title: Validate so that no special characters are allowed
body: <p>How can I validate <code>:title</code> in my model so that only the letters a-z, A-z, and 0-9 are accepted?</p>
  
  <pre><code>  validates :title, :format =&gt; { with: REGULAR EXPRESSION , :message =&gt; 'no special characters, only letters and numbers' }
  </code></pre>
  
  <p>What should the regular expression be?</p>