find and replace href in DownloadString

using System;
using System.Text.RegularExpressions;

public class Program
{
    public static void Main()
    {
        // original string
        string input = "<a href='http://oldurl.com'>somewhere</a>";
        // use Regex.Replace to replace the pattern in the input.
        string pattern = @"(?<=href=[']).+(?=')";
        string output = Regex.Replace(input, pattern,"http://newurl.com");
        // Write the output.
        Console.WriteLine(input);
        Console.WriteLine(output);
    }
}

<a href='http://oldurl.com'>somewhere</a>
<a href='http://newurl.com'>somewhere</a>