private static void appendFromEnd(final StringBuilder buffer, final String original, final int groupSize) {
    final int position = buffer.length();
    int pointer = original.length();
    while (pointer > groupSize) {
      buffer.insert(position, original.substring(pointer - groupSize, pointer));
      buffer.insert(position, '_');
      pointer -= groupSize;
    }

    if (pointer > 0) {
      buffer.insert(position, original.substring(0, pointer));
    }
  }
private static void appendFromEnd(final StringBuilder buffer, final String original, final int groupSize)