Questions regarding ES6 Unicode regular expressions

Norbert Lindenberg ecmascript at lindenbergsoftware.com
Mon Aug 25 17:16:07 PDT 2014


On Aug 25, 2014, at 1:59 , Mathias Bynens <mathias at qiwi.be> wrote:

> Norbert’s original proposal for the `u` flag (http://norbertlindenberg.com/2012/05/ecmascript-supplementary-characters/#RegExp) mentioned the following:
> 
>> Possibly the definition of the character classes `\d\D\w\W\b\B` is extended to their Unicode extensions, such as all characters in the Unicode category “Number, decimal” for `\d`, as proposed by Steven Levithan. Whether this can be done under the same flag or requires a different one still needs discussion.
> 
> Has this been discussed any further? (I couldn’t find any mention of it in the meeting notes repository.) Should I file a bug?

The “needs discussion” part actually came from the March 2012 TC39 meeting:
https://mail.mozilla.org/pipermail/es-discuss/2012-March/021919.html
We subsequently had some discussions about how to go about such a discussion, which petered out because no regular expression expert was available to work with.

I suspect this issue needs a proposal rather than a bug.

> Norbert also suggested replacing ‘characters’ with ‘code points’ in sections like https://people.mozilla.org/~jorendorff/es6-draft.html#sec-characterclassescape and https://people.mozilla.org/~jorendorff/es6-draft.html#sec-runtime-semantics-charactersetmatcher-abstract-operation when the `u` flag is set. It seems the intent was to make e.g. `/\d/u` match `/[0-9]/`, and `/\D/u` match all Unicode code points except `[0-9]`. This is different from `/\D/` which only matches BMP code points.
> 
> It seems like this change has not propagated to the spec draft, though. Is this correct, and if so, what’s the reason for that?

Technically that works out as intended because section 21.2.2 defines “character” differently depending on whether the “u” flag is used or not:
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-pattern-semantics

It is somewhat confusing though, and it might be better to use a different spec mechanism. Ideas:
- Since we're processing based on Lists anyway, we could just use "element”.
- We could map code points to UTF-32 code units (1:1), and then consistently talk about code units, which would just have different sizes in the different modes.

> The same goes for `/[^a]/u` – should this match all Unicode code points except `a` or should it only match BMP code points?

As above – the definition of CharSet depends on the “u” flag:
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-notation

Norbert



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