| TOC |
2. RULE DEFINITION
2.1. Rule Naming
2.2. Rule Form
2.3. Terminal Values
2.4. 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
3.7. 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
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
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.
To specify a rule which IS case SENSITIVE, specify the characters individually.
rulename = %d97 %d98 %d99
rulename = %d97.98.99
will match only the string which comprises only lowercased characters, abc.
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:
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 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,
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..
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
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:
char-line = %x0D.0A %x20-7E %x0D.0A
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.
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*3<element> allows exactly 3 and 1*2<element> allows one or two.
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 characters.
Square brackets enclose an optional element sequence:
is equivalent to
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 formation
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 Section 6 (APPENDIX A - CORE) (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 without DQUOTE num-val = "%" (bin-val / dec-val / hex-val) bin-val = "b" 1*BIT [ 1*("." 1*BIT) / ("-" 1*BIT) ] ; series of concatenated bit values ; or single ONEOF range dec-val = "d" 1*DIGIT [ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ] hex-val = "x" 1*HEXDIG [ 1*("." 1*HEXDIG) / ("-" 1*HEXDIG) ] prose-val = "<" *(%x20-3D / %x3F-7E) ">" ; bracketed string of SP and VCHAR without angles ; prose description, to be used as last resort
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 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 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.
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