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regex – Regular expression to match DNS hostname or IP Address? – Stack Overflow

Posted by jpluimers on 2017/08/29

I’m not fond of them, but sometimes they can do their job regex – Regular expression to match DNS hostname or IP Address? – Stack Overflow

Two notes:

  1. The first answer matches either IPv4 or hostname
    (no IPv6 support yet; reminder to self: find it one day).
  2. The second answer restricts the hostname parts to be no more than 63 characters each. It doesn’t fix the total length being 255 or less
    (reminder to self: create that restriction one day).

First answer:

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.

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])$";

Second answer:

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.


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:


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.


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