diff --git a/CHANGELOG b/CHANGELOG
index 038b1d8..ab52a6f 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,7 @@
+- 0.1.1
+	* Add optional, pluggable verifier to protect state transition
+- 0.1.0
+	* Release version bump
 - 0.0.19:
 	* Enable alias with comma separated values
 - 0.0.18
diff --git a/setup.cfg b/setup.cfg
index 43b0318..663c5ae 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = shep
-version = 0.1.0rc1
+version = 0.1.1rc1
 description = Multi-state key stores using bit masks
 author = Louis Holbrook
 author_email = dev@holbrook.no
diff --git a/shep/error.py b/shep/error.py
index bd4beb9..74223bf 100644
--- a/shep/error.py
+++ b/shep/error.py
@@ -26,3 +26,9 @@ class StateCorruptionError(RuntimeError):
     """An irrecoverable discrepancy between persisted state and memory state has occurred.
     """
     pass
+
+
+class StateTransitionInvalid(Exception):
+    """Raised if state transition verification fails
+    """
+    pass
diff --git a/shep/state.py b/shep/state.py
index 23d599e..1e00ede 100644
--- a/shep/state.py
+++ b/shep/state.py
@@ -1,12 +1,18 @@
+# standard imports
+import re
+
 # local imports
 from shep.error import (
         StateExists,
         StateInvalid,
         StateItemExists,
         StateItemNotFound,
+        StateTransitionInvalid,
         )
 
 
+re_name = r'^[a-zA-Z_]+$'
+
 class State:
     """State is an in-memory bitmasked state store for key-value pairs, or even just keys alone.
 
@@ -22,17 +28,17 @@ class State:
 
     base_state_name = 'NEW'
 
-    def __init__(self, bits, logger=None):
+    def __init__(self, bits, logger=None, verifier=None):
         self.__bits = bits
         self.__limit = (1 << bits) - 1
         self.__c = 0
         setattr(self, self.base_state_name, 0)
-        #self.NEW = 0
 
         self.__reverse = {0: getattr(self, self.base_state_name)}
         self.__keys = {getattr(self, self.base_state_name): []}
         self.__keys_reverse = {}
         self.__contents = {}
+        self.verifier = verifier
 
 
     @classmethod
@@ -54,8 +60,8 @@ class State:
 
     # validates a state name and return its canonical representation
     def __check_name_valid(self, k):
-        if not k.isalpha():
-            raise ValueError('only alpha')
+        if not re.match(re_name, k):
+            raise ValueError('only alpha and underscore')
         return k.upper()
 
 
@@ -323,6 +329,11 @@ class State:
         if current_state_list == None:
             raise StateCorruptionError(to_state)
 
+        if self.verifier != None:
+            r = self.verifier(self, from_state, to_state)
+            if r != None:
+                raise StateTransitionInvalid('{} -> {}: {}'.format(from_state, to_state, r))
+
         self.__add_state_list(to_state, key)
         current_state_list.pop(idx) 
 
diff --git a/shep/verify.py b/shep/verify.py
new file mode 100644
index 0000000..a238932
--- /dev/null
+++ b/shep/verify.py
@@ -0,0 +1,2 @@
+def default_checker(statestore, old, new):
+    return None
diff --git a/tests/test_state.py b/tests/test_state.py
index da7a06c..b5de3c6 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -18,7 +18,6 @@ class TestState(unittest.TestCase):
         for k in [
                 'f0o',
                 'f oo',
-                'f_oo',
             ]:
             with self.assertRaises(ValueError):
                 states.add(k)