registerRouter.go

package main

import (
    "fmt"
    "net/http"
    "github.com/gorilla/mux"
)

func GetExampleHandler(text string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        _, _ = w.Write([]byte(text))
    } 
}

func main() {

    mainRouter := mux.NewRouter()
    mainRouter.HandleFunc("/main", GetExampleHandler("main"))
    sub := mainRouter.PathPrefix("/sub").Subrouter()
  
    subRouter := mux.NewRouter().PathPrefix("/sub").Subrouter()
    subRouter.HandleFunc("/", GetExampleHandler("sub top"))
    for _, item := range []string{"endpoint1", "endpoint2"} {
        subRouter.HandleFunc("/" + item, GetExampleHandler(item))
    }
  
    // in mux, you need to register subrouter
    // with the same path that the handlers in the main router
    sub.Handle("/", subRouter)

    // if your subrouter has handlers that match
    // other sub paths - you also need to do this
    sub.Handle("/{_:.*}", subRouter)

    http.ListenAndServe(":5050", mainRouter)
}