confirm password in codeigniter

$this->form_validation->set_rules('first_name','First Name','trim|required');
$this->form_validation->set_rules('last_name','Last Name','trim|required');
$this->form_validation->set_rules('email','Email','trim|required|valid_email');
$this->form_validation->set_rules('username','Username','trim|required|min_length[3]');
$this->form_validation->set_rules('password', 'Password', 'required');
$this->form_validation->set_rules('confirm_password', 'Confirm Password', 'required|matches[password]');