One document matched: draft-crocker-abnf-v3-00.txt
Network Working Group D. Crocker
draft-crocker-abnf-v3-00.txt Brandenburg InternetWorking
Paul Overell
Expiration <12/02> Demon Internet Ltd
June 23, 2002
Augmented BNF for Syntax Specifications: ABNF
Status Of This Memo
This document is an Internet-Draft and is in full
conformance with all provisions of Section 10 of
RFC2026.
Internet-Drafts are working documents of the Internet
Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum
of six months and may be updated, replaced, or obsoleted
by other documents at any time. It is inappropriate to
use Internet-Drafts as reference material or to cite
them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be
accessed at http://www.ietf.org/shadow.html.
Table Of Contents
1. Introduction
2. Rule Definition
2.1 Rule Naming
2.2 Rule Form
2.3 Terminal Values
2.4 External
Encodings
3. Operators
3.1 Concatenation --
Rule1 Rule2
3.2 Alternatives --
Rule1 / Rule2
3.3 Incremental
Alternatives --
Rule1 =/ Rule2
3.4 Value Range
Alternatives --
%c##-##
3.5 Sequence Group --
(Rule1 Rule2)
3.6 Variable
Repetition --
*Rule
3.7 Specific
Repetition --
nRule
3.8 Optional Sequence
-- [Rule]
3.9 ; Comment
3.10 Operator
Precedence
4. ABNF Definition of
ABNF
5. Security
Considerations
6. Appendix A - Core
6.1 Core Rules
6.2 Common Encoding
7. Appendix B -
Enhancements
7.1 Concatenated
Terminal Values
7.2 Binary Literal
Values
8. Acknowledgements
9. References
10. Contact
0. Change Notes
Per Alvestrand:
; Suggested change to allow blank lines within rules:
c-wsp = *c-nl WSP
; This allows comments at col. 1 within rules; is this a
problem?
;c-wsp = WSP / (c-nl WSP)
and:
prose-val = "<" *(%x20-3D / %x3F-7E / c-wsp) ">"
; bracketed string of SP and VCHAR
; without angles
; prose description, to be used as
; last resort
; c-wsp added after DRUMS chair's
note 1998-08-11
; orig: prose-val = "<" *(%x20-3D / %x3F-7E ) ">"
1. Introduction
Internet technical specifications often need to define a
format syntax and are free to employ whatever notation their
authors deem useful. Over the years, a modified version of
Backus-Naur Form (BNF), called Augmented BNF (ABNF), has
been popular among many Internet specifications. It
balances compactness and simplicity, with reasonable
representational power. In the early days of the Arpanet,
each specification contained its own definition of ABNF.
This included the email specifications, [RFC733] and then
[RFC822] which have come to be the common citations for
defining ABNF. The current document separates out that
definition, to permit selective reference. Predictably, it
also provides some modifications and enhancements.
The differences between standard BNF and ABNF involve naming
rules, repetition, alternatives, order-independence, and
value ranges. Appendix A (Core) supplies rule definitions
and encoding for a core lexical analyzer of the type common
to several Internet specifications. It is provided as a
convenience and is otherwise separate from the meta language
defined in the body of this document, and separate from its
formal status.
2. Rule Definition
2.1 Rule Naming
The name of a rule is simply the name itself; that is, a
sequence of characters, beginning with an alphabetic
character, and followed by a combination of alphabetics,
digits and hyphens (dashes).
NOTE: Rule names are case-INsensitive
The names <rulename>, <Rulename>, <RULENAME> and <rUlENamE>
all refer to the same rule.
Unlike original BNF, angle brackets ("<", ">") are not
required. However, angle brackets may be used around a
rule name whenever their presence will facilitate discerning
the use of a rule name. This is typically restricted to
rule name references in free-form prose, or to distinguish
partial rules that combine into a string not separated by
white space, such as shown in the discussion about
repetition, below.
2.2 Rule Form
A rule is defined by the following sequence:
name = elements CRLF
where <name> is the name of the rule, <elements> is one or
more rule names or terminal specifications and <CRLF> is the
end-of-line indicator, carriage return followed by line
feed. The equal sign separates the name from the definition
of the rule. The elements form a sequence of one or more
rule names and/or value definitions, combined according to
the various operators, defined in this document, such as
alternative and repetition.
