regexUnnecessaryReferentialBackreferences

Reports backreferences that may reference a capturing group that was not matched.

✅ This rule is included in the ts logical presets.

It is possible to make a referenced group of a backreference not matched because some other path leads to the backreference. In that case the backreference will trivially accept (e.g. /(?:(a)|b)\1/). This can happen when the group is inside an optional quantifier, inside an alternation where some branches don’t include it, or when the group can be reset in a loop. Similarly, this can will happen if the captured text of the referenced group was reset before reaching the backreference.

This rule reports backreferences that may always be empty because the referenced capturing group might not participate in the match.

Examples

Optional Capturing Group

❌ Incorrect

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

✅ Correct

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

Zero-or-More Quantifier

❌ Incorrect

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

✅ Correct

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

Alternation Without Group

❌ Incorrect

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

✅ Correct

const pattern = /(a)\1/;

Group Reset in Loop

❌ Incorrect

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

✅ Correct

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

Lookahead Ensures Match

❌ Incorrect

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

✅ Correct

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

Options

This rule is not configurable.

When Not To Use It

If you intentionally use backreferences that may be empty to match optional content conditionally, you may want to disable this rule. You might consider using Flint disable comments and/or configuration file disables for specific cases instead of completely disabling this rule.

Further Reading

• MDN: Groups and backreferences
• MDN: Regular expressions

Equivalents in Other Linters

• ESLint: regexp/no-potentially-useless-backreference