If you are trying to figure out regular expressions, I highly recommend japhy's module YAPE::Regex::Explain.

When installing it you will have trouble unless you first install YAPE::Regex first. There is a dependency there, but he didn't properly indicate it. I reported that bug.

If you have trouble installing things (eg you are on Windows and aren't familiar with CPAN and CPANPLUS) you can get the sources by following these links to .\YAPE\Regex.pm and .\YAPE\Regex\Explain.pm. Save those files with those paths (I assumed a Windows delimiter) and then write the following script:

#! perl
use strict;
use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(shift)->explain;
[download]

And now you can get explanations like the following:

tilly@localhost:~$ perl re-explain foo    
The regular expression:

(?-imsx:foo)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  foo                      'foo'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
tilly@localhost:~$ perl re-explain '(foo|bar)'
The regular expression:

(?-imsx:(foo|bar))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    foo                      'foo'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    bar                      'bar'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

(I actually ran this under Linux. On Windows you will want to quote the RE with ", not '.)

This makes it easier to for beginners to understand what a given regular explanation should do.