解析器规则和词法分析器规则之间的替代

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问题描述

该问题被埋在另一个问题的更新部分，现在特地问一下.

我使用的是 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"和其他开头.这是我的语法:

cmds: cmd+;指令:include_cmd |其他_cmd;包含_cmd: 包括 DOUBLE_QUOTE FILE_NAME DOUBLE_QUOTE;其他_cmd:(~包括)+;包括: '#包括';DOUBLE_QUOTE:'"';文档名称: ('a'..'z' | 'A'..'Z' | '0'..'9' | '_')+;新队: ('\r' | '\n')+;WS: ('\t' | ' ')+ {$channel = HIDDEN;};

但我收到这样的警告:

Decision 可以使用多个替代项匹配输入，例如{DOUBLE_QUOTE..FILE_NAME, New_Line..WS}":1, 2因此，该输入禁用了备选方案 2

我猜这是因为双引号可以同时匹配 other_cmd 规则和 DOUBLE_QUOTE 规则，但我在这里想知道，一个是解析器规则，另一个是词法分析器规则，这个警告有意义吗?

对清除此警告有帮助吗?

附带问题 - 警告消息只是说替代方案 1,2，但我不清楚什么是 1，什么是 2，有没有办法让 antlr 提供更直接的替代方案?

解决方案

不，这不是问题，因为 include_cmd 以 other_cmd 无法匹配的内容开头.

Decision 可以匹配输入，例如{DOUBLE_QUOTE..FILE_NAME, New_Line..WS}"，使用多个替代项:1, 2

警告意味着像 foo"(一个 FILE_NAME 后跟一个 DOUBLE_QUOTE)这样的输入可以被解析器匹配到不止一个方式:

1.贪婪

2.不贪心

ANTLR 将选择贪婪解析，但由于可能出现不贪婪，因此会生成警告.如果您明确告诉解析器进行贪婪匹配，则不会再发出警告:

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

附带问题 - 警告消息只是说替代方案 1,2，但我不清楚什么是 1，什么是 2，有没有办法让 antlr 提供更直接的替代方案?

不，据我所知没有.这个警告确实相当神秘.替代项通常表示解析器可以遵循的分支:

parser_rule:替代_1|替代_2|替代_3;

但在您的情况下，似乎 ANTLR 正在谈论令牌范围是替代方案:DOUBLE_QUOTE..FILE_NAME 是替代方案，而 New_Line..WS 是第二个.

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

I am using antlr3.4.

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;}
    ;

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

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?

Any help to clear this warning?

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?

解决方案

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

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:

1. greedy

2. ungreedy

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)+
 ;

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
 ;

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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