eval of numeric strings

[Josef Eschgfaeller <esg@unife.it>]

I would like to define a function evaln
which evaluates a string to a number
only if the string contains explicitly
the decimal representation of that
number, not a variable.
--------------------------------------------------
Behavior of eval:

x=17;
s=eval("x"); print(s); \\ 17.
t=eval("17"); print(t); \\ 17.
y="John"
r=eval("y"); print(r); \\ John
--------------------------------------------------
Desired behavior of evaln:

x=17;
s=evaln("x"); print(s); \\ x.
t=evaln("17"); print(t); \\ 17.
y="John"
r=evaln("y"); print(r); \\ y.
--------------------------------------------------
Idea of evaln (x is always a string):

evaln (x) = if (isstringofdigits(x), eval(x), x)
--------------------------------------------------
I could try to program isstringofdigits
(including +,-. at the right places) by
hand, but is there a better way?

Thanks for help
Josef Eschgfaeller