string_calculator.rb

class StringCalculator
  attr_reader :array_numbers
  
  def add(string)
    return 0 if string == ''
    
    @array_numbers = extract_array_of_numbers_from string

    raise "negative not allowed #{negative_numbers.to_s}" if negative_numbers.size > 0
    
    @array_numbers.size == 1 ? string.to_i : sum_array_of_numbers
  end

  private

  def extract_delimiter(string)
    delimiter = /^\/\/(.)\n/.match string
    return delimiter[1] if delimiter != nil
    
    delimiters = extract_delimiters_with_arbitrary_length(string)
    return delimiters.join('|') unless delimiters.size == 0
    
    return ','
  end

  def extract_delimiters_with_arbitrary_length(string)
    string.scan(/\[(\w+)\]/)
  end

  def extract_array_of_numbers_from(string)
    string.split(/(#{extract_delimiter(string)}|\n)/).map {|num| num.to_i}
  end

  def negative_numbers
    @array_numbers.select {|num| num < 0}
  end

  def sum_array_of_numbers
    @array_numbers.select{|num| num < 1000}.reduce(:+)
  end
  
end

FYI, here's my (old) take on this kata: https://github.com/xpepper/string_calculator