When a regexp depends on a constant defined in a subclass, neither of the fixes suggested by Performance/ConstantRegexp work:

class C
  def self.regexp
    %r{foo #{self::BASE_PATTERN}}
  end
end

class X < C
  BASE_PATTERN = /bar/
end

class Y < C
  BASE_PATTERN = /baz/
end

If the pattern is change to use the /o modifier, the first time the method is triggered the regexp will be interpolated for both subclasses:

X.regexp
=> /foo (?-mix:bar)/
Y.regexp
=> /foo (?-mix:bar)/

If the regexp is extracted to a constant, it needs to be duplicated in every subclass (and the regexp could be considerably more complicated than the example here).

RuboCop version

Include the output of rubocop -V or bundle exec rubocop -V if using Bundler. Here's an example:

$ [bundle exec] rubocop -V
rubocop -V
1.8.1 (using Parser 3.0.0.0, rubocop-ast 1.4.0, running on ruby 2.6.6 x86_64-darwin18)
  - rubocop-performance 1.9.2
  - rubocop-rails 2.9.1
  - rubocop-rspec 2.1.0