You can use: letters without accent (a ... Z), figures (0 ... 9), apostrophe
( ' usable instead of accents), space, operation symbols (- + * / ^),
equal (=), comma (,), colon (:), semicolon (;), full, exclamation and question points (. ! ?) [but you can restrict the mode of operation to some subsets of the alphabet]
Text can be as long as you want (but it must be without line breaks).