docs: def _check_expr_filename(expr):
      """
      Split the expression and look to see if there's a specific filename that the user wants to process.
      :param str expr: Search expression
      :return str: Filename or None
      """
      expr_lst = expr.split()
      f = [x for x in expr_lst if x not in DATA_FRAMES and x.endswith(".lpd")]
      return f