ref. http://kevinvaldek.posterous.com/number-with-delimiter-in-javascript
- http://sleeplesscoding.blogspot.com/2011/02/rails-like-numberwithdelimiter-in.html

function number_with_delimiter(number, delimiter) {
  number = number + '', delimiter = delimiter || ',';
  var split = number.split('.');
  split[0] = split[0].replace(/(d)(?=(ddd)+(?!d))/g, '$1' + delimiter);
  return split.join('.');
};

var num = 12345678;  
var str = num.number_with_delimiter();  
alert(str); 

Number.prototype.number_with_delimiter = function(delimiter) {
    var number = this + '', delimiter = delimiter || ',';
    var split = number.split('.');
    split[0] = split[0].replace(
        /(d)(?=(ddd)+(?!d))/g,
        '$1' + delimiter
    );
    return split.join('.');    
};

number_with_delimiter(12345678)
> 12,345,678

number_with_delimiter(12345678, ':')
> 12:345:678

number_with_delimiter(12345678.555)
> 12,345,678.555