POSIX
From wikinotes
POSIX is a set of standards for writing code that is compatible with multiple operating systems.
Documentation
posix spec http://pubs.opengroup.org/onlinepubs/9699919799/ wikipedia https://en.wikipedia.org/wiki/POSIX
Filenames
"[A-Za-z0-9\._][A-Za-z0-9\._\-]+"