// The list of all sub categories

myList = [".a1", ".b1", ".b3", ".d2", ".e2", ".e4", ".e5"];

​

// The function to get the category of a particular subcategory

function getCategory(subCat){

    return subCat.substr(1, 1);

}

​

​

// The function to build our selector recursively

function buildSelector(categoryList, categoryIndexArray, categoryIndex)

{

    if (categoryIndex == categoryIndexArray.length - 1)

        return categoryList[categoryIndexArray[categoryIndex]];

    else {

        var results = [];

        var subList = buildSelector(categoryList, categoryIndexArray, categoryIndex + 1);

        $.each(categoryList[categoryIndexArray[categoryIndex]],function(_,e){

            $.each(subList,function(_,e1){

                results.push(e + e1);

            });

        });

        return results;

    }

}

​

var categories = {}; 

var categoryArray = [];

​

// build the categories and categoryArray 

$.each(myList,function(_,e){

    var cat = getCategory(e);

    if (categories[cat] === undefined)

    {

        categories[cat] = [];

        categoryArray.push(cat);

    }

    if (categories[cat].indexOf(e) < 0)

        categories[cat].push(e);

});

​

// build our selector as array of groups

var selectors = buildSelector(categories, categoryArray, 0);

​

// join all selector groups to get the string form

console.log(selectors.join(","));