Regular expression to match DNS hostname or IP Address?

Question

Does anyone have a regular expression handy that will match any legal DNS hostname or IP address?

It\'s easy to write one that works 95% of the time, but I\'m hoping

Answer 1

It's worth noting that there are libraries for most languages that do this for you, often built into the standard library. And those libraries are likely to get updated a lot more often than code that you copied off a Stack Overflow answer four years ago and forgot about. And of course they'll also generally parse the address into some usable form, rather than just giving you a match with a bunch of groups.

For example, detecting and parsing IPv4 in (POSIX) C:

#include 
#include 

int main(int argc, char *argv[]) {
  for (int i=1; i!=argc; ++i) {
    struct in_addr addr = {0};
    printf("%s: ", argv[i]);
    if (inet_pton(AF_INET, argv[i], &addr) != 1)
      printf("invalid\n");
    else
      printf("%u\n", addr.s_addr);
  }
  return 0;
}

---

Obviously, such functions won't work if you're trying to, e.g., find all valid addresses in a chat message—but even there, it may be easier to use a simple but overzealous regex to find potential matches, and then use the library to parse them.

For example, in Python:

>>> import ipaddress
>>> import re
>>> msg = "My address is 192.168.0.42; 192.168.0.420 is not an address"
>>> for maybeip in re.findall(r'\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}', msg):
...     try:
...         print(ipaddress.ip_address(maybeip))
...     except ValueError:
...         pass