When I get stuck on a regex, I use YAPE::Regex::Explain:

#!/usr/bin/perl

use strict;
use warnings;
use ExtUtils::MakeMaker qw(prompt);
use YAPE::Regex::Explain;

my $re  = prompt("Enter regexp: ");
my $exp = YAPE::Regex::Explain->new($re)->explain;
print $exp, "\n";
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