I think we can do it like this. Basically we just use a word boundary and a simple replace. This will find any values with numbers between 13 and 16 characters. Unless you have other long numeric values you shouldn't get too many false positives.

string ccRegEx = @"\b\d{13,16}\b";

string SomeValue = "this contains possible CC Nums 1234-5689-9875-5621 or so it seems.";

SomeValue = "blah blah... 4258-1234-5678-1. Email sent blah blah...";

Regex r = new Regex(ccRegEx);

bool found = r.IsMatch(SomeValue.Replace("-", ""));