v0.4.0 — #rx regex vocabulary tag

Backfilled release note. Originally tagged 2026-06-22; the publish date above is when
this GitHub release entry was created.

Added

• #rx core vocabulary tag (JavaScript RegExp). Validates [source] or [source, flags]
  payloads: flags must be canonical (sorted, unique, drawn from dgimsuvy, with u and v
  mutually exclusive) and the source must convert (JS escapes) and compile. Matches upstream RON
  (PR #22) and ron-go
  (PR #29). Validation-only, so #rx values
  round-trip losslessly. PHP has no RE2, so PCRE backs the source compile check; it diverges from
  ron-go only on backreferences/lookaround, which no conformance case exercises.

Changed

• Updated the pinned RON conformance corpus to include the #rx fixtures.

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Full changelog: CHANGELOG.md · v0.3.0...v0.4.0