alt+f3 should honor selection

Selecting a string (let's name it "STR") and pressing Alt+F3 selects all occurrences in the document. Then I can edit all those STR in the document. Good.

Selecting a block of text, then using Ctrl+F to find STR selects all occurrences of STR inside the block of selection only.
I'd like to be able to edit ONLY those matches inside the block, and not all occurrences throughout the document.