Python re.sub 使用非贪婪模式 (.*?) 以字符串结尾 ($) 它来贪婪！ [英] Python re.sub use non-greedy mode (.*?) with end of string ($) it comes greedy!

问题描述

代码:

str = '<br><br />A<br />B'
print(re.sub(r'<br.*?>\w$', '', str))

本应返回<br><br/>A，但返回一个空字符串''！

It is expected to return <br><br />A, but it returns an empty string ''!

有什么建议吗?

推荐答案

贪婪是从左到右起作用的，但不是相反.它的基本意思是除非匹配失败，否则不匹配".这是发生了什么:

Greediness works from left to right, but not otherwise. It basically means "don't match unless you failed to match". Here's what's going on:

1. 正则表达式引擎匹配字符串开头的 .
2. .*? 暂时忽略了，很懒.
3. 尝试匹配>，并成功.
4. 尝试匹配 \w 并失败.现在很有趣 - 引擎开始回溯，并看到 .*? 规则.在这种情况下，. 可以匹配第一个 >，所以仍然有希望匹配.
5. 这种情况一直发生，直到正则表达式到达斜线为止.然后 >\w 可以匹配，但 $ 失败.引擎再次回到惰性 .* 规则，并保持匹配，直到匹配

A
B

1. The regex engine matches <br at the start of the string.
2. .*? is ignored for now, it is lazy.
3. Try to match >, and succeeds.
4. Try to match \w and fails. Now it's interesting - the engine starts backtracking, and sees the .*? rule. In this case, . can match the first >, so there's still hope for that match.
5. This keep happening until the regex reaches the slash. Then >\w can match, but $ fails. Again, the engine comes back to the lazy .* rule, and keeps matching, until it matches<br><br />A<br />B

幸运的是，有一个简单的解决方案:通过替换 <br[^>]*>\w$ 你不会允许在你的标签之外匹配，所以它应该替换最后一次出现.
严格来说，这不适用于 HTML，因为标签属性可以包含 > 字符，但我认为这只是一个示例.

Luckily, there's an easy solution: By replacing <br[^>]*>\w$ you don't allow matching outside of your tags, so it should replace the last occurrence.
Strictly speaking, this doesn't work well for HTML, because tag attributes can contain > characters, but I assume it's just an example.

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