Regular expression to match DNS hostname or IP Address? - regex

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 to get something that's well tested to exactly match the latest RFC specs for DNS hostnames.

Answer 1

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])$";

Answer 2

The hostname regex of smink does not observe the limitation on the length of individual labels within a hostname. Each label within a valid hostname may be no more than 63 octets long.
ValidHostnameRegex="^([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])\
(\.([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]))*$"
Note that the backslash at the end of the first line (above) is Unix shell syntax for splitting the long line. It's not a part of the regular expression itself.
Here's just the regular expression alone on a single line:
^([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])(\.([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]))*$
You should also check separately that the total length of the hostname must not exceed 255 characters. For more information, please consult RFC-952 and RFC-1123.

Answer 3

To match a valid IP address use the following regex:
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}
instead of:
([01]?[0-9][0-9]?|2[0-4][0-9]|25[0-5])(\.([01]?[0-9][0-9]?|2[0-4][0-9]|25[0-5])){3}
Explanation
Many regex engine match the first possibility in the OR sequence. For instance, try the following regex:
10.48.0.200
Test
Test the difference between good vs bad

Answer 4

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

Answer 5

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

Answer 6

I think this is the best Ip validation regex. please check it once!!!
^(([01]?[0-9]?[0-9]|2([0-4][0-9]|5[0-5]))\.){3}([01]?[0-9]?[0-9]|2([0-4][0-9]|5[0-5]))$

Answer 7

def isValidHostname(hostname):
if len(hostname) > 255:
return False
if hostname[-1:] == ".":
hostname = hostname[:-1] # strip exactly one dot from the right,
# if present
allowed = re.compile("(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
return all(allowed.match(x) for x in hostname.split("."))

Answer 8

"^((\\d{1,2}|1\\d{2}|2[0-4]\\d|25[0-5])\.){3}(\\d{1,2}|1\\d{2}|2[0-4]\\d|25[0-5])$"

Answer 9

This works for valid IP addresses:
regex = '^([0-9]|[1-9][0-9]|[1][0-9][0-9]|[2][0-5][0-5])[.]([0-9]|[1-9][0-9]|[1][0-9][0-9]|[2][0-5][0-5])[.]([0-9]|[1-9][0-9]|[1][0-9][0-9]|[2][0-5][0-5])[.]([0-9]|[1-9][0-9]|[1][0-9][0-9]|[2][0-5][0-5])$'

Answer 10

>>> my_hostname = "testhostn.ame"
>>> print bool(re.match("^(([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])$", my_hostname))
True
>>> my_hostname = "testhostn....ame"
>>> print bool(re.match("^(([a-zA-Z]|[a-zA-Z][a-zA-Z0-9\-]*[a-zA-Z0-9])\.)*([A-Za-z]|[A-Za-z][A-Za-z0-9\-]*[A-Za-z0-9])$", my_hostname))
False
>>> my_hostname = "testhostn.A.ame"
>>> print bool(re.match("^(([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])$", my_hostname))
True

Answer 11

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"

Answer 12

/^(?:[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])?$/

Answer 13

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.*$

Answer 14

I found this works pretty well for IP addresses. It validates like the top answer but it also makes sure the ip is isolated so no text or more numbers/decimals are after or before the ip.
(?<!\S)(?:(?:\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5])\b|.\b){7}(?!\S)

Answer 15

AddressRegex = "^(ftp|http|https):\/\/([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}:[0-9]{1,5})$";
HostnameRegex = /^(ftp|http|https):\/\/([a-z0-9]+\.)?[a-z0-9][a-z0-9-]*((\.[a-z]{2,6})|(\.[a-z]{2,6})(\.[a-z]{2,6}))$/i
this re are used only for for this type validation
work only if
http://www.kk.com
http://www.kk.co.in
not works for
http://www.kk.com/
http://www.kk.co.in.kk
http://www.kk.com/dfas
http://www.kk.co.in/

Answer 16

try this:
((2[0-4]\d|25[0-5]|[01]?\d\d?)\.){3}(2[0-4]\d|25[0-5]|[01]?\d\d?)
it works in my case.

Answer 17

Regarding IP addresses, it appears that there is some debate on whether to include leading zeros. It was once the common practice and is generally accepted, so I would argue that they should be flagged as valid regardless of the current preference. There is also some ambiguity over whether text before and after the string should be validated and, again, I think it should. 1.2.3.4 is a valid IP but 1.2.3.4.5 is not and neither the 1.2.3.4 portion nor the 2.3.4.5 portion should result in a match. Some of the concerns can be handled with this expression:
grep -E '(^|[^[:alnum:]+)(([0-1]?[0-9]{1,2}|2[0-4][0-9]|25[0-5])\.){3}([0-1]?[0-9]{1,2}|2[0-4][0-9]|25[0-5])([^[:alnum:]]|$)'
The unfortunate part here is the fact that the regex portion that validates an octet is repeated as is true in many offered solutions. Although this is better than for instances of the pattern, the repetition can be eliminated entirely if subroutines are supported in the regex being used. The next example enables those functions with the -P switch of grep and also takes advantage of lookahead and lookbehind functionality. (The function name I selected is 'o' for octet. I could have used 'octet' as the name but wanted to be terse.)
grep -P '(?<![\d\w\.])(?<o>([0-1]?[0-9]{1,2}|2[0-4][0-9]|25[0-5]))(\.\g<o>){3}(?![\d\w\.])'
The handling of the dot might actually create a false negatives if IP addresses are in a file with text in the form of sentences since the a period could follow without it being part of the dotted notation. A variant of the above would fix that:
grep -P '(?<![\d\w\.])(?<x>([0-1]?[0-9]{1,2}|2[0-4][0-9]|25[0-5]))(\.\g<x>){3}(?!([\d\w]|\.\d))'

Answer 18

There's a further nuance here that's missing.
It's true that a HOSTNAME should match, basically, what's been given above.
What's missing is that a REFERENCE TO a hostname can be the same, plus an optional period on the end.
For example, with a trailing period, ping foo.bar.svc.cluster.local. will ping that hostname only, and not attempt any DNS search options in resolv.conf.
tldr - If you provide an input box to receive a hostname, what's entered does not actually need to be a valid hostname.

Answer 19

how about this?
([0-9]{1,3}\.){3}[0-9]{1,3}

Answer 20

on php: filter_var(gethostbyname($dns), FILTER_VALIDATE_IP) == true ? 'ip' : 'not ip'

Answer 21

Checking for host names like... mywebsite.co.in, thangaraj.name, 18thangaraj.in, thangaraj106.in etc.,
[a-z\d+].*?\\.\w{2,4}$

Answer 22

I thought about this simple regex matching pattern for IP address matching
\d+[.]\d+[.]\d+[.]\d+