For visual ease, rule definitions are left aligned. When a
rule requires multiple lines, the continuation lines are
indented, although a line containing only a comment need not
be indented. The left alignment and indentation are
relative to the first lines of the ABNF rules and need not
match the left margin of the document.
2.3 Terminal Values
Rules resolve into a string of terminal values, sometimes
called characters. In ABNF a character is merely a non-
negative integer. In certain contexts a specific mapping
(encoding) of values into a character set (such as ASCII)
will be specified.
Terminals are specified by one or more numeric characters
with the base interpretation of those characters indicated
explicitly. The following bases are currently defined:
d = decimal
x = hexadecimal
Hence:
CR = %d13
CR = %x0D
respectively specify the decimal and hexadecimal
representation of [US-ASCII] for carriage return.
ABNF permits specifying literal text string directly,
enclosed in quotation marks. Hence:
command = "command string"
Literal text strings are interpreted as a concatenated set
of printable characters.
NOTE: ABNF strings are case-INsensitive and
the character set for these strings is
US-ASCII.
Hence:
rulename = "abc"
and:
rulename = "aBc"
will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC"
and "ABC".
NOTE: To specify a rule that IS case
SENSITIVE, specify the characters
individually.
For example:
rulename = %d97 %d98 %d99
will match only the string that comprises only lowercased
characters, 'abc'.
2.4 External Encodings
External representations of terminal value characters will
vary according to constraints in the storage or transmission
environment. Hence, the same ABNF-based grammar may have
multiple external encodings, such as one for a 7-bit US-
ASCII environment, another for a binary octet environment
and still a different one when 16-bit Unicode is used.
Encoding details are beyond the scope of ABNF, although
Appendix A (Core) provides definitions for a 7-bit US-ASCII
environment as has been common too much of the Internet.
By separating external encoding from the syntax, it is
intended that alternate encoding environments can be used
for the same syntax.
Examples in the text below use the US-ASCII environment as
specified in Appendix A.
3. Operators
3.1 Concatenation -- Rule1 Rule2
A rule can define a simple, ordered string of values -- that
is, a concatenation of contiguous characters -- by listing a
sequence of rule names. For example:
foo = %x61 ; a
bar = %x62 ; b
mumble = foo bar foo
So that the rule <mumble> matches the lowercase string
"aba".
LINEAR WHITE SPACE: Concatenation is at the core of the
ABNF parsing model. A string of contiguous characters
(values) is parsed according to the rules defined in ABNF.
For Internet specifications, there is some history of
permitting linear white space (space and horizontal tab) to
be freely--and implicitly--interspersed around major
constructs, such as delimiting special characters or atomic
strings.
NOTE: This specification for ABNF does not
provide for implicit specification of
linear white space.
Any grammar that wishes to permit linear white space around
delimiters or string segments must specify it explicitly.
It is often useful to provide for such white space in "core"
rules that are then used variously among higher-level rules.
The "core" rules might be formed into a lexical analyzer or
simply might be part of the main ruleset.
3.2 Alternatives -- Rule1 / Rule2
Elements separated by forward slash ("/") are alternatives.
Therefore,
foo / bar
will accept <foo> or <bar>.
NOTE: A quoted string containing alphabetic
characters is special form for
specifying alternative characters and is
interpreted as a non-terminal
representing the set of combinatorial
strings with the contained characters,
in the specified order but with any
mixture of upper and lower case.
3.3 Incremental Alternatives -- Rule1 =/ Rule2
It is sometimes convenient to specify a list of alternatives
in fragments. That is, an initial rule may match one or
more alternatives, with later rule definitions adding to the
set of alternatives. This is particularly useful for
otherwise-independent specifications which derive from the
same parent rule set, such as often occurs with parameter
lists. ABNF permits this incremental definition through the
construct:
oldrule =/ additional-alternatives
So that the rule set:
ruleset = alt1 / alt2
ruleset =/ alt3
ruleset =/ alt4 / alt5
is the same as specifying:
ruleset = alt1 / alt2 / alt3 / alt4 / alt5
3.4 Value Range Alternatives -- %c##-##
A range of alternative numeric values can be specified
compactly, using dash ("-") to indicate the range of
alternative values. Hence:
DIGIT = %x30-39
is equivalent to:
DIGIT = "0" / "1" / "2" / "3" / "4" / "5" /
"6" / "7" / "8" / "9"
Concatenated numeric values and numeric value ranges can not
be specified in the same string. A numeric value may use
the dotted notation for concatenation or it may use the dash
notation to specify one value range. Hence, to specify one
printable character, between end of line sequences, the
specification could be:
onechar-line = %x0D %0A %x20-7E %x0D %0A
3.5 Sequence Group -- (Rule1 Rule2)
Elements enclosed in parentheses are treated as a single
element, whose contents are STRICTLY ORDERED. Thus,
elem (foo / bar) blat
which matches (elem foo blat) or (elem bar blat).
elem foo / bar blat
matches (elem foo) or (bar blat).
