callback not being called - Printable Version

Code:
class C_registro extends Controller {

        

        function C_registro()

        {

            parent::Controller();

            $this->load->helper(array('url','form'));

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

            $this->load->database();

            $this->load->model('m_user');

        }

                function index()

        {

            $this->registrar();

        }

//-------------------------------------------------------------------------------------------------        

        function mostrarRegistro()

        {

            $datos = array(

                    'titulo'     => "Registro de Usuario",

                    'url'         => $this->config->item('path_url'),

                    'jquery'     => $this->config->item('path_jquery'),

                    'jquery_ui' => $this->config->item('path_jquery_ui'),

                    'ui_css'     => $this->config->item('path_jquery_ui_css')

                );

                    

            $this->load->view('v_registro',$datos);

        }

//-------------------------------------------------------------------------------------------------        

        function registrar()

        {

            $this->form_validation->set_rules('nombre','Usuario','trim|required|min_length[3]|max_length[50]|callback_username_check');

            $this->form_validation->set_rules('pass'  ,'Contraseña','trim|required|min_length[8]|max_length[50]|alpha_numeric|MD5');

            

            $this->form_validation->set_error_delimiters('<div class="ui-state-error ui-corner-all" align="center">', '</div>');

            

            if($this->form_validation->run() == FALSE)

            {

                $this->mostrarRegistro();

            }

            else

            {

                redirect('ok');

            }        

        }

    }

//-------------------------------------------------------------------------------------------------

        function username_check($usr)

        {

            $this->form_validation->set_message('username_check', 'The callback was called.');

        return FALSE;

            /*

            $existe = $this->m_user->existeUser($usr);

            if ($existe)

            {

                $this->form_validation->set_message('username_check', 'El usuario ya existe');

                $rtrn = FALSE;

            }

            else

            {

                $this->m_user->altaUser($usr,$this->input->post('pass'));

                $rtrn = TRUE;

            }

            

            return $rtrn;

            */

        }

//--------------------------------------------------------------------------------