def split_digit(str)
      i = str.index(/.分|.割/)
      if i.nil?
        [str]
      elsif i.zero?
        [nil, str]
      else
        [str[0..i - 1], str[i..-1]]
      end
    end
def split_digit(str)
      i = str.index(/.分|.割/)
      if i.nil?
        [str]
      elsif i.zero?
        [nil, str]
      else
        [str[0..i - 1], str[i..-1]]
      end
    end
Return numerals parts

 @return [Array<String>]

 @api private
 @since 0.0.1