diff --git a/twistedchecker/configuration/pylintrc b/twistedchecker/configuration/pylintrc
index 70f03b2..5f81c4d 100644
--- a/twistedchecker/configuration/pylintrc
+++ b/twistedchecker/configuration/pylintrc
@@ -63,7 +63,7 @@ function-rgx=((([a-z])|([a-z]+_[a-z]))[a-zA-Z0-9]+)$|(__[a-z]+__)$|(\_.+)$|(test
 # Attribute names should be in mixed case, with the first letter lower case,
 # each word separated by having its first letter capitalized just like method names.
 # And all private names should begin with an underscore.
-attr-rgx=(((([a-z_])|([a-z]+_[a-z]))[a-zA-Z0-9]+)|(([A-Z_][A-Z0-9_]*)))$
+attr-rgx=(((([a-z_])|([a-z]+_[a-z]))[a-zA-Z0-9]+)|(([A-Z_][A-Z0-9_]*))|(__[a-zA-Z0-9][a-zA-Z0-9_]*[a-zA-Z0-9]+__)|(__[a-zA-Z0-9]__))$
 
 # Classes are to be named in mixed case, with the first letter capitalized,
 # each word separated by having its first letter capitalized.
diff --git a/twistedchecker/functionaltests/attributename_pass.py b/twistedchecker/functionaltests/attributename.py
similarity index 52%
rename from twistedchecker/functionaltests/attributename_pass.py
rename to twistedchecker/functionaltests/attributename.py
index 2c0fa3e..e26794e 100644
--- a/twistedchecker/functionaltests/attributename_pass.py
+++ b/twistedchecker/functionaltests/attributename.py
@@ -13,8 +13,26 @@ def __init__(self):
         """
         A init of this class
         """
+        # Bad names.
+        self.foo_bar_zar = None
+        self.FooBarZar = None
+        self.a = None
+        self._fooBar_ = None
+        self._fooBar__ = None
+        self.__fooBar_ = None
+        self.___longStart__ = None
+        self.__longEnd___ = None
+
+        # Good names.
         self.fooBarZar = None
         self._fooBarZar = None
         self.foobar = None
+        # Any __*__ name is accepted.
+        self.__foobar__ = None
+        self.__fooBar__ = None
+        self.__foo_bar__ = None
+        self.__dict__ = {}
+        self.__version__ = {}
+        self.__a__ = {}
         # In some cases constants are lazy initialized as instance members.
         self.FOOBARZAR = None
diff --git a/twistedchecker/functionaltests/attributename.result b/twistedchecker/functionaltests/attributename.result
new file mode 100644
index 0000000..6e4b434
--- /dev/null
+++ b/twistedchecker/functionaltests/attributename.result
@@ -0,0 +1,8 @@
+19:C0103
+24:C0103
+20:C0103
+22:C0103
+23:C0103
+17:C0103
+18:C0103
+21:C0103
diff --git a/twistedchecker/functionaltests/attributename_fail.py b/twistedchecker/functionaltests/attributename_fail.py
deleted file mode 100644
index 30016eb..0000000
--- a/twistedchecker/functionaltests/attributename_fail.py
+++ /dev/null
@@ -1,18 +0,0 @@
-# enable: C0103
-
-class FooClass:
-    """
-    Attribute names should be in mixed case,
-    with the first letter lower case,
-    each word separated by having its first letter capitalized
-    just like method names.
-    And all private names should begin with an underscore.
-    """
-
-    def __init__(self):
-        """
-        A init of this class
-        """
-        self.foo_bar_zar = None
-        self.FooBarZar = None
-        self.a = None
diff --git a/twistedchecker/functionaltests/attributename_fail.result b/twistedchecker/functionaltests/attributename_fail.result
deleted file mode 100644
index f64efab..0000000
--- a/twistedchecker/functionaltests/attributename_fail.result
+++ /dev/null
@@ -1,3 +0,0 @@
-18:C0103
-16:C0103
-17:C0103
diff --git a/twistedchecker/functionaltests/attributename_pass.result b/twistedchecker/functionaltests/attributename_pass.result
deleted file mode 100644
index e69de29..0000000