action-validation.xml in struts 2

<validators>

   <field name="username">
    <field-validator type="requiredstring">
    <message>Login name is required</message>
    </field-validator>

        <field-validator type="regex">
           <param name="expression"><![CDATA[(.**.*)]]></param>
       <message>Login name must contain a *</message>
      </field-validator>
    </field>

</validators>