Fixing mod_rewrite and mod_alias conflict

I have a VirtualHost setup like this :

Alias /somedir /some/other/dir

http://example.com/somedir works fine

However, I need to setup mod_rewrite for /somedir (a CodeIgniter app for which I want clean URLs). Here's the bit from the CodeIgniter wiki :

RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L]

Usually, when mod_rewrite is used in subdirs changing RewriteBase to match the name of the dir is enough to make it work :

RewriteBase /somedir

...but it doesn't seem to work with mod_alias (getting 404s). Is there a workaround for this problem ?

The following worked on my test server, so trying this might solve your problem..

In your virtual host:

Alias /somedir /some/other/dir

RewriteEngine On

# I think these conditions should work correctly; the other ones shouldn't
# because the alias has not been applied at this stage of processing yet
RewriteCond /some/other/dir%{REQUEST_URI} !-d
RewriteCond /some/other/dir%{REQUEST_URI} !-f
# L is unnecessary in this context, but be sure to PT so mod_alias picks up
# on the rewrite
RewriteRule ^/somedir/(.*)$ /somedir/index.php/$1 [PT]

Although I haven't tested your sample, in my own configurations, I haven't had much success with the PT flag, perhaps because of interactions with re-write rules in my config.

I've found it easier to restrict my use of aliases to the rewrite conditions, and always use the a file system path in the rewrite output.

Define SOME_DIR = "/somedir"
Define SOME_OTHER_PATH = "/some/other/dir"

Alias "${SOME_DIR}" "${SOME_OTHER_PATH}"

RewriteEngine On
RewriteCond ${SOME_DIR}%{REQUEST_URI} !-d
RewriteCond ${SOME_DIR}%{REQUEST_URI} !-f

RewriteRule ^${SOME_DIR}/(.*)$ ${SOME_OTHER_PATH}/index.php/$1 [NC,QSA,L]