Validating form without Doctrine Entity

See the Using a Form without a Class section — there is a subsection on validation, too.

The answer is to setup the constraints yourself, and attach them to the individual fields.

use Symfony\Component\Validator\Constraints\Length;
use Symfony\Component\Validator\Constraints\NotBlank;

$builder
   ->add('firstName', 'text', array(
       'constraints' => new Length(array('min' => 3)),
   ))
   ->add('lastName', 'text', array(
       'constraints' => array(
           new NotBlank(),
           new Length(array('min' => 3)),
       ),
   ))
;