Convert to lowercase in a mod_rewrite rule

I would like URLs like server.com/foo to be case-insensitive. But server.com/foo actually gets mod_rewrite'd to server.com/somedir/foo

(A开发者_StackOverflowssume that all the files in "somedir" are lower case.)

So the question is, how to accomplish a mod_rewrite like the following:

RewriteRule  ^([^/]+)/?$  somedir/convert_to_lowercase($1)

PS: Here's a handy mod_rewrite cheat sheet -- http://dreev.es/modrewrite -- though it fails to answer this particular question.

PPS: Thanks to Bee and Ignacio for all the help with this. Also, here's a related question: RewriteMap activation

First, put the following line in the <VirtualHost> section of your .conf file. (For me that lives at /etc/httpd/vhosts.d/00foo.conf.)

RewriteMap lc int:tolower 

You can replace lc with any name you want. Then restart apache, which you can do with sudo service httpd restart.

Finally, add this in your .htaccess file:

RewriteRule ^/(.*)$ /${lc:$1} 

RewriteMap tolower int:tolower
RewriteRule  ^([^/]+)/?$  somedir/${tolower:$1}

I would make it a 301 redirect, NOT a URL rewrite, for SEO purposes:

RewriteMap tolower int:tolower
RewriteRule  ^([^/]+)/?$  somedir/${tolower:$1} [R=301,L]