Second Subtag Rules
If the second subtag is:
Two characters long, it’s a country/region defined by ISO 3166[3]
Three to eight characters long, it may be registered with the IANA
One character long, it is illegal
The ISO 3166 country codes are summarized in Table G-3.
[3] The country codes AA, QM-QZ, XA-XZ and ZZ are reserved by ISO 3166 as user-assigned codes. These must not be used to form language tags.
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