Apparently Java\'s Regex flavor counts Umlauts and other special characters as non-\"word characters\" when I use Regex.
\"TESTÜTEST\".replaceAll( \"
Use [^\p{L}\p{Nd}]+ - this matches all (Unicode) characters that are neither letters nor (decimal) digits.
In Java:
String resultString = subjectString.replaceAll("[^\\p{L}\\p{Nd}]+", "");
Edit:
I changed \p{N} to \p{Nd} because the former also matches some number symbols like ¼; the latter doesn't. See it on regex101.com.