replaceBetween.js

// String replacement with index constraints
function replaceBetween(startIndex, endIndex, search, replace, text) {
 
  // Portion of text up until the section we want to search in
  var firstChunk = text.slice(0, startIndex);
  
  var lastChunk = (endIndex !== text.length - 1) ? text.slice(endIndex, text.length) : "";
  
  // Create the string to search within
  var searchableText = text.slice(startIndex, endIndex);
  
  // Replace the text
  var newText = searchableText.replace(search, replace);
  
  // Piece together the result
  return firstChunk + newText + lastChunk;
  
  
}

Example http://jsbin.com/zugiroj/1/edit?html,js,output