function checkPassword(str)
  {
    var re = /^(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{6,}$/;
    return re.test(str);
  }
  /*
    matches a string of six or more characters;
    that contains at least one digit (\d is shorthand for [0-9]);
    at least one lowercase character; and
    at least one uppercase character:
  */


  function checkForm(form)
  {
    if(form.username.value == "") {
      alert("Error: Username cannot be blank!");
      form.username.focus();
      return false;
    }
    re = /^\w+$/; //\w is shorthand for 'any letter, number or the underscore character'.
    if(!re.test(form.username.value)) {
      alert("Error: Username must contain only letters, numbers and underscores!");
      form.username.focus();
      return false;
    }
    if(form.pwd1.value != "" && form.pwd1.value == form.pwd2.value) {
      if(!checkPassword(form.pwd1.value)) {
        alert("The password you have entered is not valid!");
        form.pwd1.focus();
        return false;
      }
    } else {
      alert("Error: Please check that you've entered and confirmed your password!");
      form.pwd1.focus();
      return false;
    }
    return true;
  }