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BpePatterns

The four pre-tokenizer regexes real BPE models use.

public static class BpePatterns

Properties — Whitespace splits on runs of whitespace. Gpt2 is GPT-2's own pattern, which keeps a leading space with the word that follows it. Llama3 and Qwen2 are those models' patterns, which differ from GPT-2's in how they treat digits and contractions.

Example — choosing the pattern the model was trained with.

using Lodestar.Embeddings.Tokenization;

string gpt2 = BpePatterns.Gpt2;
bool declared = gpt2.Length > 0;  // => True

Remarks — the pre-tokenizer runs before any merge and decides what a "word" is; the merges then apply within each piece and never across them. So the pattern is part of the model, not a preference: encoding with GPT-2's merges under Llama-3's pattern gives ids neither model would produce.

Llama3 and Qwen2 differ from Gpt2 mainly on digit grouping — a change made because tokenizing numbers three digits at a time helps arithmetic — which is exactly the kind of difference that produces plausible, wrong ids when the pattern is mismatched.

A model whose file declares no pre-tokenizer hands the whole text to the merge loop; that is BpeVocabulary.NoPreTokenizer, and it is not the same as choosing Whitespace.

Applies to — net10.0, netstandard2.0.

See also — BpeVocabulary, BpeSplitStep.