解析器规则和词法分析器规则之间的替代 [英] Alternative between parser rule and lexer rule

问题描述

该问题埋在另一个问题的更新部分中，现在专门问这个问题.

The question is buried in the update section of another question, now specifically ask it.

我正在使用antlr3.4.

I am using antlr3.4.

我有一个简单的语法试图解析2种类型的文本，该行以"#include"开头，还有其他.这是我的语法:

I have a simple grammar trying to parse 2 types of text, the line started with "#include" and others. Here is my grammar:

cmds
    : cmd+
    ;

cmd
    : include_cmd |  other_cmd
    ;

include_cmd
    : INCLUDE  DOUBLE_QUOTE  FILE_NAME  DOUBLE_QUOTE
    ;

other_cmd
    : (~INCLUDE)+
    ;


INCLUDE
    : '#include'
    ;

DOUBLE_QUOTE
    : '"'
    ;

FILE_NAME
    : ('a'..'z' | 'A'..'Z' | '0'..'9' | '_')+
    ;

New_Line 
    : ('\r' | '\n')+   
    ;

WS 
    : ('\t' | ' ')+  {$channel = HIDDEN;}
    ;

但是我得到这样的警告:

But I get such warning:

Decision can match input such as "{DOUBLE_QUOTE..FILE_NAME, New_Line..WS}" using multiple alternatives: 1, 2

As a result, alternative(s) 2 were disabled for that input

我想这是因为双引号可以同时匹配other_cmd规则和DOUBLE_QUOTE规则，但是我想知道在这里，一个是解析器规则，另一个是lexer规则，这个警告有意义吗?

I guess this is because a double quote can match both other_cmd rule and DOUBLE_QUOTE rule, but I am wondering here, one is parser rule and the other is lexer rule, does this warning make sense?

是否有帮助清除此警告?

Any help to clear this warning?

一个附带的问题-警告消息只说替代项1,2，但是我现在还不清楚我是1，什么是2，是否有一种方法可以使antlr提供更多直接替代项?

A side question - the warning message just says alternative 1,2, but it is not immediately clear to me what is 1 and what is 2, is there a way to render antlr to give more direct alternatives?

推荐答案

我想这是因为双引号可以同时匹配other_cmd规则和DOUBLE_QUOTE规则，...

I guess this is because a double quote can match both other_cmd rule and DOUBLE_QUOTE rule, ...

否，这不是问题，因为include_cmd以other_cmd无法匹配的开头.

No, that is not the issue, since include_cmd starts with something that other_cmd cannot match.

决策可以使用多种选择来匹配输入，例如"{DOUBLE_QUOTE..FILE_NAME，New_Line..WS}":1，2

Decision can match input such as "{DOUBLE_QUOTE..FILE_NAME, New_Line..WS}" using multiple alternatives: 1, 2

该警告表示解析器可以通过多种方式匹配诸如foo"(后跟DOUBLE_QUOTE的输入)的:

The warning means that input like foo" (a FILE_NAME followed by a DOUBLE_QUOTE) can be matched by the parser in more than one way:

ANTLR将选择贪婪解析，但是由于可能出现贪婪，因此会生成警告.如果您明确告诉解析器贪婪地进行匹配，则不会再发出警告:

ANTLR will choose the greedy parse, but since an ungreedy is possible, a warning is generated. If you explicitly tell the parser to match greedily, the warning would not be issued anymore:

other_cmd
 : (options {greedy=true;} : ~INCLUDE)+
 ;

一个附带的问题-警告消息只说替代项1,2，但是我现在还不清楚我是1，什么是2，是否有一种方法可以使antlr提供更多直接替代项?

A side question - the warning message just says alternative 1,2, but it is not immediately clear to me what is 1 and what is 2, is there a way to render antlr to give more direct alternatives?

不，据我所知.这个警告确实是个神秘的事物.替代方法通常表示解析器可以遵循的分支:

No, not as far as I know. This warning is indeed rather cryptic. Alternatives usually denote the branches the parser can follow:

parser_rule
 : alternative_1 
 | alternative_2
 | alternative_3
 ;

但是在您的情况下，ANTLR似乎在谈论令牌范围是替代方案:DOUBLE_QUOTE..FILE_NAME是替代方案，New_Line..WS是第二替代方案.

But in your case, it seems ANTLR is talking about token ranges being the alternatives: DOUBLE_QUOTE..FILE_NAME being an alternative and New_Line..WS being the 2nd.

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