Let us see how to extract IP addresses from a file using Python.
 

Algorithm :  

• Import the re module for regular expression.
• Open the file using the open() function.
• Read all the lines in the file and store them in a list.
• Declare the pattern for IP addresses. The regex pattern is : 
   

r'(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})'

• For every element of the list search for the pattern using the search() function, store the IP addresses in a list.
• Display the list containing the IP addresses.

The file to be processed is test.txt : 
 

# importing the module
import re

# opening and reading the file 
with open('C:/Users/user/Desktop/New Text Document.txt') as fh:
   fstring = fh.readlines()

# declaring the regex pattern for IP addresses
pattern = re.compile(r'(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})')

# initializing the list object
lst=[]

# extracting the IP addresses
for line in fstring:
   lst.append(pattern.search(line)[0])

# displaying the extracted IP addresses
print(lst)

Output :

The above Python program displays any kind of IP addresses present in the file. We can also display the valid IP addresses.

Rules for a valid IP Address : 

• The numbers should be in a range of 0-255
• It should consist of 4 cells separated by '.'

The regular expression for valid IP addresses is :
 

((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)

Explanation of Regular Expression used for valid IP:

Since we cannot use 0-255 range in regular expression we divide the same in 3 groups:

• 25[0-5] - represents numbers from 250 to 255
• 2[0-4][0-9] - represents numbers from 200 to 249
• [01]?[0-9][0-9]?- represents numbers from 0 to 199

The file to be processed is test2.txt : 
 

# importing the module
import re

# opening and reading the file
with open('test2.txt') as fh:
  string = fh.readlines()
  
# declaring the regex pattern for IP addresses 
pattern =re.compile('''((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.)
{3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)''')

# initializing the list objects
valid =[]
invalid=[]

# extracting the IP addresses
for line in string:
    line = line.rstrip()
    result = pattern.search(line)

    # valid IP addresses
    if result:
      valid.append(line)

    # invalid IP addresses  
    else:
      invalid.append(line)

# displaying the IP addresses
print("Valid IPs")
print(valid)
print("Invalid IPs")
print(invalid)

Output :