Run-length decoder

Question

This replaces numbers with the character that appears next to them ('number' times). Is there a a more elegant or shorter way to do this?

var decode = function(str, result) {
    var regex = /\d+/,
        number = regex.exec(str);

    if (number === null) {
        return str;
    }

    var start =  number.index,
        end = number[0].length + start - 1,
        str = str.replace(/\d+/, ""),
        repeat = str.charAt(start);

    result += str.substring(0, start);
    for (var i = 0; i < number[0] - 1; i++) {
        result += repeat;
    }
    result += str.substring(start, str.length);
    number = regex.exec(result);

    if (number === null) {
        return result;
    } else {
        return decode(result, "");
    }
};

---

var str = "bob2b11a";
console.log(decode(str, ""));
// "bobbbaaaaaaaaaaa"

Answer 1

The basic concept

• Utilize string replace with regular expression
• Utilize new Array Creation with join

The code

function decode (str) {
    return str.replace(/(\d+)(\w)/g, 
        function(m,n,c){
            return new Array( parseInt(n,10)+1 ).join(c);
        }
    );
}

Answer 2

From my answer on Stack Overflow: https://stackoverflow.com/a/13481139/538551

Simply use String.replace():

function decode(str) {
    return str.replace(/(\d+)([a-zA-A])/g, function (match, num, letter) {
        var ret = '', i;
        for (i = 0; i < parseInt(num, 10); i++) {
            ret += letter;
        }
        return ret;
    });
}