A way to allow admin changeable validation for usernames created in the AccountEntry block

AccountEntry.ascx

        var usernameRegExp = new RegExp("<%= GetAttributeValue( "ValidUsernameRegularExpression" ) %>");
        var usernameValidDescription = "<%= GetAttributeValue( "ValidUsernameDescription" ) %>";

        availabilityMessageRow.hide();

        usernameTextbox.blur(function () {
            if ($(this).val() && $.trim($(this).val()) != '') {

                if (! usernameRegExp.test($(this).val())) {
                    usernameUnavailable.html('Username is not valid! ' + usernameValidDescription );
                    usernameUnavailable.addClass('alert-warning');
                    usernameUnavailable.removeClass('alert-success');
                }
                else {
                    $.ajax({
                        type: 'GET',
                        contentType: 'application/json',
                        dataType: 'json',
                        url: Rock.settings.get('baseUrl') + 'api/userlogins/available/' + escape($(this).val()),
                        success: function (getData, status, xhr) {

                            if (getData) {
                                usernameUnavailable.html('That username is available.');
                                usernameUnavailable.addClass('alert-success');
                                usernameUnavailable.removeClass('alert-warning');
                            } else {
                                availabilityMessageRow.show();
                                usernameUnavailable.html('That username is already taken!');
                                usernameUnavailable.addClass('alert-warning');
                                usernameUnavailable.removeClass('alert-success');
                            }
                        },
                        error: function (xhr, status, error) {
                            alert(status + ' [' + error + ']: ' + xhr.responseText);
                        }
                    });
                }

            } else {
                usernameUnavailable.html('Username is required!');
                usernameUnavailable.addClass('alert-warning');
                usernameUnavailable.removeClass('alert-success');
            }
            availabilityMessageRow.show();
        });

AccountEntry.ascx.cs

// two new block properties to control the valid regular expression and the human readable version
    [TextField( "Valid Username Regular Expression", "A regular expression that is used to validate good, valid usernames.", false, "^[A-Za-z0-9+.@_-]{3,128}$" )]
    [TextField( "Valid Username Description", "Human description of a valid username.", false, "It must only contain letters, numbers, +, -, _, or @. It must also be longer than three characters and less than 128." )]