using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string query = "fox";

​

            string output = "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG";

            Regex rgx = new Regex(query, RegexOptions.IgnoreCase);

            MatchCollection matches = rgx.Matches(output);

            if (matches.Count > 0)

            {

                    output = Regex.Replace(output, query, "<span class='highlight'>" + matches[0] + "</span>", RegexOptions.IgnoreCase);

            }

        Console.WriteLine(output);

    }

}