bootstrap4.css
.was-validated .form-control:valid:focus, .form-control.is-valid:focus {
  border-color: #28a745;
  box-shadow: 0 0 0 0.2rem rgba(40, 167, 69, 0.25);
}
bootstrap4.css
.was-validated .form-control:valid ~ .valid-feedback,
.was-validated .form-control:valid ~ .valid-tooltip, .form-control.is-valid ~ .valid-feedback,
.form-control.is-valid ~ .valid-tooltip {
  display: block;
}
bootstrap4.css
.was-validated textarea.form-control:valid, textarea.form-control.is-valid {
  padding-right: calc(1.5em + 0.75rem);
  background-position: top calc(0.375em + 0.1875rem) right calc(0.375em + 0.1875rem);
}
bootstrap4.css
.was-validated .form-control-file:valid ~ .valid-feedback,
.was-validated .form-control-file:valid ~ .valid-tooltip, .form-control-file.is-valid ~ .valid-feedback,
.form-control-file.is-valid ~ .valid-tooltip {
  display: block;
}
■フォーム:form-group:コンポーネント:サーバー側で入力検証を行う場合のフィードバック例

市町村名を入力してください
都道府県名を入力してください
郵便番号を入力してください
  <form>
    <div class="form-row">
      <div class="col-md-6 mb-3">
        <label for="validationServer01">姓</label>
        <input type="text" class="form-control is-valid" id="validationServer01" placeholder="First name" value="山田" required> </div>
      <div class="col-md-6 mb-3">
        <label for="validationServer02">名</label>
        <input type="text" class="form-control is-valid" id="validationServer02" placeholder="Last name" value="太郎" required> </div>
    </div>
    <div class="form-row">
      <div class="col-md-6 mb-3">
        <label for="validationServer03">市町村</label>
        <input type="text" class="form-control is-invalid" id="validationServer03" placeholder="市町村名を入力" required>
        <div class="invalid-feedback">市町村名を入力してください</div>
      </div>
      <div class="col-md-3 mb-3">
        <label for="validationServer04">都道府県</label>
        <input type="text" class="form-control is-invalid" id="validationServer04" placeholder="都道府県名を入力" required>
        <div class="invalid-feedback">都道府県名を入力してください</div>
      </div>
      <div class="col-md-3 mb-3">
        <label for="validationServer05">郵便番号</label>
        <input type="text" class="form-control is-invalid" id="validationServer05" placeholder="郵便番号を入力" required>
        <div class="invalid-feedback">郵便番号を入力してください</div>
      </div>
    </div>
    <button class="btn btn-primary" type="submit">送信する</button>
  </form>