Replace String with ignore case

I have faced a situation where I need to replace some tag in mail body template with value but tags can be created in any case so while replacing I had to do ignore case. So I created a extension method for string as follows.

//main function
        static public string ReplaceIgnoreCase(this string source, string OldText, string NewText)
        {
            source = Regex.Replace(source, OldText, NewText, RegexOptions.IgnoreCase);
            return source;
        }

//implementation
emailbody = emailbody.ReplaceIgnoreCase("<email>", "abc@ggg.com");