Periods in :id causing Apache routing error

Using Rails 1.2 and FastCGI on Apache 1.3.

I'm trying to figure out why a request like this isn't making it into
my Rails app: http://www.domain.com/users/show/a.b.c

The user id is "a.b.c", and because of the periods in there, Apache is
giving this error:

File not found
Change this error message for pages not found in public/404.html

If the :id does not have the periods, it's fine, and the page is
served by Rails.

I'm not sure where the problem lies... But, here's my .htaccess
RewriteRule ^$ index.html [QSA]
RewriteRule ^([^.]+)$ $1.html [QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ dispatch.fcgi [QSA,L]

Is this an Apache config thing, or more of a Rails routing thing?

Ah, that makes sense.

Thanks!