code: public static boolean isSignedDouble(String s) {
          if (isEmpty(s)) return defaultEmptyOK;
  
          try {
              Double.parseDouble(s);
  
              return true;
          } catch (Exception e) {
              return false;
          }
      }
docs: True if string s is a signed or unsigned floating point
  (real) number. First character is allowed to be + or -.
  
  Also returns true for unsigned integers. If you wish
  to distinguish between integers and floating point numbers,
  first call isSignedInteger, then call isSignedFloat.