using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string input = "as as asas 34 56 78 ...";

        Test(input);

    }

    private static readonly Regex Pattern = new Regex("((?<=\\s)\\d+(?=\\s))", RegexOptions.Compiled);

​

    public static void Test(string input)

    {

        var matches = Pattern.Matches(input);

        foreach (Match match in matches)

        {

            Console.WriteLine("new Match!"); 

            GroupCollection groups = match.Groups;

            Group gr = groups[0];

            Console.WriteLine("'{0}' {1}",  

                              gr.Value, 

                              gr.Index

                              );

        }

    }

}