Annie_regex

var s = "some_string_to_change",
result = s.replace(/_./g, rep); // do a global search (g) for an "_" followed by a single character (.) and run the rep function each time

function rep(v) {
  return v[1].toUpperCase(); // return the second character of the matched string, uppercased.
}

console.log(result);

I got as far as:

function up_case(str2){
return str2.replace(/_./g,'');
}

But, I didn't know you could pass in a function for the new value.