Why does this formula return a unit?

Question

I understand that F# functions return a unit if nothing else is returned, but this function returns a string. Can someone please help me understand why it returns a unit?

let rec processList aList str = 
    match aList with
    | h::t  ->  let x = Regex.Replace(str, h, h, RegexOptions.IgnoreCase)
                processList t x
    | []    ->  printfn "%s" str

Answer 1

The stopping case for this recursive function | [] -> printfn "%s" str

returns unit, and therefore the function returns a unit. The other branch only recursively call the same function.