str2syms(symstr)

str2syms evaluates a string representing a symbolic expression.

str2syms evaluates symstr where symstr is a string representing a symbolic expression.
Additionally to str2sym, str2syms declares unknown variables as symbolic in the caller workspace.
Example:
syms x y
y = x;
z = str2syms('a + y')
creating symbolic variable a
z =
a + x