The relevant configuration option is
As with many things, you can find this information in Emacs itself. For example, use
C-h r to open the Emacs manual, and use
i to search the index for
isearch. This leads to the Incremental Search topic, which includes a section on Isearch Yank with this:
Normally, when the search is case-insensitive, text yanked into the
search string is converted to lower case, so that the search remains
case-insensitive. However, if the
value of the variable ‘search-upper-case’ is other than ‘not-yanks’, that disables this down-casing.
The next logical step would be to check the documentation for the variable mentioned (
C-h v search-upper-case):
If non-nil, upper case chars disable case fold searching.
That is, upper and lower case chars must match exactly.
This applies no matter where the chars come from, but does not
apply to chars in regexps that are prefixed with
If this value is
not-yanks, text yanked into the search string
in Isearch mode is always downcased.
You can customize it using
M-x customize-option search-upper-case.
You can also just set it in your init file:
(setq search-upper-case t)