$dom = qr{
          [a-z]       # starts with a letter
          [a-z0-9-]*  # zero or more letters, numbers, hyphens
         }xi;

##</code><code>##

$au = qr{ $dom \.   # company
          $dom \.   # subdomain
          au        # ccTLD
          \. ?      # optional trailing dot
          \b        # word break
         }xi;

##</code><code>##

$com = qr{ $dom \.  # company
           com      # gTLD
           \. ?     # optional trailing dot
           \b       # word break
         }xi;

##</code><code>##

my $domain = qr{ (?:        # start of group
                   $dom \.  # company subdomain
                 ) ?        # end of group, make it optional
                 (?:        # start of group
                   $com     # .com pattern
                   |        # or
                   $au      # .au pattern
                 )          # end of group
               }xi;

##</code><code>##

use strict;
use warnings;

use Test::More 'no_plan';

my $dom = qr{
              [a-z]       # starts with a letter
              [a-z0-9-]*  # zero or more letters, numbers, hyphens
            }xi;
my $au = qr{ $dom \.   # company
             $dom \.   # subdomain
             au        # ccTLD
             \. ?      # optional trailing dot
             \b        # word break
            }xi;
my $com = qr{ $dom \.  # company
              com      # gTLD
              \. ?     # optional trailing dot
              \b       # word break
             }xi;
my $domain = qr{
                 (?:         # start of group
                   $dom \.   # company subdomain
                 ) ?         # end of group, make it optional
                 (?:         # start of group
                   $com      # .com pattern
                   |         # or
                   $au       # .au pattern
                 )           # end of group
                }xi;

my @no_match = qw( illegal_underscore.com
                   example.unreal.dom
                   too-short.au
                   0digits-allowed.com
              );

foreach my $bad ( @no_match ) {
    unlike( $bad, $domain, "no match: '$bad'" );
}

my %match_for = ( 'foo.sub.example.com'    => 'sub.example.com',
                  'bar.sub.example.com.au' => 'sub.example.com.au',
             );

while ( my ( $stuff, $find ) = each %match_for ) {
    my $test_name = "match: '$stuff'";
    if ( $stuff =~ qr{ ( $domain ) }x ) {
        my $matched = $1;
        pass( $test_name );
        is( $matched, $find, "found: '$find'" );
    }
    else {
        fail( $test_name );
    }
}