function isValidDomain($value) {
	if($value) {
		//Max 255 chars
		if(strlen($value) > 255) {
			return false;
		}
		//Remove trailing .
		if(substr($value,-1,1) == '.') {
			$value = substr($value,0,-1);
		}
		$labels = explode('.',$value);
		//More than 2 labels
		if(count($labels) >= 2) {
			foreach($labels as $label) {
				//Cannot start or end with a -
				if(substr($label,0,1) == '-' || substr($label,-1,1) == '-') {
					return false;
				}
				//Allowed characters a-z, 0-9, -
				if(!preg_match('/^[a-z\d-]{1,63}$/i',$label)) {
					return false;
				}
			}
		} else {
			return false;
		}
		return true;
	}
	return false;
}

PHP - Valid DNS name

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