Wordpress Phone + Custom field Validation - wordpress

Question

I'm a newbie in PHP, I use this PHP code that works like a charm. It's a custom form that sends to Woocommerce different fields. I want to add a "preg_match" check for each of those fields so the user can only enter:
function custom_multi_step_form_first_handler() {
$email = sanitize_text_field( strval( $_POST['email'] ) );
$first_name = sanitize_text_field( strval( $_POST['first_name'] ) );
$last_name = sanitize_text_field( strval( $_POST['last_name'] ) );
$phone = sanitize_text_field( strval( $_POST['phone'] ) );
$street = sanitize_text_field( strval( $_POST['street'] ) );
$city = sanitize_text_field( strval( $_POST['city'] ) );
$postcode = sanitize_text_field( strval( $_POST['postcode'] ) );
$person_id = sanitize_text_field( strval( $_POST['person_id'] ) );
$country = 'UK';
global $woocommerce;
$address = array(
'first_name' => $first_name,
'last_name' => $last_name,
'company' => '',
'email' => $email,
'phone' => $phone,
'address_1' => $street,
'city' => $city,
'state' => '',
'postcode' => $postcode,
'country' => $country,
);
$order = wc_create_order();
$order->set_address( $address, 'billing' );
$order->calculate_totals();
$_SESSION['current_order_id'] = $order->get_id();
update_post_meta( $order->get_id(), 'person_id', $person_id );
wp_die();
}
function custom_get_building_data_for_postcode( $postcode ) {
$building = get_page_by_title( $postcode, null, 'building' );
$building_meta = get_post_meta( $building->ID );
if ( is_null( $building ) ) {
return array();
}
return array(
'name' => get_post_meta( $building->ID, 'building_name', true ),
'city' => get_post_meta( $building->ID, 'building_city', true ),
'address' => get_post_meta( $building->ID, 'building_address', true ),
);
}
In the phone field: numbers, "+", spaces; (22 chars total)
In the postcode field: postcodes are contained in a custom post type "Buildings" with their titles as values. Only those values. How do I do that?