07-24-2008
Interesting and thoughtful question. You use "(" and ")" to mark (remember) a pattern and recall the remembered pattern with "\" followed by a single digit (back reference).
In your particular case, "(.)\1" means remember a character and recall the character.
You can extend this method to find words with multiple double letters. '(.)\1(.)\2(.)\3' will match any word with three double letters, e.g. bookkeeper.
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LEARN ABOUT DEBIAN
graphviz::regex
GraphViz::Regex(3pm) User Contributed Perl Documentation GraphViz::Regex(3pm)
NAME
GraphViz::Regex - Visualise a regular expression
SYNOPSIS
use GraphViz::Regex;
my $regex = '(([abcd0-9])|(foo))';
my $graph = GraphViz::Regex->new($regex);
print $graph->as_png;
DESCRIPTION
This module attempts to visualise a Perl regular expression. Understanding regular expressions is tricky at the best of times, and regexess
almost always evolve in ways unforseen at the start. This module aims to visualise a regex as a graph in order to make the structure clear
and aid in understanding the regex.
The graph visualises how the Perl regular expression engine attempts to match the regex. Simple text matches or character classes are
represented by.box-shaped nodes. Alternations are represented by a diamond-shaped node which points to the alternations. Repetitions are
represented by self-edges with a label of the repetition type (the nodes being repeated are pointed to be a full edge, a dotted edge points
to what to match after the repetition). Matched patterns (such as $1, $2, etc.) are represented by a 'START $1' .. 'END $1' node pair.
This uses the GraphViz module to draw the graph.
METHODS
new
This is the constructor. It takes one mandatory argument, which is a string of the regular expression to be visualised. A GraphViz object
is returned.
my $graph = GraphViz::Regex->new($regex);
as_*
The regex can be visualised in a number of different graphical formats. Methods include as_ps, as_hpgl, as_pcl, as_mif, as_pic, as_gd,
as_gd2, as_gif, as_jpeg, as_png, as_wbmp, as_ismap, as_imap, as_vrml, as_vtx, as_mp, as_fig, as_svg. See the GraphViz documentation for
more information. The two most common methods are:
# Print out a PNG-format file
print $g->as_png;
# Print out a PostScript-format file
print $g->as_ps;
BUGS
Note that this module relies on debugging information provided by Perl, and is known to fail on at least two versions of Perl: 5.005_03 and
5.7.1. Sorry about that - please use a more recent version of Perl if you want to use this module.
AUTHOR
Leon Brocard <acme@astray.com>
COPYRIGHT
Copyright (C) 2000-1, Leon Brocard
This module is free software; you can redistribute it or modify it under the same terms as Perl itself.
perl v5.14.2 2012-04-02 GraphViz::Regex(3pm)