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Private-Use Area Characters in the Wild: Signal or Noise?

The Private-Use Area (PUA) designation plays an important role in the Unicode standard. It covers several ranges of Unicode code points with no official character assignments. A PUA range is primarily used as a temporary representation mechanism for characters falling outside the official standard, to facilitate text entry and display of orthographies that are not otherwise adequately represented. The primary downside of PUA use is that characters lose their semantics if the pairing with the corresponding display font is broken, in which case they cannot be faithfully displayed in a general setting. Large-scale multilingual web corpora invariably contain PUA code points of unclear provenance, which may commonly be treated as noise and discarded. We investigate the distribution of PUA characters within large-scale web corpora, and analyze the resulting distributions across both scripts and writing systems. We show that, while the proportion of PUA-bearing paragraphs in the original corpora are small, PUA-bearing tokens can signal texts from under-represented languages. We additionally explore whether an off-the-shelf large language model (LLM) can classify PUA characters as constituting relevant orthographic signals versus punctuation or other noise. Our methods identify millions of paragraphs making use of such characters, and we argue that such data is important for the long tail of data-scarce orthographies. Moreover, as a primary Unicode mechanism for poorly represented writing systems, PUA characters are here to stay.