Validating AppleScript input text
Can you approach it like this:

Take the first two characters:

--test to see if they are numbers.
-->If not return invalid.

Take the remaining characters:

--test for minimum length?

--> if not return invalid.

--test each character to see if they are numbers.

-->  If any character is a number return invalid.

Edit to add:

Rather than checking to see if CheckName is a number try converting it to a number:

try
    set CheckName to CheckNameDigits as number
on error
    display dialog CheckName &amp; " is not a number"
end try