Sanitize custom checkout fields data in WooCommerce - php

Question

Following the WooCommerce checkout fields customization docs:
Customizing checkout fields using actions and filters
I have added a custom field to woocommerce checkout page, through functions.php.
I'm worried if I have to sanitize user input for that custom field?
I think it doesn't need sanitization since it's passed into billing fields as in: $fields['billing'], is that correct?
If not how do I sanitize this custom field?
Creating this custom field is meant to accept text strings(latin) and integers combined no longer than 50 in length.
// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
//Adding custom text field
$fields['billing']['billing_username'] = array(
'type' => 'text',
'label' => __('Your Username', 'woocommerce'),
'placeholder' => _x('', 'placeholder', 'woocommerce'),
'required' => true,
'class' => array('form-row-first'),
'clear' => true
);
return $fields;
}

Answer 1

If you look to the related Official Documentation linked in your question, you've got this snippet:
/**
* Update the order meta with field value
*/
add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );
function my_custom_checkout_field_update_order_meta( $order_id ) {
if ( ! empty( $_POST['my_field_name'] ) ) {
update_post_meta( $order_id, 'My Field', sanitize_text_field( $_POST['my_field_name'] ) );
}
}
In your case you don't need that as address fields are already processed by Woocommerce.
For custom special fields: The answer is yes (which is not your case)
As you can see in this code they use sanitize_text_field() WordPress function, when saving the submitted data to database with update_post_meta() function…
This is only for custom checkout fields and not for existing checkout fields, that already get their own process…