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