diff --git modules/user/user.module modules/user/user.module
index e8b1fe2..48dc5dc 100644
--- modules/user/user.module
+++ modules/user/user.module
@@ -390,7 +390,12 @@ 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('/[^\x80-\xBF'.            // Second, third or fourth byte of a multibyte sequence.
+                 '\xC2-\xDF'.               // Start of a 2-byte sequence.
+                 '\xE0-\xEF'.               // Start of a 3-byte sequence.
+                 '\xF0-\xF7'.               // Start of a 4-byte sequence.
+                 ' a-z0-9@_.\'-]/i',        // Desired characters from the single byte range.
+                 $name)) {
     return t('The username contains an illegal character.');
   }
   if (preg_match('/[\x{80}-\x{A0}'.          // Non-printable ISO-8859-1 + NBSP