Drupal 8 : Do something based on the referer if it's your website or another.

referer.php

 use Drupal\Core\Url;
 use Symfony\Component\HttpFoundation\Request;
 
 function my_function() {
  $previousUrl = \Drupal::request()->server->get('HTTP_REFERER');
  $fake_request = Request::create($previousUrl);
  $url_object = \Drupal::service('path.validator')->getUrlIfValid($fake_request->getRequestUri());
  if ($url_object) {
    $route_name = $url_object->getRouteName();
    // Do something with $route_name;
}

Yea this only validates the path.