[Exim] Redirecting mail to a Maildir subdirectory

My users' mail goes to ~/Maildir.

I'd like to search $header_subject: for "\[.*\]" and, if found, write
the message to ~/Maildir/.$subdir (where $subdir is what was found
between the brackets). I'd like it even better, if it would just fall
through to ~/Maildir if .$subdir doesn't exist.

I'd really appreciate it if anybody could give me some pointers as to
how (and where) this could be done. The current transport is:

local_delivery:
driver = appendfile
group = mail
mode = 0660
mode_fail_narrower = false
envelope_to_add = true
directory = "/home/$local_part/Maildir/"
maildir_format
--
derek