I couldn't find the link in that page, but the fork is here, and seems to be up-to-date: https://github.com/GerHobbelt/re2

If you need that from Go, you can probably use that to create a fork of this: https://github.com/wasilibs/go-re2

The RE2 fork from the blog post above is this one: https://github.com/epfl-systemf/re2-lookbehinds