title: Python argparse regex expression
body: <p>Is it possible to use a regex expression for parsing an argument? For example, I want to accept an argument if only it is a 32 length hex (i.e. matches <code>/[a-f0-9A-F]{32}/</code>)</p>
  
  <p>I tried</p>
  
  <pre><code>p.add_argument('hex', type=str, nargs="[a-f0-9A-F]{32}")
  </code></pre>
  
  <p>without success</p>