Editing Extended Backus–Naur form (section)

==Conventions==
{{ordered list|According to the section 4 of the ISO/IEC 14977 standard, the following conventions are used:
{{blist
| Each meta-identifier of Extended BNF is written as one or more words joined together by [[hyphen]]s. However, joining the words seems to apply only to referencing meta-identifiers outside of the metalanguage itself, as seen in the examples of the standard.
| A meta-identifier ending with ''-symbol'' is the name of a terminal symbol of Extended BNF.
}}|The normal character representing each operator of Extended BNF and its implied precedence is (highest precedence at the top):
<syntaxhighlight lang="ebnf">
 * repetition-symbol
 - except-symbol
 , concatenate-symbol
 | definition-separator-symbol
 = defining-symbol
 ; terminator-symbol
 . terminator-symbol
</syntaxhighlight>|The normal precedence is overridden by the following bracket pairs:
<syntaxhighlight lang="ebnf">
 (* start-comment-symbol          end-comment-symbol *)
 '  first-quote-symbol            first-quote-symbol  '
 (  start-group-symbol              end-group-symbol  )
 [  start-option-symbol            end-option-symbol  ]
 {  start-repeat-symbol            end-repeat-symbol  }
 ?  special-sequence-symbol  special-sequence-symbol  ?
 "  second-quote-symbol          second-quote-symbol  "
</syntaxhighlight>
The first-quote-symbol is the [[apostrophe]] as defined by ISO/IEC 646:1991, that is to say Unicode U+0027 (<code>'</code>)<!-- no apos entity for ' for HTML 4 browsers -->; the font used in ISO/IEC 14977:1996(E) renders it very much like the acute, Unicode U+00B4 (<code>&acute;</code>), so confusion sometimes arises.  However, the ISO Extended BNF standard invokes ISO/IEC 646:1991, "ISO 7-bit coded character set for information interchange", as a normative reference and makes no mention of any other character sets, so formally, there is no confusion with Unicode characters outside the 7-bit ASCII range.
}}
As examples, the following syntax rules illustrate the facilities for expressing repetition:
<syntaxhighlight lang="ebnf">
aa = "A";
bb = 3 * aa, "B";
cc = 3 * [aa], "C";
dd = {aa}, "D";
ee = aa, {aa}, "E";
ff = 3 * aa, 3 * [aa], "F";
gg = {3 * aa}, "G";
hh = (aa | bb | cc), "H";
</syntaxhighlight>
Terminal strings defined by these rules are as follows:
 aa: A
 bb: AAAB
 cc: C AC AAC AAAC
 dd: D AD AAD AAAD AAAAD etc.
 ee: AE AAE AAAE AAAAE AAAAAE etc.
 ff: AAAF AAAAF AAAAAF AAAAAAF
 gg: G AAAG AAAAAAG etc.
 hh: AH AAABH CH ACH AACH AAACH