using System;
using System.Text.RegularExpressions;
 
public class Test
{
public static void Main()
{
String input = @"start123start123
start123endstart345end
start567endstart789end";
Regex rgx = new Regex(@"(?<=start)\d+(?=end)");
foreach (Match m in rgx.Matches(input))
Console.WriteLine(m.Value);
}
}