how to select random keyword?

i have one problem with this code. it selects the first three keyword. what if i want to select the keyword in random number? currently, it has 7 keywords and it selects the first three. what if i want to select keyword 3,6,7. how can i do the random function here? thanks in advanced!

(function($) {
   $.fn.replacetext = function (target, replacement, max) {
       var limit = max || -1;


       var $textNodes = this  
           .find("*")
           .andSelf()
           .contents()
           .filter(function () {
           return this.nodeType === 3 && !$(this).parent("a").length;
       });

       $textNodes.each(function (index, element) {
           var $element = $(element);
           var words = $element.text().split(/\b/);
           var matches = 0;
           var text = words.map(function (word, index) {
               if (matches >= limit) {
                   return word;
               }

               if (word.match(target)) {
                   ++matches;
                   return word.replace(target, replacement);
               }

               return word;
           });

           $element.replaceWith(text.join(''));
       });
   };
})(jQuery);


$("p").replacetext(/\bdress\b/gi, "<a href='http://www.google.com'>$&</a>", 3);