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.
For something like locating IP addresses in text, using a regex to identify candidates is a great idea. But as you show, you don’t want to implement the full validation in it. Use regex to find dotted digit groups, but validate the actual numeric values as a separate step afterwards.