[Serusers] Distinguish between registration and unregistration SIP messages

[Jffdsaklj dsjk]

Hi,
I'd like to make some actions in the ser.cfg file when the SIP message
matches the REGISTER action, but only when it corresponds to the
unregistration of the user.
How can I distinguish both cases?. Is there any command I can include
in my ser.cfg file?.
I know that unregistration REGISTER messages include a header field:
"Expires: 0" but I don't know how to find this string in the message.
There's no $SIP_HF_EXPIRES.
Thanks