Control the number and letter input in ASP

Recently, some people have asked me how to control the input values in ASP, just like in objective Crip, only English or numbers. Now I want to sort them out as follows:
Method 1:
Str = Request ("string ")
Temp = Server. URLEncode (str)
If str = temp then
Response. write "you did not enter English letters or numbers"
End if
Grammar 2:
Str = Request ("string ")
For I = 1 to len (str)
A = mid (string, I, 1)
If (asc (a) <"0" and asc (a)> "9") or (asc (a) <asc ("a") and asc (a)> asc ("z") then
Response. write "you did not enter English letters or numbers"
End if
Next I