标记的 ANTLR 语法表达式求值，可以使用 0 次或 1 次

Question

我有一个组合的 ANTLR 语法，它应该寻找特定的标记/标记组合。有些 token 可以使用 0 次或 1 次。 我想将“找到”或“未找到”值存储到这些 token 的 HashMap 中。我已经知道，在 token 可用的情况下如何执行此操作。但是对于尚未找到的 token 我该如何做。
以下示例显示它非常简化:

grammar Expr;

@header {
import java.util.HashMap;
}

@members {
HashMap<String,String> memory = new HashMap<String,String>();
}

statements: ONE  
      (WS TWO {memory.put("two","found");})? 
      WS THREE EOF;    

ONE: 'one';
TWO: 'two';
THREE: 'three';
WS: ' ';

是否有可能直接在此语法中找出“TWO”未找到，因此将不同的值放入 HashMap 中？

memory.put("two","not found");

Answer 1

Is there a possibility to find out directly within this grammar, that "TWO" was not found and therefore put a different value to the HashMap?

当然，像这样:

statements
 : ONE WS ( TWO WS THREE EOF {memory.put("two","found");}
          | THREE EOF        {memory.put("two","not found");}
          )
 ;    

编辑

或者，如果您有更多可选 token (TWO、THREE 和 4 是可选的)，请执行以下操作:

statements
 : ONE (WS TWO memory.put("two","found");     | memory.put("two","not found");) 
       (WS THREE memory.put("three","found"); | memory.put("three","not found");) 
       (WS FOUR memory.put("four","found");   | memory.put("four","not found");) 
   EOF
 ;

或者类似的东西也应该有效:

statements
 : ONE (WS TWO)? (WS THREE)? (WS FOUR)?
   {
     memory.put("two", $TWO.text == null ? "not found" : "found");
     memory.put("three", $THREE.text == null ? "not found" : "found");
     memory.put("four", $FOUR.text == null ? "not found" : "found");
   }
 ;