How to get elements of specific class starting with a given string?

You can also try:

$(".etape").click(function () {
    var theClass = $(this).attr("class").match(/btn[\w-]*\b/);
    console.log(theClass); 
});

Uses match instead of grep...

 // Only select input which have class 
 $('input[class]').click(function(){  
   var myClass;
   // classNames will contain all applied classes
   var classNames = $(this).attr('class').split(/\s+/);  

     // iterate over all class  
     $.each(classNames, function(index, item) {
        // Find class that starts with btn-
        if(item.indexOf("btn-") == 0){
          // Store it
          myClass = item;
        }
     });

  });

Live Demo

You can use Regular Expression or split the class name.

$(".etape").click(function(){
   var classes = $.grep(this.className.split(" "), function(v, i){
       return v.indexOf('btn') === 0;
   }).join();
});

http://jsfiddle.net/LQPh6/