Subfolders and mod-rewrite

Answer 1

Add

RewriteCond ^admin/
RewriteRule (your Rule) [L]

before the other condition. That should do the trick.

Another way would be to include the condition in the rule directly via

RewriteRule ^admin/(rest_of_regex)$ (regex_stuff) [L]

Answer 2

Add this to your root .htaccess:

RewriteCond ^admin/
RewriteRule (whatever rule you need for the admin folder) [L]

Add this right after the RewriteEngine On. The key point here is [L] which tells mod-rewrite to stop executing the rest of the rules if this one matches. If you need more than one rule for the admin folder, just add more RewriteRule statements, but remember to only add the [L] to the last one.

That should do it.