Optimizing Code to Minimize Time to Process Large List of Strings

I am working on an application in C# WPF which reads a large file containing a number of HL7 formatted reports. My application takes in the file, reads and extracts any lines that starts with OBX and stores it into a List. It then tries to extract report headers from the each line, if one exists, based on a handle-full of rules:

  1. Ends with a ‘:’
  2. Is in all caps.
  3. Is less then 6 words (not include words in brackets).
  4. Contains more 4 characters.
  5. May be on its own in a line or embedded into the content of the string (always at the start).

I have the algorithm down and it works, but I am dealing with files which can contain an upward of a million lines. My initial design took about 10-15 minutes to read and process around 1 million lines. Through hours of research, I was able to optimize the code a bit, bring it to about a few minutes. However, I am hoping to optimize it even further in order to reduce the time it takes for the app to process the lines. This is where I need some help as I do not know what I can do further improve the performance of my code.

I was able to narrow down the bottleneck to this method which does the header extraction from the string collected. Below is the most recent version of my method and is as optimized as I can get it (Hopefully it will be better with your help):

    private List<string> GetHeader(List<string> FileLines)         {             List<string> headers = new List<string>();             foreach (string line in FileLines)             {                 string header = string.Empty;                 //Checks if there is a ':' and assumes that anything before that is the header except if it contains a date or a report id                 if(Regex.IsMatch(header, @"\w{2,4}[/\-]\w{2,3}[/\-]\w{2,4}", RegexOptions.Compiled) || Regex.IsMatch(header, @"^\w+, \w{2} \d{5}-{0,1}\d{0,5}", RegexOptions.Compiled))                 {                     continue;                 }                  string nobrackets = Regex.Replace(line, @".*?\(.*?\)", string.Empty, RegexOptions.Compiled);                 if (line.IndexOf(':') != -1)                 {                     string nobracks = Regex.Replace(line.Substring(0, line.IndexOf(':') + 1), @"\(.*?\)", string.Empty, RegexOptions.Compiled);                     if (nobracks.Split(' ').Length < 5 && nobracks.Length > 6)                     {                         headers.Add(line.Substring(0, line.IndexOf(':') + 1));                         continue;                     }                 }                  //Checks if a string is larger then 5 words (not including brackets)                 if (!(nobrackets.Split(' ').Length < 5 && nobrackets.Length > 6))                     continue;                 //Checks if the string is in all CAPS                 char[] letter = nobrackets.ToCharArray();                  if(letter.All(l => char.IsUpper(l))){                     headers.Add(line);                     continue;                 }                  //Checks if the string is 5 words or less                 string temp = Regex.Replace(line, @"\(.*?\)", string.Empty, RegexOptions.Compiled);                 if (temp.Split(' ').Length < 6)                 {                     headers.Add(line);                 }                  //Checks for an all caps header embedded in a string                 bool caps = true;                 string[] word = line.Split(' ');                 int lastCapWordIndex = 0;                 for (int i = 0; i < word.Length && caps; i++)                 {                     char[] char_array = word[i].ToCharArray();                      if (!letter.All(l => char.IsUpper(l)))                     {                         caps = false;                         continue;                     }                     if (caps)                         lastCapWordIndex++;                 }                 if (lastCapWordIndex > 0)                 {                     for (int i = 0; i < lastCapWordIndex; i++)                     {                         header += " " + word[i];                     }                     headers.Add(header.Trim());                     continue;                 }             }              //final check for string with less then 4 characters             string[] tempH = headers.ToArray();             headers = new List<string>();             foreach (string h in tempH)             {                 if (h.Length > 4)                 {                     headers.Add(h);                 }             }             return headers;         }