private function getExpression()
    {
        if ($this->expression === null) {
            $this->expression =
                '/(<\?xml.*\?>)?([ \t\n\r]*)(<' .
                preg_quote($this->documentElement, '/') .
                '.*xmlns="' .
                preg_quote($this->namespace, '/') .
                '".*>)/is';
        }

        return $this->expression;
    }