return CompactHashSet.create();
  }

  /**
   * Returns a new array of the given length with the same type as a reference array.
   *
   * @param reference any array of the desired type
   * @param length the length of the new array
   */
  /*
   * The new array contains nulls, even if the old array did not. If we wanted to be accurate, we

     * in Guava, at least for now).
     */
    return new JdkPatternCompiler();
  }

  private static final class JdkPatternCompiler implements PatternCompiler {
    @Override
    public CommonPattern compile(String pattern) {
      return new JdkPattern(Pattern.compile(pattern));
    }

    @Override
    public boolean isPcreLike() {
      return true;

   * A thread-local destination buffer to keep us from creating new buffers. The starting size is
   * 1024 characters. If we grow past this we don't put it back in the threadlocal, we just keep
   * going and grow as needed.
   */
  private static final ThreadLocal<char[]> DEST_TL =
      new ThreadLocal<char[]>() {
        @Override
        protected char[] initialValue() {
          return new char[1024];
        }
      };