XML Schema Regex restriction different lengths

I use in my XML schema something like that:

    <xs:restriction base="xs:string">
        <xs:pattern value="[A-Za-Z]{3,12}" />
    </xs:restriction>

Now I want my string not to have a length BETWEEN 3 and 12 but the length 3 OR 12.. Furthermore I want to check if the string has the length 3, 4, 6 or 12..

Thank you for your help!!

Kind regards