using System;
using System.Text.RegularExpressions;
public class Example
{
public static void Main()
{
string input = "this is a test";
string pattern = "is?";
MatchCollection matches = Regex.Matches(input, pattern);
foreach (Match match in matches)
Console.WriteLine("'{0}' found in the source code at position {1}.",
match.Value, match.Index);
}
}