function searchHandler()
    {
        var regex = input.value ? new RegExp(input.value.escapeForRegExp(), "i") : null;
        filterCallback(regex);
        container.classList.toggle("styles-filter-engaged", !!input.value);
    }
@param {string} placeholder
@param {!Element} container
@param {function(?RegExp)} filterCallback
@return {!Element}