But your example is not a Regular Expression, isn't it?
To make it short ... read there at end of the page:
https://docs.mp3tag.de/scripting
A search through the forum for "square brackets" will provide several questions and answers about the same problem.
Square brackets are reserved characters in MP3tag Scripting Language.
You have to put the single quote around such reserved characters to get their literal meaning.
Trial and error may help in some circumstances, but reading the manual is still the first effort someone should do before working with an unknown thing of software.
Good luck with Mp3tag. The first hurdle is taken.
DD.20100523.1924.CEST