Normally, you’d want search commands to disregard certain minor differences between the search string you type and the text being searched. For example, sequences of whitespace characters of different length are usually perceived as equivalent; letter-case differences usually don’t matter; etc. This is known as character equivalence.
This section describes the Emacs lax search features, and how to tailor them to your needs.
By default, search commands perform lax space matching: each
space, or sequence of spaces, matches any sequence of one or more
whitespace characters in the text. More precisely, Emacs matches each
sequence of space characters in the search string to a regular
expression specified by the user option
search-whitespace-regexp. The default value of this option
considers any sequence of spaces and tab characters as whitespace.
Hence, ‘foo bar’ matches ‘foo bar’, ‘foo bar’, ‘foo bar’, and so on (but not ‘foobar’). If
you want to make spaces match sequences of newlines as well as spaces
and tabs, customize the option to make its value be the regular
expression ‘[ \t\n]+’. (The default behavior of the
incremental regexp search is different; see Regular Expression Search.)
If you want whitespace characters to match exactly, you can turn lax
space matching off by typing M-s SPC
(isearch-toggle-lax-whitespace) within an incremental search.
Another M-s SPC turns lax space matching back on. To
disable lax whitespace matching for all searches, change
search-whitespace-regexp to nil; then each space in the
search string matches exactly one space.
Searches in Emacs by default ignore the case of the text they are searching through, if you specify the search string in lower case. Thus, if you specify searching for ‘foo’, then ‘Foo’ and ‘fOO’ also match. Regexps, and in particular character sets, behave likewise: ‘[ab]’ matches ‘a’ or ‘A’ or ‘b’ or ‘B’. This feature is known as case folding, and it is supported in both incremental and non-incremental search modes.
An upper-case letter anywhere in the search string makes the search
case-sensitive. Thus, searching for ‘Foo’ does not find
‘foo’ or ‘FOO’. This applies to regular expression search
as well as to literal string search. The effect ceases if you delete
the upper-case letter from the search string. The variable
search-upper-case controls this: if it is non-nil, an
upper-case character in the search string makes the search
case-sensitive; setting it to nil disables this effect of
upper-case characters. The default value of this variable is
not-yanks, which makes search case-sensitive if there are
upper-case letters in the search string, and also causes text yanked
into the search string (see Isearch Yanking) to be down-cased, so
that such searches are case-insensitive by default.
If you set the variable case-fold-search to nil, then
all letters must match exactly, including case. This is a per-buffer
variable; altering the variable normally affects only the current buffer,
unless you change its default value. See Local Variables.
This variable applies to nonincremental searches also, including those
performed by the replace commands (see Replacement Commands) and the minibuffer
history matching commands (see Minibuffer History).
Typing M-c or M-s c (isearch-toggle-case-fold)
within an incremental search toggles the case sensitivity of that
search. The effect does not extend beyond the current incremental
search, but it does override the effect of adding or removing an
upper-case letter in the current search.
Several related variables control case-sensitivity of searching and
matching for specific commands or activities. For instance,
tags-case-fold-search controls case sensitivity for
find-tag. To find these variables, do M-x
apropos-variable RET case-fold-search RET.
Case folding disregards case distinctions among characters, making
upper-case characters match lower-case variants, and vice versa. A
generalization of case folding is character folding, which
disregards wider classes of distinctions among similar characters.
For instance, under character folding the letter a matches all
of its accented cousins like ä and á, i.e., the
match disregards the diacritics that distinguish these
variants. In addition, a matches other characters that
resemble it, or have it as part of their graphical representation,
such as U+00AA FEMININE ORDINAL INDICATOR and U+24D0
CIRCLED LATIN SMALL LETTER A (which looks like a small a
inside a circle).
Similarly, the ASCII double-quote character " matches
all the other variants of double quotes defined by the Unicode
standard. Finally, character folding can make a sequence of one or
more characters match another sequence of a different length: for
example, the sequence of two characters ff matches U+FB00
LATIN SMALL LIGATURE FF and the sequence (a) matches
U+249C PARENTHESIZED LATIN SMALL LETTER A. Character sequences
that are not identical, but match under character folding are known as
equivalent character sequences.
Generally, search commands in Emacs do not by default perform
character folding in order to match equivalent character sequences.
You can enable this behavior by customizing the variable
search-default-mode to char-fold-to-regexp.
See Tailoring Search to Your Needs. Within an incremental search, typing
M-s ' (isearch-toggle-char-fold) toggles character
folding, but only for that search. (Replace commands have a different
default, controlled by a separate option; see Replace Commands and Lax Matches.)
By default, typing an explicit variant of a character, such as
ä, as part of the search string doesn’t match its base
character, such as a. But if you customize the variable
char-fold-symmetric to t, then search commands treat
equivalent characters the same and use of any of a set of equivalent
characters in a search string finds any of them in the text being
searched, so typing an accented character ä matches the
letter a as well as all the other variants like á.
You can add new foldings using the customizable variable
char-fold-include, or remove the existing ones using the
customizable variable char-fold-exclude. You can also
customize char-fold-override to t to disable all the
character equivalences except those you add yourself using
char-fold-include.