Symfony2 forms and pattern attribute

I am trying to create a form in Symfony2 with a text input field that accepts a string consisting of 6 digits followed by a dash followed by four digits (e.g. 123456-7890). It does work, but I'm getting inconsistent HTML markup depending on exactly how I add the field to the form.

This is from the entity:

class Kursist
{
...
/**
 * @ORM\Column(type="string", length=11)
 */
protected $cpr;
....
public static function loadValidatorMetadata(ClassMetadata $metadata)
{
    $metadata->addPropertyConstraint('cpr', new NotBlank());
    $metadata->addPropertyConstraint('cpr', new Regex(array(
        'pattern' => '/^\d{6}-\d{4}$/',
    )));
}

From the form definition:

class KursistType extends AbstractType
{
  public function buildForm(FormBuilderInterface $builder, array $options)
  {
    $builder->add('cpr');
    ...
  }
}

From the twig template:

<form action= ... >;
  {{ form_row(form.cpr) }}
    ...
</form>

This is what the browser source code looks like – note the pattern and maxlength attributes:

<label for="KursistType_cpr" class="required">Cpr</label><input type="text" id="KursistType_cpr" name="KursistType[cpr]" required="required" maxlength="11" pattern="\d{6}-\d{4}" value="123456-7890" />

I want to use my own label rather than the default, so I change the form field defition:

    class KursistType extends AbstractType
    {
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
        $builder->add('cpr','text', array('label' => 'CPR'));
        ...

But now when I look at the browser source code, the pattern and maxlength attributes have gone:

    <label for="KursistType_cpr" class="required">CPR</label><input type="text" id="KursistType_cpr" name="KursistType[cpr]" required="required" value="123456-7890" />

Is there some way to modify the label using the FormBuilderInterface without these attributes being dropped?