Install the "Open With" plugin, assign "Open with Sigil" to a keyboard shortcut and you will be way ahead of anything you can do in calibre when it comes to search and replace - at least for working with EPUB. I assign Alt+E in my case and it is second nature to do any editing be it css tweaks, find/replace operations, TOC manipulations etc in Sigil that way.
Personally my toes curl every time I see one of these sort of threads to look at the sort of hoops people are jumping through to try to use the calibre S&R
. It certainly has a purpose if you are not working with EPUBs (such as stripping PDF header/footers as part of a conversion to EPUB) but I would never ever use it for anything outside of that and would always recommend someone convert to EPUB (if not already), do their editing using either Sigil or Tweak ePub/HTML editor and then convert to their target format if EPUB isn't the end game.