diff --git a/.gitignore b/.gitignore
index a35a794b2..6c0a22ef7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -32,7 +32,6 @@ measurements
 
 ## BUNDLER
 .bundle
-Gemfile.lock
 Gemfile.*.lock
 
 ## PROJECT::SPECIFIC
diff --git a/Gemfile.lock b/Gemfile.lock
new file mode 100644
index 000000000..8980f2a47
--- /dev/null
+++ b/Gemfile.lock
@@ -0,0 +1,167 @@
+PATH
+  remote: .
+  specs:
+    mutant (0.8.12)
+      abstract_type (~> 0.0.7)
+      adamantium (~> 0.2.0)
+      anima (~> 0.3.0)
+      ast (~> 2.2)
+      concord (~> 0.1.5)
+      diff-lcs (~> 1.2)
+      equalizer (~> 0.0.9)
+      ice_nine (~> 0.11.1)
+      memoizable (~> 0.4.2)
+      morpher (~> 0.2.6)
+      parallel (~> 1.3)
+      parser (~> 2.3.1, >= 2.3.1.4)
+      procto (~> 0.0.2)
+      regexp_parser (~> 0.4.1)
+      unparser (~> 0.2.5)
+    mutant-rspec (0.8.12)
+      mutant (~> 0.8.12)
+      rspec-core (>= 3.4.0, < 3.6.0)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    abstract_type (0.0.7)
+    adamantium (0.2.0)
+      ice_nine (~> 0.11.0)
+      memoizable (~> 0.4.0)
+    anima (0.3.0)
+      abstract_type (~> 0.0.7)
+      adamantium (~> 0.2)
+      equalizer (~> 0.0.11)
+    ast (2.3.0)
+    axiom-types (0.1.1)
+      descendants_tracker (~> 0.0.4)
+      ice_nine (~> 0.11.0)
+      thread_safe (~> 0.3, >= 0.3.1)
+    codeclimate-engine-rb (0.4.0)
+      virtus (~> 1.0)
+    coercible (1.0.0)
+      descendants_tracker (~> 0.0.1)
+    concord (0.1.5)
+      adamantium (~> 0.2.0)
+      equalizer (~> 0.0.9)
+    descendants_tracker (0.0.4)
+      thread_safe (~> 0.3, >= 0.3.1)
+    devtools (0.1.15)
+      adamantium (~> 0.2.0)
+      anima (~> 0.3.0)
+      concord (~> 0.1.5)
+      flay (~> 2.8.1)
+      flog (~> 4.4.0)
+      mutant (~> 0.8.11)
+      mutant-rspec (~> 0.8.11)
+      procto (~> 0.0.3)
+      rake (~> 11.3.0)
+      reek (~> 4.5.0)
+      rspec (~> 3.5.0)
+      rspec-core (~> 3.5.4)
+      rspec-its (~> 1.2.0)
+      rubocop (~> 0.46.0)
+      simplecov (~> 0.12.0)
+      yard (~> 0.9.1)
+      yardstick (~> 0.9.9)
+    diff-lcs (1.2.5)
+    docile (1.1.5)
+    equalizer (0.0.11)
+    erubis (2.7.0)
+    ffi (1.9.14)
+    flay (2.8.1)
+      erubis (~> 2.7.0)
+      path_expander (~> 1.0)
+      ruby_parser (~> 3.0)
+      sexp_processor (~> 4.0)
+    flog (4.4.0)
+      path_expander (~> 1.0)
+      ruby_parser (~> 3.1, > 3.1.0)
+      sexp_processor (~> 4.4)
+    ice_nine (0.11.2)
+    json (2.0.2)
+    memoizable (0.4.2)
+      thread_safe (~> 0.3, >= 0.3.1)
+    morpher (0.2.6)
+      abstract_type (~> 0.0.7)
+      adamantium (~> 0.2.0)
+      anima (~> 0.3.0)
+      ast (~> 2.2)
+      concord (~> 0.1.5)
+      equalizer (~> 0.0.9)
+      ice_nine (~> 0.11.0)
+      procto (~> 0.0.2)
+    parallel (1.10.0)
+    parser (2.3.3.1)
+      ast (~> 2.2)
