A subtle problem with this "common knowledge": \z is different across languages.

In PCRE (Perl and Ruby), \z matches the end of a string; \Z matches the end of a string but allows for a single trailing newline.

In Python, \Z matches the end of a string. There is no \z.

In POSIX extended regexes, there is no \z or \Z. I think. Who knows, man.