Re^6: Regex::Reverse tricky test cases

This is a good example of what makes it difficult. In addition to showing me that I need to renumber my backreferences properly, I still get no match with the generated regex, qr/(.*)((?=\d+))(A)\2/, because you can't swap a lookaround for a real match.

So lookarounds in conjunction with backreferences are what really make it difficult. (Incidentally, your suggested pattern is illegal, because the lookbehind is variable-length.)

Caution: Contents may have been coded under pressure.

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Re^7: Regex::Reverse tricky test cases by dragonchild (Archbishop) on May 17, 2005 at 14:58 UTC
In addition to showing me that I need to renumber my backreferences properly, I still get no match with the generated regex, `qr/(.)((?=\d+))(A)\2/`, because you can't swap a lookaround for a real match. First, I think you want (.?), not (.). Second, the biggest problem with reversing against the string is the need to have the parsing be `( 4, 321, 'A' )`, not `('', 4321, 'A' )` as will have to happen with the \d+. The issue is you need to know what the stuff in \2 is before you match ((?=\d+)). Also, positive-lookahead is not the direct reversal of negative-lookbehind. For one thing, it's a zero-width assertion that will fail as written because you can't have numbers and letters in the same position. I think it's better reversed as `qr/(.?)\2(A)((?=\d+))/`, which makes no sense because \2 isn't populated when it needs to be. (Incidentally, your suggested pattern is illegal, because the lookbehind is variable-length.) Yes, it is. But, that's one of the main considerations you gave for wanting to have a regex reverser ... In general, if you think something isn't in Perl, try it out, because it usually is. :-) "What is the sound of Perl? Is it not the sound of a wall that people have stopped banging their heads against?"	[reply] [d/l] [select]
Re^8: Regex::Reverse tricky test cases by Roy Johnson (Monsignor) on May 17, 2005 at 15:17 UTC
I think I would reverse it as `qr/(.?)(\d+)(A)(?=\2)/`. But I'm not at all sure how to make a rule I could apply to a YAPE::Regex tree. Caution:* Contents may have been coded under pressure.	[reply] [d/l]
Re^9: Regex::Reverse tricky test cases by dragonchild (Archbishop) on May 17, 2005 at 17:25 UTC
I don't know about the YAPE::Regex tree, but the rule seems to be: If you have a negative lookbehind and a backreference to it, it stays in the same position, but becomes a positive lookahead on the backreference to its mirror'ed element. In general, if you think something isn't in Perl, try it out, because it usually is. :-) "What is the sound of Perl? Is it not the sound of a wall that people have stopped banging their heads against?"	[reply]