Skipping sub-expressions in listener

Let's say I have logical expressions in this form

((a OR b) AND c) OR (d AND e = (f OR g))

I'm interested in getting a, b, c, d and ignore e, f, g (everything that has to do with assignments).

My attempt was something like this

infixOp : OR | AND ;

assignment : Identifier EQUALS expression ;

expression  :
	Identifier |
	assignment |
	expression infixOp expression | 
	LEFTBRACKET expression RIGHTBRACKET ;

Then in my listener in enterExpression() I just printed expressionContext.getTokens(identifierType). Of course, this is too much and I got all of them.

How could "skip" over an assignment expression? Could this be done in the grammar? Or it can only be done programatically?

What you can do is create a small listener and keep track of the fact when you enter- and exit an assignment. Then inside the enterExpression method, check if you're NOT inside an assignment AND the Identifier token has a value.

A quick demo for the grammar:

grammar T;

parse
 : expression EOF
 ;

expression
 : '(' expression ')'
 | expression 'AND' expression
 | expression 'OR' expression
 | assignment
 | Identifier
 ;


assignment
 : Identifier '=' expression
 ;

Identifier : [a-z]+;
Space      : [ \t\r\n] -> skip;

and Java class:

public class Main {

    public static void main(String[] args) {

        TLexer lexer = new TLexer(CharStreams.fromString("((a OR b) AND c) OR (d AND e = (f OR g))"));
        TParser parser = new TParser(new CommonTokenStream(lexer));

        MyListener listener = new MyListener();

        ParseTreeWalker.DEFAULT.walk(listener, parser.parse());

        System.out.println(listener.ids);
    }
}

class MyListener extends TBaseListener {

    public List<String> ids = new ArrayList<String>();
    private boolean inAssignment  = false;

    @Override
    public void enterExpression(TParser.ExpressionContext ctx) {
        if (!this.inAssignment && ctx.Identifier() != null) {
            this.ids.add(ctx.Identifier().getText());
        }
    }

    @Override
    public void enterAssignment(TParser.AssignmentContext ctx) {
        this.inAssignment = true;
    }

    @Override
    public void exitAssignment(TParser.AssignmentContext ctx) {
        this.inAssignment = false;
    }
}

will print:

[a, b, c, d]

Maybe something like:

    if (!(expressionContext.parent() instanceof AssignmentContext)) {
        expressionContext.getTokens(identifierType);
    }

You can fine that by walking the parse tree structure and check the different members in the expression context.