Compute the string of a given regular expression capture.
function CaptureString(string, lastCaptureInfo, index) {
  // Scale the index.
  var scaled = index << 1;
  // Compute start and end.
  var start = lastCaptureInfo[CAPTURE(scaled)];
  // If start isn't valid, return undefined.
  if (start < 0) return;
  var end = lastCaptureInfo[CAPTURE(scaled + 1)];
  return %_SubString(string, start, end);
}