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Date: Wed, 2 Jan 2013 06:17:04 -0800 (PST)
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Subject: Re: ignore case only for a part of the regex?
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Xref: csiph.com comp.lang.python:35987

Le mercredi 2 janvier 2013 00:09:45 UTC+1, Vlastimil Brom a =C3=A9crit=C2=
=A0:
> 2013/1/1 Steven D'Aprano <steve+comp.lang.python@pearwood.info>:
>=20
> > On Sun, 30 Dec 2012 10:20:19 -0500, Roy Smith wrote:
>=20
> >
>=20
> >> The way I would typically do something like this is build my regexes i=
n
>=20
> >> all lower case and .lower() the text I was matching against them.  I'm
>=20
> >> curious what you're doing where you want to enforce case sensitivity i=
n
>=20
> >> one part of a header, but not in another.
>=20
> >
>=20
> > Well, sometimes you have things that are case sensitive, and other thin=
gs
>=20
> > which are not, and sometimes you need to match them at the same time. I
>=20
> > don't think this is any more unusual than (say) wanting to match an
>=20
> > otherwise lowercase word whether or not it comes at the start of a
>=20
> > sentence:
>=20
> >
>=20
> > "[Pp]rogramming"
>=20
> >
>=20
> > is conceptually equivalent to "match case-insensitive `p`, and case-
>=20
> > sensitive `rogramming`".
>=20
> >
>=20
> >
>=20
> > By the way, although there is probably nothing you can (easily) do abou=
t
>=20
> > this prior to Python 3.3, converting to lowercase is not the right way =
to
>=20
> > do case-insensitive matching. It happens to work correctly for ASCII, b=
ut
>=20
> > it is not correct for all alphabetic characters.
>=20
> >
>=20
> >
>=20
> > py> 'Stra=C3=9Fe'.lower()
>=20
> > 'stra=C3=9Fe'
>=20
> > py> 'Stra=C3=9Fe'.upper()
>=20
> > 'STRASSE'
>=20
> >
>=20
> >
>=20
> > The right way is to casefold first, then match:
>=20
> >
>=20
> > py> 'Stra=C3=9Fe'.casefold()
>=20
> > 'strasse'
>=20
> >
>=20
> >
>=20
> > Curiously, there is an uppercase =C3=9F in old German. In recent years =
some
>=20
> > typographers have started using it instead of SS, but it's still rare,
>=20
> > and the official German rules have =C3=9F transform into SS and vice ve=
rsa.
>=20
> > It's in Unicode, but few fonts show it:
>=20
> >
>=20
> > py> unicodedata.lookup('LATIN CAPITAL LETTER SHARP S')
>=20
> > '=E1=BA=9E'
>=20
> >
>=20
> >
>=20
> >
>=20
> > --
>=20
> > Steven
>=20
> > --
>=20
> > http://mail.python.org/mailman/listinfo/python-list
>=20
>=20
>=20
> Hi,
>=20
> just for completeness, the mentioned regex library can take care of
>=20
> casfolding in case insensitive matching (in all supported versions:
>=20
> Python 2.5-2.7 and 3.1-3.3); i.e.:
>=20
> # case sensitive match:
>=20
> >>> for m in regex.findall(ur"Stra=C3=9Fe", u" STRA=C3=9FE STRASSE STRA=
=E1=BA=9EE Strasse Stra=C3=9Fe "): print m
>=20
> ...
>=20
> Stra=C3=9Fe
>=20
>=20
>=20
> # case insensitive match:
>=20
> >>> for m in regex.findall(ur"(?i)Stra=C3=9Fe", u" STRA=C3=9FE STRASSE ST=
RA=E1=BA=9EE Strasse Stra=C3=9Fe "): print m
>=20
> ...
>=20
> STRA=C3=9FE
>=20
> STRA=E1=BA=9EE
>=20
> Stra=C3=9Fe
>=20
>=20
>=20
> # case insensitive match with casefolding:
>=20
> >>> for m in regex.findall(ur"(?if)Stra=C3=9Fe", u" STRA=C3=9FE STRASSE S=
TRA=E1=BA=9EE Strasse Stra=C3=9Fe "): print m
>=20
> ...
>=20
> STRA=C3=9FE
>=20
> STRASSE
>=20
> STRA=E1=BA=9EE
>=20
> Strasse
>=20
> Stra=C3=9Fe
>=20
> >>>
>=20
> >>>
>=20
>=20
>=20
> # after enabling the backwards incompatible modern matching behaviour,
>=20
> casefolding is by default turned on for case insensitive matches
>=20
> >>> for m in regex.findall(ur"(?V1i)Stra=C3=9Fe", u" STRA=C3=9FE STRASSE =
STRA=E1=BA=9EE Strasse Stra=C3=9Fe "): print m
>=20
> ...
>=20
> STRA=C3=9FE
>=20
> STRASSE
>=20
> STRA=E1=BA=9EE
>=20
> Strasse
>=20
> Stra=C3=9Fe
>=20
> >>>
>=20
>=20
>=20
>=20
>=20
> As a small addition, the originally posted pattern r'^Msg-(?:(?i)id):'
>=20
> would actually work as expected in this modern matching mode in regex
>=20
> - enabled with the V1 flag. In this case the flag-setting (?i) only
>=20
> affects the following parts of the pattern, not the whole pattern like
>=20
> in the current "re" and V0-compatibility-mode "regex"
>=20
>=20
>=20
> >>> regex.findall(r"(?V1)Msg-(?:(?i)id):", "the regex should match  Msg-i=
d:, Msg-Id:, ...  but not  msg-id:, MSG-ID:  and so on")
>=20
> ['Msg-id:', 'Msg-Id:']
>=20

------

Vlastimil:

Excellent.

-----

Steven:

..." It's in Unicode, but few fonts show it:" ...

Das grosse Eszett is a member of the unicode subsets MES-2, WGL-4.
Good - serious - fonts are via OpenType MES-2 or WGL-4 compliant.
So, it is a no problem.

I do not know (and I did not check) if the code point, 1e9e, is part of
the utf32 table.

jmf