RFC2482 - Language Tagging in Unicode Plain Text

Network Working Group K. Whistler
Request for Comments: 2482 Sybase
Category: Informational G. Adams
Spyglass
January 1999
Language Tagging in Unicode Plain Text
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
IESG Note:
This document has been accepted by ISO/IEC JTC1/SC2/WG2 in meeting
#34 to be submitted as a recommendation from WG2 for inclusion in
Plane 14 in part 2 of ISO/IEC 10646.
1. Abstract
This document proposed a mechanism for language tagging in [UNICODE]
plain text. A set of special-use tag characters on Plane 14 of
[ISO10646] (Accessible through UTF-8, UTF-16, and UCS-4 encoding
forms) are proposed for encoding to enable the spelling out of
ASCII-based string tags using characters which can be strictly
separated from ordinary text content characters in ISO10646 (or
UNICODE).
One tag identification character and one cancel tag character are
also proposed. In particular, a language tag identification character
is proposed to identify a language tag string specifically; the
language tag itself makes use of [RFC1766] language tag strings
spelled out using the Plane 14 tag characters. Provision of a
specific, low-overhead mechanism for embedding language tags in plain
text is aimed at meeting the need of Internet Protocols such as ACAP,
which require a standard mechanism for marking language in UTF-8
strings.
The tagging mechanism as well the characters proposed in this
document have been approved by the Unicode Consortium for inclusion
in The Unicode Standard. However, implementation of this decision
awaits formal acceptance by ISO JTC1/SC2/WG2, the working group
responsible for ISO10646. Potential implementers should be aware that
until this formal acceptance occurs, any usage of the characters
proposed herein is strictly eXPerimental and not sanctioned for
standardized character data interchange.
2. Definitions and Notation
No attempt is made to define all terms used in this document. In
particular, the terminology pertaining to the subject of coded
character systems is not explicitly specified. See [UNICODE],
[ISO10646], and [RFC2130] for additional definitions in this area.
2.1 Requirements Notation
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY"
appear capitalized, they are being used to indicate particular
requirements of this specification. A discussion of the meanings of
these terms appears in [RFC2119].
2.2 Definitions
The terms defined below are used in special senses and thus warrant
some clarification.
2.2.1 Tagging
The association of attributes of text with a point or range of the
primary text. (The value of a particular tag is not generally
considered to be a part of the "content" of the text. Typical
examples of tagging is to mark language or font of a portion of
text.)
2.2.2 Annotation
The association of secondary textual content with a point or range of
the primary text. (The value of a particular annotation *is*
considered to be a part of the "content" of the text. Typical
examples include glossing, citations, exemplication, Japanese yomi,
etc.)
2.2.3 Out-of-band
An out-of-band channel conveys a tag in such a way that the textual
content, as encoded, is completely untouched and unmodified. This is
typically done by metadata or hyperstructure of some sort.
2.2.4 In-band
An in-band channel conveys a tag along with the textual content,
using the same basic encoding mechanism as the text itself. This is
done by various means, but an obvious example is SGML markup, where
the tags are encoded in the same character set as the text and are
interspersed with and carried along with the text data.
3.0 Background
There has been much discussion over the last 8 years of language
tagging and of other kinds of tagging of Unicode plain text. It is
fair to say that there is more-or-less universal agreement that
language tagging of Unicode plain text is required for certain
textual processes. For example, language "hinting" of multilingual
text is necessary for multilingual spell-checking based on multiple
dictionaries to work well. Language tagging provides a minimum level
of required information for text-to-speech processes to work
correctly. Language tagging is regularly done on web pages, to
enable selection of alternate content, for example.
However, there has been a great deal of controversy regarding the
appropriate placement of language tags. Some have held that the only
appropriate placement of language tags (or other kinds of tags) is
out-of-band, making use of attributed text structures or metadata.
Others have argued that there are requirements for lower-complexity
in-band mechanisms for language tags (or other tags) in plain text.
