[exim] System Filter regex problem.

Hallo,

I fail to write an appropriate system filter.

The following line works:
headers add "X-Mailbox: ${sg{dummy dummy dummy}{\\N^.+\$\\N}{ersetzt}}"
result:
X-Mailbox: ersetzt

All (.+) between start (^) and end ($) is replaced with "ersetzt":

The following line does not work:
headers add "X-Mailbox: ${sg{$header_Received: }{\\N^.+\$\\N}{ersetzt}}"
result:
X-Mailbox: from michael by merkur.hd.mszet.de with local (Exim 4.63)
        (envelope-from <michael@???>)
        id 1I0Odx-0000uz-Fw
        for michael@???; Mon, 18 Jun 2007 23:20:13 +0200

Why? The Regex is the same above ...

I using Exim version 4.63 with Debian Etch.

Greetings
Michael Strauss