((this)?|(that)?|(third_thing)?)
...
... I don't even trust it. ... there's probably 4 capture groups created as a result.

Just as an aside, the  (?|(pat)|(te)|(rn)) "branch reset" pattern introduced with Perl version 5.10 will suppress the creation of a slew of captures in a case like this: 

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = 'apathetic';
 ;;
 my @captures = $s =~ m{ (pat) | (te) | (rn) }xms;
 dd \@captures;
 ;;
 @captures = $s =~ m{ (?| (pat) | (te) | (rn)) }xms;
 dd \@captures;
"
["pat", undef, undef]

["pat"]

[download]
See Extended Patterns in perlre.

Give a man a fish:  <%-{-{-{-<

In reply to Re^3: Regex result being defined when it shouldn't be(?) by AnomalousMonk
in thread Regex result being defined when it shouldn't be(?) by chenhonkhonk

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.