diff --git modules/user/user.module modules/user/user.module
index e8b1fe2..f355262 100644
--- modules/user/user.module
+++ modules/user/user.module
@@ -390,7 +390,7 @@ function user_validate_name($name) {
   if (strpos($name, '  ') !== FALSE) {
     return t('The username cannot contain multiple spaces in a row.');
   }
-  if (preg_match('/[^\x{80}-\x{F7} a-z0-9@_.\'-]/i', $name)) {
+  if (preg_match('/[^\x{80}-\x{F7} a-z0-9@_.\'-]/ui', $name)) {
     return t('The username contains an illegal character.');
   }
   if (preg_match('/[\x{80}-\x{A0}'.          // Non-printable ISO-8859-1 + NBSP