php-regexHow to use PHP regex to match an IP address?

To match an IP address using PHP regex, you can use the following code:

$ip = '127.0.0.1';
if (preg_match('/^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$/', $ip)) {
    echo 'IP address is valid';
} else {
    echo 'IP address is not valid';
}

Output example

IP address is valid

The code consists of the following parts:

1. $ip = '127.0.0.1'; - This is the IP address that we are trying to match.

2. preg_match('/^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$/', $ip) - This is the regular expression used to match the IP address. It consists of two parts:

   • ^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3} - This part matches the first three octets of the IP address. It consists of three parts:
     • [0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5] - This part matches a single octet of the IP address. It consists of five parts:
       • [0-9] - This part matches any number between 0 and 9.
       • [1-9][0-9] - This part matches any number between 10 and 99.
       • 1[0-9]{2} - This part matches any number between 100 and 199.
       • 2[0-4][0-9] - This part matches any number between 200 and 249.
       • 25[0-5] - This part matches any number between 250 and 255.
     • \ - This is an escape character.
     • \. - This part matches a single dot.
   • ([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$ - This part matches the fourth octet of the IP address. It is the same as the first part.

3. if (preg_match('/^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$/', $ip)) { - This is an if statement that checks if the IP address matches the regular expression.

4. echo 'IP address is valid'; - This is the code that is executed if the IP address matches the regular expression.

5. echo 'IP address is not valid'; - This is the code that is executed if the IP address does not match the regular expression.

Helpful links

• PHP preg_match() Function
• Regular Expression Syntax

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