How to get the first string's from the right hand side

So I have this code that would get the first string’s from the right hand side and stop whenever there is an integer but for some reason its not working with me.

Example input of fUnit is "CS_25x2u"

expected output of it after using unit is "u".

Real output is "undefined".

function buildUnit(fUnit){// wahts ginna be passed here is the gibirish unit and the output of this function is the clear unit
    fUnit = fUnit.toString;
    const regex = /[a-zA-Z]*$/;
    const unit = (x) => x.match(regex)[0];
    fUnit = unit(fUnit);

If you need more info please let me know

Thank you

>Solution :

const regex = /[a-zA-Z]*$/;
console.log(regex.exec(sample));

Assuming fUnit variable contains your string

const unit = (x) => x.match(regex)[0];
console.log(unit(fUnit));