The controversy has been muddied by the existence and widespread use
of a number of in-band text markup mechanisms (Html, text/enriched,
etc.) which enable language tagging, but which imply the use of
general parsing mechanisms which are deemed too "heavyweight" for
protocol developers and a number of other applications. The
difficulty of using general in-band text markup for simple protocols
derives from the fact that some characters are used both for textual
content and for the text markup; this makes it more difficult to
write simple, fast algorithms to find only the textual content and
ignore the tags, or vice versa. (Think of this as the algorithmic
equivalent of the difficulty the human reader has attempting to read
just the content of raw HTML source text without a browser
interpreting all the markup tags.)
The Plane 14 proposal addresses the recurrent and persistent call for
a lighter-weight mechanism for text tagging than typical text markup
mechanisms in Unicode. It proposes a special set of characters used
*only* for tagging. These tag characters can be embedded into plain
text and can be identified and/or ignored with trivial algorithms,
since there is no overloading of usage for these tag characters--they
can only express tag values and never textual content itself.
The Plane 14 proposal is not intended for general annotation of text,
such as textual citations, phonetic readings (e.g. Japanese Yomi),
etc. In its present form, its use is intended to be restriced solely
to specifying in-line language tags. Future extensions may widen
this scope of intended usage.
4.0 Proposal
This proposal suggests the use of 97 dedicated tag characters encoded
at the start of Plane 14 of ISO/IEC 10646 consisting of a clone of
the 94 printable 7-bit ASCII graphic characters and ASCII SPACE, as
well as a tag identification character and a tag cancel character.
These tag characters are to be used to spell out any ASCII-based
tagging scheme which needs to be embedded in Unicode plain text. In
particular, they can be used to spell out language tags in order to
meet the expressed requirements of the ACAP protocol and the likely
requirements of other new protocols following the guidelines of the
IAB character workshop (RFC2130).
The suggested range in Plane 14 for the block reserved for tag
characters is as follows, expressed in each of the three most
generally used encoding schemes for ISO/IEC 10646:
UCS-4
U-000E0000 .. U-000E007F
UTF-16
U+DB40 U+DC00 .. U+DB40 U+DC7F
UTF-8
0xF3 0xA0 0x80 0x80 .. 0xF3 0xA0 0x81 0xBF
Of this range, U-000E0020 .. U-000E007E is the suggested range for
the ASCII clone tag characters themselves.
4.1 Names for the Tag Characters
The names for the ASCII clone tag characters should be exactly the
ISO 10646 names for 7-bit ASCII, prefixed with the Word "TAG".
In addition, there is one tag identification character and a CANCEL
TAG character. The use and syntax of these characters is described in
detail below.
The entire encoding for the proposed Plane 14 tag characters and
names of those characters can be derived from the following list.
(The encoded values here and throughout this proposal are listed in
UCS-4 form, which is easiest to interpret. It is assumed that most
Unicode applications will, however, be making use either of UTF-16 or
UTF-8 encoding forms for actual implementation.)
U-000E0000 <reserved>
U-000E0001 LANGUAGE TAG
U-000E0002 <reserved>
U-000E001F <reserved>
U-000E0020 TAG SPACE
U-000E0021 TAG EXCLAMATION MARK
U-000E0041 TAG LATIN CAPITAL LETTER A
U-000E007A TAG LATIN SMALL LETTER Z
U-000E007E TAG TILDE
U-000E007F CANCEL TAG
4.2 Range Checking for Tag Characters
The range checks required for code testing for tag characters would
be as follows. The same range check is expressed here in C for each
of the three significant encoding forms for 10646.
