Why do you have to use a RegEx?

You can simply use indexOf and substring

  public List<String> splitOnDot(String input) {
    List<String> result = new ArrayList<>();
    int idx = -1;
    while ((idx = input.indexOf('.', idx + 1)) != -1) {
      result.add(input.substring(0, idx));
      input = input.substring(idx + 1);
    }
    return result;
  }