validExp.f90

logical function validExp(inputstring, length)
IMPLICIT NONE

	character(len=500), intent(in) :: inputstring
	integer, intent(in) :: length
	integer:: i, counter = 0
	character(len=1) :: testCharacter
	character(len=16), parameter :: allowed = "1234567890+-/$()"
	validExp = .FALSE.
	DO i = 1, length
		testCharacter = inputstring(i:i)
		if((i==1) .AND. (testCharacter /= "(")) exit
		if(index(testCharacter, allowed) == 0) exit
		! Hint: if at any point, there are more right parentheses
		! than left parentheses, we have an issue.
		if(testCharacter=="(") counter = counter + 1
		if(testCharacter==")") counter = counter - 1
		if((i == length) .AND. (counter == 0)) validExp = .TRUE.
	END DO
END FUNCTION