Looking for a Regex testing website (or tool) supporting the "\u" class

I have installed Regex Match Tracer but i can't figure out how greedy repeats work with the uppercase class. I have also googled for online regex testers unable to find one that would recognize it as uppercase instead of syntax error or literal character "u". Does anyone know of a program or online source where this class is still valid so i can test my regular expressions?

Thank you in advance!

MP3tag has some testing abilities in the shape of the function
if you enter as format string a $regexp() expression.