So, how do you avoid it? Well, say you wanted to redirect some regex (regular expression) pattern to the your index page, say index.php, but it turns out that “index.php” actually matches your pattern, so index.php redirects to index.php which redirects to index.php, and so on, and so forth.

To prevent this, simply add a RewriteCond that checks the REQUEST_URI to make sure it doesn’t match the string that you redirect to. The following is an example.

RewriteCond %{REQUEST_URI} !^/?index.php$
RewriteRule ^(regex|goes|here)$ /index.php

Just make sure the regex in the first part does not match any of the pages you want to redirect, and that’s it! You’re done.

Best of luck on getting this to work the way you want it to, and happy web developing!

You may use either httpd.conf or an .htaccess at your document root.

Make sure you have mod_rewrite turned on before you begin.
RewriteEngine On

First, use a RewriteCond like this to prevent redirection things like your favicon.ico that need to be available everywhere. Change this based on your setup.
RewriteCond %{REQUEST_URI} !^/(favicon.ico|images/.+|javascript/.*)$

Then, capture the subdomain in %1.
RewriteCond %{HTTP_HOST} ^(.+)\..+\..+$
This next line just prevents looping by making sure the REQUEST_URI doesn’t match the what’s in %1. Don’t ask how it works.
RewriteCond %1,%{REQUEST_URI} !(^[^,]+),/\1.*

This checks to see if what we have in %1 is in fact a directory under the document root.
RewriteCond /your/document/root/%1 -d

And finally, redirect ‘/anything’ to ‘/subdomain/anything’.
RewriteRule ^(.*)$ /%1/$1 [L]

And there we go, that should do it. All together, that’s:
RewriteCond %{REQUEST_URI} !^/(favicon.ico|images/.+|javascript/.*)$
RewriteCond %{HTTP_HOST} ^(.+)\..+\..+$
RewriteCond %1,%{REQUEST_URI} !(^[^,]+),/\1.*
RewriteCond /your/document/root/%1 -d
RewriteRule ^(.*)$ /%1/$1 [L]


Questions, comments, criticism, and other feedback are very welcome. If you have a better way of doing it, let me know!