the first data character in the class  (after  an  initial
       circumflex, if present) or escaped with a backslash.

       A  character  class matches a single character in the subject. In UTF-8
       mode, the character may occupy more than one byte. A matched  character
       must be in the set of characters defined by the class, unless the first
       character in the class definition is a circumflex, in  which  case  the
       subject  character  must  not  be in the set defined by the class. If a
       circumflex is actually required as a member of the class, ensure it  is
       not the first character, or escape it with a backslash.

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

       In  UTF-8 mode, characters with values greater than 255 can be included
       in a class as a literal string of bytes, or by using the  \x{  escaping
       mechanism.

       When  caseless  matching  is set, any letters in a class represent both
       their upper case and lower case versions, so for  example,  a  caseless
       [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
       match "A", whereas a caseful version would. When running in UTF-8 mode,
       PCRE  supports  the  concept of case for characters with values greater
       than 128 only when it is compiled with Unicode property support.

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

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

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

       Ranges operate in the collating sequence of character values. They  can
       also   be  used  for  characters  specified  numerically,  for  example
       [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
       are greater than 255, for example [\x{100}-\x{2ff}].

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
       character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches
       accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
       concept of case for characters with values greater than 128  only  when
       it is compiled with Unicode property support.

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

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see  the
       next  section),  and  the  terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by  [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

         [01[:alpha:]%]

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

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT  character  (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

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

       In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.


VERTICAL BAR

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

         gilbert|sullivan

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


INTERNAL OPTION SETTING

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

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

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

       When  an option change occurs at top level (that is, not inside subpat-
       tern parentheses), the change applies to the remainder of  the  pattern
       that follows.  If the change is placed right at the start of a pattern,
       PCRE extracts it into the global options (and it will therefore show up
       in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern affects only that part of the cur-
       rent pattern that follows it, so

         (a(?i)b)c

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

         (a(?i)b|c)

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

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


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

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

         cat(aract|erpillar|)

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

       2.  It  sets  up  the  subpattern as a capturing subpattern. This means
       that, when the whole pattern  matches,  that  portion  of  the  subject
       string that matched the subpattern is passed back to the caller via the
       ovector argument of pcre_exec(). Opening parentheses are  counted  from
       left  to  right  (starting  from 1) to obtain numbers for the capturing
       subpatterns.

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

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

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

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

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

       the captured substrings are "white queen" and "queen", and are numbered
       1  and 2. The maximum number of capturing subpatterns is 65535, and the
       maximum depth of nesting of all subpatterns, both  capturing  and  non-
       capturing, is 200.

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

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

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


NAMED SUBPATTERNS

       Identifying  capturing  parentheses  by number is simple, but it can be
       very hard to keep track of the numbers in complicated  regular  expres-
       sions.  Furthermore,  if  an  expression  is  modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of  sub-
       patterns,  something  that  Perl  does  not  provide. The Python syntax
       (?P<name>...) is used. Names consist  of  alphanumeric  characters  and
       underscores, and must be unique within a pattern.

       Named  capturing  parentheses  are  still  allocated numbers as well as
       names. The PCRE API provides function calls for extracting the name-to-
       number  translation table from a compiled pattern. There is also a con-
       venience function for extracting a captured substring by name. For fur-
       ther details see the pcreapi documentation.


REPETITION

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

         a literal data character
         the . metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         an escape such as \d that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (unless it is an assertion)

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

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

       In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended  sequences,  each of which may be several bytes long (and they
       may be of different lengths).

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

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

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

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

         (a?)*

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

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

         /\*.*\*/

       to the string

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

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

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

         /\*.*?\*/

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

         \d??\d

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

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

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

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  to Perl's /s) is set, thus allowing the . to match newlines, the
       pattern is implicitly anchored, because whatever follows will be  tried
       against  every character position in the subject string, so there is no
       point in retrying the overall match at any position  after  the  first.
       PCRE normally treats such a pattern as though it were preceded by \A.

       In  cases  where  it  is known that the subject string contains no new-
       lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However,  there is one situation where the optimization cannot be used.
       When .*  is inside capturing parentheses that  are  the  subject  of  a
       backreference  elsewhere in the pattern, a match at the start may fail,
       and a later one succeed. Consider, for example:

         (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth  charac-
       ter. For this reason, such a pattern is not implicitly anchored.

