String position in uicontrol

In the following code

    h=uicontrol('Style','text', 'String',str, ...
      'Units','normalized', 'Position',[0.1 0 0.9 0.7]);

the str is 'abc def - ghi, jkl'

How can I access the position of the characters a,b,c etc from the Position vector [0.1 0 0.9 0.7] ?

If it can be done in pixel units, thats also fine.