问使用jquery对提交文本框进行正则表达式验证

EN

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>

<h2>Example 1:</h2>
<form id="first_form" method="post" action="">
  <div>
    <label for="first_name">First Name:</label>
    <input type="text" id="first_name" name="first_name"></input>
  </div>
  <div>
    <label for="last_name">Last Name:</label>
    <input type="text" id="last_name" name="last_name"></input>
  </div>
  <div>
    <label for="email">Email:</label>
    <input type="email" id="email" name="email"></input>
  </div>
  <div>
    <label for="password">Password:</label>
    <input type="password" id="password" name="password"></input>
  </div>
  <div>
    <input type="submit" value="Submit" />
  </div>
</form>

$(document).ready(function() {

  $('#first_form').submit(function(e) {
    e.preventDefault();
    var first_name = $('#first_name').val();
    var last_name = $('#last_name').val();
    var email = $('#email').val();
    var password = $('#password').val();

    $(".error").remove();

    if (first_name.length < 1) {
      $('#first_name').after('<span class="error">This field is required</span>');
    }
    if (last_name.length < 1) {
      $('#last_name').after('<span class="error">This field is required</span>');
    }
    if (email.length < 1) {
      $('#email').after('<span class="error">This field is required</span>');
    } else {
      var regEx = /^[A-Z0-9][A-Z0-9._%+-]{0,63}@(?:[A-Z0-9-]{1,63}\.){1,125}[A-Z]{2,63}$/;
      var validEmail = regEx.test(email);
      if (!validEmail) {
        $('#email').after('<span class="error">Enter a valid email</span>');
      }
    }
    if (password.length < 8) {
      $('#password').after('<span class="error">Password must be at least 8 characters long</span>');
    }
  });

});