为什么正则表达式引擎选择从.X | ..X | X.中匹配模式`.X'? [英] Why regex engine choose to match pattern `..X` from `.X|..X|X.`?

问题描述

我有一个字符串

1234X5678

并且我使用此正则表达式匹配模式

and I use this regex to match pattern

.X|..X|X.

我知道了

34X

问题是为什么我没有得到4X或X5?

The question is why didn't I get 4X or X5?

为什么正则表达式选择执行第二种模式?

Why regex choose to perform the second pattern?

推荐答案

这里的重点是:

Regex引擎默认情况下会分析从左到右的输入.

因此，您有一个交替模式.X|..X|X.，并针对1234X5678运行它. 看看会发生什么:

So, you have an alternation pattern .X|..X|X. and you run it against 1234X5678. See what happens:

每个替代分支都针对字符串中从左到右的每个位置进行测试.

前1-7个步骤显示了引擎如何尝试匹配字符串开头的字符.但是，没有分支(.X，..X和X.都不匹配12或123).

The first 1-7 steps show how the engine tries to match the characters at the beginning of the string. However, none of the branches (neither .X, nor ..X, nor X. match 12 or 123).

步骤8-13只是重复相同的失败情况，因为没有一个分支匹配23或234.

Steps 8-13 just repeat the same failing scenario as none of the branches match 23 or 234.

步骤14-19显示了成功的情况，因为34X可以与分支2(..X)匹配.

Steps 14-19 show a success scenario because the 34X can be matched with Branch 2 (..X).

正则表达式引擎未到达4之前的位置，因为该位置已匹配并已被消耗 .

The regex engine does not reach the location before 4 since this location gets matched and consumed.

另一个结论:

交替的顺序很重要，在NFA regex引擎中，第一个替代匹配获胜，但该替代不必是最短的替代，而是更长的替代，它与开始可以更早匹配.

The order of alternations matters, and in NFA regex engines the first alternative matched wins, BUT this alternative does not have to be the first shortest one, a farther longer alternative that matches the same characters in the beginning can match earlier.

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