Add extended /regexp/ scheme support to untrusted regexp

lib/gitlab/untrusted_regexp.rb

@@ -9,6 +9,8 @@ module Gitlab
   # there is a strict limit on total execution time. See the RE2 documentation
   # at https://github.com/google/re2/wiki/Syntax for more details.
   class UntrustedRegexp
+    require_dependency 're2'
+
     delegate :===, :source, to: :regexp
 
     def initialize(pattern, multiline: false)
@@ -52,6 +54,27 @@ module Gitlab
       Regexp.new(pattern)
     end
 
+    def self.valid?(pattern)
+      self.fabricate(pattern)
+    rescue RegexpError
+      false
+    end
+
+    def self.fabricate(pattern)
+      matches = pattern.match(%r{^/(?<regexp>.+)/(?<flags>[ismU]*)$})
+
+      if matches
+        expression = matches[:regexp]
+        flags = matches[:flags]
+
+        expression.prepend("(?#{flags})") if flags.present?
+
+        self.new(expression, multiline: false)
+      else
+        self.new(pattern, multiline: false)
+      end
+    end
+
     private
 
     attr_reader :regexp

spec/lib/gitlab/untrusted_regexp_spec.rb

-require 'spec_helper'
+require 'fast_spec_helper'
+require 'support/shared_examples/malicious_regexp_shared_examples'
 
 describe Gitlab::UntrustedRegexp do
+  describe '.valid?' do
+    it 'returns true if regexp is valid' do
+    end
+
+    it 'returns true if regexp is invalid' do
+    end
+  end
+
+  describe '.fabricate' do
+    context 'when regexp is using /regexp/ scheme with flags' do
+      it 'fabricates regexp with a single flag' do
+        regexp = described_class.fabricate('/something/i')
+
+        expect(regexp).to eq described_class.new('(?i)something')
+        expect(regexp.scan('SOMETHING')).to be_one
+      end
+
+      it 'fabricates regexp with multiple flags' do
+        regexp = described_class.fabricate('/something/im')
+
+        expect(regexp).to eq described_class.new('(?im)something')
+      end
+
+      it 'fabricates regexp without flags' do
+        regexp = described_class.fabricate('/something/')
+
+        expect(regexp).to eq described_class.new('something')
+      end
+    end
+
+    context 'when regexp is not plain pattern' do
+      it 'fabricates regexp without flags' do
+        regexp = described_class.fabricate('something')
+
+        expect(regexp).to eq described_class.new('something')
+      end
+    end
+
+    context 'when regexp is invalid' do
+      it 'raises an error' do
+        expect { described_class.fabricate('/some ( thing/') }
+          .to raise_error(RegexpError)
+      end
+    end
+  end
+
   describe '#initialize' do
     subject { described_class.new(pattern) }