adding regex fix

Emby.Server.Implementations/Library/UserManager.cs

@@ -218,8 +218,8 @@ namespace Emby.Server.Implementations.Library
         {
             // This is some regex that matches only on unicode "word" characters, as well as -, _ and @
             // In theory this will cut out most if not all 'control' characters which should help minimize any weirdness
-            // Usernames can contain letters (a-z + whatever else unicode is cool with), numbers (0-9), dashes (-), underscores (_), apostrophes ('), and periods (.)
-            return Regex.IsMatch(username, @"^[\w-'._@]*$");
+             // Usernames can contain letters (a-z + whatever else unicode is cool with), numbers (0-9), at-signs (@), dashes (-), underscores (_), apostrophes ('), and periods (.)
+            return Regex.IsMatch(username, @"^[\w\-'._@]*$");
         }
 
         private static bool IsValidUsernameCharacter(char i)