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

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

    return $c ;
  }