I want to use JavaScript (can be with jQuery) to do some client-side validation to check whether a string matches the regex:
^([a-z0-9]{5,})$
Use regex.test() if all you want is a boolean result:
console.log(/^([a-z0-9]{5,})$/.test('abc1')); // false
console.log(/^([a-z0-9]{5,})$/.test('abc12')); // true
console.log(/^([a-z0-9]{5,})$/.test('abc123')); // true
...and you could remove the () from your regexp since you've no need for a capture.