Allow me to clarify a bit.

What I find interesting is that for the first case perl doesn't warn about it at all*. If I have a regex that uses backreferences but has no capturing parens, I'd expect that perl (with use warnings enabled) would mention something about it.

I'm not saying it's wrong behavior, and I understand why it's not matching. It just didn't provide the warning I'd expect.

*update: Or put it another way. I mean that the error message "Reference to nonexistent group in regex;" goes away if the regex is changed slightly. Consider:
print "matches\n" if $test_string =~ /(?:f)\1(.)/;
There is a valid set of capturing parens there, but after the \1 backreference is used. So this won't match -- and Perl doesn't warn about it. Surprised me a bit.

In reply to Re^4: Regular Expressions by Nkuvu
in thread Regular Expressions by y8

Title:
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, details, 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, summary, 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.