It works like this in "Edit Book" or "Search & replace" of "Convert book"

([^–«—])\xa0([^!?;:»]) => \1\x20\2
and to change normal into nbps
- before ! ? ; :»
\x20([!?;:»]) => \xa0\1
- After – — «
([–«—])\x20 => \1\xa0
The code a0 of the nbps is not found in the test (all spaces are shown).