Add Style/RedundantRegexpCharacterClass cop

```ruby
  # This cop checks for unnecessary single-element Regexp character classes.
  #
  # @example
  #
  #   # bad
  #   r = /[x]/
  #
  #   # good
  #   r = /x/
  #
  #   # bad
  #   r = /[\s]/
  #
  #   # good
  #   r = /\s/
  #
  #   # good
  #   r = /[ab]/
```

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### New features
 
 * [#6289](https://github.com/rubocop-hq/rubocop/issues/6289): Add new `CheckDefinitionPathHierarchy` option for `Naming/FileName`. ([@jschneid][])
+* [#8055](https://github.com/rubocop-hq/rubocop/pull/8055): Add new `Style/RedundantRegexpCharacterClass` cop. ([@owst][])
 * [#8069](https://github.com/rubocop-hq/rubocop/issues/8069): New option for `expect_offense` to help format offense templates. ([@marcandre][])
 * [#7908](https://github.com/rubocop-hq/rubocop/pull/7908): Add new `Style/RedundantRegexpEscape` cop. ([@owst][])
 * [#7978](https://github.com/rubocop-hq/rubocop/pull/7978): Add new option `OnlyFor` to the `Bundler/GemComment` cop. ([@ric2b][])

config/default.yml

@@ -3613,6 +3613,11 @@ Style/RedundantPercentQ:
   Enabled: true
   VersionAdded: '0.76'
 
+Style/RedundantRegexpCharacterClass:
+  Description: 'Checks for unnecessary single-element Regexp character classes.'
+  Enabled: pending
+  VersionAdded: '0.85'
+
 Style/RedundantRegexpEscape:
   Description: 'Checks for redundant escapes in Regexps.'
   Enabled: pending

docs/modules/ROOT/pages/cops.adoc

@@ -437,6 +437,7 @@ In the following section you find all available cops:
 * xref:cops_style.adoc#styleredundantinterpolation[Style/RedundantInterpolation]
 * xref:cops_style.adoc#styleredundantparentheses[Style/RedundantParentheses]
 * xref:cops_style.adoc#styleredundantpercentq[Style/RedundantPercentQ]
+* xref:cops_style.adoc#styleredundantregexpcharacterclass[Style/RedundantRegexpCharacterClass]
 * xref:cops_style.adoc#styleredundantregexpescape[Style/RedundantRegexpEscape]
 * xref:cops_style.adoc#styleredundantreturn[Style/RedundantReturn]
 * xref:cops_style.adoc#styleredundantself[Style/RedundantSelf]

docs/modules/ROOT/pages/cops_style.adoc

@@ -7116,6 +7116,40 @@ question = '"What did you say?"'
 
 * https://rubystyle.guide#percent-q
 
+== Style/RedundantRegexpCharacterClass
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| Yes
+| Yes
+| 0.85
+| -
+|===
+
+This cop checks for unnecessary single-element Regexp character classes.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+r = /[x]/
+
+# good
+r = /x/
+
+# bad
+r = /[\s]/
+
+# good
+r = /\s/
+
+# good
+r = /[ab]/
+----
+
 == Style/RedundantRegexpEscape
 
 |===

lib/rubocop.rb

@@ -476,6 +476,7 @@
 require_relative 'rubocop/cop/style/redundant_interpolation'
 require_relative 'rubocop/cop/style/redundant_parentheses'
 require_relative 'rubocop/cop/style/redundant_percent_q'
+require_relative 'rubocop/cop/style/redundant_regexp_character_class'
 require_relative 'rubocop/cop/style/redundant_regexp_escape'
 require_relative 'rubocop/cop/style/redundant_return'
 require_relative 'rubocop/cop/style/redundant_self'

lib/rubocop/cop/layout/heredoc_argument_closing_parenthesis.rb

@@ -190,7 +190,7 @@ def incorrect_parenthesis_removal_begin(node)
         def safe_to_remove_line_containing_closing_paren?(node)
           last_line = processed_source[node.loc.end.line - 1]
           # Safe to remove if last line only contains `)`, `,`, and whitespace.
-          last_line.match?(/^[ ]*\)[ ]{0,20},{0,1}[ ]*$/)
+          last_line.match?(/^ *\) {0,20},{0,1} *$/)
         end
 
         def incorrect_parenthesis_removal_end(node)

lib/rubocop/cop/migration/department_name.rb

@@ -24,7 +24,7 @@ def investigate(processed_source)
 
             offset = Regexp.last_match(1).length
 
-            Regexp.last_match(4).scan(/[^,]+|[\W]+/) do |name|
+            Regexp.last_match(4).scan(/[^,]+|\W+/) do |name|
               trimmed_name = name.strip
 
               unless valid_content_token?(trimmed_name)

lib/rubocop/cop/mixin/uncommunicative_name.rb

@@ -18,7 +18,7 @@ def check(node, args)
           full_name = arg.children.first.to_s
           next if full_name == '_'
 
-          name = full_name.gsub(/\A([_]+)/, '')
+          name = full_name.gsub(/\A(_+)/, '')
           next if (arg.restarg_type? || arg.kwrestarg_type?) && name.empty?
           next if allowed_names.include?(name)
 

lib/rubocop/cop/style/redundant_regexp_character_class.rb

@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for unnecessary single-element Regexp character classes.
+      #
+      # @example
+      #
+      #   # bad
+      #   r = /[x]/
+      #
+      #   # good
+      #   r = /x/
+      #
+      #   # bad
+      #   r = /[\s]/
+      #
+      #   # good
+      #   r = /\s/
+      #
+      #   # good
+      #   r = /[ab]/
+      class RedundantRegexpCharacterClass < Cop
+        include MatchRange
+        include RegexpLiteralHelp
+
+        MSG_REDUNDANT_CHARACTER_CLASS = 'Redundant single-element character class, ' \
+        '`%<char_class>s` can be replaced with `%<element>s`.'
+
+        PATTERN = /
+          (
+            (?<!\\)           # No \-prefix (i.e. not escaped)
+            \[                # Literal [
+            (?!\#\{)          # Not (the start of) an interpolation
+            (?:               # Either...
+             \\. |            # Any escaped character
+             [^.*+?{}()|$] |  # or one that doesn't require escaping outside the character class
+             \\[upP]\{[^}]+\} # or a unicode code-point or property
+            )
+            \]                # Literal ]
+          )
+        /x.freeze
+
+        def on_regexp(node)
+          each_redundant_character_class(node) do |loc|
+            next if whitespace_in_free_space_mode?(node, loc)
+
+            add_offense(
+              node,
+              location: loc,
+              message: format(
+                MSG_REDUNDANT_CHARACTER_CLASS,
+                char_class: loc.source,
+                element: without_character_class(loc)
+              )
+            )
+          end
+        end
+
+        def autocorrect(node)
+          lambda do |corrector|
+            each_redundant_character_class(node) do |loc|
+              corrector.replace(loc, without_character_class(loc))
+            end
+          end
+        end
+
+        def each_redundant_character_class(node)
+          each_match_range(node.loc.expression, PATTERN) do |loc|
+            yield loc
+          end
+        end
+
+        private
+
+        def without_character_class(loc)
+          loc.source[1..-2]
+        end
+
+        def whitespace_in_free_space_mode?(node, loc)
+          return false unless freespace_mode_regexp?(node)
+
+          /\[\s\]/.match?(loc.source)
+        end
+      end
+    end
+  end
+end