code: public static String exp(String exponent) {
          BigDecimal bd;
          try {
              bd = new BigDecimal(exponent);
              return Math.exp(bd.doubleValue()) + "";
          } catch (Exception e) {
              return null;
          }
      }
docs: Returns Euler's number <i>e</i> raised to the power of a {@code double}
  value.
  
  Any numeric string recognized by {@code BigDecimal} is supported.
  
  @param exponent A valid number string
  
  @return the value <i>e</i><sup>{@code value}</sup>, where <i>e</i> is the
  base of the natural logarithms.
  
  @see BigDecimal