Backward running version look-behinds

Erik Corry erik.corry at
Tue Nov 24 09:21:35 UTC 2015

I can speak only for myself.  I like the .Net-style lookbehinds, and I hope
they will be part of the standard.  For something to be in the standard we
need both implementations and someone to describe the desired behaviour in
the standards document.  It looks like implementations are being found.
Hopefully someone can write the document so this can move forward.

On Fri, Nov 20, 2015 at 3:41 PM, Nozomu Katō <noz.ka at> wrote:

> I was expecting that ES6 would come with look-behinds, because a
> proposal had been put at:
> However, ES6 does not support them. I noticed that the link to the
> proposal had been struck-through:
> I wondered what was a problem. I did research to know the situation
> about look-behinds, and I found this post:
> I realised that a spec needed to be written by someone, but "someone"
> had not appeared yet. Thus, I wrote a spec, subscribed to es-discuss,
> and posted the spec. What made me decide to post that spec was this
> post and thread.
> But now, it turns out that look-behinds similar to the proposal that has
> been struck-through have been implemented experimentally in Chromium and
> Gecko. I am confused about the ongoing situation.
> I am NOT an objector against .NET-compatible look-behinds. But I wonder
> if there is someone who writes a spec for them. I have no idea how the
> behaviours of look-behinds based on the .NET implementation are
> described in the language used by the ECMAScript spec. Introducing an
> internal direction switch might be a relatively simple way, but I have
> no concrete idea even about it.
> Nozomu
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