struts validation.xml

<?xml version="1.0" encoding="ISO-8859-1" ?>

<!DOCTYPE form-validation PUBLIC
     "-//Apache Software Foundation//DTD Commons Validator Rules Configuration 1.3.0//EN"
     "http://jakarta.apache.org/commons/dtds/validator_1_3_0.dtd">

<form-validation>

    <global>
        <constant>
            <constant-name>formatoCodigoProducto</constant-name>
            <constant-value>^\\d{4}$</constant-value>
        </constant>

    </global>

    <formset>

        <form name="">
        </form>

    </formset>

</form-validation>