(.) //match any character
\1 //if it follow by the same character
+{2 //more than 1 times
/g //global
$1 //is 1 time by $1$1 is 2 times 

     reduceString('aaaabbbb', 2) 
     reduceString('xaaabbbb', 2) 
            reduceString('aaaabbbb', 1)     
            reduceString('aaxxxaabbbb', 2) 

            function reduceString(txt,num)
            {
                var canRepeat=['$1'];
               for (i=1;i<num;i++)
               {
                  canRepeat.push('$1')
               }
                canRepeat = canRepeat.join('');

                console.log(txt.replace(/(.)\1{2,}/g, canRepeat))

            }