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 section Introduction to Buffer-Local Variables.) Alternatively, you may change the value of
default-case-fold-search, which is the default value of
case-fold-search for buffers that do not override it.
Note that the user-level incremental search feature handles case distinctions differently. When given a lower case letter, it looks for a match of either case, but when given an upper case letter, it looks for an upper case letter only. But this has nothing to do with the searching functions used in Lisp code.
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.
The function replace-match is where this variable actually has
its effect. See section Replacing the Text That Matched.
nil they do not ignore case; otherwise
they do ignore case.
case-fold-search in buffers that do not override it. This is the
same as (default-value 'case-fold-search).
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