Structure & Formats

An ISIN always has 12 characters, split into three segments:

Isin exposes each segment as a borrowed accessor:

• Isin::country_code() — the 2-character country code.
• Isin::nsin() — the 9-character National Securities Identifying Number.
• Isin::check_digit() — the check digit, as a u8 (0..=9).
• Isin::computed_check_digit() — recomputes the check digit from the first 11 characters (equal to check_digit() for any valid Isin).

ISIN has no conventional punctuated form. Its canonical rendering is simply the 12-character string, so there is no separate “compact vs. formatted” distinction. See Formatting & Display.

The check digit

The final character is a checksum computed with the modulus-10 “double-add-double” (Luhn) algorithm described in ISO 6166 Annex C. Each of the first 11 characters is first expanded to decimal digits:

• Digits contribute their own value ('0' → 0, …, '9' → 9).
• Letters contribute their two-digit ordinal value ('A' → 10, …, 'Z' → 35), i.e. two digits each.

The Luhn doubling rule is then applied to the expanded digit sequence so that the check digit, once appended, lands in the units position. Because letters expand to two digits, the country code and an alphanumeric NSIN both participate in the checksum.

use ftracker_identifiers::Isin;

// An all-numeric NSIN.
let amazon = Isin::parse("US0231351067").unwrap();

// An NSIN containing letters — same type, same validation.
let petrobras = Isin::parse("BRPETRACNOR9").unwrap();
assert_eq!(petrobras.nsin(), "PETRACNOR");
assert_eq!(petrobras.check_digit(), 9);

What the country code is (and isn’t)

The first two characters are validated structurally. They must be two uppercase ASCII letters.

This crate deliberately does not check them against the live ISO 3166-1 country registry (which changes over time and includes special allocations such as XS for international securities). Validating “is this a real, currently-assigned country code?” is out of scope for a checksum-oriented value type.

A note on ordering

Isin derives Ord directly over its underlying ASCII bytes, which matches str ordering on Isin::as_str(). This is lexicographic string order. Sorting a list of Isin values groups them by country-code prefix, but says nothing about issuance date or any numeric interpretation of the NSIN.