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Vansh Verma
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patternanchor

mixedcertificateC++ · CPUInference & servingJuly 13, 2026

the question

Does llama.cpp's JSON-Schema pattern-to-GBNF converter realize the spec's unanchored ECMA-262 semantics, or does it silently impose whole-string fullmatch?

what came out

Neither. Every unanchored pattern, the spec's normal case, throws 'Pattern must start with ^ and end with $' and emits no grammar, refuting the pre-registered fullmatch prediction. Anchored ^p$ patterns are sound and complete: over 65,532 membership checks the GBNF acceptor matches std::regex_match with zero mismatches.

method & receipts

  • Result: mixed
  • Pre-registered — the prediction was written down and committed before the run.
  • Reproducible — one script re-runs the whole thing from scratch.
  • Tested — a correctness/benchmark suite ships alongside the code.

→ read the code and re-run it

github.com/v-code01/patternanchor