Does a valid checksum prove the identifier exists?

No. It proves only internal consistency under the implemented rules.

When should separators be removed?

Only when the specification treats them as display characters.

When is Luhn insufficient?

When length, prefixes, alphabet, weights, or additional identifier rules apply.

Check-Digit and Checksum Validation

Goal and scope

Normalize an identifier, select its specified checksum path, and verify format before accepting it.
Use this guide to turn the stated scope into a reviewable result. Keep the source, intermediate outputs, and decisions together so the next person can understand what was checked and why.

Prepare evidence and make decisions

Know the identifier family.
Keep the original value and document separator rules.
Do not confuse checksum validity with real-world existence.
Decision: Which algorithm?. Use Luhn, Mod 11, or the dedicated validator named by the identifier specification.
Decision: Remove separators?. Remove spaces or hyphens only when they are presentation characters; preserve meaningful prefixes and codes.
Decision: Is a pass sufficient?. No. Length, prefix, country, issuer, registry, or business checks may still be required.

Complete the workflow

1. Classify the value

Identify card, book, retail, vehicle, device, banking, or security identifiers before choosing a path. Use credit-card-validator, isbn-validator, upc-ean-validator, vin-validator.

2. Normalize safely

Apply documented space, hyphen, case, and prefix rules while retaining the original. Use iban-validator.

3. Run the checksum

Use the named generic algorithm or the dedicated validator when structure is part of the rule. Use luhn-checksum, mod11-checksum, iban-swift-validator.

4. Check specialized rules

Inspect device, catalog, vehicle, or securities-specific failures rather than relying on a generic pass. Use imei-validator, iccid-validator, isin-validator, ean8-validator.

5. Record the outcome

Store normalized input, path, result, and limitations for correction or downstream checks. Use credit-card-validator.

Verify the result before handoff

Type and normalization are recorded.
The selected path matches the specification.
Malformed, failed, and passing results are distinct.
A pass is not presented as proof of issuance or activation.

Common questions

Does a valid checksum prove the identifier exists? No. It proves only internal consistency under the implemented rules.
When should separators be removed? Only when the specification treats them as display characters.
When is Luhn insufficient? When length, prefixes, alphabet, weights, or additional identifier rules apply.

Choose the right path

Which algorithm?

Use Luhn, Mod 11, or the dedicated validator named by the identifier specification.

Remove separators?

Remove spaces or hyphens only when they are presentation characters; preserve meaningful prefixes and codes.

Is a pass sufficient?

No. Length, prefix, country, issuer, registry, or business checks may still be required.