How can I use .htaccess to rewrite different domains to different sub-directories, masking the sub-directory?

Question

I have a shared hosting plan that allows unlimited addon domains, but they can only point to the public root (/). I use .htaccess to point different domains to their corresponding sub-directories. For example,

site1.com points to sub-directory /site1/
site2.com points to sub-directory /site2/

I don't want the user to see site1/ or site2/. My .htaccess below almost works. If the user goes to a subdirectory like site2.com/test/ (notice the trailing slash /), it works great. But if the user leaves out the trailing slash it shows site2.com/site2/test/ in the browser.

So my question boils down to, how can I mask the sub-directory whether or not the user types the trailing slash?

My root .htaccess looks like this (only with more domains, all in the same 3-lines per domain format)

RewriteEngine On

RewriteCond %{HTTP_HOST} ^(www.)?site1.com$ [NC]
RewriteCond %{REQUEST_URI} !site1/ [NC]
RewriteRule ^(.*)$ /site1/$1 [L]

RewriteCond %{HTTP_HOST} ^(www.)?site2.com$ [NC]
RewriteCond %{REQUEST_URI} !site2/ [NC]
RewriteRule ^(.*)$ /site2/$1 [L]

I've tried many things, from different online sources, but nothing works. One thing that almost worked (notice the trailing slash after $1):

RewriteRule ^(.*)$ /site2/$1/ [L]

This works correctly when the filename is omitted - site2.com/test/ - but if you go to site2.com/test/index.php it says "No input file specified."

The only file in root is .htaccess, everything else is in sub-directories.

Edit: Rewrote question, omitting irrelevant details.

Answer 1

I finally found a solution. The second RewriteCond in my above code was unnecessary, and the first RewriteRule below, found at B&T's Tips & Scripts, adds a trailing slash if none exists (unless it's a file, like index.php). Here's my new .htaccess. Notice the trailing slash directive only needs to be applied once.

.htaccess

RewriteEngine On

#ADD TRAILING SLASH TO DIRECTORY IF NONE EXISTS
RewriteRule ^([^\.]+[^/])$ http://%{HTTP_HOST}/$1/ [L]

RewriteCond %{HTTP_HOST} ^(www.)?site1.com$ [NC]
RewriteCond %{REQUEST_URI} !site1/ [NC]
RewriteRule ^(.*)$ /site1/$1 [L]

RewriteCond %{HTTP_HOST} ^(www.)?site2.com$ [NC]
RewriteCond %{REQUEST_URI} !site/ [NC]
RewriteRule ^(.*)$ /site2/$1 [L]

Edit

I'd orignally removed the second RewriteCond from each triplet above, and it worked for my main site + a couple of others. I didn't fully test, and later found a couple other domains I used infrequently weren't working (giving 500 internal errors). I added the second RewriteRule back in and they work again.

What's weird is, after I ended up deleting one of my working testing domain sub-directories and recreating it, the rewrites (without the second RewriteCond) stopped working (throwing the 500 errors). Adding the second RewriteCond made it work again.

Oh, well, I guess I'll chalk it up to the magic of mod_rewrite...as long as it's working.