From 8caec83b074121fb94e21f416b9e20bc7a4605c8 Mon Sep 17 00:00:00 2001
From: Owen Stephens <owen@owenstephens.co.uk>
Date: Wed, 24 Jun 2020 09:29:16 +0100
Subject: [PATCH] [Fix #8205] Avoid false positive in
 Style/RedundantRegexpCharacterClass

If the first element of a character class was an escaped-] our pattern
was treating it as a single element classs containing a single backslash
(which is invalid, and incorrect).
---
 CHANGELOG.md                                  |  1 +
 .../style/redundant_regexp_character_class.rb |  1 +
 .../redundant_regexp_character_class_spec.rb  | 19 +++++++++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04a1b0ac779..6afb6f8eb1f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@
 
 * [#8195](https://github.com/rubocop-hq/rubocop/issues/8195): Fix an error for `Style/RedundantFetchBlock` when using `#fetch` with empty block. ([@koic][])
 * [#8193](https://github.com/rubocop-hq/rubocop/issues/8193): Fix a false positive for `Style/RedundantRegexpCharacterClass` when using `[\b]`. ([@owst][])
+* [#8205](https://github.com/rubocop-hq/rubocop/issues/8205): Fix a false positive for `Style/RedundantRegexpCharacterClass` when using a leading escaped `]`. ([@owst][])
 
 ## 0.86.0 (2020-06-22)
 
diff --git a/lib/rubocop/cop/style/redundant_regexp_character_class.rb b/lib/rubocop/cop/style/redundant_regexp_character_class.rb
index 80c605c53bc..671afd0fcb1 100644
--- a/lib/rubocop/cop/style/redundant_regexp_character_class.rb
+++ b/lib/rubocop/cop/style/redundant_regexp_character_class.rb
@@ -38,6 +38,7 @@ class RedundantRegexpCharacterClass < Cop
              [^.*+?{}()|$] |  # or one that doesn't require escaping outside the character class
              \\[upP]\{[^}]+\} # or a unicode code-point or property
             )
+            (?<!\\)           # No \-prefix (i.e. not escaped)
             \]                # Literal ]
           )
         /x.freeze
diff --git a/spec/rubocop/cop/style/redundant_regexp_character_class_spec.rb b/spec/rubocop/cop/style/redundant_regexp_character_class_spec.rb
index df1b567c1a9..e12a9238d64 100644
--- a/spec/rubocop/cop/style/redundant_regexp_character_class_spec.rb
+++ b/spec/rubocop/cop/style/redundant_regexp_character_class_spec.rb
@@ -43,6 +43,19 @@
     end
   end
 
+  context 'with a character class containing an escaped [' do
+    it 'registers an offense and corrects' do
+      expect_offense(<<~'RUBY')
+        foo = /[\[]/
+               ^^^^ Redundant single-element character class, `[\[]` can be replaced with `\[`.
+      RUBY
+
+      expect_correction(<<~'RUBY')
+        foo = /\[/
+      RUBY
+    end
+  end
+
   context 'with a character class containing a space meta-character' do
     it 'registers an offense and corrects' do
       expect_offense(<<~'RUBY')
@@ -163,6 +176,12 @@
     end
   end
 
+  context 'with a character class with first element an escaped ]' do
+    it 'does not register an offense' do
+      expect_no_offenses('foo = /[\])]/')
+    end
+  end
+
   context 'with a negated character class with a single element' do
     it 'does not register an offense' do
       expect_no_offenses('foo = /[^x]/')