I recently found out (RegExpr für klassiche Beschriftung von Tonart) that the Mp3tag function $regexp(p1,p2,p3) expects as second parameter a literal string which contains the regular expression formular. Enclosing this literal string with single quotation characters allows the user to use any literal character in the regular expression, even those characters which needs compulsory to be escaped at other scripting places within Mp3tag.
I think it would become more clearly and handy if a user can read it from the documentation that parameter p2 of the Mp3tag scripting function $regexp(p1,p2,p3) should always be treated as a literal string which needs to be enclosed with single quotation marks.
By the way, round brackets should not to be allowed working as a string delimiter within function parameter list.
DD.20080304.2007.CET