How to manipulate columns made of strings

[dnaura]

Hi everyone,
I am facing issues when trying to do operation on strings columns.
For example I have a column which is an hexadecimal string ( '0A145C78', ...) and want to convert it to a binary string, or just want to extract the third charcater, ...
I don't find how to make it work.
Whatever my trials I get errors and script looping indefinitely.
Any guideline to work with strings columns?
Thanks.
David