在Flex中处理数字后跟字母 [英] Handle number followed by letters in Flex

问题描述

在某些Flex/Lex代码中，

  DIGIT [0-9]字母[A-Za-z]ID {LETTER} {LETTER} | {DIGIT})*NUM {DIGIT} +(\.{DIGIT} +)? 

因此， 123 匹配 NUM ，而 abc123 匹配 ID .的情况下 123abc ，它匹配 NUM ，后跟和 ID .然而，我希望它生成一条错误消息而不是匹配消息.

有没有简单的方法可以做到这一点?

解决方案

您可以添加其他模式来检测错误.

以下内容不使用宏，因为我个人认为它们会分散注意力.但是想法是一样的.

  [[:: alpha:]] [[:alnum:]] * {返回IDENTIFIER;}[[:::] +."([[:::] +)?{返回NUMBER;}[[:::] +."([[:::] +)?[[:alpha:]] {返回BAD_NUMBER;} 

只有在数字后紧跟一个字母时，最后一个模式才匹配，并且由于最长匹配规则，它将覆盖第二个模式.

顺便说一句，一个更好的数字模式是:

  [[:::]] +(." [[:::] *)?|." [[:::]] + 

这将与 23.和 .56 匹配，许多人希望这是有效数字.

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您可能还会发现此答案有趣，尤其是其他编程语言中的示例.大多数语言(除了C& family之外)都允许将 123abc 作为两个标记进行扫描，无论如何这通常都会导致语法错误，并且这是最简单且可维护性最高的解决方案.

In some Flex/Lex code we have,

DIGIT       [0-9]
LETTER      [A-Za-z]
ID          {LETTER}{LETTER}|{DIGIT})*
NUM         {DIGIT}+(\.{DIGIT}+)?

So, 123 matches NUM and abc123 matches ID. In case of 123abc, this matches NUM followed by and ID. However, I want this to generate an error message instead of matching.

Is there a simple way to do this?

解决方案

You can just add an additional pattern to detect an error.

The following does not use macros, because I personally find them distracting. But the idea is the same.

[[:alpha:]][[:alnum:]]*        { return IDENTIFIER; }
[[:digit:]]+"."([[:digit:]]+)? { return NUMBER; }
[[:digit:]]+"."([[:digit:]]+)?[[:alpha:]] { return BAD_NUMBER; }

The last pattern will only match if there is a letter immediately following a NUMBER, and will override the second pattern because of the longest-match rule.

By the way, a better pattern for a number is:

[[:digit:]]+("."[[:digit:]]*)?|"."[[:digit:]]+

That will match 23. and .56, which many people expect to be valid numbers.

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You might also find this answer interesting, particularly the examples from other programming languages. Most languages (other than C & family) do allow 123abc to be scanned as two tokens, which usually leads to a syntax error anyway, and that is both the easiest and the most maintainable solution.

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