Vue.js использование фильтров в input

let app = new Vue({ 
  el: '#root', 
  data: { 
    fam: '' 
  }, 
  methods: { 
    noNumber: function(evt) { 
      var regex = new RegExp("^[a-zA-Z ]+$"); 
      var key = String.fromCharCode(!evt.charCode ? evt.which : evt.charCode); 
      if (!regex.test(key)) { 
        event.preventDefault(); 
        return false; 
      } 
    } 
  } 
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.2.6/vue.min.js"></script> 
<div id="root"> 
  <input type="text" v-model="fam" id="fam" class="form-control test" name="thepost" placeholder="Фамилия" v-on:keypress="noNumber(event)" value=""> 
</div>