Converting a string to reals

Hi all,
I have the following problem in f90 for which I would like some help.
I would like to know
how can I convert a string, say "b**2" to a variable so given the
value b=2. the outcome will be 4.?

Imagine that there may be more complex functions like "exp(b**2)" etc
which I would like to be
able to convert them from strings to variables and given a value for b
get a real result.

Can anyone help me?

Cheers,
psycho_dad

.