|A language tag, constructed in accordance with BCP 47.
Only lowercase characters are allowed. The purpose of this
restriction is to provide unique language tags for use as
indexes. BCP 47 recommends case conventions for user
interfaces, but objects using this TEXTUAL-CONVENTION MUST
use only lowercase.
Values MUST be well-formed language tags, in conformance
with the definition of well-formed tags in BCP 47. An
implementation MAY further limit the values it accepts to
those permitted by a 'validating' processor, as defined in
In theory, BCP 47 language tags are of unlimited length.
The language tag described in this TEXTUAL-CONVENTION is of
limited length. The analysis of language tag lengths in BCP
47 confirms that this limit will not pose a problem in
practice. In particular, this length is greater than the
minimum requirements set out in Section 4.3.1.
A zero-length language tag is not a valid language tag.
This can be used to express 'language tag absent' where
required, for example, when used as an index field.