How to Insert Space Every 4 Characters for Iban Registering

document.getElementById('iban').addEventListener('input', function (e) {  e.target.value = e.target.value.replace(/[^\dA-Z]/g, '').replace(/(.{4})/g, '$1 ').trim();});

<label for="iban">iban</label><input id="iban" type="text" name="iban" />

Plain Javascript way to add a space on the fly after every 5th digit in input field

Here is a relatively short approach:

Just add an event listener for the input event (or keyup/keydown), and then use some regex.

In the example directly below, all whitespace is initially removed using .replace(/\s/g, ''), and then .replace(/(\d{5})/g, '$1 ') essentially adds a space after every 5th character.

The reason all the whitespace is removed is so that there is always a space between every 5th character (even if you go back and edit previous characters).

document.getElementById('target').addEventListener('input', function (e) {  e.target.value = e.target.value.replace(/\s/g, '').replace(/(\d{5})/g, '$1 ').trim();});

<input id="target" type="text"/>

string test = "BE45898287271283";
test = Regex.Replace(test, ".{4}", "$0 ").Trim();