option-filter.js

jQuery.fn.filterByText = function(textbox, selectSingleMatch) {
    return this.each(function() {
        var select = this;
        var options = [];
        $(select).find('option').each(function() {
            options.push({value: $(this).val(), text: $(this).text(), disabled:$(this).is(':disabled')});
        });
        $(select).data('options', options);
        $(textbox).bind('change keyup', function() {
            var options = $(select).empty().scrollTop(0).data('options');
            var search = $.trim($(this).val());
            var regex = new RegExp(search,'gi');

            $.each(options, function(i) {
                var option = options[i];
                if(option.text.match(regex) !== null) {
                    var newOption = $('<option>');
                    if(option.disabled) {
                        newOption.attr('disabled', 'disabled');
                    }
                    newOption.text(option.text).val(option.value);
                    $(select).append(newOption);
                }
            });
            if (selectSingleMatch === true &&
                $(select).children().length === 1) {
                $(select).children().get(0).selected = true;
            }
        });
    });
};

// example
$(function() {
    $('.category-filter-select').filterByText($('.category-filter'), true);
});