regex - python-re: How do I match an alpha character

Question

How can I match an alpha character with a regular expression. I want a character that is in w but is not in d. I want it unicode compatible that's why I cannot use [a-zA-Z].

Answer 1

Your first two sentences contradict each other. "in w but is not in d" includes underscore. I'm assuming from your third sentence that you don't want underscore.

Using a Venn diagram on the back of an envelope helps. Let's look at what we DON'T want:

(1) characters that are not matched by w (i.e. don't want anything that's not alpha, digits, or underscore) => W
(2) digits => d
(3) underscore => _

So what we don't want is anything in the character class [Wd_] and consequently what we do want is anything in the character class [^Wd_]

Here's a simple example (Python 2.6).

>>> import re
>>> rx = re.compile("[^Wd_]+", re.UNICODE)
>>> rx.findall(u"abc_def,k9")
[u'abc', u'def', u'k']

Further exploration reveals a few quirks of this approach:

>>> import unicodedata as ucd
>>> allsorts =u"u0473u0660u06c9u24e8u4e0au3020u3021"
>>> for x in allsorts:
...     print repr(x), ucd.category(x), ucd.name(x)
...
u'u0473' Ll CYRILLIC SMALL LETTER FITA
u'u0660' Nd ARABIC-INDIC DIGIT ZERO
u'u06c9' Lo ARABIC LETTER KIRGHIZ YU
u'u24e8' So CIRCLED LATIN SMALL LETTER Y
u'u4e0a' Lo CJK UNIFIED IDEOGRAPH-4E0A
u'u3020' So POSTAL MARK FACE
u'u3021' Nl HANGZHOU NUMERAL ONE
>>> rx.findall(allsorts)
[u'u0473', u'u06c9', u'u4e0a', u'u3021']

U+3021 (HANGZHOU NUMERAL ONE) is treated as numeric (hence it matches w) but it appears that Python interprets "digit" to mean "decimal digit" (category Nd) so it doesn't match d

U+2438 (CIRCLED LATIN SMALL LETTER Y) doesn't match w

All CJK ideographs are classed as "letters" and thus match w

Whether any of the above 3 points are a concern or not, that approach is the best you will get out of the re module as currently released. Syntax like p{letter} is in the future.