protected function unmarshallChildrenKnown(DOMElement $element, QtiComponentCollection $children)
    {
        if (($pattern = $this->getDOMElementAttributeAs($element, 'pattern')) !== null) {

            $object = new PatternMatch($children, $pattern);

            return $object;
        } else {
            $msg = "The mandatory attribute 'pattern' is missing from element '" . $element->localName . "'.";
            throw new UnmarshallingException($msg, $element);
        }
    }