So, I've thought a lot about this question since you posted it a month ago. From what I understand, you're constructing a string as the user clicks/taps buttons.
I think this is your problem.
As the user taps buttons, you should be constructing (or modifying) DDExpression
objects. This is the "pure" format of a math expression, whereas a string is lossy and difficult to manipulate. The string you show to the user should be generated from the DDExpression
tree you're building.
This is a complex problem, and I'm still not entirely sure how I would go about implementing this, but this is the root of how I'd do it. I would not just construct a string based on what the user types.