Don't do that. It doesn't work, because the substitution resets the internal position pointer used with //g, so you'll always restart from the beginning.  (And even if it didn't, it still likely wouldn't work, if you modify the string such that the remembered start position for the next match is no longer correct... — you could in theory adjust it by assigning to pos($str) = ..., but why make things more complex than they need to be?)

Simply do a global substitution without prior matching, and put the computations in a separate sub. Something like this: 

sub subst_str {
    my $match = shift;
    my $b = ...

    return "$match<tag3>$b</endtag3>";
}

..
$str =~ s/(<tag1[^>]*>\s*<tag2>\s*)<tag3>[^<]*<\/endtag3>/subst_str($1
+)/gsie;   # note the /e

[download]

In reply to Re: while loop w/match is hanging by Eliya
in thread while loop w/match is hanging by Anonymous Monk

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.