[P3] Preserve stateful RegExp lastIndex during round-trip

[samlaycock]

Priority

P3

Area

Feature enhancement: richer built-in type fidelity

Problem

The built-in RegExp handler serializes with RegExp.prototype.toString(), which preserves source and flags but drops lastIndex. For global or sticky regexes, lastIndex is observable state and can matter when a caller serializes in-progress parser/matcher state.

Current behavior:

const pattern = /item/g;
pattern.lastIndex = 4;

const result = decode<{ pattern: RegExp }>(encode({ pattern }));
result.pattern.lastIndex; // 0

Relevant code:

• src/types.ts: the RegExp handler serializes with v.toString() and deserializes by parsing /source/flags.

Expected behavior

Consider preserving lastIndex for regexes where it is meaningful, while keeping backwards compatibility with the existing /source/flags wire format.

Suggested approach

A compatible path is to support both formats during decode:

• Existing legacy string format: /source/flags
• New structured format: { source, flags, lastIndex } encoded as JSON, or another versioned representation

Acceptance criteria

• Add tests for global and sticky regexes with non-zero lastIndex.
• Existing serialized regex strings still decode correctly.
• README or changelog notes the improved fidelity if the wire format changes.