sub test_regexps2 {
    my ($seq1, $seq2, $window, $mismatch) = @_;
    my $retval = '';
    my %cache;
    my @mask = (0) x (length ($seq2) - $window);

    for my $start (0 .. (length ($seq1) - $window)) {
        my $part = substr ($seq1, $start, $window);

        $retval .= $cache{$part} ||= do {
            my $regex = build_regexp ($part, $mismatch);
            my @res = @mask;

            while ($seq2 =~ m/(?=$regex)/g) {
                $res[ pos $seq2 ] = 1;
            }
            join '', @res, "\n";
        };
    }

    $retval;
}