[antlr-interest] [SPAM] Syntax ambiguity?

[Olivier Lefevre]

Sorry, the subject is not very informational but I cannot
get the hang of the problem, so I cannot devise a better
subject. I have this small grammar:

   grammar Gr3;

   options { output=AST; }

   stat : fun1 | fun2 ;
   fun1 : 'fun1(' ID1 ')' ;
   fun2 : 'fun2(' ID2 ')' ;

   fragment DIGIT  : '0'..'9' ;
   fragment LETTER : ('a'..'z' | 'A'..'Z') ;

   ID1 : (DIGIT | LETTER)+ ;
   ID2 : (DIGIT | LETTER | '_' | '-' | '.')+ ;
   WS  : (' '|'\t')+ { skip(); } ;

It can recognize, say, fun1(AB) and fun2(AB_CD) as expected
but not fun2(AB), which should also be valid since AB matches
both ID1 or ID2. Rewriting fun2 as

   fun2 : 'fun2(' (ID1 | ID2) ')' ;

works but is not satisfactory because I want an ID2 as fun2
argument, not an ID1. So, how can I force ANTLR to "consider"
ID1 in this position?

Thanks,

-- O.L.