I believe MP3Tag uses the "Boost.Regex" library, which supports both the basic and extended standardized syntax as defined by POSIX
Although many in the forum here are very generous with their time wrt regex tutoring, that is a huge topic area of which self-teaching references abound all over the intertubes, and also note really an MP3Tag-specific issue.
You can find a comprehensive reference to Perl-style regular expressions in the Perl regex documentation, or read about the Boost.Regex-specific options on the Boost documentation pages. You can also do a web search for "regular expression syntax" and find lots more, including step-by-step tutorials and learning resources, including ones that show you matchin results "live" as you change your syntax.