.htaccess mod_rewrite - how to exclude directory from rewrite rule

What you could also do is put a .htaccess file containing

RewriteEngine Off

In the folders you want to exclude from being rewritten (by the rules in a .htaccess file that's higher up in the tree). Simple but effective.

Try this rule before your other rules:

RewriteRule ^(admin|user)($|/) - [L]

This will end the rewriting process.