var x=12345678; x=x.toString(); var lastThree = x.substring(x.length-3); var otherNumbers = x.substring(0,x.length-3); if(otherNumbers != '') lastThree = ',' + lastThree; var res = otherNumbers.replace(/\B(?=(\d{2})+(?!\d))/g, ",") + lastThree; alert(res);