Backward running version look-behinds

Gorkem Yakin goyakin at
Wed Nov 25 08:58:13 UTC 2015

Lookbehind assertions were discussed at the TC39 meeting last week and the committee is in favor of the .NET-style version.


From: es-discuss [mailto:es-discuss-bounces at] On Behalf Of Erik Corry
Sent: Tuesday, November 24, 2015 11:22 AM
To: Nozomu Katō <noz.ka at>
Cc: es-discuss <es-discuss at>
Subject: Re: Backward running version look-behinds

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<mailto: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.

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