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Comment by nickcw | Hacker News Reader

Here is a regexp to match an IPv4 address - looks quite nice and easy to understand compared to the regexp! In fact the visualisation makes it easy to spot the mistake.

https://regexper.com/#'%5Cb((25%5B0-5%5D%7C2%5B0-4%5D%5B0-9%...

(From https://stackoverflow.com/q/5284147/164234 )

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That covers the most common format accepted by the BSD and POSIX inet_* functions, but misses the less common ones.

If anyone wants to have a go at a more complete one, here are some test cases for you that it misses, using Google's well known public name server 8.8.4.4. These all work in the classic command line tools like ping on MacOS, Linux, and Windows:

  134743044
  8.525316
  8.8.1028

POSIX and BSD also allow the numbers to be written in hex:

  0x8080404
  0x8.0x8.0x404
  0x8.0x80404
  0x8.0x8.0x4.0x4

or octal:

  01002002004
  010.2002004
  010.010.02004

or mixed:

  010.8.0x404

The visualization tool shows that the regex is not correct. It allows 000.000.000.000 as an IPv4 address

I created this instead: https://regexper.com/#((%5B0-9%5D%5C.)%7C(%5B1-9%5D%5B0-9%5D...

The repetition count seems to be displayed off-by-one though.

On mobile so can't (easily) test it, but doesn't this produce a false negative for `246.{snip}`, for example?

Yes, you are right...the following better? https://regexper.com/#((%5B0-9%5D%5C.)%7C(%5B1-9%5D%5B0-9%5D...

Does not match: 10.1 -> 10.0.0.1

In IPv4 addresses, as far as I know, this is not done -- it is only IPv6 addresses that use the double colon to indicate a sequence of zeros.

Where are you seeing a double colon? He write "10.1", not "10::1".

IPv4 addresses, according to POSIX, can be written in 4 forms:

  A1.A2.A3.A4
  A1.A2.B
  A1.C
  D

where Ai is an 8 bit number, B is a 16 bit number, C is a 24 bit number, and D is a 32 bit number. "10.1" is A1.C form.

See the inet_addr man page if you are on Unix or a Unix-like system.

I referred to the double colon as something I saw used with IPv6 to indicate a sequence of 0's.

I wasn't aware that inet_aton supported all these forms and the regex I provide won't parse them. It seems like inet_aton supports specifying the numbers in octal and hex too.

Great explanation thank you! The 24 bit and 16 bit variants are highly useable but they are often overlooked by regexps, biggest problem is web apps. Being able to use octal and hex is even less common.

I don't see the problem. Though not a host address, that's known in the sockets networking API as INADDR_ANY, useful for binding a socket to listening on all networks, for instance.

The GNU C Library getaddrinfo accepts 000.000.000.000 with the leading zeros and all; I just tried.

It is important to support special addresses like 255.255.255.255 and 0.0.0.0 in the dot notation. For instance, in the configuration of some daemon, you may need to be able to specify that the bind address is 0.0.0.0. The value can't be rejected due to not being an address.

Also, you need to be able specify network as opposed to host addresses, and netmasks. You know, like 10.0.0.0 and so on.

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