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

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

       has matched "tweedledum tweedledee" the value of the captured substring
       is  "tweedledee".  However,  if there are nested capturing subpatterns,
       the corresponding captured values may have been set in previous  itera-
       tions. For example, after

         /(a|(b))+/

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


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

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

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

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  would
       give up immediately on failing to match "foo" the first time. The nota-
       tion is a kind of special parenthesis, starting with  (?>  as  in  this
       example:

         (?>\d+)foo

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

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

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

       Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists  of  an  additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

         \d++foo

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler forms of atomic group. However, there is no difference  in  the
       meaning  or  processing  of  a possessive quantifier and the equivalent
       atomic group.

       The possessive quantifier syntax is an extension to the Perl syntax. It
       originates in Sun's Java package.

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

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

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

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE and Perl have an optimization that allows  for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

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

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


BACK REFERENCES

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

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

       A back reference matches whatever actually matched the  capturing  sub-
       pattern  in  the  current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

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

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

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

       Back  references  to named subpatterns use the Python syntax (?P=name).
       We could rewrite the above example as follows:

         (?<p1>(?i)rah)\s+(?P=p1)

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

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". Because  there
       may  be  many  capturing parentheses in a pattern, all digits following
       the backslash are taken as part of a potential back  reference  number.
       If the pattern continues with a digit character, some delimiter must be
       used to terminate the back reference. If the  PCRE_EXTENDED  option  is
       set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-
       ments" below) can be used.

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

         (a|b\1)+

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


ASSERTIONS

       An assertion is a test on the characters  following  or  preceding  the
       current  matching  point that does not actually consume any characters.
       The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
       described above.

       More  complicated  assertions  are  coded as subpatterns. There are two
       kinds: those that look ahead of the current  position  in  the  subject
       string,  and  those  that  look  behind  it. An assertion subpattern is
       matched in the normal way, except that it does not  cause  the  current
       matching position to be changed.

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

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

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

         foo(?!bar)

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

         (?!foo)bar

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

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is  with  (?!)  because  an  empty  string
       always  matches, so an assertion that requires there not to be an empty
       string must always fail.

   Lookbehind assertions

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

         (?<!foo)bar

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

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This  is  an  extension  compared  with  Perl (at least for 5.8), which
       requires all branches to match the same length of string. An  assertion
       such as

         (?<=ab(c|de))

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

         (?<=abc|abde)

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the  fixed  width  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the match is deemed to fail.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode)  to appear in lookbehind assertions, because it makes it impossi-
       ble to calculate the length of the lookbehind. The \X escape, which can
       match different numbers of bytes, is also not permitted.

       Atomic  groups can be used in conjunction with lookbehind assertions to
       specify efficient matching at the end of the subject string. Consider a
       simple pattern such as

         abcd$

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

         ^.*abcd$

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

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

       or, equivalently, using the possessive quantifier syntax,

         ^.*+(?<=abcd)

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

   Using multiple assertions

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

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

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

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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


CONDITIONAL SUBPATTERNS

       It  is possible to cause the matching process to obey a subpattern con-
       ditionally or to choose between two alternative subpatterns,  depending
       on  the result of an assertion, or whether a previous capturing subpat-
       tern matched or not. The two possible forms of  conditional  subpattern
       are

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

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

       There are three kinds of condition. If the text between the parentheses
       consists of a sequence of digits, the condition  is  satisfied  if  the
       capturing  subpattern of that number has previously matched. The number
       must be greater than zero. Consider the following pattern,  which  con-
       tains  non-significant white space to make it more readable (assume the
       PCRE_EXTENDED option) and to divide it into three  parts  for  ease  of
       discussion:

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

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

       If the condition is the string (R), it is satisfied if a recursive call
       to  the pattern or subpattern has been made. At "top level", the condi-
       tion is false.  This  is  a  PCRE  extension.  Recursive  patterns  are
       described in the next section.

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

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

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


COMMENTS

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

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


RECURSIVE PATTERNS

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

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

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears. Obviously,  PCRE
       cannot  support  the  interpolation  of Perl code. Instead, it supports
       some special syntax for recursion of the entire pattern, and  also  for
       individual subpattern recursion.

       The  special item that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive call of the subpattern of
       the  given  number, provided that it occurs inside that subpattern. (If
       not, it is a "subroutine" call, which is described  in  the  next  sec-
       tion.)  The special item (?R) is a recursive call of the entire regular
       expression.

