i think the issue here guys is not how to write a clever regex solution, though it is always fun to watch great minds at work on a technical challenge
The issue for me is this: if calibre editor is to become the new improved sigil, then it needs to match or better sigil regex's ease of use.
In sigil I just pasted the unwanted line from xhtml code view into the "find" box, set replace to blank - click on replace all - job done. There was no need to faff around guessing at invisible characters, or puzzling out why it didnt work - it just worked 1st time.
Calibre is using the same regex engine, yes ? , so why does the same quick & easy method fail, when it works in sigil ?