Backwards compatibility and U+2E2F in `Identifier`s

Mathias Bynens mathias at
Wed Sep 18 13:32:50 PDT 2013

On 18 Sep 2013, at 21:05, Anne van Kesteren <annevk at> wrote:

> On Mon, Aug 19, 2013 at 5:25 AM, Mathias Bynens <mathias at> wrote:
>> After comparing the output, I noticed that both regular expressions are identical except for the following: ECMAScript 5 allows U+2E2F VERTICAL TILDE in `IdentifierStart` and `IdentifierPart`, but ECMAScript 6 / Unicode TR31 doesn’t.
> Per ES6 identifiers start with code points whose category is ID_Start
> which per includes Lm which per
> is true for
> U+2E2F. So why exactly is it disallowed?

`ID_Start` includes code points in the `Lm` category indeed, but then later explicitly disallows `Pattern_Syntax` and `Pattern_White_Space` code points. As it says on the page you linked to:

> In set notation, this is [[:L:][:Nl:]--[:Pattern_Syntax:]--[:Pattern_White_Space:]] plus stability extensions.

U+2E2F has the `Pattern_Syntax` property and is thus not a valid `ID_Start` code point.

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