my $re_mac_bUK = '(?: ( [0-9A-Z]{1,2} ) : )' x 5 . '( [0-9A-Z]{1,2} )';
  sub replace_bUK {
    s[^ $re_mac_bUK $]
     [ sprintf '%02s' x 6, $1, $2, $3, $4, $5, $6 ]ex;
  }