# Card number

Sometimes, your customer might enter a cancelled, non-issued or invalid card number in the [Payment Form](/guides/javascript/add-payment-form.md).

A card number is validated through the following steps:

* A card token is created.
* The card number field values are used, excluding any non-numeric characters.
* The remaining digits are processed by using the Luhn algorithm.

{% hint style="warning" %}
Passing the remaining digits of the credit card through the Luhn algorithm does not prove the validation but helps prevent typing errors.
{% endhint %}


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