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'esversion: 6';
function myReplace(str, before, after) {
  var re = new RegExp(before);
  return before[0] === before[0].toLowerCase() ? str.replace(re, after)
         : str.replace(re, after.charAt(0).toUpperCase() + after.slice(1));
}

myReplace("He is Sleeping on the couch", "Sleeping", "sitting");