This should not be a problem: With the /u flag, you work with code points, not code units. In particular, the `.` matches always a sequence (of code points with /u, or code units otherwise) of length 1.

—Claude


> Le 7 oct. 2015 à 10:08, Erik Corry <erik.corry at gmail.com> a écrit :
> 
> 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 <mailto: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 <mailto: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 <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 <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
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This should not be a problem: With the /u flag, you work with code points, not code units. In particular, the `.` matches always a sequence (of code points with /u, or code units otherwise) of length 1.