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7.2.5 Error Nonterminal Symbol ¶

A bottom-up template grammar can contain the special nonterminal symbol error in production right-hand sides. That nonterminal symbol represents a parse error. The nonterminal symbol makes it possible to change the flow of grammar execution on encountering a parse error. The selection of an error nonterminal symbol to handle a parse error must be deterministic.

Note: bottom-up template grammars containing the error nonterminal symbol are for direct processing by the rege-bottom-up tool after optional adding to them a source PCFG production markup by the rege-markup-cfg tool with passing the option --error-nont. The rege-vit tool ignores the error nonterminal symbol.

Example 1

S: A
 | B
 | C
 | error .
;

A: "a" ;

B: "b" "c"
 | "b" "d"
;

C: "c" "c" "c" ;

This grammar consumes any terminal symbol if a training terminal symbol sequence does not start with the terminal symbol "a", "b", and "c" or if a training terminal symbol sequence starts with the terminal symbol "b" not followed by the terminal symbol "c" or "d".

Example 2

S: "a" ( "b"
       | error . .
       )? "c"
;

This grammar is equivalent to the grammar

S: "a" ( "b"
       | [^ "b" "c" ] .
       )? "c"
;

Example 3

S: A* ( error [^ "d" ]* )? "d" ;

A: "a" "b"
 | "a" "c"
;

Normally, this grammar consumes any number of elements ‘( "a" "b" | "a" "c" )’ finalized by "d". If an element does not start with "a" or starts with "a" not followed by "b" or "c" or if a sequence of ‘( "a" "b" | "a" "c" )’ does not end with "d", the grammar consumes terminal symbols until encountering "d".