为什么不是"MOST N次，X次"? Java中的"X {，n}"中包含的正则表达式量词句法? [英] Why isn't the "X, at MOST n times" regex quantifier included in Java, in the "X{,n}" syntax?

问题描述

Java文档告诉我们您可以拥有这个贪婪的量化器:

Java docs tell us you can have this greedy quantifiers:

X{n}    X, exactly n times
X{n,}   X, at least n times
X{n,m}  X, at least n but not more than m times

没有提及拥有X, at MOST n times.

所以我做了一点测试:

boolean atMost = Pattern.matches("X{,3}", "XX");

我希望atMost为true，因为可以安全地将下限假定为零. 相反，我遇到了一个例外:

I expected atMost to be true, since it's safe to assume the lower bound to be zero. Instead, I got an exception:

Exception in thread "main" java.util.regex.PatternSyntaxException: Illegal repetition near index 0
X{,3}
^

请注意，这不是贪婪，不情愿或可能的事情:"X{,3}?"和"X{,3}+"也不起作用.

Note it's not a thing of being Greedy, Reluctant or Possessive: "X{,3}?" and "X{,3}+" also don't work.

我本可以使用"X{0,3}"来实现所需的模式，但这不是我的问题.

I could have used "X{0,3}" to achieve the pattern I'm looking for, but that's not my question.

为什么X{,n}语法中的Java不包含X, at MOST n times regex量词?

Why isn't the X, at MOST n times regex quantifier included in Java, in the X{,n} syntax?

在其他编程语言或正则表达式中的味道"如何?

How is it like in other programming languages or regex "flavors"?

推荐答案

它不是{n，m}问题，而是 PARSING 问题.

Its not a {n,m} issue, its a PARSING issue.

从左至右，{+数字是在可能范围内必须验证的所有内容
量词，否则{为文字，且解析继续.

Left to right, { + number is all that has to be validated for a possible range
quantifier, otherwise { is literal and the parse continues.

此外，paser还希望有一个简单的数字，因为范围开始与
进行比较 范围结束.它不想同时抖动/允许{min_default，max_default}或{,}

Also, the paser wants a simple number available as the range start to compare with the
range end. It doesn't want to dither/allow simultaneous {min_default,max_default} or {,}

在这样低的级别上太复杂而不能同时允许两者.

It would be too complicated at that low level to allow both.

这篇关于为什么不是"MOST N次，X次"? Java中的"X {，n}"中包含的正则表达式量词句法?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！