Variable length look-behind

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.

How about:

(?:(?<=eat_apple_)|(?<=eat_pear_)|(?<=eat_orange_))(today|yesterday)

A little bit uggly, but it works.

You can use look-ahead instead of look-behind:

/(?:eat_(apple|pear|orange)_)(?=today|yesterday)/g

and in general, there is an alternative way to describe things that naively seem to require look-behind.