I stole this

let string2opt (s:string) =
    let mutable ret = RegexOptions.ECMAScript
    for c in s do
        match c with
        | 's' -> ret <- ret ||| RegexOptions.Singleline
        | 'm' -> ret <- ret ||| RegexOptions.Multiline
        | 'i' -> ret <- ret ||| RegexOptions.IgnoreCase
        | _ -> ()
    ret