Validating numeric input in applescript

Question

I'd like to check if user has entered text with the first 2 characters as integers  (e.g. 99smith).  User names at my university are in this format.  But my first attempt does not seem to recognize the numbers. 
  display dialog "Enter CUA user name:  (like 99smith)" default answer "" giving up after 40
  set {userName, returnedButton, gaveupBoolean} to the result as list
  -->{"some text", "OK", false}
  if userName is not "" then
  # take first 2 characters for variable CHECKNAME
  set CheckName to text 1 thru 2 of userName & " --test"
  display dialog CheckName & " -- 1"
  try
  set CheckNameNumber to CheckName as number
  set isInputValid to true
  display dialog CheckName & " -- 2"
  exit repeat
  on error number errorNumber
  if (errorNumber = -1700) then
  #display dialog "INVALID INPUT. TRY AGAIN."
  display dialog "Name does not start with 2 digits  " & CheckName & " -- " & userName
  end if
  end try
  end if

Answer 1

Here's the core of it. I'm not sure what failure you were finding, but one thing I noticed is that you didn't test each integer separately.
You need to check to do that because an integer followed by a dot or a space will not return an error if you convert the first two characters to a number together.
display dialog "Enter user name" default answer ""
set checkname to the text 1 thru 2 of the text returned of the result
try
  set firstDigit to text 1 of checkname as number
  set secondDigit to text 2 of checkname as number
on error
  display dialog "Name does not start with two digits"
end try