m/(^[a-z0-9]{4})$/

##</code><code>##

#!/usr/bin/perl

use warnings;
use strict;

use YAPE::Regex::Explain;

my $re = qq[m!(^[a-z0-9]{4})$!];

my $yape = YAPE::Regex::Explain->new($re);

print $yape->explain;


__END__

The regular expression:

(?-imsx:m!(^[a-z0-9]{4}))

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):
----------------------------------------------------------------------
  m!                       'm!'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
    [a-z0-9]{4}              any character of: 'a' to 'z', '0' to '9'
                             (4 times)
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------