From the coreutils-5.97 info page:
Like I said, we're talking about grep, so you need to look at the grep manual. grep is not part of coreutils, so you're barking up the wrong tree again.
It *used* to be compatible with 'awk's regex
No, that's never been true. It wasn't true even back in the late 1970s, when I first used grep and awk. If you want to blame someone, blame the Bell Labs hackers who wrote them in the first place.
no tinfoil hat -- just a good memory, something you might find useful to work on! ;-)
In this particular case I'm afraid your memory has played tricks on you.
