Use \K as a special case.
It’s a variable length positive lookbehind assertion:
/eat_(?:apple|pear|orange)_\Ktoday|yesterday/g
Alternatively, you can list out your lookbehind assertions separately:
/(?:(?<=eat_apple_)|(?<=eat_pear_)|(?<=eat_orange_))today|yesterday/g
However, I would propose that it’s going to be a rare problem that could potentially use that feature, but couldn’t be rethought to use a combination of other more common regex features.
In other words, if you get stuck on a specific problem, feel free to share it here, and I’m sure someone can come up with a different (perhaps better) approach.