Question

What does fragment mean in ANTLR?

I've seen both rules:

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

and

DIGIT : '0'..'9';

What is the difference?

Answer 1

A fragment is somewhat akin to an inline function: It makes the grammar more readable and easier to maintain.

A fragment will never be counted as a token, it only serves to simplify a grammar.

Consider:

NUMBER: DIGITS | OCTAL_DIGITS | HEX_DIGITS;
fragment DIGITS: '1'..'9' '0'..'9'*;
fragment OCTAL_DIGITS: '0' '0'..'7'+;
fragment HEX_DIGITS: '0x' ('0'..'9' | 'a'..'f' | 'A'..'F')+;

In this example, matching a NUMBER will always return a NUMBER to the lexer, regardless of if it matched "1234", "0xab12", or "0777".

See item 3