form validation rules in CodeIgniter

<?php

class Form extends CI_Controller {

    public function index()
    {
        $this->load->helper(array('form', 'url'));

        $this->load->library('form_validation');

        $this->form_validation->set_rules('username', 'Username', 

 'callback_username_check');
        $this->form_validation->set_rules('password', 'Password', 

 'required');
        $this->form_validation->set_rules('passconf',  

'Password Confirmation', 'required');
        $this->form_validation->set_rules('email', 'Email',  

'required|is_unique[users.email]');

        if ($this->form_validation->run() == FALSE)
        {
            $this->load->view('myform');
        }
        else
        {
            $this->load->view('formsuccess');
        }
    }

    public function username_check($str)
    {
        if ($str == 'test')
        {
            $this->form_validation->set_message('username_check',  

'The %s field can not be the word "test"');
            return FALSE;
        }
        else
        {
            return TRUE;
        }
    }

}
?>