sub gen_regex {
    my ($target, $n) = @_;

    my @chars = split //, $target;
    my $ra    = Regexp::Assemble->new();
    my $iter  = combinations( $n => [ 0 .. $#chars ] );

    while (my @c = $iter->()) {
       local @_ = @chars; 
       $_[$_] = '.' for @c;
       $ra->add( join '', @_ );
    }

    $ra->re();
}