title: regex (in PHP) to match & that aren't HTML entities
body: <p>Here's the goal: to replace all standalone ampersands with &amp;amp; but NOT replace those that are already part of an HTML entity such as &amp;nbsp;.</p>
  
  <p>I think I need a regular expression for PHP (preferably for preg_ functions) that will match only standalone ampersands. I just don't know how to do that with preg_replace.</p>