static void Main(string[] args)
        {
            // input arguments: path and file name without postfix number and extension
            string pathToDelo = @"c:\delo400\";
            int delo = 1000; 

            List<string> pathes = new List<string>();
            pathes.Add(@"c:\delo100\1000.doc");
            pathes.Add(@"c:\delo200\200.doc");
            pathes.Add(@"c:\delo300\150.doc");
            pathes.Add(@"c:\delo400\1000_1.doc");   // need
            pathes.Add(@"c:\delo400\1000_2.doc");   // need 

            string pattern = String.Format(@"({0}{1})_\d+", pathToDelo, delo);                       
            var result = pathes.Where(x => Regex.IsMatch(x, pattern, RegexOptions.IgnoreCase)).ToList();          
        }