Codeigniter表单自定义验证不起作用

I'm trying to add a function to the from validation but is not working the validation is done and doesn't use the function checkPostalCode

 public function build()
    {
        return array(
            array(
                'field' => 'postal_code',
                'name' => 'Postal Code',
                'rules' => 'trim|required|numeric|exact_length[5],callback_checkPostalCode',
                'errors' => array(
                    'required' =>  trans('form.errors.required'),
                    'numeric' => trans('form.errors.numeric'),
                    'exact_length' => trans('form.errors.exact_length', array('%length%' => 5)),
                )
            )
        );
    }

    public function checkPostalCode($pc){
        if($pc == "08000")
            return true;
        else
            return false;
    }

The validation done on checkPostalCode is not working

try

'rules' => 'trim|required|numeric|exact_length[5]|callback_checkPostalCode',

instead of

'rules' => 'trim|required|numeric|exact_length[5],callback_checkPostalCode',