I'm trying to validate a form field for a college exercise, but I'm not succeeding. I do not understand clearly where is the error. Can someone tell me where is the error and why? I do not need the code to copy and paste. I need to understand what is wrong. The function checkUsername(valor) appears as it in the test document.

I appreciate any help.

<input type="text" 
  class="input" id="username" name="username" 
  placeholder="nombre de usuario" required />

function checkUsername(valor){
  valor = document.getElementById('username').value;
  if (/^[a-zA-Z0-9\_\-]{4,30}$/g.test(valor)){
    return true;
  } else {
    return false;
  }
}