Style/RedundantRegexpEscape false positive when used with multiline regexp with comments

[rafal-brize]

Provided regexp written as

        regexp = %r{
          \A[a-z#]            # letters or #
          \.[a-z]\z           # dot + letters
        }x

that enforces:

regexp.match?("#.a") => true
regexp.match?("#aa") => false

i receive

Style/RedundantRegexpEscape: Redundant escape inside regexp literal
          \.[a-z]\z           # dot + letters
          ^^

that after autocorrect changes the behaviour to:

regexp.match?("#.a") => true
regexp.match?("#aa") => true

However

regexp = %r{\A[a-z#]\.[a-z]\z}

passes the Style/RedundantRegexpEscape check

RuboCop version

$ bundle exec rubocop -V
0.92.0 (using Parser 2.7.1.5, rubocop-ast 0.7.0, running on ruby 2.6.5 x86_64-darwin18)

[rafal-brize]

@owst Tagging you, since i've read in changelog you've been working on those false positives, maybe you can pitch in.

[owst]

Hi @rafal-brize, thanks for tagging me and apologies for the bug.

It looks like the code that attempts to ignore comments is a bit overzealous and is treating the # within the character class as starting a comment, so your pattern is incorrectly processed as something like:

        regexp = %r{
          \A[a-z
          \.[a-z]\z
        }x

which makes the \. appear to be redundant as it's inside a character class.

I've recently been working on moving to use regexp_parser for this cop (rather than using a home-grown "parser") in #8625 and the parser correctly handles your pattern (I'll also add it as an explicit test). So when #8625 is merged, this bug will be fixed.

[rafal-brize]

Thanks @owst for all your work, awesome!

[bbatsov]

The fix was merged a while ago, but we forgot to close this issue.