ANTLR BNF 语法符号中 epsilon 的等价物是什么? [英] What is the equivalent for epsilon in ANTLR BNF grammar notation?

问题描述

在利用 ANTLR 3.3 的过程中，我正在更改当前的语法以支持不带括号的输入.这是我的语法的第一个版本:

During taking advantage of ANTLR 3.3, I'm changing the current grammar to support inputs without parenthesis too. Here's the first version of my grammar :

grammar PropLogic;

        NOT : '!' ;
        OR  : '+' ;
        AND : '.' ;
        IMPLIES : '->' ;
        SYMBOLS : ('a'..'z') | '~' ;
        OP : '(' ;
        CP : ')' ;

    prog    : formula EOF ;


    formula : NOT formula
        | OP formula( AND formula CP | OR formula CP | IMPLIES formula CP)
        | SYMBOLS ;


    WHITESPACE : ( '\t' | ' ' | '\r' | '\n'| '\u000C' )+    { $channel = HIDDEN; } ;

然后我以这种方式对其进行了更改以支持适当的功能:

Then I changed it this way to support the appropriate features :

grammar PropLogic;

    NOT : '!' ;
    OR  : '+' ;
    AND : '.' ;
    IMPLIES : '->' ;
    SYMBOL : ('a'..'z') | '~' ;
    OP : '(' ;
    CP : ')' ;
    EM : '' ;

prog    : formula EOF ;


formula : OP formula( AND formula CP | OR formula CP | IMPLIES formula CP)
    | ( NOT formula | SYMBOL )( AND formula | OR formula | IMPLIES formula | EM ) ;


WHITESPACE : ( '\t' | ' ' | '\r' | '\n'| '\u000C' )+    { $channel = HIDDEN; } ;

但是我遇到了以下错误:

But I've been faced with following error :

error<100>:  syntax error: invalid char literal: ''
error<100>:  syntax error: invalid char literal: ''

有人知道我怎样才能克服这个错误吗?

Does anybody know that how can I overcome this error?

推荐答案

您的 EM 令牌:

EM : '' ;

无效:您不能在词法分析器规则中匹配空字符串.

is invalid: you can't match an empty string in lexer rules.

要匹配 epsilon(无)，您应该这样做:

To match epsilon (nothing), you should do:

rule 
  :  A 
  |  B 
  |  /* epsilon */ 
  ;

当然，注释 /* epsilon */ 可以安全地删除.

Of course, the comment /* epsilon */ can safely be removed.

请注意，当您在当前语法中这样做时，ANTLR 会抱怨可以使用多种替代方法匹配规则.这是因为你的语法有歧义.

Note that when you do it like that in your current grammar, ANTLR will complain that there can be rules matched using multiple alternatives. This is because your grammar is ambiguous.

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