StringCalc in ruby

stringcalc.rb

class StringCalc
  def self.calc(str)
    # default regex to split
    re_split = %r{[\n,]}

    # regex for custom delimiter
    m = str.match(%r{^//(.)})
    re_split = %r{#{m[1]}} if m

    # sum initial val
    sum = 0

    str.split(re_split).each do |v|
      sum += v.to_i
    end

    sum
  end
end


describe StringCalc do
  describe ".calc" do
    subject do
      StringCalc
    end

    it 'returns 0 if empty input' do
      expect(subject.calc("")).to eq(0)
    end

    describe 'comma delimiter' do
      it 'returns 0 if delimiter only input' do
        expect(subject.calc(",")).to eq(0)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("1,2,3,4")).to eq(10)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("1,2")).to eq(3)
      end
    end
    describe 'newline delimiter' do
      it 'returns 0 if delimiter only input' do
        expect(subject.calc("\n\n\n")).to eq(0)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("1\n2\n3\n4")).to eq(10)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("1\n2")).to eq(3)
      end
    end
    describe 'mixed newline and comma delimiter' do
      it 'returns 0 if delimiter only input' do
        expect(subject.calc("\n,\n")).to eq(0)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("1,2\n3,4")).to eq(10)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("1\n2,3")).to eq(6)
      end
    end
    describe 'custom delimiter' do
      it 'returns correct sum of elements' do
        expect(subject.calc("//|\n1|2|3|4")).to eq(10)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("//\n\n1\n2\n3")).to eq(6)
      end
      it 'returns correct sum of elements' do
        expect(subject.calc("//-\n1-2-3-4-5")).to eq(15)
      end
    end
  end
end