#no-useless-backreference

#Configuration

import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-useless-backreference': 'error',
    },
  },
]);

#Rule Details

This rule disallows useless backreferences in regular expressions — backreferences that can only ever match the empty string regardless of input. The rule recognizes five problematic patterns:

• A backreference to a group that contains the backreference itself (the group hasn't matched yet when the backreference starts).
• A backreference that appears before the group it refers to (forward reference).
• A backreference inside a lookbehind that refers to a group appearing before it in the same lookbehind (lookbehind matches right-to-left).
• A backreference and its referenced group are in different alternatives of the same disjunction.
• A backreference to a group inside a negative lookaround when the backreference itself is outside that lookaround (the group's match was discarded).

Examples of incorrect code for this rule:

/^(?:(a)|\1b)$/;       // reference to a group in another alternative
/(?:(a)|b(?:c|\1))$/;  // reference to a group in another alternative
/\1(a)/;               // forward reference
RegExp('(a)\\2(b)');   // forward reference
/\k<foo>(?<foo>a)/;    // forward reference (named)
/(?<=(a)\1)b/;         // backward reference in lookbehind
new RegExp('(\\1)');   // nested reference
/^((a)\1)$/;           // nested reference
/a(?!(b)).\1/;         // reference into a negative lookahead
/(?<!(a))b\1/;         // reference into a negative lookbehind

Examples of correct code for this rule:

/^(?:(a)|(b)\2)$/;
/(a)\1/;
RegExp('(a)\\1(b)');
/(?<foo>a)\k<foo>/;
/(?<=\1(a))b/;
/^(?:(a)\1)$/;
/a(?!(b|c)\1)./;

#Options

This rule has no options.

#Original Documentation

• ESLint no-useless-backreference