Insert a character each specific position

Result is 123-456-789-0

var str = "1234567890";
str = str.replace(/(\S{3})/g,"$1-");
alert (str);

Inserting twice at different positions
Result 123-456-7890
var str1 = "1234567890"
var str2 = "-"

str1 = str1.substr(0,3) + str2 + str1.substr(3,3) + str2 + str1.substr(6)
alert(str1);