code: public int countIn(CharSequence sequence) {
      int count = 0;
      for (int i = 0; i < sequence.length(); i++) {
        if (matches(sequence.charAt(i))) {
          count++;
        }
      }
      return count;
    }
docs: Returns the number of matching {@code char}s found in a character sequence.
  
  <p>Counts 2 per supplementary character, such as for {@link #whitespace}().{@link #negate}().