diff --git a/homeassistant/components/script.py b/homeassistant/components/script.py
index 54490af3cfa..15df6907468 100644
--- a/homeassistant/components/script.py
+++ b/homeassistant/components/script.py
@@ -45,7 +45,7 @@ SCRIPT_ENTRY_SCHEMA = vol.Schema({
 })
 
 CONFIG_SCHEMA = vol.Schema({
-    DOMAIN: vol.Schema({cv.slug: SCRIPT_ENTRY_SCHEMA})
+    DOMAIN: cv.schema_with_slug_keys(SCRIPT_ENTRY_SCHEMA)
 }, extra=vol.ALLOW_EXTRA)
 
 SCRIPT_SERVICE_SCHEMA = vol.Schema(dict)
diff --git a/homeassistant/config.py b/homeassistant/config.py
index 10d3ce21a00..0edadf6a78d 100644
--- a/homeassistant/config.py
+++ b/homeassistant/config.py
@@ -170,10 +170,9 @@ def _no_duplicate_auth_mfa_module(configs: Sequence[Dict[str, Any]]) \
     return configs
 
 
-PACKAGES_CONFIG_SCHEMA = vol.Schema({
-    cv.slug: vol.Schema(  # Package names are slugs
-        {cv.string: vol.Any(dict, list, None)})  # Component configuration
-})
+PACKAGES_CONFIG_SCHEMA = cv.schema_with_slug_keys(  # Package names are slugs
+    vol.Schema({cv.string: vol.Any(dict, list, None)})  # Component config
+)
 
 CUSTOMIZE_DICT_SCHEMA = vol.Schema({
     vol.Optional(ATTR_FRIENDLY_NAME): cv.string,
@@ -627,7 +626,7 @@ def _identify_config_schema(module: ModuleType) -> \
     except (AttributeError, KeyError):
         return None, None
     t_schema = str(schema)
-    if t_schema.startswith('{'):
+    if t_schema.startswith('{') or 'schema_with_slug_keys' in t_schema:
         return ('dict', schema)
     if t_schema.startswith(('[', 'All(<function ensure_list')):
         return ('list', schema)
diff --git a/homeassistant/core.py b/homeassistant/core.py
index 2834730408e..02b474e3cd2 100644
--- a/homeassistant/core.py
+++ b/homeassistant/core.py
@@ -12,7 +12,6 @@ import functools
 import logging
 import os
 import pathlib
-import re
 import sys
 import threading
 from time import monotonic
@@ -43,7 +42,7 @@ from homeassistant.util.async_ import (
     fire_coroutine_threadsafe)
 from homeassistant import util
 import homeassistant.util.dt as dt_util
-from homeassistant.util import location
+from homeassistant.util import location, slugify
 from homeassistant.util.unit_system import UnitSystem, METRIC_SYSTEM  # NOQA
 
 # Typing imports that create a circular dependency
@@ -62,9 +61,6 @@ DOMAIN = 'homeassistant'
 # How long we wait for the result of a service call
 SERVICE_CALL_LIMIT = 10  # seconds
 
-# Pattern for validating entity IDs (format: <domain>.<entity>)
-ENTITY_ID_PATTERN = re.compile(r"^(\w+)\.(\w+)$")
-
 # How long to wait till things that run on startup have to finish.
 TIMEOUT_EVENT_START = 15
 
@@ -77,8 +73,12 @@ def split_entity_id(entity_id: str) -> List[str]:
 
 
 def valid_entity_id(entity_id: str) -> bool:
-    """Test if an entity ID is a valid format."""
-    return ENTITY_ID_PATTERN.match(entity_id) is not None
+    """Test if an entity ID is a valid format.
+
+    Format: <domain>.<entity> where both are slugs.
+    """
+    return ('.' in entity_id and
+            slugify(entity_id) == entity_id.replace('.', '_', 1))
 
 
 def valid_state(state: str) -> bool:
diff --git a/homeassistant/helpers/config_validation.py b/homeassistant/helpers/config_validation.py
index 92fe935085a..ef0166bc16d 100644
--- a/homeassistant/helpers/config_validation.py
+++ b/homeassistant/helpers/config_validation.py
@@ -319,7 +319,23 @@ def service(value):
                       .format(value))
 
 
-def slug(value):
+def schema_with_slug_keys(value_schema: Union[T, Callable]) -> Callable:
+    """Ensure dicts have slugs as keys.
+
+    Replacement of vol.Schema({cv.slug: value_schema}) to prevent misleading
+    "Extra keys" errors from voluptuous.
+    """
+    schema = vol.Schema({str: value_schema})
+
+    def verify(value: Dict) -> Dict:
+        """Validate all keys are slugs and then the value_schema."""
+        for key in value.keys():
+            slug(key)
+        return schema(value)
+    return verify
+
+
+def slug(value: Any) -> str:
     """Validate value is a valid slug."""
     if value is None:
         raise vol.Invalid('Slug should not be None')
@@ -330,7 +346,7 @@ def slug(value):
     raise vol.Invalid('invalid slug {} (try {})'.format(value, slg))
 
 
-def slugify(value):
+def slugify(value: Any) -> str:
     """Coerce a value to a slug."""
     if value is None:
         raise vol.Invalid('Slug should not be None')
diff --git a/tests/components/switch/test_wake_on_lan.py b/tests/components/switch/test_wake_on_lan.py
index c3f4e04057d..312a49b5183 100644
--- a/tests/components/switch/test_wake_on_lan.py
+++ b/tests/components/switch/test_wake_on_lan.py
@@ -139,11 +139,11 @@ class TestWOLSwitch(unittest.TestCase):
                 'mac_address': '00-01-02-03-04-05',
                 'host': 'validhostname',
                 'turn_off': {
-                    'service': 'shell_command.turn_off_TARGET',
+                    'service': 'shell_command.turn_off_target',
                 },
             }
         })
-        calls = mock_service(self.hass, 'shell_command', 'turn_off_TARGET')
+        calls = mock_service(self.hass, 'shell_command', 'turn_off_target')
 
         state = self.hass.states.get('switch.wake_on_lan')
         assert STATE_OFF == state.state