public string HighlightKeywords(string input, string keywords)
        {
            if (input == string.Empty || keywords == string.Empty)
            {
                return input;
            }

            var keywordChar = keywords.Split(' ');
            foreach (var keyword in keywordChar)
            {
                try
                {
                    input = Regex.Replace(input, keyword, string.Format("<span class=\"highlight\">{0}</span>", "$0"), RegexOptions.IgnoreCase);
                }
                catch (Exception)
                {
                    // Log the error.
                }
            }

            return input;
        }

Highlight keyword in string

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C#