¿How to ask for a function given by the user in a GUI?

I am currently working on a GUI. I need a code or a function that allows me to evaluate a function given by the user in a certain range of values also given by the user. For example, the user types : x^2 and evaluate it between 1 and 2. Thank you for your help Juan Pablo

[per isakson]

Hint:

    >> foo = str2func('@(x) x^2')
    foo = 
        @(x)x^2
    >> foo(3)
    ans =
         9