How to get sub-string between two repetitive keywords in Python

Question

Denote a string:

 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'

I want to extract the first three sentence, that is,

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

Apparently, the following regular expression does not work:

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

What is the correct expression for extracting text between This and the third \n?

Thanks.

I think you meant to have \nNow ... instead of \Now. I also think you could split on \n to make it simpler (and join back the first 3 elements of the split using \n) — Jerry
– Jerry, Commented Feb 27, 2019 at 6:44
Thank you, Jerry. But I want to know how to solve it using regex. — Chan
– Chan, Commented Feb 27, 2019 at 6:46
Do you have any reasons why you want to do that? Why you want to use a tool that you don't know how to use, especially when there are more efficient solutions to solve that particular problem? — Jerry
– Jerry, Commented Feb 27, 2019 at 6:50

Pushpesh Kumar Rajwanshi · Accepted Answer · 2019-02-27 09:10:34Z

1

You can use this regex for capturing three sentences starting with This text,

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

Demo

Edit:

Python code,

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())

Prints,

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

edited Feb 27, 2019 at 9:10

answered Feb 27, 2019 at 6:57

Pushpesh Kumar Rajwanshi

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6 Comments

Chan Over a year ago

How to extract text between This and the third \n?

Chan Over a year ago

I tried re.search('(?<=This)((?:[^\n]*\n){3})', string) but it returns None.

Chan Over a year ago

I also created a demo: repl.it/repls/OvercookedAdventurousDownload. Please check.

Chan Over a year ago

Thank you very much, Pushpesh. But how to include This in the result?

Pushpesh Kumar Rajwanshi Over a year ago

@Chan: For including This itself too in the match, change positive look behind to literal This and use this regex This(?:[^\n]*\n){3}. Check your modified demo

|

Nordle · Accepted Answer · 2019-02-27 06:54:42Z

0

Jerry's right, regex isn't the right tool for the job and there are much easier and more efficient ways of tackling the problem;

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.

If you just want to practice using regex, following a tutorial would be much easier.

answered Feb 27, 2019 at 6:54

Nordle

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Comments

WofWca · Accepted Answer · 2019-02-27 07:16:34Z

0

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

Make the . include newline with (?s), look for a sequence starting with This and followed by \nThis.

Don't forget that __repr__ of the search result doesn't print the whole matched string, so you'll need to

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

answered Feb 27, 2019 at 7:16

WofWca

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3 Comments

WofWca Over a year ago

My output for

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

: This is the first sentence.\nIt is the second one.\nNow, this is the third one. Is this not what you want?

Chan Over a year ago

I tried print(re.search('(?s)(This.*?)(?=\nThis)', string)[0]) and I got TypeError: 'NoneType' object is not subscriptable.

WofWca Over a year ago

May be caused by the fact that you assign a string like this string = r'blah' and not like this string = 'blah' First one is for regex strings. You need a regular one.

Martin Evans · Accepted Answer · 2019-02-27 10:14:10Z

0

Try the following:

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)

Giving you:

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

This assumes there was a missing n before Now. If you wish to keep This then you can move it inside the (

edited Feb 27, 2019 at 10:14

answered Feb 27, 2019 at 7:00

Martin Evans

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5 Comments

Chan Over a year ago

How to extract text between This and the third \n?

Martin Evans Over a year ago

.match() will return a match object, and then .group() will give you the matched text.

Chan Over a year ago

Sorry for unclear question. Please see the modified question. I want the text between This and the third \n and This is the first word of the sentence.

Chan Over a year ago

I tried your suggestions but failed. Please see repl.it/@Chan2019/OvercookedAdventurousDownload

Martin Evans Over a year ago

It wasn't clear if you actually wanted \n literally or as a newline. I have updated the solution.

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