ASCII Escaping of Unicode Characters

Network Working Group                                         J. Klensin
Request for Comments: 5137                                 February 2008
BCP: 137
Category: Best Current Practice


                  ASCII Escaping of Unicode Characters

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   There are a number of circumstances in which an escape mechanism is
   needed in conjunction with a protocol to encode characters that
   cannot be represented or transmitted directly.  With ASCII coding,
   the traditional escape has been either the decimal or hexadecimal
   numeric value of the character, written in a variety of different
   ways.  The move to Unicode, where characters occupy two or more
   octets and may be coded in several different forms, has further
   complicated the question of escapes.  This document discusses some
   options now in use and discusses considerations for selecting one for
   use in new IETF protocols, and protocols that are now being
   internationalized.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Context and Background . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Discussion List  . . . . . . . . . . . . . . . . . . . . .  4
   2.  Encodings that Represent Unicode Code Points: Code
       Position versus UTF-8 or UTF-16 Octets . . . . . . . . . . . .  4
   3.  Referring to Unicode Characters  . . . . . . . . . . . . . . .  5
   4.  Syntax for Code Point Escapes  . . . . . . . . . . . . . . . .  6
   5.  Recommended Presentation Variants for Unicode Code Point
       Escapes  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Backslash-U with Delimiters  . . . . . . . . . . . . . . .  7
     5.2.  XML and HTML . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Forms that Are Normally Not Recommended  . . . . . . . . . . .  8
     6.1.  The C Programming Language: Backslash-U  . . . . . . . . .  8
     6.2.  Perl: A Hexadecimal String . . . . . . . . . . . . . . . .  8
     6.3.  Java: Escaped UTF-16 . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Formal Syntax for Forms Not Recommended . . . . . . . 12
     A.1.  The C Programming Language Form  . . . . . . . . . . . . . 12
     A.2.  Perl Form  . . . . . . . . . . . . . . . . . . . . . . . . 12
     A.3.  Java Form  . . . . . . . . . . . . . . . . . . . . . . . . 12

1.  Introduction

1.1.  Context and Background

   There are a number of circumstances in which an escape mechanism is
   needed in conjunction with a protocol to encode characters that
   cannot be represented or transmitted directly.  With ASCII [ASCII]
   coding, the traditional escape has been either the decimal or
   hexadecimal numeric value of the character, written in a variety of
   different ways.  For example, in different contexts, we have seen
   %dNN or %NN for the decimal form, %NN, %xNN, X'nn', and %X'NN' for
   the hexadecimal form. "%NN" has become popular in recent years to
   represent a hexadecimal value without further qualification, perhaps
   as a consequence of its use in URLs and their prevalence.  There are
   even some applications around in which octal forms are used and,
   while they do not generalize well, the MIME Quoted-Printable and
   Encoded-word forms can be thought of as yet another set of escapes.
   So, even for the fairly simple cases of ASCII and standard built by
   extending ASCII, such as the ISO 8859 family, we have been living
   with several different escaping forms, each the result of some
   history.

   When one moves to Unicode [Unicode] [ISO10646], where characters
   occupy two or more octets and may be coded in several different
   forms, the question of escapes becomes even more complicated.
   Unicode represents characters as code points: numeric values from 0
   to hex 10FFFF.  When referencing code points in flowing text, they
   are represented using the so-called "U+" notation, as values from
   U+0000 to U+10FFFF.  When serialized into octets, these code points
   can be represented in different forms:

   o  in UTF-8 with one to four octets [RFC3629]

   o  in UTF-16 with two or four octets (or one or two seizets -- 16-bit
      units)

   o  in UTF-32 with exactly four octets (or one 32-bit unit)

   When escaping characters, we have seen fairly extensive use of
   hexadecimal representations of both the serialized forms and
   variations on the U+ notation, known as code point escapes.

   In accordance with existing best-practices recommendations [RFC2277],
   new protocols that are required to carry textual content for human
   use SHOULD be designed in such a way that the full repertoire of
   Unicode characters may be represented in that text.

