Morphologically Biased Byte-Pair Encoding

mBPE acts as an extension to the huggingface/tokenizers library and is designed to enhance segmentations produced by the byte-pair encoding tokenization algorithm¹. Byte-pair encoding has been shown to poorly approximate morphological boundaries², which is especially problematic for morphologically rich language. By incorporating morphological knowledge into the pre-tokenization process, we aim to improve the quality of produced segmentations through an induced bias towards morphologically motivated sub-word boundaries.

Pre-trained tokenizers and models are available on Hugging Face.

• gpt2_cx-en_00000-00000_50k
• gpt2+ts_cx-en_00000-00000_50k
• gpt2+morf_u0-30-50-x_cx-en_00000-00000_50k
• gpt2+morf_s0-30-x-2_cx-en_00000-00000_50k

Pre-Tokenizers

External

The external pre-tokenizer enables the integration custom pre-tokenization algorithms via a socket connection. Tokenization parallelism should be disabled by setting TOKENIZERS_PARALLELISM=true. Note that disabling parallelism will slow down tokenization significantly. See jonasknobloch/unimorph for a reference server implementation.

Tree-Split

The tree-split pre-tokenizer introduces additional boundaries by clustering inflected word forms retrieved from UniMorph³ dictionaries. Form clusters are aligned by constructing a suffix tree for each cluster. New boundaries are then introduced by traversing the trees and introducing boundaries at nodes with multiple children.

Morfessor

The Morfessor pre-tokenizer introduces additional boundaries retrieved using an arbitrary Morfessor⁴⁵ model. Trained Morfessor models need to be converted using the provided protobuf definition and conversion script

Intrinsic Metrics

Tokenizer Fertility

tokenizer	compounds	fertility
gpt2_cx-en_00000-00000_50k	4992469	1.32
gpt2+ts_cx-en_00000-00000_50k	4923123	1.40
gpt2+morf_u0-30-50-x_cx-en_00000-00000_50k	3630703	1.42
gpt2+morf_s0-30-x-2_cx-en_00000-00000_50k	99191	1.69

Boundary Precision and Recall

tokenizer	P	R	F1
gpt2_cx-en_00000-00000_50k	0.33	0.56	0.42
gpt2+ts_cx-en_00000-00000_50k	0.40	0.58	0.47
gpt2+morf_u0-30-50-x_cx-en_00000-00000_50k	0.45	0.61	0.52
gpt2+morf_s0-30-x-2_cx-en_00000-00000_50k	0.56	0.59	0.57

Footnotes

1. Neural Machine Translation of Rare Words with Subword Units ↩

2. Byte Pair Encoding is Suboptimal for Language Model Pretraining ↩

3. UniMorph 4.0: Universal Morphology ↩

4. Unsupervised Discovery of Morphemes ↩

5. Morfessor 2.0: Python Implementation and Extensions for Morfessor Baseline ↩