/^(?:$part:){5}$part$/;
  my @parts = split /:/;

##</code><code>##

  my @parts = /^($part):($part):($part):($part):($part):($part)$/

##</code><code>##

  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;
  }

##</code><code>##

  my $re_mac_ihb = qr/(?:$part:){5}$part/ . '';
  sub replace_ihb {
    /^ $re_mac_ihb $/x;
    $_ = sprintf '%02s' x 6, split /:/;
  }

##</code><code>##

  /^(...$)/m

##</code><code>##

  $_ = 'a ' x 1000;
  s/(\s+)$//; # Slower
  s/(\s+$)//; # Faster