private void highlightSampleGroup(int offset, @NotNull PsiFile regExpFile) {
    final HighlightManager highlightManager = HighlightManager.getInstance(regExpFile.getProject());
    removeHighlights(highlightManager);

    final List<RegExpMatch> matches = getMatches(regExpFile);
    int index = indexOfGroupAtOffset(matches, offset);
    if (index > 0) {
      @Nullable RegExpGroup group =
        SyntaxTraverser.psiTraverser(regExpFile)
          .filter(RegExpGroup.class)
          .filter(RegExpGroup::isCapturing)
          .get(index - 1);
      highlightRegExpGroup(group, highlightManager);
      highlightMatchGroup(highlightManager, matches, index);
    }
    else {
      highlightMatchGroup(highlightManager, matches, 0);
    }
  }
private void highlightSampleGroup(int offset, @NotNull PsiFile regExpFile)