LANG=en_US.UTF-8

shopt -s extglob
a="aaaäöü"
a1=${a:3:3}

[[ "${a}" == "${a1}" ]] || {
	echo cond ok 1
}

case "${a//?aa}" in
"${a1}")	echo ok 1;;
*)		echo bad 1;;
esac

case "${a//\aaa}" in
"${a1}")	echo ok 2;;
*)		echo bad 2;;
esac

case "${a//aaa}" in
"${a1}")	echo ok 3;;
*)		echo bad 3;;
esac

case "${a//@(?aa)}" in
"${a1}")	echo ok 4;;
*)		echo bad 4;;
esac

exit 0