title: Filter some special characters
body: <p>Until now, when a user is registering in my app, he can't create a username with special character. I do it like this:</p>
  
  <pre><code>if username.isalnum() is False:
  ....
  </code></pre>
  
  <p>But now I want to enable a list of special characters like: <code>! - _ $</code>.</p>
  
  <p>What is the right way to do it?</p>