I have a HTML file with a few non-ASCII characters, say encoded in UTF-8 or UTF-16. To save the file in ASCII, I would like to replace them with their (SGML/HTML/XML) entity codes. So for example, every ë should become ë and every ◊ should become ◊. How do I do that?
I use Emacs as an editor. I'm sure it has a function to do the replace, but I cannot find it. What am I missing? Or how do I implement it myself?
There is a character class which includes exactly the ASCII character set. You can use a regexp that matches its complement to find occurrences of non-ASCII characters, and then replace them with their codes using elisp:
M-x replace-regexp RET
[^[:ascii:]] RET
\,(concat "&#" (number-to-string (string-to-char \&)) ";") RET
So when, for example, á is matched: \& is "á", string-to-char converts it to ?á (= the number 225), and number-to-string converts that to "225". Then, concat concatenates "&#", "225" and ";" to get "á", which replaces the original match.
Surround these commands with C-x ( and C-x ), and apply C-x C-k n and M-x insert-kbd-macro as usual to make a function out of them.
To see the elisp equivalent of calling this function interactively, run the command and then press C-x M-: (Repeat complex command).
A simpler version, which doesn't take into account the active region, could be:
(while (re-search-forward "[^[:ascii:]]" nil t)
(replace-match (concat "&#"
(number-to-string (string-to-char (match-string 0)))
";")))
(This uses the recommended way to do search + replace programmatically.)