Restricting attribute content to not include a specific string

Hi,

I need to restrict attribute values in my Schema to not be the string
"unknown".
So I declared a simple type as follows:

	<xs:simpleType name="attrValue">
		<xs:restriction base="xs:string">
			<xs:pattern value="^(unknown)"/>
		</xs:restriction>
	</xs:simpleType>

Since I am not experienced with regular expressions I am kind of stuck
on the pattern value.
Do you have any idea, how to make that restriction in the most simple
way?
Thank you very much for your help.

best regards,
Roman