Regular expression syntax
Media Centre's expression language regex() function employs regular expressions (regexs).
Media Center 19
The regex engine is from the Boost C++ library. The Boost library regex programmers' documentation is here.
The Boost regex syntax option used is one of the three offered by Boost: Perl, POSIX extended, POSIX Basic.
Media Center 18 and earlier
The regex implementation has been reported to be the Microsoft 2010 TR1 engine. This is presumed to be a reference to the Microsoft Visual Studio 2010 Standard C++ Library TR1 engine. Programmers' documentation is here.
The TR1 implementation is reported to be based on the Boost library here.
It is supposed that Media Center versions before 2010 used the Standard C++ Library TR1 engine from earlier versions of Visual Studio. Failure of compatibility between different version of this engine have been reported, e.g. here.