function validateEmail(email)
{
    var re = /\S+@\S+\.\S+/;
    return re.test(email);
}

function next_reg_1() {
  el1 = document.getElementById("first-row");
  el2 = document.getElementById("second-row");

  var pass = document.forms["reg_form"]["password"].value;
  var username = document.forms["reg_form"]["username"].value;

  if ((pass.length < 4) || (username.length < 4)) {
    // error msg
    console.log("first row fail");
  }else{
    el1.style.display = 'none';
    el2.style.display = 'block';
  }
}

function next_reg_2() {
  el1 = document.getElementById("second-row");
  el2 = document.getElementById("third-row");

  var f_name = document.forms["reg_form"]["f_name"].value;
  var l_name = document.forms["reg_form"]["l_name"].value;
  var email = document.forms["reg_form"]["email"].value;

  if ((f_name.length < 1) || (l_name.length < 1) || (validateEmail(email) === false)) {
    // error msg
    console.log("Second row fail");
  }else{
    el1.style.display = 'none';
    el2.style.display = 'block';
  }
}