my @in_order = ($str =~ m{ 
       ^(?> .*?              ([abcd]) )
        (?> .*? (?!\1)       ([abcd]) )
        (?> .*? (?!\1|\2)    ([abcd]) )
        (?> .*? (?!\1|\2|\3) ([abcd]) )
    }x);

##</code><code>##

  sub four_or_less {
    my ($s) = @_ ;

    $s =~ tr/abcd//cd ;

    my $c = '' ;
    $c .= $1 while ($s =~ m/([^ $c])/g)  ;

    return $c ;
  }