> So b'\xe9' is mapped to \u00e9, it is `é`.

Yes but \xe9 is not strictly valid utf-8, or say not the canonical representation of "é". So there is no way to get \xe9 starting from é without leaving utf-8. So starting with é as group name, I cannot programmatically encode it into a bytes pattern.

> Of course, characters with Unicode code point greater than 0xff are impossible to appear in `bytes`.

But \xce and \x94 are both lower than \xff, yet using \xce\x94 ("Δ".encode()) in a group name fails.

According to the doc, the sole constraint on group names is that they have to be valid and unique Python identifiers. So this should work:

```
# Δ is a valid identifier
>>> "Δ".isidentifier()
True
>>> Δ = 1
>>> Δ
1
>>> import re
>>> name = "Δ"
>>> re.match(b"(?P<" + name.encode() + b">)", b"")
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    re.match(b"(?P<" + name.encode() + b">)", b"")
  File "/usr/lib/python3.8/re.py", line 191, in match
    return _compile(pattern, flags).match(string)
  File "/usr/lib/python3.8/re.py", line 304, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/usr/lib/python3.8/sre_compile.py", line 764, in compile
    p = sre_parse.parse(p, flags)
  File "/usr/lib/python3.8/sre_parse.py", line 948, in parse
    p = _parse_sub(source, state, flags & SRE_FLAG_VERBOSE, 0)
  File "/usr/lib/python3.8/sre_parse.py", line 443, in _parse_sub
    itemsappend(_parse(source, state, verbose, nested + 1,
  File "/usr/lib/python3.8/sre_parse.py", line 703, in _parse
    raise source.error(msg, len(name) + 1)
re.error: bad character in group name 'Î\x94' at position 4
re.match(b'(?P<\xce\x94>)', b'').groupdict()
```