WordNet records no synonym and no antonym for regionalism. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a feature (as a pronunciation or expression or custom) that is characteristic of a particular region
a foreign policy that defines the international interests of a country in terms of particular geographic areas
Synonyms, antonyms and the broader and narrower relations for regionalism come from Princeton University’s WordNet. Full sources and licences.
loyalty to the interests of a particular region