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Bindings to the Perl Compatibility Regular Expressions library







PCRE-OCaml - Perl Compatibility Regular Expressions for OCaml

This OCaml-library interfaces the C-library
PCRE (Perl-compatibility Regular Expressions). It can be
used for string matching with "PERL"-style regular expressions.


PCRE-OCaml offers the following functionality for operating on strings:

  • Searching for patterns

  • Extracting subpatterns

  • Splitting strings according to patterns

  • Pattern substitution

Other reasons to use PCRE-OCaml:

  • The PCRE-library by Philip Hazel has been under development for many
    years and is fairly advanced and stable. It implements just about all
    of the functionality that can be found in PERL regular expressions.
    The higher-level functions written in OCaml (split, replace, etc.),
    too, are compatible with the corresponding PERL-functions to the extent
    that OCaml allows. Most people find the syntax of PERL-style regular
    expressions more straightforward and powerful than the Emacs-style regular
    expressions used in the Str-module in the standard OCaml distribution.

  • PCRE-OCaml is reentrant and thus thread-safe, which is not the case
    for the Str-module in the OCaml standard library. Using reentrant
    libraries also means more convenience for programmers. They do not
    have to reason about states in which the library might be in.

  • The high-level functions for replacement and substitution, which are all
    implemented in OCaml, are much faster than the ones in the Str-module.
    In fact, when compiled to native code, they even seem to be significantly
    faster than those found in PERL (PERL is written in C).

  • You can rely on the data returned being unique. In other terms: if
    the result of a function is a string, you can safely use destructive
    updates on it without having to fear side effects.

  • The interface to the library makes use of labels and default arguments
    to give you a high degree of programming comfort.


Please consult the API
for details.

A general concept the user may need to understand is that most functions
allow for two different kinds of flags:

  1. "Convenience"-flags that make for readable and concise code, but which
    need to be translated to an internal representation on each call.

    let rex = Pcre.regexp ~flags:[`ANCHORED; `CASELESS] "some pattern" in
    (* ... *)

    This makes it easy to pass flags on the fly. They will be translated to
    the internal format automatically. However, if this happens to be in a
    loop, this translation will occur on each iteration. If you really need
    to save as much performance as possible, you should use the next approach.

  2. "Internal" flags that need to be defined and translated from
    "convenience"-flags before function calls, but which allow for optimum
    performance in loops. Example:

    let iflags = Pcre.cflags [`ANCHORED; `CASELESS] in
    for i = 1 to 1000 do
      let rex = Pcre.regexp ~iflags "some pattern constructed at runtime" in
      (* ... *)

    Factoring out the translation of flags for regular expressions may
    save some cycles, but don't expect too much. You can save more CPU
    time when lifting the creation of regular expressions out of loops.
    Example for what not to do:

    for i = 1 to 1000 do
      let chunks = Pcre.split ~pat:"[ \t]+" "foo bar" in
      (* ... *)


    let rex = Pcre.regexp "[ \t]+" in
    for i = 1 to 1000 do
      let chunks = Pcre.split ~rex "foo bar" in
      (* ... *)

The provided functions use optional arguments with intuitive defaults. For
example, the Pcre.split-function will assume whitespace as pattern. The
examples-directory contains a few example applications demonstrating the
functionality of PCRE-OCaml.

Restartable (partial) pattern matching

PCRE includes an "alternative" DFA match function that allows one to restart
a partial match with additional input. This is exposed by pcre-ocaml via
the pcre_dfa_exec function. While this cannot be used for "higher-level"
operations like extracting submatches or splitting subject strings, it can
be very useful in certain streaming and search use cases.

This utop interaction demonstrates the basic workflow of a partial match
that is then restarted multiple times before completing successfully:

utop # open Pcre;;
utop # let rex = regexp "12+3";;
val rex : regexp = <abstr>
utop # let workspace = Array.make 40 0;;
val workspace : int array =
  [|0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0;
    0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0|]
utop # pcre_dfa_exec ~rex ~flags:[`PARTIAL] ~workspace "12222";;
Exception: Pcre.Error Partial.
utop # pcre_dfa_exec ~rex ~flags:[`PARTIAL; `DFA_RESTART] ~workspace "2222222";;
Exception: Pcre.Error Partial.
utop # pcre_dfa_exec ~rex ~flags:[`PARTIAL; `DFA_RESTART] ~workspace "2222222";;
Exception: Pcre.Error Partial.
utop # pcre_dfa_exec ~rex ~flags:[`PARTIAL; `DFA_RESTART] ~workspace "223xxxx";;
- : int array = [|0; 3; 0|]

Please refer to the documentation of pcre_dfa_exec and check out the
dfa_restart example for more info.

Contact Information and Contributing

Please submit bugs reports, feature requests, contributions and similar to
the GitHub issue tracker.

Up-to-date information is available at: https://mmottl.github.io/pcre-ocaml