How do I add custom validations to Webform? I have created the country and state fields with the Webform module, and I want to add a field validation for the state field when user select US as country. 

How can I achieve this?
Make use of the validate function in hook_form_alter:

 if ($form_id == 'webform_client_form_14') {  
    $form['#validate'][] = 'cork_creation_validate_custom';  
 }

function cork_creation_validate_custom($form, &amp;$form_state) {
  if (!preg_match('/^[A-Za-z]+$/', $form_state['values']['submitted']['name'])) {
            // $form['submitted'][$key]['#attributes']['class'] = 'error';
            form_set_error('name', t('Please enter alphabets only for Name.'));
 } 

  }