REGEXP_FILE
  RegExpPatternImpl: <a(bc)d>
    RegExpBranchImpl: <a(bc)d>
      RegExpCharImpl: <a>
        PsiElement(CHARACTER)('a')
      RegExpGroupImpl: <(bc)>
        PsiElement(GROUP_BEGIN)('(')
        RegExpPatternImpl: <bc>
          RegExpBranchImpl: <bc>
            RegExpCharImpl: <b>
              PsiElement(CHARACTER)('b')
            RegExpCharImpl: <c>
              PsiElement(CHARACTER)('c')
        PsiElement(GROUP_END)(')')
      RegExpCharImpl: <d>
        PsiElement(CHARACTER)('d')