"Really regret" is a high standard; let's see what I can come up with:
Queen's Hot Space tour, 1982. Turned down a free ticket from the kid next door and his girlfriend. Told them I'd take them to see Queen the next time they came to town. There was no next time.
That last Soundgarden tour in 2017. I was ill that week, but goddamn, that Detroit show was their last show ever.
Mastodon's Emperor Of Sand tour, which hit town the same week Soundgarden's did. Again, illness did me in.
ELO's tour for Time, 1981 or 1982, with Hall and Oates opening. Really bad time in my life; concerts weren't really a priority for a while there...
I wish I had gone to Kiss' opening show at Tiger Stadium on the 1996-97 reunion tour just to see Alice In Chains open; I would've left right after they finished. Saw Kiss twice back in the '70s, and they got blown off the stage by their opener both times.
Juanes on his 2017 tour. Not that I give a fuck about him, but Mon Laferte was his opening act. Only time she's played here.
...damn, 2017 kinda sucked. 🤣😭