> **PR stack (3 of 3)**
> 1. #24739
> 2. #24742
> 3. **#24712 (this PR)**
>
> Review #24739 and #24742 first. This PR is based on #24742, so GitHub shows only the RegExp behavior change.
Stacked on #24742, which exposes strict engine-target capabilities to side-effect analysis.
RegExp constructor calls used for engine feature detection were removed by DCE when their pattern was valid according to the current Oxc RegExp parser. A pattern can be valid at build time but still throw a `SyntaxError` on the configured runtime, so removing the call changes the result of surrounding `try`/`catch` checks.
RegExp purity now requires both syntactic validity and support from every configured target. Consumers without concrete targets remain conservative, while an explicit target allows the minifier to remove constructors known to be safe for those engines.
Target-independent flag validation and pattern compatibility checks now live in `oxc_regular_expression`; side-effect analysis only maps target support to unsupported flags and patterns. String patterns, modern flags, named groups, property escapes, lookbehind, modifiers, invalid flags, and lone-surrogate strings are covered. A RegExp literal with replacement flags is removable only when the target supports both those flags and the dedicated `ES2015RegExpConstructorCanAlterFlags` compatibility feature. Explicitly targeting `esnext` opts into current features.
## Example
Input:
```js
export function supportsUnicodeRegExp() {
try {
new RegExp("\\p{Ll}", "u");
return true;
} catch {
return false;
}
}
```
Before:
```js
export function supportsUnicodeRegExp() {
try {
return true;
} catch {
return false;
}
}
```
After:
```js
export function supportsUnicodeRegExp() {
try {
new RegExp("\\p{Ll}", "u");
return true;
} catch {
return false;
}
}
```
Verified with all `oxc_regular_expression`, `oxc_compat`, `oxc_ecmascript`, and `oxc_minifier` tests; the no-feature/`side_effects`/`constant_evaluation` `oxc_ecmascript` matrix; affected-crate clippy; formatting; and diff checks.
Fixesrolldown/rolldown#10279.
Implemented with AI assistance (OpenAI Codex and Claude). The contributor remains responsible for reviewing, understanding, and submitting these changes under the repository AI usage policy.
## Summary
fixes#12796
* Added target `ES2026`
* Moves the explicit resource management feature from proposals to ES2026, as it is now a finalized ECMAScript 2026 feature.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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