gistfile1.php

<?php


$form = new \Zend\Form\Form;
$form->setInputFilter($formFilter = new \Zend\InputFilter\InputFilter());

$fieldSet = new \Zend\Form\FieldSet;
$fieldSet->setName('subforms');

$subForm = new \Zend\Form\Form;
$subForm->setName(0);

$subForm->add(array(
    'name' => 'text',
    'attributes' => array(
        'type' => 'text'
    ),
));
$subForm->setInputFilter($subFormFilter = new \Zend\InputFilter\InputFilter());
$subFormFilter->add(array(
    'name'        => 'text',
    'required'    => true,
    'allow_empty' => false,
));

$fieldSet->add($subForm);
$form->add($fieldSet);

$data = array(
    //note that subforms[0][text] is required and im not passing it
);
$form->setData($data));

echo $form->isValid(); //true, should be false

?>