public static boolean isXML11ValidNmtoken(String nmtoken) {
        int length = nmtoken.length();
        if (length == 0) {
            return false;
        }
        for (int i = 0; i < length; ++i) {
            char ch = nmtoken.charAt(i);
            if (!isXML11Name(ch)) {
                if (++i < length && isXML11NameHighSurrogate(ch)) {
                    char ch2 = nmtoken.charAt(i);
                    if (!XMLChar.isLowSurrogate(ch2) ||
                            !isXML11Name(XMLChar.supplemental(ch, ch2))) {
                        return false;
                    }
                } else {
                    return false;
                }
            }
        }
        return true;
    }
Check to see if a string is a valid Nmtoken according to [7]
in the XML 1.1 Recommendation

@param nmtoken string to check
@return true if nmtoken is a valid Nmtoken
stroom-util/src/main/java/stroom/util/xml/XML11Char.java