I wouldn't use a Regex for this: it is too difficult to get it right, and handle the number of days in the appropriate month.
Instead, look at using DateTime.TryParseExact:
DateTime dt;
if (DateTime.TryParseExact("12-Oct-2011 22:34", "dd-MMM-yyyy HH:mm", CultureInfo.InvariantCulture, DateTimeStyles.None, out dt))
{
Console.WriteLine(dt);
}