Mod rewrite from subdomain to directory on main

First I apologise for another mod_rewrite question. I have searched the forum and found some conditions that seem similar to mine, but I don't get it!

Here is my setup:

I currently have a wiki at a sub-domain with directory-esque pages, example:

[wiki.example.com...]
[wiki.example.com...]
[wiki.example.com...]
etc

I want to exactly map any requests from the sub-domain to a directory in the main domain, for example http://www.example.com/wiki.

So the original address of
[wiki.example.com...]
would permanently redirect to
http://www.example.com/wiki/samplepage

and the original address of
[wiki.example.com...]
would permanently redirect to
http://www.example.com/wiki/

and the original address of
[wiki.example.com...]
would permanently redirect to
http://www.example.com/wiki/sample1/sample2/sample3

etc

I think it should be simple but the answer as yet eludes me!

Any help greatly appreciated :)