code: @GwtIncompatible
      public Matcher matcher(Reader text, int length) throws IOException {
          Matcher m = new Matcher(this);
          m.setTarget(text, length);
          return m;
      }
docs: Returns a matcher taking a text stream as target.
  <b>Note that this is not a true POSIX-style stream matching</b>, i.e. the whole length of the text is preliminary read and stored in a char array.
  
  @param text   a text stream
  @param length the length to read from a stream; if <code>len</code> is <code>-1</code>, the whole stream is read in.
  @throws IOException indicates an IO problem