How to validate a domain name using Regex & Php?

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:

• IETF: RFC1035
• IETF: RFC1123
• IETF: RFC2181
• IETF: RFC952
• Wikipedia: Domain Name System

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

How about:

^(?:[-A-Za-z0-9]+\.)+[A-Za-z]{2,6}$

In my case, domain name is considered as valid if the format is stackoverflow.com or xxx.stackoverflow.com

So in addition to other stack answers, I have added checking for www. also.

function isValidDomainName($domain) {
  if (filter_var(gethostbyname($domain), FILTER_VALIDATE_IP)) {
      return (preg_match('/^www./', $domain)) ? FALSE : TRUE;
  }
  return FALSE;
}

you can test the function with this code

    $domain = array("http://www.domain.example","http://www.domain.example/folder" ,"http://domain.example", "www.domain.example", "domain.example/subfolder", "domain.example","sub.domain.example");
    foreach ($domain as $v) {
        echo isValidDomainName($v) ? "{$v} is valid<br>" : "{$v} is invalid<br>";
    }

Please try this expression:

^(http[s]?\:\/\/)?((\w+)\.)?(([\w-]+)?)(\.[\w-]+){1,2}$

What it actually does

• optional http/s://
• optional www
• any valid alphanumeric name (including - and _)
• 1 or 2 occurrences of any valid alphanumeric name (including - and _)

Validation Examples

• http://www.test.example
• test.com.mt