Epsilon definition in JavaCC

Question

How can I define Epsilon in JavaCC?? I'm trying smething like

< EPSILON: "">

but it doesn't work.

Answer 1

You can define a token to match the empty string if the token manager then switches to a lexical state in which at least one character will be consumed before returning to the state in which the empty string was matched. So:

TOKEN : {
    < EMPTY : "" >
    : MATCH_NON_EMPTY // change state, or infinite matches of the empty string could result
}

<MATCH_NON_EMPTY>
TOKEN : {
    < NON_EMPTY : ~[] >
}

void Start() : { } {
    <EMPTY> <NON_EMPTY>
}

That will work fine given any single-character input.

I should mention a caveat, however. It seems that a match of EOF is considered to be "longer" than a match of the empty string, so an empty string cannot be matched immediately before EOF without some lexical action magic when EOF is matched.

Answer 2

Since I was also looking for a way to use the empty word epsilon I came up with the following approach.

I simply used an empty production rule such as:

void Empty () : { } { System.out.prntln("epsilon"); }

Hope it works - at least for now it seems so ...

Addendum:

It works pretty fine :)