[idhanth]

Summary

Fixes #2233.

/a(?<=x|^a)/.test("a") returns false instead of true.

Regex::pushLookaround reverses a lookbehind's node list after its nested nodes were constructed. AlternationNode and LoopNode cache their children's match constraints at construction, so a ^ inside them leaves MatchConstraintAnchoredAtStart set. That flag reaches the executor as the alternation's primaryConstraints/secondaryConstraints and the loop's loopeeConstraints. satisfiesConstraints then rejects the ^a alternative, because the backwards match starts at position 1 (after the a), not at the start of the input.

LookaroundNode already clears MatchConstraintAnchoredAtStart for the top level of a lookbehind. This change clears it in AlternationNode::reverseChildren and LoopNode::reverseChildren as well. The constraint is only an early-reject optimization: the ^ itself is still checked when its LeftAnchor instruction runs, so clearing it can't make a regex match where it shouldn't.

The same bug affected loops: /a(?<=(?:^a)+)/ didn't match "a" either.

Not changed:

• MarkedSubexpressionNode caches constraints too, but only its compile-time matchConstraints() reads them, before reversal, so they never reach the executor.
• A lookahead nested in a lookbehind isn't reversed (LookaroundNode doesn't override reverseChildren), so its constraints stay correct.

Test Plan

Added 8 cases to test/hermes/regexp.js. They cover the issue's pattern, a ^ in a later alternative, a capture group, unbounded and bounded loops, a loop around an alternation, and two cases where ^ must still reject. Every expected result matches V8 (Node 22).

Built on Ubuntu 24.04 (clang 18, Release, linux/arm64 in Docker) from static_h:

• Without the fix, 6 of the 8 new cases print null where V8 returns a match. With it, all 8 match V8.
• test/hermes/regexp.js and test/hermes/string-functions.js pass when run through their RUN lines with the built hermes and FileCheck.
• LIT_FILTER="(?i)regex|regexp" ninja check-hermes: 36 of 4321 tests selected, 35 passed, 1 unsupported, 0 failed.
• clang-format --dry-run --Werror include/hermes/Regex/RegexNode.h (clang-format 18, repo .clang-format): clean.

The full check-hermes suite was not run.

🤖 Generated with Claude Code

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