How can one assigning an input on a GUI edit text box as a function for integration?

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I need to create a basic GUI that will provide a definite integral for a function the user defines in an edit text box and selected upper and lower limits of integration.

I have tried the str2func function to convert the string from the box into a function, but I receive the following errors:

* Warning: The input to STR2FUNC "(x)x.^2+2*x" is not a valid function name. This will generate an error in a future release. 
* > In HW9_gui_JTW>compute_Callback at 85
*   In gui_mainfcn at 96
*   In HW9_gui_JTW at 42
*   In @(hObject,eventdata)HW9_gui_JTW('compute_Callback',hObject,eventdata,guidata(hObject)) 
* Undefined function '(x)x.^2+2*x' for input arguments of type
* 'double'.
* 
* Error in quad (line 72)
* y = f(x, varargin{:});
* 
* Error in HW9_gui_JTW>compute_Callback (line 86)
* s = quad(f,a,b);
* 
* Error in gui_mainfcn (line 96)
*         feval(varargin{:});
* 
* Error in HW9_gui_JTW (line 42)
*     gui_mainfcn(gui_State, varargin{:});
* 
* Error in
* @(hObject,eventdata)HW9_gui_JTW('compute_Callback',hObject,eventdata,guidata(hObject))
* 
*  
* Error while evaluating uicontrol Callback
My GUI consists of 3 input text boxes (lower_lim, upper_lim, function_input) and a push button (compute)
Here is the code to the compute_Callback, The only part of the code I have added to:
*% --- Executes on button press in compute.
function compute_Callback(hObject, eventdata, handles)
% hObject    handle to compute (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
a = str2num(get(handles.lower_lim,'String'));
b = str2num(get(handles.upper_lim,'String'));
y = get(handles.enter_function,'String');
d = ['(x)' y];
f = str2func(d);
s = quad(f,a,b);
sol = num2str(s);
set(handles.solution,'String',sol);*