Ok, thanks, that explains it then. In that case, please consider this a formal request to include those features, if there’s no compelling reason not to.
Also, I’d like to suggest adding some text to the help section for action ‘Replace with regular expressions” specifying that Boost:: RegEx is the engine used, what limitations (features not enabled) there are to MP3Tag’s implementation, and maybe providing that same link to the Boost docs you included in your 30 Jan 2020 post, or to whichever version of the engine is used by the current version of MP3Tag.