With Calibre editor:
For example for the character ?.
Remove all non-break spaces or narrow-no-break spaces before ?.
Run this search with regex-function:
Search:
Function:
Code:
def replace (match, number, file_name, metadata, dictionaries, data, functions, * args, ** kwargs):
return match.group().replace ("?","@?")
Where @ would be a no-break space or narrow-no-break space.