Range check expressed in UCS-4:
if ( ( *s >= 0xE0000 ) ( *s <= 0xE007F ) )
Range check expressed in UTF-16 (Unicode):
if ( ( *s == 0xDB40 ) && ( *(s+1) >= 0xDC00 ) && ( *(s+1) <= 0xDC7F ) )
Expressed in UTF-8:
if ( ( *s == 0xF3 ) && ( *(s+1) == 0xA0 ) && ( *(s+2) & 0xE0 == 0x80 )
Because of the choice of the range for the tag characters, it would
also be possible to express the range check for UCS-4 or UTF-16 in
terms of bitmask operations, as well.
4.3 Syntax for Embedding Tags
The use of the Plane 14 tag characters is very simple. In order to
embed any ASCII-derived tag in Unicode plain text, the tag is simply
spelled out with the tag characters instead, prefixed with the
relevant tag identification character. The resultant string is
embedded directly in the text.
The tag identification character is used as a mechanism for
identifying tags of different types. This enables multiple types of
tags to coexist amicably embedded in plain text and solves the
problem of delimitation if a tag is concatenated directly onto
another tag. Although only one type of tag is currently specified,
namely the language tag, the encoding of other tag identification
characters in the future would allow for distinct tag types to be
used.
No termination character is required for a tag. A tag terminates
either when the first non Plane 14 Tag Character (i.e. any other
normal Unicode value) is encountered, or when the next tag
identification character is encountered.
All tag arguments must be encoded only with the tag characters U-
000E0020 .. U-000E007E. No other characters are valid for expressing
the tag argument.
A detailed BNF syntax for tags is listed below.
4.4 Tag Scope and Nesting
The value of an established tag continues from the point the tag is
embedded in text until either:
A. The text itself goes out of scope, as defined by the
application. (E.g. for line-oriented protocols, when reaching
the end-of-line or end-of-string; for text streams, when
reaching the end-of-stream; etc.)
or
B. The tag is explicitly cancelled by the CANCEL TAG character.
Tags of the same type cannot be nested in any way. The appearance of
a new embedded language tag, for example, after text which was
already language tagged, simply changes the tagged value for
subsequent text to that specified in the new tag.
Tags of different type can have interdigitating scope, but not
hierarchical scope. In effect, tags of different type completely
ignore each other, so that the use of language tags can be completely
asynchronous with the use of character set source tags (or any other
tag type) in the same text in the future.
4.5 Cancelling Tag Values
U-000E007F CANCEL TAG is provided to allow the specific cancelling of
a tag value. The use of CANCEL TAG has the following syntax. To
cancel a tag value of a particular type, prefix the CANCEL TAG
character with the tag identification character of the appropriate
type. For example, the complete string to cancel a language tag is:
U-000E0001 U-000E007F
The value of the relevant tag type returns to the default state for
that tag type, namely: no tag value specified, the same as untagged
text.
The use of CANCEL TAG without a prefixed tag identification character
cancels *any* Plane 14 tag values which may be defined. Since only
language tags are currently provided with an explicit tag
identification character, only language tags are currently affected.
The main function of CANCEL TAG is to make possible such operations
as blind concatenation of strings in a tagged context without the
propagation of inappropriate tag values across the string boundaries.
For example, a string tagged with a Japanese language tag can have
its tag value "sealed off" with a terminating CANCEL TAG before
another string of unknown language value is concatenated to it. This
would prevent the string of unknown language from being erroneously
marked as being Japanese simply because of a concatenation to a
Japanese string.
4.6 Tag Syntax Description
An extended BNF (Backus-Naur Form) description of the tags specified
in this proposal is found below. Note the following BNF extensions
used in this formalism:
1. Semantic constraints are specified by rules in the form of an
assertion specified between double braces; the variable $$ denotes
the string consisting of all terminal symbols matched by the this
non-terminal.
Example: {{ Assert ( $$[0] == "?" ); }}
Meaning: The first character of the string matched by this
non-terminal must be "?"
2. A number of predicate functions are employed in semantic
constraint rules which are not otherwise defined; their name is
sufficient for determining their predication.
