title: Handling backreferences to capturing groups in re.sub replacement pattern
body: <p>I want to take the string <code>0.71331, 52.25378</code> and return <code>0.71331,52.25378</code> - i.e. just look for a digit, a comma, a space and a digit, and strip out the space. </p>
  
  <p>This is my current code:</p>
  
  <pre><code>coords = '0.71331, 52.25378'
  coord_re = re.sub("(\d), (\d)", "\1,\2", coords)
  print coord_re
  </code></pre>
  
  <p>But this gives me <code>0.7133,2.25378</code>. What am I doing wrong?</p>