++ed by:

2 PAUSE users
2 non-PAUSE users.

Author image Dan Kogai


Regexp::List - builds regular expressions out of a list of words


  use Regexp::List;
  my $l  = Regexp::List->new;
  my $re = $l->list2re(qw/foobar fooxar foozap fooza/);
  # $re is now qr/foo(?:[bx]ar|zap?)/


This module offers list2re method that turns a list of words into an optimized regular expression which matches all words therein.

The optimized regular expression is much more efficient than a simple-minded '|'-concatenation thereof.


This module use Object-Oriented approach so you can use this module as a base and tweak its features. This module is a base class of Regexp::Optimizer.


Since this is an OO module there is no symbol exported.


This module offers methods below;

$l = Regexp::List->new(key=>value, ...)

Constructor. When arguments are fed in key => value, manner, it sets attributes. See $l->set for details

$re = $l->list2re(list of words ...)

Does the job. Takes a list of words and turn it into an optimal regular expresson. See "IMPLEMENTATION" to find out how it is achieved. If you want to know the underlying black magic even further, see the source.

$l->set(key => value, ...)

Sets attributes. There are many attributes supported but let me mention just a few that you may be interested.


Whether you prepend a lookahead assertion or not. Default value is 1. This module is smart enough to omit the assertion when you don't need one.

  $re = $l->list2re(qw/1 2 3 infinity/);
  # qr/(?=[123i])(?:[123]|infinity)/
  $re = $l->set(lookahead=>0)->list2re(qw/1 2 3 infinity/);
  # qr/(?:[123]|infinity)/

Whether you quote metacharacters or not. Default is 1. If you really need this feature try Regexp::Optimizer instead.

  @list = qw/3 3.14 3.14159265358979/;
  $re = $l->list2re(@list);
  # qr/3(?:\.14(?:159265358979)?)?)/
  $re = $l->set(lookahead=>0)->list2re(@list);
  # qr/3(?:.14(?:159265358979)?)?)/
  # which does match 3.14 but also "11+3=14"

Currently it accepts 'i', 'm', 's', and 'x', the same as regular expression modifiers.

  @list = qw/Perl perl BASIC basic/;
  $re = $l->list2re(@list);
  # qr/(?=[BPbp])(?:[Pp]erl|BASIC|basic)/
  $re = $l->set(modifiers => 'i')->list2re(@list);
  # qr/(?=[bp])(?:perl|basic)/i
  print $l->set(modifiers => 'x')->list2re(@list);
  # Try for yourself;  Isn't itcute ?

Utility methods to expand a regular expression. Handy when you want to check the complex regexes.


Utility methods to unexpand a regular expression.


This module optimizes the regular expression as follows. Let's see what happens when qw/foobar fooxar foozap fooza/ is fed

trie building (common prefix aggregation)

first the corresponding trie structure is built

       +- bar
  foo -+- xar
       +- za -+- p
              +- ''
common suffix aggregation

it checks if any leaf node can be optimized for common suffix. In this case we can do so to "bar" and "xar".

       +- b -+-ar
  foo -+- x -+
       +- za -+- p
              +- ''
character class conversion

If a branch contains more than two single characters, it turns it into a character class.

  foo -+- [bx] --- ar
       +- za -+-p
              +- ''
empty leaf to ?

Empty leaf is converted to a '?' quantifier

  foo -+- [bx] --- ar
       +- za -+-p?
join all leafs into a group

And the final result is reached.



This module is faily robust. You can practically use this module to find a regular expression that matches all words in a dictionary. Here is a result by on perl 5.8.0, FreeBSD 4-Stable, Pentium III 800 Mhz with 512 MB RAM.

 # Sat May 31 09:11:06 2003 (     0.000000 s) Reading /usr/share/dict/words
 # Sat May 31 09:11:07 2003 (     0.847797 s) 235881 lines read.
 # Sat May 31 09:11:07 2003 (     0.000000 s) Making regexp.
 # Sat May 31 09:13:09 2003 (   121.596928 s) Done.
 # Sat May 31 09:13:09 2003 (     0.000000 s) Saving to t/words.rx
 # Sat May 31 09:13:09 2003 (     0.000000 s) Reading t/words.rx
 # Sat May 31 09:13:13 2003 (     3.679176 s) Done.
 # Sat May 31 09:13:13 2003 (     0.000000 s) Opening /usr/share/dict/words for comparison.
 # Sat May 31 09:13:13 2003 (     0.255222 s) /usr/share/dict/words:235881 lines found.
 # Sat May 31 09:13:13 2003 (     0.000000 s) Showtime!
 # 235881/235881
 # Sat May 31 10:44:17 2003 (  5464.370409 s) Done.
 # Sat May 31 10:44:17 2003 (  5464.370624 s) 43.167 matches/s

The result of optimization is obvious as the number of alteration increases. Here is a result of a benchmark which matches randomly picked words against /usr/share/dict/words.

  ====        2 words
          Rate naive optim
  naive 1.79/s    --  -28%
  optim 2.49/s   39%    --

  ====      256 words
        s/iter naive optim
  naive   31.7    --  -81%
  optim   5.95  433%    --


Regexp::Optimizer -- uses this module as its base

eg/ directory in this package contains example scripts.

Perl standard documents

perltodo, perlre

CPAN Modules

Regexp::Presuf, Text::Trie


Mastering Regular Expressions http://www.oreilly.com/catalog/regex2/


Dan Kogai <dankogai@dan.co.jp>


Copyright 2003 by Dan Kogai, All Rights Reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.