using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var questionText = "<img src='ExtDownloadFile.axd?id=ca2c9fd3-6559-401a-8e8d-10fd95edaea4' /> whatever so <img src='ExtDownloadFile.axd?id=dcee3b68-0658-4f0f-b984-9ade30902463' />";

        var replaced = Regex.Replace(questionText, "ExtDownloadFile\\.axd\\?id=([0-9A-Fa-f]{8}-([0-9A-Fa-f]{4}-){3}[0-9A-Fa-f]{12})", new MatchEvaluator(m => {

            var guid = m.Groups[1].Value;

            // use guid to find correct datafile, then return _urlHelper.Clx().DataUrl(dataFile);

            return "newUrlWith?id=" + guid;

        }) );

        Console.WriteLine(replaced);

    }

}