Augmented BNF for Syntax Specifications: ABNFInternet Mail Consortium675 Spruce Dr.SunnyvaleCA94086US+1 408 246 8253+1 408 249 6205dcrocker@imc.orgDemon Internet Ltd.Dorking Business ParkDorkingSurreyEnglandRH4 1HNUKpaulo@turnpike.comABNFAugmentedBackus-NaurFormelectronicmail
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.
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.
A rule is defined by the following sequence:
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. 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.
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.
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.
To specify a rule which IS case SENSITIVE,
specify the characters individually.
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 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 syntax.
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:
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 freelyPand
implicitlyPinterspersed 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 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.
Elements separated by forward slash ("/") are alternatives.
Therefore,
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..
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:
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.
The sequence group notation is also used within free text to set off
an element sequence from the prose.
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.
That is, exactly <N> occurrences of <element>. Thus 2DIGIT is a
2-digit number, and 3ALPHA is a string of three alphabetic
characters.
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.
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 formationCommentValue rangeRepetitionGrouping, OptionalConcatenationAlternative
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.
This syntax uses the rules provided in (Core).
Security is truly believed to be irrelevant to this document.
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.
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.
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 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.
Coded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteStandard for the format of ARPA network text messagesThe Rand Corporation, Information Sciences Department1700 Main StSanta MonicaCA90406USDCrocker@Rand-UnixBolt Beranek and Newman Inc. (BBN)50 Moulton St.CambridgeMA02138USVittal@BBN-TenexDMassachusets Institute of Technology (MIT), Laboratory for Computer Science545 Technology SquareCambridgeMA02139USPogran@MIT-MulticsBolt Beranek and Newman Inc. (BBN)50 Moulton St.CambridgeMA02138USHenderson@BBN-TenexDStandard for the format of ARPA Internet text messagesUniversity of Delaware, Dept. of Electrical EngineeringNewarkDE19711USDCrocker@UDel-Relay