protected function buildStructuredHeaders(): void
    {
        $this->structuredHeaders = array();
        $headers = $this->entry[PoTokens::TRANSLATED];
        $headers = ($headers === null) ? array() : $headers;
        $full = implode('', $headers);
        $headers = explode("\n", $full);
        // split on ':'
        $pattern = '/([a-z0-9\-]+):\s*(.*)/i';
        foreach ($headers as $h) {
            if (preg_match($pattern, trim($h), $matches)) {
                $this->structuredHeaders[strtolower($matches[1])] = array(
                    'key' => $matches[1],
                    'value' => $matches[2],
                );
            }
        }
    }