regex_replace_sample.fs

open System
open System.Text.RegularExpressions

let private encodingMatch (m: Match) =
    match m.Value[0] with
    | '+' -> "-"
    | '/' -> "_"
    | _ -> m.Value

let private decodingMatch (m: Match) =
    match m.Value[0] with
    | '-' -> "+"
    | '_' -> "/"
    | _ -> failwith "Impossible"
  
let s = "hello+world/na=="
let r = Regex.Replace(s.TrimEnd('='), "[+/]", encodingMatch) |> Dump // LINQPad's Dump
Regex.Replace(r, "[-_]", decodingMatch) + String('=', 4 - r.Length % 4) |> Dump