Determine value

[hin]

Hi!

I have a Text field call strValue. I like to allow user to input
values like:

1
2
1.1
1.2
A
B

But not:
1.A
1.1.A
A.1

Is there a way to determine if someone inputted these kind of values?
I don't want any users to input values that have Char and Number mixed
together.

Thanks!