Match Lines Not Starting with Pattern

Regex startsWith = new Regex("^<catalog", RegexOptions.Singleline);
List<String> combinedLines = new List<string>();

            string lastline = ""; //for keepign track of the last line in case we need to merge the two

            foreach (var line in File.ReadLines(filepath))
            {

                Match match = startsWith.Match(line.Trim());
                if (!match.Success)
                {
                    found_matching_lines = true;
                    combinedLines.Add(lastline.Trim() + line.Trim());
                }
                lastline = line;
            }