Re: manipulating (capitalize, lower case) unicode bold and italic characters

[Eli Zaretskii <eliz@gnu.org>]

> From: Dan Hitt <dan.hitt@gmail.com>
> Date: Mon, 8 Jul 2019 11:50:07 -0700
> Cc: help-gnu-emacs@gnu.org
> 
> If you enter a mathematical italic small w (0x1D464) and a mathematical
> italic capital w (0x1D44A) and do 'describe-char' for each, the small one
> has the Lowercase general category, while the capital has the Uppercase
> general category.  I did not know about the concept of 'case pair' in
> unicode, so i guess it is possible that even though emacs knows one is
> Lowercase and one is Uppercase, it is possible that it does not know that
> they are in a pair.

They are not a letter-case pair because Unicode doesn't say they are.
The fact that a character has general category lowercase doesn't yet
imply that it has a defined upper-case variant, these are two separate
attributes.  Emacs defines its case pairs according to what it finds
in the Unicode Character Database.

Of course, you can teach Emacs about the letter-case pairs yourself,
like this:

  (let ((tbl (standard-case-table)))
    (set-downcase-syntax ?𝑊 ?𝑤 tbl)
    (set-upcase-syntax ?𝑤 ?𝑊 tbl))

> (How would i find out, from emacs?)

By looking at the table returned by standard-case-table, for example:

  (aref (standard-case-table) ?A)
    => 97

but

  (aref (standard-case-table) ?𝑊)
    => nil

> The commands downcase-region and upcase-region do not work on them

They won't work on letters that have no case-pairs.

Once again: I do NOT recommend using the characters from the
Mathematical Alphanumeric Symbols block for writing English text,
that's not their purpose.