Here's a validation rule that uses the following conditions:
- There has to be 2 dashes, in the exact same positions as shown.
- With the exception of the dashes, the value contains only numbers.
Step1: Select your cell and create a validation rule for it.
Step2: Choose Custom and enter the formula below (replace A1 with the cell you're referring to in your question).
=AND(ISNUMBER(MID(A1,1,2)+0), ISNUMBER(MID(A1,4,2)+0), ISNUMBER(MID(A1,7,2)+0), MID(A1,3,1)="-", MID(A1,6,1)="-")
Using this validation formula, the following sample entries will show the error "The value you entered is not valid" after the user de-selects the cell or presses Enter after typing the value.