Earlier in the subroutine, values of $f and $s where the length differed were removed. Also, if $f and $s are the same value, they are removed as well. Then comes the line:

print "$f-", substr ($s => length +(("$f" ^ "$s") =~ /^(\x00*)/) [0]),
+ "\n";

[download]

What's happening here is that an exclusive or is being applied to both terms. As you might remember from your basic discrete math class, exclusive or returns 1 if the two bits are different and 0 if they are the same. This is why the first and second conditionals are important as we can now guarantee that you won't get 28-976 when $f = 28 and $s = 28976 and we won't get 28- when $f = $s = 28. Now comes the magic part. length (...)[0] is the same as length(...)[0]. Since length only returns a scalar, length(...)[0] makes no sense. Thus the reason for a plus (+) followed by parentheses to give the compiler a hint as to what we actually mean. The parentheses give the match list context with the [0] indicating we want the first captured element of the match (note, if we didn't capture, the regex would only return 1 or boolean true). The parentheses around the "$f" ^ "$s" are there only because =~ has higher precedence than ^. The regex matches the null character (\x00) which in binary looks like 00000000. Since our exclusive or tells us where the string matches with a 0 and where it doesn't match with a 1, our null characters mean we match at those locations. Because our regex capture returns the actual match and since our match is greedy, we will get a string of \x00 returned with a length the same as the beginning matching areas of $f and $s. Then length takes over to return the length. We then have substr($s,length) which returns the substr of $s from position length to the end. Thus we end up printing out $f-(the non-matching portion of $s).

Sorry if I messed up anywhere. Hope this helps.

Thanks to PodMaster for pointing out that I put exclusive or as 1 if they are different and 1 if they are the same. Doh!

antirice    
The first rule of Perl club is - use Perl
The ith rule of Perl club is - follow rule i - 1 for i > 1

In reply to Re: Re: Re: eliding number ranges by antirice
in thread eliding number ranges by qq

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