In their replies to counting overlapping patterns, both Eimi Metamorphoumai and betterworld (Update: betterworld's example is list context so that's almost a separate question ;Update 2: actually no, it's the same issue) submitted code that looked essentially like: 
#!/usr/bin/perl

my $foo = "AAAA";
while ($foo =~ /(?=AA)/g) {
    print pos $foo;
}

[download]
which prints out "012" I am confused as to why this does not cause an infinite loop that continuously prints "0." According to perlop, following a successful match the value of pos() is set to the position after the end of the matched string, which is where the next iteration of m//g picks up. But since in this case, the matched string is zero-length, why does pos() ever advance? I ask because this seems like a cool trick, but I hesitate to use it since I'm not clear on why it works.

In reply to zero-length match increments pos() by Errto

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.