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Add regexp/no-extra-lookaround-assertions rule (#482)
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‎README.md‎

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@@ -138,6 +138,7 @@ The rules with the following star :star: are included in the `plugin:regexp/reco
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| [regexp/control-character-escape](https://ota-meshi.github.io/eslint-plugin-regexp/rules/control-character-escape.html) | enforce consistent escaping of control characters | :star::wrench: |
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| [regexp/negation](https://ota-meshi.github.io/eslint-plugin-regexp/rules/negation.html) | enforce use of escapes on negation | :star::wrench: |
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| [regexp/no-dupe-characters-character-class](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-dupe-characters-character-class.html) | disallow duplicate characters in the RegExp character class | :star::wrench: |
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| [regexp/no-extra-lookaround-assertions](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-extra-lookaround-assertions.html) | disallow unnecessary nested lookaround assertions | :wrench: |
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| [regexp/no-invisible-character](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-invisible-character.html) | disallow invisible raw character | :star::wrench: |
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| [regexp/no-legacy-features](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-legacy-features.html) | disallow legacy RegExp features | :star: |
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| [regexp/no-non-standard-flag](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-non-standard-flag.html) | disallow non-standard flags | :star: |

‎docs/rules/index.md‎

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@@ -43,6 +43,7 @@ The rules with the following star :star: are included in the `plugin:regexp/reco
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| [regexp/control-character-escape](./control-character-escape.md) | enforce consistent escaping of control characters | :star::wrench: |
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| [regexp/negation](./negation.md) | enforce use of escapes on negation | :star::wrench: |
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| [regexp/no-dupe-characters-character-class](./no-dupe-characters-character-class.md) | disallow duplicate characters in the RegExp character class | :star::wrench: |
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| [regexp/no-extra-lookaround-assertions](./no-extra-lookaround-assertions.md) | disallow unnecessary nested lookaround assertions | :wrench: |
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| [regexp/no-invisible-character](./no-invisible-character.md) | disallow invisible raw character | :star::wrench: |
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| [regexp/no-legacy-features](./no-legacy-features.md) | disallow legacy RegExp features | :star: |
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| [regexp/no-non-standard-flag](./no-non-standard-flag.md) | disallow non-standard flags | :star: |

‎docs/rules/no-extra-lookaround-assertions.md‎

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---
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pageClass: "rule-details"
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sidebarDepth: 0
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title: "regexp/no-extra-lookaround-assertions"
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description: "disallow unnecessary nested lookaround assertions"
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---
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# regexp/no-extra-lookaround-assertions
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> disallow unnecessary nested lookaround assertions
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- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> ***This rule has not been released yet.*** </badge>
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- :wrench: The `--fix` option on the [command line](https://eslint.org/docs/user-guide/command-line-interface#fixing-problems) can automatically fix some of the problems reported by this rule.
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## :book: Rule Details
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The last positive lookahead assertion within a lookahead assertion is the same without lookahead assertions.
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Also, The first positive lookbehind assertion within a lookbehind assertion is the same without lookbehind assertions.
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They can be inlined or converted to group.
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```js
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/a(?=b(?=c))/u; /* -> */ /a(?=bc)/u;
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/a(?=b(?=c|C))/u; /* -> */ /a(?=b(?:c|C))/u;
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/(?<=(?<=a)b)c/u; /* -> */ /(?<=ab)c/u;
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/(?<=(?<=a|A)b)c/u; /* -> */ /(?<=(?:a|A)b)c/u;
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```
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This rule aims to report and fix these unnecessary lookaround assertions.
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<eslint-code-block fix>
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```js
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/* eslint regexp/no-extra-lookaround-assertions: "error" */
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/* ✓ GOOD */
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var ts = 'JavaScript'.replace(/Java(?=Script)/u, 'Type');
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var java = 'JavaScript'.replace(/(?<=Java)Script/u, '');
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var re1 = /a(?=bc)/u;
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var re2 = /a(?=b(?:c|C))/u;
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var re3 = /(?<=ab)c/u;
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var re4 = /(?<=(?:a|A)b)c/u;
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/* ✗ BAD */
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var ts = 'JavaScript'.replace(/Java(?=Scrip(?=t))/u, 'Type');
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var java = 'JavaScript'.replace(/(?<=(?<=J)ava)Script/u, '');
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var re1 = /a(?=b(?=c))/u;
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var re2 = /a(?=b(?=c|C))/u;
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var re3 = /(?<=(?<=a)b)c/u;
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var re4 = /(?<=(?<=a|A)b)c/u;
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```
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</eslint-code-block>
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## :wrench: Options
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Nothing.
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## :mag: Implementation
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- [Rule source](https://github.com/ota-meshi/eslint-plugin-regexp/blob/master/lib/rules/no-extra-lookaround-assertions.ts)
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- [Test source](https://github.com/ota-meshi/eslint-plugin-regexp/blob/master/tests/lib/rules/no-extra-lookaround-assertions.ts)

