如何仅将匹配的正则表达式写入 Python 中的新文件？

Question

My goal is to extract the IP Addresses only and append them to a new file. The file I have is called error_log.txt and has lines such as:

[Sun Jun 7 16:45:56 2020] [info] [client 64.242.88.10] (104)Connection reset by peer: client stopped connection before send body completed

[Sun Jun 7 17:13:50 2020] [info] [client 64.242.88.10] (104)Connection reset by peer: client stopped connection before send body completed

The goal is to write "64.242.88.10" and the rest of the IPs to a new file.

I can get the print function to give me only the IPs, but when it writes to the file 'ip_only.txt' it prints the complete line from the error log.

How can I just get the IPs only to the new file (in a column)?

Bonus, when it does print when testing, it gives me the blank lines too. How can I omit those lines?

import re

with open('error_log.txt', 'r') as file:
    fi = file.readlines()

ip_only = open('ip_only.txt', 'w+')

re_ip = re.compile("\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}")

for line in fi:
    ip = re.findall(re_ip, line)
    ip_only.write(str(line))
    # print(ip)

Answer 1

You need to write the ip variable to the file instead of line which contains the original line:

for line in fi:
    ip = re.findall(re_ip, line)
    ip_only.write(str(ip))

# ip_only.txt:
# ['64.242.88.10']['64.242.88.10']

Additionally, to remove the brackets and quotes from your output (note that re.findall() returns a list of strings) and print each IP address to a new line:

for line in fi:
    ips = re.findall(re_ip, line)
    for ip in ips:
        ip_only.write(ip + '\n')

# ip_only.txt:
# 64.242.88.10
# 64.242.88.10

Answer 2

1. While writing into file, you are writing the whole line. instead write only the IPs as below ip_only.write(str(ip))

2. To avoid blank lines, you can have a if condition to check, whether the ip is found or not in the given line.

   for line in fi:
       ip = re.findall(re_ip, line)
       if ip:
           ip_only.write(str(ip))

Answer 3

If print(ip) gives you expected result then you should use write(ip) instead of write(line)

regex gives list so you may need to write only ip[0] . And you need to add \n to move to the next line.

        ip_only.write(ip[0] + "\n")

As for empty line - first remove all spaces, tabs, enters and next compare with empty string "" . OR use fact that empty string gives False when used in if/else

    line = line.strip()
    if line:
         # ... code ...

---

import re

fi = [
    '[Sun Jun 7 16:45:56 2020] [info] [client 64.242.88.10] (104)Connection reset by peer: client stopped connection before send body completed',
    '[Sun Jun 7 17:13:50 2020] [info] [client 64.242.88.10] (104)Connection reset by peer: client stopped connection before send body completed',
]    


ip_only = open('ip_only.txt', 'w+')

re_ip = re.compile("\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}")

for line in fi:
    line = line.strip()
    if line:
        ip = re.findall(re_ip, line)
        ip_only.write(ip[0] + "\n")
        print(ip[0])