@JustSid: Thanks for your interest.
You're right, at the moment it's a context-free expression language with a little "lite-C" touch. The goal of the game is to give non-programmers a little feeling of how programming works. But maybe, if there's enough interest, real languages will be included or used in special editons of the game.
And to your other question: I created a function of my own to check the input of the player.
There are still some bugs in it I have to deal with, but I think he's on a good way!