Variable length look-behind

Question

Is there any elegant solution to build a variable length look-behind regex such as this one ?

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

It seems Perl has a very impressive regex engine and variable length lookbehind would be very interesting. Is there a way to make it work or should I forget this bad idea ?

Answer 1

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.