   This document proposes that existing protocols being
   internationalized, and those that need an escape mechanism, SHOULD
   use some contextually appropriate variation on references to code
   points as described in Section 2 unless other considerations outweigh
   those described here.

   This recommendation is not applicable to protocols that already
   accept native UTF-8 or some other encoding of Unicode.  In general,
   when protocols are internationalized, it is preferable to accept
   those forms rather than using escapes.  This recommendation applies
   to cases, including transition arrangements, in which that is not
   practical.

   In addition to the protocol contexts addressed in this specification,
   escapes to represent Unicode characters also appear in presentations
   to users, i.e., in user interfaces (UI).  The formats specified in,
   and the reasoning of, this document may be applicable in UI contexts
   as well, but this is not a proposal to standardize UI or presentation
   forms.

   This document does not make general recommendations for processing
   Unicode strings or for their contents.  It assumes that the strings
   that one might want to escape are valid and reasonable and that the
   definition of "valid and reasonable" is the province of other
   documents.  Recommendations about general treatment of Unicode
   strings may be found in many places, including the Unicode Standard
   itself and the W3C Character Model [W3C-CharMod], as well as specific
   rules in individual protocols.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Additional Unicode-specific terminology appears in [UnicodeGlossary],
   but is not necessary for understanding this specification.

1.3.  Discussion List

   Discussion of this document should be addressed to the
   discuss@apps.ietf.org mailing list.

2.  Encodings that Represent Unicode Code Points: Code Position versus
    UTF-8 or UTF-16 Octets

   There are two major families of ways to escape Unicode characters.
   One uses the code point in some representation (see the next
   section), the other encodes the octets of the UTF-8 encoding or some
   other encoding in some representation.  Some other options are
   possible, but they have been rare in practice.  This specification
   recommends that, in the absence of compelling reasons to do
   otherwise, the Unicode code points SHOULD be used rather than a
   representation of UTF-8 (or UTF-16) octets.  There are several
   reasons for this, including:

   o  One reason for the success of many IETF protocols is that they use
      human-interpretable text forms to communicate, rather than
      encodings that generally require computer programs (or hand
      simulation of algorithms) to decode.  This suggests that the
      presentation form should reference the Unicode tables for
      characters and to do so as simply as possible.

   o  Because of the nature of UTF-8, for a human to interpret a decimal
      or hexadecimal numeral representation of UTF-8 octets requires one
      or more decoding steps to determine a Unicode code point that can
      used to look up the character in a table.  That may be appropriate
      in some cases where the goal is really to represent the UTF-8 form
      but, in general, it just obscures desired information and makes
      errors more likely and debugging harder.

   o  Except for characters in the ASCII subset of Unicode (U+0000
      through U+007F), the code point form is generally more compact
      than forms based on coding UTF-8 octets, sometimes much more
      compact.

   The same considerations that apply to representation of the octets of
   UTF-8 encoding also apply to more compact ACE encodings such as the
   "bootstring" encoding [RFC3492] with or without its "Punycode"
   profile.

   Similar considerations apply to UTF-16 encoding, such as the \uNNNN
   form used in Java (See Section 6.3).  While those forms are
   equivalent to code point references for the Basic Multilingual Plane
   (BMP, Plane 0), a two-stage decoding process is needed to handle
   surrogates to access higher planes.

3.  Referring to Unicode Characters

   Regardless of what decisions are made about escapes for Unicode
   characters in protocol or similar contexts, text referring to a
   Unicode code point SHOULD use the U+NNNN[N[N]] syntax, as specified
   in the Unicode Standard, where the NNNN... string consists of
   hexadecimal numbers.  Text actually containing a Unicode character
   SHOULD use a syntax more suitable for automated processing.

