This is a methodology-generalization demo: one deterministic-spec instance showing that the verification pattern travels outside agent transcripts, not the project's headline.
The harness treats Python re.search as the reference oracle and Python re.fullmatch as the candidate matcher configuration by default. Both sides compile the same pattern with Python re; the differential question is whether the candidate's match-mode semantics agree with the reference for each ordered input.
That pairing is intentionally small and deterministic. It creates real semantic divergences such as pattern="cat" on input="concatenate": the reference finds the substring while the candidate requires the whole input to match.
Pattern sets are JSON files with a patterns list. Each entry has an id, a pattern, and optionally an ordered inputs list. Explicit inputs run first and are never shuffled. If inputs are omitted, or generate is set to true, the harness adds a bounded deterministic corpus from literals extracted from the pattern plus fixed seed strings. There is no randomness at import or runtime.
Each input becomes a synthetic test_run activity. Agreement records 1 passed; divergence records 1 failed. The ordered inputs are parent-linked, so existing G3 gate parsing and localize_failures can report the earliest diverging input as the breaking step. The planted fixture localizes planted-search-vs-fullmatch to input index 1, input concatenate, with G3/HIGH confidence.
Python re is the oracle only for Python regex semantics. This harness does not prove POSIX, PCRE, Rust, JavaScript, or third-party regex engine behavior. Invalid patterns fail closed as unsupported. Catastrophic backtracking and ReDoS behavior are disclosed limits: the standard library does not provide a per-match timeout here, so pattern sets should avoid adversarial exponential cases. Agreement means the two configured match modes agreed on the tested finite input set, not on every possible string.
The checked fixture compares 2 patterns across 6 ordered inputs. It reports 5/6 agreements and 1/6 divergences. The first divergence is exact: planted-search-vs-fullmatch, input index 1, input concatenate.