Re: valid characters in user names- esp. compatibility characters

[Markus Scherer]

Another encoding, standardized for much longer, is what IMAP uses for mailbox names. I think it does 
not have a standard charset name, but it's described in one of the IMAP RFCs. It's a modified UTF-7, 
modified to make it filename-friendly and deterministic, and may fit the bill. It's certainly useful 
to squeeze Unicode filenames into ASCII. I am sure there are many libraries with an implementation 
(ICU has one, see convrtrs.txt).

By the way, ICU also implements IDNA and generic StringPrep.
Best regards,
markus
http://oss.software.ibm.com/cgi-bin/icu/convexp?conv=IMAP-mailbox-name&b=&s=ALL