is_number Function

   elemental function is_number(self, allow_spaces)
   !< Return true if the string contains a number (real or integer).
   !<
   !<```fortran
   !< type(string) :: astring
   !< logical      :: test_passed(7)
   !< astring = '   -1212112 '
   !< test_passed(1) = astring%is_number().eqv..true.
   !< astring = '   -121.2112 '
   !< test_passed(2) = astring%is_number().eqv..true.
   !< astring = '   -1212112'
   !< test_passed(3) = astring%is_number(allow_spaces=.false.).eqv..false.
   !< astring = '-12121.12   '
   !< test_passed(4) = astring%is_number(allow_spaces=.false.).eqv..false.
   !< astring = '+2e20'
   !< test_passed(5) = astring%is_number().eqv..true.
   !< astring = ' -2.4E13 '
   !< test_passed(6) = astring%is_number().eqv..true.
   !< astring = ' -2 E13 '
   !< test_passed(7) = astring%is_number().eqv..false.
   !< print '(L1)', all(test_passed)
   !<```
   !=> T <<<
   class(string), intent(in)           :: self         !< The string.
   logical,       intent(in), optional :: allow_spaces !< Allow leading-trailing spaces.
   logical                             :: is_number    !< Result of the test.

   is_number = (self%is_integer(allow_spaces=allow_spaces).or.self%is_real(allow_spaces=allow_spaces))
   endfunction is_number