Even something very rudimentary is fine. Like entering and evaluating something like x+1, where x is a variable I get to specify.
I was thinking of writing my own using Strings for each formula, regex for parsing them, etc., but let's assume my goal is to be as lazy as I can possibly get away with, and that if there's another option (especially a formal one), I'd rather try to use that instead, first.
Is there anything out there that does something like this?
You can think about using scripting from Java 6. The reference JDK comes with a JavaScript implementation, and you can plug in other languages too.