使用正则表达式和 Python 进行短语匹配 [英] Phrase matching using regex and Python

问题描述

我有一些想要匹配的短语.我使用了如下正则表达式:

I have some short phrases that I want to match on. I used a regex as follows:

(^|)(piston|piston ring)( |$)

使用上述内容，regex.match("piston ring") 匹配piston".如果我更改正则表达式，使较长的短语活塞环"先出现，那么它会按预期工作.

Using the above, regex.match("piston ring") matches on "piston". If I change the regex such that the longer phrase "piston ring" comes first then it work as expected.

我对这种行为感到惊讶，因为我假设正则表达式的贪婪性质会尝试免费"匹配最长的字符串.

I was surprised by this behavior as I was assuming that the greedy nature of regex would try to match the longest string "for free."

我错过了什么?有人可以解释一下吗?谢谢！

What am I missing? Can somebody explain this? Thanks!

推荐答案

在正则表达式中使用交替 (|) 时，会按从左到右的顺序尝试每个选项，直到找到匹配项.因此，在您的示例中，由于可以使用 piston 进行匹配，因此永远不会尝试 piston ring.

When using alternation (|) in regular expressions, each option is attempted in order from left to right until a match can be found. So in your example since a match can be made with piston, piston ring will never be attempted.

编写此正则表达式的更好方法是这样的:

A better way to write this regex would be something like this:

(^|)(piston( ring)?)( |$)

这将尝试匹配 'piston'，然后立即尝试匹配 ' ring'，其中 ? 使其成为可选.或者，只需确保在交替开始时出现更长的选项.

This will attempt to match 'piston', and then immediately attempt to match ' ring', with the ? making it optional. Alternatively just make sure your longer options occur at the beginning of the alternation.

您可能还想考虑使用 词边界、\b，而不是 (^|) 和 (|$).

You may also want to consider using a word boundary, \b, instead of (^|) and ( |$).

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