diff --git a/.gitmodules b/.gitmodules
index 6d61431..fd375d2 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "uap-core"]
 	path = uap-core
-	url = https://github.com/ua-parser/uap-core.git
+	url = https://github.com/ua-parser/uap-core
 	branch = master
diff --git a/.travis.yml b/.travis.yml
index 3ec68a6..5780377 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,18 +1,18 @@
 sudo: false
 language: python
-python:
-  - "3.6"
-
-env:
-  - TOX_ENV=py26
-  - TOX_ENV=py27
-  - TOX_ENV=py36
-  - TOX_ENV=docs
-  - TOX_ENV=py27-flake8
-  - TOX_ENV=py36-flake8
-
 install:
-   - pip install tox
+  - pip install -r requirements_dev.txt
+
+matrix:
+  include:
+    - python: 2.7
+      env:
+       - TOX_ENV=py27
+       - TOX_ENV=py27-flake8
+    - python: 3.6
+      env:
+        - TOX_ENV=py36
+        - TOX_ENV=py36-flake8
+        - TOX_ENV=docs
 
-script:
-   - tox -e $TOX_ENV
+script: tox -e $TOX_ENV
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..6c05a61
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+pyyaml==5.1
diff --git a/requirements_dev.txt b/requirements_dev.txt
new file mode 100644
index 0000000..eb111cc
--- /dev/null
+++ b/requirements_dev.txt
@@ -0,0 +1 @@
+tox==3.9.0
diff --git a/tox.ini b/tox.ini
index cfce717..479012c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,9 +1,9 @@
 [tox]
-envlist = py26, py27, py36, docs, py27-flake8, py36-flake8
+envlist = py27, py36, docs, py27-flake8, py36-flake8
 
 [testenv]
 deps =
-    pyyaml
+    -rrequirements.txt
 commands =
     python setup.py develop
     python ua_parser/user_agent_parser_test.py
diff --git a/ua_parser/user_agent_parser.py b/ua_parser/user_agent_parser.py
index 68824cf..05875f2 100644
--- a/ua_parser/user_agent_parser.py
+++ b/ua_parser/user_agent_parser.py
@@ -61,15 +61,15 @@ def Parse(self, user_agent_string):
             if self.v1_replacement:
                 v1 = self.v1_replacement
             elif match.lastindex and match.lastindex >= 2:
-                v1 = match.group(2)
+                v1 = match.group(2) or None
 
             if self.v2_replacement:
                 v2 = self.v2_replacement
             elif match.lastindex and match.lastindex >= 3:
-                v2 = match.group(3)
+                v2 = match.group(3) or None
 
             if match.lastindex and match.lastindex >= 4:
-                v3 = match.group(4)
+                v3 = match.group(4) or None
 
         return family, v1, v2, v3
 
@@ -125,17 +125,26 @@ def Parse(self, user_agent_string):
                 os_v1 = match.group(2)
 
             if self.os_v2_replacement:
-                os_v2 = self.os_v2_replacement
+                if re.search(r'\$2', self.os_v2_replacement):
+                    os_v2 = re.sub(r'\$2', match.group(2), self.os_v2_replacement)
+                else:
+                    os_v2 = self.os_v2_replacement
             elif match.lastindex and match.lastindex >= 3:
                 os_v2 = match.group(3)
 
             if self.os_v3_replacement:
-                os_v3 = self.os_v3_replacement
+                if re.search(r'\$3', self.os_v3_replacement):
+                    os_v3 = re.sub(r'\$3', match.group(3), self.os_v3_replacement)
+                else:
+                    os_v3 = self.os_v3_replacement
             elif match.lastindex and match.lastindex >= 4:
                 os_v3 = match.group(4)
 
             if self.os_v4_replacement:
-                os_v4 = self.os_v4_replacement
+                if re.search(r'\$4', self.os_v4_replacement):
+                    os_v4 = re.sub(r'\$4', match.group(4), self.os_v4_replacement)
+                else:
+                    os_v4 = self.os_v4_replacement
             elif match.lastindex and match.lastindex >= 5:
                 os_v4 = match.group(5)
 
diff --git a/uap-core b/uap-core
index fc570f3..71ec72c 160000
--- a/uap-core
+++ b/uap-core
@@ -1 +1 @@
-Subproject commit fc570f378e41063bad3bdf0532967743efc75b4b
+Subproject commit 71ec72cd37a940683cd40af3f7ab738ecbdcc45b