Example: IsRFC1766LanguageIdentifier ( tag-argument )
Meaning: tag-argument is a valid RFC1766 language identifier
3. A lexical expander function, TAG, is employed to denote the tag
form of an ASCII character; the argument to this function is
either a character or a character set specified by a range or
enumeration expression.
Example: TAG("-")
Meaning: TAG HYPHEN-MINUS
Example: TAG([A-Z])
Meaning: TAG LATIN CAPITAL LETTER A ...
TAG LATIN CAPITAL LETTER Z
4. A macro is employed to denote terminal symbols that are character
literals which can"t be directly represented in ASCII. The
argument to the macro is the UNICODE (ISO/IEC 10646) character
name.
Example: "${TAG CANCEL}"
Meaning: character literal whose code value is U-000E007F
5. Occurrence indicators used are "+" (one or more) and "*" (zero or
more); optional occurrence is indicated by enclosure in "[" and
"]".
4.6.1 Formal Tag Syntax
tag : language-tag
cancel-all-tag
;
language-tag : language-tag-introducer language-tag-argument
;
language-tag-argument : tag-argument
{{ Assert ( IsRFC1766LanguageIdentifier ( $$ ); }}
tag-cancel
;
cancel-all-tag : tag-cancel
;
tag-argument : tag-character+
;
tag-character : { c : c in
TAG( { a : a in printable ASCII characters or SPACE } ) }
;
language-tag-introducer : "${TAG LANGUAGE}"
;
tag-cancel : "${TAG CANCEL}"
;
5.0 Tag Types
5.1 Language Tags
Language tags are of general interest and should have a high degree
of interoperability for protocol usage. To this end, a specific
LANGUAGE TAG tag identification character is provided. A Plane 14
tag string prefixed by U-000E0001 LANGUAGE TAG is specified to
constitute a language tag. Furthermore, the tag values for the
language tag are to be spelled out as specified in RFC1766, making
use only of registered tag values or of user-defined language tags
starting with the characters "x-".
For example, to embed a language tag for Japanese, the Plane 14
characters would be used as follows. The Japanese tag from RFC1766
is "ja" (composed of ISO 639 language id) or, alternatively, "ja-JP"
(composed of ISO 639 language id plus ISO 3166 country id). Since
RFC1766 specifies that language tags are not case significant, it is
recommended that for language tags, the entire tag be lowercased
before conversion to Plane 14 tag characters. (This would not be
required for Unicode conformance, but should be followed as general
practice by protocols making use of RFC1766 language tags, to
simplify and speed up the processing for operations which need to
identify or ignore language tags embedded in text.) Lowercasing,
rather than uppercasing, is recommended because it follows the
majority practice of expressing language tag values in lowercase
letters.
Thus the entire language tag (in its longer form) would be converted
to Plane 14 tag characters as follows:
U-000E0001 U-000E006A U-000E0061 U-000E002D U-000E006A U-000E0070
The language tag (in its shorter, "ja" form) could be expressed as
follows:
U-000E0001 U-000E006A U-000E0061
The value of this string is then expressed in whichever encoding form
(UCS-4, UTF-16, UTF-8) is required and embedded in text at the
relevant point.
5.2 Additional Tags
Additional tag identification characters might be defined in the
future. An example would be a CHARACTER SET SOURCE TAG, or a GENERIC
TAG for private definition of tags.
In each case, when a specific tag identification character is
encoded, a corresponding reference standard for the values of the
tags associated with the identifier should be designated, so that
interoperating parties which make use of the tags will know how to
interpret the values the tags may take.
6.0 Display Issues
All characters in the tag character block are considered to have no
visible rendering in normal text. A process which interprets tags may
choose to modify the rendering of text based on the tag values (as
for example, changing font to preferred style for rendering Chinese
versus Japanese). The tag characters themselves have no display; they
may be considered similar to a U+200B ZERO WIDTH SPACE in that
regard. The tag characters also do not affect breaking, joining, or
any other format or layout properties, except insofar as the process
interpreting the tag chooses to impose such behavior based on the tag
value.
