diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index 0641bc85ea62c9f739d094dc4c33d95dab947974..ebd3553c36a612fa89cc105d58ce17febca4e591 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -54,7 +54,7 @@ class ExtensionDiscovery {
    *
    * @see http://php.net/manual/functions.user-defined.php
    */
-  const PHP_FUNCTION_PATTERN = '/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$/';
+  const PHP_FUNCTION_PATTERN = '/^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$/';
 
   /**
    * Previously discovered files keyed by origin directory and extension type.