What's the best way to store an expression so that you can reevaluate it with, say, every update of your game's logic?. I'm talking of an expression like the following:

var = (1+2+3+XPos)*10

The expression is read from an external file and var and XPos are references to variables in my code. I already have code that can do the parsing of the expression and handle the references and I thought that storing the expression is Reverse Polish Notation would allow for easy evaluation and that is true for an expression like the one above but things get complicated as I try to introduce function calls where function arguments can be expressions as in:

var = 2*Min(E1, E2)

RPN is one way to go

It's not too hard to write a recursive descent parser, I wrote at one point a parser that could handle most Excel expressions.

For a more thorough approach with functions and variables, you could look into embedding Lua - Lua is designed for that type of thing.

I have a function parser that handles unary, binary, and arbitrary function expressions. I based everything upon a base function class, with a constant function as well. It would be too hard to rip code out, but you can do it with some effort.