[Tipewryter]

The solution...

    not_this|(but_this)

... is interesting. But since it returns the match in a submatch I would say the \K approach is better:

    (?:not_this.*?)*\Kbut_this

Because usually when you try hard to accomplish something with a regex, you do not have the luxury to say "And then please disregard the match and look at the submatch instead".

[lifthrasiir]

That doesn't work. `(?:"Tarzan".*?)*\KTarzan` should behave identically without `\K`, and it will match `"Tarzan" "Tarzan"` because the ungreedy quantifier ? still allows backtracking (it just changes the search order). You want the possessive quantifier + instead; `not_this|(but_this)` is equivalent because regexp engines will not look back into once matched string.

[Tipewryter]

Interesting. I took the \K solution right from the article without trying it.

Now that I try it, it indeed does not work.

Maybe the author reads this and can look at it.