title: Rails format validation -- alphanumeric, but not purely numeric
body: <p>Whats the best way to test a format validation of lets says a username, with a regex for alphanumeric, but not purely numeric?</p>
  
  <p>I've been using the following validation in my model</p>
  
  <pre><code>validates :username, :format =&gt; { :with =&gt; /^[a-z0-9]+[-a-z0-9]*[a-z0-9]+$/i }
  </code></pre>
  
  <p>Numeric username's such as '342' pass the validation, which I don't want.</p>