var sb=new StringBuilder();
foreach(char c in s)
{
if(c>='0'&&c<='9')
{
sb.Append(c);
}
}
return sb.ToString()

------解决方案--------------------
正则表达式！

------解决方案--------------------
            string s = "d$a#31#d$fc$r%22%f$e$r$43@e$65%g$g&d$z";
           string pattern = @"\d+";
  
           MatchCollection matches = Regex.Matches(s, pattern,RegexOptions.Multiline);    // Split on hyphens
           foreach (Match match in matches)
           {
               Console.WriteLine("'{0}'", match);
               Console.ReadLine();
           }


这样的结果是：31 22 43 65

如果string pattern = @"\d";
则结果是3 1 2 2 4 3 6 5  

具体知识上面有提的，正则表达式