gistfile1.txt

/**
 * 
 * All internal links should open in a new window with rel=noopener
 * 
 */

if ( ! function_exists( 'force_external_links' ) ) {

	add_filter('the_content', 'force_external_links');
	add_filter('acf_the_content', 'force_external_links');

	function force_external_links($content) {
		return preg_replace_callback('/<a[^>]+/', 'force_external_links_callback', $content);
	}

	function force_external_links_callback($matches) {
		$link = $matches[0];
		$parsed_site_link = parse_url(get_bloginfo('url'));
		$site_host = $parsed_site_link['host'];

		// is our domain in the host or is the scheme empty (anchor link, truncated link)
		if ( strpos($link, $site_host ) === false && strpos( $link, 'http' ) !== false ) {
			$link = preg_replace("%(href=\S(?!$site_host))%i", 'target="_blank" $1', $link);
			// is noopener in the rel attribute
			if ( strpos($link, 'noopener' ) === false) {
				$link = preg_replace("%(href=\S(?!$site_host))%i", 'rel="noopener" $1', $link);
			}
		}
		return $link;
	}

} 

Note - breaks rel=noreferrer links