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

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 '(?([0-1]?[0-9]{1,2}|2[0-4][0-9]|25[0-5]))(\.\g){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 '(?([0-1]?[0-9]{1,2}|2[0-4][0-9]|25[0-5]))(\.\g){3}(?!([\d\w]|\.\d))'