For my own reference. Hope it’s helpful to you as well.

Given this text pattern:
Keyword blah blah keyword blah
Line I don’t care.
Keyword blah blah keyword blah
I want everything after this line.
Good stuff
Good stuff

Recipe 1: Returning all lines with keyword:

        //string pattern = @"keyword .+ keyword .+";
        public string GetKeywordLines(string pattern, string inputText)
        {
            StringBuilder sb = new StringBuilder();
            foreach (Match m in Regex.Matches(inputText, pattern))
            {
                sb.AppendLine(m.Groups[1].Value);
            }
            return sb.ToString();
        }

Recipe 2: Returning all lines after “I want everything after this line.”

       //string pattern = @"^.+I want everything after this line\.(.+)$";
       public string SplitStatusUpdate(string pattern, string inputText)
        {
            Regex regex = new Regex(pattern, RegexOptions.IgnoreCase | RegexOptions.Singleline);
            Match m = regex.Match(inputText);
            return m.Groups[1].Value;
        }

]