Editing Augmented Backus–Naur form (section)

==Operators==

===White space===
White space is used to separate elements of a definition; for space to be recognized as a delimiter, it must be explicitly included. The explicit reference for a single whitespace character is <code>WSP</code> (linear white space), and <code>LWSP</code> is for zero or more whitespace characters with newlines permitted. The <code>LWSP</code> definition in RFC5234 is controversial<ref name="RFC5234 Errata">[http://www.rfc-editor.org/errata_search.php?rfc=5234&eid=3096 RFC Errata 3096].</ref> because at least one whitespace character is needed to form a delimiter between two fields.

Definitions are left-aligned. When multiple lines are required (for readability), continuation lines are indented by whitespace.

===Comment===
<code>; comment</code>

A semicolon (<code>;</code>) starts a comment that continues to the end of the line.

===Concatenation===
<code>Rule1 Rule2</code>

A rule may be defined by listing a sequence of rule names.

To match the string “aba”, the following rules could be used:
* {{code|1=fu = %x61	; a|2=abnf}}
* {{code|1=bar = %x62	; b|2=abnf}}
* {{code|1=mumble = fu bar fu|2=abnf}}

===Alternative===
<code>Rule1 / Rule2</code>

A rule may be defined by a list of alternative rules separated by a [[Solidus (punctuation)|solidus]] (<code>/</code>).

To accept the rule ''fu'' or the rule ''bar'', the following rule could be constructed:
* {{code|1=fubar = fu / bar|2=abnf}}

===Incremental alternatives===
<code>Rule1 =/ Rule2</code>

Additional alternatives may be added to a rule through the use of <code>=/</code> between the rule name and the definition.

The rule
* {{code|1=ruleset = alt1 / alt2|2=abnf}}
* {{code|1=ruleset =/ alt3|2=abnf}}
* {{code|1=ruleset =/ alt4 / alt5|2=abnf}}
is therefore equivalent to 
* {{code|1=ruleset = alt1 / alt2 / alt3 / alt4 / alt5|2=abnf}}

===Value range===
<code>%c##-##</code>

A range of numeric values may be specified through the use of a hyphen (<code>-</code>).

The rule
* {{code|1=OCTAL = %x30-37|2=abnf}}
is equivalent to
* {{code|1=OCTAL = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"|2=abnf}}

===Sequence group===
<code>(Rule1 Rule2)</code>

Elements may be placed in parentheses to group rules in a definition.

To match "a b d" or "a c d", the following rule could be constructed:
* {{code|1=group = a (b / c) d|2=abnf}}

To match “a b” or “c d”, the following rules could be constructed:
* {{code|1=group = a b / c d|2=abnf}}
* {{code|1=group = (a b) / (c d)|2=abnf}}

===Variable repetition===
<code>n*nRule</code>

To indicate repetition of an element, the form <code>&lt;a&gt;*&lt;b&gt;element</code> is used. The optional <code>&lt;a&gt;</code> gives the minimal number of elements to be included (with the default of 0). The optional <code>&lt;b&gt;</code> gives the maximal number of elements to be included (with the default of infinity).

Use <code>*element</code> for zero or more elements, <code>*1element</code> for zero or one element, <code>1*element</code> for one or more elements, and <code>2*3element</code> for two or three elements, cf. [[regular expression]]s <code>e*</code>, <code>e?</code>, <code>e+</code> and <code>e{2,3}</code>.

===Specific repetition===
<code>nRule</code>

To indicate an explicit number of elements, the form <code>&lt;a&gt;element</code> is used and is equivalent to <code>&lt;a&gt;*&lt;a&gt;element</code>.

Use <code>2DIGIT</code> to get two numeric digits, and <code>3DIGIT</code> to get three numeric digits. (<code>DIGIT</code> is defined below under "[[#Core rules|Core rules]]". Also see ''zip-code'' in the example below.)

===Optional sequence===
<code>[Rule]</code>

To indicate an optional element, the following constructions are equivalent:
* {{code|1=[fubar snafu]|2=abnf}}
* {{code|1=*1(fubar snafu)|2=abnf}}
* {{code|1=0*1(fubar snafu)|2=abnf}}

===Operator precedence===
The following operators have the given precedence from tightest binding to loosest binding:
#Strings, names formation
#Comment
#Value range
#Repetition
#Grouping, optional
#Concatenation
#Alternative

Use of the alternative operator with concatenation may be confusing, and it is recommended that grouping be used to make explicit concatenation groups.

===Core rules===

[[File:ABNF core rules.png|thumb|alt=ABNF syntax diagram of core rules|ABNF [[syntax diagram]] of core rules]]

The core rules are defined in the ABNF standard.
{|class="wikitable"
|- style="background-color: #efefef;"
!Rule!!Formal definition!!Meaning
|-
|ALPHA	|| {{codett|%x41–5A / %x61–7A		|abnf}} ||Upper- and lower-case ASCII letters (A–Z, a–z)
|-
|DIGIT 	|| {{codett|%x30–39			|abnf}} ||Decimal digits (0–9)
|-
|HEXDIG	|| {{codett|DIGIT / "A" / "B" / "C" / "D" / "E" / "F"	|abnf}} ||Hexadecimal digits (0–9, A–F, a–f)
|-
|DQUOTE 	|| {{codett|%x22				|abnf}} ||Double quote
|-
|SP		|| {{codett|%x20				|abnf}} ||Space
|-
|HTAB		|| {{codett|%x09				|abnf}} ||Horizontal tab
|-
|WSP		|| {{codett|SP / HTAB			|abnf}} ||Space and horizontal tab
|-
|LWSP		|| {{codett|*(WSP / CRLF WSP)	|abnf}} ||Linear white space (past newline)
|-
|VCHAR	|| {{codett|%x21–7E			|abnf}} ||Visible (printing) characters
|-
|CHAR	|| {{codett|%x01–7F			|abnf}} ||Any ASCII character, excluding NUL
|-
|OCTET	|| {{codett|%x00–FF			|abnf}} ||8 bits of data
|-
|CTL 		|| {{codett|%x00–1F / %x7F		|abnf}} ||Controls
|-
|CR		|| {{codett|%x0D				|abnf}} ||Carriage return
|-
|LF		|| {{codett|%x0A				|abnf}} ||Linefeed
|-
|CRLF		|| {{codett|CR LF			|abnf}} ||Internet-standard newline
|-
|BIT		|| {{codett|"0" / "1"			|abnf}} ||Binary digit
|-
|}

Note that in the core rules diagram the '''CHAR2''' charset is inlined in '''char-val''' and '''CHAR3''' is inlined in '''prose-val''' in the RFC spec. They are named here for clarity in the main syntax diagram.