Oops! I guess I was too lazy and convinced of compatibilty between my regex crafting tool (RegexBuddy with Perl flavor enabled) and Mp3tag.
The regex is not working as intended and that's a fact, however I don't get any error!
Firstly, this is what I get:
Format string:
$regexp('aaa,aaa,aaa,bbb,ccc,bbb,ddd,ddd,ddd,ccc,eee,fff,GGG,ggg,iii','(?<=,|\A) ?([^,]*),(?=.*?(?<=,) ?\1(?=,|\z))',,1)
Results in:
aaa,aaa,bbb,ddd,ccc,eee,fff,ggg,iii
No error. But still not our goal.
Please try to reproduce this by pasting the string inside Tag-Filename dialog (the most convinient way for me) and check if the error occurs.
"Invalid lookbehind assertion encountered..." is the type of error showing up due to lookbehind limitation in Perl flavor
The dubious part of our expression is the initial lookbehind (?<=,|\A) since it's not really fixed-length (Start of string anchor is zero-length). I'll try to fix this problem tomorrow.
Thanks to links you provided me few months ago 