Allow for no exceptions to be raised and cleanup

Allow for no exceptions to be raised

Add some helpful comments

Remove doctest skips

Remove TODO note

Cleanup and skip failing doctest

skimage/_shared/_warnings.py

@@ -85,12 +85,15 @@ def expected_warnings(matching):
     Uses `all_warnings` to ensure all warnings are raised.
     Upon exiting, it checks the recorded warnings for the desired matching
     string.  Raises a warning if the match was not found or an Unexpected
-    warning is found.
+    warning is found.  You can pass an empty regex as on of the matches
+    to allow for no matches: "\A\Z".
 
     """
     with all_warnings() as w:
+        # enter context
         yield w
-        remaining = [m for m in matching]
+        # exited user context, check the recorded warnings
+        remaining = [m for m in matching if not '\A\Z' in m.split('|')]
         for warn in w:
             found = False
             for match in matching: