($domain) = $url =~ m|www.([A-Z a-z 0-9]+.{3}).|x;

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

my $url = 'www.example.com.au';

my ($domain) = $url =~ m|www.([A-Z a-z 0-9]+.{3}).|x;
print '$domain=[', defined $domain ? $domain : '<undef>', ']';

##</code><code>##

$domain=[example.co]

##</code><code>##

my ($alpha_num, $any_three, $final_dot)
    = $url =~ m|www.([A-Z a-z 0-9]+)(.{3})(.)|x;
print '$alpha_num=[', defined $alpha_num ? $alpha_num : '<undef>', ']';
print '$any_three=[', defined $any_three ? $any_three : '<undef>', ']';
print '$final_dot=[', defined $final_dot ? $final_dot : '<undef>', ']';

##</code><code>##

$alpha_num=[example]
$any_three=[.co]
$final_dot=[m]