htaccess Rewrite path to subdomain except the Index Document - apache

Question

I have the following structure in my website and I need to redirect all the traffic to a subdomain
Structure:
domain.com/subdomain/
.../folder1/folder/1.txt
.../folder1/folder/2.txt
.../folder2/folder/1.txt
.../folder2/folder/1.txt
.../index.php
I want to redirect all the traffic except the index.php or (domain.com/subdomain/) to a subdomain
Example:
domain.com/subdomain/folder1/folder/1.txt ---> subdomain.domain.com/folder1/folder/1.txt
domain.com/subdomain/folder1/folder/2.txt ---> subdomain.domain.com/folder1/folder/1.txt
But
domain.com/subdomain/index.php ---> No redirect
domain.com/subdomain/ ---> No redirect
This is what I came up with:
Options +FollowSymlinks
RewriteEngine on
RewriteRule ^(.*)$ http://subdomain.domain.com/$1 [NC]
This works for all the request but I want to exclude both / & index.php from this RewriteRule
Thanks!

Answer 1

You'll need a bit more than you have there. You must first check the domain with RewriteCond. The pattern ^subdomain/?(index\.php)?$ should match requests to the subdomain root or index.php with or without /. Technically it would also match the invalid subdomainindex.php without a / between but that would result in a 404 anyway.
RewriteEngine On
# If the requested domain isn't already the subdomain...
RewriteCond %{HTTP_HOST} !^subdomain\. [NC]
# Rewrite it to the subdomain
# unless it is a request for index.php or /
RewriteRule ^subdomain/?(index\.php)?$ - [L]
RewriteRule ^subdomain/(.*) http://subdomain.com/$1 [L,R=301,NC]