Highlight something in string using Regex (C#)

highlighter.cs

using System.Text.RegularExpressions;

public static class Highlighter
{
    private const string MatchPattern = "(?<!<[^>]*)(?<matched>{0})";
    private const string ReplacePattern = "<span class='highlight'>${matched}</span>";

    public static string Highlight(string value, string pattern)
    {
        var patternEsc = string.Format(MatchPattern, Regex.Escape(pattern));
        return Regex.Replace(value, patternEsc, ReplacePattern, RegexOptions.IgnoreCase);
    }
}