the thing is that i never know how does the string gonna look like as it depends from user user input..

If you don't know how the input looks, you're screwed. Processing natural language is very, very hard and usually works only partially.

is there a way of converting string to a list?

Many ways, the easiest involving split and a regular expression.

Anyway, it might help if you'd describe us what problem you are trying to solve, and in which way you are trying to solve it - maybe there's a better way.