ReplaceFormat string extension

StringExtensions.cs

    /// <summary>
    /// This class is used for replacing parts of a string with other content.
    /// </summary>
    public static class StringExtensions
    {
        public static string ReplaceFormat(this string str, string format, string replacement)
        {
            return ReplaceFormat(str, format, match => replacement);
        }

        public static string ReplaceFormat(this string str, string format, MatchEvaluator evaluator)
        {
            int pos = format.IndexOf("{0}");
            if (pos < 0)
                throw new ArgumentException("Format argument must contain '{0}'.");

            var prefix = format.Substring(0, pos);
            var suffix = format.Substring(pos + 3);
            var pattern = BuildExpression(prefix, suffix);

            return Regex.Replace(str, pattern, evaluator);
        }

        private static string BuildExpression(string prefix, string suffix)
        {
            return string.Format("(?<={0})(.*?)(?={1})", Regex.Escape(prefix), Regex.Escape(suffix));
        }
    }