code: public int indexIn(CharSequence sequence, int start) {
      int length = sequence.length();
      checkPositionIndex(start, length);
      for (int i = start; i < length; i++) {
        if (matches(sequence.charAt(i))) {
          return i;
        }
      }
      return -1;
    }
docs: Returns the index of the first matching BMP character in a character sequence, starting from a
  given position, or {@code -1} if no character matches after that position.
  
  <p>The default implementation iterates over the sequence in forward order, beginning at {@code
  start}, calling {@link #matches} for each character.
  
  @param sequence the character sequence to examine
  @param start the first index to examine; must be nonnegative and no greater than {@code
  sequence.length()}
  @return the index of the first matching character, guaranteed to be no less than {@code start},
  or {@code -1} if no character matches
  @throws IndexOutOfBoundsException if start is negative or greater than {@code
  sequence.length()}