GNU Emacs Lisp Reference Manual

30.7: Searching and Case

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. Regexps, and in particular character sets, are included: 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 Intro to Buffer-Local.) 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 Lisp functions use.

User Option: case-replace

This variable determines whether the 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.

The function replace-match is where this variable actually has its effect. See Replacing Match.

User Option: case-fold-search

This buffer-local variable determines whether searches should ignore case. If the variable is nil they do not ignore case; otherwise they do ignore case.

Variable: default-case-fold-search

The value of this variable is the default value for case-fold-search in buffers that do not override it. This is the same as (default-value 'case-fold-search).