oxc-project/oxc · warning · OxcDiagnostic

Backreference ' ' will be ignored. It references group ' '…

Error message

Backreference '{back_reference}' will be ignored. It references group '{group}' which appears before in the same lookbehind.

What it means

Diagnostic from `no-useless-backreference` (variant `Backward`). Inside a lookbehind, matching proceeds right-to-left, so a backreference that textually follows a group is evaluated before that group has captured; conversely a reference to a group that appears before it in the same lookbehind can never see a captured value in the way the author expects. Oxc flags the lookbehind-internal reference as always matching empty.

Solutions

Restructure the pattern so the capture happens outside the lookbehind and the reference sits in the forward direction, e.g. `/(?<=(a))\1/`.
Repeat the subpattern literally inside the lookbehind instead of using a backreference.
Use a lookahead after the group: `/(a)(?<=\1)/`-style ordering where the engine evaluates captures first.
Add an inline disable comment if you specifically target an engine with different lookbehind semantics.

Example fix

// before
const re = /(?<=(a)\1)x/;

// after
const re = /(?<=(a))\1x/;

Defensive patterns

Strategy: validation

Validate before calling

// Avoid backreferences inside lookbehinds; assert with a quick sanity grep before commit
const usesBackrefInLookbehind = /\(\?<=.*(\\\d+|\\k<[^>]+>).*\)/.test(reSource);

Prevention

Capture groups outside the lookbehind, reference them after it.
Remember lookbehind evaluates right-to-left; references to in-lookbehind groups are dead.
Test regexes against a runner (regex101 with the JS flavor) before committing.

When it happens

Trigger: A regex containing a lookbehind where a backreference and its group both live inside the same lookbehind, e.g. `/(?<=(a)\1)/` or `/(?<=\1(a))/`. The rule fires when the referenced group's span sits inside the same lookbehind as the backreference.

Common situations: Porting PCRE-style patterns that rely on right-to-left capture semantics; trying to assert repeated content immediately before a position using a backreference inside `(?<=...)`.

Related errors

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/156bd643ea7dd591. Report an issue: GitHub.

Appendix: source

Thrown at crates/oxc_linter/src/rules/eslint/no_useless_backreference.rs:29

fn no_useless_backreference_diagnostic(
    span: Span,
    problem: &Problem,
    back_reference: &str,
    group: &str,
) -> OxcDiagnostic {
    let diagnostic = match problem {
        Problem::Nested => OxcDiagnostic::warn(format!(
            "Backreference '{back_reference}' will be ignored. It references group '{group}' from within that group."
        )),
        Problem::Disjunctive => OxcDiagnostic::warn(format!(
            "Backreference '{back_reference}' will be ignored. It references group '{group}' which is in another alternative."
        )),
        Problem::Forward => OxcDiagnostic::warn(format!(
            "Backreference '{back_reference}' will be ignored. It references group '{group}' which appears later in the pattern."
        )),
        Problem::Backward => OxcDiagnostic::warn(format!(
            "Backreference '{back_reference}' will be ignored. It references group '{group}' which appears before in the same lookbehind."
        )),
        Problem::IntoNegativeLookaround => OxcDiagnostic::warn(format!(
            "Backreference '{back_reference}' will be ignored. It references group '{group}' which is in a negative lookaround."
        )),
    };
    diagnostic
        .with_help("Consider revising the pattern to remove or relocate the backreference so it points to a group that can be matched at the time of evaluation.")
        .with_label(span)
}

#[derive(Debug, Default, Clone)]
pub struct NoUselessBackreference;

declare_oxc_lint!(
    /// ### What it does
    ///
    /// Disallows backreferences in regular expressions that will always be ignored
    /// because the capture group they refer to has not matched and cannot match

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