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 [\W\d_] and consequently what we do want is anything in the character class [^\W\d_]

Here's a simple example (Python 2.6).

>>> import re >>> rx = re.compile("[^\W\d_]+", 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"\u0473\u0660\u06c9\u24e8\u4e0a\u3020\u3021" >>> 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.

Answer 2

What about:

\p{L} 

You can to use this document as reference: Unicode Regular Expressions

EDIT: Seems Python doesn't handle Unicode expressions. Take a look into this link: Handling Accented Characters with Python Regular Expressions -- [A-Z] just isn't good enough (no longer active, link to internet archive)

Another references:

• re.UNICODE
• python and regular expression with unicode
• Unicode Technical Standard #18: Unicode Regular Expressions

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For posterity, here are the examples on the blog:

import re string = 'riché' print string riché  richre = re.compile('([A-z]+)') match = richre.match(string) print match.groups() ('rich',)  richre = re.compile('(\w+)',re.LOCALE) match = richre.match(string) print match.groups() ('rich',)  richre = re.compile('([é\w]+)') match = richre.match(string) print match.groups() ('rich\xe9',)  richre = re.compile('([\xe9\w]+)') match = richre.match(string) print match.groups() ('rich\xe9',)  richre = re.compile('([\xe9-\xf8\w]+)') match = richre.match(string) print match.groups() ('rich\xe9',)  string = 'richéñ' match = richre.match(string) print match.groups() ('rich\xe9\xf1',)  richre = re.compile('([\u00E9-\u00F8\w]+)') print match.groups() ('rich\xe9\xf1',)  matched = match.group(1) print matched richéñ