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34.7 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. 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 11.10.1 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.

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.

This variable is used by passing it as an argument to the function replace-match. See section 34.6.1 Replacing the Text that Matched.

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).


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