+    path_expander (1.0.1)
+    powerpack (0.1.1)
+    procto (0.0.3)
+    rainbow (2.1.0)
+    rake (11.3.0)
+    reek (4.5.2)
+      codeclimate-engine-rb (~> 0.4.0)
+      parser (~> 2.3.1, >= 2.3.1.2)
+      rainbow (~> 2.0)
+    regexp_parser (0.4.1)
+    rspec (3.5.0)
+      rspec-core (~> 3.5.0)
+      rspec-expectations (~> 3.5.0)
+      rspec-mocks (~> 3.5.0)
+    rspec-core (3.5.4)
+      rspec-support (~> 3.5.0)
+    rspec-expectations (3.5.0)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.5.0)
+    rspec-its (1.2.0)
+      rspec-core (>= 3.0.0)
+      rspec-expectations (>= 3.0.0)
+    rspec-mocks (3.5.0)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.5.0)
+    rspec-support (3.5.0)
+    rubocop (0.46.0)
+      parser (>= 2.3.1.1, < 3.0)
+      powerpack (~> 0.1)
+      rainbow (>= 1.99.1, < 3.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (~> 1.0, >= 1.0.1)
+    ruby-progressbar (1.8.1)
+    ruby_parser (3.8.3)
+      sexp_processor (~> 4.1)
+    sexp_processor (4.7.0)
+    simplecov (0.12.0)
+      docile (~> 1.1.0)
+      json (>= 1.8, < 3)
+      simplecov-html (~> 0.10.0)
+    simplecov-html (0.10.0)
+    thread_safe (0.3.5)
+    unicode-display_width (1.1.1)
+    unparser (0.2.5)
+      abstract_type (~> 0.0.7)
+      adamantium (~> 0.2.0)
+      concord (~> 0.1.5)
+      diff-lcs (~> 1.2.5)
+      equalizer (~> 0.0.9)
+      parser (~> 2.3.0)
+      procto (~> 0.0.2)
+    virtus (1.0.5)
+      axiom-types (~> 0.1)
+      coercible (~> 1.0)
+      descendants_tracker (~> 0.0, >= 0.0.3)
+      equalizer (~> 0.0, >= 0.0.9)
+    yard (0.9.5)
+    yardstick (0.9.9)
+      yard (~> 0.8, >= 0.8.7.2)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  bundler (~> 1.10)
+  devtools (~> 0.1.12)
+  ffi (~> 1.9.6)
+  mutant!
+
+BUNDLED WITH
+   1.13.6
diff --git a/circle.yml b/circle.yml
index 24ef33c8a..8267a2936 100644
--- a/circle.yml
+++ b/circle.yml
@@ -1,10 +1,7 @@
 ---
 machine:
   ruby:
-    version: '2.3.0'
-dependencies:
-  pre:
-    - bundle -v | grep -Fx "Bundler version 1.10.6" || gem install bundler --version 1.10.6
+    version: '2.3.3'
 test:
   override:
     - bundle exec rake ci
diff --git a/config/flay.yml b/config/flay.yml
index 860f29761..cb7eb82bf 100644
--- a/config/flay.yml
+++ b/config/flay.yml
@@ -1,3 +1,3 @@
 ---
 threshold: 16
-total_score: 1317
+total_score: 1321
diff --git a/lib/mutant/ast/regexp.rb b/lib/mutant/ast/regexp.rb
index 0d44bca44..545e80817 100644
--- a/lib/mutant/ast/regexp.rb
+++ b/lib/mutant/ast/regexp.rb
@@ -12,10 +12,7 @@ module Regexp
       #
       # @return [Regexp::Expression]
       def self.parse(regexp)
-        ::Regexp::Parser.parse(
-          regexp,
-          "ruby/#{RUBY_VERSION.split('.').first(2).join('.')}"
-        )
+        ::Regexp::Parser.parse(regexp)
       end
 
