https://github.com/ocaml/ocaml-re

Pure OCaml regular expressions, with support for Perl and POSIX-style strings
https://github.com/ocaml/ocaml-re

dfa ocaml regular-expression

Last synced: about 1 month ago
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Pure OCaml regular expressions, with support for Perl and POSIX-style strings

• Host: GitHub
• URL: https://github.com/ocaml/ocaml-re
• Owner: ocaml
• License: other
• Created: 2012-01-05T03:14:49.000Z (almost 13 years ago)
• Default Branch: master
• Last Pushed: 2024-04-14T00:06:47.000Z (7 months ago)
• Last Synced: 2024-04-14T02:36:35.304Z (7 months ago)
• Topics: dfa, ocaml, regular-expression
• Language: OCaml
• Homepage:
• Size: 1.41 MB
• Stars: 224
• Watchers: 8
• Forks: 63
• Open Issues: 37
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGES.md
  • License: LICENSE.md

Awesome Lists containing this project

• awesome-list - ocaml-re - style strings | ocaml | 180 | (OCaml)

README

        
Description

===========

Re is a regular expression library for OCaml.

[![Build status](https://github.com/ocaml/ocaml-re/actions/workflows/main.yml/badge.svg)](https://github.com/ocaml/ocaml-re/actions/workflows/main.yml)

Contact

=======

This library has been written by Jerome Vouillon

([email protected]).

It can be downloaded from 

Bug reports, suggestions and contributions are welcome.

Features

========

The following styles of regular expressions are supported:

- Perl-style regular expressions (module `Re.Perl`);

- Posix extended regular expressions (module `Re.Posix`);

- Emacs-style regular expressions (module `Re.Emacs`);

- Shell-style file globbing (module `Re.Glob`).

It is also possible to build regular expressions by combining simpler regular

expressions (module `Re`).

The most notable missing features are **back-references** and

look-ahead/look-behind **assertions**.

There is also a subset of the PCRE interface available in the `Re.Pcre` module.

This makes it easier to port code from that library to Re with minimal changes.

Performances

============

The matches are performed by lazily building a DFA (deterministic

finite automaton) from the regular expression. As a consequence,

matching takes linear time in the length of the matched string.

The compilation of patterns is slower than with libraries using

back-tracking, such as PCRE.  But, once a large enough part of the

DFA is built, matching is extremely fast.

Of course, for some combinations of regular expression and string, the

part of the DFA that needs to be build is so large that this point is

never reached, and matching will be slow.  This is not expected to

happen often in practice, and actually a lot of expressions that

behaves badly with a backtracking implementation are very efficient

with this implementation.

The library is at the moment entirely written in OCaml.  As a

consequence, regular expression matching is much slower when the

library is compiled to bytecode than when it is compiled to native

code.

Here are some timing results (Pentium III 500Mhz):

* Scanning a 1Mb string containing only `a`s, except for the last

  character which is a `b`, searching for the pattern `aa?b`

  (repeated 100 times):

    - RE: 2.6s

    - PCRE: 68s

* Regular expression example from http://www.bagley.org/~doug/shootout/ [1]

    - RE: 0.43s

    - PCRE: 3.68s

  [1] this page is no longer up but is available via the Internet Archive 

  http://web.archive.org/web/20010429190941/http://www.bagley.org/~doug/shootout/bench/regexmatch/

* The large regular expression (about 2000 characters long) that

  Unison uses with my preference file to decide whether a file should

  be ignored or not.  This expression is matched against a filename

  about 20000 times.

    - RE: 0.31s

    - PCRE: 3.7s

  However, RE is only faster than PCRE when there are more than about

  300 filenames.