public function parseKeys($data, $offset)
    {
        $element   = \preg_quote($this->getIndex()->getElement());
        $attribute = \preg_quote($this->getIndex()->getAttribute());
        $pregExp = '/<' . $element . '\s([^>]*\s)?'
            . $attribute . '\s*=\s*([\'"])(.+?)\2[^>]*>/si';

        \preg_match_all(
            $pregExp,
            $data,
            $matches,
            PREG_OFFSET_CAPTURE | PREG_SET_ORDER
        );

        $keys = array();

        foreach ($matches as $match) {
            $keys[] = new FoundKey($match[0][1], $match[3][0]);

        }

        return $keys;
    }