class BaseCustomForm extends BaseFormDoctrine
{

$this->setValidators(array(
.....
'employer_postcode'     => new sfValidatorString(array('max_length' => 10, 'required' => false)),
....


class CustomForm extends BaseIncomeInformationForm
{
......

$this->validatorSchema['employer_postcode'] =  new sfValidatorOr(array(
      new UKPostCodeValidator(),
      $this->validatorSchema['employer_postcode'],
      ));

......


class UKPostCodeValidator extends sfValidatorRegex
{
  protected function configure($options = array(), $messages = array())
  {
    parent::configure($options, $messages);

    $this->setOption('pattern', '/^([A-PR-UWYZ0-9][A-HK-Y0-9][AEHMNPRTVXY0-9]?[ABEHMNPRVWXY0-9]? {1,2}[0-9][ABD-HJLN-UW-Z]{2}|GIR 0AA)$/');
  }
}