‎lib/rules/no-extra-lookaround-assertions.ts‎

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import type { LookaroundAssertion } from "regexpp/ast"
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import type { RegExpVisitor } from "regexpp/visitor"
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import type { RegExpContext } from "../utils"
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import { createRule, defineRegexpVisitor } from "../utils"
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export default createRule("no-extra-lookaround-assertions", {
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meta: {
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docs: {
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description: "disallow unnecessary nested lookaround assertions",
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category: "Best Practices",
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// TODO Switch to recommended in the major version.
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// recommended: true,
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recommended: false,
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},
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fixable: "code",
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schema: [],
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messages: {
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canBeInlined:
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"This {{kind}} assertion is useless and can be inlined.",
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canBeConvertedIntoGroup:
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"This {{kind}} assertion is useless and can be converted into a group.",
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},
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type: "suggestion",
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},
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create(context) {
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/**
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* Create visitor
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*/
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function createVisitor(
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regexpContext: RegExpContext,
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): RegExpVisitor.Handlers {
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return {
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onAssertionEnter(aNode) {
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if (
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aNode.kind === "lookahead" ||
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aNode.kind === "lookbehind"
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) {
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verify(regexpContext, aNode)
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}
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},
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}
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}
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/** Verify for lookaround assertion */
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function verify(
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regexpContext: RegExpContext,
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assertion: LookaroundAssertion,
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) {
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for (const alternative of assertion.alternatives) {
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const nested = at(
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alternative.elements,
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assertion.kind === "lookahead"
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? // The last positive lookahead assertion within
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// a lookahead assertion is the same without the assertion.
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-1
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: // The first positive lookbehind assertion within
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// a lookbehind assertion is the same without the assertion.
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0,
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)
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if (
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nested?.type === "Assertion" &&
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nested.kind === assertion.kind &&
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!nested.negate
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) {
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reportLookaroundAssertion(regexpContext, nested)
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}
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}
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}
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/** Report */
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function reportLookaroundAssertion(
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{ node, getRegexpLocation, fixReplaceNode }: RegExpContext,
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assertion: LookaroundAssertion,
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) {
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let messageId, replaceText
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if (assertion.alternatives.length === 1) {
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messageId = "canBeInlined"
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// unwrap `(?=` and `)`, `(?<=` and `)`
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replaceText = assertion.alternatives[0].raw
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} else {
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messageId = "canBeConvertedIntoGroup"
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// replace `?=` with `?:`, or `?<=` with `?:`
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replaceText = `(?:${assertion.alternatives
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.map((alt) => alt.raw)
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.join("|")})`
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}
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context.report({
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node,
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loc: getRegexpLocation(assertion),
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messageId,
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data: {
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kind: assertion.kind,
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},
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fix: fixReplaceNode(assertion, replaceText),
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})
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}
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return defineRegexpVisitor(context, {
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createVisitor,
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})
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},
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})
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// TODO After dropping support for Node < v16.6.0 we can use native `.at()`.
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/**
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* `.at()` polyfill
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* see https://github.com/tc39/proposal-relative-indexing-method#polyfill
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*/
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function at<T>(array: T[], n: number) {
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// ToInteger() abstract op
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let num = Math.trunc(n) || 0
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// Allow negative indexing from the end
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if (num < 0) num += array.length
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// OOB access is guaranteed to return undefined
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if (num < 0 || num >= array.length) return undefined
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// Otherwise, this is just normal property access
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return array[num]
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}

‎lib/utils/rules.ts‎

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@@ -16,6 +16,7 @@ import noEmptyCharacterClass from "../rules/no-empty-character-class"
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import noEmptyGroup from "../rules/no-empty-group"
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import noEmptyLookaroundsAssertion from "../rules/no-empty-lookarounds-assertion"
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import noEscapeBackspace from "../rules/no-escape-backspace"
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import noExtraLookaroundAssertions from "../rules/no-extra-lookaround-assertions"
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import noInvalidRegexp from "../rules/no-invalid-regexp"
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import noInvisibleCharacter from "../rules/no-invisible-character"
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import noLazyEnds from "../rules/no-lazy-ends"
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noEmptyGroup,
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noEmptyLookaroundsAssertion,
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noEscapeBackspace,
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noExtraLookaroundAssertions,
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noInvalidRegexp,
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noInvisibleCharacter,
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noLazyEnds,

