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RFC2234 - Augmented BNF for Syntax Specifications: ABNF

王朝other·作者佚名  2008-05-31
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Network Working Group D. Crocker, Ed.

Request for Comments: 2234 Internet Mail Consortium

Category: Standards Track P. Overell

Demon Internet Ltd.

November 1997

Augmented BNF for Syntax Specifications: ABNF

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1997). All Rights Reserved.

TABLE OF CONTENTS

1. INTRODUCTION .................................................. 2

2. RULE DEFINITION ............................................... 2

2.1 RULE NAMING .................................................. 2

2.2 RULE FORM .................................................... 3

2.3 TERMINAL VALUES .............................................. 3

2.4 EXTERNAL ENCODINGS ........................................... 5

3. OPERATORS ..................................................... 5

3.1 CONCATENATION RULE1 RULE2 ............................. 5

3.2 ALTERNATIVES RULE1 / RULE2 ................................... 6

3.3 INCREMENTAL ALTERNATIVES RULE1 =/ RULE2 .................... 6

3.4 VALUE RANGE ALTERNATIVES %C##-## ........................... 7

3.5 SEQUENCE GROUP (RULE1 RULE2) ................................. 7

3.6 VARIABLE REPETITION *RULE .................................... 8

3.7 SPECIFIC REPETITION NRULE .................................... 8

3.8 OPTIONAL SEQUENCE [RULE] ..................................... 8

3.9 ; COMMENT .................................................... 8

3.10 OPERATOR PRECEDENCE ......................................... 9

4. ABNF DEFINITION OF ABNF ....................................... 9

5. SECURITY CONSIDERATIONS ....................................... 10

6. APPENDIX A - CORE ............................................. 11

6.1 CORE RULES ................................................... 11

6.2 COMMON ENCODING .............................................. 12

7. ACKNOWLEDGMENTS ............................................... 12

8. REFERENCES .................................................... 13

9. CONTACT ....................................................... 13

10. FULL COPYRIGHT STATEMENT ..................................... 14

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. 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:

b = binary

d = decimal

x = hexadecimal

Hence:

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

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".

To specify a rule which IS case SENSITIVE,

specify the characters individually.

For example:

rulename = %d97 %d98 %d99

or

rulename = %d97.98.99

will match only the string which 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 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.

3. OPERATORS

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 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.

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:

char-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

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

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. ACKNOWLEDGMENTS

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.

8. REFERENCES

[US-ASCII] Coded Character Set--7-Bit American Standard Code for

Information Interchange, ANSI X3.4-1986.

[RFC733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,

"Standard for the Format of ARPA Network Text Message," RFC733,

November 1977.

[RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text

Messages", STD 11, RFC822, August 1982.

9. CONTACT

David H. Crocker Paul Overell

Internet Mail Consortium Demon Internet Ltd

675 Spruce Dr. Dorking Business Park

Sunnyvale, CA 94086 USA Dorking

Surrey, RH4 1HN

UK

Phone: +1 408 246 8253

Fax: +1 408 249 6205

EMail: dcrocker@imc.org paulo@turnpike.com

10. Full Copyright Statement

Copyright (C) The Internet Society (1997). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 
 
 
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