The problem can also be played in reverse, maybe it is more telling:

```
# consider the following bytestring pattern
>>> p = b"(?P<\xc3\xba>)"

# what character does the group name correspond to?
# maybe we can try to infer it by decoding the bytestring?
# let's try to do it with the default encoding... that natural, right?
>>> p.decode()
'(?P<ú>)'

# so we can reasonably expect the group name to be ú, right?
>>> list(re.compile(p).groupindex.keys()).pop()
'ú'

# Fail.
```