Regular expression to match DNS hostname or IP Address?

You can use the following regular expressions separately or by combining them in a joint OR expression.

ValidIpAddressRegex = "^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$";

ValidHostnameRegex = "^(([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z0-9]|[A-Za-z0-9][A-Za-z0-9\-]*[A-Za-z0-9])$";

ValidIpAddressRegex matches valid IP addresses and ValidHostnameRegex valid host names. Depending on the language you use \ could have to be escaped with \.

ValidHostnameRegex is valid as per RFC 1123. Originally, RFC 952 specified that hostname segments could not start with a digit.

http://en.wikipedia.org/wiki/Hostname

The original specification of hostnames in RFC 952, mandated that labels could not start with a digit or with a hyphen, and must not end with a hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with digits.

Valid952HostnameRegex = "^(([a-zA-Z]|[a-zA-Z][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z]|[A-Za-z][A-Za-z0-9\-]*[A-Za-z0-9])$";

I don't seem to be able to edit the top post, so I'll add my answer here.

For hostname - easy answer, on egrep example here -- http: //www.linuxinsight.com/how_to_grep_for_ip_addresses_using_the_gnu_egrep_utility.html

egrep '([[:digit:]]{1,3}\.){3}[[:digit:]]{1,3}'

Though the case doesn't account for values like 0 in the fist octet, and values greater than 254 (ip addres) or 255 (netmask). Maybe an additional if statement would help.

As for legal dns hostname, provided that you are checking for internet hostnames only (and not intranet), I wrote the following snipped, a mix of shell/php but it should be applicable as any regular expression.

first go to ietf website, download and parse a list of legal level 1 domain names:

tld=$(curl -s http://data.iana.org/TLD/tlds-alpha-by-domain.txt |  sed 1d  | cut -f1 -d'-' | tr '\n' '|' | sed 's/\(.*\)./\1/')
echo "($tld)"

That should give you a nice piece of re code that checks for legality of top domain name, like .com .org or .ca

Then add first part of the expression according to guidelines found here -- http: //www.domainit.com/support/faq.mhtml?category=Domain_FAQ&question=9 (any alphanumeric combination and '-' symbol, dash should not be in the beginning or end of an octet.

(([a-z0-9]+|([a-z0-9]+[-]+[a-z0-9]+))[.])+

Then put it all together (PHP preg_match example):

$pattern = '/^(([a-z0-9]+|([a-z0-9]+[-]+[a-z0-9]+))[.])+(AC|AD|AE|AERO|AF|AG|AI|AL|AM|AN|AO|AQ|AR|ARPA|AS|ASIA|AT|AU|AW|AX|AZ|BA|BB|BD|BE|BF|BG|BH|BI|BIZ|BJ|BM|BN|BO|BR|BS|BT|BV|BW|BY|BZ|CA|CAT|CC|CD|CF|CG|CH|CI|CK|CL|CM|CN|CO|COM|COOP|CR|CU|CV|CX|CY|CZ|DE|DJ|DK|DM|DO|DZ|EC|EDU|EE|EG|ER|ES|ET|EU|FI|FJ|FK|FM|FO|FR|GA|GB|GD|GE|GF|GG|GH|GI|GL|GM|GN|GOV|GP|GQ|GR|GS|GT|GU|GW|GY|HK|HM|HN|HR|HT|HU|ID|IE|IL|IM|IN|INFO|INT|IO|IQ|IR|IS|IT|JE|JM|JO|JOBS|JP|KE|KG|KH|KI|KM|KN|KP|KR|KW|KY|KZ|LA|LB|LC|LI|LK|LR|LS|LT|LU|LV|LY|MA|MC|MD|ME|MG|MH|MIL|MK|ML|MM|MN|MO|MOBI|MP|MQ|MR|MS|MT|MU|MUSEUM|MV|MW|MX|MY|MZ|NA|NAME|NC|NE|NET|NF|NG|NI|NL|NO|NP|NR|NU|NZ|OM|ORG|PA|PE|PF|PG|PH|PK|PL|PM|PN|PR|PRO|PS|PT|PW|PY|QA|RE|RO|RS|RU|RW|SA|SB|SC|SD|SE|SG|SH|SI|SJ|SK|SL|SM|SN|SO|SR|ST|SU|SV|SY|SZ|TC|TD|TEL|TF|TG|TH|TJ|TK|TL|TM|TN|TO|TP|TR|TRAVEL|TT|TV|TW|TZ|UA|UG|UK|US|UY|UZ|VA|VC|VE|VG|VI|VN|VU|WF|WS|XN|XN|XN|XN|XN|XN|XN|XN|XN|XN|XN|YE|YT|YU|ZA|ZM|ZW)[.]?$/i';

    if (preg_match, $pattern, $matching_string){
    ... do stuff
    }

You may also want to add an if statement to check that string that you checking is shorter than 256 characters -- http://www.ops.ietf.org/lists/namedroppers/namedroppers.2003/msg00964.html

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 <arpa/inet.h>
#include <stdio.h>

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

The new Network framework has failable initializers for struct IPv4Address and struct IPv6Address which handle the IP address portion very easily. Doing this in IPv6 with a regex is tough with all the shortening rules.

Unfortunately I don't have an elegant answer for hostname.

Note that Network framework is recent, so it may force you to compile for recent OS versions.

import Network
let tests = ["192.168.4.4","fkjhwojfw","192.168.4.4.4","2620:3","2620::33"]

for test in tests {
    if let _ = IPv4Address(test) {
        debugPrint("\(test) is valid ipv4 address")
    } else if let _ = IPv6Address(test) {
        debugPrint("\(test) is valid ipv6 address")
    } else {
        debugPrint("\(test) is not a valid IP address")
    }
}

output:
"192.168.4.4 is valid ipv4 address"
"fkjhwojfw is not a valid IP address"
"192.168.4.4.4 is not a valid IP address"
"2620:3 is not a valid IP address"
"2620::33 is valid ipv6 address"

Here is a regex that I used in Ant to obtain a proxy host IP or hostname out of ANT_OPTS. This was used to obtain the proxy IP so that I could run an Ant "isreachable" test before configuring a proxy for a forked JVM.

^.*-Dhttp\.proxyHost=(\w{1,}\.\w{1,}\.\w{1,}\.*\w{0,})\s.*$