Поиск подстроки

let searchInString str pat =
    let mutable i = 0
    let mutable j = 0
    let strLen = String.length str
    let patLen = String.length pat
    while i <= (strLen - patLen) && j < patLen  do
        if str.[i + j] = pat.[j] 
        then  j <-  j + 1
        else i <- i + 1; j <- 0;
    if i <= (strLen - patLen) then Some i else None

let s = "My name is Anonymous."
let f = searchInString s " is"
match f with
| Some v -> printfn "%c %i" (s.[v]) v
| None -> printfn "pat not found"