Network Working Group D. Crocker (editor)
Internet-Draft: DRAFT-DRUMS-ABNF-04.txt Internet Mail
Expiration <4/98> Consortium
Demon Internet Ltd
Augmented BNF for Syntax Specifications: ABNF
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TABLE OF CONTENTS
2. RULE DEFINITION
2.1 RULE NAMING
2.2 RULE FORM
2.4 TERMINAL VALUES
2.5 EXTERNAL ENCODINGS
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
3.6 SPECIFIC REPETITION NRULE
3.7 OPTIONAL SEQUENCE [RULE]
3.8 ; COMMENT
3.8 OPERATOR PRECEDENCE
4. ABNF DEFINITION OF ABNF
5. APPENDIX A - CORE
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, lists, and
value ranges. Appendix A (Core) supplies rule definitions 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
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 reference
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
where <name> is the name of the rule and <elements> is one or
more rule names or terminal specifications. 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.
Formally the grammar requires a one-token look-ahead to find the
"=" token, which indicates that the previous token is the name of
a new rule. For visual ease, rule definitions are left aligned.
When a rule requires multiple lines, the continuation lines are
2.4 Terminal Values
Rules resolve into a string of terminal values, sometimes called
characters. Values within ABNF are represented as decimal
numbers. Hence, an ABNF parser processes a sequence of
characters. Each character is represented as a decimal number.
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.
Terminals are specified by one or more numeric characters with
the base interpretation of those characters indicated explicitly.
The following bases are currently defined:
b = binary
d = decimal
x = hexadecimal
CR = %d13
CR = %x0D
respectively specify the decimal and hexadecimal representation
of [US-ASCII] for carriage return.
A concatenated string of such values is specified compactly,
using a period (".") to indicate separation of characters within
that value. Hence:
CRLF = %d13.10
ABNF permits specifying literal text string directly, enclosed in
command = "command string"
Literal text strings are interpreted as a concatenated set of
ABNF STRINGS ARE CASE-INSENSITIVE AND THE
CHARACTER SET FOR THESE STRINGS IS US-ASCII.
rulename = "abc"
rulename = "aBc"
will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC" and
TO SPECIFY A RULE WHICH IS CASE SENSITIVE,
SPECIFY THE CHARACTERS INDIVIDUALLY.
rulename = %d97 %d9 %d99
rulename = %d97.98.99
will match only the string which comprises only lowercased
2.5 External Encodings
External representations of these 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 to much of the Internet.
By separating external encoding from the syntax, it is intended
that alternate encoding environments can be used for the same
3.1 Concatenation Rule1 Rule2
A rule can define a simple, ordered string of values -- i.e., 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 lower-case 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 - interspered around major constructs, such as
delimiting special characters or atomic strings.
THIS SPECIFICATION FOR ABNF DOES NOT PROVIDE
FOR IMPLICIT SPECIFICATION OF LINEAR WHITE SPACE
Any grammar which 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 be part
of the main ruleset.
3.2 Alternatives Rule1 / Rule2
Elements separated by forward slash ("/") are alternatives.
foo / bar
will accept <foo> or <bar>.
REMINDER: A string containing alphabetic
characters is a non-terminal representing the set
of combinatorial strings with 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.
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 a line containing one
printable character, the specification could be:
onechar-line = %x0D.OA %x20-7E %xOD.OA
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).
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
Hence it is strongly recommended that instead of the above form,
(elem foo) / (bar blat)
be used. It will avoid misinterpretation by casual readers.
The local grouping 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:
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*3element> allows exactly 3 and <1*2element> allows one or two.
3.7 Specific Repetition nRule
A rule of the form:
is equivalent to
That is, exactly <N> occurrences of <element>. Thus 2DIGIT is
a 2-digit number, and 3ALPHA is a string of three alphabetic
3.8 Optional Sequence [RULE]
Square brackets enclose an optional element sequence:
is equivalent to
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
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
Use of the alternative operator, freely mixed with concatenations
can be confusing.
IT IS STRONGLY RECOMMENDED THAT THE GROUPING
OPERATOR BE USED TO MAKE EXPLICIT
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
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
num-val = "%" (bin-val / dec-val / hex-val)
bin-val = "b" 1*BIT
*("." 1*BIT) / ["-" 1*BIT]
; series of concatenated bit values
; or single ONEOF range
dec-val = "d" 1*DIGIT
*("." 1*DIGIT) / ["-" 1*DIGIT]
hex-val = "x" 1*HEXDIG
*("." 1*HEXDIG) / ["-" 1*HEXDIG]
prose-val = "<" *(%x20-3D / %x3F-7E) ">"
; bracketed string of SP and VCHAR
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.
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,
CR = %x0D
; carriage return
CRLF = CR LF
; Internet standard newline
CTL = %x00-1F / %x7F
DIGIT = %x30-39
DQUOTE = %x22
; " (Double Quote)
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
HTAB = %x09
; horizontal tab
LF = %x0A
LWSP = *(WSP / CRLF WSP)
; linear white space (past newline)
OCTET = %x00-FF
; 8 bits of data
SP = %x20
VCHAR = %x21-7E
; visible (printing) characters
WSP = SP / HTAB
; white space
Externally, data are represented as "network virtual ASCII",
namely 7-bit US-ASCII in an 8th bit field, with the high (8th)
bit set to zero.
The syntax for ABNF was originally specified in RFC #733. 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 RFC 822 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 Roger Fajman, Bill
McQuillan, Keith Moore, Pete Resnick, Jerome Abela and Chris
[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.
David H. Crocker Paul Overell
Internet Mail Consortium Demon Internet Ltd
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