Your proposal for look-behind relies on being able to count the match
length of the look-behind in order to step back that far.  This presupposes
that atoms like . and character classes have a fixed length.

However, with the /u flag, the . and some character classes can be either 1
or two code units.  This means you don't know how far to step back.  This
needs to be fixed in a way that is not incompatible with the "correct" .NET
way of doing things.

Eg matching /a.(?<!x..)/ against "xa😹"  (x, a, cat-face-with-tears-of-joy,
which is a surrogate pair).  The back reference has an apparent width of 3,
so we step back 3 code units, but that hits the 'a', not the 'x' and so the
back reference fails to spot the 'x'.


On Sun, Oct 4, 2015 at 1:52 PM, Nozomu Katō <noz.ka at akenotsuki.com> wrote:

> Apparently my proposal for adding the look-behind assertions to RegExp
> has been in trouble. I would like to ask anyone for help.
>
> The following story is what I know about the proposal after my previous
> post:
>
> I created a pull request for the proposal in July and sent an email to
> Brendan Eich asking if I can put his name as a champion:
> https://github.com/tc39/ecma262/pull/48
>
> I have not received a reply to my email, but I received a notification
> email in September that replying to the pull request, the proposal was
> moved to stage 0. Today, however, I just noticed that the proposal had
> been dropped from stage 0, stating "RegExp lookbehind has no champion".
> https://github.com/tc39/ecma262/commits/master/stage0.md (Oct 4, 2015)
>
> I am uncertain about what happened. Does this mean that Brendan Eich is
> no longer a champion or did not take a champion on from the beginning or
> ...?
>
>
> Regards,
>   Nozomu
> _______________________________________________
> es-discuss mailing list
> es-discuss at mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
>
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Your proposal for look-behind relies on being able to count the match
length of the look-behind in order to step back that far.  This presupposes
that atoms like . and character classes have a fixed length.

However, with the /u flag, the . and some character classes can be either 1
or two code units.  This means you don't know how far to step back.  This
needs to be fixed in a way that is not incompatible with the "correct" .NET
way of doing things.

Eg matching `/a.(?<!x..)/u` against `"xa😹"`  (x, a, cat-face-with-tears-of-joy,
which is a surrogate pair).  The back reference has an apparent width of 3,
so we step back 3 code units, but that hits the 'a', not the 'x' and so the
back reference fails to spot the 'x'.

Your proposal for look-behind relies on being able to count the match
length of the look-behind in order to step back that far.  This presupposes
that atoms like . and character classes have a fixed length.

However, with the /u flag, the . and some character classes can be either 1
or two code units.  This means you don't know how far to step back.  This
needs to be fixed in a way that is not incompatible with the "correct" .NET
way of doing things.

Eg matching `/a.(?<!x..)/` against `"xa😹"`  (x, a, cat-face-with-tears-of-joy,
which is a surrogate pair).  The back reference has an apparent width of 3,
so we step back 3 code units, but that hits the 'a', not the 'x' and so the
back reference fails to spot the 'x'.