Beefy Boxes and Bandwidth Generously Provided by pair Networks
more useful options

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??
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

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others contemplating the Monastery: (9)
    As of 2015-11-26 12:01 GMT
    Find Nodes?
      Voting Booth?

      What would be the most significant thing to happen if a rope (or wire) tied the Earth and the Moon together?

      Results (697 votes), past polls