you need namespace using System.Text.RegularExpressions;


on form load event


{
         
            char[] delimiterChars = { '.', ',', '.', ':','?', '\t' };

            string text = "Hello world. How are you? I am fine. This is a difficult sentence because I use I.D.Newlines should also be accepted. Numbers should not cause sentence breaks.";
       

            string[] words = text.Split(delimiterChars);
         

            foreach (String match in words)
            {
           
                comboBox1.Items.Add(match);
            }
}

Output:

Hello world

How are you

I am fine

and so on......