diff -u b/core/includes/install.inc b/core/includes/install.inc
--- b/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -135,6 +135,7 @@
   // without modifying the installer.
   require_once DRUPAL_ROOT . '/core/includes/database.inc';
   // Allow any valid PHP identifier.
+  // @see http://www.php.net/manual/en/language.variables.basics.php.
   $mask = '/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$/';
   $files = file_scan_directory(DRUPAL_ROOT . '/core/lib/Drupal/Core/Database/Driver', $mask, array('recurse' => FALSE));
   if (is_dir(DRUPAL_ROOT . '/drivers/lib/Drupal/Driver/Database')) {