bootstrap4.css
::-webkit-file-upload-button {
  font: inherit;
  -webkit-appearance: button;
}
bootstrap4.css
.form-control-file,
.form-control-range {
  display: block;
  width: 100%;
}
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;
}
bootstrap4.css
.was-validated .custom-file-input:valid ~ .custom-file-label, .custom-file-input.is-valid ~ .custom-file-label {
  border-color: #28a745;
}
bootstrap4.css
.was-validated .custom-file-input:valid ~ .valid-feedback,
.was-validated .custom-file-input:valid ~ .valid-tooltip, .custom-file-input.is-valid ~ .valid-feedback,
.custom-file-input.is-valid ~ .valid-tooltip {
  display: block;
}
bootstrap4.css
.was-validated .custom-file-input:valid:focus ~ .custom-file-label, .custom-file-input.is-valid:focus ~ .custom-file-label {
  border-color: #28a745;
  box-shadow: 0 0 0 0.2rem rgba(40, 167, 69, 0.25);
}
bootstrap4.css
.was-validated .form-control-file:invalid ~ .invalid-feedback,
.was-validated .form-control-file:invalid ~ .invalid-tooltip, .form-control-file.is-invalid ~ .invalid-feedback,
.form-control-file.is-invalid ~ .invalid-tooltip {
  display: block;
}
bootstrap4.css
.was-validated .custom-file-input:invalid ~ .custom-file-label, .custom-file-input.is-invalid ~ .custom-file-label {
  border-color: #dc3545;
}
bootstrap4.css
.was-validated .custom-file-input:invalid ~ .invalid-feedback,
.was-validated .custom-file-input:invalid ~ .invalid-tooltip, .custom-file-input.is-invalid ~ .invalid-feedback,
.custom-file-input.is-invalid ~ .invalid-tooltip {
  display: block;
}
bootstrap4.css
.was-validated .custom-file-input:invalid:focus ~ .custom-file-label, .custom-file-input.is-invalid:focus ~ .custom-file-label {
  border-color: #dc3545;
  box-shadow: 0 0 0 0.2rem rgba(220, 53, 69, 0.25);
}
bootstrap4.css
.input-group > .form-control,
.input-group > .form-control-plaintext,
.input-group > .custom-select,
.input-group > .custom-file {
  position: relative;
  -ms-flex: 1 1 auto;
  flex: 1 1 auto;
  width: 1%;
  margin-bottom: 0;
}
bootstrap4.css
.input-group > .form-control + .form-control,
.input-group > .form-control + .custom-select,
.input-group > .form-control + .custom-file,
.input-group > .form-control-plaintext + .form-control,
.input-group > .form-control-plaintext + .custom-select,
.input-group > .form-control-plaintext + .custom-file,
.input-group > .custom-select + .form-control,
.input-group > .custom-select + .custom-select,
.input-group > .custom-select + .custom-file,
.input-group > .custom-file + .form-control,
.input-group > .custom-file + .custom-select,
.input-group > .custom-file + .custom-file {
  margin-left: -1px;
}
bootstrap4.css
.input-group > .form-control:focus,
.input-group > .custom-select:focus,
.input-group > .custom-file .custom-file-input:focus ~ .custom-file-label {
  z-index: 3;
}
bootstrap4.css
.input-group > .custom-file .custom-file-input:focus {
  z-index: 4;
}
bootstrap4.css
.input-group > .custom-file {
  display: -ms-flexbox;
  display: flex;
  -ms-flex-align: center;
  align-items: center;
}
bootstrap4.css
.input-group > .custom-file:not(:last-child) .custom-file-label,
.input-group > .custom-file:not(:last-child) .custom-file-label::after {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
bootstrap4.css
.input-group > .custom-file:not(:first-child) .custom-file-label {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.custom-file {
  position: relative;
  display: inline-block;
  width: 100%;
  height: calc(1.5em + 0.75rem + 2px);
  margin-bottom: 0;
}
bootstrap4.css
.custom-file-input {
  position: relative;
  z-index: 2;
  width: 100%;
  height: calc(1.5em + 0.75rem + 2px);
  margin: 0;
  opacity: 0;
}
bootstrap4.css
.custom-file-input:focus ~ .custom-file-label {
  border-color: #80bdff;
  box-shadow: 0 0 0 0.2rem rgba(0, 123, 255, 0.25);
}
bootstrap4.css
.custom-file-input:disabled ~ .custom-file-label {
  background-color: #e9ecef;
}
bootstrap4.css
.custom-file-input:lang(en) ~ .custom-file-label::after {
  content: "Browse";
}
bootstrap4.css
.custom-file-input ~ .custom-file-label[data-browse]::after {
  content: attr(data-browse);
}
bootstrap4.css
.custom-file-label {
  position: absolute;
  top: 0;
  right: 0;
  left: 0;
  z-index: 1;
  height: calc(1.5em + 0.75rem + 2px);
  padding: 0.375rem 0.75rem;
  font-weight: 400;
  line-height: 1.5;
  color: #495057;
  background-color: #fff;
  border: 1px solid #ced4da;
  border-radius: 0.25rem;
}
bootstrap4.css
.custom-file-label::after {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  z-index: 3;
  display: block;
  height: calc(1.5em + 0.75rem);
  padding: 0.375rem 0.75rem;
  line-height: 1.5;
  color: #495057;
  content: "Browse";
  background-color: #e9ecef;
  border-left: inherit;
  border-radius: 0 0.25rem 0.25rem 0;
}
bootstrap4.css
.custom-control-label::before,
.custom-file-label,
.custom-select {
  transition: background-color 0.15s ease-in-out, border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
}
bootstrap4.css
@media (prefers-reduced-motion: reduce) {
  .custom-control-label::before,
  .custom-file-label,
  .custom-select {
    transition: none;
  }
}
■フォーム:form-group:コンポーネント:フォームの基本要素

