Uri：

Uri uri = new Uri(@"\\servername\d$\directory");
string[] segs = uri.Segments;
string s = "http://" + uri.Host + "/" + 
    string.Join("/", segs, 2, segs.Length - 2) + "/";

只是另一个选项，为了显示不同的选项：

(?<=^//)[^/]++

服务器名称将是\0或$0，或者仅仅是函数的结果，这取决于您如何调用它以及您的语言提供的内容。
regex注解模式下的解释：

(?x)      # flag to enable regex comments
(?<=      # begin positive lookbehind
^         # start of line
//        # literal forwardslashes (may need escaping as \/\/ in some languages)
)         # end positive lookbehind
[^/]++    # match any non-/ and keep matching possessively until a / or end of string found.
          # not sure .NET supports the possessive quantifier (++) - a greedy (+) is good enough here.

这个应该可以了

^//([^/]+).*

服务器名称位于第一个捕获组中

匹配servername的正则表达式：

^//(\w+)

很丑，但它只是工作：

var host = uncPath.Split(new [] {'\\'}, StringSplitOptions.RemoveEmptyEntries).FirstOrDefault();

使用System.Uri主机属性：https://learn.microsoft.com/en-us/dotnet/api/system.uri.host?view=net-7.0

var path = @"\\servername\d$\directory";
var uncPath = new Uri(path);
var serverName = uncPath.Host; // returns "servername"