Question

I would like to know, why

expr:
a=(.*) 'ing' { System.out.println($a.text};

Does not work.

Answer 1

For one, you’re missing ); in your embedded code. It’s not:

{ System.out.println($a.text}

but:

{ System.out.println($a.text); }

You’re also not really clear as to what “does not work” means, but I can make an educated guess. You’re probably seeing null printed to the console. This is because you cannot assign a label to .* (a=(.*) is invalid). What you can do is move what a is supposed to point to, to a sub-rule:

expr
 : a=sub_rule 'ing' { System.out.println($a.text); }
 ;

sub_rule
 : /* ... something other than '.*' ... */
 ;

You should not be using .* inside parser rules, unless you know exactly what you’re doing.