use strict;
while (<DATA>) {
  chomp;

  my $match = "'$1'"
    if m#^<p>((?:[^<]*|<(?!b>)[^>]+>)*?)</p>$#ism;
  
  $match = "<nothing>" unless defined $match;
  
  print "$_ matches $match\n"
};

__DATA__
<p>this should match</p>
<p>This should <b>not</b> match</p>
<p>What about <a href="http://www.example.com">this</a>?</p>
<p>And <p>this</p> malformed piece?</p>

##</code><code>##

//p[not descendant::b()]/text()