javascript实现身份证号的验证

<html xmlns="http://www.w3.org/1999/xhtml" ><head><title>标题页</title><SCRIPT LANGUAGE="JavaScript">     function isCardNo(num)      {        if (isNaN(num)) {alert("输入的不全是数字！"); return false;}        var len = num.length, re;         if (len == 15)                                  //15位身份证的判断          re = new RegExp(/^(\d{6})()?(\d{2})(\d{2})(\d{2})(\d{3})$/);        else if (len == 18)                            //18位身份证的判断          re = new RegExp(/^(\d{6})()?(\d{4})(\d{2})(\d{2})(\d{3})(\d)$/);        else {alert("输入的数字位数不对！"); return false;}        var a = num.match(re);                        //判断是否有符合条件的表达式        if (a != null)        {          if (len==15)          {                                                 //对15位身份证中日期的判断            var D = new Date("19"+a[3]+"/"+a[4]+"/"+a[5]);            var B = D.getYear()==a[3]&&(D.getMonth()+1)==a[4]&&D.getDate()= =a[5];          }          else          {                                                 //对18位身份证中日期的判断            var D = new Date(a[3]+"/"+a[4]+"/"+a[5]);            var B = D.getFullYear()==a[3]&&(D.getMonth()+1)==a[4]&&D.getDate ()==a[5];          }          if (!B) {alert("输入的身份证号 "+ a[0] +" 里出生日期不对！"); return false;}        }        return true;      }</script></head><body><input type=text name="txt1" value="1110120780423003"><input type=button value="判断身份证号" onClick="javascript:alert(isCardNo(txt1.value))"></body></html>