Tab functionality to navigate inside the input positions in masked text box control

Hi,
 
I have a masked text box custom control which accepts text in the format
" _ _ w _ _ d _ _ h _ _ m". My requirement is to have the tab functionality working inside the control. Say if my cursor is in the first input position ( first underscore before the w letter ( _ _ w )). Then on pressing tab, it should go to the next input position. Please guide.

Thanks,

Divya