What characters can SystemInfo.deviceUniqueIdentifier return?

I am integrating a pvp system for my game. And i want to use the device identifier and store the battles in text. I will keep scores of each player using that but i want to learn if characters like "@" , " " , "_" , are used in any uid return by SystemInfo.deviceUniqueIdentifier? Are they safe? (Will be never used in the returning string?)