Here is the CodeIgniter .htaccess that I use:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
#Removes access to the system folder by users.
#Additionally this will allow you to create a System.php controller,
#previously this would not have been possible.
#'system' can be replaced if you have renamed your system folder.
RewriteCond %{REQUEST_URI} ^system.*
RewriteRule ^(.*)$ /index.php?/$1 [L]
#When your application folder isn't in the system folder
#This snippet prevents user access to the application folder
#Submitted by: Fabdrol
#Rename 'application' to your applications folder name.
RewriteCond %{REQUEST_URI} ^application.*
RewriteRule ^(.*)$ /index.php?/$1 [L]
#Checks to see if the user is attempting to access a valid file,
#such as an image or css document, if this isn't true it sends the
#request to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^public.*
RewriteRule ^(.*)$ index.php?/$1 [L]
</IfModule>
<IfModule !mod_rewrite.c>
# If we don't have mod_rewrite installed, all 404's
# can be sent to index.php, and everything works as normal.
# Submitted by: ElliotHaughin
ErrorDocument 404 /index.php
</IfModule>
Generally speaking, the
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
rules should prevent existing files or directories from being rewritten to index.php, but you can also duplicate the
RewriteCond %{REQUEST_URI} !^public.*
rule to be
RewriteCond %{REQUEST_URI} !^forums.*
and it should accomplish the same goal, in your case.