public function testFilter()
{
$filter = new Alpha();
$this->assertEquals(true, $filter->apply('foo'));
$this->assertEquals(false, $filter->apply('foo123'));
$this->assertEquals(false, $filter->apply('123'));
$this->assertEquals(false, $filter->apply('12 3'));
$this->assertEquals(false, $filter->apply(''));
$this->assertEquals(false, $filter->apply('foo%&/'));
// test error message
$this->assertErrorMessage($filter->getErrorMessage());
}