struts-sandbox/struts2/apps/starter/src/main/java/com/foo/example/HelloMatrixAction-helloMatrixSubmit-validation.xml

<!DOCTYPE validators PUBLIC "-//OpenSymphony Group//XWork Validator 1.0.2//EN" "http://www.opensymphony.com/xwork/xwork-validator-1.0.2.dtd">

<validators>
    <field name="hello">
        <field-validator type="requiredstring">
            <message>You must enter a name</message>
        </field-validator>
    </field>
</validators>