How to allow only letters in a symfony2 form in twig?

I want to know how to configure a textbox in twig to allow only letters, like a field type to write only names for example. I have something like this on my view:

<div class="control-group">
   <label class="control-label">{{form_label(form.name,'Name: ')}}</label>
   <div class="controls">
      {{form_widget(form.name,{'attr':{'placeholder':'Insert only text','min':'1'}, 'id':'1'})}}          
   </div>

sorry, here´s my symfony php form code

public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder
            ->add('name')
            ->add('surname')
            ->add('address')
            ->add('idSex')
            //->add('birthdate', 'date',array('required'=>true))
            ->add('birthdate', 'date', [
                'widget' => 'single_text',
                'format' => 'yyyy-MM-dd',
                'attr' => [
                    'class' => 'form-control input-inline datepicker',
                    'data-provide' => 'datepicker',
                    'data-date-format' => 'yyyy-mm-dd'
                ]
            ])
            ->add('observation')
            ->add('idCmf')
    ;
}

You can do this with HTML 5 by using the input's pattern attribute.

Here's an example.

The pattern attribute uses regex, which is really good for this sort of task. One where you want a certain kind of answer, but don't care about specifics.

In your example:

<div class="control-group">
    <label class="control-label">{{ form_label(form.name, 'Name: ') }}</label>
    <div class="controls">
       {{ form_widget(form.name, {'attr':{'pattern': '[a-zA-Z]*', ...} }) }}          
    </div>
</div>

Or in the formType:

// formType.php
//...

public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder
        ->add('name', 'text', array(
            'attr' => ['pattern' => '[a-zA-Z]*']
        ))
        // I'm assuming you'd use it for both.
        ->add('surname', 'text', array(
            'attr' => ['pattern' => '[a-zA-Z]*']
        ))
        //...
}