Helper function for replacing regular expressions with the result of a function application in String.prototype.replace.
function StringReplaceGlobalRegExpWithFunction(subject, regexp, replace) {
  var resultArray = reusableReplaceArray;
  if (resultArray) {
    reusableReplaceArray = null;
  } else {
    // Inside a nested replace (replace called from the replacement function
    // of another replace) or we have failed to set the reusable array
    // back due to an exception in a replacement function. Create a new
    // array to use in the future, or until the original is written back.
    resultArray = new InternalArray(16);
  }
  var res = %RegExpExecMultiple(regexp,
                                subject,
                                RegExpLastMatchInfo,
                                resultArray);
  regexp.lastIndex = 0;
  if (IS_NULL(res)) {
    // No matches at all.
    reusableReplaceArray = resultArray;
    return subject;
  }
  var len = res.length;
  if (NUMBER_OF_CAPTURES(RegExpLastMatchInfo) == 2) {
    // If the number of captures is two then there are no explicit captures in
    // the regexp, just the implicit capture that captures the whole match.  In
    // this case we can simplify quite a bit and end up with something faster.
    // The builder will consist of some integers that indicate slices of the
    // input string and some replacements that were returned from the replace
    // function.
    var match_start = 0;
    for (var i = 0; i < len; i++) {
      var elem = res[i];
      if (%_IsSmi(elem)) {
        // Integers represent slices of the original string.
        if (elem > 0) {
          match_start = (elem >> 11) + (elem & 0x7ff);
        } else {
          match_start = res[++i] - elem;
        }
      } else {
        var func_result = replace(elem, match_start, subject);
        // Overwrite the i'th element in the results with the string we got
        // back from the callback function.
        res[i] = TO_STRING(func_result);
        match_start += elem.length;
      }
    }
  } else {
    for (var i = 0; i < len; i++) {
      var elem = res[i];
      if (!%_IsSmi(elem)) {
        // elem must be an Array.
        // Use the apply argument as backing for global RegExp properties.
        var func_result = %reflect_apply(replace, UNDEFINED, elem);
        // Overwrite the i'th element in the results with the string we got
        // back from the callback function.
        res[i] = TO_STRING(func_result);
      }
    }
  }
  var result = %StringBuilderConcat(res, len, subject);
  resultArray.length = 0;
  reusableReplaceArray = resultArray;
  return result;
}