improvements-2.x-dev/src/Plugin/Filter/NofollowExternalLinksFilter.php

<?php

namespace Drupal\improvements\Plugin\Filter;

use Drupal\Component\Utility\Html;
use Drupal\Component\Utility\UrlHelper;
use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;

/**
 * @Filter(
 *   id = "nofollow_external_links_filter",
 *   title = @Translation("Nofollow external link"),
 *   description = @Translation("Add attribute rel=nofollow to external links."),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_TRANSFORM_REVERSIBLE,
 *   weight = 20,
 * )
 */
class NofollowExternalLinksFilter extends FilterBase {

  /**
   * {@inheritdoc}
   */
  public function process($text, $langcode): FilterProcessResult {
    $result = new FilterProcessResult($text);

    if (stripos($text, '<a') !== FALSE) {
      $html_dom = Html::load($text);
      $links = $html_dom->getElementsByTagName('a');

      foreach ($links as $link) {
        $link_href = $link->getAttribute('href');
        if (UrlHelper::isExternal($link_href)) {
          $link->setAttribute('rel', 'nofollow');
        }
      }

      $result->setProcessedText(Html::serialize($html_dom));
    }

    return $result;
  }

}