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

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