using System;

using System.Text.RegularExpressions;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string input = "xxoxoxoxoxoxooxxxxox";

        int[] result = Regex.Matches(input, "(?<=x)o(?=x)").Cast<Match>().Select(x => x.Index).ToArray();

        foreach (int item in result)

        {

            Console.WriteLine(item);

        }

    }

}