str.isidentifier() works. The regex answers incorrectly fail to match some valid python identifiers and incorrectly match some invalid ones.
str.isidentifier() Return true if the string is a valid identifier
according to the language definition, section Identifiers and
keywords.
Use keyword.iskeyword() to test for reserved identifiers such as def
and class.
@martineau's comment gives the example of '℘᧚' where the regex solutions fail.
>>> '℘᧚'.isidentifier()
True
>>> import re
>>> bool(re.search(r'^[^\d\W]\w*\Z', '℘᧚'))
False
Why does this happen?
Lets define the sets of code points that match the given regular expression, and the set that match str.isidentifier.
import re
import unicodedata
chars = {chr(i) for i in range(0x10ffff) if re.fullmatch(r'^[^\d\W]\w*\Z', chr(i))}
identifiers = {chr(i) for i in range(0x10ffff) if chr(i).isidentifier()}
How many regex matches are not identifiers?
In [26]: len(chars - identifiers)
Out[26]: 698
How many identifiers are not regex matches?
In [27]: len(identifiers - chars)
Out[27]: 4
Interesting -- which ones?
In [37]: {(c, unicodedata.name(c), unicodedata.category(c)) for c in identifiers - chars}
Out[37]:
set([
('\u1885', 'MONGOLIAN LETTER ALI GALI BALUDA', 'Mn'),
('\u1886', 'MONGOLIAN LETTER ALI GALI THREE BALUDA', 'Mn'),
('℘', 'SCRIPT CAPITAL P', 'Sm'),
('℮', 'ESTIMATED SYMBOL', 'So'),
])
What's different about these two sets?
They have different Unicode "General Category" values.
In [31]: {unicodedata.category(c) for c in chars - identifiers}
Out[31]: set(['Lm', 'Lo', 'No'])
From wikipedia, that's Letter, modifier; Letter, other; Number, other. This is consistent with the re docs, since \d is only decimal digits:
\d Matches any Unicode decimal digit (that is, any character in Unicode character category [Nd])
What about the other way?
In [32]: {unicodedata.category(c) for c in identifiers - chars}
Out[32]: set(['Mn', 'Sm', 'So'])
That's Mark, nonspacing; Symbol, math; Symbol, other.
Where is this all documented?
Where is it implemented?
https://github.com/python/cpython/commit/47383403a0a11259acb640406a8efc38981d2255
I still want a regular expression
Look at the regex module on PyPI.
This regex implementation is backwards-compatible with the standard ‘re’ module, but offers additional functionality.
It includes filters for "General Category".
