rfc5646extension top-level property

Implementation

final rfc5646extension = rule(
  'extension',
  concatenation([
    rfc5646singleton,
    variableRepetition(
      concatenation([
        literal('-'),
        variableRepetition(alphanum, min: 2, max: 8),
        negativeLookahead(alphanum),
      ]),
      min: 1,
    ),
  ]),
);