Update 2012/07/15 all code updated with the new .htaccess rules (changed in WP 3.0). The code in this article should work with all versions of WordPress. </update>

I recently performed a series of tests on a fresh installation of WordPress 2.8.6 to determine the exact htaccess rewrite rules that WordPress writes to its htaccess file for various permalink configurations. Under the WP admin option menu, WordPress lists four choices for permalink structure:

1. Default: http://perishablepress.com/press/?=123
2. Date and name based: http://perishablepress.com/press/index.php/2006/06/14/sample-post/
3. Numeric: http://perishablepress.com/press/index.php/archives/123
4. Custom: /%year%/%monthnum%/%day%/%postname%/

The "default" option is to not use permalinks. The "date and name based" setting invokes the /index.php/%year%/%monthnum%/%day%/%postname%/ rule pattern. The "numeric" setting invokes the /index.php/archives/%post_id% rule pattern. And the "custom" setting invokes whatever pattern is specifically chosen. For our "custom" option, we chose the /%category%/%author%/%postname%/ pattern.

For the test, we began with the common "date and name based" permalink configuration. Then, after invoking the chosen permalink structure, the htaccess file was downloaded and the new WP rules were recorded. After this, the admin permalink settings were switched back to the "default" configuration. Finally, we deleted the new rules and uploaded the htaccess file. This process was diligently repeated for each different permalink configuration. It should also be noted that blog performance was checked during each round of testing.

The results indicate conclusively that WordPress uses the exact same set of htaccess rules for all permalink configurations. Surely this information is available elsewhere on the internet, however we were experiencing several inconsistencies related to permalink structure that inspired us to determine for ourselves the precise htaccess rules for WordPress permalinks.

Without further ado, the htaccess rules for all WordPress permalinks¹ are precisely either #1 or #2:

[ #1 ] If WordPress installed in the root directory »

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

[ #2 ] If WordPress installed in a subdirectory called "foo" »

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /foo/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /foo/index.php [L]
</IfModule>
# END WordPress

¹ Note: These results are valid for standard permalink structures invoked via a standard WordPress install and may not operate effectively for non-standard or highly specialized configurations. We assume that if you are hard at work hacking and tweaking, you must know what you are doing.