是否经常EX pression引擎跳过，比模式更短的字符串？ [英] Does the regular expression engine skip over strings that are shorter than the pattern?

问题描述

我要通过一组字符串循环。在每个字符串我想遍历一组正EX pression，以确定哪些EX pressions对我的字符串匹配。但是，如果该字符串长度比图案串的最大可能长度短，我想要的正则表达式引擎跳过它。

I want to loop through a set of strings. On each string i want to loop through a set of regular expression to determine which expressions match on the string I'm on. However, If the string length is shorter than the maximum possible length of the pattern string, I want the regex engine to skip over it.

例如说我停在一个字符串abc 与此正则表达式进行测试。

For example say I stop on a string "abc" and test it with this regex.

(?i)[A-Z]{3}

和它匹配。然后，我的下一个前pression测试就像

and it matches. Then my next expression to test is like

(?i)[A-Z]+(?=123)

将发动机仍然开始研究从字符串的开始，即使在第二种情况永远不会成为一个匹配？

Will the engine still start examining the string from the beginning even though the second case will never be a match?

如果是这样的话，有没有办法得到它跳过不符合最小长度要求的字符串？

If this is the case, is there a way to get it to skip over strings that don't meet minimum length requirement?

推荐答案

当你后的实施细则，并在源$ C ​​$ C可，说最好的办法是简单地看它。 ：）

When you're after implementation details, and when the source code is available, the best way to tell is to simply look at it. :)

简短的回答是：不完全的

在.NET正则表达式执行实施的优化是一个博耶 - 穆尔字符串搜索作为匹配的第一阶段的如果可能的话。看看的<一个href="http://referencesource.microsoft.com/#System/regex/system/text/regularex$p$pssions/RegexBoyerMoore.cs"相对=nofollow>来源$ C ​​$ C ，在血淋淋的细节。

The optimization implemented in the .NET regex implementation is a Boyer-Moore string search as the first phase of matching when possible. Take a look at the source code for the gory details.

从code本身：

// The RegexBoyerMoore object precomputes the Boyer-Moore
// tables for fast string scanning. These tables allow
// you to scan for the first occurance of a string within
// a large body of text without examining every character.
// The performance of the heuristic depends on the actual
// string and the text being searched, but usually, the longer
// the string that is being searched for, the fewer characters
// need to be examined.

这需要一个固定的 preFIX ，这是搜索由<一个href="http://referencesource.microsoft.com/#System/regex/system/text/regularex$p$pssions/RegexFCD.cs,5559c2458a83d8ee,references"相对=nofollow>这个功能，其评论说：

This requires an anchoring prefix, which is searched for by this function, whose comment says:

/*
 * This is the one of the only two functions that should be called from outside.
 * It takes a RegexTree and computes the set of chars that can start it.
 */

匹配算法包含code它返回的不敌的结果立即如果输入字符串比计算preFIX短。

The matching algorithm contains code which returns a no match result immediately if the input string is shorter than the computed prefix.

请注意，这也是在寻找的锚的优化，当然这些。

Note that it's also looking for anchors and optimizing for these, of course.

我没有找到一个的最小长度的在code优化，但我承认我没有仔细阅读<子>（必须这样做，有一天）。但我知道其他的正则表达式的实现，它们做这样的优化（PCRE想到）。无论如何，.NET实现有自己的优化的事情来说，你应该依赖于这一点。

I did not find a minimum length optimization in the code, but I admit I didn't read it thoroughly _{(gotta do that one day)}. But I know other regex implementations which do this kind of optimization (PCRE comes to mind). Anyway, the .NET implementation has its own way of optimizing things, you should rely on that.

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