For debugging or other operations which must render the tags
themselves visible, it is advisable that the tag characters be
rendered using the corresponding ASCII character glyphs (perhaps
modified systematically to differentiate them from normal ASCII
characters). But, as noted below, the tag character values are chosen
so that even without display support, the tag characters will be
interpretable in most debuggers.
7.0 Unicode Conformance Issues
The basic rules for Unicode conformance for the tag characters are
exactly the same as for any other Unicode characters. A conformant
process is not required to interpret the tag characters. If it does
not interpret tag characters, it should leave their values
undisturbed and do whatever it does with any other uninterpreted
characters. If it does interpret them, it should interpret them
according to the standard, i.e. as spelled-out tags.
So for a non-TagAware Unicode application, any language tag
characters (or any other kind of tag expressed with Plane 14 tag
characters) encountered would be handled exactly as for uninterpreted
Tibetan from the BMP, uninterpreted Linear B from Plane 1, or
uninterpreted Egyptian hieroglyphics from private use space in Plane
15.
A TagAware but TagPhobic Unicode application can recognize the tag
character range in Plane 14 and choose to deliberately strip them out
completely to produce plain text with no tags.
The presence of a correctly formed tag cannot be taken as a guarantee
that the data so tagged is correctly tagged. For example, nothing
prevents an application from erroneously labelling French data as
Spanish, or from labelling JIS-derived data as Japanese, even if it
contains Greek or Cyrillic characters.
7.1 Note on Encoding Language Tags
The fact that this proposal for encoding tag characters in Unicode
includes a mechanism for specifying language tag values does not mean
that Unicode is departing from one of its basic encoding principles:
Unicode encodes scripts, not languages.
This is still true of the Unicode encoding (and ISO/IEC 10646), even
in the presence of a mechanism for specifying language tags in plain
text. There is nothing obligatory about the use of Plane 14 tags,
whether for language tags or any other kind of tags.
Language tagging in no way impacts current encoded characters or the
encoding of future scripts.
It is fully anticipated that implementations of Unicode which already
make use of out-of-band mechanisms for language tagging or "heavy-
weight" in-band mechanisms such as HTML will continue to do exactly
what they are doing and will ignore Plane 14 tag characters
completely.
8.0 Security Considerations
There are no known security issues raised by this document.
References
[ISO10646] ISO/IEC 10646-1:1993 International Organization for
Standardization. "Information Technology -- Universal
Multiple-Octet Coded Character Set (UCS) -- Part 1:
Architecture and Basic Multilingual Plane", Geneva, 1993.
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC1766, March 1995.
[RFC2070] Yergeau, F., Nicol, G. Adams, G. and M. Duerst,
"Internationalization of the Hypertext Markup Language",
RFC2070, January 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC2130] Weider, C. Preston, C., Simonsen, K., Alvestrand, H.,
Atkinson, R., Crispin, M. and P. Svanberg, "The Report of
the IAB Character Set Workshop held 29 February - 1 March,
1996", RFC2130, April 1997.
[UNICODE] The Unicode Standard, Version 2.0, The Unicode Consortium,
Addison-Wesley, July 1996.
Acknowledgements
The following people also contributed to this document, directly or
indirectly: Chris Newman, Mark Crispin, Rick McGowan, Joe Becker,
John Jenkins, and Asmus Freytag. This document also was reviewed by
the Unicode Technical Committee, and the authors wish to thank all of
the UTC representatives for their input. The authors are, of course,
responsible for any errors or omissions which may remain in the text.
Authors" Addresses
Ken Whistler
Sybase, Inc.
6475 Christie Ave.
Emeryville, CA 94608-1050
Phone: +1 510 922 3611
EMail: kenw@sybase.com
Glenn Adams
Spyglass, Inc.
One Cambridge Center
Cambridge, MA 02142
Phone: +1 617 679 4652
EMail: glenn@spyglass.com
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