Add regexp/no-unused-capturing-group rule (#65)

* Add `regexp/no-unused-capturing-group` rule

* Update

Author: ota-meshi

README.md

@@ -95,6 +95,7 @@ The rules with the following star :star: are included in the `plugin:regexp/reco
 | [regexp/no-invisible-character](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-invisible-character.html) | disallow invisible raw character | :star::wrench: |
 | [regexp/no-legacy-features](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-legacy-features.html) | disallow legacy RegExp features |  |
 | [regexp/no-octal](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-octal.html) | disallow octal escape sequence | :star: |
+| [regexp/no-unused-capturing-group](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-unused-capturing-group.html) | disallow unused capturing group |  |
 | [regexp/no-useless-backreference](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-backreference.html) | disallow useless backreferences in regular expressions |  |
 | [regexp/no-useless-character-class](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-character-class.html) | disallow character class with one character | :wrench: |
 | [regexp/no-useless-dollar-replacements](https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-dollar-replacements.html) | disallow useless `$` replacements in replacement string |  |

docs/rules/README.md

@@ -23,6 +23,7 @@ The rules with the following star :star: are included in the `plugin:regexp/reco
 | [regexp/no-invisible-character](./no-invisible-character.md) | disallow invisible raw character | :star::wrench: |
 | [regexp/no-legacy-features](./no-legacy-features.md) | disallow legacy RegExp features |  |
 | [regexp/no-octal](./no-octal.md) | disallow octal escape sequence | :star: |
+| [regexp/no-unused-capturing-group](./no-unused-capturing-group.md) | disallow unused capturing group |  |
 | [regexp/no-useless-backreference](./no-useless-backreference.md) | disallow useless backreferences in regular expressions |  |
 | [regexp/no-useless-character-class](./no-useless-character-class.md) | disallow character class with one character | :wrench: |
 | [regexp/no-useless-dollar-replacements](./no-useless-dollar-replacements.md) | disallow useless `$` replacements in replacement string |  |

docs/rules/no-unused-capturing-group.md

@@ -0,0 +1,63 @@
+---
+pageClass: "rule-details"
+sidebarDepth: 0
+title: "regexp/no-unused-capturing-group"
+description: "disallow unused capturing group"
+---
+# regexp/no-unused-capturing-group
+
+> disallow unused capturing group
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> ***This rule has not been released yet.*** </badge>
+
+## :book: Rule Details
+
+This rule aims to optimize regular expressions by replacing unused capturing groups with non-capturing groups.
+
+<eslint-code-block>
+
+```js
+/* eslint regexp/no-unused-capturing-group: "error" */
+
+/* ✓ GOOD */
+var replaced = '2000-12-31'.replace(/(\d{4})-(\d{2})-(\d{2})/, '$1/$2/$3') // "2000/12/31"
+var replaced = '2000-12-31'.replace(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/u, '$<y>/$<m>/$<d>') // "2000/12/31"
+var replaced = '2000-12-31'.replace(/(\d{4})-(\d{2})-(\d{2})/, (_, y, m, d) => `${y}/${m}/${d}`) // "2000/12/31"
+
+var isDate = /(?:\d{4})-(?:\d{2})-(?:\d{2})/.test('2000-12-31') // true
+
+var matches = '2000-12-31 2001-01-01'.match(/(\d{4})-(\d{2})-(\d{2})/)
+var y = matches[1] // "2000"
+var m = matches[2] // "12"
+var d = matches[3] // "31"
+
+var index = '2000-12-31'.search(/(?:\d{4})-(?:\d{2})-(?:\d{2})/) // 0
+
+/* ✗ BAD */
+var replaced = '2000-12-31'.replace(/(\d{4})-(\d{2})-(\d{2})/, 'Date') // "Date"
+var replaced = '2000-12-31'.replace(/(\d{4})-(\d{2})-(\d{2})/, '$1/$2') // "2000/12"
+var replaced = '2000-12-31'.replace(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/u, '$<y>/$<m>') // "2000/12"
+var replaced = '2000-12-31'.replace(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/u, '$1/$2/$3') // "2000/12/31"
+var replaced = '2000-12-31'.replace(/(\d{4})-(\d{2})-(\d{2})/, (_, y, m) => `${y}/${m}`) // "2000/12"
+
+var isDate = /(\d{4})-(\d{2})-(\d{2})/.test('2000-12-31') // true
+
+var matches = '2000-12-31 2001-01-01'.match(/(\d{4})-(\d{2})-(\d{2})/g) // ["2000-12-31", "2001-01-01"]
+
+var index = '2000-12-31'.search(/(\d{4})-(\d{2})-(\d{2})/) // 0
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :couple: Related rules
+
+- [regexp/no-useless-dollar-replacements](./no-useless-dollar-replacements.md)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/ota-meshi/eslint-plugin-regexp/blob/master/lib/rules/no-unused-capturing-group.ts)
+- [Test source](https://github.com/ota-meshi/eslint-plugin-regexp/blob/master/tests/lib/rules/no-unused-capturing-group.ts)