searchAndReplace(string, pattern, step): step through RegExp matches and generate replacements with callback that has full access to string indexes.

search-and-replace.js

// Splice in a string between `begin` and `end` indexes.
// Returns a new string.
function spliceString(string, insert, begin, end) {
  return string.slice(0, begin) + insert + string.slice(end);
}

// Search a string with a regular expression, where `step` callback will
// be called at each match, allowing you to perform extra logic and create a
// replacement string.
//
// Abstracts away all the scary janky footguns of `RegExp.exec`.
// 
// See http://alexyoung.org/2011/08/01/lightweight-markup/
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec
function searchAndReplace(string, pattern, step) {
  // Clone RegExp/pattern string. We're going to mutate its state.
  var re = new RegExp(pattern);
  // Initialize variables we'll use in the loop.
  var match, replacement, diff, substring, startIndex, endIndex;

  while ((match = re.exec(string)) !== null) {
    // Calculate match start index. `lastIndex` contains the index to start
    // next search on, so we actually want to back up by the `length` of the
    // matched substring.
    substring = match[0];
    startIndex = match.index;
    endIndex = startIndex + substring.length;
    // Create replacement string given the environmental variables.
    replacement = step(substring, string, startIndex);
    // Splice replacement string into place of old substring.
    string = spliceString(string, replacement, startIndex, endIndex);
    // Calculate the difference in length between the substring and
    // its replacement.
    diff = replacement.length - substring.length;
    // We changed the width of the string, so we have to adjust the `lastIndex`
    // property so the `RegExp` object stays in sync.
    re.lastIndex = re.lastIndex + diff;
  }

  return string;
}