var computeRoll = function(diceAndMods) {

  var numberOfDie = 0,

      sidesOfDie = 0,

      modifier = 0,

      temp,

      result = 0,

      errorMsg = "Requires a string of the format #d#[+-]#",

      regEx = /^(\d+)d(\d+)([+-]\d+)?$/;

  if(!regEx.test(diceAndMods)) {

    console.log(errorMsg);

    return result;

  }

  temp = regEx.exec(diceAndMods);

  numberOfDie = parseInt(temp[1], 10);

  sidesOfDie = parseInt(temp[2], 10);

  if(temp[3]) {

    modifier = parseInt(temp[3], 10);

  }

  for(var i = 0; i < numberOfDie; i++) {

    result += Math.floor(Math.random() * (sidesOfDie) + 1);

  }

  result += modifier;

  return result;

};

​

console.log(computeRoll("4d3"));

console.log(computeRoll("-4d8-1"));

console.log(computeRoll("2d6+3"));

console.log(computeRoll("2d8&3"));