[Vlasenko Fedor]

bes,

function calc(arr) {
    var a = +arr[1],
      b = +arr[3];
    return {
      '+': a + b,
      '-': a - b,
      '*': a * b,
      '/': a / b
    }
    [arr[2]];
  }
  var mas = "example 5-15 =twenty, example 5+15 =twenty, example 15/5 =twenty, example 5*15 =twenty".split(",");
  for (var i = 0; i < mas.length; i++) {
    alert(calc(/.*?(\d+)(.)(\d+)/.exec(mas[i])));
  }