The experimental implementation [0] in V8 landed a few days ago and is included in the latest Canary build (49.0.2568.0 and later). You can test it after enabling it with the command line flag --js-flags="--harmony-regexp-lookbehind". Apparently the port to SpiderMonkey is already underway [1].
This implementation supports variable length lookbehind similar to .NET's semantics. It does so by emitting code to read backwards inside the lookbehind. The size of the change without platform ports and tests is about 600 lines. [0] https://codereview.chromium.org/1418963009/ [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1225665
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