in reply to Re: Regex::Reverse tricky test cases
in thread Regex::Reverse tricky test cases

See my answer to dragonchild for why. I'm sorry, I thought everybody knew about sexeger, which was probably a pretty foolish assumption. Here are a couple of regexes I've run through my program: 
Before: (?i-msx:a(ab)(cd)\2\1)
After: (?i-msx:(ba)(dc)\2\1a)
Before: (?-imsx:\d+[abc]def(foo|bar))
After: (?-imsx:(oof|rab)fed[abc]\d+)

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Note that the 2nd is from japhy's seminal work on the subject, and differs from his reversal: alternatives should maintain their order (foo|bar becomes oof|rab, not rab|oof).
Caution: Contents may have been coded under pressure.

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Re^3: Regex::Reverse tricky test cases
by dragonchild (Archbishop) on May 17, 2005 at 13:31 UTC
    Ah. I understand now.

    How would you reverse the following: /(?<\d+)(A)\1/

    • 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?"
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      Is that supposed to be a variable-length lookbehind (it lacks an =)? Perhaps ironically, I haven't put any handling for lookahead/behind in my program.

      What I would need to do is to convert lookbehinds to lookaheads, and vice-versa. Other than that, the normal reversing would work. Ok, I've done that. Program says: 

      Before: (?-imsx:(?<=\d+)(A)\1)
After: (?-imsx:(A)\1(?=\d+))

      [download]
      Caution: Contents may have been coded under pressure.
[reply]
[d/l]
        You're right, I should have been more careful. The regex I was trying to write (if I'd actually tested it) should have been: (?-imsx: ((?<=\d+))(A)\1) - I forgot that (?<=) is non-capturing.

        Your reversal fails, in large part because I didn't give you any test input. 

        my $string = "123A1234";
my $regex  = qr/((?<=\d+))(A)\1(.*)/;

my ($num, $letter, $leftover) = $string =~ $regex;
ok( $num eq '123' );
ok( $letter eq 'A' );
ok( $leftover eq '4' );

----------------------------------------------------------

my $rev_string = "4321A321";
my $rev_regex  = qr/.../; # <-- what goes here??

my ($leftover, $letter, $num) = $rev_string =~ $rev_regex;

ok( $num eq '123' );
ok( $letter eq 'A' );
ok( $leftover eq '4' );

        [download]
        • 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?"
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