Fix an incorrect auto-correct for `Performance/RedundantSplitRegexpAr…

…gument`

Follow rubocop#190.

This PR fixes the following incorrect auto-correct for `Performance/RedundantSplitRegexpArgument`
when using consecutive special string chars.

```console
% cat example.rb
"foo\nbar\nbaz\n".split(/\n\n/)

% bundle exec rubocop example.rb --only Performance/RedundantSplitRegexpArgument -a
Inspecting 1 file
C

Offenses:

example.rb:1:25: C: [Corrected]
Performance/RedundantSplitRegexpArgument: Use string as argument instead of regexp.
"foo\nbar\nbaz\n".split(/\n\n/)
                        ^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

## Before

```console
% cat example.rb
"foo\nbar\nbaz\n".split("nn")
```

## After

```console
% cat example.rb
"foo\nbar\nbaz\n".split("\n\n")
```

lib/rubocop/cop/performance/redundant_split_regexp_argument.rb

@@ -18,7 +18,7 @@ class RedundantSplitRegexpArgument < Base
         MSG = 'Use string as argument instead of regexp.'
         RESTRICT_ON_SEND = %i[split].freeze
         DETERMINISTIC_REGEX = /\A(?:#{LITERAL_REGEX})+\Z/.freeze
-        STR_SPECIAL_CHARS = %w[\n \" \' \\ \t \b \f \r].freeze
+        STR_SPECIAL_CHARS = %w[\n \" \' \\\\ \t \b \f \r].freeze
 
         def_node_matcher :split_call_with_regexp?, <<~PATTERN
           {(send !nil? :split {regexp})}
@@ -40,8 +40,19 @@ def determinist_regexp?(first_argument)
         end
 
         def autocorrect(corrector, node)
-          new_argument = node.first_argument.source[1..-2]
-          new_argument.delete!('\\') unless STR_SPECIAL_CHARS.include?(new_argument)
+          regexp_content = node.first_argument.content
+          stack = []
+          chars = regexp_content.chars.each_with_object([]) do |char, strings|
+            if stack.empty? && char == '\\'
+              stack.push(char)
+            else
+              strings << "#{stack.pop}#{char}"
+            end
+          end
+          new_argument = chars.map do |char|
+            char.dup.delete!('\\') unless STR_SPECIAL_CHARS.include?(char)
+            char
+          end.join
 
           corrector.replace(node.first_argument, "\"#{new_argument}\"")
         end

spec/rubocop/cop/performance/redundant_split_regexp_argument_spec.rb

@@ -37,6 +37,39 @@
     RUBY
   end
 
+  it 'registers an offense when the method is split and corrects correctly consecutive special string chars' do
+    expect_offense(<<~RUBY)
+      "foo\\n\\nbar\\n\\nbaz\\n\\n".split(/\\n\\n/)
+                                    ^^^^^^ Use string as argument instead of regexp.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      "foo\\n\\nbar\\n\\nbaz\\n\\n".split("\\n\\n")
+    RUBY
+  end
+
+  it 'registers an offense when the method is split and corrects correctly consecutive backslash escape chars' do
+    expect_offense(<<~RUBY)
+      "foo\\\\\\.bar".split(/\\\\\\./)
+                         ^^^^^^ Use string as argument instead of regexp.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      "foo\\\\\\.bar".split("\\\\\.")
+    RUBY
+  end
+
+  it 'registers an offense when the method is split and corrects correctly complex special string chars' do
+    expect_offense(<<~RUBY)
+      "foo\\nbar\\nbaz\\n".split(/foo\\n\\.\\n/)
+                              ^^^^^^^^^^^ Use string as argument instead of regexp.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      "foo\\nbar\\nbaz\\n".split("foo\\n.\\n")
+    RUBY
+  end
+
   it 'registers an offense when the method is split' do
     expect_offense(<<~RUBY)
       'a,b,c'.split(/,/)