COSESAI

Luhn Checksum Field Guide: When One mod-10 Walk Beats Eight Different Identifier Pipelines

By xiezixing 2026年10月11日

A field guide to the four-step Luhn algorithm, the eight identifier families that depend on it, and the three failure modes that silently ship past unit tests.

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Currency by Country Field Guide: When One Country Lookup, Three ISO Codes, and Two Output Modes Decide Whether Your Invoice Is Right or Your Bank Rejects the Wire

By xiezixing 2026年9月25日

Currency by Country answers one question: what is the money called in a given country,…

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IBAN & SWIFT Code Validator Field Guide: When Two Bank Identifiers Share One Input Box But Use Different Checksums

By xiezixing 2026年9月2日

The cross-border payment hits your invoice queue at 4:47 AM on a Tuesday. The customer swears the IBAN is correct, your bank says the SWIFT is misrouted, and your finance team is going to spend the...

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ABA Routing Number Validator Field Guide: When MICR Modulus-10 and Federal Reserve Districts Decide What Nine Digits Mean

By xiezixing 2026年9月1日

A nine-digit ABA routing number is not really nine digits. It is three pieces of…

Article cover poster for Words to Number text-processing tool article

When Your Auditor Writes Numbers in English

By xiezixing 2026年7月17日

Number words are the kind of data no one notices until they break the pipeline.…

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IBAN & SWIFT Validator: four checks, one verifier

By xiezixing 2026年7月17日

Cross-border payments need a four-step validator: strip whitespace, check length against country, run mod-97 for IBANs, and verify the 8 or 11 character SWIFT/BIC structure. Here is the exact recipe and the five traps that break it in production.

SEDOL validation explained with seven characters one check digit and a weighted mod 10 control

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Seven Characters, One Check Digit: Catch a Bad SEDOL Before It Reaches Your Books

By xiezixing 2026年7月13日

A practical guide to SEDOL-7 format rules, weighted mod-10 validation, and where securities-data pipelines should catch a bad identifier.

Credit Card Generator - Luhn-valid test card numbers for development

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Test Card Numbers That Actually Pass Luhn: A Credit Card Generator Walkthrough

By xiezixing 2026年7月12日

BIN-correct, brand-correct, check-digit-valid test card numbers for Visa, MasterCard, Amex, Discover, JCB, and China UnionPay — without leaking real PII into your fixtures.

Credit Card Validator cover image — why Luhn is a typo catcher not a security check, by Elysia Tools blog

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Why the Luhn Check Is a Typo Catcher, Not a Security Check: Inside a Modern Credit Card Validator

By xiezixing 2026年7月5日

The four-part output every payment form's credit card validator should surface — Luhn, brand, length, IIN — and the prefix-table traps that old validators miss.

IBAN Validator cover - three decisions that decide whether a bank account number is real

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IBAN Validator: The Three Decisions That Decide Whether a Bank Account Number Is Real

By xiezixing 2026年7月4日

Country code, length, mod-97. Three decisions in a strict order — the only structure an IBAN validator needs to catch every typo without rejecting valid numbers.