could you just explain what's going on inside the map function please?

Sure.

sub fuzzyMatch {
    my( $rHay, $rNee, $misses ) = @_;
    my $lNee = length $$rNee;
    my $min = $lNee - $misses;

    map {
        (
             ( substr( $$rHay, $_, $lNee ) ^ $$rNee ) =~ tr[\0][] 
        ) >= $min ? $_ : ()
    } 0 .. length( $$rHay ) - $lNee;
}
[download]

• We need to compare the needle against the haystack at each position.

  Hence the map counter runs from 0 to length( haystack) - length( needle ).

• We need to compare the same number of characters from the haystack as there are in the needle.

  Hence, the substr presents a needle length substring of haystack at each of those counter positions.

• We don't just want a yes/no comparison; we need a count of the differences.

  So we bit-wise xor (^) the substring and the needle.

  The result is a string that has a 0 (null) byte wherever the two strings match; and some other byte value where they do not.

• We need to count the zero bytes.

  tr[\0][] does that efficiently.

• If the count of matched bytes is greater than the minimum required (length( needle ) - misses)

  return the position where the match occurred ($_), otherwise return nothing (()).

Hope that clarifies things little. Continue to ask about anything that isn't clear.

Hi, Thank you again for all your patience and helping me. Can you tell me why the map counter is from 0 to length(hay)-length(nee) and why not just from 0 to length(nee)?

why not just from 0 to length(nee)?

Because if you compare at position lenght( hay), you aren't comparing anything.

Take the case of a 20-byte haystack:acgtacgtacgtacgtacgt and a 4-byte needle: acct; at position 20:

000000001111111111112
012345678901234567890
acgtacgtacgtacgtacgt
                    acct
[download]

The last position you can get a full match is at 20-4 position 16:

000000001111111111112
012345678901234567890
acgtacgtacgtacgtacgt
                acct
[download]

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