LVRUG Kata 6/19/13 with many thanks to @marksim

string_calculator.rb

module StringCalculator
  String.class_eval do 
    
    def calc
      raise NoNegativeNumbers.new(numbers.select{|n| n < 0}) if numbers.any?{|n| n < 0}
      numbers.inject(0, &:+)
    end

    private

    def numbers
      if self.start_with?("//")
        parts = self.split("\n")
        parts.last.split(delimiter).map(&:to_i).reject{|n| n > 999}
      else
        self.split(delimiter).map(&:to_i).reject{|n| n > 999}
      end
    end

    def delimiter
      if self.start_with?("//")
        parts = self.split("\n")
        regex = parts.first.sub("//", "").sub("[]","|").sub("[", "").sub("]", "")
        /[#{regex}]/
      else
        /[\n|,]/
      end
    end

  end
end

class NoNegativeNumbers < Exception

  def initialize(numbers)
    @negative_numbers = numbers
  end

  def message
    "No Negative Numbers: #{@negative_numbers.join(", ")}"
  end
end

string_calculator_spec.rb

require 'spec_helper'

using StringCalculator

describe StringCalculator do

  it 'should return zero for an empty string' do 
    "".calc.should eq 0
  end

  it 'should return 1 for "1"' do 
    "1".calc.should eq 1
  end

  it 'should return 3 for "1,2"' do 
    "1,2".calc.should eq 3
  end

  it 'should return 6 for "1,2,3"' do 
    "1,2,3".calc.should eq 6
  end

  it 'should return 6 for "1\n2,3"' do 
    "1\n2,3".calc.should eq 6
  end

  it 'should let one define their delimiter' do 
    "//;\n1;2;3".calc.should eq 6
    "//s\n1s2s3".calc.should eq 6
  end

  it 'should let one define a delimiter within brackets' do 
    "//[n]\n1n2n3".calc.should eq 6
  end

  it 'should let one define multiple delimiters within brackets' do 
    "//[n][s]\n1n2s3".calc.should eq 6
    "//[n][s][y]\n1n2s3y4".calc.should eq 10
  end

  it 'should raise NoNegativeNumbers for "-1,2,-3"' do
    expect{"-1,2,-3".calc}.to raise_error NoNegativeNumbers, /-1, -3/
  end

  it 'should show correct error message for "-1,2,-4"' do 
    begin
      "-1,2,-4".calc 
    rescue NoNegativeNumbers => e 
      e.message.should eq "No Negative Numbers: -1, -4"
    end
  end

  it 'should reject numbers more than 999' do
    "1,1000,2".calc.should eq 3
  end 

end

websites.txt

Sources

The Kata http://osherove.com/tdd-kata-1/
The String Class http://ruby-doc.org/core-2.0/String.html
The Array Class http://ruby-doc.org/core-2.0/Array.html
Symbol to Proc http://blog.thoughtfolder.com/2008-02-25-a-detailed-explanation-of-ruby-s-symbol-to-proc.html
The Exception Class http://www.ruby-doc.org/core-2.0/Exception.html
Rescuing Exceptions http://rubylearning.com/satishtalim/ruby_exceptions.html