Backward running version look-behinds

Claude Pache claude.pache at
Tue Nov 24 17:35:12 UTC 2015

> Le 20 nov. 2015 à 15:41, Nozomu Katō <noz.ka at> a écrit :
> 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

I've amended the spec in order to add .NET-style lookbehinds. It proved to be indeed relatively simple, once you get how it works. Here is the result (with diffs): <>

The most difficult part was to manage to output the token `<!` in ecmarkdown :-P


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