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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Cake - C23 to C99 transpiler
Date: Wed, 14 Sep 2022 06:45:44 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
>> David Brown <david.brown@hesbynett.no> writes:
>>
>>> I believe (but I might have this wrong) that the use of dollar signs
>>> in identifiers is undefined behaviour according to the standard.
>>
>> I don't think it's undefined.  Recent C drafts permit "other
>> implementation defined characters" in the syntax, so I'm not sure how it
>> could be undefined.
>
> Right.  So if the implementation defines identifier-nondigit to include
> '$', then using '$' in an identifier is well defined;  if it doesn't,
> it's simply a syntax error.
>
>> The C23 draft permits an XID_Start character followed by XID_Continue
>> characters.  These may, in fact, include $ but I got lost down the
>> rabbit hole of referenced standards so I can't be sure.
>
> I don't believe '$' is included in XID_Start or XID_Continue, but I'm
> not 100% certain of that.
>
> https://unicode.org/reports/tr31/
> https://unicode.org/reports/tr44/

In addition to these documents I have done some web searching,
and I think the evidence is pretty strong that neither '$' nor
'@' is in XID_Start or XID_Continue, as the Unicode documentation
details them.

Editorial comment:  the two Unicode reference documents may be
the very worst reference documentation I have ever read.