Python to find and replace string within certain lines of file

Question

I have file

Mod top (o i)
In a
Op a
End


Mod mid (o i)
In a
Op a
End

Mod bottom (o i)
In a
Op a
End

I want to replace “a” with “z” only in mid.

with open(‘data.txt’,'r') as file:
read_lines = file.readlines()
for line in read_lines:
    print line.replace(string_to_be_identified,string_to_be_replace)
file.close()

I tried to match using regex and then replace but it will repalce all lines after it but i want only in that “mid”

Expected

Mod top (o i)
In a
Op a
End


Mod mid (o i)
In z
Op z
End

Mod bottom (o i)
In a
Op a
End

Could you post expected output?

Bijay Regmi
– Bijay Regmi

2022-12-06 21:25:10 +00:00
Commented Dec 6, 2022 at 21:25 — Bijay Regmi
– Bijay Regmi, Commented Dec 6, 2022 at 21:25

Andrej Kesely · Accepted Answer · 2022-12-06 21:38:53Z

1

Try:

import re

with open("your_file.txt", "r") as f_in:
    data = f_in.read()

data = re.sub(
    r"(?s)Mod mid.*?End", lambda g: g.group(0).replace(" a", " z"), data
)
print(data)

Prints:

Mod top (o i)
In a
Op a
End


Mod mid (o i)
In z
Op z
End

Mod bottom (o i)
In a
Op a
End

answered Dec 6, 2022 at 21:38

Andrej Kesely

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2 Comments

flash Over a year ago

What does lambda g.group(0) mean and r in beginning of re.sub is regexp ??

Andrej Kesely Over a year ago

@flash r"string" means raw-string in Python (prefix that disables most escape sequence processing). g.group(0) in re.sub means that I want to get the whole matched string (group 0)

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