How to compare input characters for loop which are in string

Question

Ok I am writing a code and I have a row and column. Row has values from A to J and column has values 0 to 9. Now I am trying to put a for loop to check if the input is valid character and not integer for row. How am I gonna check it?? Please tell me what condition I require to only allow in put in form or A B C D till J.