First of all, thank you very much for your answer.
I am a Python newbie… How do I insert this in a workflow ? Through "RunPython script", I suppose (coming after an "Extend Selection" to the whole document ?), but how ?
How come a sequence \b\n\b in Find and a space in Replace does not work, although it works perfectly well in Regex101 ?
Could it be a bug ? I doubt it. I most certainly make a mistake, but where ?
I do not (entirely) share your condemnation of Regexes, but, as you know, nobody is perfect…