Regex Isn't Hard (2023)

I love regular expressions but one thing I've learned over the years is the syntax is dense enough that even people who are confident enough to start writing regex tutorials often can't write a regex that matches an IP address.

    (
      (
      25[0-5] # 250-255
      |
      2[0-4][0-9] # 200-249
      |
      1[0-9]{2} # 100-199
      |
      [1-9][0-9] # 10-99
      |
      [0-9]
      )
      \.
    ){3}
    (
    25[0-5] # 250-255
    |
    2[0-4][0-9] # 200-249
    |
    1[0-9]{2} # 100-199
    |
    [1-9][0-9] # 10-99
    |
    [0-9]
    )
    

The problem is that expressing “an integer from 0-255” is surprisingly complicated for regex engines to express. And that’s not even accounting for IP addresses that don’t use dots (which is legal as an argument to most software that connects to an IP address), as other commenters have pointed out.