using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string[] strings = {"msl_0123456789_otherstuff.csv", "msl_test.xml","anythingShort.w1", "123456testxxxxxxxx"};
        string pattern = @"^(?:msl_)?(\S{1,10})\S*\.[^\W_]{2,}$";
        foreach (String s in strings) {
            Match match = Regex.Match(s, pattern);
            if (match.Success)
            {
                Console.WriteLine(match.Groups[1]);
            }                            
        }
	}
}