With the latest hourly build (antlr-03-16-2007.10): lexer grammar x; LT: '<' ' '* | ('<' IDENT) => '<' IDENT '>'; IDENT: 'a'+; Produces the following error: error(100): x.g:0:0: syntax error: buildnfa: <AST>:5:8: unexpected AST node: synpred1