We adopted, in Varna, a paper that adds $ to the basic character set. The
week after, the C committee decided (for the C23 DIS) that $ can be used
within identifiers (including in the initial position).

We should consider keeping C and C++ in sync.

Note that we already have examples of u"text", etc. where identifier
non-digits form the initial portion of non-identifier pp-tokens.

-- HT