title: Regular expression for crossword solution
body: <p>This is a crossword problem. Example:</p>
  
  <ul>
  <li>the solution is a 6-letter word which starts with "r" and ends with "r"</li>
  <li>thus the pattern is "r....r"</li>
  <li>the unknown 4 letters must be drawn from the pool of letters "a", "e", "i" and "p"</li>
  <li>each letter must be used exactly once</li>
  <li>we have a large list of candidate 6-letter words </li>
  </ul>
  
  <p>Solutions: "rapier" or "repair".</p>
  
  <p>Filtering for the pattern "r....r" is trivial, but finding words which also have [aeip] in the "unknown" slots is beyond me.</p>
  
  <p>Is this problem amenable to a regex, or must it be done by exhaustive methods?</p>