search-upper-case

search-upper-case is a variable defined in `isearch.el'.
Its value is

not-yanks

Documentation:
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 this variable.