Quoting Olaf Meeuwissen (paddy-hack@???):

> On any Unix-like system I've come across the '/' (and '\0') are about
> the only character that cannot be used in a filename.

That and null are the only disallowed characters. It's in the Single
Unix Specification.
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_170