正则表达式性能与纯粹迭代的最佳实践 [英] Best practices for regex performance VS sheer iteration

问题描述

我想知道什么时候使用正则表达式VS "string".contains("anotherString")和/或其他String API调用有任何通用准则?

I was wondering if there are any general guidelines for when to use regex VS "string".contains("anotherString") and/or other String API calls?

虽然上面给定的.contains()决定是微不足道的(如果可以在单个调用中执行此操作，那么为什么要对正则表达式进行打扰)，现实生活中会做出更复杂的选择.例如，最好执行两个.contains()调用或单个正则表达式?

While above given decision for .contains() is trivial (why bother with regex if you can do this in a single call), real life brings more complex choices to make. For example, is it better to do two .contains() calls or a single regex?

我的经验法则是始终使用正则表达式，除非可以用单个API调用代替它.这可以防止代码膨胀，但是从代码可读性的角度来看可能不是那么好，尤其是如果正则表达式倾向于变大的话.

My rule of thumb was to always use regex, unless this can be replaced with a single API call. This prevents code against bloating, but is probably not so good from code readability point of view, especially if regex tends to get big.

另一个经常被忽略的论点是性能.我如何知道此正则表达式需要进行多少次迭代(如"Big O"中所述)?它会比纯粹的迭代更快吗?每个人都以某种方式认为，一旦正则表达式看起来少于5个if语句，它肯定会更快.但是，总是这样吗?如果无法预先预编译正则表达式，则尤其重要.

Another, often overlooked, argument is performance. How do I know how many iterations (as in "Big O") does this regex require? Would it be faster than sheer iteration? Somehow everybody assumes that once regex looks shorter than 5 if statements, it must be quicker. But is this always the case? This is especially relevant if regex cannot be pre-compiled in advance.

推荐答案

答案(通常)取决于它.

The answer (as usual) is that it depends.

在您的特定情况下，我想替代方法是执行正则表达式"this | that"，然后进行查找.这种特殊的构造确实戳了正则表达式的弱点.在这种情况下，"OR"并不真正知道子模式要尝试执行的操作，因此无法轻松地进行优化.最终完成了(用伪代码)的等效操作:

In your particular case, I guess the alternative would be to do the regex "this|that" and then do a find. This particular construct really pokes at regex's weaknesses. The "OR" in this case doesn't really know what the sub-patterns are trying to do and so can't easily optimize. It ends up doing the equivalent of (in pseudo code):

for( i = 0; i < stringLength; i++ ) {
    if( stringAt pos i starts with "this" )
       found!
    if( stringAt pos i starts with "that" )
       found!
}

几乎没有较慢的方法可以做到这一点.在这种情况下，两个contains()调用会更快.

There almost isn't a slower way to do it. In this case, two contains() calls will be much faster.

另一方面，完全匹配:".*this.*|.*that.*"可能会更好地优化.

On the other hand, a full match on: ".*this.*|.*that.*" may optimize better.

对我来说，当代码复杂或笨拙时，应使用正则表达式.因此，如果要在目标字符串中找到两个或三个字符串之一，则只需使用contains即可.但是，如果您想查找以'A'或'B'开头并以'g'-'m'结尾的单词，请使用正则表达式.

To me, regex should be used when the code to do otherwise is complicated or unwieldy. So if you want to find one of two or three strings in a target string then just use contains. But if you wanted to find words starting with 'A' or 'B' and ending in 'g'-'m'... then use regex.

然后您就不必担心这里和那里的几个周期了.

And then you won't be so worried about a few cycles here and there.

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