regexUnnecessaryBackreferences

Reports backreferences in regular expressions that will always match empty or fail.

✅ This rule is included in the ts logical presets.

Reports backreferences in regular expressions that will always match empty or fail to match. These backreferences are useless because they reference capturing groups that haven’t captured anything at the time the backreference is evaluated.

Examples

Nested Backreference

A backreference inside its own capturing group will always match empty since the group hasn’t finished capturing yet.

❌ Incorrect

const pattern = /(a\1)/;

✅ Correct

const pattern = /(a)\1/;

Forward Reference

A backreference that appears before the group it references will always match empty since the group hasn’t been matched yet.

❌ Incorrect

const pattern = /\1(a)/;

✅ Correct

const pattern = /(a)\1/;

Disjunctive Backreference

A backreference in a different alternative from its group will always match empty since the group’s alternative wasn’t taken.

❌ Incorrect

const pattern = /(a)|\1/;

✅ Correct

const pattern = /(a)\1|b/;

Negative Lookaround

A backreference to a group inside a negative lookaround will always match empty since the group wasn’t matched.

❌ Incorrect

const pattern = /(?!(a))\w\1/;

✅ Correct

const pattern = /(?=(a))\w\1/;

Lookbehind Direction

In lookbehind assertions, matching proceeds right-to-left, so a backreference after its group is a backward reference.

❌ Incorrect

const pattern = /(?<=(a)\1)b/;

✅ Correct

const pattern = /(?<=\1(a))b/;

Options

This rule is not configurable.

When Not To Use It

If you are intentionally using backreferences that match empty strings for specific pattern-matching logic, you might want to disable this rule. However, in most cases, these patterns indicate a mistake in the regular expression.

Further Reading

• MDN: Backreferences
• MDN: Lookahead and lookbehind assertions

Equivalents in Other Linters

• Biome: noUselessRegexBackrefs
• ESLint: no-useless-backreferenceregexp/no-useless-backreference
• Oxlint: eslint/no-useless-backreference