StringCalc.rb

module StringCalculator
  class << self
    def add string_numbers = nil
      return 0 unless string_numbers

      delimeters = get_delimeters string_numbers
      num_array = string_numbers.split delimeters

      raise 'negative numbers not allowed' if num_array.join().match(/-[0-9]/)

      num_array.select{|i| i.to_i < 1000}
        .map(&:to_i)
        .reduce(&:+)
    end

    def get_delimeters string
      user_delimeter = unless string.chars.first.match(/\d/)
        "|" + string.chars.first
      else
        ""
      end

      /\n|\,#{user_delimeter}/
    end
  end
end