complex AST rewrite rule in ANTLR - antlr

Question

After the problem about AST rewrite rule with devide group technique at AST rewrite rule with " * +" in antlr.
I have a trouble with AST generating in ANTLR, again :).Here is my antlr code :
start : noun1+=n (prep noun2+=n (COMMA noun3+=n)*)*
-> ^(NOUN $noun1) (^(PREP prep) ^(NOUN $noun2) ^(NOUN $noun3)*)*
;
n : 'noun1'|'noun2'|'noun3'|'noun4'|'noun5';
prep : 'and'|'in';
COMMA : ',';
Now, with input : "noun1 and noun2, noun3 in noun4, noun5", i got following unexpected AST:
Compare with the "Parse Tree" in ANLRwork:
I think the $noun3 variable holding the list of all "n" in "COMMA noun3+=n". Consequently, AST parser ^(NOUN $noun3)* will draw all "n" without sperating which "n" actually belongs to the "prep"s.
Are there any way that can make the sepration in "(^(PREP prep) ^(NOUN $noun2) ^(NOUN $noun3))". All I want to do is AST must draw exactly, without token COMMA, with "Parse Tree" in ANTLRwork.
Thanks for help !

Answer 1

Getting the separation that you want is easiest if you break up the start rule. Here's an example (without writing COMMAs to the AST):
start : prepphrase //one prepphrase is required.
(COMMA! prepphrase)* //"COMMA!" means "match a COMMA but don't write it to the AST"
;
prepphrase: noun1=n //You can use "noun1=n" instead of "noun1+=n" when you're only using it to store one value
(prep noun2=n)?
-> ^(NOUN $noun1) ^(PREP prep)? ^(NOUN $noun2)?
;
A prepphrase is a noun that may be followed by a preposition with another noun. The start rule looks for comma-separated prepphrases.
The output appears like the parse tree image, but without the commas.
If you prefer explicitly writing out ASTs with -> or if you don't like syntax like COMMA!, you can write the start rule like this instead. The two different forms are functionally equivalent.
start : prepphrase //one prepphrase is required.
(COMMA prepphrase)*
-> prepphrase+ //write each prepphrase, which doesn't include commas
;