This came up while writing a regex to match characters that are valid in Python identifiers for Jinja. https://github.com/pallets/jinja/pull/731 `\w` matches all valid identifier characters except for 4 special cases:

import unicodedata
import re
import sys

cre = re.compile(r'\w')

for cp in range(sys.maxunicode + 1):
    s = chr(cp)

    if s.isidentifier() and not cre.match(s):
        print(hex(cp), unicodedata.name(s))

0x1885 MONGOLIAN LETTER ALI GALI BALUDA
0x1886 MONGOLIAN LETTER ALI GALI THREE BALUDA
0x2118 SCRIPT CAPITAL P
0x212e ESTIMATED SYMBOL

Python < 3.6 matches the two Mongolian characters, not sure why 3.6 stopped matching them.

For our case, we just added them to a character set, `[\w\u1885\u1886\u2118\u212e]`.

It can cause unexpected behavior when using `\b`, since that's defined as the transition from `\w` to `\W` and those 4 characters aren't in `\w`. `re.match(r'\b[\w\u212e', '℮')` fails to match.