<DataSource ID="users">
    <fields>
        <field name="firstName" title="First Name" type="text" required="true" length="50"/>
        <field name="lastName" title="Last Name" type="text" required="true" length="50"/>
        <field name="email" title="Email" type="text" required="true" length="100">
            <validators>
                <validator type="regexp" 
                           expression="^([a-zA-Z0-9_.\-+])+@(([a-zA-Z0-9\-])+\.)+[a-zA-Z0-9]{2,4}$"/>
            </validators>
        </field>
        <field name="password" title="Password" type="password" required="true" length="20"/>
    </fields>
</DataSource>