REGEXP_FILE
  RegExpPatternImpl: <(|)>
    RegExpBranchImpl: <(|)>
      RegExpGroupImpl: <(|)>
        PsiElement(GROUP_BEGIN)('(')
        RegExpPatternImpl: <|>
          RegExpBranchImpl: <>
            <empty list>
          PsiElement(UNION)('|')
          RegExpBranchImpl: <>
            <empty list>
        PsiElement(GROUP_END)(')')