I have a TextBox for the ZIP and a ComboBox for the country (selectable: germany, switzerland and austria).
I am not much familiar with RegEx, I get it to build a simple one for ZIP of germany, but how to build one for allowed ZIPs of germany, switzerland and austria?
You should mention what rules the ZIP codes follow. A quick peek tells me they have 4 (Switzerland, Austria) or 5 (Germany) numbers.
would match that. If you want a more specific answer, then be more specific in your post.