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During the first iteration, the first of those lines will come down to:
In words, the first of the query strings is exclusive-Or'd (^) with the first 5 characters of the first target string.
Where the aligned characters in those two string are the same, the result of xoring them will be zero. Where the two characters differ, the result will be non-zero:
In order to know how many characters matched, all we need to do is count the zeros, which is efficiently done using the transliteration operator:
The result of the first line is that $matched is assigned the number of characters that matched between the two strings.
As you want all characters, except one in any position, to match, the second line tests the number that matched against the length of the query string minus 1:
If you know all your query strings are always five characters, then you could hard code the value 4 instead of ( length( $q ) -1 ). Ditto for the usage of length in the substr, but it never hurts to be flexible if there is little cost involved in doing so.
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In reply to Re^3: string match using with an N in any position