       For example, this PCRE pattern solves the  nested  parentheses  problem
       (assume  the  PCRE_EXTENDED  option  is  set  so  that  white  space is
       ignored):

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

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

       If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

         ( \( ( (?>[^()]+) | (?1) )* \) )

       We have put the pattern into parentheses, and caused the  recursion  to
       refer  to them instead of the whole pattern. In a larger pattern, keep-
       ing track of parenthesis numbers can be tricky. It may be  more  conve-
       nient  to use named parentheses instead. For this, PCRE uses (?P>name),
       which is an extension to the Python syntax that  PCRE  uses  for  named
       parentheses (Perl does not provide named parentheses). We could rewrite
       the above example as follows:

         (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )

       This particular example pattern contains nested unlimited repeats,  and
       so  the  use of atomic grouping for matching strings of non-parentheses
       is important when applying the pattern to strings that  do  not  match.
       For example, when this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

       At the end of a match, the values set for any capturing subpatterns are
       those from the outermost level of the recursion at which the subpattern
       value is set.  If you want to obtain  intermediate  values,  a  callout
       function can be used (see the next section and the pcrecallout documen-
       tation). If the pattern above is matched against

         (ab(cd)ef)

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

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

       the string they capture is "ab(cd)ef", the contents of  the  top  level
       parentheses.  If there are more than 15 capturing parentheses in a pat-
       tern, PCRE has to obtain extra memory to store data during a recursion,
       which  it  does  by  using pcre_malloc, freeing it via pcre_free after-
       wards. If  no  memory  can  be  obtained,  the  match  fails  with  the
       PCRE_ERROR_NOMEMORY error.

       Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

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

       In  this  pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and  non-recursive  cases.
       The (?R) item is the actual recursive call.


SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern reference (either by number or
       by name) is used outside the parentheses to which it refers,  it  oper-
       ates  like  a  subroutine in a programming language. An earlier example
       pointed out that the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Such references must, however, follow  the  subpattern  to
       which they refer.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout  points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the  number
       of  the callout, the position in the pattern, and, optionally, one item
       of data originally supplied by the caller of pcre_exec().  The  callout
       function  may cause matching to proceed, to backtrack, or to fail alto-
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE

       In  normal  use  of  PCRE,  if  the  subject  string  that is passed to
       pcre_exec() matches as far as it goes, but is too short  to  match  the
       entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances
       where it might be helpful to distinguish this case from other cases  in
       which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error as soon as a mistake is made, possibly beeping and not
       reflecting the character that has been typed. This  immediate  feedback
       is  likely  to  be a better user interface than a check that is delayed
       until the entire string has been entered.

       PCRE supports the concept of partial matching by means of the PCRE_PAR-
       TIAL  option,  which  can be set when calling pcre_exec(). When this is
       done,  the   return   code   PCRE_ERROR_NOMATCH   is   converted   into
       PCRE_ERROR_PARTIAL  if  at  any  time  during  the matching process the
       entire subject string matched part of the pattern. No captured data  is
       set when this occurs.

       Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
       the last literal byte in a pattern, and abandons  matching  immediately
       if  such a byte is not present in the subject string. This optimization
       cannot be used for a subject string that might match only partially.


RESTRICTED PATTERNS FOR PCRE_PARTIAL

       Because of the way certain internal optimizations  are  implemented  in
       PCRE,  the  PCRE_PARTIAL  option  cannot  be  used  with  all patterns.
       Repeated single characters such as

         a{2,4}

       and repeated single metasequences such as

         \d+

       are not permitted if the maximum number of occurrences is greater  than
       one.  Optional items such as \d? (where the maximum is one) are permit-
       ted.  Quantifiers with any values are permitted after  parentheses,  so
       the invalid examples above can be coded thus:

         (a){2,4}
         (\d)+

       These  constructions  run more slowly, but for the kinds of application
       that are envisaged for this facility, this is not felt to  be  a  major
       restriction.

       If  PCRE_PARTIAL  is  set  for  a  pattern that does not conform to the
       restrictions, pcre_exec() returns the error code  PCRE_ERROR_BADPARTIAL
       (-13).


