I tried and its works:
List<string> list = new List<string>();
list.Add("ASD");
list.Add("ASFsdsd");
list.Add("adsASFsdsd");
string pattern = @"[A-Z]{3}";
Regex regex = new Regex(pattern);
List<string> result = new List<string>();
foreach (string val in list)
{
Match match = regex.Match(val);
result.Add(match.Value);
}
If you replace 3 with 17 it must work for you.