Vue Js Input allow only small letter

<!DOCTYPE html>
<html>

<body>
  <div id="app">
    <h3>Vue Js Textbox allow only small letters</h3>
    <input type="text" v-model="inputValue" @keydown="restrictInput" />
    <div v-if="!isInputValid" class="error-message">Only small letters are allowed.</div>
  </div>

.error-message {
      color: red;
      font-size: 14px;
      margin-top: 5px;
    }

input[type="text"] {
      padding: 10px;
      font-size: 16px;
      border: 1px solid #ccc;
      border-radius: 4px;
      outline-color: blue;
      /* Change the color as desired */