How to make a character("/" or "-") as default character at a position in the text field in iPhone?

Question

I have to implement the date format like, "2010/08/03" or "2010-08-03". In this, the user will enter only the numeric in the date field.(User will enter, 20100803). As the user enter the 5th character(here in the example "0") the "/" or "-" has to be added automatically.

Please suggest me how can I do it? Thanks in advance for your help.