Got something wrong on StringRegExp

#include <IE.au3>
$oHTTP = _IECreate("http://invoice.etax.nat.gov.tw/", 0, 0, 1, 0)
$HTMLSource = _IEDocReadHTML($oHTTP)
$open = FileOpen("test.txt",1)
FileWrite("test.txt",$HTMLSource)
FileClose($open)
_IEQuit($oHTTP)

$getDate = StringRegExp(FileRead("test.txt"), '<h2>(.*?)</h2>', 3)
MsgBox(0,"",$getDate[0])