title: Form validation rules for regex_match
body: <p>I'm trying to figure out what I'm doing wrong with this validation rule because its saying this error.</p>
  
  <p>Validation rule:</p>
  
  <pre><code>$this-&gt;form_validation-&gt;set_rules('username', 'Username', 'trim|required|xss_clean|regex_match[a-z0-9]');
  </code></pre>
  
  <p>Error:</p>
  
  <blockquote>
    <p>Severity: Warning<br>
       Message:  preg_match() [function.preg-match]: Delimiter must not be alphanumeric or backslash</p>
  </blockquote>