validate_date.sh

function validate_date() {
   if [[ $1 =~ ^[0-9]{4}/[0-9]{2}/[0-9]{2}$ ]]; then 
   	return 0
   else    	
   	return 1
   fi
}

while true; do
  echo
  echo "Please input the date of release (e.g., 2017/11/06)"
  echo -n ">>"
  read release_date
  validate_date ${release_date}
  if [ $? -eq 0 ]; then
    break
  else
  	echo "Date ${release_date} is in an invalid format (not YYYY/MM/DD)" 1>&2
  fi
done