Play with this by sending requests for /customer, /customer/, and /customer/foo with curl:

gorilla_mux_playground.go

package main

import (
	"fmt"
	"net/http"

	"github.com/gorilla/mux"
)

func customersGetHandleFunc(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte(fmt.Sprintf("handling customers\n")))
}

func customerGetByIDHandleFunc(w http.ResponseWriter, r *http.Request) {
	vars := mux.Vars(r)

	w.Write([]byte(fmt.Sprintf("handling ID %s\n", vars["id"])))
}

func main() {
	rtr := mux.NewRouter()
	rtrCust := rtr.PathPrefix("/customer").Subrouter()
	rtrCust.Path("").
		Methods(http.MethodGet).
		HandlerFunc(customersGetHandleFunc)
	rtrCust.Path("/").
		Methods(http.MethodGet).
		HandlerFunc(customersGetHandleFunc)
	rtrCust.
		Path("/{id}").
		Methods(http.MethodGet).
		HandlerFunc(customerGetByIDHandleFunc)

	http.ListenAndServe("localhost:8080", rtr)
}