## How to check if a number or a string is a valid port number in JavaScript?

June 10, 2021 - 2 min read

To check if a number or a string is a valid port number, we can use a regex expression to match for all the numbers from `0` till `65535` in JavaScript.

### TL;DR

``````// Regular expression to check if number is a valid port number
const regexExp = /^((6553[0-5])|(655[0-2][0-9])|(65[0-4][0-9]{2})|(6[0-4][0-9]{3})|([1-5][0-9]{4})|([0-5]{0,5})|([0-9]{1,4}))\$/gi;

// a number
const num = 8080;

regexExp.test(num); // true
``````

This is the regex expression for matching almost all the test cases for a valid port number from `0` till `65535` in JavaScript.

``````// Regular expression to check if number is a valid port number
const regexExp = /^((6553[0-5])|(655[0-2][0-9])|(65[0-4][0-9]{2})|(6[0-4][0-9]{3})|([1-5][0-9]{4})|([0-5]{0,5})|([0-9]{1,4}))\$/gi;
``````

Now let's write a number which is a valid port number like this,

``````// Regular expression to check if number is a valid port number
const regexExp = /^((6553[0-5])|(655[0-2][0-9])|(65[0-4][0-9]{2})|(6[0-4][0-9]{3})|([1-5][0-9]{4})|([0-5]{0,5})|([0-9]{1,4}))\$/gi;

// a number
const num = 8080;
``````

Now to test the number, we can use the `test()` method available in the regular expression we defined. It can be done like this,

``````// Regular expression to check if number is a valid port number
const regexExp = /^((6553[0-5])|(655[0-2][0-9])|(65[0-4][0-9]{2})|(6[0-4][0-9]{3})|([1-5][0-9]{4})|([0-5]{0,5})|([0-9]{1,4}))\$/gi;

// a number
const num = 8080;

regexExp.test(str); // true
``````
• The `test()` method will accept a `number` type as an argument to test for a match.
• The method will return boolean `true` if there is a match using the regular expression and `false` if not.

See the above example live in JSBin.

If you want this as a utility function which you can reuse, here it is,

``````/* Check if number is a valid port number */
function checkIfValidPortnumber(num) {
// Regular expression to check if number is a valid port number
const regexExp = /^((6553[0-5])|(655[0-2][0-9])|(65[0-4][0-9]{2})|(6[0-4][0-9]{3})|([1-5][0-9]{4})|([0-5]{0,5})|([0-9]{1,4}))\$/gi;

return regexExp.test(num);
}

// Use the function
checkIfValidPortnumber(8080); // true
checkIfValidPortnumber(65536); // false
``````

That's all! ðŸ˜ƒ