.htaccess cascading conditions - .htaccess

Question

I'm trying to figure out how to manage potentially conflicting conditions in .htaccess
My setup is the following:
- I have a CMS running on a server that can be accessed through myCMSdomain.com where myCMSdomain.com would be CMS home page and myCMSdomain.com/admin would be the admin interface.
- Sites using this CMS should be pointing to myCMSdomain.com/sites/index.php
- Images for all sites are available somewhere behind myCMSdomain.com/admin/images/sitename/...
So here is how I tried to tackle this problem:
RewriteCond %{HTTP_HOST} !^(www.)?myCMSdomain.com$ [NC]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule . sites/index.php
With this, all incoming requests coming from other domain names are treated correctly by the index.php file but as images are hosted somewhere else, I'd like to use a rule saying that images should be fetched somewhere else like this:
Visible url format for images is: images-[sitename]/[image_path]
Real location of the images is: admin/site/[sitename]/[image_path]
The following rule works but not in combination with the first rule
RewriteRule images-([a-zA-Z0-9]+)/(.*)$ admin/site/$1/images/$2
Images end up calling index.php instead of using the rule I defined for them.
I have tried to excluse the image directory from the conditions but it doesn't work either:
RewriteCond %{REQUEST_URI} !(images-([a-zA-Z0-9]+)/(.*))
I might have similar issues in the future with other exception so I was wondering if there was a way to handle this.
Thanks!
Laurent
UPDATE 1:
If I use the following rule on top of all other
it works only if I'm using myCMSdomain.com domain name
if I use any other domain like anotherdomain.com, the rule leads to a http 500
RewriteRule images-([a-zA-Z0-9]+)/(.*)$ manager/site/$1/images/$2
So http://www.myCMSdomain.com/images-test/test.jpg leads me to the correct image
But http://www.anotherdomain.com/images-test/test.jpg leads me to a 500 http error code while this domain is pointing correctly to sites/index.php
UPDATE 2:
On Justin's request, here is a view on the physical directory structure on the server
/admin/
/admin/site
/admin/site/site_name/
/admin/site/site_name/images/
/sites/
/sites/js
/sites/css

Answer 1

You can rearrange your conditions logic.
# if www.myCMSdomain.com or myCMSdomain.com -> do nothing
RewriteCond %{HTTP_HOST} ^(?:www\.)?myCMSdomain\.com$ [NC]
RewriteRule ^ - [L]
# if we reach here, this means it's a subdomain/another domain
# images rule
RewriteRule ^images-([^/]+)/(.+)$ /admin/site/$1/images/$2 [L]
# not a file/directory -> sites/index.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^ /sites/index.php [L]
It is also possible the way you did but it would be longer to write.
Also, keep in mind that conditions (RewriteCond) are non-cumulative: they are for next rule (RewriteRule) only.

Answer 2

I've been struggling with this issue for some time now but at last I think I have found the issue. It looks like there was something wrong in the domain name configuration, I don't know exactly what but once I had re-saved the mapping of all domains, the htaccess worked the way it should.
In the end I have used Justin's proposition, it looks more future proof than mine.
Thanks for your help and happy new year to all
Laurent