my $string = 'tu0   1500  <Link>      00-00-f8-05-c8-20';

$string =~ m/^.+
            ( # Start back reference capture for MAC Address
            ((?:(\d{1,2}|[a-fA-F]{1,2}){2})(?::|-*)){6}
            ) # End back reference
         $/xms;

print "$1\n" if (defined $1);