In this tutorial, we demonstrate how to implement jQuery-based form validation in WordPress, including AJAX-based username and email existence checks.

Step 1: Load jQuery and Validator Plugin

<!-- jQuery library -->
<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>

<!-- jQuery Form Validator -->
<script src="https://cdn.jsdelivr.net/jquery.validation/1.16.0/jquery.validate.min.js"></script>

Step 2: Form Markup

<form id="myForm">
  <label for="username">Username:</label>
  <input type="text" name="username" id="username" required pattern="[A-Za-z0-9_]+">

  <label for="email">Email:</label>
  <input type="email" name="email" id="email" required>

  <label for="password">Password:</label>
  <input type="password" name="password" id="password" required minlength="6">

  <label for="confirmPassword">Confirm Password:</label>
  <input type="password" name="confirmPassword" id="confirmPassword" required equalTo="#password">

  <button type="submit">Submit</button>
</form>

Step 3: jQuery Validator Setup

<script>
  $(document).ready(function() {
    $("#myForm").validate({
      rules: {
        username: {
          required: true,
          pattern: /^[A-Za-z_]+$/, // Only letters and underscores allowed
          remote: {
            url: ajaxurl,
            type: "post",
            data: {
              action: "check_username_exists",
              username: function() {
                return $("#username").val();
              }
            }
          }
        },
        email: {
          required: true,
          email: true,
          remote: {
            url: ajaxurl,
            type: "post",
            data: {
              action: "check_email_exists",
              email: function() {
                return $("#email").val();
              }
            }
          }
        },
        password: {
          required: true,
          minlength: 6
        },
        confirmPassword: {
          required: true,
          equalTo: "#password"
        }
      },
      messages: {
        username: {
          required: "Please enter a username.",
          pattern: "Only letters and underscores are allowed.",
          remote: "This username is already taken."
        },
        email: {
          required: "Please enter your email address.",
          email: "Please enter a valid email address.",
          remote: "This email address already exists."
        },
        password: {
          required: "Please enter a password.",
          minlength: "Password must be at least 6 characters long."
        },
        confirmPassword: {
          required: "Please confirm your password.",
          equalTo: "Passwords do not match."
        }
      }
    });
  });
</script>

Step 4: PHP Functions for AJAX

<script>
  $(document).ready(function() {
    $.validator.addMethod(
      "noNumeric",
      function(value, element) {
        return this.optional(element) || /^[^0-9]+$/i.test(value);
      },
      "Please enter a value without numeric characters."
    );

    $("#myForm").validate({
      rules: {
        // Other rules here
        noNumericField: {
          noNumeric: true
        }
      },
      messages: {
        // Other messages here
        noNumericField: {
          noNumeric: "Please enter a value without numeric characters."
        }
      }
    });
  });
</script>

Step 5: Enqueue Script with AJAX URL

// Check if username exists in the database
add_action('wp_ajax_check_username_exists', 'check_username_exists');
add_action('wp_ajax_nopriv_check_username_exists', 'check_username_exists');

function check_username_exists() {
    $username = $_POST['username'];
    $user_id = username_exists($username);

    if ($user_id) {
        echo 'false';
    } else {
        echo 'true';
    }
    exit;
}

// Check if email exists in the database
add_action('wp_ajax_check_email_exists', 'check_email_exists');
add_action('wp_ajax_nopriv_check_email_exists', 'check_email_exists');

function check_email_exists() {
    $email = $_POST['email'];
    $user_id = email_exists($email);

    if ($user_id) {
        echo 'false';
    } else {
        echo 'true';
    }
    exit;
}

// Check if username contains no numeric value
add_action('wp_ajax_check_username_no_numeric', 'check_username_no_numeric');
add_action('wp_ajax_nopriv_check_username_no_numeric', 'check_username_no_numeric');

function check_username_no_numeric() {
    $username = $_POST['username'];

    if (preg_match('/\d/', $username)) {
        echo 'false';
    } else {
        echo 'true';
    }
    exit;
}

add_action('wp_enqueue_scripts', 'my_enqueue_scripts');

function my_enqueue_scripts() {
    wp_enqueue_script('my-custom-script', get_stylesheet_directory_uri() . '/js/custom-script.js', array('jquery', 'jquery-validate'), '1.0', true);
    wp_localize_script('my-custom-script', 'ajaxurl', admin_url('admin-ajax.php'));
}

  That’s it! You now have a functional registration form with advanced validation for both the client and server side in WordPress.