NOTE: It is strongly advised to use grouping
notation, rather than to rely on proper
reading of "bare" alternations, when
alternatives consist of multiple rule
names or literals.
Hence it is recommended that instead of the above form, the
form:
(elem foo) / (bar blat)
be used. It will avoid misinterpretation by casual readers.
The sequence group notation is also used within free text to
set off an element sequence from the prose.
3.6 Variable Repetition -- *Rule
The operator "*" preceding an element indicates repetition.
The full form is:
<a>*<b>element
where <a> and <b> are optional decimal values, indicating at
least <a> and at most <b> occurrences of element.
Default values are 0 and infinity so that *<element> allows
any number, including zero; 1*<element> requires at least
one; 3*3<element> allows exactly 3 and 1*2<element> allows
one or two.
3.7 Specific Repetition -- nRule
A rule of the form:
<n>element
is equivalent to
<n>*<n>element
That is, exactly <n> occurrences of <element>. Thus
2DIGIT is a 2-digit number, and 3ALPHA is a string of three
alphabetic characters.
3.8 Optional Sequence -- [Rule]
Square brackets enclose an optional element sequence:
[foo bar]
is equivalent to
*1(foo bar).
3.9 ; Comment
A semi-colon starts a comment that continues to the end of
line. This is a simple way of including useful notes in
parallel with the specifications.
3.10 Operator Precedence
The various mechanisms described above have the following
precedence, from highest (binding tightest) at the top, to
lowest and loosest at the bottom:
Strings, Names formation
Comment
Value range
Repetition
Grouping, Optional
Concatenation
Alternative
Use of the alternative operator, freely mixed with
concatenations can be confusing.
Again, it is recommended that the grouping
operator be used to make explicit concatenation
groups.
4. ABNF Definition of ABNF
This syntax uses the rules provided in Appendix A (Core).
rulelist = 1*( rule / (*c-wsp c-nl) )
rule = rulename defined-as elements c-nl
; continues if next line
; starts with white space and
; is not comment
rulename = ALPHA *(ALPHA / DIGIT / "-")
defined-as = *c-wsp ("=" / "=/") *c-wsp
; basic rules definition and
; incremental alternatives
elements = alternation *c-wsp
c-wsp = WSP / (c-nl WSP)
c-nl = comment / CRLF
; comment or newline
comment = ";" *(WSP / VCHAR) CRLF
alternation = concatenation
*(*c-wsp "/" *c-wsp concatenation)
concatenation = repetition *(1*c-wsp repetition)
repetition = [repeat] element
repeat = 1*DIGIT / (*DIGIT "*" *DIGIT)
element = rulename / group / option /
char-val / num-val / prose-val
group = "(" *c-wsp alternation *c-wsp ")"
option = "[" *c-wsp alternation *c-wsp "]"
char-val = DQUOTE *(%x20-21 / %x23-7E) DQUOTE
; quoted string of SP and
; VCHAR without DQUOTE
num-val = "%" (bin-val / dec-val / hex-val)
bin-val = "b" 1*BIT [ "-" 1*BIT ]
; a single bit string or a
; range of alternative bit
; values
dec-val = "d" 1*DIGIT [ "-" 1*DIGIT ]
hex-val = "x" 1*HEXDIG [ "-" 1*HEXDIG ]
prose-val = "<" *(%x20-3D / %x3F-7E / c-wsp) ">"
; bracketed string of SP and VCHAR
; without angles
; prose description, to be used as
; last resort
5. Security Considerations
Security is truly believed to be irrelevant to this
document.
6. Appendix A - Core
This Appendix is provided as a convenient core for specific
grammars. The definitions may be used as a core set of
rules.
