Re: Subsets of Unicode and coding systems

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 6 Dec 2011 12:38:57 +0100
> From: Ulrich Mueller <ulm@gentoo.org>
> 
> Sometimes I want to check if a file saved in UTF-8 encoding contains
> only characters from a certain subset of the Unicode character
> repertoire, like the MES-* [1] or WGL4 [2] subsets. (For example, for
> things published on the WWW one might rather avoid exotic characters
> for better compatibility.)
> 
> My idea was now to define a coding system e.g. for MES-2 and set the
> buffer-file-coding-system accordingly, so that Emacs would check it
> upon saving the file.
> 
> Now I have the following questions:
> 1. Is using coding systems for this purpose a reasonable approach
>    at all, or is there a better way to achieve this?

I don't think defining a new coding-system is the best way to go about
this.  I would suggest to define a new category instead, see
"(elisp)Categories" (which includes a working example), and then make
a category table (which is just a special type of char-table) where
the relevant characters are marked with the appropriate categories.
Then you can simply scan the buffer with a regexp that uses "\\Cx"
where "x" is the letter you assign to your category.  (You will
probably need a separate letter for each of the MES-* and WGL4 sets.)

> 2. Where can I find documentation how to define a coding system?

You don't want to know that ;-)