C# - Text in Textdateien suchen

public static List<string> SearchStringInTextFiles(string pfad, string search)
{
  List<string> matchFiles = new List<string>();
  if (search != "")
  {
    string pattern = search;
    string[] metaChars = new string[] { @"\", @"*", @"+", @"?", @".", @"(", @")", @"[", @"]", @"{", @"}", @"/", @"|", @"^", @"$" };
    foreach (string s in metaChars)
    {
      pattern = pattern.Replace(s, @"\" + s);
    }
    foreach (string fileName in Directory.GetFiles(pfad, "*.txt", SearchOption.AllDirectories))
    {
      string[] lines = File.ReadAllLines(fileName);
      if (lines.Length > 0)
      {
        if (System.Text.RegularExpressions.Regex.IsMatch(String.Join(Environment.NewLine, lines), pattern, System.Text.RegularExpressions.RegexOptions.IgnoreCase))
        {
          matchFiles.Add(fileName);
        }
      }
    }
  }
  return matchFiles;
}