diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 0e88d594b..15a3e3ba3 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -112,11 +112,11 @@ class Item(object):
 
     def update(self, update_expression, expression_attribute_names, expression_attribute_values):
         parts = [p for p in re.split(r'\b(SET|REMOVE|ADD|DELETE)\b', update_expression) if p]
-        for action, valstr in zip(parts[:-1:1], parts[1::1]):
+        for action, valstr in zip(parts[:-1:2], parts[1::2]):
             values = valstr.split(',')
             for value in values:
                 # A Real value
-                value = value.lstrip(":").rstrip(",")
+                value = value.lstrip(":").rstrip(",").strip()
                 for k, v in expression_attribute_names.items():
                     value = re.sub(r'{}\b'.format(k), v, value)
 
diff --git a/moto/dynamodb2/responses.py b/moto/dynamodb2/responses.py
index eea2bace5..39cdaae4e 100644
--- a/moto/dynamodb2/responses.py
+++ b/moto/dynamodb2/responses.py
@@ -286,7 +286,8 @@ class DynamoHandler(BaseResponse):
 
                 index_hash_key = [key for key in index if key['KeyType'] == 'HASH'][0]
                 hash_key_var = reverse_attribute_lookup.get(index_hash_key['AttributeName'], index_hash_key['AttributeName'])
-                i, hash_key_expression = ((i, e) for i, e in enumerate(expressions) if re.search(r'[\s(]#n1\b'.format(hash_key_var), e)).next()
+                hash_key_regex = r'(^|[\s(]){}\b'.format(hash_key_var)
+                i, hash_key_expression = ((i, e) for i, e in enumerate(expressions) if re.search(hash_key_regex, e)).next()
                 hash_key_expression = hash_key_expression.strip('()')
                 expressions.pop(i)