mod_rewrite does not rewrite - apache

Question

I have the following code:
RewriteEngine on
RewriteRule ^user/([a-z]+)$ index.php?Agent=$1 [QSA]
theoretically any visitor to my site using the format:
www.site.com/user/username
should be directed to:
www.site.com/index.php?Agent=username
they are instead being treated as if /user is a real directory and as if /user/username is a valid file and exists (which it does not). Any ideas?
EDIT #1
I have modified my htaccess slightly and even stripped off the /user requirement, the following .htaccess file causes and 500 error, even when visiting the primary domain alone (www.site.com):
ErrorDocument 404 /404.php
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*)$ $1.php
RewriteRule ^(.+)$ index.php?Agent=$1 [QSA]

Answer 1

they are instead being treated as if /user is a real directory and as if /user/username is a valid file and exists (which it does not).
What exactly do you mean like this? What does it show?
Do you want to rewrite or **redirect?
Rewrite = call www.site.com/index.php?Agent=username when the user enters www.site.com/user/username
Redirect = redirect the user to the other URL so their address field says www.site.com/index.php?Agent=username
Right now, your code should rewrite, not redirect, which is probably what you want. Just checking...
Your problem might be that the username you're testing with contains capital letters. Add NC to make the match case insensitive.
RewriteRule ^user/([a-z]+)$ index.php?Agent=$1 [QSA,NC]
If you want to allow other characters than just the letters a-z, you can either add more characters to character class, or just write (.+) to match any character.
Edit
As per the question below, I think the infinite redirect happens because the rule is triggered again (and again) because the rewrites matches the new index.php request. Simply checking that the target of the redirect isn't a file should solve that. (-f = "is file". ! = negate condition.)
RewriteCond %{REQUEST_FILENAME} !-f

Answer 2

It should work. I am using pretty much that exact line for my rewrite. Here's what I've got:
RewriteRule ^history/(.+)?$ history.php?display_name=$1 [QSA,L]
Note the "L" flag, which means mod_rewrite should stop at this point, and not process any other rewrite rules. Do you have any other rewrite rules after this one?
Are you putting this in .htaccess? Make sure it's being processed at all. Put some random junk in there and load a page. Apache should show you an error page, complaining about a server misconfiguration. If it doesn't, it's not even looking at your .htaccess file. Make sure in your apache configuration, "AllowOverride" is set to "FileInfo Options" for the directory in which this webpage lives.

Answer 3

This is a stab in the dark but I suggest that you recheck your Apache configuration file apache2.conf file to make sure that mod_rewrite is enabled.
Look for a line like this :
#LoadModule rewrite_module modules/mod_rewrite.so
and change it to:
LoadModule rewrite_module modules/mod_rewrite.so
EDIT: Then restart your web server for the changes to take effect!
Hope this helps!