acutally my english is not good but i want to detect forbidden letters in a variable that comes from html input but its not working. My code:

nextBtnFirst.addEventListener("click", function(event) {
  event.preventDefault();
  //Validating the required Fields//
  var fname = document.getElementById("fname_id").value;
  var lname = document.getElementById("lname_id").value;
  var letters = "/^[0-9a-zA-Z]+$/";
  if(fname =="" || lname=="") {
    updateError("First name and Last name is required!");
  } else {
    slidePage.style.marginLeft = "-25%";
    bullet[current - 1].classList.add("active");
    progressCheck[current - 1].classList.add("active");
    progressText[current - 1].classList.add("active");
    current += 1;
  } 
});

What i have tried but didnt work:

if(fname.includes("!")) { 
  updateError("Dont use forbidden Characters!"); 
}

Please help. Thanks!