Linq-GroupBy问题

我需要对列表中的数据进行分组,但不能对groupBy子句进行硬编码,因为它必须由用户定义。

List<ThirdPartyExportTransaction> transactions = new List<ThirdPartyExportTransaction>();
transactions.Add(new ThirdPartyExportTransaction { DeviceId = "TEST01-1", Unit = 1 });
transactions.Add(new ThirdPartyExportTransaction { DeviceId = "TEST01-2", Unit = 2 });
transactions.Add(new ThirdPartyExportTransaction { DeviceId = "TEST01-2", Unit = 3 });
transactions.Add(new ThirdPartyExportTransaction { DeviceId = "TEST01-1", Unit = 4 });
transactions.Add(new ThirdPartyExportTransaction { DeviceId = "TEST01-1", Unit = 5 });
transactions.Add(new ThirdPartyExportTransaction { DeviceId = "TEST01-2", Unit = 6 });


var s= transactions.GroupBy(r => extractText(r.DeviceId, "TEST01-.")); // Need to change this

// At this point s now holds the grouped list

private static string extractText(string fieldText, string regExp)
{
   Match m = Regex.Match(fieldText, regExp);

   return m.Success ? m.Value : "";
}