how can I convert a string to an expression which "symvar" can search for identifiers?

how can I convert a string to an expression which "symvar" can search for identifiers?

I use this: symvar 'cos(pi*x - beta1)'

ans =

    'beta1'
    'x'

and it works, but when I use this:

      insertfunction='cos(pi*x - beta1)'
symvar insertfunction

it doesn't have the same results. How can I make it?

Answer 1

[Paulo Silva]

insertfunction='cos(pi*x - beta1)'
symvar(insertfunction)

Answer 2

[Ivan van der Kroon]

insertfunction='cos(pi*x - beta1)'
>> symvar(insertfunction)

ans = 

    'beta1'
    'x'