//validate
$(function(){

  $.validator.addMethod('strongPassword', function(value, element) {
    return this.optional(element)
      || value.length >= 8
      && /d/.test(value)
      && /[a-z]/i.test(value);
  }, 'Your password must be at least 8 characters long and contain at least one number and one char'.')
  $('#register-form').validate({
    rules:{
      email: {
        required: true,
        email: true
      },
      password:{
        required: true,
        strongPassword: true

      },
      confirmPassword:{
        required: true,
        equalTo: '#password'
      },
      firstname: {
        required: true,
        nowhitespace: true,
        lettersonly: true
      },
      lastname: {
        required: true,
        nowhitespace: true,
        lettersonly: true
      },
      mobileNumber: {
        required: true,
        digits: true,
        phonesUK: true
      },
      username:{
        required:true
      }
    },
    messages: {
      email: {
        required: 'Please enter an email address',
        email: 'Please enter a <i>valid</i> email address'
      }
    }

  }); //valdate end
}); //function end

<form class="article" id="article-form" name="article" method="post">
                    <ol>
                        <li>
                            <label for="heading">Heading</label> <span id="headingMessage"></span>
                            <input name="heading" id="heading" class="form-control" type="text">
                        </li>

                        <li>
                            <label for="topic">Topic</label> <span id="topicMessage"></span>
                            <input name="topic" id="topic" class="form-control" type="text" list="football">
                            <datalist id="football">
                                <option value="Scotland"></option>
                                <option value="England"></option>
                                <option value="Spain"></option>
                            </datalist>

                        </li>

                        <li>
                            <label for="summary">Summary</label> <span id="summaryMessage"></span>
                            <input name="summary" id="summary" class="form-control" type="text">
                        </li>

                        <li>
                            <label for="thumbnail">Thumbnail Link</label> <span id="thumbnailMessage"></span>
                            <input name="thumbnail" id="thumbnail" class="form-control" type="text">
                        </li>

                        <li>
                            <label for="video">Video</label> <span id="videoMessage"></span>
                            <input name="video" id="video" class="form-control" type="text" >
                        </li>

                        <li>
                            <label for="articleText">Text</label>
                            <textarea name="articleText" height:300px id="articleText" class="form-control" type="text"></textarea><span id="articleTextMessage"></span>

                        </li>
                    </ol>
                        <input class="btn btn-info" id="formButtonSubmit" type="submit" value="Submit" name="submit">
                </form>