def _string_substr(self, start, length=None):
    """
    Pull substrings out of each string value by position and maximum
    length.

    Parameters
    ----------
    start : int
      First character to start splitting, indices starting at 0 (like
      Python)
    length : int, optional
      Maximum length of each substring. If not supplied, splits each string
      to the end

    Returns
    -------
    substrings : type of caller
    """
    op = ops.Substring(self, start, length)
    return op.to_expr()