EXAMPLE OF PARTIAL MATCHING USING PCRETEST

       If  the  escape  sequence  \P  is  present in a pcretest data line, the
       PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
       uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04P
          0: 25jun04
          1: jun
         data> 25dec3P
         Partial match
         data> 3juP
         Partial match
         data> 3jujP
         No match
         data> jP
         No match

       The  first  data  string  is  matched completely, so pcretest shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete pattern, but the first two are partial matches.

Last updated: 08 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

SAVING AND RE-USING PRECOMPILED PCRE PATTERNS

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit more complicated.

       If you save compiled patterns to a file, you can copy them to a differ-
       ent  host  and  run them there. This works even if the new host has the
       opposite endianness to the one on which  the  patterns  were  compiled.
       There  may  be a small performance penalty, but it should be insignifi-
       cant.


SAVING A COMPILED PATTERN
       The value returned by pcre_compile() points to a single block of memory
       that  holds  the compiled pattern and associated data. You can find the
       length of this block in bytes by calling pcre_fullinfo() with an  argu-
       ment  of  PCRE_INFO_SIZE. You can then save the data in any appropriate
       manner. Here is sample code that compiles a pattern and writes it to  a
       file. It assumes that the variable fd refers to a file that is open for
       output:

         int erroroffset, rc, size;
         char *error;
         pcre *re;

         re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

       In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

       Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

       If  the pattern has been studied, it is also possible to save the study
       data in a similar way to the compiled  pattern  itself.  When  studying
       generates  additional  information, pcre_study() returns a pointer to a
       pcre_extra data block. Its format is defined in the section on matching
       a  pattern in the pcreapi documentation. The study_data field points to
       the binary study data,  and  this  is  what  you  must  save  (not  the
       pcre_extra  block itself). The length of the study data can be obtained
       by calling pcre_fullinfo() with  an  argument  of  PCRE_INFO_STUDYSIZE.
       Remember  to check that pcre_study() did return a non-NULL value before
       trying to save the study data.


RE-USING A PRECOMPILED PATTERN

       Re-using a precompiled pattern is straightforward. Having  reloaded  it
       into main memory, you pass its pointer to pcre_exec() in the usual way.
       This should work even on another host, and even if that  host  has  the
       opposite endianness to the one where the pattern was compiled.

       However,  if  you  passed a pointer to custom character tables when the
       pattern was compiled (the tableptr  argument  of  pcre_compile()),  you
       must now pass a similar pointer to pcre_exec(), because the value saved
       with the compiled pattern will obviously be  nonsense.  A  field  in  a
       pcre_extra()  block is used to pass this data, as described in the sec-
       tion on matching a pattern in the pcreapi documentation.

       If you did not provide custom character tables  when  the  pattern  was
       compiled,  the  pointer  in  the compiled pattern is NULL, which causes
       pcre_exec() to use PCRE's internal tables. Thus, you  do  not  need  to
       take any special action at run time in this case.

       If  you  saved study data with the compiled pattern, you need to create
       your own pcre_extra data block and set the study_data field to point to
       the  reloaded  study  data. You must also set the PCRE_EXTRA_STUDY_DATA
       bit in the flags field to indicate that study  data  is  present.  Then
       pass the pcre_extra block to pcre_exec() in the usual way.


COMPATIBILITY WITH DIFFERENT PCRE RELEASES

       The  layout  of the control block that is at the start of the data that
       makes up a compiled pattern was changed for release 5.0.  If  you  have
       any  saved  patterns  that  were compiled with previous releases (not a
       facility that was previously advertised), you will  have  to  recompile
       them  for  release  5.0. However, from now on, it should be possible to
       make changes in a compabible manner.

Last updated: 10 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE PERFORMANCE

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

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a  character's  property.
       If  you  can  find  an  alternative pattern that does not use character
       properties, it will probably be faster.

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

         .*second

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

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

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

         (a+)*

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

       An optimization catches some of the more simple cases such as

         (a+)*b

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

         (a+)*\d

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

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions.

SYNOPSIS OF POSIX API

       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION

       This  set  of  functions provides a POSIX-style API to the PCRE regular
       expression package. See the pcreapi documentation for a description  of
       PCRE's native API, which contains additional functionality.

       The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h  header  file,  and  on  Unix systems the library itself is
       called pcreposix.a, so can be accessed by  adding  -lpcreposix  to  the
       command  for  linking  an application that uses them. Because the POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I have implemented only those option bits that can be reasonably mapped
       to  PCRE  native  options.  In  addition,  the options REG_EXTENDED and
       REG_NOSUB are defined with the value zero. They  have  no  effect,  but
       since  programs that are written to the POSIX interface often use them,
       this makes it easier to slot in PCRE as a  replacement  library.  Other
       POSIX options are not even defined.

