Searches in Emacs normally ignore the case of the text they are searching through, if you specify the text 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’.
An upper-case letter anywhere in the incremental 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 string search. The effect ceases if you delete the upper-case letter from the search string.
Typing M-c within an incremental search toggles the case sensitivity of that search. The effect does not extend beyond the current incremental search to the next one, but it does override the effect of adding or removing an upper-case letter in the current search.
If you set the variable
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 Locals.
This variable applies to nonincremental searches also, including those
performed by the replace commands (see Replace) and the minibuffer
history matching commands (see Minibuffer History).
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>.