[Exim] splitting up local_part

Is there a nice way to do the following?

For mail delivered to userid.folder.subfolder@domain
I would like to split up $local_part into $userid and $folder where
$folder = folder.subfolder and $userid = userid
and use the following transport

local_delivery:
driver = pipe
command = "/usr/cyrus/bin/deliver -m ${folder} ${userid}"
user = cyrus
group = mail
return_output
return_path_add

How would this be implemented?

Is it fair to say that this facility would not be available if using
deliver in batch/lmtp mode?


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P Christie
ccsphc@???