Prevent wordpress autoredirect

Not really a WP-specific question, but I’ll help you out. That regex will match ex.com/helloworld/ but not ex.com/helloworld.

To handle both you’d want RewriteRule ^helloworld/?$ /careers/helloworld-recruiting/ [L,R=301]

The question mark makes the preceding token optional and the single slash rather than double in the rewritten URL makes it relative to the root.