public static @NotNull String extractStringToFind(@NotNull String regexp, @NotNull Project project) {
    return ReadAction.compute(() -> {
      List<PsiElement> topLevelRegExpChars = getTopLevelRegExpChars("a", project);
      if (topLevelRegExpChars.size() != 1) return " ";

      // leave only top level regExpChars

      Class<?> regExpCharPsiClass = topLevelRegExpChars.get(0).getClass();
      return getTopLevelRegExpChars(regexp, project)
        .stream()
        .map(psi -> {
          if (regExpCharPsiClass.isInstance(psi)) {
            String text = psi.getText();
            if (!text.startsWith("\\")) return text;
          }
          return " ";
        })
        .collect(Collectors.joining());
    });
  }
public static @NotNull String extractStringToFind(@NotNull String regexp, @NotNull Project project)