If a question is specifically about the use of regex in program code, then it belongs on Stack Overflow. If it's about regex find-and-replace in, say, a text editor, then as far as I'm aware it's very much on topic for SU.
This seems to be more of an issue with Notepad++ questions than regex questions. As somebody who regularly looks at notepad++ questions (it's one of my favorite tags on both sites), I'm quite torn. Namely, the software itself was made for use by programmers, but it is often used in non-programming contexts such as general text editing purposes, as you describe. Combined with the general unawareness that questions about programming tools are, in fact, on topic for SO, we get tons of unfulfilled close votes for notepad++ questions on SO. I'm not active on SU though, so I'm not sure how often questions get migrated from there to here, if ever.
But personally, I consider questions about Notepad++ on topic for both SO and SU; wherever a question is asked, I don't see much of a need to migrate it to one or the other unless explicitly requested by the asker. Since they're on SU already, I'd say it's OK to keep them where they are. It just happens by coincidence that the nature of Notepad++ as a code editor means questions about it are on topic for SO as well as SU.