using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{

          
String s = @"abcde   my first line
fghij   my second line
klmno   my third line";

String mySubstring = "fghij";

Match m = Regex.Match(s, "^" + mySubstring + ".*", RegexOptions.Multiline);

if (m.Success) {
   Console.WriteLine(m.Value);
} 


    }
}