RegEx's not working

Hello,

Ever since updating to 2.94, I have not been able to get RegEx's working.

Original title tag:
Starstrukk (feat. Katy Perry)

Action: Guess values
Source format:
%TITLE%$regexp(%ARTIST%,'^(.+?)\s+[[({<]?\s*(?:featuring|feat\.?|ft\.?)\s*([^])}>]+)[])}>]?(.*)
Guessing pattern:
%TITLE%+++%ARTIST%

Has something been changed in the RegEx engine?

No, nothing has changed.
But if the data is in the field TITLE and your regular expression tries to use the data from ARTIST then I would say that nothing in this constellation of data and regular expression matches.
So you are right: this regular expression does not work for the source data.

You can test regular expressions in the function Convert>tag-Tag.

For the data

and the split into TITLE and ARTIST, the action of the type "Guess value" use
Source: %title%
Target: %title% (feat. %artist%)

I need to swap the tags around then?

It's been so long since I've used mp3tag... could you please break down the above RegEx?

I see no need for swapping, only for splitting ...
And I also see no need for a regular expression if you want to deal with a pattern of the type
TITLE (feat. ARTIST)

Ok. What would you recommend then?

See what I wrote about "Guess value"