Preserving Morphemes: Morphology-Guided Pre-Tokenization for Nepali

Abstract

A byte-level BPE vocabulary learns each inflected form of a Nepali word as a separate string, so a noun stem is spelled differently in each of its case-marked forms. We test whether splitting words into stem and affixes before BPE helps, with the corpus, vocabulary size, model and number of training steps held fixed. Our pre-tokenizer, Papaya, uses a finite-state transducer built from a published grammar of Nepali, falls back to regular expressions, and leaves the BPE trainer unchanged. On 607 words annotated by seven native speakers its segmenter reaches 0.96 boundary F1, and the resulting tokens keep stems intact far more often than plain BPE does. In a 17M-parameter language model it lowers bits per byte by about 1% at equal training steps; most of the larger gain seen at equal epochs comes from the extra steps that longer token sequences buy, and an unsupervised Morfessor segmentation gives the same improvement. Downstream the effect is small: NER improves only on entities that contain words unseen in training, POS tagging and news classification do not change, and published Nepali tokenizers perform about as well. We release the annotated boundary set, a 556-affix dataset and the code.