code: private boolean isFastPath(String regex) {
          char ch;
          return (((regex.length() == 1 && ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) || (regex.length() == 2
                  && regex.charAt(0) == '\\' && (((ch = regex.charAt(1)) - '0') | ('9' - ch)) < 0 && ((ch - 'a') | ('z' - ch)) < 0 && ((ch - 'A') | ('Z' - ch)) < 0)) && (ch < Character.MIN_HIGH_SURROGATE || ch > Character.MAX_LOW_SURROGATE));
      }
docs: Since JDK 1.7 there's a special branch in String.split which works very fast for one-character pattern
  We do not report a bug if this case takes place
  (in fact precompilation will make split much slower since this fast path doesn't use regexp engine at all)
  @param regex regex to test whether it's suitable for the fast path
  @return true if fast path is possible