Backward running version look-behinds

Nozomu Katō noz.ka at
Fri Nov 20 14:41:41 UTC 2015

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.


More information about the es-discuss mailing list