What does “fragment” mean in ANTLR?

Question

What does fragment mean in ANTLR?

I\'ve seen both rules:

fragment DIGIT : \'0\'..\'9\';

and

DIGIT : \'0\'         


        

Answer 1

According to the Definitive Antlr4 references book :

Rules prefixed with fragment can be called only from other lexer rules; they are not tokens in their own right.

actually they'll improve readability of your grammars.

look at this example :

STRING : '"' (ESC | ~["\\])* '"' ;
fragment ESC : '\\' (["\\/bfnrt] | UNICODE) ;
fragment UNICODE : 'u' HEX HEX HEX HEX ;
fragment HEX : [0-9a-fA-F] ;

STRING is a lexer using fragment rule like ESC .Unicode is used in Esc rule and Hex is used in Unicode fragment rule. ESC and UNICODE and HEX rules can't be used explicitly.