codice:
nome_campo: {            
    required: function(element) {
        return $("#id_checkbox").val() == "Y";                                        
    },
    regex_hhmm: /^([0-1]?[0-9]|2[0-4]):([0-5][0-9])(:[0-5][0-9])?$/,
    maxlength: 5
}