嵌入注释的 antlr 问题

Question

我正在尝试在 D 中实现嵌套注释。

nestingBlockComment
 : '/+' (options {greedy=false;} :nestingBlockCommentCharacters)* '+/' {$channel=HIDDEN;}; // line 58

  nestingBlockCommentCharacters
   :  (nestingBlockComment| '/'~'+' | ~'/' ) ; //line 61

对我来说，这应该可行是合乎逻辑的...

This is the error message I get:
[21:06:34] warning(200): d.g:58:64: Decision can match input such as "'+/'" using multiple alternatives: 1, 2
As a result, alternative(s) 1 were disabled for that input
[21:06:34] warning(200): d.g:61:7: Decision can match input such as "'/+'" using multiple alternatives: 1, 3
As a result, alternative(s) 3 were disabled for that input

有人可以向我解释这些错误消息和修复方法吗？

谢谢。

Answer 1

AFAIK，错误是因为 nestingBlockCommentCharacters can 匹配 +/ （~'/' 两次）。

就我个人而言，我会将nestingBlockComment保留为词法分析器规则而不是解析器规则。您可以通过在词法分析器类中添加一些辅助方法来做到这一点：

public boolean openOrCloseCommentAhead() {
  // return true iff '/+' or '+/' is ahead in the character stream
}

然后在词法分析器注释规则中，使用 门控语义谓词，使用该辅助方法作为谓词内的布尔表达式：

// match nested comments
Comment
  :  '/+' (Comment | {!openOrCloseCommentAhead()}?=> Any)* '+/'
  ;

// match any character
Any
  :  .
  ;

一些演示语法：

grammar DComments;

@lexer::members {
  public boolean openOrCloseCommentAhead() {
    return (input.LA(1) == '+' && input.LA(2) == '/') ||
           (input.LA(1) == '/' && input.LA(2) == '+');
  }
}

parse
  :  token+ EOF
  ;

token
  :  Comment {System.out.println("comment :: "+$Comment.text);}
  |  Any
  ;

Comment
  :  '/+' (Comment | {!openOrCloseCommentAhead()}?=> Any)* '+/'
  ;

Any
  :  .
  ;

以及一个用于测试它的主类：

import org.antlr.runtime.*;

public class Main {
    public static void main(String[] args) throws Exception {
        ANTLRStringStream in = new ANTLRStringStream(
            "foo /+ comment /+ and +/ comment +/ bar /+ comment +/ baz");
        DCommentsLexer lexer = new DCommentsLexer(in);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        DCommentsParser parser = new DCommentsParser(tokens);
        parser.parse();
    }
}

然后是以下命令：

java -cp antlr-3.2.jar org.antlr.Tool DComments.g 
javac -cp antlr-3.2.jar *.java
java -cp .:antlr-3.2.jar Main

(对于 Windows，最后一个命令是：java -cp .;antlr-3.2.jar Main)

产生以下输出：

comment :: /+ comment /+ and +/ comment +/
comment :: /+ comment +/