using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var searchTerm = "text";
		var textToFormat = "This is some Text: <a href='/text.html'><img src='/images/text.png'/></a>";
		var regex = new Regex($@"(<(?:img|a)(?:\s[^>]*>)?)|{Regex.Escape(searchTerm)}", RegexOptions.IgnoreCase);

		var highlightedText = regex.Replace(textToFormat, m =>
    		m.Groups[1].Success ? m.Groups[1].Value : $"<span class='highlight'>{m.Value}</span>");
		Console.WriteLine(highlightedText);
	}
}