How to validate a domain name using Regex & Php?

Question

I want a solution to validate only domain names not full urls, The following example is what i'm looking for:

domain.com -> true
domain.net -> true
domain.org -> true
domain.biz -> true
domain.co.uk -> true
sub.domain.com -> true
domain.com/folder -> false
domμ*$ain.com -> false

Answer 1

The accepted answer is incomplete/wrong.

The regex pattern;

• should NOT validate domains such as:
  -domain.com, domain--.com, -domain-.-.com, domain.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-Zd][a-zA-Zd-]{0,61})?[a-zA-Zd].){1,126}(?!d+)[a-zA-Zd]{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.