Clarify regexp syntax allowed for use in CI config

lib/gitlab/config/entry/validators.rb

@@ -213,7 +213,7 @@ module Gitlab
 
           def validate_each(record, attribute, value)
             unless validate_regexp(value)
-              record.errors.add(attribute, 'must be a regular expression')
+              record.errors.add(attribute, 'must be a regular expression with re2 syntax')
             end
           end
 
@@ -238,12 +238,16 @@ module Gitlab
         class ArrayOfStringsOrRegexpsValidator < RegexpValidator
           def validate_each(record, attribute, value)
             unless validate_array_of_strings_or_regexps(value)
-              record.errors.add(attribute, 'should be an array of strings or regexps')
+              record.errors.add(attribute, validation_message)
             end
           end
 
           private
 
+          def validation_message
+            'should be an array of strings or regexps using re2 syntax'
+          end
+
           def validate_array_of_strings_or_regexps(values)
             values.is_a?(Array) && values.all?(&method(:validate_string_or_regexp))
           end
@@ -259,6 +263,19 @@ module Gitlab
         class ArrayOfStringsOrRegexpsWithFallbackValidator < ArrayOfStringsOrRegexpsValidator
           protected
 
+          # TODO
+          #
+          # Remove ArrayOfStringsOrRegexpsWithFallbackValidator class too when
+          # you are removing the `:allow_unsafe_ruby_regexp` feature flag.
+          #
+          def validation_message
+            if ::Feature.enabled?(:allow_unsafe_ruby_regexp, default_enabled: :yaml)
+              'should be an array of strings or regexps'
+            else
+              super
+            end
+          end
+
           def fallback
             true
           end

lib/gitlab/untrusted_regexp/ruby_syntax.rb

@@ -36,7 +36,7 @@ module Gitlab
           create_untrusted_regexp(matches[:regexp], matches[:flags])
         rescue RegexpError
           raise unless fallback &&
-              Feature.enabled?(:allow_unsafe_ruby_regexp, default_enabled: false)
+              Feature.enabled?(:allow_unsafe_ruby_regexp, default_enabled: :yaml)
 
           if Feature.enabled?(:ci_unsafe_regexp_logger, type: :ops, default_enabled: :yaml)
             Gitlab::AppJsonLogger.info(

spec/lib/gitlab/ci/yaml_processor_spec.rb

@@ -9,6 +9,10 @@ module Gitlab
 
       subject { described_class.new(config, user: nil).execute }
 
+      before do
+        stub_feature_flags(allow_unsafe_ruby_regexp: false)
+      end
+
       shared_examples 'returns errors' do |error_message|
         it 'adds a message when an error is encountered' do
           expect(subject.errors).to include(error_message)
@@ -609,13 +613,13 @@ module Gitlab
           context 'when it is an array of integers' do
             let(:only) { [1, 1] }
 
-            it_behaves_like 'returns errors', 'jobs:rspec:only config should be an array of strings or regexps'
+            it_behaves_like 'returns errors', 'jobs:rspec:only config should be an array of strings or regexps using re2 syntax'
           end
 
           context 'when it is invalid regex' do
             let(:only) { ["/*invalid/"] }
 
-            it_behaves_like 'returns errors', 'jobs:rspec:only config should be an array of strings or regexps'
+            it_behaves_like 'returns errors', 'jobs:rspec:only config should be an array of strings or regexps using re2 syntax'
           end
         end
 
@@ -633,13 +637,13 @@ module Gitlab
           context 'when it is an array of integers' do
             let(:except) { [1, 1] }
 
-            it_behaves_like 'returns errors', 'jobs:rspec:except config should be an array of strings or regexps'
+            it_behaves_like 'returns errors', 'jobs:rspec:except config should be an array of strings or regexps using re2 syntax'
           end
 
           context 'when it is invalid regex' do
             let(:except) { ["/*invalid/"] }
 
-            it_behaves_like 'returns errors', 'jobs:rspec:except config should be an array of strings or regexps'
+            it_behaves_like 'returns errors', 'jobs:rspec:except config should be an array of strings or regexps using re2 syntax'
           end
         end
       end