REGEXP_FILE
  RegExpPatternImpl: <a[a[b[c]][d]]>
    RegExpBranchImpl: <a[a[b[c]][d]]>
      RegExpCharImpl: <a>
        PsiElement(CHARACTER)('a')
      RegExpClassImpl: <[a[b[c]][d]]>
        PsiElement(CLASS_BEGIN)('[')
        RegExpCharImpl: <a>
          PsiElement(CHARACTER)('a')
        RegExpClassImpl: <[b[c]]>
          PsiElement(CLASS_BEGIN)('[')
          RegExpCharImpl: <b>
            PsiElement(CHARACTER)('b')
          RegExpClassImpl: <[c]>
            PsiElement(CLASS_BEGIN)('[')
            RegExpCharImpl: <c>
              PsiElement(CHARACTER)('c')
            PsiElement(CLASS_END)(']')
          PsiElement(CLASS_END)(']')
        RegExpClassImpl: <[d]>
          PsiElement(CLASS_BEGIN)('[')
          RegExpCharImpl: <d>
            PsiElement(CHARACTER)('d')
          PsiElement(CLASS_END)(']')
        PsiElement(CLASS_END)(']')