openerp.YOUR_CUSTOM_MOUDLE_NAME = function (instance) {
    var _t = instance.web._t;
    var QWeb = instance.web.qweb;
    var module = instance.point_of_sale;

    module.PosDB.include({
        init: function(options){
            var self = this;
            this._super();
        },

       search_product_in_category: function(category_id, query){

		//You can add your custom code here

            try {
                query = query.replace(/[\[\]\(\)\+\*\?\.\-\!\&\^\$\|\~\_\{\}\:\,\\\/]/g,'.');
                query = query.replace(/ /g,'.+');
                var re = RegExp("([0-9]+):.*?"+query,"gi");
            }catch(e){
                return [];
            }
            var results = [];
            for(var i = 0; i < this.limit; i++){
                r = re.exec(this.category_search_string[category_id]);
                if(r){
                    var id = Number(r[1]);
                    results.push(this.get_product_by_id(id));
                }else{
                    break;
                }
            }
            return results;
        },
	});


};

Dear Vinod,
you can try the above method.