Antlr 奇怪的括号语法

Question

Cant understand this round bracket meaning. Its not necessary to write it, but sometimes it can produce left-recursion error. Where should we use it in grammar rules?

Answer 1

Its not necessary to write it,

That is correct, it is not necessary. Just remove them.

but sometimes it can produce left-recursion error.

If that really is the case, you can open an issue here: https://github.com/antlr/antlr4/issues

EDIT

Seeing kaby76's comment, just to make sure: you cannot just remove them from a grammar file regardless. They can be removed from your example rule.

When used like this:

rule
 : ID '=' ( NUMBER | STRING ) // match either `ID '=' NUMBER` 
                              //           or `ID '=' STRING`
 ;

they cannot be removed because removing them wold result in:

rule
 : ID '=' NUMBER | STRING // match either `ID '=' NUMBER` 
                          //           or `STRING`
 ;

Or with repetition:

rule
 : ( ID STRING )+ // match: `ID STRING ID STRING ID STRING ...`
 ;

and this:

rule
 : ID STRING+ // match: `ID STRING STRING STRING ...`
 ;