Urgent doubt.

I have a doubt regarding forming a SgExpression.

Assume that I use 'gets(char *)' to get an expression from the user.
So the program asks something like:

    Enter Expression:

for which the user would enter
    n1 * n2 / n3

Now, how can I make this as an instance of (SgExpression *)
example: SgExpression *exp;

	  I want 'exp' to be (n1 * n2 / n3)

 Is there a direct way by which I can do this. I tried several
 methods, but nothing seems to work fine.

 An early reply would be very much appreciated.