Taking a system as input in MATLAB App Designer

I need to extract a system function from the data typed by the user into a text box.

function out_sig = systemSelector(app,x,t)
            sys_strstart = '@(x,t)';
            sys_string = app.SystemyEditField.Value;
            sys_string = strcat(sys_strstart,sys_string);
            sys_handler = str2func(sys_string);
            out_sig = sys_handler(x,t);
end

This is the function I wrote which takes the input provided by the user in terms of x and t.

But, for this to work, I have to provide strings like 't.*x' and '(t.*3).*(x.^2)' which are not user-friendly. Besides the user cannot enter systems which have a delay, like 'x(t-5)'.

Can anyone please suggest a new function or update my existing function so that I can take the input in a better manner?