Javadoc for java.util.regex.Pattern specifies that these special costructs are allowed:
(?idmsuxU-idmsuxU) Nothing, but turns match flags i d m s u x U on - off
(?idmsux-idmsux:X) X, as a non-capturing group with the given flags i d m s u x on - off

In the second case, the flag U is not listed as an allowed option.
In fact, this flag is recognized by the parser and can be turned on/off for non-capturing groups.

Below is a testcase, which demonstrates that the flag U is indeed recognized:
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import java.util.regex.*;
public class U {
    public static void main(String[] args) throws Throwable {
        Pattern p = Pattern.compile("(?U:\\d)");
        Matcher m = p.matcher("\uFF11");
        System.out.println(p + " " + m.matches());
    }
}
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Prints out:
(?U:\d) true
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The javadoc has to be updated accordingly.