}

  public void testRetainFrom() {
    assertThat(is('a').retainFrom("bazaar")).isEqualTo("aaa");
    assertThat(is('z').retainFrom("bazaar")).isEqualTo("z");
    assertThat(is('!').retainFrom("!@#$%^&*()-=")).isEqualTo("!");
    assertThat(is('x').retainFrom("bazaar")).isEqualTo("");
    assertThat(is('a').retainFrom("")).isEqualTo("");
  }

  public void testPrecomputedOptimizations() {

     *
     * <p>ASCII characters in the part are expected to be valid per RFC 1035, with underscore also
     * being allowed due to widespread practice.
     */

    String asciiChars = CharMatcher.ascii().retainFrom(part);

    if (!PART_CHAR_MATCHER.matchesAllOf(asciiChars)) {
      return false;
    }

    // No initial or final dashes or underscores.

    if (DASH_MATCHER.matches(part.charAt(0))

   * Returns a string containing all matching BMP characters of a character sequence, in order. For
   * example:
   *
   * {@snippet :
   * CharMatcher.is('a').retainFrom("bazaar")
   * }
   *
   * ... returns {@code "aaa"}.
   */
  public String retainFrom(CharSequence sequence) {
    return negate().removeFrom(sequence);
  }

  /**

   * Returns a string containing all matching BMP characters of a character sequence, in order. For
   * example:
   *
   * {@snippet :
   * CharMatcher.is('a').retainFrom("bazaar")
   * }
   *
   * ... returns {@code "aaa"}.
   */
  public String retainFrom(CharSequence sequence) {
    return negate().removeFrom(sequence);
  }

  /**