‎tests/lib/rules/no-extra-lookaround-assertions.ts‎

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import { RuleTester } from "eslint"
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import rule from "../../../lib/rules/no-extra-lookaround-assertions"
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const tester = new RuleTester({
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parserOptions: {
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ecmaVersion: 2020,
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sourceType: "module",
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},
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})
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tester.run("no-extra-lookaround-assertions", rule as any, {
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valid: [
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`console.log('JavaScript'.replace(/Java(?=Script)/u, 'Type'))`,
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`console.log('JavaScript'.replace(/(?<=Java)Script/u, ''))`,
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],
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invalid: [
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{
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code: `console.log('JavaScript'.replace(/Java(?=Scrip(?=t))/u, 'Type'))`,
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output: `console.log('JavaScript'.replace(/Java(?=Script)/u, 'Type'))`,
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errors: [
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{
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message:
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"This lookahead assertion is useless and can be inlined.",
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column: 47,
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},
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],
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},
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{
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code: `console.log('JavaScript'.replace(/(?<=(?<=J)ava)Script/u, ''))`,
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output: `console.log('JavaScript'.replace(/(?<=Java)Script/u, ''))`,
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errors: [
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{
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message:
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"This lookbehind assertion is useless and can be inlined.",
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column: 39,
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},
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],
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},
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// Within negate
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{
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code: `console.log('JavaScript Java JavaRuntime'.replace(/Java(?!Scrip(?=t))/gu, 'Python'))`,
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output: `console.log('JavaScript Java JavaRuntime'.replace(/Java(?!Script)/gu, 'Python'))`,
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errors: [
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{
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message:
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"This lookahead assertion is useless and can be inlined.",
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column: 64,
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},
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],
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},
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{
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code: `console.log('JavaScript TypeScript ActionScript'.replace(/(?<!(?<=J)ava)Script/gu, 'ScriptCompiler'))`,
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output: `console.log('JavaScript TypeScript ActionScript'.replace(/(?<!Java)Script/gu, 'ScriptCompiler'))`,
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errors: [
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{
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message:
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"This lookbehind assertion is useless and can be inlined.",
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column: 63,
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},
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],
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},
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// Multiple alternatives
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{
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code: `console.log('JavaScriptChecker JavaScriptLinter'.replace(/Java(?=Script(?=Checker|Linter))/gu, 'Type'))`,
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output: `console.log('JavaScriptChecker JavaScriptLinter'.replace(/Java(?=Script(?:Checker|Linter))/gu, 'Type'))`,
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errors: [
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{
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message:
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"This lookahead assertion is useless and can be converted into a group.",
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column: 72,
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},
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],
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},
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{
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code: `console.log('JavaScriptChecker JavaScriptLinter'.replace(/Java(?=Script(?=(?:Check|Lint)er))/gu, 'Type'))`,
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output: `console.log('JavaScriptChecker JavaScriptLinter'.replace(/Java(?=Script(?:Check|Lint)er)/gu, 'Type'))`,
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errors: [
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{
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message:
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"This lookahead assertion is useless and can be inlined.",
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column: 72,
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},
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],
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},
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{
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code: `console.log('ESLint JSLint TSLint'.replace(/(?<=(?<=J|T)S)Lint/gu, '-Runtime'))`,
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output: `console.log('ESLint JSLint TSLint'.replace(/(?<=(?:J|T)S)Lint/gu, '-Runtime'))`,
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errors: [
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{
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message:
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"This lookbehind assertion is useless and can be converted into a group.",
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column: 49,
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},
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],
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},
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{
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code: `console.log('JavaScriptChecker JavaScriptLinter'.replace(/Java(?=Script(?=Checker)|Script(?=Linter))/gu, 'Type'))`,
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output: `console.log('JavaScriptChecker JavaScriptLinter'.replace(/Java(?=ScriptChecker|ScriptLinter)/gu, 'Type'))`,
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errors: [
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{
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message:
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"This lookahead assertion is useless and can be inlined.",
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column: 72,
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},
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{
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message:
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"This lookahead assertion is useless and can be inlined.",
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column: 90,
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},
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],
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},
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{
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code: `console.log('ESLint JSLint TSLint'.replace(/(?<=(?<=J)S|(?<=T)S)Lint/gu, '-Runtime'))`,
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output: `console.log('ESLint JSLint TSLint'.replace(/(?<=JS|TS)Lint/gu, '-Runtime'))`,
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errors: [
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{
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message:
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"This lookbehind assertion is useless and can be inlined.",
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column: 49,
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},
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{
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message:
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"This lookbehind assertion is useless and can be inlined.",
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column: 57,
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},
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],
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},
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],
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})

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