The grammar of a language is defined via a syntax definition, expressed in EBNF.
The PILL EBNF (“Extended Backus-Naur-Form”) supports a variety of BNF extensions and follows ISO/IEC 14977:1996(E) (ISO), with some further very useful extensions expected in industrial environments.
Following major changes have been applied to the ISO standard:
- Lists are explicitly supported by PILL-EBNF language elements
- A Non-Terminal is an identifier, removing the need of the “,” terminal within EBNF
- The empty string is an explicit keyword (EPSILON) instead of an absence of expression for improved readability.
- The ISO terminal “-“ is not supported.
|
Description |
Specific PILL EBNF |
Generic PILL EBNF |
ISO/IEC 14977:1996(E) |
|
Terminal |
“+” |
“+” |
“+” |
|
Non-Terminal |
someNT |
someNT |
someNT |
|
Syntax Rule |
NT = expression; |
NT = expression; |
NT = expression; |
|
Group Element |
( expr ) |
( expr ) |
( expr ) |
|
Optional Element |
[ expr ] |
[ expr ] |
[ expr ] |
|
Optional Repeat |
{ expr } |
{ expr } |
{ expr } |
|
Repeat at least 1 |
{ expr ; 1 } |
{ expr ; 1} |
expr , { expr } |
|
Limited Repeat |
{ expr ; 0..N } { expr ; 1..N } |
{ expr ; 0..N } { expr ; 1..N } |
N * { expr } expr , N-1 * { expr } |
|
Optional List |
< expr1 ; expr2 > |
{ expr1 ; expr2 } |
[ expr1 { expr2 , expr1 } ] |
|
List at least 1 |
< expr1 ; expr2; 1> |
{ expr1 ; expr2; 1 } |
expr1 { expr2 , expr1 } |
|
Limited List |
< expr1 ; expr2; 0..N > < expr1 ; expr2; 1..N > |
{ expr1 ; expr2; 0..N > { expr1 ; expr2; 1..N > |
[ expr1 , N-1 * { expr2 , expr1 } ] expr1 , N-1 * { expr2 , expr1 } |
. term = { factor } . factor = “EPSILON” | nonTerminal | terminal | “(“ expression “)” | “[“ expression “]” | “{“ expression [ “;” limits ] “}” | “<“ expression “;” expression [ “;” limits ] “>” | “<<“ expression “;” [ expression ] “;” [ limits ] “>>” . limits = ( “0” | “1” ) [ “..” ( integer | “*” ) ] . terminal = string_literal . nonTerminal = identifier . " v:shapes="Text_x0020_Box_x0020_2">
The extensions of PILL EBNF are defined in BNF (Wikipedia) as follows.
|
Description |
Generic PILL EBNF |
BNF alternative construction |
|
|
Terminal |
String_literal |
String_literal |
|
|
Non-Terminal |
identifier |
Identifier |
|
|
Syntax Rule |
NT = expression ; |
NT = expression . |
|
|
Epsilon |
NT = EPSILON; |
NT = . |
|
|
Alternatives |
NT = expr1 | expr2 ; |
NT = expr1 . NT = expr2 . |
|
|
Group Element |
NT = expr1 ( expr ) expr2 ; |
NT = expr1 NT_NEW expr2 . NT_ NEW = expr . |
|
|
Embedded EBNF Elements |
NT = expr1 [ expr ] expr2 ; NT = expr1 { expr } expr2 ; |
NT = expr1 NT_NEW expr2 . NT_ NEW = [ expr ] . NT_ NEW = { expr } . |
|
|
Optional Element |
NT = [ expr ] ; |
NT = expr . NT = . |
|
|
Optional Repeat |
NT = { expr } ; |
NT = expr NT . NT = . |
|
|
Repeat at least 1 |
NT = { expr ; 1} ; |
NT = expr NT . NT = expr . |
|
|
Optional List |
NT = { expr1 ; expr2 } ; |
NT = expr1 NT_NEW . NT = . NT_NEW = expr2 expr1 NT_NEW . NT_NEW = . |
|
|
List at least 1 |
NT = { expr1 ; expr2; 1 } ; |
NT = expr1 NT_NEW . NT_NEW = expr2 expr1 NT_NEW . NT_NEW = . |
|
|
Optional Limited Repeat |
NT = { expr ; 0..N } ; |
Limitation can be expressed in BNF via explicit enumeration, e.g. NT = { expr ; 0..3 } ; à NT = . NT = expr . NT = expr expr . NT = expr expr expr . |
|
|
Limited Repeat at least 1 |
NT = { expr ; 1..N } ; |
Limitation can be expressed in BNF via explicit enumeration |
|
|
Optional Limited List |
NT = { expr1 ; expr2; 0..N > ; |
Limitation can be expressed in BNF via explicit enumeration |
|
|
Limited List at least 1 |
NT = { expr1 ; expr2; 1..N > ; |
Limitation can be expressed in BNF via explicit enumeration |
|