By default, searches in Emacs ignore the case of the text they are searching through; if you specify searching for ‘FOO’, then ‘Foo’ or ‘foo’ is also considered a match. This applies to regular expressions, too; thus, ‘[aB]’ would match ‘a’ or ‘A’ or ‘b’ or ‘B’.
If you do not want this feature, set the variable
case-fold-search to nil. Then all letters must match
exactly, including case. This is a buffer-local variable; altering the
variable affects only the current buffer. (See Introduction to Buffer-Local Variables.) Alternatively, you may change the default value.
In Lisp code, you will more typically use let to bind
case-fold-search to the desired value.
Note that the user-level incremental search feature handles case distinctions differently. When the search string contains only lower case letters, the search ignores case, but when the search string contains one or more upper case letters, the search becomes case-sensitive. But this has nothing to do with the searching functions used in Lisp code. See Incremental Search in The GNU Emacs Manual.
This buffer-local variable determines whether searches should ignore
case. If the variable is nil they do not ignore case; otherwise
(and by default) they do ignore case.
This variable determines whether the higher-level replacement
functions should preserve case. If the variable is nil, that
means to use the replacement text verbatim. A non-nil value
means to convert the case of the replacement text according to the
text being replaced.
This variable is used by passing it as an argument to the function
replace-match. See Replacing the Text that Matched.