キャリアメールは届かない場合があります。
  <form>
    <div class="form-group">
      <label for="email1">メールアドレス</label>
      <input type="email" class="form-control" id="email1" aria-describedby="emailHelp" placeholder="メールアドレスを入力">
      <small id="emailHelp" class="form-text text-muted">キャリアメールは届かない場合があります。</small>
    </div>
    <div class="form-group">
      <label for="password1">パスワード</label>
      <input type="password" class="form-control" id="password1" placeholder="パスワードを入力"> </div>
    <div class="form-group">
      <label for="select1">プルダウンメニューから選んでください</label>
      <select class="form-control" id="select1">
        <option>メニュー1</option>
        <option>メニュー2</option>
        <option>メニュー3</option>
        <option>メニュー4</option>
        <option>メニュー5</option>
      </select>
    </div>
    <!-- 複数選択のプルダウンメニュー -->
    <div class="form-group">
      <label for="select2">複数選択のプルダウンメニュー</label>
      <select multiple class="form-control" id="select2">
        <option>複数選択可A</option>
        <option>複数選択可B</option>
        <option>複数選択可C</option>
        <option>複数選択可D</option>
        <option>複数選択可E</option>
      </select>
    </div>
    <div class="form-group">
      <label for="textarea1">複数行のテキスト入力欄</label>
      <textarea class="form-control" id="textarea1" rows="3"></textarea>
    </div>
    <div class="form-group">
      <label for="file1">ファイルを選択</label>
      <input type="file" class="form-control-file" id="file1"> 
    </div>
    <div class="form-check">
      <input type="checkbox" class="form-check-input" id="checkbox01">
      <label class="form-check-label" for="">チェックする</label>
    </div>
    <button type="submit" class="btn btn-primary">送信</button>
  </form>
■フォーム:form-group:コンポーネント:ファイル選択

<div class="container">
  <form>
    <div class="default-file mb-5">
      <input type="file" id="dafaultFile" lang="ja">
      <label for="dafaultFile">ファイル選択...</label>
    </div>
    <div class="custom-file">
      <input type="file" class="custom-file-input" id="customFile" lang="ja">
      <label class="custom-file-label" for="customFile">ファイル選択...</label>
    </div>
  </form>
■ナビゲーション:nav:JavaScript:タブとコンテンツ(データ属性API)

ホームのコンテンツ
プロフィールのコンテンツ
コンタクトのコンテンツ
  <!-- タブ部分 -->
  <ul class="nav nav-tabs" id="myTab" role="tablist">
    <li class="nav-item"><a class="nav-link active" id="home-tab" data-toggle="tab" href="#home" role="tab" aria-controls="home" aria-selected="true">ホーム</a></li>
    <li class="nav-item"><a class="nav-link" id="profile-tab" data-toggle="tab" href="#profile" role="tab" aria-controls="profile" aria-selected="false">プロフィール</a></li>
    <li class="nav-item"><a class="nav-link" id="contact-tab" data-toggle="tab" href="#contact" role="tab" aria-controls="contact" aria-selected="false">コンタクト</a></li>
  </ul>
  <!-- パネル部分 -->
  <div class="tab-content mt-3" id="myTabContent">
    <div class="tab-pane fade show active" id="home" role="tabpanel" aria-labelledby="home-tab">ホームのコンテンツ</div>
    <div class="tab-pane fade" id="profile" role="tabpanel" aria-labelledby="profile-tab">プロフィールのコンテンツ</div>
    <div class="tab-pane fade" id="contact" role="tabpanel" aria-labelledby="contact-tab">コンタクトのコンテンツ</div>
  </div>
■ナビゲーション:nav:JavaScript:タブ型ナビゲーション(JavaScript経由)

ホームのコンテンツが入ります。
プロフィールのコンテンツが入ります。
コンタクトのコンテンツが入ります。
  <ul class="nav nav-tabs" id="myTab_js" role="tablist">
    <li class="nav-item"><a class="nav-link active" id="home-tab" href="#home_js" role="tab" aria-controls="home" aria-selected="true">ホーム</a></li>
    <li class="nav-item"><a class="nav-link" id="profile-tab" href="#profile_js" role="tab" aria-controls="profile" aria-selected="false">プロフィール</a></li>
    <li class="nav-item"><a class="nav-link" id="contact-tab" href="#contact_js" role="tab" aria-controls="contact" aria-selected="false">コンタクト</a></li>
  </ul>
  <!-- パネル部分 -->
  <div class="tab-content mt-3" id="myTabContent">
    <div class="tab-pane fade show active" id="home_js" role="tabpanel" aria-labelledby="home-tab">ホームのコンテンツが入ります。</div>
    <div class="tab-pane fade" id="profile_js" role="tabpanel" aria-labelledby="profile-tab">プロフィールのコンテンツが入ります。</div>
    <div class="tab-pane fade" id="contact_js" role="tabpanel" aria-labelledby="contact-tab">コンタクトのコンテンツが入ります。</div>
  </div>
<script>
  $('#myTab_js a').on('click', function(e) {
    e.preventDefault()
    $(this).tab('show')
  })
</script>