title: Escaping numbers in regex
body: <p>Is the following regex valid? And if so what does it do?</p>
  
  <pre><code>.*(.)(.)(.)(.)\4\3\2\1.*
  </code></pre>
  
  <p>Can't seem to find any info on escaping numbers, and why four (.) 's?</p>
  
  <p>Abstracted from the Regex Sudoku/Crossword puzzle listed recently on <a href="http://games.slashdot.org/story/13/02/13/2346253/can-you-do-the-regular-expression-crossword" rel="nofollow">Slashdot</a></p>