4.  Syntax for Code Point Escapes

   There are many options for code point escapes, some of which are
   summarized below.  All are equivalent in content and semantics -- the
   differences lie in syntax.  The best choice of syntax for a
   particular protocol or other application depends on that application:
   one form may simply "fit" better in a given context than others.  It
   is clear, however, that hexadecimal values are preferable to other
   alternatives: Systems based on decimal or octal offsets SHOULD NOT be
   used.

   Since this specification does not recommend one specific syntax,
   protocol specifications that use escapes MUST define the syntax they
   are using, including any necessary escapes to permit the escape
   sequence to be used literally.

   The application designer selecting a format should consider at least
   the following factors:

   o  If similar or related protocols already use one form, it may be
      best to select that form for consistency and predictability.

   o  A Unicode code point can fall in the range from U+0000 to
      U+10FFFF.  Different escape systems may use four, five, six, or
      eight hexadecimal digits.  To avoid clever syntax tricks and the
      consequent risk of confusion and errors, forms that use explicit
      string delimiters are generally preferred over other alternatives.
      In many contexts, symmetric paired delimiters are easier to
      recognize and understand than visually unrelated ones.

   o  Syntax forms starting in "\u", without explicit delimiters, have
      been used in several different escape systems, including the four
      or eight digit syntax of C [ISO-C] (see Section 6.1), the UTF-16
      encoding of Java [Java] (see Section 6.3), and some arrangements
      that may follow the "\u" with four, five, or six digits.  The
      possible confusion about which option is actually being used may
      argue against use of any of these forms.

   o  Forms that require decoding surrogate pairs share most of the
      problems that appear with encoding of UTF-8 octets.  Internet
      protocols SHOULD NOT use surrogate pairs.

5.  Recommended Presentation Variants for Unicode Code Point Escapes

   There are a number of different ways to represent a Unicode code
   point position.  No one of them appears to be "best" for all
   contexts.  In addition, when an escape is needed for the escape
   mechanism itself, the optimal one of those might differ from one
   context to another.

   Some forms that are in popular use and that might reasonably be
   considered for use in a given protocol are described below and
   identified with a current-use context when feasible.  The two in this
   section are recommended for use in Internet Protocols.  Other popular
   ones appear in Section 6 with some discussion of their disadvantages.

5.1.  Backslash-U with Delimiters

   One of the recommended forms is a variation of the many forms that
   start in "\u" (See, e.g., Section 6.1, below>), but uses explicit
   delimiters for the reasons discussed elsewhere.

   Specifically, in ABNF [RFC5234],

   EmbeddedUnicodeChar =  %x5C.75.27 4*6HEXDIG %x27
      ; starting with lowercase "\u" and "'" and ending with "'".
      ; Note that the encodings are considered to be abstractions
      ; for the relevant characters, not designations of specific
      ; octets.

   HEXDIG =  "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" /
      "A" / "B" / "C" / "D" / "E" / "F"
      ; effectively identical with definition in RFC 5234.

   Protocol designers of applications using this form should specify a
   way to escape the introducing backslash ("\"), if needed. "\\" is one
   obvious possibility, but not the only one.

5.2.  XML and HTML

   The other recommended form is the one used in XML.  It uses the form
   "&#xNNNN;".  Like the Perl form (Section 6.2), this form has a clear
   ending delimiter, reducing ambiguity.  HTML uses a similar form, but
   the semicolon may be omitted in some cases.  If that is done, the
   advantages of the delimiter disappear so that the HTML form without
   the semicolon SHOULD NOT be used.  However, this format is often
   considered ugly and awkward outside of its native HTML, XML, and
   similar contexts.

   In ABNF:

   EmbeddedUnicodeChar =   %x26.23.78 2*6HEXDIG %x3B
      ; starts with "&#x" and ends with ";"

   Note that a literal "&" can be expressed by "&" when using this
   style.

6.  Forms that Are Normally Not Recommended

6.1.  The C Programming Language: Backslash-U

   The forms

      \UNNNNNNNN (for any Unicode character) and

      \uNNNN (for Unicode characters in plane 0)

   are utilized in the C Programming Language [ISO-C] when an ASCII
   escape for embedded Unicode characters is needed.

