$ 表示尋找指定字符串的結尾的位置，\b 表示單詞的邊界例如：

 var reg4=/are$/g;
   var reg5=/are\b/g;

   var str="we are thinking that      that cars      are";
   console.log(str.match(reg4)); //["are"]
   console.log(str.match(reg5));  //["are", "are"]

　　反向引用查找重復的單詞：

 var reg3=/([A-Za-z]+)\s+\1\b/g;
   var str="we are thinking that      that cars      are
 
";
   console.log(str.match(reg3)); //["that      that"]

正則表達式1