I want to know which RegEx-flavour is used for Python? Is it PCRE, Perl compatible or is it ICU or something else?

It's compatible enough w/ Perl, meaning most Perl expressions will work unmodified. It aims to be Perl compatible but, of course, there are some minor differences.

It, technically, uses its own flavor of regular expressions. For instance, named groups were offered in Python regex long-before other implementations adopted the syntax. It also supports Unicode out of the box. Most Python extensions are supported elsewhere these days. See http://docs.python.org/library/re.html