6.1 Core Rules
Certain basic rules are in uppercase, such as SP, HTAB,
CRLF, DIGIT, ALPHA, etc.
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
BIT = "0" / "1"
CHAR = %x01-7F
; any 7-bit US-ASCII
; character, excluding NUL
CR = %x0D
; carriage return
CRLF = CR LF
; Internet standard newline
CTL = %x00-1F / %x7F
; controls
DIGIT = %x30-39
; 0-9
DQUOTE = %x22
; " (Double Quote)
HEXDIG = DIGIT /
"A" / "B" / "C" / "D" / "E" / "F"
HTAB = %x09
; horizontal tab
LF = %x0A
; linefeed
LWSP = *(WSP / CRLF WSP)
; linear white space
; (past newline)
OCTET = %x00-FF
; 8 bits of data
SP = %x20
; space
VCHAR = %x21-7E
; visible (printing)
; characters
WSP = SP / HTAB
; white space
6.2 Common Encoding
Externally, data are represented as "network virtual ASCII",
namely 7-bit US-ASCII in an 8-bit field, with the high (8th)
bit set to zero. A string of values is in "network byte
order" with the higher-valued bytes represented on the left-
hand side and being sent over the network first.
7. Appendix B - Enhancements
This section provides some additional features for ABNF that
are not part of the official specification. The features
contained here are expected to have benefit, eventually, but
did not gain immediate use. Writers of specifications
wishing to use these features may cite the relevant sub-
sections to this appendix.
7.1 Concatenated Terminal Values
A concatenated string of such values is specified compactly,
using a period (".") to indicate separation of characters
within that value. Hence:
CRLF = %d13.10
Therefore,
rulename = %d97.98.99
will match only the string that comprises only lowercased
characters, abc.
If a grammar uses concatenated terminal values, then the
"ABNF Definition of ABNF" will modify two rules to be:
dec-val = "d" 1*DIGIT
[ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ]
hex-val = "x" 1*HEXDIG
[ 1*("." 1*HEXDIG)
/ ("-" 1*HEXDIG) ]
7.2 Binary Literal Values
Terminals are specified by one or more numeric characters
with the base interpretation of those characters indicated
explicitly. The following, additional base is defined:
b = binary
If binary literal values are used with the concatenation
mechanism defined above, then the "ABNF Definition of ABNF"
modifies the relevant rule to be:
bin-val = "b" 1*BIT [ "-" 1*BIT ]
; a single bit string or a
; range of alternative bit
; values
8. Acknowledgements
The syntax for ABNF was originally specified in [RFC733].
Ken L. Harrenstien, of SRI International, was responsible
for re-coding the BNF into an augmented BNF that makes the
representation smaller and easier to understand.
This recent project began as a simple effort to cull out the
portion of [RFC822], which has been repeatedly cited by non-
email specification writers, namely the description of
augmented BNF. Rather than simply and blindly converting
the existing text into a separate document, the working
group chose to give careful consideration to the
deficiencies, as well as benefits, of the existing
specification and related specifications available over the
last 15 years and therefore to pursue enhancement. This
turned the project into something rather more ambitious than
first intended. Interestingly the result is not massively
different from that original, although decisions such as
removing the list notation came as a surprise.
The current round of specification was part of the DRUMS
working group, with significant contributions from Jerome
Abela , Harald Alvestrand, Robert Elz, Roger Fajman, Aviva
Garrett, Tom Harsch, Dan Kohn, Bill McQuillan, Keith Moore,
Chris Newman , Pete Resnick and Henning Schulzrinne.
9. References
[US-ASCII] Coded Character Set--7-Bit American Standard
Code for Information Interchange, ANSI X3.4-1986.
[RFC733] Crocker, D.H., Vittal, J.J., Pogran, K.T.,
Henderson, D.A. "Standard for the Format of ARPA
Network Text Message," RFC 733, November 1977.
[RFC822] Crocker, D., "Standard for the Format of ARPA
Internet Text Messages", RFC 822, August, 1982.
10. Contact
David H. Crocker Paul Overell
Brandenburg InternetWorking Demon Internet Ltd
675 Spruce Drive Dorking Business Park
Sunnyvale, CA 94086, USA Dorking
Surrey, RH4 1HN, UK
<dcrocker@brandenburg.com>
Phone: +1 408 246 8253 <paulo@turnpike.com>
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