Edit box - Want an error message when a negative value is entered (Beginner)

Can anyone explain to me why this is not working for me. There are no error messages - it just doesn't give me any error box.

function edit1_Callback(hObject, eventdata, handles)

function edit1_CreateFcn(hObject, eventdata, handles)

strng=get(gcbo,'string');

if ~isempty(strng)
val=str2num(strng);
    if val < 0 
      errordlg('Value cannot be negative','Error','modal');
      set(gcbo,'string','Try again');
    end
end

Answer 1

[Mathew]

This worked

function edit2_Callback(hObject, eventdata, handles)
strng=get(gcbo,'string');

      val=str2num(strng);
      if val < 0 
        errordlg('Value cannot be negative','Error','modal');
        set(gcbo,'string','Try again');
      end