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copied!<p>The selected answer is incomplete/wrong.</p> <p>The regex pattern; </p> <ul> <li><p><strong>should NOT</strong> validate domains such as:<br> <code>-domain.com</code>, <code>domain--.com</code>, <code>-domain-.-.com</code>, <code>domain.000</code>, etc... </p></li> <li><p><strong>should</strong> validate domains such as:<br> <code>schools.k12</code>, <code>newTLD.clothing</code>, <code>good.photography</code>, etc...</p></li> </ul> <p>After some further research; below is the most correct, cross-language and compact pattern I could come up with: </p> <pre class="lang-js prettyprint-override"><code>^(?!\-)(?:[a-zA-Z\d\-]{0,62}[a-zA-Z\d]\.){1,126}(?!\d+)[a-zA-Z\d]{1,63}$ </code></pre> <p>This pattern conforms with most* of the rules defined in the specs: </p> <ul> <li>Each label/level (splitted by a dot) may contain up to <strong>63 characters</strong>.</li> <li>The full domain name may have up to <strong>127 levels</strong>.</li> <li>The full domain name may not exceed the length of <strong>253 characters</strong> in its textual representation.</li> <li>Each label can consist of <strong>letters, digits and hyphens</strong>.</li> <li>Labels cannot <strong>start</strong> or <strong>end</strong> with a hyphen.</li> <li>The top-level domain (extension) cannot be <strong>all-numeric</strong>.</li> </ul> <p><strong><em>Note 1</strong>: The full domain length check is not included in the regex. It should be simply checked by native methods e.g. <code>strlen(domain) <= 253</code>.</em><br> <strong><em>Note 2</strong>: This pattern works with most languages including PHP, Javascript, Python, etc...</em> </p> <p>See <a href="https://regex101.com/r/ZIMkXh/1" rel="noreferrer"><strong>DEMO here</strong></a> (for JS, PHP, Python)</p> <h3>More Info:</h3> <ul> <li><p>The regex above does not support <a href="http://en.wikipedia.org/wiki/Internationalized_domain_name" rel="noreferrer">IDN</a>s.</p></li> <li><p>There is no spec that says the extension (TLD) should be between 2 and 6 characters. It actually supports 63 characters. See the current <a href="http://data.iana.org/TLD/tlds-alpha-by-domain.txt" rel="noreferrer"><strong>TLD list</strong> here</a>. Also, some networks do internally use custom/pseudo TLDs. </p></li> <li><p>Registration authorities might impose some extra, <a href="https://www.register.com/policy/domain-extension-rules.rcmx" rel="noreferrer">specific rules</a> which are not explicitly supported in this regex. For example, <code>.CO.UK</code> and <code>.ORG.UK</code> 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.</p></li> <li><p>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 <a href="https://docs.python.org/2/library/urlparse.html" rel="noreferrer"><code>urlparse()</code></a> method or PHP's <a href="http://php.net/manual/en/function.parse-url.php" rel="noreferrer"><code>parse_url()</code></a> method...</p></li> <li><p>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.</p></li> </ul> <h3>Specs & References:</h3> <ul> <li><a href="http://tools.ietf.org/html/rfc1035" rel="noreferrer">IETF: RFC1035</a></li> <li><a href="http://tools.ietf.org/html/rfc1123#section-2.1" rel="noreferrer">IETF: RFC1123</a></li> <li><a href="http://tools.ietf.org/html/rfc2181#page-13" rel="noreferrer">IETF: RFC2181</a></li> <li><a href="http://tools.ietf.org/html/rfc952" rel="noreferrer">IETF: RFC952</a></li> <li><a href="http://en.wikipedia.org/wiki/Domain_Name_System#cite_ref-rfc1034_1-2" rel="noreferrer">Wikipedia: Domain Name System</a></li> </ul>

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