CodeIgniter Postcode Validation Form Validation (and global call back functionality), read more at http://wlkns.co/codeigniter/global-form-validation-callback-functionality-and-postcode-checker-in-codeigniter/

MY_Form_validation.php

<?php defined('BASEPATH') or die('Direct access to this file is not permitted.')

class MY_Form_validation extends CI_Form_validation
{
  
	/**
	 * Postcode Validation Callback
	 */
	public function valid_postcode( $string )
	{
		if ( ! preg_match('/^[A-Z]{1,2}[0-9]{1,2}[A-Z]?[\s]*[0-9]{1}[A-Z]{1,2}$/') )
		{
			$this->set_message(__FUNCTION__, 'Invalid Postcode entered.');			
			return FALSE;
		}

		return TRUE;
	}

}