实例代码：
if (strNum.length <= 3) {
return strNum;
}
if (!/^(\+|-)?(\d+)(\.\d+)?$/.test(strNum)) {
return strNum;
}
var a = RegExp.$1, b = RegExp.$2, c = RegExp.$3;
var re = new RegExp();
re.compile("(\\d)(\\d{3})(,|$)");
while (re.test(b)) {
b = b.replace(re, "$1,$2$3");
}
return a + "" + b + "" + c;
}
