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\._\-]+"