   There are disadvantages of this form that may be significant.  First,
   the use of a case variation (between "u" for the four-digit form and
   "U" for the eight-digit form) may not seem natural in environments
   where uppercase and lowercase characters are generally considered
   equivalent and might be confusing to people who are not very familiar
   with Latin-based alphabets (although those people might have even
   more trouble reading relevant English text and explanations).
   Second, as discussed in Section 4, the very fact that there are
   several different conventions that start in \u or \U may become a
   source of confusion as people make incorrect assumptions about what
   they are looking at.

6.2.  Perl: A Hexadecimal String

   Perl uses the form \x{NNNN...}.  The advantage of this form is that
   there are explicit delimiters, resolving the issue of having
   variable-length strings or using the case-change mechanism of the
   proposed form to distinguish between Plane 0 and more general forms.
   Some other programming languages would tend to favor X'NNNN...' forms
   for hexadecimal strings and perhaps U'NNNN...' for Unicode-specific
   strings, but those forms do not seem to be in use around the IETF.

   Note that there is a possible ambiguity in how two-character or low-
   numbered sequences in this notation are understood, i.e., that octets
   in the range \x(00) through \x(FF) may be construed as being in the
   local character set, not as Unicode code points.  Because of this
   apparent ambiguity, and because IETF documents do not contain
   provision for pragmas (see [PERLUniIntro] for more information about
   the "encoding" pragma in Perl and other details), the Perl forms
   should be used with extreme caution, if at all.

6.3.  Java: Escaped UTF-16

   Java [Java] uses the form \uNNNN, but as a reference to UTF-16
   values, not to Unicode code points.  While it uses a syntax similar
   to that described in Section 6.1, this relationship to UTF-16 makes
   it, in many respects, more similar to the encodings of UTF-8
   discussed above than to an escape that designates Unicode code
   points.  Note that the UTF-16 form, and hence, the Java escape
   notation, can represent characters outside Plane 0 (i.e., above
   U+FFFF) only by the use of surrogate pairs, raising some of the same
   issues as the use of UTF-8 octets discussed above.  For characters in
   Plane 0, the Java form is indistinguishable from the Plane 0-only
   form described in Section 6.1.  If only for that reason, it SHOULD
   NOT be used as an escape except in those Java contexts in which it is
   natural.

7.  Security Considerations

   This document proposes a set of rules for encoding Unicode characters
   when other considerations do not apply.  Since all of the recommended
   encodings are unambiguous and normalization issues are not involved,
   it should not introduce any security issues that are not present as a
   result of simple use of non-ASCII characters, no matter how they are
   encoded.  The mechanisms suggested should slightly lower the risks of
   confusing users with encoded characters by making the identity of the
   characters being used somewhat more obvious than some of the
   alternatives.

   An escape mechanism such as the one specified in this document can
   allow characters to be represented in more than one way.  Where
   software interprets the escaped form, there is a risk that security
   checks, and any necessary checks for, e.g., minimal or normalized
   forms, are done at the wrong point.

8.  Acknowledgments

   This document was produced in response to a series of discussions
   within the IETF Applications Area and as part of work on email
   internationalization and internationalized domain name updates.  It
   is a synthesis of a large number of discussions, the comments of the
   participants in which are gratefully acknowledged.  The help of Mark
   Davis in constructing a list of alternative presentations and
   selecting among them was especially important.

   Tim Bray, Peter Constable, Stephane Bortzmeyer, Chris Newman, Frank
   Ellermann, Clive D.W. Feather, Philip Guenther, Bjoern Hoehrmann,
   Simon Josefsson, Bill McQuillan, der Mouse, Phil Pennock, and Julian
   Reschke provided careful reading and some corrections and suggestions
   on the various working drafts that preceded this document.  Taken
   together, their suggestions motivated the significant revision of
   this document and its recommendations between version -00 and version
   -01 and further improvements in the subsequent versions.