       When  PCRE  is  called  via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE options, as described below. "POSIX-like in  style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-byte encoding  domains  it  is  probably
       even less compatible.

       The  header for these functions is supplied as pcreposix.h to avoid any
       potential clash with other POSIX  libraries.  It  can,  of  course,  be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two structure types, regex_t for  compiled  internal  forms,  and  reg-
       match_t  for  returning  captured substrings. It also defines some con-
       stants whose names start  with  "REG_";  these  are  used  for  setting
       options and identifying error codes.


COMPILING A PATTERN

       The  function regcomp() 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. The preg argument is a pointer to a
       regex_t structure that is used as a base for storing information  about
       the compiled expression.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_ICASE

       The PCRE_CASELESS option is set when the expression is passed for  com-
       pilation to the native function.

         REG_NEWLINE

       The PCRE_MULTILINE option is set when the expression is passed for com-
       pilation to the native function. Note that  this  does  not  mimic  the
       defined POSIX behaviour for REG_NEWLINE (see the following section).

       In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the  regex  is  compiled  with  PCRE  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they  aren't)  or
       by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE to obey  POSIX  semantics,  but
       then  PCRE was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching  newline  characters  in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl,  there  is
       no way to stop newline from matching [^a].

       The   default  POSIX  newline  handling  can  be  obtained  by  setting
       PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to  make  PCRE
       behave exactly as for the REG_NEWLINE action.


MATCHING A PATTERN

       The  function  regexec()  is  called  to  match a compiled pattern preg
       against a given string, which is terminated by a zero byte, subject  to
       the options in eflags. These can be:

         REG_NOTBOL

       The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEOL

       The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

       The  portion of the string that was matched, and also any captured sub-
       strings, are returned via the pmatch argument, which points to an array
       of  nmatch  structures of type regmatch_t, containing the members rm_so
       and rm_eo. These contain the offset to the first character of each sub-
       string and the offset to the first character after the end of each sub-
       string, respectively. The 0th element of  the  vector  relates  to  the
       entire  portion  of string that was matched; subsequent elements relate
       to the capturing subpatterns of the regular expression. Unused  entries
       in the array have both structure members set to -1.

       A  successful  match  yields  a  zero  return;  various error codes are
       defined in the header file, of  which  REG_NOMATCH  is  the  "expected"
       failure code.


ERROR MESSAGES

       The regerror() function maps a non-zero errorcode from either regcomp()
       or regexec() to a printable message. If preg is  not  NULL,  the  error
       should have arisen from the use of that structure. A message terminated
       by a binary zero is placed  in  errbuf.  The  length  of  the  message,
       including  the  zero, is limited to errbuf_size. The yield of the func-
       tion is the size of buffer needed to hold the whole message.


MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with  the preg structure. The function regfree() frees all such
       memory, after which preg may no longer be used as  a  compiled  expres-
       sion.


AUTHOR

       Philip Hazel <ph10@cam.ac.uk>
       University Computing Service,
       Cambridge CB2 3QG, England.

Last updated: 07 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------

PCRE(3)                                                                PCRE(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE SAMPLE PROGRAM

       A simple, complete demonstration program, to get you started with using
       PCRE, is supplied in the file pcredemo.c in the PCRE distribution.

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

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       If PCRE is installed in the standard include  and  library  directories
       for  your  system, you should be able to compile the demonstration pro-
       gram using this command:

         gcc -o pcredemo pcredemo.c -lpcre

       If PCRE is installed elsewhere, you may need to add additional  options
       to  the  command line. For example, on a Unix-like system that has PCRE
       installed in /usr/local, you  can  compile  the  demonstration  program
       using a command like this:

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

       Once  you  have  compiled the demonstration program, you can run simple
       tests like this:

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

       Note that there is a  much  more  comprehensive  test  program,  called
       pcretest,  which  supports  many  more  facilities  for testing regular
       expressions and the PCRE library. The pcredemo program is provided as a
       simple coding example.

       On some operating systems (e.g. Solaris), when PCRE is not installed in
       the standard library directory, you may get an error like this when you
       try to run pcredemo:

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

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

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.

Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
-----------------------------------------------------------------------------