如何在Python中的两个重复关键字之间获取子字符串

Question

表示一个字符串：

 string = 'Other unwanted text here and start here: This is the first sentence.\nIt is the second one.\nNow, this is the third one.\nThis is not I want.\n'

我想提取前三个句子，即

This is the first sentence.\nIt is the second one.\nNow, this is the third one.

显然，以下正则表达式不起作用：

re.search('(?<=This)(.*?)(?=\n)', string)

在This和第三个\\n之间提取文本的正确表达式是什么？

谢谢。

Answer 1

你可以使用这个表达式，用于捕获三句话开始This文本，

This(?:[^\n]*\n){3}

演示

编辑：

Python代码

import re

s = 'Other unwanted text here and start here: This is the first sentence.\nIt is the second one.\nNow, this is the third one.\nThis is not I want.\n'

m = re.search(r'This(?:[^\n]*\n){3}',s)
if (m):
 print(m.group())

打印，

This is the first sentence.
It is the second one.
Now, this is the third one.

Answer 2

杰里（Jerry）说的对，正则表达式（Regex）不是解决问题的正确工具，并且有很多更容易，更有效的方法来解决问题。

this = 'This is the first sentence.\nIt is the second one.\nNow, this is the third one.\nThis is not I want.\n'

print('\n'.join(this.split('\n', 3)[:-1]))

OUTPUT：

This is the first sentence.

It is the second one.

Now, this is the third one.

如果您只想练习使用正则表达式，那么遵循教程会容易得多。

Answer 3

请尝试以下操作：

import re

string = 'Other unwanted text here and start here: This is the first sentence.\nIt is the second one.\nNow, this is the third one.\nThis is not I want.\n'
extracted_text = re.search(r'This(.*?\n.*?\n.*?)\n', string).group(1)
print(extracted_text)

给你：

 is the first sentence.
It is the second one.
Now, this is the third one.

假设Now之前缺少n 。 如果您希望保留This则可以将其移至(

Answer 4

(?s)(This.*?)(?=\\nThis)

使. 包括带有(?s)换行符，寻找一个以This开头，然后是\\nThis的序列。

不要忘记搜索结果__repr__不会打印出整个匹配的字符串，因此您需要

print(re.search('(?s)(This.*?)(?=\nThis)', string)[0])