A regex queries

I want a simple regex to delete the first instance of everything in a parenthesis, i.e. capture everything after the parenthesis. I wrote a simple code which works: regex101: build, test, and debug regex
But running it on mp3tag gives strange results:


Please test your expressions in MP3tag with Convert>Tag-Tag and the scripting function $regexp()

To replace everything leading in parenthesis, try Convert>Tag-Tag for TITLE:
Format string: $regexp(%title%,^\(.*?\)\s*,)

In general, to address the captured string part, use $1 instead of \1

... and there are a lot of threads that deal with the removal of parenthesis and their contents, e.g.