assertEscaping(xmlEscaper, "&#x" + Integer.toHexString(ch).toUpperCase() + ";", ch);
        } else {
          assertUnescaped(xmlEscaper, ch);
        }
      } else {
        // and everything else is replaced with FFFD.
        assertEscaping(xmlEscaper, "\uFFFD", ch);
      }
    }

    // Test _all_ allowed characters (including surrogate values).
    for (char ch = 0x20; ch <= 0xFFFD; ch++) {

        return null;
    }

    /**
     * Extracts the actual ID from a config ID by removing the type prefix.
     * Config IDs are formatted as [type_prefix][actual_id], so this method
     * returns everything after the first character.
     *
     * @param configId the configuration ID to process
     * @return the actual ID without the type prefix, or null if the config ID is invalid
     */