regular expressions and the LOCALE flag
in [Python]
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From: Baz Walter on 3 Aug 2010 13:56 the python docs say that re.LOCALE makes certain character classes "dependent on the current locale". here's what i currently see on my system: >>> import re, locale >>> locale.getdefaultlocale() ('en_GB', 'UTF8') >>> locale.getlocale() (None, None) >>> re.findall(r'\w', u'a b c \xe5 \xe6 \xe7', re.L) [u'a', u'b', u'c'] >>> locale.setlocale(locale.LC_ALL, 'en_GB.ISO 8859-1') 'en_GB.ISO 8859-1' >>> re.findall(r'\w', u'\xe5 \xe6 \xe7 a b c', re.L) [u'\xe5', u'\xe6', u'\xe7', u'a', u'b', u'c'] >>> locale.setlocale(locale.LC_ALL, 'en_GB.UTF-8') 'en_GB.UTF-8' >>> re.findall(r'\w', u'a b c \xe5 \xe6 \xe7', re.L) [u'a', u'b', u'c'] it seems wrong to me that re.LOCALE fails to give the "right" result when the local encoding is utf8 - i think it should give the same result as re.UNICODE. is this a bug, or does the documentation just need to be made clearer?
From: MRAB on 3 Aug 2010 14:40 Baz Walter wrote: > the python docs say that re.LOCALE makes certain character classes > "dependent on the current locale". > > here's what i currently see on my system: > > >>> import re, locale > >>> locale.getdefaultlocale() > ('en_GB', 'UTF8') > >>> locale.getlocale() > (None, None) > >>> re.findall(r'\w', u'a b c \xe5 \xe6 \xe7', re.L) > [u'a', u'b', u'c'] > >>> locale.setlocale(locale.LC_ALL, 'en_GB.ISO 8859-1') > 'en_GB.ISO 8859-1' > >>> re.findall(r'\w', u'\xe5 \xe6 \xe7 a b c', re.L) > [u'\xe5', u'\xe6', u'\xe7', u'a', u'b', u'c'] > >>> locale.setlocale(locale.LC_ALL, 'en_GB.UTF-8') > 'en_GB.UTF-8' > >>> re.findall(r'\w', u'a b c \xe5 \xe6 \xe7', re.L) > [u'a', u'b', u'c'] > > it seems wrong to me that re.LOCALE fails to give the "right" result > when the local encoding is utf8 - i think it should give the same result > as re.UNICODE. > > is this a bug, or does the documentation just need to be made clearer? re.LOCALE just passes the character to the underlying C library. It really only works on bytestrings which have 1 byte per character. UTF-8 encodes codepoints outside the ASCII range to multiple bytes per codepoint, and the re module will treat each of those bytes as a separate character. And, BTW, none of your examples pass a UTF-8 bytestring to re.findall: all those string literals starting with the 'u' prefix are Unicode strings! Locale encodings are more trouble than they're worth. Unicode is better. :-)
From: Baz Walter on 3 Aug 2010 15:40 On 03/08/10 19:40, MRAB wrote: > Baz Walter wrote: >> the python docs say that re.LOCALE makes certain character classes >> "dependent on the current locale". > > re.LOCALE just passes the character to the underlying C library. It > really only works on bytestrings which have 1 byte per character. the re docs don't specify 8-bit encodings: they just refer to the 'current locale'. > And, BTW, none of your examples pass a UTF-8 bytestring to re.findall: > all those string literals starting with the 'u' prefix are Unicode > strings! not sure what you mean by this: if the string was encoded as utf8, '\w' still wouldn't match any of the non-ascii characters. > Locale encodings are more trouble than they're worth. Unicode is better. > :-) yes, i'm really just trying to decide whether i should offer 'locale' as an option in my program. given the unintuitive way re.LOCALE works, i'm not sure that i should. are you saying that it only really makes sense for *bytestrings* to be used with re.LOCALE? if so, the re docs certainly don't make that clear.
From: MRAB on 3 Aug 2010 16:24 Baz Walter wrote: > On 03/08/10 19:40, MRAB wrote: >> Baz Walter wrote: >>> the python docs say that re.LOCALE makes certain character classes >>> "dependent on the current locale". >> >> re.LOCALE just passes the character to the underlying C library. It >> really only works on bytestrings which have 1 byte per character. > > the re docs don't specify 8-bit encodings: they just refer to the > 'current locale'. > >> And, BTW, none of your examples pass a UTF-8 bytestring to re.findall: >> all those string literals starting with the 'u' prefix are Unicode >> strings! > > not sure what you mean by this: if the string was encoded as utf8, '\w' > still wouldn't match any of the non-ascii characters. > Strings with the 'u' prefix are Unicode strings, not bytestrings. They don't have an encoding. A UTF-8 string is a bytestring in which the bytes represent Unicode codepoints encoded as UTF-8. >> Locale encodings are more trouble than they're worth. Unicode is better. >> :-) > > yes, i'm really just trying to decide whether i should offer 'locale' as > an option in my program. given the unintuitive way re.LOCALE works, i'm > not sure that i should. > > are you saying that it only really makes sense for *bytestrings* to be > used with re.LOCALE? > > if so, the re docs certainly don't make that clear. The re module can match against 3 types of string: 1. ASCII (default in Python 2): bytestring with characters in the ASCII range (1 byte per character). However, it doesn't complain if it sees bytes/characters outside the ASCII range. 2. LOCALE: bytestring with characters in the current locale (but only 1 byte per character). Characters are categorised according to the underlying C library; for example, 'a' is a letter if isalpha('a') returns true. 3. UNICODE (default in Python 3): Unicode string.
From: Baz Walter on 3 Aug 2010 19:27
On 03/08/10 21:24, MRAB wrote: >>> And, BTW, none of your examples pass a UTF-8 bytestring to re.findall: >>> all those string literals starting with the 'u' prefix are Unicode >>> strings! >> >> not sure what you mean by this: if the string was encoded as utf8, >> '\w' still wouldn't match any of the non-ascii characters. >> > Strings with the 'u' prefix are Unicode strings, not bytestrings. They > don't have an encoding. well, they do if they are given one, as i suggested! to be explicit, if the local encoding is 'utf8', none of the following will get a hit: (1) re.findall(r'\w', '\xe5 \xe6 \xe7', re.L) (2) re.findall(r'\w', u'\xe5 \xe6 \xe7'.encode('utf8'), re.L) (3) re.findall(r'\w', u'\xe5 \xe6 \xe7', re.L) so i still don't know what you meant about passing a 'UTF-8 bytestring' in your first comment :) only (3) could feasibly get a hit - and then only if the re module was smart enough to fall back to re.UNICODE for utf8 (and any other encodings of unicode it might know about). > 2. LOCALE: bytestring with characters in the current locale (but only 1 > byte per character). Characters are categorised according to the > underlying C library; for example, 'a' is a letter if isalpha('a') > returns true. this is actually what my question was about. i suspected something like this might be the case, but i can't actually see it stated anywhere in the docs. maybe it's just me, but 'current locale' doesn't naturally imply 'only 8-bit encodings'. i would have thought it implied 'whatever encoding is discovered on the local system' - and these days, that's very commonly utf8. is there actually a use case for it working the way it currently does? it seems just broken to have it depending so heavily on implementation details. > 3. UNICODE (default in Python 3): Unicode string. i've just read the python3 re docs, and they do now make an explicit distinction between matching bytes (with the new re.ASCII flag) and matching textual characters (i.e. unicode, the default). the re.LOCALE flag is still there, and there are now warnings about it's unreliability - but it still doesn't state that it can only work properly if the local encoding is 8-bit. |