string input = "News-1.html";
 string pattern = @"(?<=News)-(\d+)(?=\.html)";
 input = System.Text.RegularExpressions.Regex.Replace(input, pattern, @"/$1");

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探讨

C# code


 string input = "News-1.html";
 string pattern = @"(?<=News)-(\d+)(?=\.html)";
 input = System.Text.RegularExpressions.Regex.Replace(input, pattern, @"/$1");


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try...

按你的需求
C# code
//要么是这样
void Application_BeginRequest(object sender, EventArgs e)
{
    Regex reg = new Regex(@".+News-(\d+).html");
    HttpContext.Current.RewritePath(reg.Replace(HttpContext.Current.Request.Url.AbsolutePath, "News/$1.html"));
｝
//或者是这样
void Application_BeginRequest(object sender, EventArgs e)
{
    Regex reg = new Regex(@"News-(\d+).html");
    HttpContext.Current.RewritePath(reg.Replace(HttpContext.Current.Request.Url.AbsolutePath, "News/$1.html"));
｝

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整这么麻烦干嘛？
用伪静态，直接写在webconfig就行。