using System.Text.RegularExpressions;
public static void Main()
string output = "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG";
Regex rgx = new Regex(query, RegexOptions.IgnoreCase);
MatchCollection matches = rgx.Matches(output);
output = Regex.Replace(output, query, "<span class='highlight'>" + matches[0] + "</span>", RegexOptions.IgnoreCase);
Console.WriteLine(output);