This question already has an answer here:
I'm trying to figure out why a domain name (example1.commerce) isn't matching the following regular expression:
if (!preg_match('([a-z\d]){1,63}(\.)([a-z\d]){2,16}',$_POST['domain']))
{
echo '<p>Error: domain is not valid.</p>';
}
else {echo '<p>Domain looks good.</p>';}
The pattern should only match the domain (1~63 characters) and domain suffix (2~16 characters); no subdomains, uppercase, etc.
All the online regex testers seem to work though
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'([a-z\d]){1,63}(\.)([a-z\d]){2,16}'
should be
'/([a-z\d]){1,63}(\.)([a-z\d]){2,16}/i'
Most online regex testers add this here for you.