Targeting single directory for rewrite rule

Targeting single directory for rewrite rule

I need a rewrite rule to act on just one directory. I have tried:
RewriteCond %{REQUEST_URI} ^/test_db RewriteRule ^([^/])/([^/])/$
/test_db/main.php?page=$1&dep_id=$2 [L]
The conditional is doing something as the rewrite rule is not being
applied outside of test_db but its also not doing anything to files
requested in test_db either. Any idea how to make this work?