String calculator kata

calc.rb

class StringCalculator
  def calculate string = ""
    string.split(/\D+/).map do |number|
      number.to_i
    end.reduce(&:+).to_i
  end
end

calc_spec.rb

require_relative 'calc'

describe StringCalculator do
  before do
    @calc = StringCalculator.new
  end

  it "should return zero with no arguments" do
    @calc.calculate.should be 0
  end

  it "should return zero with empty string" do
    @calc.calculate("").should be 0
  end

  it "should return zero with 0 as argument" do
    @calc.calculate("0").should be 0
  end

  it "should return a bare number" do
    @calc.calculate("1").should be 1
  end

  it "should return the sum of space separated numbers" do
    @calc.calculate("2 3").should be 5
  end

  it "should return the sum of any whitespace separated numbers" do
    @calc.calculate("2  4").should be 6
    @calc.calculate("2\t4").should be 6
  end

  it "should accept a custom separator" do
    @calc.calculate("//+1+2+3+4").should be 10
    @calc.calculate("//a1a2a3a4").should be 10
  end
end