Dictionary 内容类型匹配

我试图使用nginx映射模块来检测传入请求的内容类型是否为application/x-www-form-urlencoded。映射中的示例正则表达式代码不起作用。谁能看出我做错了什么？任何帮助都将不胜感激

map $content_type $ct {
    ~^/application\/x-www-form-urlencoded/ "application/x-www-form-urlencoded";
    default "application/json";
}

server {
    listen 80;
    server_name www.example.com;
    location / {
        proxy_pass http://localhost:8000;
        proxy_set_header Content-Type $ct;
    }
}

---

我最终没有使用map函数，而是：

server {
    listen 80;
    server_name www.example.com;
    location / {
        set $ct "application/json";
        if ($content_type = "application/x-www-form-urlencoded") {
            set $ct "application/x-www-form-urlencoded";
        } 
        proxy_pass http://localhost:8000;
        proxy_set_header Content-Type $ct;
    }
}