js正则 replace 反向引用的问题。

var app = express();
// Rewrite the URL before it gets to Express' static middleware. 
app.use('/public/', function(req, res, next) {
        req.url = req.url.replace(/\/([^\/]+)\.[0-9a-f]+\.(css|js|jpg|png|gif|svg)$/, "/$1.$2");
        next();
});
app.use('/public/', express['static'](__dirname + '/public', { maxAge: 30 }));

//现在想把 ：

var url = 'http://localhost:8080/newcss/act/act1/index.html/aaa';          //替换为：
          'http://localhost:8080/newcss/act/act1/index.html';
          
> url.replace(/(\/newcss\/.+?index.html)/,"$1");
> "http://localhost:8080/newcss/act/act1/index.html/aaa"          //不生效

//我们来看下match的

url.match(/(\/newcss\/.+?index.html)/)
>[ "/newcss/act/act1/index.html",
   "/newcss/act/act1/index.html",          //$1
   index: 21,
   input: "http://localhost:8080/newcss/act/act1/index.html/aaa"
 ]


//match的结果完全正常，而且 $1 就是我们想要的，但是为什么就replace不行了呢？？？？？



//我在replace后再加一个括号匹配剩下的url就正常了。如下：

> url.replace(/(\/newcss\/.+?index.html)(.*)?/,"$1")
> "http://localhost:8080/newcss/act/act1/index.html"