stringZ="    "

if [[ "$stringZ" == ^[[:blank:]][[:blank:]]*$ ]];then
  echo  string is  blank
else
  echo string is not blank
fi

string is not blank   # wrong

case $string in
  (*[![:blank:]]*) echo "string is not blank";;
  ("") echo "string is empty";;
  (*) echo "string is blank"
esac

if [[ "$stringZ" =~ ^[[:blank:]][[:blank:]]*$ ]];then
  echo  string is  blank
else
  echo string is not blank
fi

if [[ "$stringZ" =~ ^[[:blank:]]+$ ]]; then