There are some issues with handling the -W option:

1. A traceback is printed for some invalid category names.

$ ./python -Wignore::0
'import warnings' failed; traceback:
Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 542, in <module>
    _processoptions(sys.warnoptions)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 208, in _processoptions
    _setoption(arg)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 224, in _setoption
    category = _getcategory(category)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 271, in _getcategory
    if not issubclass(cat, Warning):
TypeError: issubclass() arg 1 must be a class

$ ./python -Wignore::0a 
'import warnings' failed; traceback:
Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 542, in <module>
    _processoptions(sys.warnoptions)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 208, in _processoptions
    _setoption(arg)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 224, in _setoption
    category = _getcategory(category)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 256, in _getcategory
    cat = eval(category)
  File "<string>", line 1
    0a
     ^
SyntaxError: unexpected EOF while parsing

$ ./python -Wignore::=
'import warnings' failed; traceback:
Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 542, in <module>
    _processoptions(sys.warnoptions)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 208, in _processoptions
    _setoption(arg)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 224, in _setoption
    category = _getcategory(category)
  File "/home/serhiy/py/cpython/Lib/warnings.py", line 264, in _getcategory
    m = __import__(module, None, None, [klass])
ValueError: Empty module name

In normal case Python just complains:

$ ./python -Wignore::unknown
Invalid -W option ignored: unknown warning category: 'unknown'

2. For non-ascii warning names Python complains about a module and strips the last character:

$ ./python -Wignore::Wärning
Invalid -W option ignored: invalid module name: 'Wärnin'

3. The re module is always imported is the -W option is used, even if this is not needed.

New changeset 41ec17e45d54473d32f543396293256f1581e44d by Serhiy Storchaka in branch 'master':
bpo-39056: Fix handling invalid warning category in the -W option. (GH-17618)
https://github.com/python/cpython/commit/41ec17e45d54473d32f543396293256f1581e44d

New changeset e1caa49f68dd63b534774aebad0c240143e6fb5d by Miss Islington (bot) in branch '3.8':
bpo-39056: Fix handling invalid warning category in the -W option. (GH-17618)
https://github.com/python/cpython/commit/e1caa49f68dd63b534774aebad0c240143e6fb5d

New changeset 43fbc70360b2a934ea809b2175d7e99031db2df3 by Miss Islington (bot) in branch '3.7':
bpo-39056: Fix handling invalid warning category in the -W option. (GH-17618)
https://github.com/python/cpython/commit/43fbc70360b2a934ea809b2175d7e99031db2df3