In emacs 25.1, if I customize search-default-mode to "Char-Fold Search", then an isearch-forward for
and one for
But what character should I type to match
By default, this is not possible (easiest way to see this is with
C-h v char-fold-table RET and searching for ß — which you won't find).
However, if you modify
char-fold-table (the variable responsible for determining how folding occurs), then you can enable this.
char-fold-table is a char-table — quoting the emacs manual:
A char-table is a one-dimensional array of elements of any type, indexed by character codes.
In this case, the "elements" are regular expressions for what the given character is supposed to match when Char-Fold Search is on (i.e. more-or-less a list of the characters that are the same as the given character). For instance, by default, the element corresponding to s, obtained with:
(char-table-range char-fold-table ?s)
(aref char-fold-table ?s) is:
char-fold-table also has an "extra slot", which is itself a char-table, and which lists the character combinations (starting with the given letter) that should match specific regexps (e.g.
fi should match the unicode character
ﬁ). For instance, the alist corresponding to f, found using:
(aref (char-table-extra-slot char-fold-table 0) ?f)
(("fl" . "ﬄ") ("fi" . "ﬃ") ("l" . "ﬂ") ("i" . "ﬁ") ("f" . "ﬀ") ("m" . "㎙") ("̇" . "ḟ"))
which means that
ffl should match
This is probably the most logical possibility. To make this possible, we need to modify the alist in the extra slot of the
char-fold-table to also contain
("s" . "ß"):
(set-char-table-extra-slot char-fold-table 0 (let* ((multi-char-table (char-table-extra-slot char-fold-table 0)) (s-alist (aref multi-char-table ?s)) (modified-s-alist (cons '("s" . "ß") s-alist))) (aset multi-char-table ?s modified-s-alist) multi-char-table))
Unfortunately, searching isn't too smooth, since sometimes you need to jog isearch, by pressing
C-s again, after typing the entire search string, to convince it that the search is not actually failing (the same issue occurs with the default combinations).
(To see the issue, insert
gru grüßen in a fresh buffer, place the mark at the start and try to search for
grussen, typing — not copy-pasting — the search string. After you've typed
grus, isearch concludes that the search is failing and won't do anything more unless prompted. An equivalent problem, using the default configuration would be with
an aﬃne and searching for
(aset char-fold-table ?s "\\(?:s[̧̣̦́̂̇̌]\\|[sśŝşšſșˢṡṣₛⓢｓ𝐬𝑠𝒔𝓈𝓼𝔰𝕤𝖘𝗌𝘀𝘴𝙨𝚜ß]\\)")
ß inserted into the end of the default regexp.
In principle, you could do both, partially circumventing the above mentioned issue.
If you believe that the absence of eszett in the default
char-fold-table is a bug, you could report it.
The reason why it's absent is because
char-fold-table is automatically generated from the Unicode list of decomposable characters and Unicode believes that
ß does not "decompose" into
s in the same way that
á does decompose into
LATIN SMALL LETTER SHARP S).