How to validate a domain name using Regex & PHP?

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The accepted answer is incomplete/wrong.

The regex pattern;

  • should NOT validate domains such as:
    -example.com, example--.com, -example-.-.com, example.000, etc…

  • should validate domains such as:
    schools.k12, newTLD.clothing, good.photography, etc…

After some further research; below is the most correct, cross-language and compact pattern I could come up with:

^(?!\-)(?:(?:[a-zA-Z\d][a-zA-Z\d\-]{0,61})?[a-zA-Z\d]\.){1,126}(?!\d+)[a-zA-Z\d]{1,63}$

This pattern conforms with most* of the rules defined in the specs:

  • Each label/level (splitted by a dot) may contain up to 63 characters.
  • The full domain name may have up to 127 levels.
  • The full domain name may not exceed the length of 253 characters in its textual representation.
  • Each label can consist of letters, digits and hyphens.
  • Labels cannot start or end with a hyphen.
  • The top-level domain (extension) cannot be all-numeric.

Note 1: The full domain length check is not included in the regex. It should be simply checked by native methods e.g. strlen(domain) <= 253.
Note 2: This pattern works with most languages including PHP, Javascript, Python, etc…

See DEMO here (for JS, PHP, Python)

More Info:

  • The regex above does not support IDNs.

  • There is no spec that says the extension (TLD) should be between 2 and 6 characters. It actually supports 63 characters. See the current TLD list here. Also, some networks do internally use custom/pseudo TLDs.

  • Registration authorities might impose some extra, specific rules which are not explicitly supported in this regex. For example, .CO.UK and .ORG.UK must have at least 3 characters, but less than 23, not including the extension. These kinds of rules are non-standard and subject to change. Do not implement them if you cannot maintain.

  • Regular Expressions are great but not the best effective, performant solution to every problem. So a native URL parser should be used instead, whenever possible. e.g. Python’s urlparse() method or PHP’s parse_url() method…

  • After all, this is just a format validation. A regex test does not confirm that a domain name is actually configured/exists! You should test the existence by making a request.

Specs & References:

UPDATE (2019-12-21): Fixed leading hyphen with subdomains.

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