Need help regarding form validation

Hi i need help regarding form validation. I have this textfield named idNum and users are supposed to key in their id numbers into the field eg (S1234567D, F1234567Z). The id numbers either start with a S or F followed by 7 numbers and then end with an alphabet (A-Z). How can i check if the id number the user keyed in is valid? I'm using VBScript to code and the format is something like that:
If Request.Form("idNum") = "" Then
validateForm=False
errorMsg = errorMsg & "Please key in your ID number.<br>"
End If

Thanks! Hope to get a reply soon.

What I would do would be create a function that tests it for you.

Something like (Note I have not coded in a while bear witht he sytax and such it is the idea)

Sub cmdSubmit_OnClick

dim first as string
dim numeric as int
dim last as string
dim correct as boolean

first = mid$(idNum, 0, 1)
numeric = mid$(idNum, 1, 7)
last = mid$(idNum, 8,1)

correct = isNumeric(first)
if correct == true then
Msgbox "Invalid Data in the ID Box",, "Invalid Data"
exit sub
end if

correct = isNumeric(numeric)
if correct == false then
Msgbox "Invalid Data in the ID Box",, "Invalid Data"
exit sub
end if

correct = isNumeric(last)
if correct == true then
Msgbox "Invalid Data in the ID Box",, "Invalid Data"
exit sub
end if

end sub

If it makes it through something like that (There are better ways to do this I am pretty sure but I am having a brain fart) then the ID Number is valid.

hope that helps a bit.. if any.