i need an input mask which does this:

input: 1234567890
display: 12 3456 7890

BUT also does this at the same time:
input: 12345678
display: 1234 5678

In other words, it has to be able to tell that how many digits are there. If there are 10 digits, it will automatically format the digits as shown. If 8 digits, it'll show the latter.

Hope someone can help. Thanks.