[_ck_]

Apparently WordPress allows dots (periods) in usernames as well?! (it did at some point, not sure if it still does but I have users with nicknames like that)

So I had to modify this plugin with a dot

return preg_replace('/[^ ta-z0-9_.-]/i', '', str_replace("%20"," ",$raw));

apparently php is very sensitive as to what order the pattern is in, that dot has to be right there in the sequence or it throws a shedload of errors.