Count repeated letters in a string

Question

I\'m stuck with the following problem: I need to find repeated characters in a string. Basically what I want is regular expression that will match like that

Answer 1

var obj = {};
var str = "this is my string";
for (var i = 97; i < 97 + 26; i++) 
  obj[String.fromCharCode(i)] = 0;
for (var i = 0; i < str.length; i++) {
  obj[str.charAt(i).toLowerCase()]++;
}

From there you can say obj["a"] to get the number of occurrences for any particular letter.