Look-behind proposal in trouble

Erik Corry erik.corry at gmail.com
Wed Oct 7 08:08:30 UTC 2015

Oops forgot the /u on the regexp in the example.

On Wed, Oct 7, 2015 at 10:06 AM, Erik Corry <erik.corry at gmail.com> wrote:

> 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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