What characters should be restricted from a Unix file name?

Consider a Save As dialog with a free text entry where the user enters a file name as free text, then clicks a Save button. The software then validates the file name, and saves the file if the name is valid. On a Unix file system, what rules should be applied in the validation such that: - The name will not be difficult to manipulate later in terms of escaping special characters, etc. - The rules are not so restrictive that saving a file becomes non-user-friendly. So basically, what is the minimum set of characters that should be restricted from a Unix file name?