Remove non-numeric characters from a field

Multi-select picklists are stored as value1;value2;value3;etc. Knowing that, you can get a List of these values using the String split() method. You can also view this string splitting example. To access it, you would then tweak your code as:

trigger AutoCreateInterviewer on Position__c (after insert) {
    List<Interviewer__c> interviewers = new List<Interviewer__c>();

    //For each position processed by the trigger, add a new  

    //interviewer record for the specified hiring manager.  

    //Note that Trigger.New is a list of all the new positions  

    //that are being created.  

    for (Position__c newPosition: Trigger.New) {
        if (newPosition.Hiring_Manager__c != null) {
            // split out the multi-select picklist using the semicolon delimiter
            for(String hiringManager: newPosition.Hiring_Manager__c.split(';')){
                interviewers.add(new Interviewer__c(
                        Name = '1',
                        Position__c = newPosition.Id,
                        Employee__c = (Id)hiringManager,
                        Role__c = 'Managerial'));
            }
        }
    }
    insert interviewers;
}

@isTest
public with sharing class AutoCreateInterviewerTriggerTest {

    static testMethod void testGetAdvisor() {
        // First, set up your test User.
        User testUser = generateTestUser();

        // Generate your position but don't perform DML yet
        Position__c pos = new Position(Hiring_Manager__c = testUser.Id);

        // Start your test and insert your position
        Test.startTest();
        insert pos;
        Test.stopTest();

        // Get the interviewers from the DB to ensure they were created
        List<Interviewer__c> interviewers = [SELECT
                                                 Id
                                             FROM
                                                 Interviewer__c
                                             WHERE
                                                 Position__c = :pos.Id
        ];

        // There should be 1 interviewer because only one user ID was provided
        System.assert(1, interviewers.size(), 'One interviewer object is expected');
    }

    private User generateTestUser(){
        // We use the System Admin profile because it will always be there
        Profile sysAdmin = [SELECT 
                               Id 
                            FROM 
                               Profile 
                            WHERE 
                               Name = 'System Administrator'
                            LIMIT 1
        ];

        // Generate the user
        User newUser = new User(
            UserName = 'unit.tester@example.com',
            LastName = 'Test',
            FirstName = 'Jane',
            Email = 'unit.tester@example.com',
            phone = '555-555-5555',
            MobilePhone = '555-555-5554',
            Street = '123 Fake Street',
            City = 'Los Angeles',
            State = 'CA',
            PostalCode = '12345',
            CommunityNickName = 'unit.test',
            Alias = 'abcd',
            ProfileId = sysAdmin.Id,
            emailencodingkey = 'UTF-8',
            languagelocalekey = 'en_US',
            localesidkey = 'en_US',
            timezonesidkey = 'America/Los_Angeles'
        );

        // Insert that user
        insert user;

        return user;
    }
}

I know this is old, but here's how I'm doing it.

$(function () {
    // Create the tree inside the <div id="tree"> element.
    $("#tree").fancytree({
        source: { url: "/Home/GetData", cache: true }
        , checkbox: true
        , icons: false
        , cache: true
        , lazyLoad: function (event, data) {
            var node = data.node;
            data.result = {
                url: "/Home/GetTreeViewData/" + node.key
                , data: { mode: "children", parent: node.key }
                , cache: true
            };
        }
        , selectMode: 3
        , select: function (event, data) { 
        }
        , strings: {
            loading: "Grabbing places and events…",
            loadError: "Load error!"
        },
    })
});

Basically, if you were to browse to mysite/Home/GetData, it returns a json result that looks like this

[{"key":1,"title":"Accommodations","lazy":true,"parentId":null},{"key":5,"title":"Attractions","lazy":false,"parentId":null},{"key":6,"title":"Dining","lazy":false,"parentId":null},{"key":7,"title":"Entertainment","lazy":false,"parentId":null},{"key":18,"title":"1","lazy":true,"parentId":null},{"key":23,"title":"2","lazy":true,"parentId":null}]

Now say you expand the first node, it's ID in the database is 1, and any children underneath it have a column called parentId with a value of one. When you browse to mysite/Home/GetTreeViewData/1 you get this

[{"key":2,"title":"Hotels","lazy":false,"parentId":1},{"key":3,"title":"Bed \u0026 Breakfast","lazy":false,"parentId":1}]

So I just pull the node.key as my Id and use it to load the children. Hope it helps.