使用lex对转义序列进行正则表达式说明 [英] Regex clarification on escape sequences with lex

问题描述

我正在创建一个lexer.l文件，该文件按预期工作，但其中一部分除外.我有规则:

I'm creating a lexer.l file that is working as intended except for one part. I have the rule:

[\(\*.*\*\)] {}

我想这样做，所以当我在文件中遇到(* this is a test *)时，我什么都不做.但是，当我运行lex lexer.l时，我在规则\(，\*和\)的行上收到警告，指出它们永远无法满足.所以我想我的问题是，为什么[\(\*.*\*\)] {}会干扰\(和其他?如何捕捉(* this is a test *)?

which I want to make it so when I encounter (* this is a test *) in a file, I simply do nothing with it. However when I run lex lexer.l I get warning on lines with rules \(, \*, and \) stating that they can never be met. So I guess my question is why would [\(\*.*\*\)] {} interfere with \( and the others? How can I catch (* this is a test *)?

推荐答案

注释语法为(*…*)的语言通常允许嵌套注释，并且(f)lex无法轻松识别嵌套注释，因为嵌套要求上下文无关语法，而词法扫描程序仅实现常规语言.

Languages with the comment syntax (*…*) typically allow nested comments, and nested comments cannot easily be recognized by (f)lex because the nesting requires a context-free grammar, and the lexical scanner only implements regular languages.

如果注释不嵌套(因此(* something (* else *)是注释，而不是较长注释的前缀)，则可以使用正则表达式

If your comments do not nest (so that (* something (* else *) is a comment, rather than the prefix of a longer comment), then you can use the regular expression

[(][*][^*]*[*]+([^*)][^*]*[*]+)*[)]

如果确实需要嵌套注释，则可以使用开始条件和堆栈(或如下所示的模拟堆栈):

If you do require nested comments, you can use start conditions and a stack (or a simulated stack, as below):

%x SC_COMMENT

%%
  int comment_nesting = 0;

"(*"             { BEGIN(SC_COMMENT); }
<SC_COMMENT>{
  "(*"           { ++comment_nesting; }
  "*"+")"        { if (comment_nesting) --comment_nesting;
                   else BEGIN(INITIAL); }
  "*"+           ; 
  [^(*\n]+       ;
  [(]            ; 
  \n             ; 
}

该摘要摘录自此答案，并进行了少量调整，因为该答案可识别嵌套的/*…*/注释.此处显示了更完整的代码说明.

That snippet was taken from this answer, with a small adjustment because that answer recognizes nested /*…*/ comments. A fuller explanation of the code appears there.

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