Question

I am trying to pick paticular strings in an array and add them to a new array e.g. I want to grab all strings in the array that contain .txt and .rtf and add them to a new array e.g. filteredStrings[]

Answer 1

You do not need regex for something that simple: Contains works faster, and is easier to understand:

var filteredStrings = myStrings.Where(s => s.Contains(".txt") || s.Contains(".rtf")).ToArray();

If you insist on using regex, you can do this:

var regexp = new Regex("[.](txt|rtf)");
var filteredStrings = myStrings.Where(s => regexp.IsMatch(s)).ToArray();