javacc Expansion within "(...)+" can be matched by empty string error

Question

I'm working on a parser and require custom errors to be thrown for every keyword. My code is the following.

SKIP:  { " " | "\t" | "\n" | "\r" }
TOKEN: { "DEF" | "MAIN" | <NAME: (["A"-"Z"])+> | <PARAM: (["a"-"z"])+> | <NUM: (["0"-"9"])+> }

void Start(): {} {(Def() Func())+ <EOF>}

void Def(): {} {"DEF" | { throw new ParseException("expected keyword DEF"); }}

void Func(): {} {"MAIN" | Name() Param() | { throw new ParseException("Expected MAIN or NAME PARAM"); }}

void Name(): {} {<NAME> | { throw new ParseException("invalid function name"); }}

void Param(): {} { <PARAM> | { throw new ParseException("invalid PARAM"); }}

The Start() function is giving me an error and tells me that Expansion within "(...)+" can be matched by empty string error . I think the problem is in the Name() Param() part of Func() but I do not know how to change this while still throwing custom error messages. Can anyone provide some pointers?

Answer 1

While I agree with the comment from user207421, you could maybe do the following

void oneOrMoreThings() : {} {
    (Thing() | (throw new ParseException( ... ) ; }
    ( Thing() )*
}

Answer 2

将 DEF 设为可选，然后检查它是否已找到，如果没有则引发异常。

Start(): {Token tk=null;} {tk="DEF"? {if (tk==null) throw ...} "MAIN" etc