You need to slice the string and push each character to an array. Then, use for loop to apply your function to each element in your array.
Here's a sample function I made using AS3:
function splitString(str:String):Array{
var arr = [];
for(var i=0;i<str.length;i++){
arr.push(str.substr(i,1));
}
return arr;
}
to apply:
for(var a=0;a<str_split.length;a++){
//trace(str_split[a]);
//apply here your function
}