title: Store boolean variable in url dispatching
body: <p>The following url definition should pass whether <code>results/</code> is present in the url:</p>
  
  <pre><code>url(r'^(?P&lt;question_id&gt;[0-9]+)/(?P&lt;results&gt;(results/)?)shorten/$', views.shorten, name='shorten')
  </code></pre>
  
  <p>Currently it passes <code>results/</code> or <code>None</code> which is sufficient for a simple:</p>
  
  <pre><code>if results:
      pass
  </code></pre>
  
  <p>But it would be more elegant to have <code>True</code> and <code>False</code>. How could this be done?</p>