Mozilla's secure coding guidelines for web developers
<p><pre><code>  Example A field accepts a username. A good regex would
  be to verify that the data consists of the following
  [0-9a-A-Z]{3,10}. The data is rejected if it doesn't
  match.
</code></pre>
I guess then that pg won't be able to sign up for your service... Nor will donfernandovillaverde79.