正则表达式字符类减法与负组 [英] Regex character class subtraction with negative groups

问题描述

这个问题与正则表达式(regex)中的字符类减法有关.我参考了 XPATH 2.0 第二版的正则表达式.

This question relates to character class subtraction in regular expression (regex). I refer to the regex flavour of XPATH 2.0 second edition.

当字符类减法中存在负组时，减法运算符 (-) 是否出现在之前?还是在负组运算符 (^) 之后?

When there are negative groups within a character class subtraction, does the subtract operator (-) occur before? or after the negative group operator (^)?

XPATH/XML 模式规范的文本如下.但在我看来，它读起来含糊不清.

The text of the XPATH/ XML schema specification is below. But to my mind, it reads ambiguously.

对于任何·正字符组·或·负字符组·G，和任何·字符类表达式·C，G-C 是有效的·字符类减法·，识别 C(G) 中所有字符的集合不在 C(C) 中.

For any ·positive character group· or ·negative character group· G, and any ·character class expression· C, G-C is a valid ·character class subtraction·, identifying the set of all characters in C(G) that are not also in C(C).

更具体地说，请考虑以下三个正则表达式:

To be more specific, consider the following three regexes:

1. [^abc-[ad]]
2. [^abc-[^ad]]
3. [abc-[^ad]]

与以下的干草堆文本匹配:

being matched against the haystack text of:

• abcdef

可能的匹配文本是什么(第一个和后续的)?

What are the possible match texts (first and subsequent)?

推荐答案

我不认为文本有歧义，如果我们足够宽容将 GC 解读为 [G-[C]]，以及一个否定组，^G，如[^G].现在，插入符号显然是第一组的一部分，并且不会否定这两个组.

I don't think that text is ambiguous, if we are lenient enough to read G-C as [G-[C]], and a negative group, ^G, as [^G]. Now, it looks clear that the caret is part of the first group, and does not negate both groups.

因此，[^abc-[ad]] 会匹配:

{除a、b 和c之外的所有字符} \ {a 和 d} = { 除 a、b、c 和 d}

{All Characters Besides a, b and c} \ {a and d} = { All Characters Besides a, b, c and d}

请记住，您可以轻松测试以查看行为 :).
作为奖励，.Net 正则表达式也支持此功能，使得 在线测试.
另请参阅:字符类减法

Keep in mind, you can easily test to see the behavior :).
As a bonus, .Net regular expressions also support this feature, making it a little easier to test online.
See also: Character Class Subtraction

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