Semantic Predicate / Guard Function

About

Semantic Predicate are boolean expressions that enable or disable the rule that follows.

They are written in the target language making the grammar language dependent

Example

RegularExpressionLiteral : {isRegexPossible()}? '/' RegularExpressionBody '/' RegularExpressionFlags ;

Python

TEXT : {self._input.LA(-1) == ord('[')}? ~[\])]+ ;

where:

• self._input.LA(-1) get the character before the current one

Usage

• support different versions of the same language

Documentation / Reference

• https://github.com/antlr/antlr4/blob/4.9.2/doc/predicates.md
• guard function explained taken from Ecma script (ie javascript) grammar
• semantic predicate
• Different Lexer State