Use | to combine different case
using System; using System.Text.RegularExpressions; class RXOptionsApp { static void Main(string[] args) { Regex r = new Regex("in|In|IN|iN"); string s = "The KING Was In His Counting House"; MatchCollection mc = r.Matches(s); for (int i = 0; i < mc.Count; i++) { Console.WriteLine("Found '{0}' at position {1}",mc[i].Value, mc[i].Index); } } }