When typing in a number, how can I automatically have it insert a / (slash)?

I've been trying to figure out how to automatcially insert a forward slash (/) after I have entered a 6-digit number.  I'd like to have the slash inserted after the 3 number from the left?   For example; if after entering 123456 the expected outcome would be 123/456.  Any help would be great appreciated. I'd like to give away some points if anyone has the correct answer!!!