Regex for ipv4 address without 0.x.x.x if x != 0

Question

I'm doing a regex for ipv4 address, It's very interesting that Ubuntu and some RFC references state that 0.xxx if x != 0 are reserved, so invalid. What should be the more optimal regex for this? I have this one:

import re
matcher = re.compile(r'^(25[0-5]|2[0-4][0-9]|[0-1]?[0-9][0-9]|[1-9])(\.(25[0-5]|2[0-4][0-9]|[0-1]?[0-9]?[0-9])){3}\Z^(0)(.0){3}\Z')

For example:

0.1.2.3 => should be invalid
1.0.0.0 => should be valid
0.0.0.0 => should be valid

Answer 1

I think the regex you looking for is:

^(?:[01][0-9]?[0-9]?|2[0-4][0-9]|25[0-5])(?:\\.0{1,3}){3}$

You can test it here

Answer 2

Here is the regex that meets the requirements:

^(?!0+(?:\.0*[1-9][0-9]*){3}$)(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$

See its online demo .

The main points are:

• (?!0+(?:\\.0*[1-9][0-9]*){3}$) - a (?!...) is a negative lookahead that fails the match if its pattern matches:
  • 0+ - 1+ zeros
  • (?:\\.0*[1-9][0-9]*){3} - 3 consecutive occurrences of
  • \\. - a dot
  • 0* - 0+ zeros
  • [1-9] - a digit from 1 to 9
  • [0-9]* - any 0+ digits
  • $ - end of string.

Also, the octet regex now matches 0 , too:

• (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?) :
  • 25[0-5]| - 250 till 255
  • 2[0-4][0-9]| - 200 till 249
  • [01]?[0-9][0-9]? - 1 or 0 (optionally, 1 or 0 times) then any digit and then any 1 or 0 digits (an optional digit).

A Python demo :

import re

rx = """(?x)^   # start of string
         (?!    # start of the negative lookahead that fails the match if 
            0+(?:\.0*[1-9][0-9]*){3}$  # 0s appear only in the first octet
         )      # end of the lookahead
         (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)  # First octet regex
         (?:    # start of the non-capturing group
            \.  # a dot
            (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?) # octet
         ){3}   # repeated 3 times
         $      # end of string
"""
lst = ['0.1.2.3','1.0.0.0','0.0.0.0']
for s in lst:
    m = re.match(rx, s)
    if m:
        print("{} matched!".format(s))
    else:
        print("{} did not match!".format(s))

Output:

0.1.2.3 did not match!
1.0.0.0 matched!
0.0.0.0 matched!