Negative lookahead regex to include subdirectory of excluded directory includes the whole directory

[Void-025]

I'm trying to exclude everything in my /mnt/ directory except a specific drive ("HDD") using this exclude pattern:
re:/mnt/(?!HDD/)
This doesn't seem to be working, based on the size difference of using the above compared to just excluding the entire mount directory (about 5TB difference, even though "HDD" is only 2TB in capacity), it seems to be including the entire /mnt/ directory.
What's wrong with the exclude pattern I'm using?

[real-yfprojects]

about 5TB difference, even though "HDD" is only 2TB in capacity

The vorta GUI currently doesn't support all the pattern types borg supports. That's why the GUI won't show you the correct backup size at the moment. In contrast the backup itself should respect the exclude pattern.

[Void-025]

Oh I see, I'm assuming this means all the other normal exclude patterns I have setup (like: "/proc/") also need to have their leading slashes removed (to: "proc/")?

[Void-025]

With re:mnt/(?!HDD/) it seems to instead be excluding the entire /mnt/ directory instead of including it, even the /mnt/HDD/ which I want included (this is in the backups themselves, which are now 2TB smaller than they should be)...

[real-yfprojects]

That makes sense since (?!HDD/) will match the empty string. I would try the regular expression mnt/(?!HDD/|$).

[Void-025]

Ah I see, after fiddling around with it a bit I've settled on re:mnt/(?!HDD|$).*(?!/$)/.* which seems to work for me, thanks for the help!

[real-yfprojects]

I've settled on re:mnt/(?!HDD|$).*(?!/$)/.*

That's a very overly complicated pattern. It is aquivalent to re:mnt/(?!HDD).*/.+ to begin with. As far as I understand it this will only exclude items in subdirectories (unequal to HDD) of /mnt, not files in /mnt or the subdirectories themselves.

Rereading the borg docs you should prepend ^ to you pattern which matches the beginning of the path so that a mnt folder in /mnt/HDD isn't excluded.