How do I implement a Antlr4 parser that resolve ambiguity when both rules use "=="?-CodePudding

I know the title is not clear enough. Here is the details. I need to parse something like below:

Client->iFrame.Initialise() {
  res = Server.loadStaticResource()
  if (res.status == 404 || res.status == 503) {  <1>
     throw Error()
  }
}
== Initialisation done! ==                       <2>
Client->iFrame.load(data) {
   moreStatements()
}

It is a Java-like syntax, except that we support a Divider (shown as == Initialisation done! == in the example). Note that:

both <1> and <2> uses ==.
It is treated as a Divider only if it appears at the beginning of the line (after spaces removed).
Any character can be used between the starting == and ending == except for changelines.
There could be more = in the Divider component, such as === 3 equals ===.

How should I implement that?

To add more context, this is to be used to render a sequence diagram like below. It is an opensource project and can be found here:

What I have tried?

I was looking for a way to use lookahead in the Lexer, but could not find any good examples. I am currently limiting the content between starting == and ending == to ONE word only.

My implementation is here:

CodePudding user response：

OK, here's the answer evolved from the question and comments, and from my experience writing little parsers for a long time.

Rule 1: find a way to cheat

In the case of ambiguities like in this question, looking for a solution in the lexer can be a useful avenue to explore. In this case, because newlines apparently have some degree of meaning, introducing a token that matches a newline followed by a couple of = signs means that the parser sees a token ("newline equal equal") that directly indicates the start of that production.

The particular "cheat" is a \n== token, so that the grammar can have that as a separator or statement start (whatever makes sense). If it appears at a weird point in the grammar somehow, the parser can throw an error that says "unexpected start of the == thing" because it knows that's what's going on.