Challenges don't accept ES8?

I used a ES8 regular expression feature, specifically, the look behind (?<(!)) expression for one of the challenge in Basic Algorithm Scripting module and, it didn’t pass any test. But, it works in other websites like regexp101, etc.

I’m not sure regex you wrote would work anywhere - at least to my knowledge. The correct syntax would be:

(?<=) - positive lookbehind
(?<!) - negative lookbehind