XML Schema pattern which allows no data at all

I'm receiving xml data in these two forms from an external company

<currencydate>20110910</currencydate>
<currencydate/>

I want to verify using a pattern that this date indeed has the开发者_StackOverflow format YYYYMMDD like this

<xs:element name="currencydate" type="dateType"/>

<xs:simpleType name="dateType">
    <xs:restriction base="xs:string">
        <xs:pattern value="[0-2][0-9]{3}[0-1][0-9][0-3][0-9]"/>
    </xs:restriction>
</xs:simpleType>

This works fine. But the validation breaks on the empty element

So I added the minOccurs like this

<xs:element name="currencydate" type="dateType" minOccurs="0"/>

No success so I added nillable

<xs:element name="currencydate" nillable="true" type="dateType" minOccurs="0"/>

No success, I guess the element is there so it checks the pattern. So I changed the pattern

        <xs:pattern value="[0-2][0-9]{3}[0-1][0-9][0-3][0-9]|"/>

I only added the pipe indicating the value can be empty. But still no success.

So my question is: how can I check the data pattern but also allow the value

    <currencydate/>

Please note I'm receiving this data from an external company which does not provide an xsd nor are they willing to change anything for me.

---

Did you already try

<xs:pattern value="|([0-2][0-9]{3}[0-1][0-9][0-3][0-9])" />

as suggested in how to validate empty string value tag in xsd?

I only tried it in VS 2010 Express but it seems to work even if a comment in the linked post tells otherwise.