连续数字转换为正则表达式？

例如有数字2001 2002 2003 2004 3001 3002 3003 3004 4201 4202 4203 4204 要转变为：200[1-4]|300[1-4]|420[1-4]该怎么做？

正则不是只有一种写法
var a = [2001 ,2002, 2003 ,2004 ,3001 ,3002 ,3003 ,3004 ,4201 , 4202, 4203,4204];
var reg = new RegExp('^' + a.join('|') + '$');
我写了几个代码，你看看吧................
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
  <script type="text/javascript">
   function loadend(){
   var req = "";
   var tempReq = "";
   var flag = false;
var array = [2001,2002,2003,2004,3001,3002,3003,3004,4201,4202,4203,4204,50,51,69,70];
for(i = 1;i < array.length;i ++){
//如果连续
if(array[i]-array[i-1] == 1){
//加那个“|“符号
if(req != "" && tempReq == "") tempReq = "|";
//如果十位上的数相同
if(Math.floor(array[i]/10) == Math.floor(array[i-1]/10)){
if(tempReq == "" || tempReq == "|"){
tempReq += Math.floor(array[i]/10) + "[" + array[i-1]%10;
flag = true;
}
tempReq += array[i]%10;
}else{
//4楼的问题我就没考虑了...
}
}else{
tempReq += "]";
req += tempReq;
tempReq = "";
flag = false;
}
}
if(flag){
tempReq += "]";
req += tempReq;
}
document.write(req);
   }
  </script>
    <title>MyHtml.html</title>

    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
    <meta http-equiv="description" content="this is my page">
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">

      </head>

  <body onload="loadend();">
  </body>
</html>