Using different validation rules based on user input.

Question

I have a simple form: a combobox and a textbox.
My combobox has 2 values: A and B. When value A is selected I want textbox to use a validation rule. When value B is selected there should be no validation rules applied to the textbox.

I've read an article that has a solution and I'm trying to use it, but had no luck so far, and I think there might be a more elegant solution.

Has anyone done anything like this before?

Any ideas are highly appreciated.