diff --git a/core/lib/Drupal/Component/Utility/String.php b/core/lib/Drupal/Component/Utility/String.php
index 2a15a12..901c12b 100644
--- a/core/lib/Drupal/Component/Utility/String.php
+++ b/core/lib/Drupal/Component/Utility/String.php
@@ -162,7 +162,7 @@ public static function upperCase($text) {
       // Use C-locale for ASCII-only uppercase
       $text = strtoupper($text);
       // Case flip Latin-1 accented letters
-      $text = Unicode::caseFlip('/\xC3[\xA0-\xB6\xB8-\xBE]/', $text);
+      $text = preg_replace_callback('/\xC3[\xA0-\xB6\xB8-\xBE]/', '\Drupal\Component\Utility\Unicode::caseFlip', $text);
       return $text;
     }
   }
@@ -184,7 +184,7 @@ public static function lowerCase($text) {
       // Use C-locale for ASCII-only lowercase
       $text = strtolower($text);
       // Case flip Latin-1 accented letters
-      $text = Unicode::caseFlip('/\xC3[\x80-\x96\x98-\x9E]/', $text);
+      $text = preg_replace_callback('/\xC3[\x80-\x96\x98-\x9E]/', '\Drupal\Component\Utility\Unicode::caseFlip', $text);
       return $text;
     }
   }
diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index f0c9d33..e8c2dad 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -286,21 +286,16 @@ public static function mimeHeaderDecode($header) {
   }
 
   /**
-   * Case flip Latin-1 accented letters.
+   * Flip U+C0-U+DE to U+E0-U+FD and back. Can be used as preg_replace callback.
    *
-   * @param string $text
-   *   The text to case flip.
+   * @param array $matches
+   *   An array of matches by preg_replace_callback().
    *
    * @return string
-   *   The case flipped text.
+   *   The flipped text.
    */
-  public static function caseFlip($pattern, $text) {
-    $matches = array();
-    $callback = function ($matches) {
-      // Flip U+C0-U+DE to U+E0-U+FD and back.
-      return $matches[0][0] . chr(ord($matches[0][1]) ^ 32);
-    };
-    return preg_replace_callback($pattern, $callback , $text);
+  public static function caseFlip($matches) {
+    return $matches[0][0] . chr(ord($matches[0][1]) ^ 32);
   }
 
 }