Regular expression for specific string and numeric combo

Question

I am looking for a specific regular expression that will validate my employee number:

1. Employee number will always be 8 characters in length
2. First 2 characters will always be either a DF or an EH (need to cater for case combos too)
3. Last 6 characters will always be numeric

I have the following:

Regex regex = new(@"^[a-zA-Z]{2}[0-9]{6}$");

And the check:

if (!regex.IsMatch(viewModel.EmployeeNumber))
    ModelState.AddModelError("EmployeeNumber", "Please provide a valid employee number");

It seems to work, but doesn't do like what I need it for, it only checks for letters in any combo, I need the specifics like explained above.

Can someone please assist and please correct my expression if not correct.