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I fixed a quirky bug in the regex engine this morning, thanks to converter's test case. It was due to an over-active optimization. The optimization is "if the regex starts with .*, pretend it started with ^.*" and it makes sense -- since .* will have exhausted itself by the time the regex has failed, there's no sense in trying to match anywhere later in the string.

However, this happens even if the .* is being captured and referenced later in the regex (a back-reference). That's no good, as shown by the test case: "abc123bc" =~ /(.*)\d+\1/;
We'd like $1 to be "bc", but Perl implicitly anchors this regex to the beginning of the string, and thus fails.

I've patched Perl, but in retrospect, I should make sure I find a backreference too. That shouldn't be too bad, though.

Ruby has this bug too. Both can side-step it with a little trick: put .{0} or (?=) as the first thing in your regex; that way, the dot-star isn't the first thing the regex engine sees. Really.

Python is free from this bug as of its latest release, 2.2.

Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

In reply to Quirky regex bug by japhy

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