{{h2. Overview
Add normalization for regex pattern slash escaping in the aggregation fuzzer result comparison
logic to prevent false-positive failures caused by the SERVER-116052 bug fix.

Background

SERVER-116052 fixes a bug in the MozJS WASM engine's regex serialization. The old code used
RegExp.prototype.toString() to extract a regex pattern before writing it to BSON. Per the
ECMAScript spec, toString() calls EscapeRegExpPattern, which escapes any unescaped /
in the pattern to \/. This means a BSON regex with pattern hello/world would round-trip
as hello\/world — a different byte sequence, even though both patterns are semantically
identical (\/ is an IdentityEscape in ECMAScript regex syntax and matches the same strings
as /).

The fix in SERVER-116052 replaces the toString() call with JS::GetRegExpSource(), a
SpiderMonkey API that returns the raw pattern without any delimiter escaping. After the fix, the
same BSON regex correctly round-trips as hello/world.

This causes 27 agg_fuzzer failures in multiversion testing, where old mongod (with the bug) and
new mongod (with the fix) disagree on the BSON representation of regex fields returned by
$function and $accumulator expressions.

Scope of Work

Changes are in 10gen/jstestfuzz on branch calvin.nguyen/mozjs-regex-fix:

• src/fuzzers/agg/results_comparison_lib.ts — Two additions:
  • In normalizeDoc(): strip unnecessary \/ escapes from RegExp.source before result
    comparison, so both serializations of the same regex compare equal.
  • In compareDocsInternal(): add explicit instanceof RegExp handling that compares
    source+flags after normalizing \/ → /, ensuring correct behaviour even when
    bsonBinaryEqual does not handle native RegExp objects.
• src/fuzzers/agg/results_comparison_lib.spec.ts — 7 new test cases covering:
  escaped-vs-unescaped slash equality, multiple slashes, same-pattern equality, genuinely
  different patterns (must still be unequal), different flags (must still be unequal), and
  normalization inside nested arrays and subdocuments.

Acceptance Criteria

• The 7 new spec tests pass with no regressions in the existing test suite.
• After this change is merged and the jstestfuzz SHA is bumped in the mongo repo (see linked
  SERVER-116052 ticket), the 27 agg_fuzzer Evergreen failures on the SERVER-116052 branch
  are resolved.

Technical Notes

• The \/ → / normalization is semantically safe: in ECMAScript regex patterns
  constructed via new RegExp(string), \/ is an IdentityEscape — the backslash has no
  effect and the regex matches identically to /.
• Follows the same pattern as DEVPROD-31535 (Decimal128 normalization) and SERVER-123514
  (NaN string normalization).
• A companion SHA-bump ticket in the SERVER project should link to this ticket.}}