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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.


 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

Klensin Best Current Practice [Page 1] RFC 5137 Unicode Escapes February 2008

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

Klensin Best Current Practice [Page 2] RFC 5137 Unicode Escapes February 2008

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
 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
 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.

Klensin Best Current Practice [Page 3] RFC 5137 Unicode Escapes February 2008

 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
 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
 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",
 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 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

Klensin Best Current Practice [Page 4] RFC 5137 Unicode Escapes February 2008

 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
 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"
 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.

Klensin Best Current Practice [Page 5] RFC 5137 Unicode Escapes February 2008

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
 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.

Klensin Best Current Practice [Page 6] RFC 5137 Unicode Escapes February 2008

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.

Klensin Best Current Practice [Page 7] RFC 5137 Unicode Escapes February 2008

 EmbeddedUnicodeChar =   %x26.23.78 2*6HEXDIG %x3B
    ; starts with "&#x" and ends with ";"
 Note that a literal "&" can be expressed by "&" when using this

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

Klensin Best Current Practice [Page 8] RFC 5137 Unicode Escapes February 2008

 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

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
 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.

Klensin Best Current Practice [Page 9] RFC 5137 Unicode Escapes February 2008

 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.

Klensin Best Current Practice [Page 10] RFC 5137 Unicode Escapes February 2008

 [Java]             Sun Microsystems, Inc., "Java Language
                    Specification, Third Edition", 2005, <http://
 [PERLUniIntro]     Hietaniemi, J., "perluniintro", Perl
                    documentation  5.8.8, 2002,
 [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,
 [W3C-CharMod]      Duerst, M., "Character Model for the World Wide
                    Web 1.0", W3C Recommendation, February 2005,

Klensin Best Current Practice [Page 11] RFC 5137 Unicode Escapes February 2008

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
 Phone: +1 617 245 1457

Klensin Best Current Practice [Page 12] RFC 5137 Unicode Escapes February 2008

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 retain all their rights.
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Klensin Best Current Practice [Page 13]

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