JSFiddle - React, Tailwind, and code Playground

by Boba Poshtar

HTML

<input name="test" class="test-input" />

<p>
  56X
  <select>
    <option checked>M</option>
    <option>N</option>
  </select>
  <input maxlength="12"><br>
</p>

JavaScript

let input = document.querySelector('.test-input');
input.value = '56X';
input.addEventListener('input', codeRestraint);

function codeRestraint(){
	let input = this;
  let val = input.value.substr(0, 16);
  let res = '56X';
  let reg1 = new RegExp(`^[MN]$`);
  if (reg1.test(val[3])) {
  	res += val[3] + val.substr(4).replace(/[^A-Za-z0-9]/g, '');
  }
  input.value = res;
}