calculator.rb

require "mathn"

module Calculator
  extend self

  def call(expression)
    eval(sanitize(expression)).to_f
  end

  private

  def sanitize(expression)
    # Remove non-numeric and non-operational characters
    expression.gsub(/[^()*+\-\/0-9]/, '')
  end
end

calculator_spec.rb

require_relative "../lib/calculator"

describe Calculator do
  let(:calculator) { described_class }

  it "adds" do
    expect(calculator.("1 + 1")).to eq 2
  end

  it "subracts" do
    expect(calculator.("1 - 1")).to eq 0
  end

  it "multiplies" do
    expect(calculator.("1 * 1")).to eq 1
  end

  it "divides" do
    expect(calculator.("5 / 2")).to eq 2.5
  end

  it "understands order of operations" do
    expect(calculator.("5 + 3 / 2")).to eq 6.5
  end

  it "evaluates parentheses first" do
    expect(calculator.("(5 + 3) / 2")).to eq 4
  end

  it "evaluates exponents second" do
    expect(calculator.("(5 + 3) / 2 ** 2")).to eq 2
  end

  it 'disregards non-numeric and non-operational characters' do
    expect(calculator.("2abc + 1abc")).to eq 3
  end
end