fr 
fr-Latn
fr-fra # Extended tag
fr-Latn-FR
fr-Latn-419
fr-FR
ax-TZ # Not in the registry, but well-formed
fr-shadok # Variant
fr-y-myext-myext2
fra-Latn # ISO 639 can be 3-letters
fra
fra-FX
i-klingon # grandfathered with singleton
no-bok # grandfathered without singleton
fr-Lat # Extended
mn-Cyrl-MN
mN-cYrL-Mn
fr-Latn-CA
en-US
fr-Latn-CA
i-enochian # Grand fathered
x-fr-CH 
sr-Latn-CS
es-419
sl-nedis
de-CH-1996
de-Latg-1996
sl-IT-nedis
en-a-bbb-x-a-ccc
de-a-value
en-Latn-GB-boont-r-extended-sequence-x-private
en-x-US
az-Arab-x-AZE-derbend
es-Latn-CO-x-private
en-US-boont
ab-x-abc-x-abc # anything goes after x
ab-x-abc-a-a # ditto
i-default # grandfathered
i-klingon # grandfathered