Bash based regex domain name validation

Question

I want to create a script that will add new domains to our DNS Servers. I found that Fully qualified domain name validation REGEX. However, when I use it with sed, it is not working as I would expect:

echo test | sed  '/(?=^.{5,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(:[a-zA-Z]{2,})$)/p'  
--------
Output is: 
test
echo test.com | sed  '/(?=^.{5,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(:[a-zA-Z]{2,})$)/p'  
--------
Output is: 
test.com

I expected that the output of the first command should be a blank line. What do I do wrong?

Answer 1

I find this to be a more comprehensive regex:

(?=^.{4,253}$)(^(?:[a-zA-Z0-9](?:(?:[a-zA-Z0-9\-]){0,61}[a-zA-Z0-9])?\.)+([a-zA-Z]{2,}|xn--[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9])$)

• RFC 1034§3: Allows for a length of 4-253, with the shortest operational domain I'm aware of, "t.co", still matching where the other answers don't. 255 bytes is the maximum length, minus the length octet for each label (TLD and "primary" subdomain) gives us 253: (?=^.{4,253}$)
  • RFC 3696§2: Single-letter TLDs are technically permitted, meaning the minimum length would be 3, but as there are currently no single-letter TLDs a minimum length of 4 is practical.
• RFC 1034§3: Allows numbers in subdomains, which Conor Clafferty's apparently doesn't (by not distinguishing other subdomains from "primary" subdomains -- i.e. the domain you register -- which the DNS spec doesn't)
• RFC 1034§3: Restricts individual labels to 63 characters, permitting hyphens in the middle while restricting the beginning and end to alphanumerics (?:[a-zA-Z0-9](?:(?:[a-zA-Z0-9\-]){,61}[a-zA-Z0-9])?\.)
• Requires a two-letter or larger TLD, but may be punycoded ([a-zA-Z]{2,}|xn--[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9])

  • RFC 3696§2: The DNS spec technically permits numerics in the TLD, as well as single-letter TLDs; however, there are currently no single-letter TLDs or TLDs with numbers currently, and all-numeric TLDs are not permitted, so this part of the regex has been simplified to [a-zA-Z]{2,}.

    --OR--

  • RFC 3490§5: an internationalized domain name ccTLD (IDN ccTLD) may be punycoded, as indicated by an "xn--" prefix, after which it may contain letters, numbers, or hyphens. This approximates to xn--[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9]

    Be aware that this pattern does not validate a punycode TLD! Invalid punycode will be tolerated, e.g. "xn--qqqq", because attempting to validate punycode against the appropriate encoding mechanisms is beyond the scope of a regular expression. While punycode itself technically permits an encoded string ending in a hyphen, RFC 3492§5 observes and respects the IDNA limitation that labels may not end in a hyphen.

EDIT 02/2021: Hat tip to user2241415 for pointing out that IDN ccTLDs did not match the previously-specified regex.

Answer 2

You are missing a question mark in your regex :

(?=^.{5,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{2,})$)

You can test your regex here

You can do what you want with grep :

$ echo test.com | grep -P '(?=^.{5,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{2,})$)'
test.com
$ echo test | grep -P '(?=^.{5,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{2,})$)'
$