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UPC/EAN Barcode Validator Field Guide: When the Mod-10 Catches the Bad One
A field guide to the mod-10 weighted checksum baked into the last digit of every GTIN-8, GTIN-12, GTIN-13, and GTIN-14 barcode on every shelf.
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Mod-11 Checksum Calculator Field Guide: ISBN-10, fodselsnummer, and the X check digit
The Mod-11 check digit is the quiet workhorse behind ISBN-10, ISBT-128 blood bags, Norwegian fødselsnummer,…
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Your CSV Migration Silently Drops 5% of Records. Here’s the Foreign Key Check That Catches Them.
A finance team I worked with moved 18,000 customer records from a legacy SQL Server…
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Seven Characters, One Check Digit: Catch a Bad SEDOL Before It Reaches Your Books
A practical guide to SEDOL-7 format rules, weighted mod-10 validation, and where securities-data pipelines should catch a bad identifier.
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IBAN Validator: The Three Decisions That Decide Whether a Bank Account Number Is Real
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.
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Why your API endpoint breaks the moment the JSON drifts
Your API accepts a JSON body. The shape looks fine in Postman. It looks fine…