The program will accept all letters of the alphabet (non-case-sensetive), logical connectives, and parentheses. Using numbers will throw an error. The program only evaluates well formed formulas, and will inform you if yours is not properly formatted. Well formed means that all variables and connectives are in appropriate parentheses (the full formula should also be surrounded), there are no incomplete sets of brackets, and that in the initial half of the proposition, the amount of left brackets is greater than that of the right brackets.