The asterisk is not a valid regex character, but rather a shell expander known from MS DOS times.
If you want to use any character, you must allow any character [ . ] any number of times [ * ].
.*@.*\.domain\.de
The dot must be escaped [ \. ] so that it does not stand for any character [ . ].
However, in my opinion, the probability of incorrect filtering if you leave it as 'any character' is low if the domain is long enough.
.*@.*.domain.de
I don't see any advantage over filtering for .*domain\.de or even .*domain.de instead.
If you want to learn regex (you should!), you need PCRE, because Spamassassin is a Perl Programm.
ERE are similar (but less complete), BRE are quite different (and ugly in my opinion), but you should know the differences between BRE and ERE and also know, where to use what.