Re: extending case-fold-search to remove nonspacing marks (diacritics etc.)

[Juri Linkov]

> Something essentially identical to this was being discussed here a
> couple of weeks ago. Look for the thread "Single quotes in Info". I
> wrote a small elisp solution for building this into isearch (which you
> can find on the "scratch/isearch-character-group-folding" branch). It
> took a different approach to yours, relating characters to regexp, but
> it works.

I see that your branch contains nothing more than was already implemented
a long time ago in bug#13041 where the major stumbling block was
an inefficiency of the regexp-based solution.  Could you help to improve it?

> The bright side is that I think this two-char way of writing latin
> accents is much less common (not 100% sure though, it's hard to tell
> the difference). The downside is that I know nothing about other
> languages, so maybe using two chars to represent one char is the
> default behavior in some other languages?

As https://emacs.stackexchange.com/q/7992/478 indicates,
other languages require insertion/deletion of special characters
like diacritics/accents from the search string/buffer for normalization.

When looking for a solution I recommend you to check ucs-normalize.
For example, evaluating:

  (require 'ucs-normalize)
  ucs-normalize-combining-chars

you can see exactly the same characters

  1616 1615 1619 1648 1618 1612 1613 1611 1617 1614

mentioned in https://emacs.stackexchange.com/a/8001/478

Using its corresponding regexp `ucs-normalize-combining-chars-regexp'
is easy in isearch, e.g.:

  ;; Decomposition search for accented letters.
  (define-key isearch-mode-map "\M-sd" 'isearch-toggle-decomposition)

  (defun isearch-toggle-decomposition ()
    "Toggle Unicode decomposition searching on or off."
    (interactive)
    (setq isearch-word (unless (eq isearch-word 'isearch-decomposition-regexp)
                         'isearch-decomposition-regexp))
    (if isearch-word (setq isearch-regexp nil))
    (setq isearch-success t isearch-adjusted t)
    (isearch-update))

  (defun isearch-decomposition-regexp (string &optional _lax)
    "Return a regexp that matches decomposed Unicode characters in STRING."
    (let ((accents (substring ucs-normalize-combining-chars-regexp 0 -1)))
      (mapconcat
       (lambda (c0)
         (concat (string c0) accents "?"))
       (replace-regexp-in-string accents "" string) "")))

  (put 'isearch-decomposition-regexp 'isearch-message-prefix "deco ")

But this is more inefficient than properly implementing it using case tables.