Question

Is it even possible?

• I know it’s possible to do a restriction based on regex, but that’s not it
• I know it’s possible to declare an attribute as a foreign key calculated by an XPath, but it seems it has to be unique

Exemple:

<root children="2">
    <child />
    <child />
</root>

Answer 1

W3C Schema 1.0 doesn’t have the ability to constrain the attribute values based upon the instance document.

Schematron is a great tool for validating that documents adhere to such custom validation scenarios.

For example:

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://purl.oclc.org/dsdl/schematron">
    <pattern>
        <rule context="root[@children]">
            <assert 
                id="children-value" 
                test="@children=count(child)" 
                flag="error">
                The root/@children value must be equal to the number of child elements.
                </assert>
            </rule>
    </pattern>
</schema>