diff --git a/pyls/_utils.py b/pyls/_utils.py
index fa7d6cd5..1980b28f 100644
--- a/pyls/_utils.py
+++ b/pyls/_utils.py
@@ -202,4 +202,5 @@ def is_process_alive(pid):
 
 
 def camel_to_underscore(camelcase):
-    return re.sub('([A-Z]+)', r'_\1', camelcase).lower()
+    s1 = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', camelcase)
+    return re.sub('([a-z0-9])([A-Z])', r'\1_\2', s1).lower()
diff --git a/test/test_utils.py b/test/test_utils.py
index bf48920d..f8ece3a3 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -93,3 +93,9 @@ def test_camel_to_underscore():
     assert _utils.camel_to_underscore('camelCase') == 'camel_case'
     assert _utils.camel_to_underscore('hangClosing') == 'hang_closing'
     assert _utils.camel_to_underscore('ignore') == 'ignore'
+    assert _utils.camel_to_underscore('CamelCase') == 'camel_case'
+    assert _utils.camel_to_underscore('SomeLongCamelCase') == 'some_long_camel_case'
+    assert _utils.camel_to_underscore('already_using_underscore') == 'already_using_underscore'
+    assert _utils.camel_to_underscore('Using_only_someUnderscore') == 'using_only_some_underscore'
+    assert _utils.camel_to_underscore('Using_Only_Some_underscore') == 'using_only_some_underscore'
+    assert _utils.camel_to_underscore('ALL_UPPER_CASE') == 'all_upper_case'