Add anti-corruption layer above expressions pattern matching

lib/gitlab/ci/pipeline/expression/lexeme/pattern.rb

@@ -6,14 +6,14 @@ module Gitlab
           require_dependency 're2'
 
           class Pattern < Lexeme::Value
-            PATTERN = %r{/(?<regexp>.+)/}.freeze
+            PATTERN = %r{^(?<regexp>/.+/[ismU]*)$}.freeze
 
             def initialize(regexp)
               @value = regexp
             end
 
             def evaluate(variables = {})
-              Gitlab::UntrustedRegexp.new(@value)
+              Gitlab::UntrustedRegexp.fabricate(@value)
             rescue RegexpError
               raise Expression::RuntimeError, 'Invalid regular expression!'
             end

spec/lib/gitlab/ci/pipeline/expression/lexeme/pattern_spec.rb

@@ -42,6 +42,34 @@ describe Gitlab::Ci::Pipeline::Expression::Lexeme::Pattern do
 
       expect(token).to be_nil
     end
+
+    it 'support single flag' do
+      scanner = StringScanner.new('/pattern/i')
+
+      token = described_class.scan(scanner)
+
+      expect(token).not_to be_nil
+      expect(token.build.evaluate)
+        .to eq Gitlab::UntrustedRegexp.new('(?i)pattern')
+    end
+
+    it 'support multiple flags' do
+      scanner = StringScanner.new('/pattern/im')
+
+      token = described_class.scan(scanner)
+
+      expect(token).not_to be_nil
+      expect(token.build.evaluate)
+        .to eq Gitlab::UntrustedRegexp.new('(?im)pattern')
+    end
+
+    it 'does not support arbitrary flags' do
+      scanner = StringScanner.new('/pattern/x')
+
+      token = described_class.scan(scanner)
+
+      expect(token).to be_nil
+    end
   end
 
   describe '#evaluate' do