accepted characters for 'auth code'

[Pflege]

I have tried to change some auth codes which had been automatically asigned
for biz domains and had the following problem:

The assigned code was f.ex. 1234$4321

and I changed it to: ab$cdefgh

When I came back to check the changes,
I found that the auth code was stored like
ab????cdefgh
(my input of '$' was changed to '????')

Similar problem with '�'

Are there any rules concerning the 'auth code'?
I could not find an answer at the official site.

Matthias