How GSTIN Checksum Validation Works (and Why It Isn't Enough)

Every Indian GST invoice carries a 15-character GSTIN. If you've ever had to validate one in code, you've probably written a regex like this and called it done:

/^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z]{1}[1-9A-Z]{1}Z[0-9A-Z]{1}$/

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That regex checks the shape of a GSTIN. It says nothing about whether the number is real. A typo that happens to land on a syntactically valid string will sail straight through it — and if that typo ends up on an invoice, it's the kind of thing that gets flagged months later during a GST audit.

How GSTIN Checksum Validation Works (and Why It Isn't Enough)

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