String to a mathematical expression

how can i convert a String to a mathematical expression , that i can use in a function's body ?

[per isakson]

That depends. What does the String look like?

[Elia]

for example sin(x)-x^2

Answer 1

[Matt J]

If you display the string at the command line, you can then copy/paste it into the function's body.

[Elia]

i want to get the string from an edit text feld in a gui . it will be a function's body that i want to plot it .

[Matt J]

If the string will always be a function of a known letter, e.g. 'x', you can do

 theFunc = str2func(['@(x)' theString])