unset var
YES='[Yy]?([Ee][Ss])'
NO='[Nn]?([Oo])'
while [[ "$var" != $YES && "$var" != $NO ]]
do
        printf "Enter value: "
        read var

        case $var in
        $YES)
                echo "Yes"
        ;;
        $NO)
                echo "No"
        ;;
        esac
done