The Hostname Regex

Question

I'm looking for the regex to validate hostnames. It must completely conform to the standard. Right now, I have

^[0-9a-z]([0-9a-z\-]{0,61}[0-9a-z])?(\.[0-9a-z](0-9a-z\-]{0,61}[0-9a-z])?)*$

but it allows successive hypens and hostnames longer than 255 characters. If the perfect regex is impossible, say so.

Edit/Clarification: a Google search didn't reveal that this is a solved (or proven unsolvable) problem. I want to to create the definitive regex so that nobody has to write his own ever. If dialects matter, I want a a version for each one in which this can be done.

Answer 1

Take a look at the following question. A few of the answers have regex expressions for host names

• http://stackoverflow.com/questions/106179/regular-expression-to-match-hostname-or-ip-address

Could you specify what language you want to use this regex in? Most languages / systems have slightly different regex implementations that will affect people's answers.

Answer 2

Your answer was relatively close.

But see

• RFC 2396 Section 3.2.2
• JaredPar's reference to this answer is referring to Regexp/Common/URI/RFC2396.pm source.

For a hostname RE, that perl module produces

(?:(?:(?:(?:[a-zA-Z0-9][-a-zA-Z0-9]*)?[a-zA-Z0-9])[.])*(?:[a-zA-Z][-a-zA-Z0-9]*[a-zA-Z0-9]|[a-zA-Z])[.]?)

I would modify to be more accurate as:

(?:(?:(?:(?:[a-zA-Z0-9][-a-zA-Z0-9]{0,61})?[a-zA-Z0-9])[.])*(?:[a-zA-Z][-a-zA-Z0-9]{0,61}[a-zA-Z0-9]|[a-zA-Z])[.]?)

Optionally anchoring the ends with ^$ to ONLY match hostnames.

I don't think a single RE can accomplish an full validation because, according to Wikipedia, there is a 255 character length restriction which i don't think can be included within that same RE, at least not without a ton of changes, but it's easy enough to just check the length <= 255 before running the RE.

Answer 3

^(?=.{1,255}$)[0-9A-Za-z](?:(?:[0-9A-Za-z]|\b-){0,61}[0-9A-Za-z])?(?:\.[0-9A-Za-z](?:(?:[0-9A-Za-z]|\b-){0,61}[0-9A-Za-z])?)*\.?$