Issue #3302931: Don't override existing attribute.

src/Plugin/Filter/TargetAttributesFilter.php

@@ -52,6 +52,19 @@ class TargetAttributesFilter extends FilterBase implements ContainerFactoryPlugi
     );
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function setConfiguration(array $configuration) {
+    parent::setConfiguration($configuration);
+    $this->settings = $this->settings + [
+      'filter_target_attribute' => '_blank',
+      'filter_target_method' => 'all',
+      'filter_target_replace' => 1,
+    ];
+    return $this;
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -79,6 +92,12 @@ class TargetAttributesFilter extends FilterBase implements ContainerFactoryPlugi
       '#default_value' => $this->settings['filter_target_method'],
       '#required' => TRUE,
     ];
+    $form['filter_target_replace'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Replace target attribute'),
+      '#description' => $this->t('If checked, any existing target attributes value is replaced.'),
+      '#default_value' => $this->settings['filter_target_replace'],
+    ];
     return $form;
   }
 
@@ -89,6 +108,10 @@ class TargetAttributesFilter extends FilterBase implements ContainerFactoryPlugi
     $dom = Html::load($text);
     $internalHost = $this->requestStack->getMainRequest()->getHttpHost();
     foreach ($dom->getElementsByTagName('a') as $link) {
+      /** @var \DOMElement $link */
+      if (!$this->settings['filter_target_replace'] && $link->hasAttribute('target')) {
+        continue;
+      }
       if ($href = trim($link->getAttribute('href'))) {
         $url = parse_url($href);
         $host = preg_replace('/^www\./i', '', $url['host'] ?? $internalHost);