Using preg_match

With this code I can block special characters in my input , however I want it to still accept . , - and _ how to proceed?

elseif (!preg_match('/^[a-z A-Z0-9]+$/', $username)) {
    echo json_encode(array(
        'login'     => false,
        'message' => 'Existem caracteres especiais no seu nome de usuário, se estiver utilizando <strong>@</strong>, remova-o.'
    ));
}