http://nabetani.sakura.ne.jp/kanagawa.rb/evalex/

hoge_spec.rb

def foo(str)
  result = str.scan(/(\*|\+|&|\||\d+)/).flatten
  while result.size != 1
    result = hoge(result)
  end
  result.first
end

def hoge(ary)
  %w(| & + *).each do |op|
    if ary.include? op
      pos = ary.index(op)
      res = ary[pos-1].to_i.send(op, ary[pos+1].to_i)
      ary[pos-1..pos+1] = res
      return ary
    end
  end
end

describe "all" do
  it do
    [
        # [ id, input, expected ],
      [ 0, "4*5+6&7|8", "44" ],
      [ 1, "15*5", "75" ],
      [ 2, "15+5", "20" ],
      [ 3, "15&5", "5" ],
      [ 4, "15|5", "15" ],
      [ 5, "30*15*5", "2250" ],
      [ 6, "30*15+5", "600" ],
      [ 7, "30*15&5", "150" ],
      [ 8, "30*15|5", "450" ],
      [ 9, "30+15*5", "225" ],
      [ 10, "30+15+5", "50" ],
      [ 11, "30+15&5", "35" ],
      [ 12, "30+15|5", "45" ],
      [ 13, "30&15*5", "70" ],
      [ 14, "30&15+5", "19" ],
      [ 15, "30&15&5", "4" ],
      [ 16, "30&15|5", "14" ],
      [ 17, "30|15*5", "155" ],
      [ 18, "30|15+5", "36" ],
      [ 19, "30|15&5", "5" ],
      [ 20, "30|15|5", "31" ],
      [ 21, "1+2+3+4+5+6+7+8+9+10", "55" ],
      [ 22, "1*2*3*4*5*6*7*8*9*10", "3628800" ],
      [ 23, "1+2+3+4+5*6+7+8+9+10", "600" ],
      [ 24, "1*2*3*4*5+6*7*8*9*10", "1330560" ],
      [ 25, "1|2|4|8|16|32|64|128|256|512", "1023" ],
      [ 26, "2046&2045&2043&2039&2031&2015&1983&1919&1791&1535", "1024" ],
      [ 27, "0+1|7*6", "42" ],
      [ 28, "6|4+2&9+4", "10" ],
      [ 29, "0&6+1&6|4*2", "0" ],
      [ 30, "4|4*7+7+4&9", "56" ],
      [ 31, "9&8*2+3*1|2|7", "280" ],
      [ 32, "230+83*751&176", "50080" ],
      [ 33, "89+62465*94&84", "5254536" ],
      [ 34, "668&925+398*562", "599092" ],
      [ 35, "15|9+348*302&23&77", "1452" ],
      [ 36, "3&3&6|7+3|5*3|2&4*4", "0" ],
      [ 37, "7*6|7|7*1&7|7&3&8*3", "0" ],
      [ 38, "896+316*209*264&728", "2026464" ],
      [ 39, "1844+41*64|7906|66842", "138965970" ],
      [ 40, "2&41&6884*69857+68083", "0" ],
      [ 41, "2+3*9*3|6|7&0+3+3*6&8", "0" ],
      [ 42, "895+400*988|549&237+488", "938875" ],
      [ 43, "30*48&99036+140&33+75|645", "22050" ],
      [ 44, "278+2033+53*96*56|3303|3&14", "3177216" ],
      [ 45, "5380&27|643*2+1888&74+30|16", "0" ],
      [ 46, "0&3845+6645*4293+78&78*3102|9|3", "90127550385" ],
      [ 47, "78&44956&67*974|413+13237*5588|54*668", "0" ],
      [ 48, "500*206+145|167|163|465&668+662+806*681&458", "123896000" ],
      [ 49, "82+14&0*344+34+542916&18*11|844|64*873223|840993", "23813260003764" ],
      [ 50, "374958|6727+53965&53*954&29|6*138572+59|547783&43*8998", "12178274756590800" ],
    ].each do |q|
      expect(foo(q[1])).to eq q[2].to_i
    end
  end
end

describe "896+316*209*264&728" do
  it do
    expect(foo("896+316*209*264&728")).to eq 2026464
  end
end

describe "30*15+5" do
  it do
    expect(foo("30*15+5")).to eq 600
  end
end

describe "30*15*5" do
  it do
    expect(foo("30*15*5")).to eq 2250
  end
end

describe "15*5" do
  it do
    expect(foo("15*5")).to eq 75
  end
end

describe "15+5" do
  it do
    expect(foo("15+5")).to eq 20
  end
end

describe "15&5" do
  it do
    expect(foo("15&5")).to eq 5
  end
end

describe "15|5" do
  it do
    expect(foo("15|5")).to eq 15
  end
end