using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string text = "<table>\n  <thead><tr><th style=\"\"><div class=\"th-inner \">Login Name</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner sortable\">Registered</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner \">Registered Date <br>Time</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner sortable\">User Response Count</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner \">Test Start Date Time</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner \">Test End Date Time</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner \">Time Remaining</div><div class=\"fht-cell\"></div></th><th style=\"\"><div class=\"th-inner \">User Status</div><div class=\"fht-cell\"></div></th></tr></thead><tbody><tr data-index=\"9\"><td style=\"\">njuser14</td><td style=\"\">Yes</td><td style=\"\">-</td><td style=\"\">0</td><td style=\"\">29-Feb-2020 15:27:58</td><td style=\"\">29-Feb-2020 15:28:03</td><td style=\"\">179</td><td style=\"\">Paused</td></tr><tr data-index=\"10\"><td style=\"\">njuser15</td><td style=\"\">Yes</td><td style=\"\">-</td><td style=\"\">0</td><td style=\"\">29-Feb-2020 15:27:32</td><td style=\"\">29-Feb-2020 15:27:42</td><td style=\"\">179</td><td style=\"\">Paused</td></tr></tbody></table>";

        string text2 = " dasd arew 29-Mar-2020 12:25:56 29-Mar-2020 12:25:56 das tfgwe 29-Feb-2020 12:25:56 ";

        //string pattern = @"\d{4}\-\d{2}\-\d{2}\s\d{2}\:\d{2}\:\d{2}";

        //string pattern = @"\d{4}\-^[a-zA-Z]{3}-\d{2}\s\d{2}\:\d{2}\:\d{2}";

        //string pattern = @"\d{4}\-([JAN|FEB|MAR|APR|MAY|JUN|JULY|AUG|SEP|OCT|NOV|DEC])-\d{2}\s\d{2}\:\d{2}\:\d{2}";

        string pattern = @"\d{4}\-\d{2}\-\d{2}\s\d{2}\:\d{2}\:\d{2}|\d{2}\-[a-zA-Z]{3}\-\d{4}\s\d{2}\:\d{2}\:\d{2}";

        Regex r = new Regex(pattern);

        var res = r.Replace(text, new MatchEvaluator(ConvertDateFormat));

        var res2 = r.Replace(text2, new MatchEvaluator(ConvertDateFormat));

        Console.WriteLine(res);

        Console.WriteLine("-------------------------------------------------------");

        Console.WriteLine(res2);

    }

    static string ConvertDateFormat(Match m)

    {

        var mydate = DateTime.Parse(m.Value);

        //return mydate.ToString("=yyyy-MM-dd hh:mm:ss");

        return string.Format("=\"{0}\"", mydate.ToString("yyyy-MM-dd hh:mm:ss"));

    }

}

​

// 29-Feb-2020 15:27:58 need to be replaced with ="29-Feb-2020 15:27:58"