I need to create a cell format that will automatically
translate this:

123123123412345612312123

INTO THIS:

123.123.1234.123456.123.12.123

BUT, the values in each segment will span the range
involved. For example, the first segment could be 123 or
it could be 999; and the same for all the other segments.

Ideally, the user should be able to enter the info with or
without the periods.

I can't figure it out. Can someone please help?

Thanks in advance for your assistance.