Python extremely puzzling regex unicode behaviour

Question

I use a tokenizer to split french sentences into words and had problems with words containing the french character â.

I tried to isolate the problem and it eventually boiled down to this simple fact:

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>>> re.match(r"’", u'â', re.U)
>>> re.match(r"[’]", u'â', re.U)
<_sre.SRE_Match object at 0x21d41d0>

â is matched by a pattern containing ’ if it's put in an ensemble matcher.

Is there something wrong on my part regarding UTF-8 handling or is it a bug?

My python version is:

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Python 2.7.3 (default, Jan  2 2013, 13:56:14) 
[GCC 4.7.2] on linux2

EDIT:

Hum, embarassingly enough, it seems that replacing the r prefixing the pattern with a u fixes the issue.

I wonder why the official documentation uses extensively r then :((

Answer 1

Your pattern should be a unicode string too:

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 >>> re.match(ur"’", u'â', re.U)
 >>> re.match(ur"[’]", u'â', re.U)

Otherwise apparently sre encodes â to latin-1 and finds the resulting byte in the three bytes that is a utf-8 ’.

"[’]" is equivalent to "[\xe2\x80\x99]", and u'â'.encode('latin-1') is \xe2.