问题描述
我正在学习正则表达式，我在这里有点困惑。我有一个字符串，我想从中提取一个至少4位且最多7位的int。我尝试了如下：
I am just learning regex and I'm a bit confused here. I've got a string from which I want to extract an int with at least 4 digits and at most 7 digits. I tried it as follows:
>>> import re
>>> teststring = 'abcd123efg123456'
>>> re.match(r"[0-9]{4,7}$", teststring)
在那里我期待123456，不幸的是，这根本没有任何结果。有人可以帮我一点吗？
Where I was expecting 123456, unfortunately this results in nothing at all. Could anybody help me out a little bit here?
推荐答案
@ExplosionPills是正确的，但你的正则表达式仍有两个问题。
@ExplosionPills is correct, but there would still be two problems with your regex.
首先， $ 匹配字符串的 end 。我猜你也希望能够在字符串中间提取一个int，例如 abcd123456efg789 返回 123456 。要解决这个问题，你需要这个：
First, $ matches the end of the string. I'm guessing you'd like to be able to extract an int in the middle of the string as well, e.g. abcd123456efg789 to return 123456. To fix that, you want this:
r"[0-9]{4,7}(?![0-9])"
^^^^^^^^^
增加的部分是一个否定的先行断言，意思是......后面没有更多的数字。让我通过使用 \d 来简化：但是：
The added portion is a negative lookahead assertion, meaning, "...not followed by any more numbers." Let me simplify that by the use of \d though:
r"\d{4,7}(?!\d)"
这样更好。现在，第二个问题。你的正则表达式的左边没有约束，所以给定一个像 abcd123efg123456789 的字符串，你实际上匹配 3456789 。所以，你还需要一个否定的lookbehind断言：
That's better. Now, the second problem. You have no constraint on the left side of your regex, so given a string like abcd123efg123456789, you'd actually match 3456789. So, you need a negative lookbehind assertion as well:
r"(?<!\d)\d{4,7}(?!\d)"
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