title: How to allow empty or numeric values in codeigniter's form_validation class?
body: <p>I'd like to have input validated with form_validation class, that'll allow me to put numeric or empty value in the field.<br/>
  Something like this:</p>
  
  <pre><code>$this-&gt;form_validation-&gt;set_rules('field[]','The field','numeric or empty|xss_clean');
  </code></pre>
  
  <p>Is this possible to achieve?</p>