Thank you, DetlevD, for your comments to make a file filter easier to read. I like the [:cntrl:] and [:unicode:] expressions - hadn't seen them before. What regex flavor are these from?
Still, I feel inclined to stick to my conclusion that there is something wrong in Mp3tag's regex parsing in the file filter. As said some moments ago, my regexes have been tested in two other programs using a.o. the Perl regex flavor, plus they work fine in Mp3tag's "Ersetzen mit regulärem Ausdruck". I am including a screenshot of Regex Buddy's analysis of my two regexes, chosing Perl as the regex flavor.
So, I'll wait some more time before escalating this to a bug report.