htaccess mod_rewrite problem

The problem is, I have multiple sites pointing to one site and the settings are loaded depending on the http host.
All of the sites should be looking at a folder with in my root directory.

I've done this once before and got it working but can;t remember what the code is for it.
This is what I have so far but I keep getting a server error.

RewriteCond %{HTTP_HOST} ^(m|mobile)\. [NC]
RewriteRule ^(.*)$ /mobile/$1 [L]

Code (markup):

So any thing like http://m.domain.com or http://mobile.domain.com should be a masked URL to http://m.domain.com/mobile/ or http://mobile.domain.com/mobile/

 

Any one got any suggestions?

The above isn't the only code in my htaccess file.
It start with the following:

RewriteEngine On
RewriteBase /

Code (markup):

 

I've got it working, there was a line I was missing by the looks of it.

RewriteCond %{HTTP_HOST} ^(m|mobile)\. [NC]
RewriteCond %{REQUEST_URI} !^/mobile/
RewriteRule ^(.*)$ /mobile/$1 [L]

Code (markup):