$username = $this->addElement('text', 'username', array(
            'filters'    => array('StringTrim', 'StringToLower'),
            'validators' => array(
                'Alpha',
                array('StringLength', false, array(3, 20)),
            ),
            'required'   => true,
            'label'      => 'Username:',
        ));

        $password = $this->addElement('password', 'password', array(
            'filters'    => array('StringTrim'),
            'validators' => array(
                'Alnum',
                array('StringLength', false, array(6, 20)),
            ),
            'required'   => true,
            'label'      => 'Password:',
        ));