嵌入注释的antlr问题

好吧,这个错误是因为 nestingBlockCommentCharacters 可以 比赛 +/ (the) ~'/' 两次)。

nestingBlockComment 作为lexer规则而不是解析器规则。可以通过在lexer类中添加一个小helper方法来实现:

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

然后在lexer注释规则中,使用 gated semantic predicates 将该helper方法作为谓词中的布尔表达式:

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

// match any character
Any
  :  .
  ;

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
  :  .
  ;

还有一个主要的类来测试它:

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();
    }
}

然后执行以下命令:

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 +/