code: 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);
  }
docs: Compute the string of a given regular expression capture.