If I understand your problem correctly (i.e. looking for files which have a specific length and consist only of numbers) this would be a possible solution - in this example desired length is 4 characters
$ ls
1 1234 2 23456 3456 45 45678 567 bl23 bluh
12 12345 234 3 34567 456 5 5678 blah
123 1bla 2345 345 4 4567 56 56789 bloh
$ find /home/tom/Dokumente -type d -regextype posix-egrep -regex ".*/[^/][0-9]{3}"
/home/mrjazz/Dokumente/5678
/home/mrjazz/Dokumente/4567
/home/mrjazz/Dokumente/2345
/home/mrjazz/Dokumente/3456
/home/mrjazz/Dokumente/1234
so files blah, bloh, bluh, and also 1bla, bl12 are not found