Use different delimiter and comment modifier.

core/modules/path_alias/src/Entity/PathAlias.php

@@ -63,16 +63,15 @@ class PathAlias extends ContentEntityBase implements PathAliasInterface {
    *   The trimmed alias.
    */
   public static function trimAlias(string $alias): string {
-    $filteredAlias = preg_replace('%
+    $filteredAlias = preg_replace('/
       (         # At start
         [^\/]   # never trim slash.
       )
       (         # Then
         [\\\/]* # trim slashes
         \s*     # and whitespaces
-      )*        # in zero or more repetitions
-      $         # at the end.
-      %', '\1', $alias);
+      )*        # in zero or more repetitions at the end.
+      $/x', '\1', $alias);
     if ($filteredAlias === NULL) {
       throw new \Exception('A problem occurred while trimming the path alias.');
     }