Good examples

https://www.w3schools.com/howto/howto_js_password_validation.asp

Remove non permitted characters from an input box as user types

  <script>
    function VerifyProfileText() {
      var a = document.getElementById("Nickname").value;
      a = a.replace(/[^a-zA-Z ]/g, '');   //Permit a to z, AtoZ and space only
      document.getElementById("Nickname").value = a;
    }
  </script>

<input type="text" id="MyNickname" name="Nickname" maxlength="20" value="$Nickname" onkeyup="VerifyProfileText();" paste="VerifyProfileText();" >