Imagine two tags, X and Y. Is it possible to search for something like "all posts that aren't tagged X, unless they're also tagged Y"?

harrybenson said: "all posts that aren't tagged X, unless they're also tagged Y"

By that, do you mean:
NOT X && NOT Y: "posts that are not X, except those that also have Y", or
NOT X || ( X && Y): "posts that are not X, plus posts that are both X and Y"

The former is just -x -y. I don't think that the latter is possible with the current search engine.

bitwolfy said:
By that, do you mean:
NOT X && NOT Y: "posts that are not X, except those that also have Y", or
NOT X || ( X && Y): "posts that are not X, plus posts that are both X and Y"

The former is just -x -y. I don't think that the latter is possible with the current search engine.

I think he means the second. Also NOT X || ( X && Y) can be simplified to NOT X || Y

add X -Y to your blacklist for the duration of the search, and then remove it. That's the closest you'll get.

I'm parsing this to mean "(NOT X) OR Y", but I believe there's no way to describe this in a search query. You can search for -X (NOT X), you can search for Y -X (Y AND NOT X), but the ~ syntax does not work as expected when searching for -x ~y. ~ is not "OR", per se. It's more like "at least one of these".