Before-and-after regex review: compare match sets

To review a regex change, run the old and new expressions with the same flags over the same labeled examples. Compare which strings match, where matches start and end, and what capture groups or replacements produce. A new pattern can fix one case while silently broadening another. Add any newly discovered counterexample to the test set. If the expression runs on user-controlled text, compare behavior on long repetitions too; performance regressions may not appear in tiny examples.