private function getSubmittedVersionNumber(RequestInterface $request)
    {
        $accept  = $request->getHeader('Accept');
        $matches = array();

        preg_match('/^application\/vnd\.([a-z.-_]+)\.v([\d]+)\+([a-z]+)$/', $accept, $matches);

        return isset($matches[2]) ? $matches[2] : null;
    }