public static String checkNamespacePrefix(final String prefix) {
    // Manually do rule, since URIs can be null or empty
    if ((prefix == null) || (prefix.isEmpty())) {
      return null;
    }

    if (checkJDOMName(prefix) != null) {
      // will double-check null and empty names, but that's OK
      // since we have already checked them.
      return checkJDOMName(prefix);
    }

    // Cannot start with "xml" in any character case
		/* See Issue 126 - https://github.com/hunterhacker/jdom/issues/126
		if (prefix.length() >= 3) {
			if (prefix.charAt(0) == 'x' || prefix.charAt(0) == 'X') {
				if (prefix.charAt(1) == 'm' || prefix.charAt(1) == 'M') {
					if (prefix.charAt(2) == 'l' || prefix.charAt(2) == 'L') {
						return "Namespace prefixes cannot begin with " +
								"\"xml\" in any combination of case";
					}
				}
			}
		}
		*/

    // If we got here, everything is OK
    return null;
  }
public static String checkNamespacePrefix(final String prefix)