[muk@kunkku-tki tmp]$ tar --exclude='.//a/dir' -cvf test.tar .
./
tar: ./test.tar: file is the archive; not dumped
./a/
./a/b
./a/dir/
./a/dir/c
[muk@kunkku-tki tmp]$ tar --exclude='./a/dir' -cvf test.tar .
./
tar: ./test.tar: file is the archive; not dumped
./a/
./a/b
[muk@kunkku-tki tmp]$