Zend_Filter_Alpha example

$this->_setValue($value);

        if ('' === $value) {
            $this->_error(self::STRING_EMPTY);
            return false;
        }

        if (null === self::$_filter) {
            /** * @see Zend_Filter_Alpha */
            self::$_filter = new Zend_Filter_Alpha();
        }

        self::$_filter->allowWhiteSpace = $this->allowWhiteSpace;

        if ($value !== self::$_filter->filter($value)) {
            $this->_error(self::NOT_ALPHA);
            return false;
        }

        return true;
    }

}