$a =~ m%<div\b[^>]*>((?:(?!</div>)(?!<div\b).)*)</div>%;
$a =~ m%<div\b[^>]*>((?:(?!<div\b).)*?)</div>%;


##</code><code>##

#!perl;
$a = 'A<div class="X">B<div class="Y">C<span class="Z">D</span>E</div>F</div>G';
print "before: $a\n";
$a =~ s%<div\b[^>]*>((?:(?!<div\b).)*?)</div>%\1%g;
print "after: $a\n";