function woohoo($content)
{
  if($content == '<!--blah-->')
    {
    	$content = preg_replace_callback('|<!--blah-->|i', 'foobar', $content);
  	}
  elseif($content == '<p><!--blah--></p>')
    {
    	$content = preg_replace_callback('|(<p>)?<!--blah-->(</p>)?|i', 'foobar', $content);
  	}
  return $content;
}