If you really mean you have an expression created using sym objects from
Symbolic Math Toolbox when you say "a symbolic expression" then look here:
If you mean that you have a string that you'd like to convert, I don't
believe there's a ready-made function to do what you want, though creating
the correct patterns [the hard part] and applying REGEXP to your expression
should do the trick.
comp.soft-sys.matlab (CSSM) FAQ: http://www.yqcomputer.com/