get rid of x if no extension given for phone field in Jquery masked input plugin

Im using jquery Masked input plugin.

I have the following mask on a field:

 "?999-999-9999 x9999"

Why ? in begining? because if user entered incomplete number, i dont want the field to clear out so i made the whole thing optional. now when user is entering the numbers is able to see x always whether or not a phone has extension. How do i get rid of that x if no extension was given?

Again i do not want to clear out the field, if i use

  "999-999-9999? x9999" 

yes it works it adds x only when it has extension but if incomplete phone entered it empties the field which is not the goal.

Any help is appreciated thanks alot.

after trying alot i fixed it myself to accomplish the goal.

Goal was to keep text not disappearing if incomplete input was given and to get rid of x if no extension was given.

I added this test at the end of code:

var splitString = $("[name=myinput]").val().split("x");
//if splitString array length is more than 1 means splitString[1] is set
//if splitString[1].length is 0 means its only x by itself so get rid of it
if (splitString.length > 1 && splitString[1].length == 0) {
    $("[name=myinput]").val($("[name=myinput]").val().replace("x", ""));
}

examples:

 entered text: 718-111-123 x -> turns into -> 718-111-123
 entered text: 718-111     x -> turns into -> 718-111
 entered text: 718-111-123 x555 -> turns into -> 718-111-123 x555