title: Regex in RequestParser for Python Flask-RESTful
body: <p>Wondering if it's possible in <a href="http://flask-restful-cn.readthedocs.io/en/0.3.5/" rel="nofollow noreferrer">Python Flask-RESTful</a>, to use regular expressions while <a href="http://flask-restful-cn.readthedocs.io/en/0.3.5/reqparse.html" rel="nofollow noreferrer">parsing arguments</a>?</p>
  
  <p>For example, per the <a href="http://flask-restful-cn.readthedocs.io/en/0.3.5/reqparse.html#multiple-values-lists" rel="nofollow noreferrer">documentation for multiple values</a>, I have the following argument I'm expecting in our API:</p>
  
  <pre><code>parser.add_argument('fq', type=str, action='append', help='expecting filter query (fq) (multiple)')
  </code></pre>
  
  <p>As is evident in the description and parameters, this field can repeat.  Unfortunately, I'm working with PHP libraries (namely, <a href="http://php.net/manual/en/function.http-build-query.php" rel="nofollow noreferrer"><code>http_build_query</code></a> via <a href="http://docs.guzzlephp.org/en/latest/" rel="nofollow noreferrer">Guzzle</a>) that return repeating values in the following form:</p>
  
  <pre><code>?fq[0]=foo&amp;fq[1]=bar&amp;fq[2]=baz
  </code></pre>
  
  <p>With an unknown number of <code>fq</code> parameters to parse, it would be invaluable to be able to define a regex parser, something akin to:</p>
  
  <pre><code>parser.add_argument('(fq)\[[0-9]+\]', type=str, action='append', help='expecting filter query (fq) (multiple)')
  </code></pre>
  
  <p>that would capture anything in the form <code>fq[#]</code>, drop the brackted <code>[]</code> suffix, and append all those to an <code>fq</code> list in the <code>args</code> dictionary.</p>
  
  <p>Is that possible with Flask-RESTful?  I'm not seeing anything in the documentation, save some comments about potentially moving to something like Marshmallow.</p>