[DiapDealer]

Oh, my bad. I was thinking the bundled python available for plugins with the newer Sigil. You are definitely limited to standard pcre regex when using Sigil's search and replace. So lookaheads can be variable width, but not lookbehinds. Sorry.

You still should be able to get Kevin's search working though. Why do you need to capture JUST the "were'? What's the end goal? Are you looking to replace "were" with something else, including nothing (deleting)?

If the lookbehind is what's holding you back, try refactoring it with \K instead. \K tells the engine to pretend the match starts immediately after.