Positive lookbehind after negative lookbehind

Question

Regex: (?<!xx)(?<=34)56(?=78) with global flag

12345678

xx345678

Why does it match 56 on both lines? Shouldn't (?<!xx) exclude second line?

Answer 1

You can use this regex with a nested negative lookbehind:

(?<=(?<!xx)34)56(?=78)

RegEx Demo

(?<!xx) inside the (?<=(?<!xx)34) will assert failure if xx is positioned just before 34.