Try this one:
((?:(\d{1,2}|[a-fA-F]{1,2}){2})(?::|-*)){6}
Example Usage (extract MAC address):

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);
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NB: This will also work with MAC like:
my $string = 'tu0   1500  <Link>  00:00:f8:05:c8:20';