Why Can\'t I Validate Domains in PHP Without Regular Expressions?

PHP domain name verification

Question: How to verify domain name in PHP without using regular expressions?

Verification requirements:

• Starts with a letter or number
• Can contain letters, numbers and hyphens
• Starts with Ending with a letter or number
• Length between 1 and 253 characters
• Each label length between 1 and 63 characters

Possible answers:

Domain name validation without using regular expressions is not possible as multiple rules and restrictions need to be checked.

Regular expression pattern:

/^[a-z\d](-*[a-z\d])*)(\.([a-z\d](-*[a-z\d])*))*$/i

Meaning:

• ^ and $ anchor strings the beginning and end of.
• [a-zd] matches letters or numbers.
• (-*[a-zd])* matches zero or more letters or numbers separated by hyphens.
• . Matches a period.
• {} specifies the number of repetitions.

Sample code:

<code class="php">function is_valid_domain_name($domain_name)
{
    return (preg_match("/^([a-z\d](-*[a-z\d])*)(\.([a-z\d](-*[a-z\d])*))*$/i", $domain_name) //valid chars check
            &amp;&amp; preg_match("/^.{1,253}$/", $domain_name) //overall length check
            &amp;&amp; preg_match("/^[^\.]{1,63}(\.[^\.]{1,63})*$/", $domain_name)   ); //length of each label
}</code>

Test case:

域名	有效性
a	Y
0	Y
a.b	Y
localhost	Y
google.com	Y
news.google.co.uk	Y
xn--fsqu00a.xn--0zwm56d	Y
goo gle.com	N
google..com	N
google.com	N
google-.com	N
.google.com	N
	N
alert(	N
.	N
..	N
	N
-	N
[]	N

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