9.  References

9.1.  Normative References

   [ISO10646]         International Organization for Standardization,
                      "Information Technology -- Universal Multiple-
                      Octet Coded Character Set (UCS)", ISO/
                      IEC 10646:2003, December 2003.

   [RFC2119]          Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

   [RFC3629]          Yergeau, F., "UTF-8, a transformation format of
                      ISO 10646", STD 63, RFC 3629, November 2003.

   [RFC5234]          Crocker, D. and P. Overell, "Augmented BNF for
                      Syntax Specifications: ABNF", STD 68, RFC 5234,
                      January 2008.

   [Unicode]          The Unicode Consortium, "The Unicode Standard,
                      Version 5.0", 2006.
                      (Addison-Wesley, 2006.  ISBN 0-321-48091-0).

9.2.  Informative References

   [ASCII]            American National Standards Institute (formerly
                      United States of America Standards Institute),
                      "USA Code for Information Interchange", ANSI X3.4-
                      1968, 1968.

                      ANSI X3.4-1968 has been replaced by newer versions
                      with slight modifications, but the 1968 version
                      remains definitive for the Internet.

   [ISO-C]            International Organization for Standardization,
                      "Information technology --  Programming languages
                      -- C", ISO/IEC 9899:1999, 1999.

   [Java]             Sun Microsystems, Inc., "Java Language
                      Specification, Third Edition", 2005, <http://
                      java.sun.com/docs/books/jls/third_edition/html/
                      lexical.html#95413p>.

   [PERLUniIntro]     Hietaniemi, J., "perluniintro", Perl
                      documentation  5.8.8, 2002,
                      <http://perldoc.perl.org/perluniintro.html>.

   [RFC2277]          Alvestrand, H., "IETF Policy on Character Sets and
                      Languages", BCP 18, RFC 2277, January 1998.

   [RFC3492]          Costello, A., "Punycode: A Bootstring encoding of
                      Unicode for Internationalized Domain Names in
                      Applications (IDNA)", RFC 3492, March 2003.

   [UnicodeGlossary]  The Unicode Consortium, "Glossary of Unicode
                      Terms", June 2007,
                      <http://www.unicode.org/glossary>.

   [W3C-CharMod]      Duerst, M., "Character Model for the World Wide
                      Web 1.0", W3C Recommendation, February 2005,
                      <http://www.w3.org/TR/charmod/>.

Appendix A.  Formal Syntax for Forms Not Recommended

   While the syntax for the escape forms that are not recommended above
   (see Section 6) are not given inline in the hope of discouraging
   their use, they are provided in this appendix in the hope that those
   who choose to use them will do so consistently.  The reader is
   cautioned that some of these forms are not defined precisely in the
   original specifications and that others have evolved over time in
   ways that are not precisely consistent.  Consequently, these
   definitions are not normative and may not even precisely match
   reasonable interpretations of their sources.

   The definition of "HEXDIG" for the forms that follow appears in
   Section 5.1.

A.1.  The C Programming Language Form

   Specifically, in ABNF [RFC5234],

   EmbeddedUnicodeChar =  BMP-form / Full-form

   BMP-form =  %x5C.75 4HEXDIG ; starting with lowercase "\u"
      ; The encodings are considered to be abstractions for the
      ; relevant characters, not designations of specific octets.

   Full-form =  %x5C.55 8HEXDIG ; starting with uppercase "\U"

A.2.  Perl Form

   EmbeddedUnicodeChar =   %x5C.78 "{" 2*6HEXDIG "}" ; starts with "\x"

A.3.  Java Form

   EmbeddedUnicodeChar =   %x5C.7A 4HEXDIG ; starts with "\u"

Author's Address

   John C Klensin
   1770 Massachusetts Ave, #322
   Cambridge, MA  02140
   USA

   Phone: +1 617 245 1457
   EMail: john-ietf@jck.com

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