title: Validate: Only letters, numbers and -
body: <p>I would like to validate my users, so they can only use a-z and - in their username. </p>
  
  <pre><code>validates_format_of :username, :with =&gt; /[a-z]/
  </code></pre>
  
  <p>However this rule also allows spaces ._@</p>
  
  <pre><code>Username should use only letters, numbers, spaces, and .-_@ please.
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p>Best regards.
  Asbjørn Morell</p>