RedundantRegexpEscape tells me backslashes are redundant even though they are not

[betesh]

Regexp literals that contain [^:] confuse the RedundantRegexpEscape cop into thinking that later backslashes are redundant.

---

Expected behavior

Rubocop should not identify any offenses in the example below. It should recognize that the backslashes are meaningful in context.

Actual behavior

Rubocop reports that backslashes are redundant even though they are not.

Steps to reproduce the problem

$ cat test.rb 
# These 2 have different results, so the backslash 
# before the dot is not redundant
puts /[^:]+./.match?('ab')
puts /[^:]+\./.match?('ab')

# These 2 have different results, so the backslashes 
# before the opening and closing parentheses are not redundant
puts /[^:]\(u\)/.match?(':(u)')
puts /[^:](u)/.match?(':(u)')

$ ruby test.rb 
true
false
false
true

$ rubocop --only Style/RedundantRegexpEscape
Inspecting 1 file
C

Offenses:

test.rb:4:12: C: Style/RedundantRegexpEscape: Redundant escape inside regexp literal
puts /[^:]+\./.match?('ab')
           ^^
test.rb:8:11: C: Style/RedundantRegexpEscape: Redundant escape inside regexp literal
puts /[^:]\(u\)/.match?(':(u)')
          ^^
test.rb:8:14: C: Style/RedundantRegexpEscape: Redundant escape inside regexp literal
puts /[^:]\(u\)/.match?(':(u)')
             ^^

1 file inspected, 3 offenses detected

RuboCop version

$ rubocop -V
0.88.0 (using Parser 2.7.1.4, rubocop-ast 0.3.0, running on ruby 2.5.0 x86_64-linux)

[betesh]

This is fixed in master. Probably duplicates #8335