[Bug regex/12045] regex range semantics outside of POSIX should be documented and consistent

["eblake at redhat dot com" <sourceware-bugzilla@sourceware.org>]

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------- Additional Comments From eblake at redhat dot com  2010-09-21 22:17 -------
Actually, according to POSIX 2008, there was a requirement in older POSIX that
range expressions be treated as CEO (collating element order) for all locales,
but this was specifically relaxed in POSIX 2001.  If glibc is going to insist on
CEO ordering because of a version of POSIX two editions ago, it would be nice to
see that documented.  Then again, other glibc interfaces no longer comply with
the stricter requirements in older POSIX that have since been relaxed (for
example, whether getopt() must include an error message with "illegal" in the
string). so I see no reason to tie regex to the older standard's CEO ordering
either.

XRAT A.9.3.5: 
http://www.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap09.html

Historical implementations used native character order to interpret range
expressions. The ISO POSIX-2:1993 standard instead required collating element
order (CEO): the order that collating elements were specified between the
order_start and order_end keywords in the LC_COLLATE category of the current
locale. CEO had some advantages in portability over the native character order,
but it also had some disadvantages:

    * CEO could not feasibly be mimicked in user code, leading to
inconsistencies between POSIX matchers and matchers in popular user programs
like Emacs, ksh, and Perl.
    * CEO caused range expressions to match accented and capitalized letters
contrary to many users' expectations. For example, "[a-e]" typically matched
both 'E' and 'á' but neither 'A' nor 'é' .
    * CEO was not consistent across implementations. In practice, CEO was often
less portable than native character order. For example, it was common for the
CEOs of two implementation-supplied locales to disagree, even if both locales
were named "da_DK" .

Because of these problems, some implementations of regular expressions continued
to use native character order. Others used the collation sequence, which is more
consistent with sorting than either CEO or native order, but which departs
further from the traditional POSIX semantics because it generally requires
"[a-e]" to match either 'A' or 'E' but not both. As a result of this kind of
implementation variation, programmers who wanted to write portable regular
expressions could not rely on the ISO POSIX-2:1993 standard guarantees in practice.

While revising the standard, lengthy consideration was given to proposals to
attack this problem by adding an API for querying the CEO to allow user-mode
matchers, but none of these proposals had implementation experience and none
achieved consensus. Leaving the standard alone was also considered, but rejected
due to the problems described above.

The current standard leaves unspecified the behavior of a range expression
outside the POSIX locale. This makes it clearer that conforming applications
should avoid range expressions outside the POSIX locale, and it allows
implementations and compatible user-mode matchers to interpret range expressions
using native order, CEO, collation sequence, or other, more advanced techniques.
The concerns which led to this change were raised in IEEE PASC interpretation
1003.2 #43 and others, and related to ambiguities in the specification of how
multi-character collating elements should be handled in range expressions. These
ambiguities had led to multiple interpretations of the specification, in
conflicting ways, which led to varying implementations. As noted above, efforts
were made to resolve the differences, but no solution has been found that would
be specific enough to allow for portable software while not invalidating
existing implementations.


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http://sourceware.org/bugzilla/show_bug.cgi?id=12045

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