Antlr: Unintended behavior

Question

Why this simple grammar

grammar Test;

expr
    :    Int | expr '+' expr;

Int
:    [0-9]+;

doesn't match the input 1+1 ? It says "No method for rule expr or it has arguments" but in my opition it should be matched.

Answer 1

It looks like I haven't used ANTLR for a while... ANTLRv3 did not support left-recursive rules, but ANTLRv4 does support immediate left recursion. It also supports the regex-like character class syntax you used in your post. I tested this version and it works in ANTLRWorks2 (running on ANTLR4):

grammar Test;

start : expr
      ;
expr  : expr '+' expr
      | INT
      ;
INT   : [0-9]+
      ;

If you add the start rule then ANTLR is able to infer that EOF goes at the end of that rule. It doesn't seem to be able to infer EOF for more complex rules like expr and expr2 since they're recursive...

---

There are a lot of comments below, so here is (co-author of ANTLR4) Sam Harwell's response (emphasis added):

You still want to include an explicit EOF in the start rule. The problem the OP faced with using expr directly is ANTLR 4 internally rewrote it to be expr[int _p] (it does so for all left recursive rules), and the included TestRig is not able to directly execute rules with parameters. Adding a start rule resolves the problem because TestRig is able to execute that rule. :)

I've posted a follow-up question with regard to EOF: When is EOF needed in ANTLR 4?