How to make an If-Then-Else statement that accepts only Capitals

I have a UNIX script written in korn shell. I need to make it so that this statement:

while true
            do
                echo "What is the last name of the person you would like to modify:"
                read last_name
                if line=$(grep -i "^${last_name}:" "$2")
                then
                    IFS=: read c1 c2 c3 c4 rest <<< "$line"
                    echo -e "Last Name: $c1\nFirst Name: $c2\nState: $c4"
                    while true
                    do
                        echo "What would you like to change the state to?:"
                        read state
                            if [[ $state -eq [A-Z] ]];then
                                echo "State: $state"
                                echo "This is a valid input"
                                break
                            else
                                echo "Not a valid input:"
                            fi
                    done                        
                else
                    echo "ERROR: $last_name is not in database"
                    echo "Would you like to search again (y/n):"
                    read delete_choice
                    case $delete_choice in [Nn]) break;; esac
                fi  
            done
 ;;

Specifically, I am having trouble with this code:

if [[ $state -eq [A-Z] ]];then

The point of this program is to modify a record in a text file but will only take the input of state abbreviations such as (MI, WA, KS, ....).