       # Check if expression is supported by mapper
diff --git a/mutant.gemspec b/mutant.gemspec
index 5a069f635..b33fce51f 100644
--- a/mutant.gemspec
+++ b/mutant.gemspec
@@ -36,7 +36,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency('equalizer',     '~> 0.0.9')
   gem.add_runtime_dependency('anima',         '~> 0.3.0')
   gem.add_runtime_dependency('concord',       '~> 0.1.5')
-  gem.add_runtime_dependency('regexp_parser', '~> 0.3.6')
+  gem.add_runtime_dependency('regexp_parser', '~> 0.4.1')
 
   gem.add_development_dependency('devtools', '~> 0.1.12')
   gem.add_development_dependency('bundler',  '~> 1.10')
diff --git a/spec/integrations.yml b/spec/integrations.yml
index a1525bd83..45e6e6ea0 100644
--- a/spec/integrations.yml
+++ b/spec/integrations.yml
@@ -2,6 +2,7 @@
 - name: rubyspec
   namespace: Rubyspec
   repo_uri: 'https://github.com/ruby/rubyspec.git'
+  repo_ref: 'origin/master'
   mutation_coverage: false
   mutation_generation: true
   expected_errors:
@@ -16,18 +17,23 @@
 - name: regexp_parser
   namespace: Regexp
   repo_uri: 'https://github.com/ammar/regexp_parser.git'
+  repo_ref: 'v0.4.1'
   mutation_coverage: false
   mutation_generation: true
-  expected_errors: {}
+  expected_errors:
+    "Regexp::Syntax::Ruby::V233 does not implement: [escape:codepoint]":
+     - regexp_parser/test/parser/test_escapes.rb
 - name: auom
   namespace: AUOM
   repo_uri: 'https://github.com/mbj/auom.git'
+  repo_ref: 'origin/master'
   mutation_coverage: true
   mutation_generation: true
   expected_errors: {}
 - name: axiom
   namespace: Axiom
   repo_uri: 'https://github.com/dkubb/axiom.git'
+  repo_ref: 'origin/master'
   mutation_coverage: false
   mutation_generation: true
   expected_errors: {}
diff --git a/spec/support/corpus.rb b/spec/support/corpus.rb
index a9e9c6bd4..b129045b6 100644
--- a/spec/support/corpus.rb
+++ b/spec/support/corpus.rb
@@ -37,7 +37,8 @@ class Project
         :mutation_generation,
         :name,
         :namespace,
-        :repo_uri
+        :repo_uri,
+        :repo_ref
       )
 
       # Verify mutation coverage
@@ -95,18 +96,23 @@ def verify_mutation_generation
       def checkout
         return self if noinstall?
         TMP.mkdir unless TMP.directory?
+
         if repo_path.exist?
           Dir.chdir(repo_path) do
             system(%w[git fetch origin])
             system(%w[git reset --hard])
             system(%w[git clean -f -d -x])
-            system(%w[git checkout origin/master])
-            system(%w[git reset --hard])
-            system(%w[git clean -f -d -x])
           end
         else
           system(%W[git clone #{repo_uri} #{repo_path}])
         end
+
+        Dir.chdir(repo_path) do
+          system(%W[git checkout #{repo_ref}])
+          system(%w[git reset --hard])
+          system(%w[git clean -f -d -x])
+        end
+
         self
       end
       memoize :checkout
@@ -300,6 +306,7 @@ def to_h
               s(:guard, s(:primitive, Hash)),
               s(:hash_transform,
                 s(:key_symbolize, :repo_uri,            s(:guard, s(:primitive, String))),
+                s(:key_symbolize, :repo_ref,            s(:guard, s(:primitive, String))),
                 s(:key_symbolize, :name,                s(:guard, s(:primitive, String))),
                 s(:key_symbolize, :namespace,           s(:guard, s(:primitive, String))),
                 s(:key_symbolize, :mutation_coverage,
diff --git a/spec/unit/mutant/ast/regexp/parse_spec.rb b/spec/unit/mutant/ast/regexp/parse_spec.rb
index 624c6e3e8..2e22f456a 100644
--- a/spec/unit/mutant/ast/regexp/parse_spec.rb
+++ b/spec/unit/mutant/ast/regexp/parse_spec.rb
@@ -1,6 +1,4 @@
 RSpec.describe Mutant::AST::Regexp, '.parse' do
-  before { stub_const('RUBY_VERSION', '2.3.9') }
-
   it 'parses using minor ruby version' do
     expect(described_class.parse(/foo/).to_re).to eql(/foo/)
   end