Find does n't use regular expressions. You can do it the other way around. If you want the numerics, remove the non-numeric with
replaceFirst($Nummer, '([A-Za-z]+)', '')
if you want the non-numeric characters
replaceFirst($Nummer, '([0-9]+)', '')
You can try Regular Expressions, although I think you'll have to resort to Java to actually get the value from the attribute.
Something like this I suppose:
String regex = "^[^0-9]*([0-9]*).*$";
String test = "some words 234 and more 678";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(test);
matcher.matches();
System.out.println(matcher.group(1));