There's more than that possible that's still valid... "The string may contain digits, a comma (thousands separator), a decimal point, a leading + or - sign, an 'e' preceding a power of 10 scale factor, and an 'i' for a complex unit." And, while not mentioned explicitly, there's at minimum the 'E' variant for 'e'. I'm not sure w/o testing whether may/may not handle the Fortran-style 'd/D' double precision exponential character or not. Also, one can have a malformed string that consists of only the allowable characters so just testing membership in the club isn't sufficient.
All in all, you can presume it was not valid if strdouble returns NaN, but probably more overall effort in testing the string than verifying the result.
There's always the route of a small helper function if you're going to be doing this in many locations; if it's just a single entry box, a small logic expression ain't all that bad...
isOK=isfinite(x) & isreal(x)
OTOMH w/ your expressed want don't think there's anything else--of course, this doesn't prevent user from entering something from the range of realmin to realmax but that's a different question than whether it's a valid double or not. There, indeed, you're to the point of limiting a value.
If that's the case of there being some allowable range, there's always using a slider or other input mechanism than an edit box.
