using System;

public class Program

{

    public static void Main()

    {

        string htmlPageContent = "Some HTML <br> DefaultStyle.css?x=839ua9";

        string pageContent = "";

        System.Text.RegularExpressions.Regex regEx = null;

​

        regEx = new System.Text.RegularExpressions.Regex(@"DefaultStyle\.css\?x=839ua9");

        pageContent = regEx.Replace(htmlPageContent, "DefaultStyleSnap.css?x=742zh2");

        Console.WriteLine(pageContent);

    }

}