NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

INTRODUCTION

       PCRE2 is the name used for a revised API for the PCRE library, which is a set of functions, written in C,
       that implement regular expression pattern matching using the same syntax and semantics as Perl, with just
       a  few  differences.  Some features that appeared in Python and the original PCRE before they appeared in
       Perl are also available using the Python syntax. There is also some support  for  one  or  two  .NET  and
       Oniguruma  syntax  items,  and  there  are  options  for  requesting  some minor changes that give better
       ECMAScript (aka JavaScript) compatibility.

       The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit  code  units,  which  means
       that  up  to  three  separate libraries may be installed.  The original work to extend PCRE to 16-bit and
       32-bit code units was done by Zoltan Herczeg and Christian Persch,  respectively.  In  all  three  cases,
       strings can be interpreted either as one character per code unit, or as UTF-encoded Unicode, with support
       for Unicode general category properties. Unicode support is optional at build time (but is the  default).
       However,  processing  strings  as  UTF  code units must be enabled explicitly at run time. The version of
       Unicode in use can be discovered by running

         pcre2test -C

       The three libraries contain identical  sets  of  functions,  with  names  ending  in  _8,  _16,  or  _32,
       respectively (for example, pcre2_compile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
       32, a program that  uses  just  one  code  unit  width  can  be  written  using  generic  names  such  as
       pcre2_compile(), and the documentation is written assuming that this is the case.

       In addition to the Perl-compatible matching function, PCRE2 contains an alternative function that matches
       the same compiled patterns in a different way. In certain circumstances,  the  alternative  function  has
       some advantages.  For a discussion of the two matching algorithms, see the pcre2matching page.

       Details of exactly which Perl regular expression features are and are not supported by PCRE2 are given in
       separate documents. See the pcre2pattern and  pcre2compat  pages.  There  is  a  syntax  summary  in  the
       pcre2syntax page.

       Some  features  of  PCRE2  can  be  included,  excluded,  or  changed  when  the  library  is  built. The
       pcre2_config() function makes it possible for a client to discover  which  features  are  available.  The
       features  themselves are described in the pcre2build page. Documentation about building PCRE2 for various
       operating systems can be found in the README and NON-AUTOTOOLS_BUILD files in the source distribution.

       The libraries contains a number of undocumented internal functions and data tables that are used by  more
       than  one  of  the  exported  external functions, but which are not intended for use by external callers.
       Their names all begin with "_pcre2",  which  hopefully  will  not  provoke  any  name  clashes.  In  some
       environments,  it  is  possible  to  control which external symbols are exported when a shared library is
       built, and in these cases the undocumented symbols are not exported.

SECURITY CONSIDERATIONS

       If you are using PCRE2 in a non-UTF application that permits  users  to  supply  arbitrary  patterns  for
       compilation,  you  should  be  aware  of a feature that allows users to turn on UTF support from within a
       pattern. For example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8  mode,  which  interprets
       patterns  and subjects as strings of UTF-8 code units instead of individual 8-bit characters. This causes
       both the pattern and any data against which it is matched to be checked for UTF-8 validity. If  the  data
       string  is  very long, such a check might use sufficiently many resources as to cause your application to
       lose performance.

       One way of guarding against this possibility is to use the pcre2_pattern_info()  function  to  check  the
       compiled  pattern's  options  for  PCRE2_UTF.  Alternatively, you can set the PCRE2_NEVER_UTF option when
       calling pcre2_compile(). This causes a compile time error if the pattern contains a UTF-setting sequence.

       The use of Unicode properties for character types such as \d can also be enabled from within the pattern,
       by specifying "(*UCP)". This feature can be disallowed by setting the PCRE2_NEVER_UCP option.

       If  your application is one that supports UTF, be aware that validity checking can take time. If the same
       data string is to be matched many times, you can use the PCRE2_NO_UTF_CHECK option  for  the  second  and
       subsequent matches to avoid running redundant checks.

       The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to problems, because it may leave
       the current matching point in the middle of  a  multi-code-unit  character.  The  PCRE2_NEVER_BACKSLASH_C
       option  can  be  used  by an application to lock out the use of \C, causing a compile-time error if it is
       encountered. It is also possible to build PCRE2 with the use of \C permanently disabled.

       Another way that performance can be hit is by running a pattern that has a very large search tree against
       a  string  that  will  never  match.  Nested  unlimited  repeats in a pattern are a common example. PCRE2
       provides some protection against this: see the pcre2_set_match_limit() function  in  the  pcre2api  page.
       There  is  a  similar  function called pcre2_set_depth_limit() that can be used to restrict the amount of
       memory that is used.

USER DOCUMENTATION

       The user documentation for PCRE2 comprises a number of different sections. 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, the descriptions of the pcre2grep and pcre2test programs are  in  files  called
       pcre2grep.txt  and  pcre2test.txt, respectively. The remaining sections, except for the pcre2demo section
       (which is a program listing),  and  the  short  pages  for  individual  functions,  are  concatenated  in
       pcre2.txt, for ease of searching. The sections are as follows:

         pcre2              this document
         pcre2-config       show PCRE2 installation configuration information
         pcre2api           details of PCRE2's native C API
         pcre2build         building PCRE2
         pcre2callout       details of the callout feature
         pcre2compat        discussion of Perl compatibility
         pcre2demo          a demonstration C program that uses PCRE2
         pcre2grep          description of the pcre2grep command (8-bit only)
         pcre2jit           discussion of just-in-time optimization support
         pcre2limits        details of size and other limits
         pcre2matching      discussion of the two matching algorithms
         pcre2partial       details of the partial matching facility
         pcre2pattern       syntax and semantics of supported regular
                              expression patterns
         pcre2perform       discussion of performance issues
         pcre2posix         the POSIX-compatible C API for the 8-bit library
         pcre2sample        discussion of the pcre2demo program
         pcre2syntax        quick syntax reference
         pcre2test          description of the pcre2test command
         pcre2unicode       discussion of Unicode and UTF support

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

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

       Putting an actual email address here is a spam magnet. If you want to email  me,  use  my  two  initials,
       followed by the two digits 10, at the domain cam.ac.uk.

REVISION

       Last updated: 01 April 2017
       Copyright (c) 1997-2017 University of Cambridge.