title: use of the ampersand here means pre_match?
body: <p>What is the ampersand doing in the code below?</p>
  
  <pre><code>s.reverse.gsub( /\d{3}(?=\d)/, '\&amp;,' ).reverse
  </code></pre>
  
  <p>One would think, after attempting to look up such things, that it is a special variable meaning post_match or pre_match, but the docs say nothing about ampersands - only dollar signs either followed by or preceded by a tick mark.</p>