It does do negative lookahead though - https://regexper.com/#%2F%5Cd%2B(%3F!%5C.)%2F

Negative lookbehind only appeared in Chrome in v.62 (https://v8project.blogspot.co.uk/2017/09/v8-release-62.html) it's not too surprising that tools haven't caught up yet.

I'm sure they'd appreciate a ticket - There's already this which is similar: https://github.com/javallone/regexper-static/issues/26