use Modern::Perl '2014'; use Algorithm::Permute; my @elements = qw/ A C T G /; my @permutations; Algorithm::Permute::permute { push @permutations, ( join '(?:', @elements ) . ')?)?)?'} @elements; my $regex = join '|', @permutations; $regex = qr/^(?:$regex)$/; say "Testing"; for my $length ( 1 .. 4 ) { my $p = new Algorithm::Permute( [ qw/ A C T G A C T G A C T G A C T G X /, ' ' ], $length ); while ( my @res = $p->next ) { my $test = join( '', @res ); print "$test\t"; say $test=~ /$regex/ ? 'Accept' : 'Reject'; } }