shell-popd-regexp not working?

[Chad Brown]

I've long used the alias `bd' for popd, but I've never bothered to
make it work in emacs' shell mode before. Today, I ran into the
issue again, and I decided to fix it. After looking at the doc, I
customized shell-popd-regexp from `popd' to `bd'. It doesn't seem
to do what I thought it should do: if I use `bd' in a shell buffer,
emacs does not change the current directory.

Am I doing something wrong here?  I'm not very familiar with
shell-mode.