#!/bin/bash
validinput=0

while [ "$validinput" != "1" ]
do
        echo "Please enter an RFC number"
        read rfcnumber

        case $rfcnumber in
                "")
                        echo Either type in your RFC number, or enter N/A if one is unavailable.
                        ;;
                N/A)
                        echo You have indicated an RFC number is not available.
                        validinput=1
                        ;;
                *)
                        if echo "$rfcnumber" | /bin/grep -E "^[0-9]{5,}$" >/dev/null 2>/dev/null
                        then
                                echo Valid RFC number entered.
                                validinput=1
                        else
                                echo "Invalid RFC number entered"
                                echo "Please enter a minimum of five digits only, or N/A if you don't have one"
                        fi
                        ;;
        esac
done

echo "You provided the following valid input:"
echo $rfcnumber

$ ./script.sh
Please enter an RFC number

Either type in your RFC number, or enter N/A if one is unavailable.
Please enter an RFC number
N/A
You have indicated an RFC number is not available.
You provided the following valid input:
N/A
$ ./script.sh
Please enter an RFC number
123
Invalid RFC number entered
Please enter a minimum of five digits only, or N/A if you don't have one
Please enter an RFC number
ABC
Invalid RFC number entered
Please enter a minimum of five digits only, or N/A if you don't have one
Please enter an RFC number
a12345
Invalid RFC number entered
Please enter a minimum of five digits only, or N/A if you don't have one
Please enter an RFC number
12345a
Invalid RFC number entered
Please enter a minimum of five digits only, or N/A if you don't have one
Please enter an RFC number
1234-567
Invalid RFC number entered
Please enter a minimum of five digits only, or N/A if you don't have one
Please enter an RFC number
123456
Valid RFC number entered.
You provided the following valid input:
123456