使用交替或字符类进行单字符匹配? [英] Using alternation or character class for single character matching?

问题描述

(注意:标题似乎不清楚——如果有人可以改写这个，我完全赞成！)

(Note: Title doesn't seem to clear -- if someone can rephrase this I'm all for it!)

给定这个正则表达式:(.*_e.txt)，它匹配一些文件名，除了 e 之外，我还需要添加一些其他的单字符后缀.我应该选择一个字符类还是应该为此使用替代?(或者真的重要吗??)

Given this regex: (.*_e.txt), which matches some filenames, I need to add some other single character suffixes in addition to the e. Should I choose a character class or should I use an alternation for this? (Or does it really matter??)

也就是说，以下哪两个看起来更好"，为什么:

That is, which of the following two seems "better", and why:

a) (.*(e|f|x).txt)，或

b) (.*[efx].txt)

推荐答案

使用 [efx] - 这正是字符类的设计目的:匹配包含的字符之一.因此它也是最易读、最短的解决方案.

Use [efx] - that's exactly what character classes are designed for: to match one of the included characters. Therefore it's also the most readable and shortest solution.

我不知道它是否更快，但如果不是，我会非常惊讶.绝对不会慢.

I don't know if it's faster, but I would be very much surprised if it wasn't. It definitely won't be slower.

我的推理(从来没有写过正则表达式引擎，所以这纯粹是猜想):

My reasoning (without ever having written a regex engine, so this is pure conjecture):

正则表达式标记 [abc] 将在正则表达式引擎的一个步骤中应用:下一个字符是 a, b，还是c?"

The regex token [abc] will be applied in a single step of the regex engine: "Is the next character one of a, b, or c?"

(a|b|c) 但是告诉正则表达式引擎

(a|b|c) however tells the regex engine to

• 如有必要，记住字符串中的当前位置以进行回溯
• 检查是否可以匹配a.如果是这样，成功.如果不是:
• 检查是否可以匹配b.如果是这样，成功.如果不是:
• 检查是否可以匹配c.如果是这样，成功.如果不是:
• 放弃.

• remember the current position in the string for backtracking, if necessary
• check if it's possible to match a. If so, success. If not:
• check if it's possible to match b. If so, success. If not:
• check if it's possible to match c. If so, success. If not:
• give up.

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