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Luhn / Card Check
Check whether a number passes the Luhn checksum — the formula used by credit-card and many ID numbers. It validates the checksum only, not whether an account exists.
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Worked example
✓ valid (Luhn passes)
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Frequently asked
Does this mean the card is real?
No — Luhn only checks the checksum digit, not that the card is issued or active.
Are spaces okay?
Yes, non-digits are ignored.