title: Rails 3 - validation rules with regex
body: <p>With this rule </p>
  
  <pre><code>validates_format_of :numbers, :with =&gt; /^[0-9]{15}$/, :message =&gt; "error message"
  </code></pre>
  
  <p>will be the item <code>:numbers</code> valid only with the regex above. But is possible to set, that the item <code>:numbers</code> could be send as an empty or like a 15 digits? I can't find the right combination of validation rules.</p>