UpperCase Username

add_filter('validate_username','custom_allow_uppercase_username',20,2);

function custom_allow_uppercase_username($is_valid,$user_name){
    if($is_valid)
        return $is_valid;//it is already valid, no need to waste the precious cpu cycles
    $sanitized=sanitize_user($user_name,false);

   if(strtolower($sanitized)==strtolower($user_name))
       return true;
   return false;
}