title: TCL Regexp Explanation
body: <p>What does the below regexp mean. I could see that some value has to get assigned to "z"</p>
  
  <p><code>regexp ${a}(.+?)($x) $y NULL p1 p2 p3 z p5</code></p>
  
  <p>but what are these <code>p1</code>, <code>p2</code> etc..</p>
  
  <p>Thanks in advance.</p>