PHP regex for validating a URL

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I have created a solution for validating the domain. While it does not specifically cover the entire URL, it is very detailed and specific. The question you need to ask yourself is, “Why am I validating a domain?” If it is to see if the domain actually could exist, then you need to confirm the domain (including valid TLDs). The problem is, too many developers take the shortcut of ([a-z]{2,4}) and call it good. If you think along these lines, then why call it URL validation? It’s not. It’s just passing the URL through a regex.

I have an open source class that will allow you to validate the domain not only using the single source for TLD management (iana.org), but it will also validate the domain via DNS records to make sure it actually exists. The DNS validating is optional, but the domain will be specifically valid based on TLD.

For example: example.ay is NOT a valid domain as the .ay TLD is invalid. But using the regex posted here ([a-z]{2,4}), it would pass. I have an affinity for quality. I try to express that in the code I write. Others may not really care. So if you want to simply “check” the URL, you can use the examples listed in these responses. If you actually want to validate the domain in the URL, you can have at the class I created to do just that. It can be downloaded at:
http://code.google.com/p/blogchuck/source/browse/trunk/domains.php

It validates based on the RFCs that “govern” (using the term loosely) what determines a valid domain. In a nutshell, here is what the domains class will do:
Basic rules of the domain validation

  • must be at least one character long
  • must start with a letter or number
  • contains letters, numbers, and hyphens
  • must end in a letter or number
  • may contain multiple nodes (i.e. node1.node2.node3)
  • each node can only be 63 characters long max
  • total domain name can only be 255 characters long max
  • must end in a valid TLD
  • can be an IP4 address

It will also download a copy of the master TLD file iana.org only after checking your local copy. If your local copy is outdated by 30 days, it will download a new copy. The TLDs in the file will be used in the REGEX to validate the TLD in the domain you are validating. This prevents the .ay (and other invalid TLDs) from passing validation.

This is a lengthy bit of code, but very compact considering what it does. And it is the most accurate. That’s why I asked the question earlier. Do you want to do “validation” or simple “checking”?

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