/**
 * Modifies internal/external links. Strips target and rel from internal links
 * so that they are not opened in a new window. Adds target="_blank" to external
 * links so that they are opened in a new window.
 *
 * If this behavior is not desired on a link add the CSS class NO_MODIFY to the
 * link.
 */
function modifyLinks(string $htmlString, array $internalDomains) {
    $html = new DOMDocument();
    @$html->loadHTML(''.$htmlString);
    foreach($html->getElementsByTagName('a') as $a) {
        // skip links that have a CSS class of NO_MODIFY
        if (strpos($a->getAttribute('class'),'NO_MODIFY') !== false) {
            continue;
        }
        $url = parse_url($a->getAttribute('href'));
        if (empty($url['host']) || in_array($url['host'], $internalDomains)) {
            // Internal Link
            $a->removeAttribute('target');
            $a->removeAttribute('rel');
        } else {
            // External Link
            $a->setAttribute('target','_blank');
        }
    }
    return $html->saveHTML($html->documentElement);
}
add_filter('the_content', static function($content) {
    return modifyLinks($content, array('domain.com','subdomain.domain.com'));
},99);