It seems to me that Google once had wonderful regular expression searches. That all got blown up when they introduced "Google+", thereby destroying REs.
Could you explain more about this "Basic" free tier? When I go to https://groups.io/static/pricing, I see a free 30-day account, after which it becomes $20/month. You're not the first person who has said there's a free plan at groups.io but I don't seem able to get one myself. Have their plans changed recently?
It has seemed to me for quite some time that Amazon deliberately tries NOT to return what you are searching for. Oh, they'll give you one or two items that fit your search criteria, but they'll also return lots of stuff that they think you'll be interested in and possibly buy.