diff --git a/main.py b/main.py
index 9175192..14bd4a4 100644
--- a/main.py
+++ b/main.py
@@ -3,14 +3,21 @@
 import argparse
 
 from src.lib import get_api_envs
-from src.utils import get_common_terms
+from src.utils import get_common_terms, apply_regex
 
 
 def create_parser():
     parser = argparse.ArgumentParser(description='Quizlet Utils')
-    commands = parser.add_subparsers(title="Commands", dest="command")
-    commands.add_parser("common", help="Find duplicate terms across all sets")
+    commands = parser.add_subparsers(title='Commands', dest='command')
     commands.required = True
+
+    commands.add_parser('common', help='Find duplicate terms across all sets')
+
+    apply = commands.add_parser('apply', help='Apply regex replace to set')
+    apply.add_argument('pattern', type=str, help='Pattern to search for')
+    apply.add_argument('repl', type=str, help='Replacement for pattern')
+    apply.add_argument('set_name', type=str, help='Word set to apply to')
+
     return parser
 
 
@@ -20,6 +27,8 @@ def main():
     api_envs = get_api_envs()
     if args.command == 'common':
         get_common_terms(*api_envs)
+    elif args.command == 'apply':
+        apply_regex(args.pattern, args.repl, args.set_name, *api_envs)
 
 
 if __name__ == '__main__':
diff --git a/src/utils.py b/src/utils.py
index d8e0296..fdef752 100644
--- a/src/utils.py
+++ b/src/utils.py
@@ -1,4 +1,5 @@
 """User-available utilities."""
+# ToDo: rename the module
 
 from itertools import combinations
 
@@ -19,3 +20,8 @@ def get_common_terms(*api_envs):
         common_terms.append(
             (word_set_1, word_set_2, word_set_1.has_common(word_set_2)))
     return common_terms
+
+
+def apply_regex(pattern, repl, set_name, *api_envs):
+    """Apply regex replace to all terms in word set."""
+    raise NotImplementedError  # ToDo: complete the utility
diff --git a/tests/src/test_lib.py b/tests/src/test_lib.py
index 322b1c5..af57a01 100644
--- a/tests/src/test_lib.py
+++ b/tests/src/test_lib.py
@@ -1,23 +1,8 @@
-import os
 import unittest
-from functools import wraps
 from unittest import mock
 
 from src.lib import api_call, get_api_envs
-
-
-def mock_envs(**envs):
-    """Mock environment variables for test."""
-
-    def decorator(func):
-        @wraps(func)
-        def wrapper(*args, **kwargs):
-            with mock.patch.dict(os.environ, envs, clear=True):
-                func(*args, **kwargs)
-
-        return wrapper
-
-    return decorator
+from tests.utils import mock_envs
 
 
 class TestGetApiEnvs(unittest.TestCase):
diff --git a/tests/src/test_utils.py b/tests/src/test_utils.py
index e69de29..fe9909b 100644
--- a/tests/src/test_utils.py
+++ b/tests/src/test_utils.py
@@ -0,0 +1 @@
+# ToDo: add tests
diff --git a/tests/test_main.py b/tests/test_main.py
index 0817884..5b99553 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -2,6 +2,7 @@
 from unittest import mock
 
 from main import create_parser, main
+from tests.src.test_lib import mock_envs
 
 
 class TestCreateParser(unittest.TestCase):
@@ -20,11 +21,30 @@ def test_common_command(self):
         args = self.parser.parse_args(['common'])
         self.assertEqual(args.command, 'common')
 
+    def test_apply_command_without_args(self):
+        with self.assertRaises(SystemExit):
+            self.parser.parse_args(['apply'])
+
+    def test_apply_command_with_all_args(self):
+        args = self.parser.parse_args(['apply', 'pattern', 'repl', 'set_name'])
+        self.assertEqual(args.command, 'apply')
+        self.assertEqual(args.pattern, 'pattern')
+        self.assertEqual(args.repl, 'repl')
+        self.assertEqual(args.set_name, 'set_name')
+
 
+@mock_envs(CLIENT_ID='client_id', USER_ID='user_id')
 class TestMain(unittest.TestCase):
     @mock.patch('main.get_common_terms')
-    @mock.patch('main.get_api_envs', mock.Mock(return_value=[]))
     @mock.patch('sys.argv', ['', 'common'])
     def test_common(self, mock_get_common_terms):
         main()
         mock_get_common_terms.assert_called_once()
+
+    @mock.patch('main.apply_regex')
+    @mock.patch('sys.argv', ['', 'apply', 'pattern', 'repl', 'set_name'])
+    def test_apply(self, mock_apply_regex):
+        main()
+        mock_apply_regex.assert_called_once()
+
+# ToDo: add decorator for mocking system arguments
diff --git a/tests/utils.py b/tests/utils.py
new file mode 100644
index 0000000..c179d06
--- /dev/null
+++ b/tests/utils.py
@@ -0,0 +1,30 @@
+"""Utilities for tests."""
+
+import inspect
+from functools import wraps
+from unittest import mock
+
+
+def mock_envs(**envs):
+    """Mock environment variables for test.
+
+    Can be applied to:
+      - any function
+      - test class - all test methods (starts with "test_*" will be decorated)
+    """
+
+    def decorator(obj):
+        @wraps(obj)
+        def wrapper(*args, **kwargs):
+            with mock.patch.dict('os.environ', envs, clear=True):
+                obj(*args, **kwargs)
+
+        if inspect.isclass(obj):
+            for name, method in inspect.getmembers(obj, inspect.isfunction):
+                if name.startswith('test_'):
+                    setattr(obj, name, decorator(method))
+            return obj
+        else:
+            return wrapper
+
+    return decorator