i;unicode-casemap - Simple Unicode Collation Algorithm
RFC 5051
| Document | Type |
RFC - Proposed Standard
(October 2007; No errata)
Was draft-crispin-collation-unicasemap (individual in app area)
|
|
|---|---|---|---|
| Last updated | 2015-10-14 | ||
| Stream | IETF | ||
| Formats | plain text pdf html bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 5051 (Proposed Standard) | |
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Lisa Dusseault | ||
| Send notices to | alexey.melnikov@isode.com | ||
Network Working Group M. Crispin
Request for Comments: 5051 University of Washington
Category: Standards Track October 2007
i;unicode-casemap - Simple Unicode Collation Algorithm
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes "i;unicode-casemap", a simple case-
insensitive collation for Unicode strings. It provides equality,
substring, and ordering operations.
1. Introduction
The "i;ascii-casemap" collation described in [COMPARATOR] is quite
simple to implement and provides case-independent comparisons for the
26 Latin alphabetics. It is specified as the default and/or baseline
comparator in some application protocols, e.g., [IMAP-SORT].
However, the "i;ascii-casemap" collation does not produce
satisfactory results with non-ASCII characters. It is possible, with
a modest extension, to provide a more sophisticated collation with
greater multilingual applicability than "i;ascii-casemap". This
extension provides case-independent comparisons for a much greater
number of characters. It also collates characters with diacriticals
with the non-diacritical character forms.
This collation, "i;unicode-casemap", is intended to be an alternative
to, and preferred over, "i;ascii-casemap". It does not replace the
"i;basic" collation described in [BASIC].
2. Unicode Casemap Collation Description
The "i;unicode-casemap" collation is a simple collation which is
case-insensitive in its treatment of characters. It provides
equality, substring, and ordering operations. The validity test
operation returns "valid" for any input.
Crispin Standards Track [Page 1]
RFC 5051 i;unicode-casemap October 2007
This collation allows strings in arbitrary (and mixed) character
sets, as long as the character set for each string is identified and
it is possible to convert the string to Unicode. Strings which have
an unidentified character set and/or cannot be converted to Unicode
are not rejected, but are treated as binary.
Each input string is prepared by converting it to a "titlecased
canonicalized UTF-8" string according to the following steps, using
UnicodeData.txt ([UNICODE-DATA]):
(1) A Unicode codepoint is obtained from the input string.
(a) If the input string is in a known charset that can be
converted to Unicode, a sequence in the string's charset
is read and checked for validity according to the rules of
that charset. If the sequence is valid, it is converted
to a Unicode codepoint. Note that for input strings in
UTF-8, the UTF-8 sequence must be valid according to the
rules of [UTF-8]; e.g., overlong UTF-8 sequences are
invalid.
(b) If the input string is in an unknown charset, or an
invalid sequence occurs in step (1)(a), conversion ceases.
No further preparation is performed, and any partial
preparation results are discarded. The original string is
used unchanged with the i;octet comparator.
(2) The following steps, using UnicodeData.txt ([UNICODE-DATA]),
are performed on the resulting codepoint from step (1)(a).
(a) If the codepoint has a titlecase property in
UnicodeData.txt (this is normally the same as the
uppercase property), the codepoint is converted to the
codepoints in the titlecase property.
(b) If the resulting codepoint from (2)(a) has a decomposition
property of any type in UnicodeData.txt, the codepoint is
converted to the codepoints in the decomposition property.
This step is recursively applied to each of the resulting
codepoints until no more decomposition is possible
(effectively Normalization Form KD).
Example: codepoint U+01C4 (LATIN CAPITAL LETTER DZ WITH CARON)
has a titlecase property of U+01C5 (LATIN CAPITAL LETTER D
WITH SMALL LETTER Z WITH CARON). Codepoint U+01C5 has a
decomposition property of U+0044 (LATIN CAPITAL LETTER D)
U+017E (LATIN SMALL LETTER Z WITH CARON). U+017E has a
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