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

问题描述

(注意:标题似乎不清楚-如果有人可以改写我的意思，就是这样！)

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

鉴于此正则表达式:(.*_e\.txt)(与某些文件名匹配)，除了e外，我还需要添加一些其他单字符后缀.我应该选择字符类还是为此使用替代字符? (还是真的很重要吗?)

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，以下两个中的哪一个看起来更好"，以及原因:

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:匹配其中一个字符.因此，它也是最易读和最短的解决方案.

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，但是如果不是更快的话，我会感到非常惊讶.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(从来没有编写过正则表达式引擎，所以这纯粹是推测):

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

正则表达式令牌[abc]将在正则表达式引擎的单个步骤中应用:下一个字符是a，b还是c之一?"

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

• 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，那就成功了.如果没有:
• 检查是否可以匹配b.如果是这样，那就成功了.如果没有:
• 检查是否可以匹配c.如果是这样，那就成功了.如果没有:
• 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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