Augmented BNF for Syntax Specifications: ABNF

Internet technical specifications often need to define a formal 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.

RFC 5234

Contents

! []byte(...) should be UTF-8 encoded!

Function Generator

A way to generate the operators in memory.

g := ParserGenerator{
	RawABNF: rawABNF,
}
functions := g.GenerateABNFAsOperators()
// e.g. functions["ALPHA"]([]byte("a"))

Code Generator

Both the Core ABNF and the ABNF Definition contained within this package where created by the generator.

corePkg := externalABNF{
	operator:    true,
	packageName: "github.com/elimity-com/abnf/core",
}
g := Generator{
	PackageName:  "definition",
	RawABNF:      rawABNF,
	ExternalABNF: map[string]ExternalABNF{
		"ALPHA":  corePkg,
		"BIT":    corePkg,
		// etc.
	},
}
f := g.GenerateABNFAsAlternatives()
// e.g. ioutil.WriteFile("./definition/abnf_definition.go", []byte(fmt.Sprintf("%#v", f)), 0644)

(Currently) Not Supported

• free-form prose
• incremental alternatives

Core ABNF

"Core" rules that are used variously among higher-level rules. The "core" rules might be formed into a lexical analyzer or simply be part of the main ruleset.

Operators

Elements form a sequence of one or more rule names and/or value definitions, combined according to the various operators defined in this package, such as alternative and repetition.

HEXDIG

In the spec HEXDIG is case insensitive.
i.e. 0x6e != 0x6E

HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

In this implementation it is so that 0x6e == 0x6E.

HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
               / "a" / "b" / "c" / "d" / "e" / "f"

EOL

Text files created on DOS/Windows machines have different line endings than files created on Unix/Linux. DOS uses carriage return and line feed (\r\n) as a line ending, which Unix uses just line feed (\n).

This is why this package also allows LF which is NOT compliant with the specification.

CRLF = CR LF / LF

Operator Precedence

RFC 5234 3.10

highest

1. Rule name, prose-val, Terminal value
2. Comment
3. Value range
4. Repetition
5. Grouping, Optional
6. Concatenation
7. Alternative

lowest